Germania-Based Sol-Gel Organic-Inorganic Hybrid Coatings ...

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Germania-Based Sol-Gel Organic-Inorganic Hybrid Coatings for Germania-Based Sol-Gel Organic-Inorganic Hybrid Coatings for

Capillary Microextraction Capillary Microextraction

Li Fang University of South Florida

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Germania-Based Sol-Gel Organic-Inorganic Hybrid Coatings for Capillary

Microextraction

by

Li Fang

A dissertation submitted in partial fulfillment

of the requirements for the degree of

Doctor of Philosophy

Department of Chemistry

College of Arts and Sciences

University of South Florida

Major Professor: Abdul Malik, Ph.D.

Milton D. Johnston, Ph.D.

Jennifer Lewis, Ph.D.

Robert Potter, Ph.D.

Date of Approval:

April 1, 2009

Keywords: Sol-Gel Germania Coating, CME, SPME, In-Tube SPME, Gas

Chromatography, Stationary Phase

© Copyright 2009 , Li Fang

DEDICATION

To

my parents Duohui Li and Fengshan Fang

my sister Ying Fang

who made this possible, for their endless love and support.

ACKNOWLEDGEMENT

I would like to express my gratitude to many people who gave me the possibility

to complete this thesis. I am deeply grateful to my major professor, Dr. Abdul Malik, for

his supervision, patience and encouragement in the past five years. I also want to present

my sincere thanks to my dissertation committee members: Dr. Milton D. Johnston, Dr.

Jennifer Lewis, and Dr. Robert Potter for their valuable advice, thoughtful comments and

support.

I would like to thank all my former and current colleagues, Dr. Khalid Alhooshani,

Dr. Abuzar Kabir, Dr. Tae-Young Kim, Dr. Sameer M. Kulkarni, Dr. Wen Li, Anne M.

Shearrow, Erica Turner and Scott Segro for their continuous assistance, encouragement,

and friendship. I also want to extend thanks to the Department of Chemistry for financial

support over the past five years.

i

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................ vii

LIST OF FIGURES ........................................................................................................... ix

LIST OF SCHEMES......................................................................................................... xii

LIST OF SYMBOLS AND ABBREVIATIONS ............................................................ xiii

ABSTRACT .......................................................................................................................xv

CHAPTER ONE: BACKGROUND OF SOLID PHASE MICROEXTRACTION

(SPME) .......................................................................................................................1

1.1 Introduction to sample preparation .......................................................................1

1.2 Introduction to classical sample preparation approaches ......................................2

1.2.1 Liquid-liquid extraction (LLE) ....................................................................2

1.2.2 Soxhlet extraction ........................................................................................3

1.3 Solvent-free extraction ..........................................................................................3

1.3.1 Gas-phase extraction ....................................................................................5

1.3.2 Supercritical fluid extraction (SFE) .............................................................6

1.3.3 Membrane extraction ...................................................................................7

1.3.4 Solid phase extraction (SPE) .......................................................................8

1.4 Introduction of exhaustive-extraction and non-exhausive extraction

techniques .............................................................................................................8

1.5 Introduction to solid phase microextraction .......................................................10

ii

1.5.1 SPME device and application ....................................................................11

1.5.2 Theoretical aspect of SPME.......................................................................13

1.6 Conventional SPME coating ...............................................................................16

1.6.1 Homegeneous polymers .............................................................................19

1.6.2 Composite extraction media ......................................................................20

1.6.3 Emerging extraction media ........................................................................20

1.7 Different formats of SPME .................................................................................21

1.7.1 Fiber SPME ................................................................................................21

1.7.1.1 Direct immersion SPME (DI) .........................................................22

1.7.1.2 Headspace fiber SPME (HS-SPME) ...............................................22

1.7.2 In-tube SPME (also termed capillary microextraction (CME)) .................23

1.7.2.1 Comparison of fiber SPME and in-tube SPME ..............................24

1.7.2.2 Theoretical aspect of in-tube SPME ...............................................30

1.7.2.3 Parameters influencing in-tube SPME ............................................32

1.7.3 Stir bar sorptive extraction (SBSE)............................................................32

1.8 References for chapter one..................................................................................34

CHAPTER TWO: SOLID PHASE MICROEXTRACTION AND SOL-GEL

TECHNOLOGY .......................................................................................................39

2.1 Introduction to sol-gel process and some basic definitions ................................39

2.2 Historical background of sol-gel technology ......................................................42

2.3 The potential of sol-gel chemistry in coating technology for analytical

microextraction .................................................................................................43

2.3.1 Problems with traditional SPME coating technology ................................43

iii

2.3.2 The advantages sol-gel technology for use in analytical

microextraction ..........................................................................................44

2.4 Fundamentals of sol-gel technology ...................................................................46

2.5 Creation of sol-gel CME sorbents.......................................................................49

2.5.1 Pre-treatment of the fused-silica capillary .................................................49

2.5.2 Preparation of the sol solution ...................................................................52

2.5.3 Sol-gel coating process ..............................................................................55

2.5.4 Further treatment of sol-gel-coated CME capillay ....................................55

2.6 Characterization of sol-gel extracting phase and its morphology .......................56

2.7 Reported sol-gel coating materials in SPME and CME......................................57

2.8 Drawbacks of silica-based sol-gel materials .......................................................62

2.9 Sol-gel process using transition metal alkoxide..................................................62

2.10 Non-silica sol-gel materials in analytical microextraction and

chromatographic separation ..............................................................................67

2.11 References for chapter two ...............................................................................71

CHAPTER THREE: GERMANIA-BASED SOL-GEL HYBRID

ORGANIC-INORGANIC COATINGS FOR CAPILLARY

MICROEXTRACTION AND GAS CHROMATOGRAPHY .................................80

3.1 Introduction of germanosilicate glasses material ................................................80

3.2 Preparation of sol-gel GeO2/ormosil organic-inorganic hybrid material ............82

3.3 Ormosil precursors at low treatment temperature ...............................................83

3.4 Problems of germanosilicate material during sol-gel process ............................85

iv

3.5 Introduction of Germania –based sol-gel organic-inorganic hybrid

coatings for capillary microextraction and gas chromatography ........................86

3.6 Experimental .......................................................................................................91

3.6.1 Equipment ..................................................................................................91

3.6.2 Chemicals and materials ............................................................................92

3.6.3 Preparation of sol-gel germanis-PDMS coated capillaries for CME .........92

3.6.4 Preparation of silica-based sol-gel PDMS columns for GC analysis ........94

3.6.5 Preparation of aqueous samples for CME-GC...........................................94

3.6.6 Gravity-fed sample dispenser for capillary microextraction .....................94

3.6.7 Sol-gel capillary microextraction ...............................................................95

3.6.8 Safety precautions ......................................................................................96

3.7 Results and discussion ........................................................................................96

3.7.1 Chemical reactions involved in the preparation of sol-gel germania

coatings ......................................................................................................96

3.7.2 IR Characterization ..................................................................................103

3.8 CME profile for sol-gel Germania coated capillaries .......................................103

3.9 Extraction characteristics of sol-gel germania coatings in CME ......................106

3.9.1 Extraction of non-polar and moderately polar compounds by sol-gel

germania-PDMS coated capillary for CME-GC analysis ........................106

3.9.2 pH stability of sol-gel germania-PDMS coated capillary ........................113

3.9.3 Extraction of polar compounds by sol-gel germania CME-GC...............114

3.9.4 GC separation of sol-gel germania-PDMS coated column ......................119

3.10 Conclusion ......................................................................................................122

v

3.11 References for chapter four .............................................................................122

CHAPTER FOUR: MIXED GERMANIA-SILICA SOL-GEL

CYANOPROPYL-POLY(DIMETHYLSILOXANE) COATING FOR

CAPILLARY MICROEXTRACTION OF POLAR ORGANIC TRACE

CONTAMINANTS IN AQUEOUS MEDIA .........................................................128

4.1 Introduction .......................................................................................................128

4.2 Experimental .....................................................................................................132

4.2.1 Equipment ................................................................................................132

4.2.2 Chemicals and materials ..........................................................................155

4.2.3 Preparation of sol-gel germanis-CN/PDMS-coated capillaries for

CME .........................................................................................................133

4.2.4 Preparation of aqueous samples for CME-GC.........................................134

4.2.5 Sol-gel capillary microextraction by an in-lab gravity-fed sample

dispenser ..................................................................................................135

4.3 Results and discussion ......................................................................................136

4.3.1 Chemical reactions involved in the preparation of sol-gel

germania-CN/PDMS coatings .................................................................136

4.3.2 Extraction profiles for sol-gel germania-CN/PDMS capillaries ..............140

4.3.3 Extraction characteristics of sol-gel germania-CN/PDMS in CME ........144

4.3.4 Thermal stability of sol-gel germania coatings in CM ............................154

4.4 Conclusion ........................................................................................................156

4.5 References for chapter five ...............................................................................156

APPENDICES .................................................................................................................160

vi

Appendix A: Germia-based, sol-gel hybird organic-inorganic coatings for

capillary microextraction and gas chromatography ..........................................161

Appendix B: Sol-gel immobilized cyano-polydimethylsiloxane coatings for

capillary microextraction of aqueous trace analytes ranging from

polycyclic aromatic hydrocarbons to free fatty acids .......................................172

ABOUT THE AUTHOR ................................................................................... END PAGE

vii

LIST OF TABLES

Table 1.1 Commercial SPME coatings and their properties ..............................................18

Table 2.1 Organic sorbents and their applications .............................................................58

Table 3.1 Chemical ingredients of the coating solution used in preparing sol-gel

germania organic-inorganic hybrid coatings ............................................................98

Table 3.2 GC peak area and retention time repeatability data for PAHs, aldehydes,

and ketones extracted from aqueous samples by CME using a sol-gel

germania-PDMS coated microextraction capillary .................................................107

Table 3.3 Detection limit data for PAHs, aldehydes, and ketones extracted from

aqueous samples by CME-GC using a sol-gel germania-PDMS coated

microextraction capillary ........................................................................................108

Table 3.4 Capillary-to-capillary reproducibility (n=3) data for extracted amounts in

CME-GC experiments conducted on sol-gel germania-PDMS coated capillaries

using a PAH (phenanthrene), an aldehyde (undecanal), and a ketone

(hexanophenone) as test solutes ..............................................................................109

Table 4.1 Chemical ingredients of the coating solution used in preparing sol-gel

germania organic-inorganic hybrid coatings chemically anchored to the inner

surface of a fused silica capillary ............................................................................138

viii

Table 4.2 GC peak area and retention time repeatability data (n=4) for

chlorophenols,alcohols, free fatty acids and ketones extracted from aqueous

samples by CME using sol-gel germania-CN/PDMS coated microextraction

capillary...................................................................................................................145

Table 4.3 GC-FID detection limit data (n=4) for chlorophenols, alcohols, free fatty

acids and ketones extracted from aqueous samples by CME using sol-gel

germania-CN/PDMS coated microextraction capillary ..........................................146


CME-GC experiments conducted on sol-gel germania-CN/PDMS coated

capillaries using a chlorophenol (pentachlorophenol), an alcohol (undecanol),

a free fatty acid (undecanoic acid) and a ketone (heptanophenone) as test

solutes .....................................................................................................................147

ix

LIST OF FIGURES

Figure 1.1 Classification of Solvent-free extraction techniques ..........................................5

Figure 1.2 Classification of the extraction techniques .......................................................10

Figure 1.3 Schematic of a SPME device ...........................................................................12

Figure 1.4 Extraction step for SPME .................................................................................13

Figure 1.5 Principle of SPME ............................................................................................14

Figure 1.6 Modes of SPME operation ...............................................................................23

Figure 1.7 Comparison SPME fiber with in-tube SPME ...................................................24

Figure 1.8 Comparison of extraction process for the transfer of analytes in fiber

SPME and in- tube SPME.........................................................................................25

Figure 1.9 Illustration of extraction procedure of extraction by fiber SPME and

analyte desorption for HPLC analysis ......................................................................28

Figure 1.10 Schematic diagrams of in-tube SPME-LC-MS systems ................................29

Figure 1.11 Distribution of analyte between sample matrix and the extraction phase ......30

Figure 1.12 Schematic of a stir bar of SBSE .....................................................................33

Figure 2.1 Overview of the Sol-gel process.......................................................................41

Figure 2.2 Homemade capillary filling/purging device .....................................................51

Figure 3.1-A FTIR spectra representing: pure PDMS .....................................................104

Figure 3.1-B FTIR spectra representing: sol-gel germania PDMS hybrid material

used as a sorbent in capillary microextraction ........................................................104

x

Figure 3.2 Capillary microextraction profiles of pyrene and heptanophenone on a

sol-gel germania coated capillary ...........................................................................105

Figure 3.3 CME-GC trace analysis of a mixture of PAHs using a sol-gel

germania-PDMS coated microextraction capillary .................................................111

Figure 3.4 CME-GC trace analysis of a mixture of aldehydes and ketones using a

sol-gel germania-PDMS coated microextraction capillary .....................................112

Figure 3.5 CME-GC trace analysis of alcohols using a sol-gel germania-APTMS

coated microextraction capillary .............................................................................115

Figure 3.6 Excellent stability of sol-gel germania-PDMS coating under highly basic

conditions demonstrated through CME-GC analysis of a mixture of PAHs

using a sol-gel germania-PDMS coated microextraction capillary before

(3.6-A) and after (3.6-B) continuously rinsing the capillary with a 0.1 M

NaOH solution (pH = 13) for 24h ...........................................................................116

Figure 3.7 Excellent stability of sol-gel germania-PDMS coating under highly

acidic conditions demonstrated through CME-GC analysis of aldehydes using

a germania-PDMS coated microextraction capillary before (3.7-A) and after

(3.7-B) continuously rinsing the capillary with a 0.05 M HCl solution

for 24 h ....................................................................................................................117

Figure 3.8 CME-GC trace analysis of 2, 4, 6 Trichlorophenol using a sol-gel

germania-PDMDPS coated microextraction capillary............................................118

Figure 3.9 CME-GC analysis of free fatty acids using a sol-gel germania-APTMS

coated microextraction capillary .............................................................................120

xi

Figure 3.10 Gas chromatogram of a natural gas sample separated on sol-gel

germania-PDMS stationary phase...........................................................................121

Figure 4.1 Capillary microextraction profiles of undecanol, heptanophenone,

pentachlorophenol on a sol-gel germania-CN/PDMS coated capillary ..................143

Figure 4.2 CME-GC analysis of a mixture of alcohols using a sol-gel


Figure 4.3 CME-GC analysis of a mixture of acids using a sol-gel


Figure 4.4 CME-GC analysis of a mixture of chlorophenols using a sol-gel


Figure 4.5 CME-GC analysis of a mixture of ketones using a sol-gel


Figure 4.6 Effect of conditioning temperature on the performance of sol–gel

germania-CN/PDMS microextraction capillary in the extraction of nonanol ........155

xii

LIST OF SCHEMES

Scheme 2.1 Illustration of hydrolysis and condensation of tetramethoxysilane

(TMOS) .....................................................................................................................49

Scheme 2.2 Deactivating reagents .....................................................................................53

Scheme 2.3 Deactivation of silica surface with HMDS ....................................................54

Scheme 2.4 General sol-gel process for transitional metal alkoxides ...............................64

Scheme 2.5 Mechanisms of sol-gel hydrolysis and condensation reactions of metal

alkoxides ...................................................................................................................65

Scheme 3.1 Typical hydrolytic ploycondensation of germanium alkoxide to form

germania ....................................................................................................................82

Scheme 3.2 Hydrolysis of sol-gel germania precursor, TMOG ........................................99

Scheme 3.3 Polycondensation of the hydrolyzed precursor and chemical bonding

of the sol-gel-active organic ligand (Y) to the evolving sol-gel network ...............100

Scheme 3.4 Chemical anchoring of the evolving sol-gel germania hybrid

organic-inorganic polymer to the inner walls of a fused silica capillary ................101

Scheme 4.1 Hydrolysis of sol-gel germania precursor (TEOG) ......................................140

Scheme 4.2 Polycondensation of the hydrolyzed precursor and chemical bonding

of the sol-gel-active organic ligand (Y) to the evolving sol-gel network ...............141


organic-inorganic polymer to the inner walls of a fused silica capillary ................142

xiii

LIST OF SYMBOLS AND ABBREVIATIONS

AN Nucleophilic Addition

AFM Atomic Force Microscopy

AMTEOS Anilinemethyltriethoxysilane

APTMS 3-Aminopropyltrimethoxysilane

BMA Butyl methacrylate

C8-TEOS n-Octyltriethoxysilane

CE Capillary Electrophoresis

CEC Capillary Electrochromatography

CME Capillary Microextraction

CME-GC Capillary Microextraction - Gas Chromatography

CPs Chlorophenols

CPTES 3-Cyanopropyltriethoxysilane

CVD Chemical Vapour Deposition

CW/DVB Carbowax/divinylbenzene

CW/TPR Carbowax/templated resin

CW Carbowax

DCCA Drying Control Chemical Additive

DEOS Diethylorthosilicate

DHS Dynamic Headspace

DVB Divinyl benzene

EPA Environmental Protection Agency

EVP-ICP-MS Environmental Vaporization - Inductively Coupled Plasma - Mass

Spectrometry

FA Formic Acid

FID Flame Ionization Detector

GC Gas Chromatography

GC-MS Gas Chromatography - Mass Spectrometry

GLYMO Glycidoxypropyltrimethoxysilane

GODC Germanium Oxygen Deficient Centers

HMDS 1,1,1,3,3,3-Hexamethyldisilazane

HPLC High - Performance Liquid Chromatography

HS-SPME Headspace - Solid Phase Microextraction

ICP-MS Inductively Coupled Plasma - Mass Spectrometry

INCAT Inside Needle Capillary Adsorption Trap

LC-MS Liquid Chromatography - Mass Spectrometry

LLE Liquid - Liquid Extraction

MA Methyl acrylate

MMA Methyl methacrylate

xiv

MS Mass Spectrometry

MSDS Material Safety Data Sheet

MTES Methyltrimethoxysilane

MTMOS Methyltrimethoxysilane

NMR Nuclear Magnetic Resonance

OH-TSO Hydroxyl-terminated polydimethylsiloxane

ppq Parts Per Quadrillion

ppt Parts Per Trillion

poly-THF Polytetrahydrofuran

PA Polyacrylate

PAHs Polycyclic Aromatic Hydrocarbons

PDMDPS Polydimethyldiphenylsiloxane

PDMS Polydimethylsiloxane

PDMS/DVB Polydimethylsiloxane/divinylbenzene

PEG Polyethylene glycol

PheDMS Phenyldimethylsiane

PMHS Poly(methylhydrosiloxane)

PMPS Polymethylphenylsiloxane

PMPVS Poly (methylphenylvinylsiloxane)

PTFPMS Poly(trifluoropropyl)methylsiloxane

PVA Poly (vinyl alcohol)

R.S.D Relative Standard Deviation

SN Nucleophilic Substitution

SBSE Stir Bar Sorptive Extraction

SCF Supercritical Fluids

SEM Scanning Electron Microscopy

SFE Supercritical Fluid Extraction

SHG Second Harmonic Generation

SHS Static Headspace

SPE Solid - Phase Extraction

SPME Solid - Phase Microextraction

SPME-GC Solid Phase Microextraction - Gas Chromatography

SPME-HPLC Solid Phase Microextraction - High Performance Liquid

Chromatography

SPME-HPLC-MS Solid Phase Microextraction - High Performance Liquid

Chromatography - Mass Spectrometry

TEOG Tetraethyoxygermane

TEOS Tetraethoxysilane

TFA Trifluoroacetic Acid

THF Tetrahydrofuran

TMOG Tetramethoxygermane

TP-i-OG Germanium iso-propoxide

TPOG Tetrapropyloxygermane

VOCs Volatile Organic Compounds

XPS X-ray Photoelectron Spectroscopy

xv

GERMANIA-BASED SOL-GEL ORGANIC-INORGANIC HYBRID COATINGS

FOR CAPILLARY MICROEXTRACTION

Li Fang

ABSTRACT

For the first time, germania-based hybrid organic-inorganic sol-gel materials were

developed for analytical sample preparation and chromatographic separation. Being an

isostructural analog of silica (SiO2), germania (GeO2) is compatible with silica network.

This structural similarity, which is reflected by the relative positions of germanium and

silicon in the periodic table, stimulated our investigation on the development of

germania-based sol-gel hybrid organic-inorganic coatings for analytical applications.

Sol-gel sorbents and stationary phases reported to date are predominantly

silica-based. Poor pH stability of silica-based materials is a major drawback. In this work,

this problem was addressed through development of germania-based organic-inorganic

hybrid sol-gel materials. For this, tetramethoxygermane (TMOG) and

tetraethyoxygermane (TEOG) were used as precursors to create a sol-gel network via

hydrolytic polycondensation reactions to provide the inorganic component (germania) of

the organic-inorganic hybrid coating. The growing sol-gel germania network was

simultaneously reacted with an organic ligand that contained sol-gel-active sites in its

chemical structure. Hydroxy-terminated polydimethylsiloxane (PDMS) and

xvi

3-cyanopropyltriethoxysilane (CPTES) served as sources of nonpolar and polar organic

components, respectively. The sol-gel reactions were performed within fused silica

capillaries. The prepared sol-gel germania coatings were found to provide excellent pH

and thermal stability. Their extraction characteristics remained practically unchanged

after continuous rinsing of the sol-gel germania-PDMS capillary for 24 hours with highly

basic (pH = 13) and/or acidic (pH = 1.3) solution. They were very efficient in extracting

non-polar and moderately polar analytes such as polycyclic aromatic hydrocarbons,

aldehydes, ketones. Possessing the polar cyanopropyl moiety, sol-gel germania

cyanopropyl-PDMS capillaries were found to effectively extract polar analytes such as

alcohols, fatty acids, and phenols. Besides, they also showed superior thermal stability

compared with commercial cyano-PDMS coatings thanks to the covalent attachment of

the coating to capillary surface achieved through sol-gel technology. Their extraction

characteristics remained unchanged up to 330 ºC which is significantly higher than the

maximum operation temperature (< 280 ºC) for commercial cyano-PDMS coatings.

Low ng/L detection limits were achieved for both non-polar and polar test solutes. Our

preliminary results also indicated that sol-gel hybrid germania coatings have the potential

to offer great analytical performance as GC stationary phases.

1

CHAPTER ONE

BACKGROUND OF SOLID PHASE MICROEXTRACTION (SPME)

1.1 Introduction to sample preparation

The analytical procedure for complex samples consists of several typical steps:

sampling, sample preparation, separation, quantification, statistical evaluation, and

decision making. These analytical steps follow one after another, and a subsequent step

cannot begin until the preceding one has been completed. Therefore, the slowest step

determines the overall speed of the analytical process, and each step is critical for

obtaining correct and informative results. To improve analysis speed, all the steps should

be considered.

Sample preparation as a part of the analysis takes about 80 % of analysis time for

complex samples [1]. One reason is the difficulty to couple sampling and sample

preparation steps with today’s sophisticated instruments such as Gas

Chromatography/Mass Spectrometry (GC/MS) or Liquid Chromatography/Mass

Spectrometry (LC/MS). Those sophisticated instruments, can separate and quantify

complex mixtures and automatically apply chemometric methods to statistically evaluate

results. Despite the advances in separation and quantitation techniques, many sample

preparation techniques are based on classical technologies such as Soxhlet extraction

method are time consuming and involve multi-step procedures that are prone to lose

analyte loss use of the toxic organic solvents. These characteristics make it extremely

difficult to integrate such methods with today’s sophisticated separation and quantitation

2

instruments for the purpose of automation. Another reason is that often one has to deal

with real world complex or natural samples, therefore the fundamentals of extraction

techniques involving them are much less well developed and understood compared well

defined physicochemical principles governing instrument used in separation and

quantification steps. Lastly, a new awareness of the hazards has resulted in international

initiatives [2] to develop new sample preparation techniques aimed at complete

elimination of using organic solvents that we used in large volume by classical sample

preparation techniques.

1.2 Introduction to classical sample preparation approaches

It is not an exaggeration to say that choice of sample preparation method greatly

influences the analytical reliability, efficiency and accuracy. Soxhlet extraction for solid

samples [3] and liquid-liquid extraction (LLE) for liquid samples [4-6] are two most

commonly used classical sample preparation approaches.

1.2.1 Liquid-liquid extraction (LLE)

Liquid-liquid extraction involves the use of an organic solvent to extract analytes

from aqueous samples in reparatory funnels or other vessels. It is one of the oldest

techniques and still is widely used. Many of the US EPA standard methods for organic

trace analysis still utilize LLE. The main disadvantage of the technique is the use of large

quantity of organic solvents, which might be expensive to buy and dispose of, and cause

environmental problem and potential health concerns. It is also very difficult to couple

LLE technique with any other instrumental analysis systems. LLE is not easily automated.

3

Lastly, the enrichment factor is quite limited and for many sample types there are

problems with emulsion formation and precipitation.

1.2.2 Soxhlet extraction

Soxhlet extraction technique has been the most widely used method for

exhaustive extraction analytes from solid samples. Typically, a Soxhlet extraction is used

where the target analyte has a limited solubility in a solvent, and the matrixes are possible

impurities are insoluble in that solvent. Because a simple filtration can be used to

separate the compound easily from the insoluble substance it is desirable that the target

analytes has a high solubility in the solvent. Soxhlet extraction normally involves placing

the solid sample in the porous cellulous thimble placed in a holder. During operation the

thimble is filled with fresh warm organic solvent from a distillation flask. During the

process of extraction, the extracted analytes accumulate in the solvent and are

automatically siphoned into the distillation flask regularly. These steps are repeated until

exhaustive extraction of the target is achieved. Although the soxhlet technique uses

inexpensive equipment to operate, the processes involved are quite slow and may require

the use of large amounts of hazardous solvents causing concerns about the environment

and human health. At the same time, the used solvents are highly costly to be disposed of.

1.3 Solvent-free extraction

Classical sample preparation techniques consume large amounts of solvents.

During the volume reduction step, waste solvents are often disposed into the environment.

Solvent disposal into the atmosphere causes pollution and contributes to unwanted

http://en.wikipedia.org/wiki/Solubility

http://en.wikipedia.org/wiki/Solvent

http://en.wikipedia.org/wiki/Insoluble

http://en.wikipedia.org/wiki/Filtration

4

atmospheric effects such as smog and ozone holes. In addition, solvent disposal adds

extra cost to the overall analytical process by complicating work of the regulatory

agencies, and it increases health hazards to laboratory personnel. The desire for efficient

extractions while consuming less solvent has led to a trend on increased research on new

and practical solvent-free alternatives. A few of these research areas include Gas-phase

extractions [7-8] supercritical fluid extraction (SFE) [9-11], membrane extraction [12-13]

and solid-phase extraction (SPE) [14-17].

Solvent-free extraction techniques [6] [18] have been an important direction to

address the problems associated with classical sample preparation methods. Solvent-free

sample preparation techniques use little or no organic solvent. This direction not only

addresses health and pollution prevention issues, but also in most cases provides an easier

way to implement on-site monitoring in field conditions. This direction has been creating

a lot of interests and research opportunities recently and it is expected to continue to be a

very active area in the near future. Solvent-free extraction techniques can be broadly

classified, as shown in the Figure 1.1, into gas phase, membrane, and sorbent extraction

techniques.

5

Figure 1.1 Classification of Solvent-free extraction techniques [19]

1.3.1 Gas-phase extraction

Gas-phase extractions can be carried out in two different modes static head space

(SHS) and dynamic head space (DHS) (also known as purge and trap) [7, 8]. In static

head space mode, the volatile analytes in the sample matrix diffuse into the headspace in

a vial, and the concentration of the analyte in the headspace reaches equilibrium with the

concentration in the sample matrix. After the equilibrium is established, a small volume

of the head space gas is injected into a GC. Using static headspace method has the

advantages of simplicity. It is more convenient for qualitative analysis since the sample

can be placed directly into the headspace vial and analyzed with no additional preparation.

However, this technique suffers from low sensitivity because there is no mechanism of

sample preconcentration. DHS potentially provides improved sensitivity when compared

with SHS, due to the constant depletion of the analytes from the sample. In dynamic head

Solvent-Free Sample Preparation Methods

Membrane

Extraction Gas-Phase extraction Sorbent Extraction

SPE SPME

Cartridge

Disk

Direct

Headspace

In-tube

Headspace

Dynamic

Static

Dynamic

Static

SFE

6

space, carrier gas is bubbled through solid or liquid samples containing volatile organic

compounds (VOCs), are subsequently in a sorbent trap. The carrier gas sweeps volatile

organic compounds into the sorbent trap. Desorption of trapped analytes for subsequent

analysis can be performed either with small volumes of appropriate solvents or using an

on-line automated thermal desorption device, the latter is suitable for routine procedures.

1.3.2 Supercritical fluid extraction (SFE)

Supercritical fluid extraction [9-11] is an attractive technique because it is fast and

selective. This extraction technique is based on the fact that near critical conditions of the

solvent, its properties change rapidly with only slightly variations of pressure. A

supercritical-fluid extractor typically consists of a tank of the extracting fluid, usually

liquid CO2, a pump to pressurize CO2, an oven containing the extraction vessel, a

restrictor to maintain a high pressure in the extraction line, and a trapping vessel.

Analytes are trapped by letting the solute-containing supercritical fluid decompress into

an empty vial, through a solvent, or onto a solid sorbent material. Extractions are done in

dynamic-, static-, or combination modes. In a dynamic SFE, the supercritical fluid

continuously flows through the sample in the extraction vessel and out the restrictor to

the trapping vessel. In static mode the supercritical fluid circulates in a loop containing

the extraction vessel for some period of time before being released through the restrictor

to the trapping vessel. In the combination mode, a static extraction is performed for some

period of time, followed by dynamic extraction. Several compounds have been examined

as SFE solvents. For example, hydrocarbons such as hexane, pentane and butane, nitrous

oxide, sulphur hexafluoride and fluorinated hydrocarbons [20]. However, carbon dioxide

7

(CO2) is the most popular SFE solvent because it is safe, readily available in highly pure

form and has a low cost. It allows supercritical operations at relatively low pressures and

at near-room temperatures. The main drawback of CO2 is its lack of polarity for the

extraction of polar analytes [21]. Compared with other traditional extraction techniques,

it is a flexible process due to the possibility of continuous modulation of the solvent

power/selectivity of the supercritical fluids (SCF), and it is also environmental friendly

than organic solvents and avoids the expensive post-processing of the extracts for solvent

elimination. Because of these advantages, carbon dioxide (CO2) is the most widely used

supercritical solvent. A serious drawback of SFE is the higher investment costs compared

to traditional atmospheric pressure extraction techniques. The interest in SFE has

dramatically declined because of the dependence of the extraction on sample condition.

This dependency makes the technique difficult for routine analysis due to the tedious

optimization procedure which is required to overcome this problem. Furthermore, SFE

requires liquid-like compressed CO2 as the extraction medium. The high cost of pure CO2

and overall design of SFE makes the technique incompatible with field analyses [22-24].

1.3.3 Membrane extraction

Membrane extraction [12, 13] can be divided into two categories, porous and

non-porous membrane techniques. The porous membrane techniques involve filtration

and dialysis in different formats. In these techniques the solutions on both sides of the

membrane are in physical contact through the membrane pores, so this is in fact a

one-phase system. Non-pores membrane techniques involve a membrane that is either a

polymeric material or a liquid separating the donor and acceptor. In these techniques, it

8

could be two-phase or three-phase system. Membrane extraction consists of two

simultaneous processes. In the first step, a membrane is used to extract analyte from the

matrix. This step is followed by extracting the analyte from the membrane surface with a

stripping phase. This method is only suitable for non-polar volatile and semivolatile

compounds. The major limitations of membrane extraction include slow response,

carryover, and difficulty in interfacing the technique with other instrumentation [17, 18,

25, 26].

1.3.4 Solid phase extraction (SPE)

Solid phase extraction [14-17] is based on the selective accumulation of target

analytes from a liquid matrix onto a sorbent bed. SPE has several formats including

cartridge tube, flat disk membrane, monolithic bed, and flow-through membrane

containing a sorbent on a particulate support. The major advantages of the technique are

the significant reduction in the use of organic solvent, ease in operation, and low cost.

However, in some cases the extraction solvent may not be compatible with an analytical

system. To overcome this, the solvent is evaporated and the remaining residue is

dissolved in a compatible solvent. Furthermore, SPE has poor reproducibility and high

carry over problem.

1.4 Introduction of exhaustive-extraction and non-exhaustive extraction techniques

The objective of exhaustive technique is to completely remove analytes from a

sample matrix and transfer them to the extracting phase. The fundamental advantage of

exhaustive methods is that, in principle, they do not require calibration since the vast

9

majority of analytes are transferred to the extracting phase. To reduce the amounts of

solvents and time required to accomplish exhaustive removal, batch equilibrium

techniques (for example, liquid-liquid extractions) are frequently replaced by

flow-through techniques. SPE is an example in this category. Alternatively, a sample

(typically a solid sample) can be packed in the bed and the extraction phase can be used

to remove and transport the analytes to the collection point. In SFE, supercritical fluid is

used to wash analytes from the sample matrix, and in Soxhlet, the solvent continuously

removes the analytes from the matrix at the boiling point of solvent.

On the other hand, non-exhaustive approaches can be designed based on

equilibrium, pre-equilibrium and permeation principles. Equilibrium non-exhaustive

techniques are fundamentally analogues to equilibrium exhaustive techniques; however,

the capacity of the extracting phase is smaller, and in most cases is not sufficient to

remove the majority of analytes from the sample matrix. This is caused by the use of a

small volume of the extracting phase relative to sample volume. Microextraction, such as

solvent microextraction or solid phase microextraction is in this category. Pre-equilibrium

conditions are accomplished by breaking the contact between the extracting phase and

sample matrix before the point of equilibrium with the extracting phase has been reached.

The devices frequently used are identical to the microextraction system, but extraction

times are shorter. The general classification of the extraction techniques based on these

fundamental principles is in Figure 1.2 [19].

10

Figure 1.2 Classification of the extraction techniques [19]

1.5 Introduction of solid-phase microextraction

Solid-phase microextraction (SPME) is an important and practical approach to

sampling and sample preparation. It was developed in 1989 by Belardi and Pawliszyn [27]

to overcome the limitations inherent in solid-phase extraction (SPE) and liquid-liquid

extraction (LLE). SPME facilitates rapid sample preparation for both laboratory and field

analyses. SPME has been successfully applied to a variety of analytes in environmental

[28-30], biological [31-33], food [34], drug [35-37], flavor [38], and other types of

samples in hyphenation with gas chromatography (GC) [39], high-performance liquid

chromatography (HPLC) [40], capillary electrophoresis (CE) [41], and Mass

Extraction techniques

Batch equilibrium

and Pre-equilibrium

Flow through Equilibrium

and Pre-equilibrium

Steady State Exhaustive

and non-Exhaustive

Exhaustive Non-Exhaustive

LLE

Soxhlet

Sorbents

Headspace

LLME

SPME

Exhaustive

Purge and Trap

Solvent

SPE

HSE

SFE

Non-Exhaustive

In-tube SPME

Membrane

11

spectrometry (MS) [42]. Solid-phase microextraction provides many advantages over

other sample preparation techniques including:

(1) It was a simple device which enables in situ sampling. Low set-up cost.

(2) SPME techniques eliminate the use of toxic organic solvents (green

chemistry).

(3) SPME is shorten the analysis time integrating sampling, extraction, sample

preconcentration, and sample introduction into a single step (no intermediate step

is necessary between extraction and analysis).

(4) High sensitivity (detection limit down to part per quadrillion (ppq) level) can

be easily achieved using SPME.

(5) SPME extraction is an equilibrium process that requires small (sample

volume require).

(6) The technique can be easily automated with analytical instruments (GC,

HPLC, and CE).

(7) SPME is portable which makes it attractive for field sampling.

1.5.1 SPME device and application

In SPME, the extracting phase is coated on the surface of a fused-silica fiber

(fiber format) or capillary (in-tube format). The extraction in SPME is an equilibrium

process of analyte distribution between the extracting phase and sample matrix. The

conventional SPME device (Figure 1.3) is very simple; it is based on a fiber which is

housed in a specially designed syringe like apparatus. The retractable metal rod, serving

as the syringe piston, is replaced by stainless steel microtubing to hold and protect the

12

SPME fiber. Typically, 1.5-cm long silica rod (fiber) is used. The protective external

coating of this SPME fiber is removed from about 0.5 cm length of the fiber of one end,

and this end is attached to the syringe plunger using epoxy glue. The microtubing (needle

of the syringe) protects the SPME fiber coating from mechanical damage and provides a

suitable way for SPME fiber insertion and withdrawing during extraction and analysis.

Sampling and sample injection occur in a manner which is identical to typical syringe

injections.

Figure 1.3 Schematic of a SPME device. Adapted from [43]

The fiber solid phase microextraction technique is best suited for GC (SPME-GC).

SPME-GC generally consists of two steps: (1) extraction: the needle of the SPME device

at first pierces the septum of the sample container. After adjusting the depth of needle to a

13

proper position, the fiber is exposed to the sample matrix for a certain time. During this

period the analytes are extracted by the SPME coating on the outer surface of the fiber

(Figure 1.4); (2) Injection: the analytes are thermally desorbed from the fiber and are

transferred by the carrier gas (mobile phase) to the GC separation column for further

separation and analysis. In liquid chromatography, the extracted analytes are desorbed by

the organic solvents (LC mobile phase) and passed into the separation solution.

Figure 1.4 Extraction step for SPME [44]

1.5.2 Theoretical aspects of SPME

The transport of analytes from the matrix into the coating begins as soon as the

coated fiber has been placed in contact with the sample (Figure 1.5). SPME is a non

exhaustive extraction technique, and it is based on an equilibrium distribution. Analytes

continue to sorb onto the SPME coating until the equilibrium between the sample matrix

and the SPME fiber coating is achieved. Typically, SPME extraction is considered to be

14

completed when the distribution equilibrium has been reached. In practice, this means

that once equilibrium is reached, the extracted amount is constant within the limits of

experimental error and it is independent of further increase of extraction time. The

thermodynamic and kinetic fundamentals of solid phase extraction have detailed by

Pawliszyn and co-workers [45-48]. SPME can be used for extraction from both gas and

liquid samples. In both cases, analyte molecules are distributed between the sample

matrix and the sorbent coating on the SPME fiber.

Figure 1.5 Principle of SPME [45]

The distribution constant represents the ratio of analyte concentrations in the

sorbent coating and in the sample. The analyte distribution equilibrium for liquid

polymeric sorbent extraction phase is represented as:

analytesample analytefiber

15

The distribution constant can be described mathematically as:

(1.1)

where,

Kfs is distribution constant between fiber coating and sample matrix,

Cf is analyte concentration in the sorbent coating,

Co is analyte concentration in the sample.

The amount of analyte in the sorbent coating (nf) at equilibrium condition is as

follows [45]:

(1.2)

Where,

Vs is the volume of the sample,

Vf is the volume of the sorbent coating on the fiber

Since Vf is very small (µL), generally (Kfs Vf << Vs) the amount of analyte in the sorbent

coating can be written as :

n = Kfs Vf Co

(1.3)

This equation (1.3) shows that the amount of the extracted analyte in the sorbent

coating for direct SPME (no head space) is independent of the sample volume. SPME is a

non-exhaustive extraction technique. SPME doesn’t exhaustively extract target analytes

existing in the sample matrix, and the extraction is based on the equilibrium. This is also

n =Kfs Vf Vs

Kfs Vf + Vs

Co

16

a theoretical basis for SPME to be used as a portable extraction technique in field

analysis. In practice, there is no need to collect a defined sample prior to analyses as the

fiber can be exposed directly to the ambient air, water, production steam, etc. The amount

of extracted analytes will correspond directly to its concentration in the matrix, without

being dependent on the sample volume. When the sampling step is eliminated, the whole

analytical process can be accelerated, and errors associated with analyte losses through

decomposition or adsorption on the sampling container walls will be prevented. This

advantage of SPME still needs to be explored practically, by developing portable field

device on a commercial scale. This equation also shows that there is a direct

proportionality between the sample concentration and the amount of analyte extracted,

and this is the basis for analyte quantification. Furthermore, it implies that the extraction

efficiency and sensitivity depend on the distribution constant. Thus, a high distribution

constant, Kfs , is desirable to enhance selectivity and sensitivity. This can be achieved by

careful selection of the fiber coating material, changing sample pH, and through the

derivatization of target analyte (s).

1.6 Conventional SPME coating

Equation (1.3) indicates that the efficiency of the extraction process is dependent

on the distribution constant. Distribution constant is an important parameter that

characterizes properties of a certain coating and its selectivity toward a class of analyte

with certain polarity over others. Coating volume is also related with method sensitivity,

for example, thicker coatings will increase the sample load and increase the extraction

17

sensitivity, however sometimes that will result in longer extraction times. Therefore, it is

important to use the appropriate coating for a given application. Coating selection and

design can be based on chromatographic experience, and specific coatings can be and

should be designed for specific applications.

Conventional sorbents for SPME can be divided into two groups [45] (a)

homogeneous polymers and (b) composite extraction media consisting of a partially

cross-linked polymers embedded with porous particles of a second component. However,

the newly emerged polymers such as sol-gel polymers and molecular imprinted polymers

etc can be classified as new categories of SPME sorbents. To achieve good selectivity for

the analytes of interest, the choice of most suitable coating can be based on the principle

of ―like dissolve like‖. According to the polarity of the analytes, a coating with similar

polarity will provide a better affinity. Therefore from the polarity point, it is also useful to

classify conventional SPME coatings into four categories: (1) non-polar, (2) moderately

polar, (3) polar, and (4) highly polar. Table 1.1 shows some commercial SPME coating

and their properties [49].

18

Table 1.1 Commercial SPME coatings and their properties

Fibre coating

Film

thickness

(µm) Polarity

Coating

method

Maximum

operating

temperature

(°C) Technique

Compounds

to be

analysed

PDMS 100 Non-polar Non-bonded 280 GC/HPLC Volatiles

PDMS 30 Non-polar Non-bonded 280 GC/HPLC Non-polar

semivolatiles

PDMS 7 Non-polar Bonded 340 GC/HPLC Medium- to

non-polar

semivolatiles

PDMS-DVB 65 Bipolar Cross-linked 270 GC Polar

volatiles

PDMS-DVB 60 Bipolar Cross-linked 270 HPLC General

purpose

PDMS-DVBa 65 Bipolar Cross-linked 270 GC Polar

volatiles

PA 85 Polar Cross-linked 320 GC/HPLC Polar

semivolatiles

(phenols)

Carboxen-PDMS 75 Bipolar Cross-linked 320 GC Gases and

volatiles

Carboxen-PDMSa 85 Bipolar Cross-linked 320 GC Gases and

volatiles

Carbowax-DVB 65 Polar Cross-linked 265 GC Polar

analytes

(alcohols)

Carbowax-DVBa 70 Polar Cross-linked 265 GC Polar

analytes

(alcohols)

Carbowax- TPR 50 Polar Cross-linked 240 HPLC Surfactants

DVB-PDMS-Carboxena 50/30 Bipolar Cross-linked 270 GC Odors and

flavors aStableflex type is 2 cm length fiber

19

1.6.1 Homogeneous polymers

PDMS coated SPME fibers are commercially available in three different

thicknesses (7, 30, and 100 µm), and commercial polyacrylate (PA) fibers have a coating

thickness of 85 µm [50, 51]. These coatings extract analytes through an absorption

mechanism, where the analytes dissolve and diffuse onto the coating material.

PDMS fibers are the most popular and frequently used polymer in SPME because

of its inherent versatility, ruggedness, and high thermal stability. The PDMS fiber with 7

µm coating thickness is cross-linked, while the other two are not. This is because it is

very difficult to stabilize thick coatings through cross-linking reactions. Compared with

maximum operating temperature (280 ºC) of 30 and 100 µm PDMS coatings, 7 µm

PDMS coating possesses higher operating temperature (340 ºC). Cross-linked SPME

fiber coatings are more rigid and are characterized by relatively high thermal stability

compared to non-cross-linked or partially cross-linked counterparts. That may explain the

different thermal stability of these three types of PDMS coatings. However, cross-linked

PDMS coating possesses smaller sample capacity due to the smaller coating thickness of

7 µm. Since PDMS is non-polar in nature, it can only extract non-polar analytes very well.

More polar compounds can be extracted by optimizing extraction conditions such as pH,

salt effect, and temperature however they are not efficient for extraction of polar analytes.

Polyacrylate (PA) based coatings [52, 53] are more suitable for the extraction of

polar analytes. Commercial PA is available in 85µm thickness, and it is a highly polar

sorbent, immobilized by partial crosslinking. Because of its high polarity, PA coated fiber

frequently recommended for extracting polar analytes from different matrices. Unlike

20

other sorbents, at room temperature, polyacrylate is a rigid, low density solid polymer.

Therefore diffusion of analytes into this coating requires longer time resulting in longer

extraction time. Moreover, higher temperatures are required for complete desorption of

the extracted analytes.

1.6.2 Composite extraction media

Composite extraction coatings are prepared using a blend of two components in

which the particulate component is embedded in the partially crosslinked polymeric

component. Among the mixed coatings are carbowax/divinylbenzene (CW/DVB) [54],

polydimethylsiloxane/divinylbenzene (PDMS/DVB) [55]

polydimethylsiloxane/carboxane (PDMS/carboxane) [56], and carbowax/templated resin

(CW/TPR) [56]. Such composite coatings usually extract via adsorption, which is a

competitive process. In mixed phases the analytes stay on the surface of SPME coatings.

The mixed phases provide better selectivity due to the existence of two sorbents in the

coating. However unlike the preparation of homogeneous polymeric coatings, the process

to make composite coatings is difficult to automate and it is usually done manually.

1.6.3 Emerging extraction media

Commercially available fibers are limited to the scale of polarity. Consequently,

this either limits the selectivity in structural analogues analysis or prevents the sensitivity

of trace level analysis in complex sample analysis [57]. In recent years homemade

coatings with different chemical and physical characteristics [58-62] have been reported.

These include fullerene [58, 59], crown ester [60, 61], and calixarene-based [62] coatings

that provide increased sensitivity and selectivity. Fiber coating procedures have been

21

recently reviewed [63], and include sol-gel technology [64], electrochemical methods [65,

66], and physical deposition [67]. Among them, sol-gel technology is gradually turning to

be an important coating technology to prepare all kinds of customized extraction media,

and the resultant sol-gel organic-inorganic polymeric materials have been widely used to

meet application selectivity and sensitivity. Molecularly imprinted polymers have also

been introduced as extracting phases for SPME. They have a great potential for selective

extraction of target analytes from various complex matrices.

1.7 Different formats of SPME

1.7.1 Fiber SPME

As the dominant format of Solid-phase microextraction, Fiber SPME possesses a

number of inherent deficiencies arising from the fiber construction and its syringe-based

mechanical operation. In fiber SPME, the protective polyimide coating is removed from

the external surface of a fused silica fiber end segment (~ 1 cm). Therefore the fiber and

the outside sorbent layer is vulnerable to mechanical damage during operation and

handling of the SPME device due to the lack of external protective coating. The short

length (~ 1 cm) of the coated segment results in low sorbent loading, which leads to

reduced extraction sensitivity. Moreover, fiber SPME is difficult to couple to

high-performance liquid chromatography (HPLC) and other liquid-phase separation

techniques. A number of SPME variants have been proposed to overcome these

difficulties such as membrane extraction, in-tube microextraction, and stir bar sorptive

extraction (SBSE), etc.

22

1.7.1.1 Direct immersion SPME (DI)

Fiber SPME can be carried out in two different modes: (1) direct immersion

SPME and (2) headspace SPME (Figure 1.6). In direct SPME, the coated segment of

SPME fiber is inserted into the sample matrix containing the target analytes directly.

Under agitation, the target analytes distribute between the sample matrix and solid phase

on the fiber. The equilibrium will be reached after a certain period of time. When

extraction is carried out in aqueous solution, different means of agitation such as stirring

or sonication of the solution, rapid fiber or vial shaking, and fast flow of the aqueous

solution may be employed to fasten the extraction. When the sample matrix is gaseous

sample, natural convection of the air or the use of a fan may be employed to speed up the

extraction.

1.7.1.2 Headspace fiber SPME (HS-SPME)

In headspace fiber SPME, the coated fiber stays in the headspace (the gaseous

phase) above the liquid sample in a sealed container. The analyte distribution equilibrium

is first reached between the sample matrix (either liquid or solid) and the headspace of

the closed container. After the coated fiber is introduced, the extraction will be carried out

from gaseous headspace to the extracting phase on the fiber. Headspace SPME can avoid

unwanted interferences from sample matrix such as compounds having high molecular

mass (e.g., humic materials, poteins, etc). During extraction, those high molecular mass

molecules, characterized by low vapor pressure, stay in the liquid or solid but the small

target anaytes and other small molecules characterized by big vapor pressure distribute

themselves between the sample matrix the headspace. Compared with direct SPME, a

23

certain temperature is often used to help the evaporation of target analytes into the

gaseous headspace of the container. Headspace SPME allows the modification of the

matrix such as pH change, addition of salts etc to improve the extraction efficiency of the

coatings without any harm to the coating on the fiber. However, for this extraction mode,

the target analytes cannot be non-volatile, only volatile or semi-volatile compounds can

be extracted from headspace with enough sensitivity.

Figure 1.6 Modes of SPME operation: (a) direct extraction, (b) headspace SPME

1.7.2 In-tube SPME (also termed capillary microextraction (CME))

In-tube SPME (also termed capillary microextraction (CME)) [68-70], is

practically free form the shortcomings inherent in conventional fiber SPME. Unlike fiber

SPME, where a sorbent coating on the outer surface of a small-diameter solid rod serves

as the extraction medium, in CME a piece of fused silica capillary with a stationary phase

24

coating on its inner surface (e.g., a short piece of GC column) is typically used to perform

extraction (Figure 1.7). In the capillary format, the protective polyimide coating on the

external surface of the capillary remains intact and provides reliable protection against

mechanical damage to the capillary. Moreover, in-tube SPME provides a simple, easy,

and convenient way to couple SPME to gas- as well as to liquid-phase separation

techniques.

Figure 1.7 Comparison SPME fiber with in-tube SPME

1.7.2.1 Comparison of fiber SPME and in-tube SPME

Although the theory of fiber and in-tube SPME methods are similar, the

significant difference between these methods is that in fiber SPME the extraction of

25

analytes is performed with the coating on the outer surface of the fiber using mechanical

agitation of the sample while in-tube SPME method uses a sorbent coating on the inner

surface of a capillary to perform extraction by a flow-though process that involves

agitation by sample flow in and out of the extraction capillary. Figure 1.8 illustrates the

comparison of extraction process for the transfer of the analytes in fiber SPME and

in-tube SPME. For in-tube SPME, it is necessary to prevent plugging of the extraction

capillary and flow lines during extracting process and the particles in the sample matrix

must be removed by filtration before extraction.

Figure 1.8 Comparison of extraction process for the transfer of the analytes in fiber

SPME and in-tube SPME [44]

26

Both SPME and in-tube SPME can be performed as: (a) active (dynamic) and (b)

passive (static) mode. To use in-tube SPME in active mode, the matrix containing the

analytes is passed through the extraction capillary and the analytes are extracted by

sorbent coating on the capillary inner walls as sample passes through. To use fiber SPME

in active mode, a stir bar is used to agitate the sample, thereby speeding up the extraction

of analytes from sample matrix by the extracting phase coating on the surface of the fiber.

In static in-tube SPME mode, the capillary is filled with sample matrix and the high

affinity of the analytes for the sorbent material serves as the driving force for their

extraction. Static fiber SPME is carried out mainly in the headspace mode. In this case,

the fiber and extracting phase resides inside the protective tubing (needle), is not exposed

directly to the matrix sample. The extraction occurs through diffusion of the gaseous

sample contained in the tubing. The static fiber SPME is suitable for field analysis of

gaseous samples.

The SPME fiber should be handled carefully, because it is fragile, and the fiber

coating can be damaged during insertion and agitation. In in-tube SPME, on the other

hand, the protective polyimide coating on the external surface of the capillary remains

intact and provides reliable protection against mechanical damage to the capillary.

Therefore in-tube SPME provides safe and convenient way to couple SPME to gas- as

well as to liquid-phase separation techniques.

The introduction of in-tube SPME was primarily for coupling SPME to high

performance liquid chromatography (HPLC) for automated applications. For fiber

SPME-HPLC, analysts are currently limited to performing manual extractions and

27

desorptions. Analytes extracted by fiber SPME must be desorbed into a suitable receiving

solvent prior to HPLC analysis with the help of a SPME-HPLC interface constructed

with a special desorption chamber and switching valve. Figure 1.9 illustrates the

procedure of extraction by fiber SPME and desorption for HPLC. Analytes are directly

extracted by the fiber coating and then introduced to the chromatography is system for

further analysis. The extraction is carried out by exposure of the SPME fiber in the

headspace or in the sample solution. An SPME-HPLC interface (Figure 1.9) equipped

with a special desorption chamber is utilized for solvent desorption prior to HPLC

analysis. The analytes extracted in the fiber are released in the desorption chamber by

external addition of solvent or mobile phase, and then introduced to the HPLC column.

SPME-HPLC (-MS) has been applied to the analysis of various polar compounds such as

drugs and pesticides, and has been reviewed [44].

However, for automated extraction and analysis, in-tube SPME is relatively

simple to implement, and it can continuously perform extraction, desorption, and

injection using a conventional autosampler without any special SPME-HPLC interface

for the desorption of analytes. A schematic diagram of the automated in-tube SPME-LC

system is illustrated in Figure 1.10. For in-tube SPME system, the extraction is completed

by injection syringe that repeatedly draws and ejects sample from the vial under

computer control. The analytes partition from the sample matrix into the extracting phase

coating until equilibrium is reached. Subsequently, the extracted analytes are directly

desorbed from the capillary coating by mobile phase flow or by aspirating desorption

solvent after switching the six-port valve. Therefore, in-tube SPME provides automated

28

and high precision extraction for HPLC analysis by setting sample to autosampler. The

application of in-tube SPME technique to the determination of pesticides, environmental

pollutants, drugs, and food contaminants by hyphenation with HPLC, LC-MS has been

reviewed [44].

Figure 1.9 Illustration of the extraction procedure of extraction by fiber SPME and

analyte desorption for HPLC analysis [44]

29

Figure 1.10 Schematic diagrams of in-tube SPME-LC-MS systems [44]

Both SPME and in-tube SPME are also compatible with GC. For fiber SPME-GC,

the extraction procedure includes [71]: (1) piecing the septum of sample container using

the needle of SPME device, (2) exposing the SPME fiber to the sample and perform

extraction either by direction immersion or in the headspace (3) retracting fiber back into

the needle and withdraw needle. The extracted analytes are thermally desorbed in the GC

injection port for further transport into the GC column by the carrier gas. For in-tube

SPME-GC, the capillary with the extracted analytes is connected to the inlet end of the

GC column using a two-way press-fit fused-silica connector housed inside the GC

injection port [69] or by placing inside needle capillary adsorption trap (INCAT) device

[72].

30

1.7.2.2 Theoretical aspect of in-tube SPME

The fundamental thermodynamic principle to all extraction techniques commonly

involves distribution of analyte between sample matrix and the extracting phase. When a

liquid is used as the extracting phase, then the distribution constant Kes defines the

equilibrium conditions and enrichment factors achievable in the technique. The

partitioning is between aqueous sample matrix and organic extraction phase, which is

illustrated in Figure 1.11.

Figure 1.11 Distribution of analyte between sample matrix and the extraction phase [19]

When a liquid is used as an extracting phase, the distribution constant Kes can be

expressed as:

Kes

ae

as

Ce

Cs (1.4)

Where,

Kes: analyte distribution constant between liquid extracting phase and sample

matrix,

31

ae: activity of an analyte in the liquid extracting phase,

as: activity of an analyte in the sample matrix,

Ce: analyte concentration in the liquid extracting phase,

Cs: analyte concentration in the sample matrix.

In-tube SPME uses a flow-through system where solid phase is located on the

inner surface of capillary and is used as extracting phase. When a solid phase is used as

the extracting phase, a similar equation will be expressed [19]:

Kses

Se

Cs (1.5)

Where,

Kses: analyte distribution constant between solid extracting phase and sample

matrix,

Se: surface concentration of adsorbed analytes in the solid extracting phase,

For the in-tube SPME using flow-through technique system, the minimum

extraction time at equilibrium condition can be expressed [19]:

(1.6)

Where,

L is the length of the capillary holding the extraction phase,

Ve is the volume of the extracting phase,

Vv is the void volume of the capillary,

32

u is linear flow rate of the sample.

From the above equation, it can be seen that the extracting time is proportional to

the length of the capillary and inversely proportional to the linear flow rate of the sample.

Extraction time increases with an increase in the distribution constant between extracting

phase and sample matrix. Extraction time also increases with the volume of the extracting

phase (Ve) but decreases with an increase of the void volume of the capillary. In another

word, under a certain distribution constant, thinner coating with bigger capillary void

volume, less length of capillary and higher linear flow rate will help to fasten the

extraction equilibrium, therefore the extraction time can be reduced. It should be

emphasized that the above equation can be used only for direct extraction when the

sample matrix passes through capillary [19].

1.7.2.3 Parameters influencing in-tube SPME

In-tube SPME is an extraction method based on the transfer of analyte from the

sample to sorbent coating, in accordance with the analyte distribution between the two

phases. To obtain rapid and high efficiency of extraction, the following parameters

influencing in-tube SPME should be considered: (1) the selection of capillary coating (2)

sample solution (3) volume of draw/eject cycles and, (4) desorption of compounds from

capillary.

1.7.3 Stir bar sorptive extraction (SBSE)

Stir bar sorptive extraction (SBSE) is another sorptive technique which can

overcomes the limitations of traditional SPME fibers. The theory of SBSE is rather

straightforward and similar to that of SPME [73], which is based on distribution between

33

the sample and the extracting phase and the phase ratio. Stir bar sorptive extraction was

developed and used for the determination of volatile and semivolatile organic compounds

[73-75]. In SBSE, the coating covers a magnetic stir bar, varying in length from 1 to 4 cm,

coated with a relatively thick layer of PDMS (03.-1 mm), resulting in PDMS volumes

varying from 55 to 220 uL [74] (Figure 1.12). This allows for an extracting phase volume

50-250 times greater than that of SPME fibers [76]. The coated stir bar is added to an

aqueous sample for stirring and extraction. After a certain stirring time, the bar is

removed from the aqueous sample, and the sample is thermally desorbed by loading the

stir bar into the injection port of a gas chromatography.

Figure 1.12 Schematic of a stir bar of SBSE [71].

The extracting phase on the stir bar in SBSE is critical for performances of both

extraction and thermal desorption. However, the only commercially available phase for

SBSE is PDMS, marketed by Gerstel. A stir bar has three essential parts [76]. The

innermost part is a magnetic stirring rod, which is necessary for transferring the rotating

movement of a stirring plate to the liquid sample. The second part is a thin glass jacket

34

that covers the magnetic stirring rod. The outermost part is the layer of PDMS sorbent

into which the analytes are extracted. The glass layer is essential for the construction of a

high-quality stir bar as it effectively prevents the decomposition of the PDMS layer due

to catalytic effects of the metals in the magnetic stirring rod.

1.8 References for chapter one

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Inc, NY, 1999.

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500.

39

CHAPTER TWO

SOLID PHASE MICROEXTRACTION AND SOL-GEL TECHNOLOGY

2.1 Introduction to sol-gel process and some basic definitions

Generally, the sol-gel process involves the transition of a system from a liquid

"sol" (mostly colloidal) into a solid ―gel‖ phase. The starting materials used in the

preparation of the ―sol‖ are usually inorganic metal salts or metal organic compounds

such as metal alkoxides. Typically, in sol-gel process the precursor is subjected to a

series of hydrolysis and polymerization reactions to form a colloidal suspension, or a

―sol‖. When the ―sol‖ is cast into a mold, a wet ―gel‖ will form. Further drying and

heat-treatment convert the ―gel‖ into dense ceramic or glass material. If the liquid in a

wet ―gel‖ is removed under a supercritical condition, a highly porous and extremely low

density material called ―aerogel‖ is obtained. Here are some terms that need to be

defined [1]. A colloid is a suspension in which the dispersed phase is so small (~ 1-1000

nm) that the gravitational forces are negligible and interactions are dominated by

short-range forces, such as van der Waals attraction and surface charges. The inertia of

the disperse phase is small enough to exhibit Brownian motion, a random walk drive by

momentum imparted by collision with molecules of the suspending medium. A sol is a

colloidal suspension of solid particles in a liquid. An aerosol is a colloidal suspension of

particles in a gas. In sol-gel process, the precursors (starting compounds) for preparation

of a colloid consist of a metal or metalloid element surrounded by various ligands

40

(appendage not including another metal or metalloid atom). Metal alkoxides are popular

precursors because they react readily with water. The reaction is called hydrolysis. Two

partially hydrolyzed molecules can link together, which is called condensation reaction.

Condensation liberates a small molecule, such as water or alcohol. This type of reaction

can continue to build larger and larger silicon containing molecules by the process of

polymerization.

Applying the sol-gel process, it is possible to fabricate ceramic or glass materials

in a wide variety of forms: ultra-fine or spherical particles in the form of powders [2],

thin film coatings [3], ceramic fibers [4], microporous inorganic membranes [5],

monolithic ceramics and glasses, or extremely porous aerogel materials etc. Sol-gel

technology has found interesting applications in development of new materials for

catalysis [6, 7], chemical sensors [8, 9], membrane [10], fibers [11], optical gain media

[12], photochromic and nonlinear applications [13, 14], solid state electrochemical

devices [15], nuclear industry [16], and electronic industry [17] in various scientific and

engineering fields. A simple overview of the sol-gel process is graphically presented in

Figure 2.1.

41

Figure 2.1 Overview of the sol-gel process. Adapted from [1, 18]

42

2.2 Historical background of sol-gel technology

Sol-gel research on ceramic and glass materials can be traced back to the mid

1800s when Ebelmen [19] found that solutions of certain compounds gelled on exposure

to atmosphere. However, these materials presented interest only to chemists. It was

finally in the 1930s, that systematic improvement of sol-gel technology was carried out

by Geffcken and Dislich of Schott Glass Company in Germany [20]. The network

structure of silica gels was widely accepted in the 1930s when Hurd [21] showed that

they must consist of a polymeric skeleton of silicic acid enclosing a continuous liquid

phase. In the 1950s, the method for the preparation of homogeneous powder was

popularized by Roy in ceramics research [22, 23]. However, that work was not directed

toward an understanding of sol-gel reaction mechanism. In 1970s, controlled hydrolysis

and condensation of alkoxides for preparation of multicomponent glasses was

independently developed by Levene and Thomas [24] and Dislich [25] due to an

increased interest from ceramics industry. Tremendous attentions in sol-gel research have

been obtained ever since Yoldas [26] and Yamane et al. [27] showed that large monoliths

could be made by the sol-gel method through the careful drying of gels in the 1980s.

In 1987, Cortes et al. [28] prepared sol–gel monolithic ceramic beds within

small-diameter capillaries as separation columns in liquid chromatography (LC). In 1993,

Crego et al. [29] prepared a thin layer of silica gel with chemically bonded C18 moieties

on the inner walls of fused-silica capillaries for use in reversed-phase HPLC. Guo and

Colon [30] used a similar sol-gel technology to prepare stationary phase coatings for

open-tubular LC and electrochromatography. Malik and co-workers introduced sol-gel

43

coated columns for capillary GC [31], sol-gel coated fibers for solid-phase

microextraction (SPME) [32] and in tube SPME [33]. Gbatu [34] also prepared SPME

fiber with sol-gel technology. These preceding works led to sol-gel research aiming at

developing novel sorbent materials for solid-phase microextraction [35-39] and

solid-phase extraction [40-43].

2.3 The potential of sol-gel chemistry in coating technology for analytical

microextraction

2.3.1 Problems with traditional SPME coating technology

In chapter one, some drawbacks of fiber SPME inherited from physical

construction of SPME device such as the breakage of the fiber and bending of needles

were pointed out. The introduction of an alternative format of SPME seems to be a

promising direction in solving those problems. The other drawbacks arise from the

traditional coatings and coating technology [44]. First, the traditional SPME coatings are

designed to extract either polar or non-polar analytes for a given matrix. It is important to

have a sorbent that can extract both polar and non-polar compounds with high extraction

sensitivity needed for trace analysis. Second, in traditional SPME only a short length of

the fiber is coated with sorbent. The short length of the fiber coated with sorbent

translates into low sorbent loading which in turn leads to low sample capacity, which

significantly limits the sensitivity of traditional SPME coating. This is particularly

important for analyzing ultra-trace contaminants. Increasing the coating thickness [45, 46]

seems one possible way of improving extraction sensitivity. However, the increased

coating thickness requires longer equilibrium time, which translates into a long analysis

44

time. Moreover, immobilization of thicker coating on fused silica surface is difficult to

achieve by conventional approaches [47, 48]. Therefore, developing an effective

alternative approach to immobilize thick coatings is important for reliable and effective

SPME performance. Third, low thermal and solvent stability of traditional SPME

coatings represents a major drawback of conventional SPME technology. In traditional

approaches, relatively thin coatings can be immobilized on the capillary inner surface

though free-radical cross-linking reactions [49, 50]. Because of the absence of direct

chemical bonding between the coating and the substrate, the thermal and solvent

stabilities of such coatings are typically poor. The problem is more severe in the case of

thicker coatings. When coupled to GC, such extracting coatings may cause reduced

thermal stability of conventional coatings leads to incomplete sample desorption and

sample carryover problems. Low solvent stability of conventionally prepared extraction

coatings presents an obstacle to the hyphenation of in-tube SPME with liquid-phase

separation techniques because HPLC employs organic mobile-phase systems for

desorption of analytes. Evidently, all above problems are related to coating materials and

coating technology. Novel approaches to sorbent chemistry and coating technology are

very important for solving these problems. Development of new methods for the

preparing of chemically immobilized coatings form advanced material systems to achieve

desired selectivity and performance in SPME is an important direction in SPME research.

2.3.2 The advantages sol–gel technology for use in analytical microextraction

Sol-gel technology is one approach to address most of the above mentioned

problems through the creation of chemically bonded sorbent coatings [31-33, 51].

45

Sol–gel chemistry provides a simple and convenient pathway to synthesize advanced

material systems that can be used to prepare surface coatings. The advantages sol–gel

technology in the area of fiber based SPME or capillary-based SPME (CME) are as

follows [44, 52]:

(1) it combines surface treatment, deactivation, coating, and sorbent immobilization into

a single-step procedure making the whole SPME fiber/capillary manufacturing process

very efficient and cost-effective; (2) it creates chemical bonds between the fused silica

surface and the created sorbent coating, therefore it can achieve enhanced extracting

phase stability (solvent stability and thermal stability) in sample preparation and

chromatographic separations (3) it provides surface-coatings with high operational

stability ensuring reproducible performance of the sorbent coating under operation

conditions involving high temperature and/or organic solvents, and thereby it expands the

SPME application range towards both higher-boiling as well as thermally labile analytes;

(4) it provides the possibility to combine organic and inorganic material properties in

extraction sorbents providing tunable selectivity; (5) it offers the opportunity to create

sorbent coatings with a porous structure which significantly increases the surface area of

the extracting phase and provides acceptable stationary phase loading and sample

capacity using thinner coatings; (6) it reaches a better homogeneity from raw material on

the molecular level; (7) it provides mild reaction temperature; (8) it provides the

possibility to design the material structure and property through proper selection of

sol-gel precursor and other components; (9) it can easily provide different products such

as coatings, and monolithic bed, particles etc.

46

2.4 Fundamentals of sol-gel technology

To design and create proper sol-gel material, understanding the general chemical

reactions involved in the sol-gel process is very important. Chemical reagents for the

preparation of sol-gel sorbents normally include (1) sol-gel precursor(s); (2) a solvent for

dispersing and mixing all the reagents; (3) a polymer which provides selective functional

group; (4) deactivating reagents; (5) a catalyst, and (6) water.

At least one sol-gel precursor is used in sol-gel synthesis. Sol-gel precursors are

usually alkoxide-based materials [53]. However, alkoxides of various metals and

metalloids (Si, Al, Ti, Zr, Ge, etc.) [54-59] have been utilized to create sol-gel sorbents,

and they have been applied in analytical microextraction. Among them, silicon-based

alkoxides, especially tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTMOS)

are the most commonly used precursors [60-64]. Conventionally, tetraalkoxysilanes are

used as sol-gel precursors [65]. However, the use of alkyl or aryl derivatives of

tetraalkoxysilanes as precursors may provide important advantages in coating technology

because sol-gels obtained by using these derivatives are reported to be less prone to

cracking and shrinking [31, 65].

An organic polymer is usually used to form a network and provide the pendent

functional groups for sol-gel materials for selective interaction with the target analytes.

The resultant microextraction performance of sol-gel stationary phase will mainly be

affected by the characteristics of the pendent functional group. They provide the sol-gel

material with different affinity to certain classes of analytes based on the structure and

polarity of pendent functional groups acquired from the polymer(s). Polysiloxanes are

47

important SPME coating materials due to its high thermal stability and good film forming

characteries. Poly(dimethlysiloxane) (PDMS) has been commonly used in sol-gel

microextraction sorbent either as a mono-polymer [54] or co-polymers [63].

Hydroxy-termanited, PDMS has been used to synthesize sol-gel sorbents that have been

used in a variety of microextraction applilcation. Other used polymers such as poly (vinyl

alcohol) (PVA) [63], polyethylene glycol (PEG) [66, 67], hydroxyfullerene [68], Ucon

[69] and poly (methylphenylvinylsiloxane) (PMPVS) [70] etc. have been used to produce

sorbent coatings.

All the above mentioned precursors, polymers, and deactivating reagents (if

necessary) are dispersed and mixed well in solvent(s). Here the solvent can be a pure

organic solvent or a mixture of organic solvents. The purpose of the solvent is usually for

the dissolution of all components of sol solution to achieve a molecule level uniformity in

the mixed system. After mixing, water and a proper catalyst are added to the system to

initiate hydrolysis of the precursor(s).

The catalysts for sol-gel reactions usually include (I) acids [60, 71-72], (II) bases

[73] (III) or ions (e.g., F-), [74, 75]. A radical initiator is used in conjunction with

ultraviolet (UV) light induced polymerization. In sol-gel synthesis, trifluoroacetic acid is

the most popular catalyst since its initial use in 1997 [31]. It serves not only as a catalyst

but also an organic solvent and a source of water (1% water). As a strong organic acid of

pKa value of 0.3, it provides for relatively the short gelling times. Other than TFA, HCl is

the second most commonly used catalyst for sol-gel reactions [60]. Acid-catalyzed sol-gel

reactions are more popular in the preparation of microextraction sorbents. However,

48

NaOH, and F- are sometimes used for comparison purposes in sol-gel CME techniques

[76]. Generally, acid-catalyzed sol-gel processes are believed to produce linear polymers

while base-catalyzed processes produce highly condensed particulate structures.

Acid-catalyzed material also has a tendency to be highly porous and possesses higher

surface areas in comparison to base-catalyzed sol-gel materials. High surface area and

narrowly distributed pore structure of acid-catalyzed sol-gel material make them very

suitable for being used as sorbent coatings in analytical microextraction.

Sol-gel synthesis consists of two processes: (1) hydrolysis of the precursor(s) and

(2) water or alcohol polycondensation of the sol-gel-active species present in the sol

solution [1]. The sol-gel-active species include fully or partially hydrolyzed precursors

and polymers which include silica-based species, analogous non-silica species, silanol

group or any other species chemically reactive to them. Scheme 2.1 illustrates hydrolysis

and condensation of tetramethoxysilane (TMOS) as an example (Scheme 2.1). During the

process, a liquid like colloidal suspension (the sol) is transformed by hydrolysis. Then a

3-D network, a gel form via polycondensation reactions. In sol-gel process, hydrolysis

and condensation reactions occur simultaneously — not stepwise. A wet

three-dimensional sol-gel network of porous material is formed as the polycondensation

reaction progresses.

49

Si

OCH3

OCH3

OCH3

CH3O + H2Ohydrolysis

Si

OCH3

OCH3

OCH3

HO + CH3OH

Si

OCH3

OCH3

OCH3

CH3O + Si

OCH3

OCH3

OCH3

HOalcohol condensation

Si

OCH3

O

OCH3

CH3O Si

OCH3

OCH3

OCH3

CH3OH+

Si

OCH3

OH

OCH3

CH3O + Si

OCH3

OCH3

OCH3

HOwater condensation

Si

OCH3

O

OCH3

CH3O Si

OCH3

OCH3

OCH3

H2O+

Scheme 2.1 Illustration of hydrolysis and condensation of tetramethoxysilane (TMOS).

Adapted from [18]

2.5 Creation of sol-gel CME sorbents

Further advancements in the area of analytical microextraction are greatly

dependent on improvements and new breakthroughs in sorbent development and coating

technology to replace traditional coating technology. The development of sol-gel coatings

and/or monolithic beds has proved to be an important step in this direction. Generally,

preparation of the sol-gel stationary phase coatings for CME involves four typical steps:

(1) pre-treatment of the fused-silica capillary, (2) preparation of the sol solution, (3)

sol-gel coating process, and (4) treatment of coated capillary.

2.5.1 Pre-treatment of the fused-silica capillary

The purposes of capillary pretreatment are: (1) cleaning the possible contaminants

inside the fuse silica capillary, (2) increasing the concentration of surface silanol group.

50

Silanol group concentration on the capillary surface may be low due to the formation of

siloxane bridges due to condensation of neighboring silanol groups at high drawing

temperatures (about 2000 ºC), and may vary from one piece to another piece. Since

silanol groups on the capillary surface represent the binding sites for in situ created

sol-gel extracting phases, higher surface concentration of these binding sites is desirable

to facilitate the formation of chemical bonds between sol-gel extracting phase and the

capillary inner surface. Alkali solutions are traditional approaches for pretreatment of

fused silica capillaries [77-83]. The pretreatment usually starts with rinsing by 1 M of

NaOH, followed by diluted HCl to neutralize excessive NaOH, and rinsing with purified

water. After rinsing, the capillary is usually purged with helium or nitrogen. This

traditional pretreatment method has been used for prepare sol-gel CE preconcentration

columns [79], sol-gel SPME fibers [11, 35] and stir bars for sorptive extraction [77]. It

also was used in preparing CME [78, 81-83] sorbents.

Hayes and Malik [84, 85] reported the use of hydrothermal treatment of the inner

surface of fused silica capillary prior to in situ creation of surface coated and monolithic

capillaries. This hydrothermal treatment was performed for several reasons. First, the

water serves to clean the inner capillary surface, removing any contaminants originating

from the capillary drawing process or postdrawing manipulation and handling. Moreover,

this pretreatment with water enhances surface silanol concentrations, thereby providing a

higher percentage of bonding sites for anchoring the sol-gel coating. A homemade

gas-pressure-operated capillary filling/purging device [69] was used to realize this

(Figure 2.2). The fused silica capillary was sequentially rinsed with two organic solvents

51

of different polarities: CH2Cl2 was used first to clean the non-polar and moderately polar

contaminations inside the capillary. Then CH3OH was used to substitute CH2Cl2 and also

to clean the relatively polar contaminations on the inner surface. Deionized water was

then passed through the capillary under helium pressure. This was followed by purging of

the capillary with helium for 5 min. Both capillary ends were then sealed using an

oxyacetylene torch and the capillary was placed in a GC oven at 250 ºC for thermal

conditioning for 2 h. Following this, the capillary was removed from the GC oven, it ends

were cut open using an alumina wafer, and the capillary was further purged with helium

for an additional 30 min at ambient temperature.

Figure 2.2 Homemade capillary filling/purging device. Adapted from [69]

52

2.5.2 Preparation of the sol solution

Designing the sol solution is the most critical step in preparing sol-gel CME

capillary. Judicious selection of the sol solution ingredients is essential for a successful

creation of the desired sol-gel sorbent. Chemical reagents for the preparation of sol-gel

sorbents normally includes sol-gel precursor(s), a solvent for dispersing and mix all the

reagents, a polymer which provides the functional group deactivation reagent(s), a

catalyst (or inhibitor) and water. The role of each ingredient plays has been explained in

section 2.4.

In addition to typical ingredients in the sol solution, various additives are often

used, such as surface deactivating reagents or a drying control chemical additive (DCCA).

An important attribute of sol-gel for coating technology is the simplified deactivation step

compared with traditional coating technology [31]. In sol-gel chemistry, deactivation is

achieved by adding to the sol solution traditional deactivating reagents such as

poly(methylhydrosiloxane) (PMHS), Hexamethyldisilazane (HMDS), etc. that contain

chemically reactive hydrogen atoms. These chemicals are not sol-gel reactive but they get

physically incorporated in the coating and deactivate residual silanol groups on the

created sol-gel material during the thermal step. This is in contrast with traditional

coating approaches, where silica surface is deactivated prior to coating.

Malik and coworkers reported the use of various deactivation reagents for sol-gel

stationary phases, such as phenyldimethylsiane (PheDMS) [84, 86]

1,1,1,3,3,3-hexamethyldisilazane (HMDS) [50, 51, 57, 58], and polyethylhydeosiloxane

(PMHS) [31, 54]. Some deactivating reagents and deactivation reactions of HMDS are

53

shown in Scheme 2.2 and Scheme 2.3.

A drying control chemical additive (DCCA) such as formamide, glycerin,

dimethyl ether, and oxalic acid is usually used as a sol-solvent and it may play other

critical roles in the sol-gel reactions [87, 88]. During sol-gel process, stress may be

developed in the sol-gel network as the solvent escapes or as the pore size changes during

these processes. This stress may cause problems during the formation of the uniform

sol-gel coatings. To overcome this problem, DCCA is incorporated in the starting mixture

of sol solution, and they can release the capillary stress generated during of the coated

surface [1, 77, 89, 90]. Unfortunately, the exact way in which the DCCA improves the

drying process is still unclear [90].

Scheme 2.2 Deactivating reagents

H3C Si NH Si

CH3 CH3

CH3 CH3

CH3

Hexamethyldisilazane, HMDS

Si OH3C

CH3

CH3

Si O

CH3

CH3

Si O

CH3

Hn

Si O

CH3

CH3 m

CH3

Polymethylhydrosiloxane, PMHS

54

Si O Si O Si OSi O

OHOH O

(Bare silica surface)

+ SiCH3

CH3

CH3

NH Si

CH3

CH3

CH3

Si O Si O Si OSi O

O

(Derivatized silica surface)

O

SiCH3

CH3

CH3

O

SiCH3

CH3

CH3

+ NH3

HMDS

Scheme 2.3 Deactivation of silica surface with HMDS

The homogeneity of the sol is very important. Therefore careful selection of a

solvent system which is compatible with other ingredients is necessary. A mixture of

solvents compatible with inorganic and organic components in the sol system needs to be

used to achieve the homogeneity. The viscosity of the system is another important

parameter for in-tube SPME coatings. The water/precursors ratio in the sol solution also

affects the speed of reaction as well as the physical properties of the obtained sol-gel

materials. The ratio should be carefully controlled to adjust the reaction speed and to fine

tune structures of sol-gel materials.

55

2.5.3 Sol gel coating process

After preparing sol solution, the sol solution was filled into the capillaries. Sol

solution was kept inside of the capillary for a controlled period of time to facilitate the

formation of sol-gel reactions inside the capillary. During this process, the evolving

sol-gel organic-inorganic hybrid material gets chemically bonded to the inner walls of the

capillary via condensation with the surface silanol groups. After this, the unbonded

portion of the sol solution is expelled from the capillaries under inert gas leaving behind a

surface-bonded sol-gel thin coating within the capillary. In the end, this thin coating is

further purged with inert gas for an additional period to facilitate the evaporation of the

remaining volatile organic solvents. The in-capillary residence time of the sol solution is

various and it can be adjusted for application purpose. It can be adjusted to prepare

sol-gel coatings with various thicknesses.

A sol-gel in-tube SPME coating technology was first introduced by Malik and

co-workers [69]. A homemade capillary filling/purging device (Figure 2.2) was used to

fill the capillary with the coating sol solution under helium. Then an additional period of

purging of helium was applied to facilitate the evaporation of the remaining organic

solvents. Hu and coworkers [78, 81] used a syringe to introduce sol solutions into the

capillary to prepare CME sorbents followed by purging nitrogen to remove the sol

solution.

2.5.4 Further treatment of sol-gel-coated CME capillary

After coating, the capillary is further thermally treated for aging, drying,

conditioning and cleaning. Different temperature programs have be been developed to

56

achieve the desired performance. Malik and coworkers [31, 57] installed the capillary in a

GC oven using a temperature program of 40 to 300 ºC or even higher at a rate of 1

ºC/min under the purging of helium. Purging with inert gas helped to remove the solvent

and unbonded materials from the coating at high temperature. The prepared CME

capillary was coupled with both GC and HPLC [44, 47]. Hu and coworkers [81]

connected CME capillary in a GC oven under nitrogen, the solvent was removed through

the capillary at 1 bar for 10 min and then at 0.2 bar for 60 min to prepare a zirconia

coating, then heated at 573 K for 8 h in a muffle furnace. The prepared zirconia coating

was used for on-line hyphenation with ICP-MS. In prepare another

N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPTS)-silica coating for on-line

hyphenation with Inductively Coupled Plasma Mass Spectrometry (ICP-MS), they also

placed the capillary in a muffle furnace at 120 ºC for 8 h with the heating program of

1 °C min−1

rising from the ambient temperature for the formation of the coating. When

making a monolithic column for CME, Hu and coworkers [78] sealed both ends of the

capillary with silicon rubber followed by further reaction at 40 °C for 12 h. The prepared

monolithic column was for on-line CME coupled to HPLC. In the end, the CME capillary

was rinsed with ethanol in order to remove the surfactant [78] or template materials [91].

2.6 Characterization of sol-gel extracting phase and its morphology

Various characterization methods are employed to understand sol-gel coatings.

Scanning electron microscopy (SEM) is one of the most popular techniques to evaluate

the morphology of sol-gel coatings thanks to its combination of higher magnification,

57

larger depth of focus, greater resolution, and ease of sample observation [92]. SEM can

reveal the uniformity of the coating thickness and structural details or defects through

cross sectional and surface views of sol-gel coatings.

In addition to SEM, Infrared (IR) spectroscopy is also an important and simple

tool to follow the evolution of the sol-gel material and microstructure of sol–gel silica

films [93, 94]. FTIR spectroscopy is most commonly used for identifying specific

chemical bonds on the sol-gel sorbent coating. Besides IR, various techniques such as

mass spectrometry (MS), fluorescence microscopy and nuclear magnetic resonance

(NMR) have been used to study chemical bonds in sol-gel structure [95, 96]. NMR is

another important and powerful analytical technique to investigate the structural features

for sol-gel material. NMR, SEM and FTIR are the most frequently used techniques to

study gel formation and for surface coating characterization. Other techniques atomic

force microscopy (AFM) [97], small angel X-ray photoelectron spectroscopy (XPS) [98],

etc. are also used to study the morphology of sol-gel materials.

2.7 Reported sol-gel coating materials in SPME and CME

Sol-gel chemistry provides a simple and convenient pathway to synthesis

advanced coating materials for fiber-based SPME or capillary SPME (CME). Various

types of polymers and functionalized organic compounds have been used. Table 2.1

summarized polymer and organic components that have been used to prepare sorbents in

the area of SPME and CME in recent years.

58

Table 2.1 Organic sorbents and their applications. Adapted from ref [99]

Name of the stationary

phase

Functional Group or Repeating

Functional Group structure

SPME

Techniques Ref.

Polyvinyl chloride

(PVC)

CH2 CH

Cl

n

HP-SPME [100-102]

Poly(ethylene glycol)

(PEG)

CH2 CH2n

O

SPME

CME

[103-105]

Poly(ethylenepropylene

glycol) monobutyl ether

(Ucon)

CH2 CH2 O CH2 CH

CH3

On

SPME [104]

3-Aminopropyltrimetho

xysilane (APTMS)

Si

OCH3

OCH3

H2NCH2CH2CH2 OCH

SPME,

CME

[59, 106]

N-(2-aminoethyl)-3-ami

nopropyltrimethoxysilan

e (AAPTS)

HN2

N Si OCH33H

CME,

Monolithic

- CME

[91,107]

Hydroxy-terminated

polydimethylsiloxane

(PDMS)

Si

CH3

CH3

On

HS-SPME

SPME

CME

[32,59,

105, 108]

Hydroxy-terminated

polymethylphenylsiloxa

ne

(PMPS)

Si

CH3

CH3

O Si

CH3

CH3

O Si O

x y

SPME

CME

[109 -111]

59

Table 2.1 (Continued)

Polyacrylate (PA)

CH CH2

n

COOH

SPME

[112]

Divinyl benzene

(DVB)

CH2 CH CH CH2

HS-SPME [113]

Calixarenes

HS-SPME

[114-118]

β-cyclodextrins

HS-SPME

[119-123]

Poly(trifluoropropyl)me

thylsiloxane (PTFPMS)

Si

CH3

CH2

O

CH2CF3

n

HS-SPME [124]

Poly(butylacrylate)

CH2 CH

C O

n

SPME [125]

60


Methyl acrylate (MA)

CHCH2C O CH3

O

HS-SPME [126]

Methyl methacrylate

(MMA)

CCH2 C O CH3

O

CH3

HS-SPME [126]

Butyl methacrylate

(BMA)

CCH2 C O C4H9

O

CH3

HS-SPME

[126 -128]

Anilinemethyltriethoxys

ilane (AMTEOS)

NH Si C2H5O3

SPME [129]

Crown Ethers

SPME

[11,

130-132]

Calixcrowns

SPME [35, 133]

61


Poly(methylphenylvinyl

siloxane)

O Si O

CH3

Si O

CH3

CH3

Si

CH3

HC CH2

x y z

CME,

HS-SPME [37, 134]

Methyl groups

CME [135]

Cyano-Polydimethylsilo

xane

CNCH2CH2CH2

CME [67]

Polytetrahydrofuran

(poly-THF)

CME [44]

n-Octyltriethoxysilane

(C8-TEOS)

Si C2H5O3

C8H17

Monolithic

- CME

[78]

Dendrimers

CME [51]

Hydroxyfullerene

(fullerol) -OH-TSO

C60 OH n

HS-SPME [68]

Poly(vinyl alcohol)

(CH2 CH)n

OH

HS-SPME [63, 136]

62

2.8 Drawbacks of silica-based sol-gel materials

Sol-gel microextraction sorbents reported so far are predominantly silica-based.

Despite many attractive material properties such as mechanical strength, surface

characteristics, catalytic inertness, surface derivatization possibilities, etc., silica-based

sol gel materials have some inherent shortcomings. The main shortcoming of silica-based

sorbents is the narrow range of pH stability. Siloxane bond (Si-O-Si) on the silica surface

hydrolyzes at pH > 8 [137] and it happens rapidly under elevated temperatures [138].

Under acidic conditions, siloxane bond is also unstable [139], and it gets worse with

elevated temperatures [140, 141]. A number of methods such as multiple covalent bonds

[142], multi-dentate synthetic approach [143], and end-capping [144] have been used to

solve this problem. None of them, however, seems to work effectively. Therefore,

developing sorbents with a wide range of pH stability is fundamentally important for

chromatographic separation and sample preparation technology. From this point of view,

metal oxides such as zirconia, titania, and alumina are interesting materials to offer better

chemical and thermal stabilities [138]. Due to these advantages, they have attracted

attention of separation scientists and they have been used in chromatography for many

years.

2.9 Sol-gel process using transition metal alkoxide

Transition metal alkoxides, M (OR)z, (M is Ti, Zr, and Hf etc.) are used sol-gel

precursors to glasses and ceramics. These transitional metal alkoxides are distinguished

from silicon alkoxides (Si(OR)4) in two aspects [1]: (1) they have greater chemical

63

reactivity resulting from the lower electronegativity of transition elements, which causes

them to be more electrophilic and thus less stable toward hydrolysis, (2) they have the

ability to exhibit several coordination states, so that coordination expansion occurs

spontaneously when water or other nucleophilic reagents are added, and they are able to

expand their coordination via olation, oxolation, alkoxy bridging, or other nucleophilic

association mechanisms, (3) the greater reactivity of transition metal alkoxides requires

that they be processed with stricter control of moisture and conditions of hydrolysis in

order to prepare homogeneous gels rather than precipitates, (4) the generally rapid

kinetics of nucleophilic reactions cause fundamental studies of hydrolysis and

condensation of transition metal alkoxides to be much more difficult than for Si(OR)4.

Similar to silicon alkoxides (Si(OR)4), two main reactions typically are involved

in the sol-gel process including transition metal alkoxides: (1) hydrolysis and (2) water or

alcohol condensation reactions. The general scheme for sol-gel hydrolysis and

condensation reactions of metal alkoxide precursors are illustrated in Scheme 2.4.

For coordinatively saturated metals in the absence of catalysts, hydrolysis and

condensation both occur by nucleophilic substitution (SN) mechanism involving

nucleophilic addition (AN) followed by proton transfer from the attacking molecule to an

alkoxide or hydroxo-ligand within the transition state and removal of protonated species

as either alcohol or water. The condensation process by removal of alcohol molecule is

called alcoxolation, and that by removal of water molecule is called oxolation. Scheme

2.5 illustrates the mechanism for hydrolysis, alcoxolation, oxolation or olation [1].

64

(A) Hydrolysis

M

OR

OR

OR

RO + H2O + ROHM

OR

OH

OR

RO

(B) Water condensation

M

OR

OH

OR

RO M

OR

OR

OR

HO+ M

OR

O

OR

RO M

OR

OR

OR

+ H2O

(C) Alcohol condensation

M

OR

OR

OR

HO+M

OR

OR

OR

RO M

OR

O

OR

RO M

OR

OR

OR

+ ROH

Scheme 2.4 General sol-gel process for transitional metal alkoxides. Adapted from [1,

145]

65

(A) Hydrolysis

O

H

H + M OR O

H

H

M OR O

H

R

MHO

M OH + ROH

a b c

d

(B) Oxolation

+ M OHOM

H

M O

H

M OH O

H

H

MOM

a b c

M O +M

d

H2O

(C) Alcoxolation

+ M OROM

H

M O

H

M OR O

H

R

MOM

a b c

M O + ROHM

d

(D) Olation

+ ROHM O M + O

R

H

M M O M

H

+ H2OM O M + O

H

H

M M O M

H

Scheme 2.5 Mechanisms of sol-gel hydrolysis and condensation reactions of metal

alkoxides [1]

66

In hydrolysis, the first step (a) for hydrolysis is a nucleophilic addition, which

leads to a transition state (b), where the coordination number of metal atom has increased

by one. The second step involves a proton transfer within (b) leading to the intermediate

(c). The third step is the departure of the leaving group which should be the most

positively charged species within the transition state. The entire process, (a) to (d) follows

a nucleophilic substitution mechanism.

The oxolation is a reaction by which an oxo bridge (―O―) is formed between

metal atoms through the elimination of a water molecule. Basically, oxolation follows the

same mechanism as that for condensation process: the metal atom replacing hydrogen

atom in the entering group. When the metal is unsaturated in terms of coordination

number, oxolation occurs by AN with rapid kinetics. Alcoxolation follows pretty much the

same mechanism as oxolation except the leaving group is an alcohol molecule.

Alcoxolation and oxolation are two competitive processes. The thermodynamics of

hydrolysis, alcoxolation, and oxolation are governed by the strength of the entering

nucleophile, the electrophilicity of the metal, and the partial charge and the stability of

the leaving group. In addition, condensation can also occur by olation when the transfer

ability of the proton is large. The thermodynamics of olation depend on the strength of

the entering nucleophile and the electrophilicity of the metal. The kinetics of the olation

are systematically fast because there is no proton transfer in the transition state.

Acid or base catalyst can influence both the hydrolysis and condensation rates and

the structure of the condensed product. Acids serve to protonate alkoxide groups,

enhancing the reaction kinetics by producing good leaving group, and eliminating the

67

requirement for proton transfer within the transition sate. Hydrolysis goes to completion

when sufficient water is added.

2.10 Non-silica sol-gel materials in analytical microextraction and chromatographic

separation

Metal oxides such as zirconia, titania and alumina offer much better chemical

stability than silica which has attracted a great deal of interest in the separation science

community [138]. They have been used in chromatography for many years.

The literature shows that zirconia has been the most systematically studied

transition metal oxide. Zirconia appears possess thermal exceptionally high stability. It

has been extensively for capturing and storing radiochemical wastes. Zirconia possesses

superior alkali resistance than other metal oxides, such as alumina, silica and titania [2]

[138]. It is practically insoluble within a wide pH range (1-14) [146- 148]. Zirconia also

shows outstanding resistance to dissolution at high temperatures [149]. Besides the

extraordinary pH stability, excellent chemical inertness and high mechanical strength are

two other attractive features that add value to zirconia as a support material in

chromatography and membrane-based separations. Extensive research has been done on

zirconia particles and their surface modifications for use as HPLC stationary phase [150,

151]. A number of reports have also recently appeared in the literature on the use of

zirconia-modified fused silica capillary electrophoresis (CE) [152, 153]. A zirconia-based

hybrid organic-inorgnic sol-gel polydimethyldiphenylsiloxane (PDMDPS) coating was

developed for CME [58]. The sol-gel zirconia-PDMDPS coating was immobilized on the

inner surface of a fused silica microextraction capillary. The resultant CME coating

68

efficiently extracted polycyclic aromatic hydrocarbons, ketones, and aldehydes from

aqueous samples, and it performance remained practically unchanged even after

continuous rinsing with a 0.1 M NaOH solution for 24 h. Recently, a zirconia hollow

fiber was prepared by sol-gel technology and applied in microextraction [56]. For this, a

polypropylene hollow fiber was employed as the template, and it was immersed in the

proper zirconia sol precursor for specify followed by a drying process. After burning the

template off, the zirconia hollow fiber was obtained. The zirconia hollow fiber is similar

to its propylene hollow fiber template in terms of morphology. However, it exhibits a

bimodal porous structure consisting of: through- pores and skeleton mesopores. The pore

structure and wall thickness can be easily controlled during the coating process and heat

treatment. The zirconia hollow fiber was a highly selective for phosphonic

acid-containing compounds. Pinacolyl methylphosphonic acid is a nerve agent

degradation product and it was used as a model analyte.

Titania has also attracted interest recently in separation science due to its superior

pH stability and mechanical strength compared with silica [154-157]. Several studies

have been conducted on the application of titania in chromatographic separations

[156-161]. A sol-gel titania-poly(dimethylsiloxane) (TiO2-PDMS) coating was

successfully developed for capillary microextraction (CME) by Malik and coworkers in

2004 [57]. This is the first report of the use of sol-gel titania-based organic-inorganic

material as a sorbent in capillary microextraction. The sol-gel titania-PDMS coating was

immobilized on the inner surface of a fused silica capillary. It showed good affinity for

many non-polar and moderately polar compounds, and it was used for on-line extraction

69

in hyphenation with HPLC analysis of polycyclic aromatic hydrocarbons, ketones, and

alkylbenzenes. This newly developed coating demonstrated excellent pH stability.

Extraction characteristics remained unchanged after continuous rinsing with a 0.2 M

NaOH solution for 12 hours. Besides those neutral compounds, Zeng and coworkers

utilized sol-gel titania in conjunction with hydroxy-terminated silicone oil as a sorptive

coating for SPME fibers [162]. The sol-gel coating provided significant extraction and

had sensitivity to polar compounds in SPME-GC analysis [162]. The application

demonstrated extraction and analysis of six aromatic amines in dye process wastewater.

The developed sol-gel hybrid titania coating was extremely pH stable, and its extraction

characteristic remained unchanged after rinsing with a 3 M HCl or NaOH solution for 12

hours. The thermal stability of this coating was as high as 320 ºC. An ordered,

mesoporous sol-gel titania film was developed for preconcentration/separation of trace V,

Cr, and Cu by CME. The sorbed analytes were eluted for electrothermal

vaporization-inductively coupled plasma mass spectrometry (EVT-ICP-MS)

determination of the trace element ions in environmental and biological samples [82]. In

this work, the properties, stability, and other factors of the sol-gel titania coating affecting

the sorption behaviors of V, Cr, and Cu were investigated.

Alumina, due to its high surface area, mechanical strength, thermal and chemical

stability, has been widely used in the area of sample purification [163] and analytical

separations [164, 165]. The surface chemistry of alumina is quite different from that of

silica. Silica has a weak Brønsted acidity due to the silanol groups, while alumina

provides well Lewis acidity and basicity as well as a low concentration of Brønsted acid

70

sites [138]. Both Brønsted and Lewis acid–base characteristics are responsible for the

chromatography performance of alumina. The surface chemistry of alumina is more

sililar to that of zirconia than to silica. Thus, it is reasonable to expect rather more

complex surface properties. As compared to silica, alumina is capable of both ion and

ligand exchange [166]. The ligand exchange ability of alumina originates from the

presence of Lewis acid sites on the surface, i.e., coordinatively unsaturated Al3+

, and

water molecules or other easily displaced ligands coordinatively bond to the sites [138,

166]. Lewis basic analytes containing polar functional groups can substitute for the

surface hydroxyl groups or coordinated water molecules and form complexes with the

metal ions of the oxide surface [154, 167] and so ligand exchange interaction can play an

important role for selective extraction of these compounds by alumina-based sorbents.

Alumina-based sol-gel sorbents were prepared from a highly reactive precursor, alumina

sec-butoxide, and a sol-gel active organic polymer (hydroxyl-terminated

polydimethylsiloxane) [167]. The resultant sol-gel alumina-OH-TSO coating had

superior ligand exchange ability, thus it extracted polar compounds such as fatty acids,

phenols, alcohols, aldehydes, and amines. It also demonstrated outstanding pH usage

range, good thermal stability, and good coating preparation reproducibility. When applied

in analysis of volatile alcohols and fatty acids in beer, the alumina-OH-TSO coating

provided recoveries ranging from 85.7% to 104% and the relative standard deviation

values of less than 9% for all the analytes [167].

71

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80

CHAPTER THREE

GERMANIA-BASED SOL-GEL HYBRID ORGANIC-INORGANIC COATINGS

FOR CAPILLARY MICROEXTRACTION AND GAS CHROMATOGRAPHY

3.1 Introduction of germanosilicate glasses material

In recent years silica glasses containing germanium dioxide (germanosilicate

glasses) have attracted considerable interests due to their various appealing properties.

Most of them are related to the composition and to the structure of the glass [1] . As an

isostructural analogue of SiO2, GeO2 is well compatible with the silica network; it is

employed as a refractive index modifier in binary xSiO2-(1-x) GeO2 glasses for optics.

The presence of Ge atoms modifies the atomic structure and causes both permanent and

photo-induced variation of the refractive index [2-3]. Ge-doped silica glasses have

attracted much attention for giving rise to phenomena such as photosensitive grating

effects and second harmonic generation (SHG) [4, 5]. The low phonon energy of

germania is also another interesting property because it can be used to produce low losses

optical waveguides and amplifiers [6]. For their intrinsic characteristics such as insulating

behavior, low phonon energy with respect to the silicate glasses, transparency and

homogeneity, SiO2-GeO2 glasses are good candidates as host matrixes for metal or

semiconductor clusters for optical applications. Significant attention has been further

devoted to these remarkable optical properties of these binary systems manifested in

different optical and opto-electronic phenomena.

81

The photosensitivity exhibited by these materials is ascribable to the presence of

germanium dioxide and to the occurrence of germania-related defect center. From a

structural point of view, each Ge atom could be substitutional to a Si atom and could be

bonded to four O atoms, or it could be under coordinated giving rise to oxygen deficient

centers (GODC) [7]. It is well known that there is an UV absorption band near 5 eV for

germanosilicate material and that this absorption is associated with an oxygen-deficient

defect [8, 9]. A 5 eV absorption band of germania-related oxygen-deficient defects is

responsible for the photosensitivity of germanium oxide based materials [10, 11].

Both silica and germania are prototypes of simple glasses consisting of

corner-shared TO4 (T = Si, Ge) tetrahedra linked in a random network structure. T-O-T

bond angles are estimated by X-ray diffraction to be 144 º in SiO2 glass and 133 º in

GeO2 glass. Besides the structural similarities, a striking difference between these two

systems is the concentration of defects. In pure silica, they are of order of 10 -9

of the total

Si atoms [12], while in pure germania or in binary SiO2-GeO2 glasses the concentration

is around 10 -3

-10 -4

of the total germanium sites. This difference has been ascribed to

different lead of thermodynamic stability for SiO2 and GeO2. From a thermodynamic

point of view, at room temperature, formation of SiO2 quartz is favored with respect to

GeO2 in the hexagonal form. Moreover, the latter is much less stable than SiO2. GeO2

may occur in either the 4-fold coordination of the α-quartz structure, or the 6-fold

coordination of the rutile structure. However, although both GeO2 polymorphs exist at

ambient conditions, rutile is the stable one, whereas the α-quartz structure is metastable

[13].

82

3.2 Preparation of sol-gel GeO2/ormosil organic-inorganic hybrid material

Various methods including melting, flame hydrolysis, vapour axial deposition

(VAD), sputtering deposition, chemical vapour deposition (CVD), ion implantation and

sol-gel technology have been employed to fabricated GeO2 based materials [14]. Among

the synthetic methods, the sol-gel route has gained great interest during recent years for

making advanced materials and in particular for preparation oxide-based coatings and it

has gained great interest in preparing Ge-doped SiO2 glasses. Germanium alkoxide

typically undergo polymerization via hydrolysis and condensation reactions similar to

those seen in the case of alkoxysilanes (Scheme 3.1). When GeO2 is mixed with vitreous

silica at a molecular level, Ge atoms would randomly substitute Si atoms in SiO4

tetrahedra so that the microstructure of the glass would remain a continuous

three-dimensional network consisting of interconnected SiO4 and GeO4 tetrahedea [15].

Scheme 3.1 Typical hydrolytic ploycondensation of germanium alkoxide to form

germania [16]

Ge OR4

+ x H2O GeRO4-x

OHx

+ x ROH

Ge OR GeHO+ Ge GeO ROH+

Ge OH GeHO+ Ge GeO + H2O

83

Sol-gel method is a solution-based chemical process from which

inorganic-organic hybrid materials can be synthesized through hydrolysis and

condenstation reactions. The components involved are well mixed at the molecular level,

providing good control of chemical compositions film homogeneity. Interests in using the

sol-gel method to fabricate germanosilicate coatings originate from the advantages that it

offers: homogeneity, ease of composition control, ability to fabricate large area coatings,

and low equipment coat. Materials for photonics require a very high level of purity and

homogeneity. Sol-gel processing is therefore an important route to fabricating

GeO2-based materials with high degree of purity, and homogeneity as well as advanced

and controlled properties [7]. Compared with the conventional glasses and glass-ceramic

processes, the sol-gel technique has the notable advantages of providing chemical

homogeneity at relatively low process temperatures. In the case of multicomponent

systems it provides a more uniform phase distribution and the possibility of preparing

new crystalline and non-crystalline materials [14]. In addition, because sol-gel based

glass can be prepared on the basis of chemical reactions involving a solution, this process

is very suitable to fabricate glassy and crystalline films or coatings. It has been widely

used to produce various kinds of functional films, coatings, hollow-core fibers,

concavo-convex cavity waveguides, etc. and organic or inorganic materials [14].

3.3 Ormosil precursors at low treatment temperature

Sol-gel germanosilicate synthesis usually starts from solutions of silicon and

germanium alkoxides at a low germania content. In reported GeO2-based material system,

84

tetraethylorthosilicate (TEOS) is generally employed as a SiO2 source, and TEOG is

generally used as GeO2 source. However, the heat treatment temperature for TEOS is

quite high (around 1000 ºC). Therefore, it is difficult to produce a film thicker than 0.2

μm by a single-coating process in a single purely inorganic sol-gel layer [17]. This is

because high curing and sintering temperatures necessary for densification of the final

materials causes a large shrinkage of the film during the thermal densification process

which leads to the cracking in a thicker layer and then leads to possible stress fracture [18,

19]. The use of organically modified alkoxysilane precursors helps overcome this

problem. Since silica precursors can produce a relatively thick single-coating layer at a

low temperature or even room temperature [20-12] without cracking.

Methyltrimethoxysilane (MTES) has been used as precursors for the hybrid material to

provide SiO2 source [23] A GeO2/Methyltrimethoxysilane sol-gel processed hybrid thin

film has been fabricated by acid catalyzed solutions of methyltrimethoxysilane (MTES)

mixed with germanium isopropoxide. A single layer coating with a thickness of about 1.3

μm film has been obtained on silica-on-silicon substrate by single spin-coating process at

a heat treatment temperature of 100 ºC. The result indicated that a heat treatment

temperature below 300 ºC is expected to produce a dense, low absorption, high

transparent hybrid waveguide film for photonic applications. The introduction of MTES

provides the advantage of producing thicker and denser films at low temperature.

Germania/γ-glycidoxypropyltrimethoxysilane orgnic-inorganic hybrid has also been

prepared by a sol-gel technique and a spin coating process. The result indicated that a

heat treatment temperature below 300 ºC is expected to produce a dense, low absorption,

85

highly transparent hybrid film. The introduction of ormosinl GLYMO, provides the

advantage of producing thick and dense films at low temperature [24-28].

Ormosil-based organic-inorganic hybrid materials have been studied as a

promising system for photonic applications in recent years. This is because organic

groups are integrated in the glass and the bulky organic components fill the pores

between the inorganic oxide chains.

3.4 Problems of germanosilicate material during sol-gel process

There are disadvantages to fabricate germanosilicate using sol-gel process. It is

well known that cracks and crystallization could be easily occor in the sol-gel materials.

Since germanium ethoxide hydrolyzes extremely fast and the resulting sol-gel materials

are likely to crack, the production of thick, crack-free GeO2-SiO2 sol-gel films with high

GeO2 content is very difficult. It has been reported by Kozuka et al. that the maximum

thickness achievable without crack formation via non-repetitive deposition is often below

0.1 μm for non-silicate oxide films [29]. It was also reported that although

germanosilicate films containing up to 45 mol % germanium oxide were fabricated, the

film thickness was usually less than 1 μm [30]. Thus, cycles of dip-coating are widely

used to deposit thick sol-gel films to overcome this problem. However, generally the

lifetime of GeO2-SiO2 sol with a high TEOG content is too short to perform cycles of

dip-coating. Therefore, a coating sol solution with a long gelation time is needed. To

reach this goal, techniques capable of suppressing the increase in sol viscosity and the

crack formation in sol-gel films with a high GeO2 content are in great demand. Jing et al.

86

developed a new sol-gel route to fabricate germansilicate film by using

diethylorthosilicate (DEOS) and TEOG as the precursors for SiO2 and GeO2 [30]. A thick

(3.6 μm), crack-free SiO2-GeO2 film containing up to 70 mol % germanium oxide were

obtained. By comparing TEOS and DEOS, they pointed out different precursors differed

greatly during ageing although they had the same molar ratio of SiO2 to GeO2. The sol

synthesized using TEOS and TEOG as raw material exhibited very poor stability and the

use of DEOS as precursor for silica is of great help in stabilizing the sol by maintaining

low viscosity for 280 h. In the work, they also explained the reason from the structural

point of view of the colloidal particles. On the other hand, sol-gel processing of oxide

materials, bulk and thin films, requires a high firing temperature to achieve a full

densification. During this process some oxides show a tendency to crystallize. This is

actually the case of several oxides such as titania [31]. The crystallization of germania

generally took place at temperatures higher than 500 ºC, and a full densification cannot

be achieved without avoiding a partial crystallization of the system. Therefore it is a

difficult problem to solve when sol-gel processing is employed for fabrication process.

The limitation in a wider application of the hybrid materials is the lack of a deeper

knowledge and control of their structure. It was observed that control of the hybrid

structure through an appropriate and reproducible synthesis is a difficult task.

3.5 Introduction of germania-based sol-gel organic-inorganic hybrid coatings for

capillary microextraction and gas chromatography

Solid-phase microextraction (SPME) [32] is an effective approach to solvent-free

sampling and sample preparation for laboratory and field analyses. Developed in 1989 by

87

Belardi and Pawliszyn, [33] SPME integrates sampling, sample preparation, and analyte

preconcentration into a single-step procedure characterized by ease and simplicity. By

eliminating the use of organic solvents, SPME overcomes the limitations inherent in

conventional sample preparation techniques like liquid-liquid extraction (LLE), Soxhlet

extraction, solid-phase extraction (SPE), etc. As a simple, sensitive, time-effective,

easy-to-automate, and portable sample preparation technique, SPME has been

successfully applied to a variety of analytes in environmental [43-36], biological [37-39],

food [40], drug [41-43], flavor [44] and other types of samples in hyphenation with gas

chromatography (GC) [45], high-performance liquid chromatography (HPLC) [46],

capillary electrophoresis (CE) [47], and Mass spectrometry (MS) [48].

In spite of its popularity and wide acceptance, fiber SPME (the dominant format

of SPME) possesses a number of inherent deficiencies arising from the fiber design and

its syringe-based mechanical operation. In fiber SPME, the protective polyimide coating

is removed from the external surface of a fused silica fiber end segment (~ 1 cm). A

sorbent layer created and on this end segment, serves as the SPME extracting phase. The

lack of external protective coating makes the end segment as well as the sorbent phase

coated on it vulnerable to mechanical damage during operation and handling of the

SPME device. The short length (~ 1 cm) of the coated segment results in low sorbent

loading, which leads to reduced extraction sensitivity. Moreover, fiber SPME is difficult

to couple to high-performance liquid chromatography (HPLC) and other liquid-phase

separation techniques. A number of SPME variants [49-53] have been proposed to

overcome these difficulties. Among them, in-tube SPME [54-56] holds great promise,

88

especially for effective hyphenation with liquid-phase separation techniques.

In-tube SPME (also termed capillary microextraction (CME)) [57], is practically

free form the shortcomings inherent in conventional fiber SPME. Unlike fiber SPME,

where a sorbent coating on the outer surface of a small-diameter solid rod serves as the

extraction medium, CME typically uses a piece of fused silica capillary with a stationary

phase coating on its inner surface (e.g., a short piece of GC column) to perform extraction.

In CME, the protective polyimide coating on the external surface of the capillary remains

intact and provides reliable protection against mechanical damage to the capillary.

Moreover, in-tube SPME provides a simple, easy, and convenient way to couple SPME to

gas- as well as to liquid-phase separation techniques.

Despite all these advantageous features, in-tube SPME still suffers from other

shortcomings that originate from the stationary phase coating on the inner surface of a

short piece of conventionally coated GC capillary column typically used as in in-tube

SPME device. First, the thin (usually < 1 μm) coating in such a capillary often results in

low stationary phase loading which leads to low sample capacity and reduced extraction

sensitivity of the technique. Increasing the coating thickness is a possible way to solve

this problem [58, 59]. However, it is extremely difficult to reliably immobilize thicker

coatings using conventional approaches [60]. Second, conventionally prepared GC

coatings are not chemically bonded to the fused silica capillary inner surface. This lack of

chemical bonds is mainly responsible for the low thermal and solvent stability of

conventionally prepared coatings [61]. When used in hyphenation with GC, low thermal

stability of the SPME coatings forces one to use low desorption temperatures to preserve

89

coating integrity, which in turn, results in incomplete sample desorption and sample

carry-over problems. Low solvent stability of the sorbent coating, on the other hand,

prevents effective hyphenation of in-tube SPME with liquid-phase separation techniques

that employ organic or organo-aqueous mobile phases [62]. Obviously, sorbent coating

stability plays a fundamentally important role in solid-phase microextraction — both

fiber-based and capillary-based. Further advancements in these areas will greatly depend

on improvements and new breakthroughs in sorbent development and coating technology.

In recent years, sol-gel technology has attracted considerable interest among

analytical chemists. Sol-gel chemistry provides an effective pathway for the synthesis of

advanced organic-inorganic hybrid materials [63] in a wide variety of forms. The mild

thermal conditions typical of sol-gel reactions not only allow chemical incorporation of a

wide range of chemical species in the created material systems but also ease the

requirements for laboratory operation and safety. Sol-gel technology allows the use of a

wide range of chemicals (both organic and inorganic) to integrate desirable properties in a

single material system. Thanks to all these advantages, sol-gel chemistry is receiving

increased attention as an effective tool for the synthesis of advanced stationary phases

and extraction media for modern analytical techniques. In recent years, sol-gel chemistry

has been the used to prepare highly stable surface-bonded coatings or monolithic beds

within capillaries for use as chromatographic stationary phases and sorbents for

microextraction techniques [57, 63, 64-70].

Silica-based stationary phases and sorbents are predominantly used in modern

separation and sample preparation techniques. Silica surface chemistry and surface

90

modification process are well understood. However, the limitations of siliceous materials

are also well known. The pH stability problem is a major drawback of silica-based

stationary phases and extraction media. Siloxane bond (Si-O-Si) on the silica surface

hydrolyzes at pH > 8 [71] and it happens rapidly under elevated temperatures [72].

Under acidic conditions, siloxane bond is also unstable [73], and it gets worse with

elevated temperatures [74]. Transitional metal oxides such as zirconia, titania, and

alumina are interesting materials to offer better chemical and thermal stabilities [72], and

have attracted attention of separation scientists.

Although titania-, alumina and zirconia-based sol-gel systems [75-77] that

demonstrated high chemical and pH stabilities in applications, surface modification of

these systems is rather hard. The precursor selections for transitional metal systems are

limited. Therefore, we are interested in investigating novel non-silica sol-gel system and

their application in separation material. Germania as an isostructural analog of silica, can

be expected to offer advanced material properties for analytical separation and sample

preparation. However, the potentials and promises of these novel materials in analytical

chemistry remained practically unexplored. In this work, for the first time, the

preparation of germania-based hybrid organic-inorganic sol-gel coatings for capillary

microextraction was explored. The great performance of sol-gel germania-based coatings

in CME, and the preliminary GC results that point to the analytical potential of such

coatings as GC stationary phases was presented.

91

3.6 Experimental

3.6.1 Equipment

Capillary microextraction - gas chromatography (CME-GC) experiments with

sol-gel germania coated capillaries were carried out on a Shimadzu Model 14A GC

system equipped with a flame ionization detector (FID) and a split/splitless injector. An

in-lab designed liquid sample dispenser [57] was used to perform capillary

microextraction via gravity flow of aqueous samples through the sol-gel microextraction

capillary. A Fisher Model G-560 Genie 2 vortex (Fisher Scientific, Pittsburgh, PA) was

used for thorough mixing of sol solution ingredients. A Micromax model 3590F

microcentrifuge (Thermo IEC, Needham Heights, MA) was used for centrifugation at

14,000 rpm (15,682 x g) to separate out the precipitate from the sol solution. A

home-built gas pressure-operated filling/purging device [78] was used to perform a

number of operations: (a) filling the extraction capillary with the sol solution, (b)

expelling the solution from the capillary after predetermined period of in-capillary

residence, (c) rinsing the capillary with a suitable solvent to clean the coated surface, and

(d) purging the microextraction capillary with helium. Ultra pure water (17.0 MΩ) was

obtained from a Barnstead Model 04741 Nanopure deionized water system

(Barnstead/Thermodyne, Dubuque, IA). Online data collection and processing were

accomplished using ChromPerfect (version 3.0) computer software (Justice Laboratory

Software, Denville, NJ).

92

3.6.2 Chemicals and materials

Fused-silica capillary (250 μm, i.d.) with a protective polyimide external coating

was purchased from Polymicro Technologies Inc. (Phoenix, AZ). HPLC-grade solvents

(methylene chloride, methanol, and tetrahydrofuran (THF)) were purchased from Fisher

Scientific (Pittsburgh, PA). Fluorene, phenanthrene, naphthalene, pyrene, chrysene,

butyrophenone, heptanophenone, hexanophenone, decanophenone, trans-chalcone,

nonanal, decanal, undecanal, dodecanal, tridecanal and trifluoroacetic acid (TFA, 99%)

were purchased from Acros Organics (Pittsburgh, PA). Hydroxy-terminated

polydimethylsiloxane (PDMS) and hydroxy-terminated polydimethyldiphenylsiloxane

(PDMDPS) were purchased from United Chemical Technologies Inc. (Bristol, PA).

Tetramethoxygermane (TMOG) and 3-aminopropyltrimethoxysilane (APTMS) were

obtained from Gelest, Inc (Morrisville, PA).

3.6.3 Preparation of sol-gel germania-PDMS coated capillaries for CME

The coating sol solution was prepared in a clean polypropylene centrifuge tube

using 100 mg of hydroxy-terminated PDMS, 20 μL of TMOG, and 30μL of TFA (with

5% of water) in 100 μL of methylene chloride. The mixture was then vortexed for 3 min

followed by centrifugation for 5 min to remove possible precipitates. The clean

supernatant was transferred to another clean vial, discarding the precipitate. A

hydrothermally pretreated [79] fused silica capillary (1 m) was filled with the clear sol

solution using pressurized helium (50 psi) in a filling/purging device. The sol solution

was allowed to stay inside the capillary for 15 min to facilitate the formation of a sol-gel

coating. During this in-capillary residence period, a 3-dimensional sol-gel network was

93

formed in the sol solution due to hydrolytic polycondensation reactions. In this process,

patches of the sol-gel network evolving in the vicinity of the capillary inner surface had

the favorable conditions to get chemically bonded to the capillary walls due to

condensation reactions between the sol-gel active groups on the sol-gel network

fragments and the silanol groups on the capillary surface. Following this, the unbonded

portion of the sol solution in the central part of the capillary was expelled under helium

pressure (50 psi) and the coated surface was purged with helium (50 psi) for an hour,

leaving behind a dry surface-bonded sol-gel coating on the capillary inner walls. The

capillary was further conditioned under helium purge by programming the temperature

from 40 ºC to 300 ºC at 1 ºC/min and held at the final temperature for 5 hours. The

sol-gel coated microextraction capillary was then rinsed with a 1:1 (v/v) mixture of

methylene chloride and methanol (2 mL), and then the capillary was conditioned again

from 40 ºC - 300 ºC at 5 ºC/min. The conditioned capillary was then cut into 10-cm long

pieces that were ready for use in capillary microextraction. Sol-gel germania-PDMDPS

and sol-gel germania-APTMS coated capillaries were prepared in an analogous way by

replacing hydroxyl-terminated PDMS in the sol solution either by hydroxyl-terminated

PDMDPS or APTMS.

A sol-gel germania-PDMS coated GC column was prepared following a similar

procedure carried out within a 5-m piece of hydrothermally treated fused silica capillary

using a more dilute coating sol solution containing PDMS (200 mg), TEOG (60 μL), TFA

(50 μL containing 5% H2O), and CH2Cl2 (600 μL).

94

3.6.4 Preparation of silica-based sol-gel PDMS columns for GC analysis

The extracted analytes were separated by GC using a silica-based sol-gel capillary

column that was also prepared in-house following a modified version of the original

procedure described by us elsewhere [63]. Instead of using 100 μL each of methylene

chloride and TFA (with 5% of water) [63], in the present work we used 300 μL methylene

chloride and 200 μL TFA (with 5% of water) to prepare the sol solutions used in coating

the GC columns.

3.6.5 Preparation of aqueous samples for CME-GC

Sample solutions were prepared in several stages. Solute chemical standards were

first dissolved in methanol or THF to obtain the stock solutions (10 mg/mL), which was

further diluted to 0.1 mg/mL in the same solvent. The final aqueous solution was

prepared by additionally diluting this solution in water to achieve ng/mL level

concentrations. The aqueous solutions were freshly prepared prior to performing the

microextraction experiments. The used glassware was properly deactivated to avoid

analyte loss due to adsorption on the glassware surface [57].

3.6.6 Gravity-fed sample dispenser for capillary microextraction

An in-lab designed gravity-fed sample dispenser [57] was used for capillary

microextraction. It offered a consistent means to extract analytes by passing the liquid

sample through the microextraction capillary under gravity, as well as a convenient way

to clean the capillary via rising and purging. It was built by modifying a Chromaflex AQ

column (Kontes Glass Co., Vineland, NJ) consisting of a thick-walled Pyrex glass

cylinder concentrically placed in an acrylic jacket. The inner surface of the glass cylinder

95

was deactivated by treating with HMDS solution as described previously [57].

3.6.7 Sol-gel capillary microextraction.

A 10 cm long segment of previously conditioned hybrid sol-gel germania coated

fused silica capillary (250 μm i.d.) was vertically connected to the bottom end of an

empty sample dispenser. A 50 mL volume of the aqueous sample solution was then

poured into the dispenser from the top. The sample solution was allowed to pass through

the microextraction capillary under gravity. During this process, the analyte molecules

were extracted by the sol-gel germania hybrid coating residing on the inner walls of the

capillary. With the progression of this microextraction process, concentration equilibrium

was established between the sol-gel sorbent coating and the sample solution (typically

within 30-40 min). The capillary was then removed from the sample dispenser, briefly

purged with helium, and connected to the inlet end of the GC column using a two-way

press-fit connector. The free end of the microextraction capillary was introduced into the

GC injection port (from the bottom end of the port previously cooled down to 30 ºC) so

that ~8 cm of the capillary resided inside the quartz liner in the injection port. A graphite

ferrule was used to secure a leak-free connection between the capillary and the lower end

of injection port. The extracted analytes were then thermally desorbed from the

capillary in the splitless injection mode by rapidly raising the temperature of the injector

(from 30ºC to 300ºC in five min) and the desorbed analytes were transported to the GC

column by the helium flow through the injector. The desorbed analytes were focused at

the inlet of the GC column held at 30 ºC. Analyte desorption and focusing was performed

over a 5 min period. The GC column temperature was then raised @ 20 ºC/min to

96

facilitate transportation of the focused solutes through the column providing their GC

separation. A flame ionization detector (FID), maintained at 350 ºC, was used for analyte

detection.

3.6.8 Safety precautions

The presented work involved the use of various chemicals (organic and inorganic)

that may be environmentally hazardous with adverse health effects. In accordance with

material safety data sheet (MSDS) proper safety measures were taken in handling strong

acids, bases and organic solvents such as methanol, methylene chloride, and acetonitrile.

All used chemicals were disposed in the proper waste container to ensure personnel and

environmental safety.

3.7 Results and discussion

3.7.1 Chemical reactions involved in the preparation of sol-gel germania coatings

Sol-gel chemistry provides a powerful tool for the design and synthesis of novel

materials in desirable formats including thin films, monolithic beds, spherical particles,

etc. It provides the ease and flexibility in synthesis thanks to mild reaction conditions.

In general, a sol-gel process involves chemical reactions that convert simple sol-gel

active precursor molecules into an intermediate colloid state called the sol and then into a

three dimensional polymeric network structure (with solvent-filled pores) called the gel.

The sol-gel active components in the sol solution include one or more sol-gel precursors

(typically alkoxides), various hydrolyzed forms of the precursors and certain hydroxy- or

alkoxy-terminated polymers. Two key chemical reactions take place in the sol-gel process:

97

(a) hydrolysis of the sol-gel precursor, and (b) polycondensation of the hydrolyzed

precursors among themselves and/or with other sol-gel active components of the sol

solution. Besides silica-based precursors, metal alkoxide–based sol-gel precursors are

often used together with other sol-gel active components in the coating solution to create

a wide variety of material systems with desired properties. Zirconia-, titania- and

alumina- based sol-gel materials have already been prepared, and their various appealing

properties (e.g., excellent pH stability) attracted considerable interest [72]. Since GeO2 is

well established as an isostructural analog of SiO2 [80] and is compatible with the silica

network, it presents great potential in materials synthesis. In this work, our aim was to

use germanium alkoxide precursors in sol-gel reactions to create germania-based

surface-bonded organic-inorganic hybrid coatings within fused silica capillaries and

evaluate their performances as CME sorbents and GC stationary phases. Table 3.1 lists

the chemical ingredients used in the sol solution to in situ create germania-based sol-gel

hybrid organic-inorganic coatings within fused silica capillaries for use either in

solvent-free microextraction or in GC separations.

98

Table 3.1 Chemical ingredients of the coating solution used in preparing sol-gel germania

organic-inorganic hybrid coatings.

Chemical Name Function Structure

Tetramethoxygermane (TMOG)

Hydroxy-terminated

polydimethylsiloxane (PDMS)

Hydroxy-terminated

polydimethyldiphenylsiloxane

(PDMDPS)

3-Aminopropyltrimethoxysilane

(APTMS)

Methylene chloride

Trifluoroacetic acid (99%)

Water

Sol-gel

precursor

Sol-gel active

polymer

Sol-gel active

polymer

Sol-gel active

polymer

Solvent

Catalyst

Hydrolysis

reagent

Ge

OCH3

OCH3

OCH3

CH3O

Si

CH3

H

CH3

HO

n

O

Si

CH3

CH3

HO O Si

CH3

CH3

O Si O

x

Si

OCH3

OCH3

H2NCH2CH2CH2 OCH

CH2Cl2

CF3COOH

H2O

99

Here, tetramethoxygermane served as the sol-gel precursor and generated the

inorganic component of the resulting hybrid sol-gel organic-inorganic coating. The

hydroxy-terminated PDMS or PDMDPS served as the source of the organic component

of the hybrid coating, while APTMS played a dual role: it served as a sol-gel precursor

and also as the source of the organic (aminopropyl) ligand. An appropriate choice from

these sol-gel-active organic ligands was made for use in the sol solution to create a

sol-gel germania coating with the desired organic ligand. For example,

hydroxy-terminated PDMS was incorporated in the sol solution to create a sol-gel

germania PDMS coating. A general symbol ―Y‖ has been used in the reaction schemes

(3.2-3.3) to denote the residuals of these ligands incorporated into the sol-gel structure.

The following illustrates the creation of sol-gel hybrid germania coatings using

sol-gel GeO2-PDMS as an example, and involves the following chemical processes: (1)

hydrolysis of the alkoxide precursor (TMOG) (Scheme 3.2), (2) polycondensation of the

partially and/or fully hydrolyzed precursor molecules among themselves and with other

sol-gel active components of the solution (Scheme 3.3), (3) chemical bonding of PDMS

to the evolving sol-gel germania network, and (4) chemical anchoring of the evolving

hybrid organic-inorganic polymer to the inner walls of the capillary (Scheme 3.4).

Ge

OCH3

OCH3

OCH3

CH3O + 4H2O GeCH3O4-n

OHn

+ nCH3OH

TMOG

where : n=0,1,2,3 or 4

Scheme 3.2 Hydrolysis of germania precursor (TMOG)

100

+ +GeCH3O4-n

OHn

GeCH3O4-m

OHm Y

Where : Y =condensation residues from PDMS, PDMDPS or APTMS

Ge O

O

HO Ge

O

O

O q

O Ge

O

O

OGe

O

O

Y

OGe

O

Y

O

O Ge

O

OGe

O

O

YO

O

Y

Y

Scheme 3.3 Polycondensation of the hydrolyzed precursor and chemical bonding of the

sol-gel-active organic ligand (Y) to the evolving sol-gel network

101


organic-inorganic polymer to the inner walls of a fused silica capillary

102

The hydrolysis (partial or complete) of TMOG produces a variety of reactive

hydroxyl-containing species capable of undergoing condensation reactions with

neighboring sol-gel active chemical species in the sol solution including TMOG, APTMS

and its hydrolyzed forms, hydroxy-terminated polymers (PDMS and PDMDPS).

Condensation of these reactive species leads to a hybrid organic-inorganic material via

chemical integration of an inorganic component (germania, originating from TMOG) and

an organic residual (―Y‖, originating from one of the above mentioned sources of organic

ligands). The silanol groups residing on the inner walls of the fused silica capillary are

also sol-gel active, and can take part in the condensation reactions resulting in chemical

anchoring of the portion of the sol-gel material evolving in the vicinity of the capillary

walls. This surface bonded portion of the sol-gel material is left on the column surface in

the form of an organic-inorganic hybrid coating and serves as the sorbent for solvent-free

microextraction.

Sol-gel process is complex and proper control of the processing parameters (e.g.,

drying temperature, pressure, pH, precursor/water molar ratio, solvent, and the structure

of alkoxy groups, among others) is necessary during various stages of the process

(polymerization, gelation, drying) to prepare sol-gel materials with desired structural

characteristics [81, 82]. In this work, experimental conditions were carefully optimized to

obtain the germania-based hybrid gel within 2 hours. This was accomplished through

variation of relative proportions of various components of the solution. After coating and

thermal conditioning, the capillary was rinsed with a mixture of methylene chloride and

methanol to get rid of unreacted chemical species physically adhering to the coating.

103

3.7.2 IR Characterization

Figure 3.1 represents two FTIR spectra for: (A) pure PDMS, (B) sol-gel

Germania-PDMS Coating. According to literature data [83], Ge-O-Si vibration provides a

characteristic IR stretch at around 1000 cm-1

[84], which is determined by the vibrational

modes associated with O in Ge-O-Si. Based on FTIR measurements in germania-doped

SiO2 film sample, Mei et al. [83] assigned the peak at 987 cm-1

to the Ge-O-Si vibration.

However, they also noted that a shift in the position of this characteristic peak is possible

due to a difference in the proportions of Ge, Si, and O in the sample [85]. The FTIR

spectrum for our sol-gel germania-PDMS (Figure 3.1-B) coating contains a peak at

960.43 cm-1

. This peak is absent in the spectrum for pure PDMS (Figure 3.1-A) and, and

we attribute this to successful creation of sol-gel germania-PDMS hybrid material

containing Ge-O-Si bonds.

3.8 CME profile for sol-gel germania coated capillaries

In capillary microextraction, it is desirable that the extraction equilibrium is

reached in the shortest possible time. In this work, we investigated the relationship

between extraction time and the amount of analytes extracted in CME with sol-gel

germania-PDMS coating (Figure 3.2).

As can be seen in Figure 3.2, for both heptanophenone (a moderately polar solute)

and pyrene (a nonpolar analytes), the extraction equilibrium was reached within 40 min.

Based on this observation, 40 min extraction time was used in CME experiments

presented in this paper.

104

Figure 3.1-A FTIR spectra representing: pure PDMS

Figure 3.1-B FTIR spectra representing: sol-gel germania PDMS hybrid material used as

a sorbent in capillary microextraction

105

0

20000

40000

60000

80000

100000

120000

10 30 50 70

Extraction time (min.)

Peak a

rea (

arb

itra

ry u

nit

s)

pyrene

hepatanophenone

Figure 3.2 Capillary microextraction profiles of pyrene and heptanophenone on a sol-gel

germania coated capillary. Extraction conditions: 10 cm x 0.25 mm i.d. microextraction

capillary, gravity-fed sample dispenser for extraction at room temperature. Other

conditions: 5 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary column; splitless

analyte desorption by rapidly increasing the GC injector temperature from 30 ºC to 300

ºC, 5 min; GC oven temperature programmed from 30 ºC to 300 ºC at a rate of 20 ºC/min;

helium carrier gas; FID 350 ºC

106

3.9 Extraction characteristics of sol-gel germania coatings in CME

By atomic structure, Ge and Si are closest neighbors located in Group 14 of the

periodical table, and they have similar properties. GeO2 is well known as the isostructural

analogue of SiO2 which makes it compatible with the silica network. In this work, we

prepared Germania-based sol-gel PDMS coatings and tested their performance in

capillary microextraction coupled to GC-FID. Our results suggest that sol-gel Germania

hybrid coatings have the ability to extract a wide range of analytes from aqueous media.

Such sol-gel coatings also provide outstanding solvent and pH stability which is difficult

to achieve on pure silica based extraction material.

3.9.1 Extraction of non-polar and moderately polar compounds by sol-gel

germania-PDMS coated capillary for CME-GC analysis

The consistency in the performance of sol-gel germania-PDMS capillaries was

characterized by the run-to-run and capillary-to-capillary RSD values for the extracted

amounts in CME-GC analysis. For this, aqueous samples of three different classes of

analytes (PAHs, aldehydes, and ketones) were used. GC peak areas obtained from the

extracted analytes were used as a measure of the extracted amounts in CME. As we can

see from Table 3.2, the peak area run-to-run R.S.D values for PAH samples were below

6%, and those for GC retention time were less than 0.33 %. For aldehydes, the run-to-run

peak area and GC retention time R.S.D values were under 6.5 % and 0.2 % respectively.

For ketones, the run-to-run peak area R.S.D value was under 7.2 % and that for retention

time was within 0.23. The detection limit value for different classes of analytes is shown

in Table 3.3.

107

Table 3.2 GC peak area and retention time repeatability data for PAHs, aldehydes, and

ketones extracted from aqueous samples by CME using a sol-gel germania-PDMS coated

microextraction capillary

Analyte

Peak area repeatability

(n=3)

tR Repeatability

(n=3)

Class Name

Mean peak

area

(aribitary

unit)

R.S.D. (%) Mean tR

(min)

R.S.D.

(%)

PAHs

Aldehyde

Ketones

Naphthalene

Fluorene

Phenanthrene

Pyrene

Nonanal

Decanal

Undecanal

Valerophenone

Heptanophenone

Hexanophenone

Decanophenone

5.2

19.9

28.2

15.7

1.5

3.3

2.5

3.2

11.0

11.8

10.1

2.84

4.82

5.05

2.33

4.91

6.46

1.32

2.69

3.97

7.18

2.38

7.07

9.75

10.94

12.87

6.31

7.08

7.80

8.40

9.02

9.12

9.79

0.12

0.33

0.17

0.26

0.18

0.01

0.21

0.16

0.23

0.13

0.17

Extraction conditions: 10 cm x 0.25 mm i.d. microextraction capillary, 40 min extraction

time; Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS open tubular GC column;

splitless desorption of the extracted analytes in the GC injector, 30 ºC -300 ºC, 5 min; GC

oven temperature rose from 30 to 300 ºC at rate of 20 ºC/min; helium carrier gas; FID

350 ºC.

108

Table 3.3 Detection limit data for PAHs, aldehydes, and ketones extracted from aqueous

samples by CME-GC using a sol-gel germania-PDMS coated microextraction capillary

Analyte Class Name Detection Limit,

(ng/L), (S/N =3)

PAHs

Aldehyde

Ketones

Naphthalene

Fluorene

Phenanthrene

Pyrene

Nonanal

Decanal

Undecanal

Valerophenone

Heptanophenone

Hexanophenone

Decanophenone

83.8

20.0

11.4

20.1

139.7

89.7

97.7

92.3

46.1

35.8

30.6





350 ºC

109

Capillary-to-capillary reproducibility is another important characteristic which

shows the consistency of the method used to prepare the microextraction capillaries. In

this work, we used one ketone, one aldehyde and a polycyclic aromatic hydrocarbon as

test solutes for capillary-to-capillary reproducibility in CME performed on sol-gel

germania-PDMS coated microextraction capillaries (Table 3.4). For all the tested analytes,

the peak area R.S.D. values for the extracted amounts were below 6%, which means

germania-based sol-gel CME in conjunction with GC-FID shows good reproducibility,

and that the capillary preparation method possesses acceptable consistency.


CME-GC experiments conducted on sol-gel germania-PDMS coated capillaries using a

PAH (phenanthrene), an aldehyde (undecanal), and a ketone (hexanophenone) as test

solutes

Analytes Mean peak area (arbitrary

unit)

R.S.D. (%)

Phenanthrene

Undecanal

Hexanophenone

27.9

3.5

13.2

5.4

5.5

5.9





350 ºC

110

PAHs present potential health hazards because of their toxic, mutagenic, and

carcinogenic properties. A chromatogram of PAHs obtained in a CME-GC-FID

experiment using a sol-gel germania-PDMS coated CME capillary is presented in Figure

3.3. Parts per trillion (ppt) level detection limits were achieved. Mention should be made

here that, sol-gel coating technology can easily produce stable coating with different

thicknesses through manipulation of the in-capillary residence time of the sol solution

during preparation of the microextraction capillary. The use of microextraction capillaries

with thicker coatings should provide higher sensitivity in CME [57].

Aldehydes and ketones also present health and environmental concerns. Polar

nature of these carbonyl compounds often requires their derivatization prior to their

extraction or chromatographic analysis [86]. In CME-GC experiments, in which we used

sol-gel technology to prepare both the CME capillaries and GC columns, no

derivatization step was necessary either for extraction or for GC analysis [57, 87]. Sol-gel

germania-PDMS capillary provided highly efficient extraction for aldehydes and ketones.

As can be seen in Figure 3.4-A and 3.4-B, the GC peaks provided by the sol-gel column

are sharp and symmetrical, and this is indicative of effective focusing of the analytes at

the GC column inlet after their desorption form the CME capillary, as well as excellent

performance of the in-lab prepared sol-gel PDMS column used for GC separation. For

both solute types, parts per trillion level detection limits were achieved.

111

Figure 3.3 CME-GC trace analysis of a mixture of PAHs using a sol-gel germania-PDMS

coated microextraction capillary. Extraction conditions: 10 cm x 0.25 mm i.d.

microextraction capillary, extraction time, 40 min (Gravity fed at room temperature).

Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary column;

splitless analyte desorption by rapidly increasing the GC injector temperature from 30 ºC

to 300 ºC, 5 min; oven temperature programmed from 30 ºC to 300 ºC at a rate of 20

ºC/min; helium carrier gas; FID 350 ºC Peaks: (1) Naphthalene (2) Chrysene (3) Fluorene

(4) Phenanthrene (5) Pyrene

112

Figure 3.4 -A Figure 3.4 -B

Figure 3.4 CME-GC trace analysis of mixture of aldehydes and ketones using a sol-gel

germania-PDMS coated microextraction capillary. Extraction Conditions: 10 cm x 0.25

mm i.d. microextraction capillary, extraction time, 40 min (gravity fed sample delivery at

room temperature). Other conditions: 5 m x 0.25 mm i.d. Sol-gel PDMS column; splitless

analyte desorption by rapidly increasing the GC injector temperature from 30 ºC to 300

ºC, 5 min; GC oven temperature programmed from 30 ºC to 300 ºC at a rate of 20 ºC/min;

helium carrier gas; FID 350 ºC; GC Peaks in 3.6 -A: (1) Nonanal (2) Decanal (3)

Undecanal (4) Dodecanal; GC Peaks in 3.6 -B: (1)Valerophenone (2) Heptanophenone (3)

Decanophenone (4) Anthraquinone

113

Being highly polar compounds, alcohols demonstrate higher affinity for water and

are usually difficult to extract from an aqueous matrix. In the present work, these highly

polar analytes were extracted from aqueous sample using sol-gel germania-APTMS

coated capillary (Figure 3.5) that did not require any analyte derivatization, pH

adjustment, or salting out effect. Our results demonstrate that sol-gel germania-APTMS

microextraction capillary has a pronounced affinity for these highly polar analytes.

3.9.2 pH stability of sol-gel germania-PDMS coated capillary

One serious shortcoming of conventional silica-based stationary phases and

sorbents is their poor pH stability under both acidic and basic conditions. In this work,

the pH stability of the prepared sol-gel germania coated capillaries was evaluated by

comparing the GC peak areas obtained in CME-GC-FID experiments carried out on

sol-gel germania-PDMS coated microextraction capillaries before and after prolonged

treatment of the coated capillary surface under extreme pH conditions.

Sol-gel germania-PDMS coating showed excellent pH stability. After coating and

conditioning, we conducted two sets of experiments in which we rinsed the sol-gel

germania-PDMS capillaries for 24 hours with (a) 0.05M HCl and (b) 0.1M NaOH

(pH=13). The results suggest that sol-gel germania-PDMS coating retain its excellent

extraction capability after being subjected to prolonged rinsing with a strong base (Figure

3.6) or a strong acid (Figure 3.7). For example, after continuously rinsing a sol-gel

germania-PDMS coated capillary for 24-hours with 0.1M NaOH, the GC peak areas for

extracted amounts of phenanthrene and nonanal were found to increase by 5.0 % and

7.1 %, respectively. Similarly, after a continuous 24-hours rinse of the microextraction

114

capillary with a 0.05 M HCl, the observed peak area changes for fluorene and

heptanophenone were +4.1 % and +3.3 %, respectively.

This data suggests that the new hybrid sol-gel germania coated capillary has

excellent pH stability. Rinsing with NaOH also led to improved extraction characteristics.

For most solutes, after rinsing the capillary with NaOH, the peak area increased. In some

instances, these increases were statistically significant and in other cases they were well

within experimental errors. A significant increase in the peak area for some solutes can be

explained by assuming that NaOH helps cleaning the sol-gel germania-PDMS coating by

removing from its pores and surfaces various sorbed species that hinder the extraction

process, and that getting rid of those sorbed species is often difficult via rinsing with

commonly used organic solvent.

3.9.3 Extraction of polar compounds by Sol-gel germania CME-GC

Chlorophenols are one of the major classes of environmental pollutants.

Analytical capability to preconcentrate traces of these compounds in aqueous samples,

therefore, presents significant interest in environmental science and technology. Figure

3.8 shows an example highlighting the ability of a sol-gel germania-PDMDPS coated

CME capillary to accomplish preconcentration of 2, 4, 6-triclorophenol—a typical

member of the chlorophenol family.

115

Figure 3.5 CME-GC trace analysis of alcohols using a sol-gel germania-APTMS coated

microextraction capillary. Extraction Conditions: 10 cm x 0.25 mm i.d. microextraction

capillary, extraction time, 40 min (Gravity fed at room temperature). Other conditions: 5

m x 0.25 mm i.d. Sol-gel PDMS GC capillary column; splitless analyte desorption by

rapidly increasing the GC injector temperature from 30 ºC to 300 ºC, 5 min; GC oven

temperature programmed from 30 ºC to 300 ºC at rate of 20 ºC/min; helium carrier gas;

FID 350 ºC. GC peaks: (1) decanol (2) undecanol

116

Figure 3.6-A Figure 3.6-B

Figure 3.6. Excellent stability of sol-gel germania-PDMS coating under highly basic

conditions demonstrated through CME-GC analysis of a mixture of PAHs using a sol-gel

germania-PDMS coated microextraction capillary before (3.6-A) and after (3.6-B)

continuously rinsing the capillary with a 0.1 M NaOH solution (pH = 13) for 24h.

Extraction conditions: 10 cm x 0.25 mm i.d. microextraction capillary, extraction time, 30

min (Gravity fed at room temperature). Other conditions: 5 m x 0.25 mm i.d. sol-gel

PDMS GC capillary column; splitless analyte desorption by rapidly increasing the GC

injector temperature from 30 ºC to 300 ºC, 5 min; GC oven temperature rose from 30 to

300 ºC at rate of 20 ºC/min; helium carrier gas; FID 350 ºC. GC peaks: (1) naphthalene,

(2) fluorene, (3) phenanthrene, (4) pyrene

117

Figure 3.7-A Figure 3.7-B

Figure 3.7 Excellent stability of sol-gel germania-PDMS coating under highly acidic

conditions demonstrated through CME-GC analysis of aldehydes using a

germania-PDMS coated microextraction capillary before (3.7-A) and after (3.7-B)

continuously rinsing the capillary with a 0.05 M HCl solution for 24 h. Extraction

conditions: 10 cm x 0.25 mm i.d. microextraction capillary, extraction time, 40 min

(gravity-fed at room temperature). Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS

coated GC capillary column; splitless desorption splitless analyte desorption by rapidly

increasing the GC injector temperature from 30 ºC to 300 ºC, 5 min; GC oven

temperature programmed from 30 ºC to 300 ºC at a rate of 20 ºC/min; helium carrier gas;

FID 350 ºC. GC peaks: (1) nonanal, (2) decanal (3) undecanal (4) dodecanal

118

Figure 3.8 CME-GC trace analysis of 2, 4, 6 Trichlorophenol using a sol-gel

germania-PDMDPS coated microextraction capillary. Extraction Conditions: 10 cm x

0.25 mm i.d. microextraction capillary, extraction time, 40 min (gravity fed at room

temperature). Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary

column; splitless analyte desorption from the microextraction capillary by rapidly

increasing the GC injector temperature from 30 ºC to 300 ºC, 5 min; GC oven

temperature programmed from 30 to 300 ºC at rate of 20 ºC/min; helium carrier gas; FID

350 ºC. GC peak: (1) 2, 4, 6 Trichlorophenol

119

In this example, a detection limit of 49.2 ppt was achieved for this chlorophenol

in CME-GC-FID. Extraction of free fatty acids and their analysis by GC are often met

with serious analytical difficulties which can be explained by the presence of highly polar

carboxyl group in the structure of these molecules and by the propensity of these

molecules toward adsorption. To overcome these difficulties, analytical chemists often

derivatize these molecules to methyl esters and analyze the generated esters. However,

quantitative derivatization of fatty acids especially when they are present in trace

concentrations may be problematic. Figure 3.9 presents a gas chromatogram highlighting

the possibility of extracting traces of free fatty acids from an aqueous medium using a

sol-gel germania-APTMS coated CME capillary and direct GC analysis of free fatty acids

using an in-lab prepared sol-gel PDMS open tubular GC column.

3.9.4 GC separation of sol-gel germania-PDMS coated column

Gas chromatographic performances of a sol-gel germania-PDMS coated columns

was investigated using a natural gas sample. Figure 3.10 represents a chromatogram

illustrating the possibility of analyzing a natural gas sample on a sol-gel germania-PDMS

coated open tubular GC column. The symmetrical and sharp peaks indicate the potential

of sol-gel germania-PDMS coating for use as a stationary phase in GC, especially for

low-boiling analytes.

120

Figure 3.9 CME-GC analysis of free fatty acids using a sol-gel germania-APTMS coated

microextraction capillary. Extraction Conditions: 10 cm x 0.25 mm i.d. microextraction

capillary, extraction time, 40 min (gravity fed at room temperature). Other conditions: 5

m x 0.25 mm i.d. sol-gel PDMS coated GC capillary column; splitless analyte desorption

from the microextraction capillary by rapidly increasing the GC injector temperature

from 30 ºC to 300 ºC, 5 min; GC oven temperature programmed from 30 ºC to 300 ºC at

rate of 20 ºC/min; helium carrier gas; FID 350 ºC. GC peaks: (1) decanoic acid, (2)

undecanoic acid, and (3) dodecanoic acid

121

Figure 3.10 Gas chromatogram of a natural gas sample separated on sol-gel

germania-PDMS stationary phase. Conditions: 5 m x 0.25 mm i.d. sol-gel

germania-PDMS column; carrier gas, helium; injection, split (100:1, 300 ºC); FID 350 ºC,

constant column temperature, 30 ºC. GC peaks: (1) methane and (2) ethane

122

3.10 Conclusion

Germania-based sol-gel organic-inorganic hybrid coatings were developed for

high-performance capillary microextraction. Run-to-run and capillary-to-capillary

reproducibility (peak area RSD) were below 6.5 % and 7.2 %, respectively. The

developed method can be applied to chemically bind non-polar (e.g., PDMS), moderately

polar (e.g., PDMDPS) and highly polar (e.g., APTMS) ligands to the sol-gel germania

network in the course of its evolution from the sol solution. The sol-gel germania coated

capillaries are effective in extracting broad classes of analytes ranging from non-polar

PAHs to polar compounds such as alcohols and fatty acids. Parts per trillion level

detection limits were achieved in CME-GC-FID using sol-gel germania hybrid coatings

for microextraction. Sol-gel germania-PDMS coating also showed promising

chromatographic performance as a GC stationary phase. To our knowledge, this is the

first report on the use of sol-gel germania hybrid coatings as sorbents in microextraction

or as a GC stationary phase.

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128

CHAPTER FOUR

MIXED GERMANIA-SILICA SOL-GEL

CYANOPROPYL-POLY(DIMETHYLSILOXANE) COATING FOR CAPILLARY

MICROEXTRACTION OF POLAR ORGANIC TRACE CONTAMINANTS IN

AQUEOUS MEDIA

4.1 Introduction

Solid-phase microextraction (SPME) [1] is an effective approach to sampling and

sample preparation for both laboratory and field analyses. Developed in 1989 by Belardi

and pawliszyn [2], SPME integrates sampling, sample preparation, analyte preparation,

and sample introduction step to the analytical cycle providing a simple and efficient

solvent-free method for the extraction and preconcentration of analytes from various

sample matrices. A number of shortcomings inherent in traditional fiber SPME stem from

design and physical construction of the fiber and syringe like SPME device. As one of the

variants of SPME which has been proposed to overcome these difficulties [3-7], in-tube

SPME also called capillary microextraction, CME is promising, especially for effective

hyphenation with liquid-phase separation techniques [8-10].

The sorptive coating is the most important component fiber for both conventional

SPME and in-tube SPME, and its nature determines the fiber selectivity toward different

compound classes. Polydimethylsiloxane (PDMS) phase was one of the first polymers

used in SPME [11] over the years. SPME has served as the most commonly used

non-polar coating [12-13]. Other polymeric coatings such as polyacrylate (PA) and

129

carbowax (CW) have been used to extract polar compounds for SPME. However, all

these commercially available coatings present important drawbacks such as their

relatively low thermal stability (200 ºC - 270 ºC) and low organic solvent stability. The

lack of chemical bonding between the coating material and the substrate greatly

restrained the hyphenation of SPME with HPLC.

Sol-gel coating technology has been shown to overcome these problems by

synthesizing chemically bonded microextraction sorbents with different functional groups

[14-16]. Sol-gel chemistry offers an effective methodology to chemical incorporation of

organic components into evolving inorganic polymeric structures under extraordinarily

mild thermal conditions; therefore it provides a simple and efficient pathway to the

synthesis of advanced materials for use as extraction sorbents. An important aspect

direction of this coating technology is that the selectivity of a sol–gel coating can be

easily fine tuned by changing the composition of the used sol solution. Sol-gel chemistry

was applied to prepare a number of coatings including polyethylene glycol (PEG) [17],

hydroxyfullerene [18], and Crown ether [13] that provided extraction selectivity in SPME

for both polar and non-polar compounds.

Silica-based stationary phases and sorbents are dominantly used in modern

separation and sample preparation techniques. Silica surface chemistry and surface

modification process are well understood. However, the limitations of siliceous materials

are also well known. Under acidic pH conditions, coatings containing of siloxane bonds

(Si-O-Si) become unstable [19-20]. On the other hand, under a pH value of higher than 8,

siloxane bond on the silica surface hydrolyzes [21] and it happens rapidly under elevated

130

temperatures [22]. A number of methods such as multiple covalent bonds [23],

multi-dentate synthetic approach [24], and end-capping [25] have been used to solve this

problem. None of them, however, seems to work effectively. Therefore, developing

sorbents with a wide range of pH stability is a fundamentally important area of research

in chromatographic separation and sample preparation. Metal oxides such as zirconia,

titania, and alumina are alternative materials to offer better chemical and thermal

stabilities [22], and have attracted attention of separation scientists. The resultant metal

oxide-based sol-gel stationary phases have been applied in separation science due to their

superior pH stability and mechanical strength [22]. Besides, in recent years new research

accomplishments have been made in the development and application of metal

oxide-based sol-gel extraction sorbents. Titania- [26-28], zirconia- [29-30] and

alumina-based [31] sol-gel sorbents have been synthesized and applied in analytical

microextraction. Results revealed that those materials offer better thermal and pH

stability compared to silica-based systems [26, 27, 29, 31].

Our work focuses on developing organic-inorganic hybrid sol-gel coatings and

monolithic beds [32-38] for various substrates such as capillary, fiber or particle surface

to accomplish enhanced performance in analytical microextraction and separation. In past

several years, we worked on titania- and zirconia-based sol-gel systems that demonstrated

high chemical and pH stabilities in applications involving CME in hyphenation with both

GC and HPLC [26, 29]. As an isostructural analogue of SiO2 [39], GeO2 is compatible

with the silica network and they have similar properties. Tetramthyoxygermane (TMOG)

[39], tetraethyoxygermane (TEOG) [41], tetrapropyloxygermane (TPOG) [42],

131

germanium iso-propoxide (TP-i-OG) [41, 43], and ClGeCH2CH2COOH [44] etc have

been reported to introduce prepare mixed GeO2-SiO2 thin films [42, 43], monolithic

aerogel [40] and glasses [41] by sol-gel technology. From a structural point of view, each

Ge atom could be substitutional to a Si and be bonded to four O atoms, or it could be

under-coordinated giving rise to oxygen deficient centers (GODC) [45]. Those properties

attracted considerable attention on its optical properties and optoelectronic applications

[46-48]. Our preliminary research [32] has suggested that sol-gel germania hybrid

coatings not only have the ability to extract a wide range of analytes from aqueous media

but also showed outstanding solvent and pH stability which is difficult to achieve on pure

silica based extraction material. Because of high affinity of water for highly polar

compounds, direct extraction of such analytes is difficult. Solution of this problem

requires a highly polar extracting phase that can compete with water in term of polarity

and provide a favorable environment for the transfer of highly polar analytes from the

aqueous medium into the extracting phase. Cyanopropyl-based stationary phases provide

extremely high chromatographic polarity [49]. Cyanopropylsiloxanes exhibit both polar

and polarizable characteristics which are responsible for increased affinity of these phases

for alcohols, ketones, esters, and analytes bearing π-electrons. Cyano stationary phases

have been used in GC and HPLC [50-52], capillary electrochromatography (CEC) [53]

and as an extraction medium in solid-phase extraction (SPE) [54-56]. Sol–gel CN-PDMS

coating have been reported in our previous work [38]. The prepared cyano-based sol-gel

coating provided highly stable performance at elevated temperatures compared with

conventionally prepared cyano coatings (having no chemical bonds to the substrate).

132

Here we report on the development of cyanopropyl-based sol-gel germania hybrid

coatings for the extraction of highly polar analytes from aqueous media.

4.2 Experimental

4.2.1 Equipment

Sol-gel germania-based capillary microextraction-gas chromatography (CME-GC)

experiments were carried out on a Shimadzu Model 14A GC system equipped with a

flame ionization detector (FID) and a split/splitless injector. An in-lab designed liquid

sample dispenser was used to perform capillary microextraction via gravity flow of

aqueous samples through the sol-gel microextraction capillary connected at the bottom

end of the dispenser. A Fisher Model G-560 Genie 2 vortex (Fisher Scientific, Pittsburgh,

PA) was used for thorough mixing of sol solution ingredients. A Micromax model 3590F

microcentrifuge (Thermo IEC, Needham Heights, MA) was used for centrifugation at

14,000 rpm (15,682 x g) to separate out the precipitate from the sol solution. A

home-built gas pressure-operated filling/purging device was used to perform a number of

operations: (a) filling the extraction capillary with the sol solution, (b) expelling the

solution from the capillary after predetermined period of in-capillary residence, (c)

rinsing the capillary with a suitable solvent to clean the coated surface, and (d) purging

the microextraction capillary with helium. Ultra pure water (17.0 MΩ) was obtained from

a Barnstead Model 04741 Nanopure deionized water system (Barnstead/Thermodyne,

Dubuque, IA). Online data collection and processing were accomplished using

ChromPerfect (version 3.0) computer software (Justice Laboratory Software, Denville,

133

NJ).

4.2.2 Chemicals and materials

Fused-silica capillary (250 μm, i.d.) with a protective polyimide external coating was

purchased from Polymicro Technologies Inc. (Phoenix, AZ). HPLC-grade solvents

(methanol, and tetrahydrofuran (THF)), were purchased from Fisher Scientific

(Pittsburgh, PA). Valerophenone heptanophenone, hexanophenone, decanoic acid,

undecanoic acid, dodecanoic acid nonanal, decanal, undecanal, 2,4,6-trichlorophenol and

Pentachlorophenol, 1,1,1,3,3,3-hexamethyldisilazane, and formic acid (FA, 95%) were

purchased from Aldrich (Milwaukee, WI). Hydroxy-terminated polydimethylsiloxane

(PDMS) was purchased from United Chemical Technologies Inc. (Bristol, PA).

Tetraethoxygermane (TEOG), 3-cyanopropyltriethoxysilane (CPTES) were obtained

from Gelest, Inc (Morrisville, PA).

4.2.3 Preparation of sol-gel germanis-CN/PDMS-coated capillaries for CME

The coating sol solution was prepared in a clean polypropylene centrifuge tube

using 100 μL CPTES, 5 mg of hydroxy-terminated PDMS, 15 μL of TEOG, and 65 μL of

TFA (with 38 % of water) and 50 μL of methanol. The mixture was then vortexed for 5

min followed by centrifugation for 4 min to remove possible precipitates. The clean

supernatant was transferred gently to another clean vial. A hydrothermally pretreated

fused silica capillary (1 m) was filled with the clear sol solution using pressurized helium

(40 psi) in a filling/purging device. The sol solution was allowed to stay inside the

capillary for 40 min to facilitate the formation of a sol-gel coating. During this

in-capillary residence period, a 3-dimensional sol-gel network was formed in the sol

134

solution due to hydrolytic polycondensation reactions. During this process, patches of the

sol-gel network evolving in the vicinity of the capillary inner surface got chemically

bonded to the capillary walls due to condensation reactions between the sol-gel active

groups on the sol-gel network fragments and the silanol groups on the surface of fused

silica capillary. Following this, the capillary was purged with helium (50 psi) for an hour

to expel the unbonded portion of the sol solution leaving behind a dry surface-bonded

sol-gel coating on the capillary inner walls. The capillary was further conditioned under

helium purge by programming the temperature from 40 ºC to 250 ºC at 1 ºC/min and held

at the final temperature for 4 hours. The sol-gel coated microextraction capillary was then

rinsed with a 1:1 (v/v) mixture of methylene chloride and methanol (2 mL), and then the

capillary was conditioned again from 40 ºC - 250 ºC at 5 ºC/min, from 250 ºC to 300 ºC

at 1 ºC/min and held at final temperature for 1 hour. The conditioned capillary was then

cut into 10-cm long pieces that were ready for use in capillary microextraction.

4.2.4 Preparation of aqueous samples for CME-GC

Solute standards were first dissolved in methanol or THF to obtain the stock

solutions (10 mg/mL), which was further diluted to 0.1 mg/mL in the same solvent and

stored in the freezer in amber vials. The fresh aqueous sample was prepared daily by

further diluting this solution with DI water. All the glassware used was deactivated by

treating with HMDS solution to avoid analyte loss due to adsorption on the glassware

surface.

135

4.2.5 Sol-gel capillary microextraction by an in-lab gravity-fed sample dispenser

An in-lab designed gravity-fed sample dispenser was used for capillary

microextraction. It was constructed by modifying a Chromaflex AQ column (Kontes

Glass Co., Vineland, NJ) consisting of a thick-walled Pyrex glass cylinder concentrically

placed in an acrylic jacket. The inner surface of the glass cylinder was deactivated as

described previously [57]. In-lab dispenser provided a consistent means to pass the liquid

through the microextraction capillary under gravity. Therefore, it provides a convenient

means for (1) the flow-through microextraction to take place and (2) cleaning or rinsing

the inside of the capillary.

A 10 cm long segment of previously conditioned cyanopropyl-based sol-gel

germania coated fused silica capillary (250 μm i.d.) was vertically connected to the

bottom end of an empty sample dispenser. A 50 mL volume of the aqueous sample

solution was then filled into the dispenser, and the sample solution was allowed to flow

through the microextraction capillary under gravity. During this flow-through process, the

analyte molecules were extracted by the sol-gel germania hybrid coating residing on the

inner walls of the capillary. With the progression of this microextraction process, an

equilibrium was established between the sol-gel sorbent coating and the sample solution

(typically within 30-40 min). The capillary was then removed from the sample dispenser,

briefly purged with helium, and connected to the inlet end of the GC column using a

two-way press-fit connector. The free end of the microextraction capillary was introduced

into the GC injection port (from the bottom end of the port previously cooled down to 30

ºC) so that ~8 cm of the capillary resided inside the quartz liner in the injection port. A

136

graphite ferrule was used to secure a leak-free connection between the capillary and the

lower end of injection port. The extracted analytes were then thermally desorbed from the

capillary in the splitless injection mode by rapidly raising the temperature of the injector

(from 30ºC to 300ºC in five min) and the desorbed analytes were transported to the GC

column by the helium flow through the injector. The desorbed analytes were focused at

the inlet of the GC column held at 30 ºC. Analyte desorption and focusing was performed

over a 5 min period. The GC column temperature was then raised @ 20 ºC/min to

facilitate transportation of the focused solutes through the column providing their GC

separation. A flame ionization detector (FID), maintained at 350 ºC, was used for analyte

detection. The PDMS-based sol-gel GC silica-based sol-gel capillary column was also

prepared in-house following the procedure described by us elsewhere [32].

4.3 Results and discussion

4.3.1 Chemical reactions involved in the preparation of sol-gel Germania-CN/PDMS

coatings

Table 4.1 lists the chemical ingredients used in the sol solution to in situ create

germania-based sol-gel hybrid organic-inorganic coatings within fused silica capillaries

for use in solvent-free microextraction. All the chemicals presented in table 4.1 except

catalyst were dispersed and thoroughly mixed in solvent(s) first, and the catalyst was

introduced at the end to initiate the sol-gel process. Sol-gel process consists of two main

reactions: (1) hydrolysis of the precursor(s) and (2) polycondensation of hydrolyzed the

sol-gel-active species present in the sol solution. Silica-base sol gel system is the most

widely used, however, analogous non-silica material such as those based on zirconia-,

137

titania- and alumina have already been prepared [22, 26, 29, 58]. Since GeO2 is well

established as an isostructural analog of SiO2 it is compatible with the silica network [39].

Our previous work [32], germanium alkoxide precursor has been successfully used in

conjunction with hydroxyl-terminated polysiloxanes to prepare Germania-based sol-gel

extraction for CME as well as Ge stationary phase.

TEOG served as the sol-gel precursor and generated the inorganic component of

the resulting hybrid sol-gel organic-inorganic coating. The hydroxy-terminated PDMS,

CPTES served as the source of the organic component of the hybrid coating. CPTES

played a dual role: it served as a sol-gel co-precursor and also served as the source of the

organic (cyanopropyl) ligands providing the cyano functional group to the system.

Because of the high thermal stability and good film forming performance of PDMS, a

small amount hydroxy-terminated PDMS was incorporated in the sol solution to help for

the formation of stable the sol-gel coating.

138

Table 4.1 Chemical ingredients of the coating solution used in preparing sol-gel germania

organic-inorganic hybrid coatings chemically anchored to the inner surface of a fused

silica capillary

Ingredient Function Chemical structure

Tetraethoxygermane (TEOG)

Hydroxy-terminated

polydimethylsiloxane

(PDMS)

3-Cyanopropyltriethoxysilane

(CPTES)

Methanol

Formic acid (96%)

Water

Sol-gel

precursor

Sol-gel

active

polymer

Sol-gel

active

polymer

Solvent

Catalyst

Hydrolysis

reagent

Ge

OCH2CH3

OCH2CH3

OCH2CH3

CH3CH2O

Si

CH3

H

CH3

HO

n

O

Si

OC2H5

OC2H5

CNCH2CH2CH2 OC2H5

CH3OH

HCOOH

H2O

139

The creation of sol-gel hybrid germania coatings involved the following chemical

processes: (1) hydrolysis of the alkoxide-based precursors (Scheme 4.1), (2)

polycondensation of the partially and/or fully hydrolyzed precursors among themselves

and with other sol-gel active components of the solution (Scheme 4.2), (3) chemical

bonding of hydrolyzed CPTES and hydroxyl-terminated PDMS to the evolving sol-gel

germania network, and (4) chemical anchoring of the evolving hybrid organic-inorganic

polymer to the inner walls of the capillary (Scheme 4.3). The hydrolysis (partial or

complete) of TEOG produces a variety of reactive hydroxyl-containing species which are

capable of undergoing condensation reactions with neighboring sol-gel active chemical

species in the sol solution such as CPTES and hydroxyl-terminated PDMS. Condensation

of these reactive species leads to a hybrid organic-inorganic network via chemical

integration of an inorganic component (germania, originating from TEOG) and a

cyanopropyl-containing organic residual (cyanopropyl, hydrolyzed forms of CPTES).

Addition of PDMS into the system promoted the network formation and incorporation

and bridging of the chemical species arising from hydrolysis of TEOG, CPTES. The

silanol groups residing on the inner walls of the fused silica capillary are also sol-gel

active and under the catalyst they can take part in the condensation reactions with the

sol-gel network fragments in their vicinity resulting in chemical anchoring of a portion of

the sol-gel material on the surface of the capillary walls. This portion of surface bonded

sol-gel material is left on the capillary surface in the form of an organic-inorganic hybrid

coating and served as the sorbent for solvent-free microextraction. After coating and

thermal conditioning, the capillary was rinsed with a mixture of methylene chloride and

140

methanol to get rid of unreacted chemical species physically adhering to the coating.

Sol-gel hybrid germania coated capillaries were then ready for microextraction.

4.3.2 Extraction profiles for sol-gel germania-CN/PDMS capillaries

In capillary microextraction, it is desirable that the extraction equilibrium is

reached in the shortest possible time. In this work, we investigated the relationship

between extraction time and the amount of analytes extracted in CME with sol-gel

germania-CN/PDMS coating (Figure 4.1).

As can be seen in Figure 4.1, for undecanol, the extraction equilibrium was

reached within 20 min. For pentachlorophenol, a highly polar solute, the time to reach

equilibrium was around 30 min. However, for heptanophenone, which is moderately

polar compound compared with phenols and alcohols, the time to reach equilibrium was

at 40 min. These results indicated that polar solutes present stronger affinity to the

developed polar germania-CN/PDMS coating, which makes sorption time for them

shorter. Based on this observation, 40 min extraction time was used in CME experiments

presented in this paper.

Ge

OC2H5

OC2H5

OC2H5

C2H5O + 4H2O GeC2H5O4-n

OHn

+ nC2H5OH

TEOG

where : n=0,1,2,3 or 4

Scheme 4.1 Hydrolysis of germania precursor (TEOG)

141

+ + Si

CH3

O

CH3

HO

p

Ge O

O

HO Ge

O

O

O q

Si

CH3

O

CH3p

O Ge

O

O

Si

CH3

O

CH3

p

GeCH3O4-n

OHn

GeCH3O4-m

OHm

O Ge

O

Si

CH3

O

CH3

pO

+ Si

OC2H5

OC2H5

CN(CH2)3 OC2H5

Si

O

(CH2)3CN

O

Si

O

(CH2)3CN

O

Si (CH2)3CN

O

O

Scheme 4.2 Polycondensation of the hydrolyzed precursor and chemical bonding of the

sol-gel-active organic ligand (Y) to the evolving sol-gel network

142

OH

OH

OH

+

O

OH

OH

Ge O

O

HO Ge

O

O

O q

Si

CH3

O

CH3p

O Ge

O

O

Si

CH3

O

CH3

p

O Ge

O

Si

CH3

O

CH3

pO

Si

O

(CH2)3CN

O

Si

O

(CH2)3CN

O

Si (CH2)3CN

O

O

Ge O

O

Ge

O

O

O q

Si

CH3

O

CH3p

O Ge

O

O

Si

CH3

O

CH3

p

O Ge

O

Si

CH3

O

CH3

pO

Si

O

(CH2)3CN

O

Si

O

(CH2)3CN

O

Si (CH2)3CN

O

O


organic-inorganic polymer to the inner walls of a fused silica capillary

143

Figure 4.1 Capillary microextraction profiles of undecanol, heptanophenone,

pentachlorophenol on a sol-gel germania-CN/PDMS coated capillary. Extraction

conditions: 10 cm x 0.25 mm i.d. microextraction capillary, gravity-fed sample dispenser

for extraction at room temperature. Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS

coated GC capillary column; splitless analyte desorption by rapidly increasing the GC

injector temperature from 30 ºC to 300 ºC, 5 min; GC oven temperature programmed

from 30 ºC to 300 ºC at a rate of 20 ºC/min; helium carrier gas; FID 350 ºC.

144

4.3.3 Extraction characteristics of sol-gel germania-CN/PDMS in CME

Aqueous samples of different classes of analytes (ketones, alcohols, acids, and

phenols) were used. GC peak areas obtained from the extracted analytes were used as a

measure of the extracted amounts in CME. As can be seen from Table 4.2, the peak

area run-to-run R.S.D values for phenol samples were under 3.18 %, and those for GC

retention times were less than 0.2 %. For ketones, the RSD values for run-to-run peak

area and GC retention time were under 6.89 % and 0.07 %, respectively. For acids and

alcohols, the run-to-run peak area R.S.D value were under 4.51 %, 3.52 % and that for

retention times were within 0.07, and 0.54 %, respectively.

Using the developed sol-gel germania hybrid coatings for capillary

microextraction, ng/L level detection limits were achieved in CME-GC-FID for most

analytes. The detection limit value for different classes of analytes is shown in Table 4.3.

Capillary-to-capillary reproducibility is another important criterion to characterize

the consistency in capillary preparation. The results are presented in Table 4.4. For each

analyte, extractions were made in triplicates on each capillary and the mean of the

measured peak areas were used in Table 4.4 to evaluate capillary-to-capillary

reproducibility. The presented data showed that the capillary-to-capillary reproducibility

is characterized by an RSD value of less than 5.5 % for all four classes of compounds

used for evaluation. The low RSD value is indicative of excellent reproducibility.

145

Table 4.2 GC peak area and retention time repeatability data (n=4) for chlorophenols,

alcohols, free fatty acids and ketones extracted from aqueous samples by CME using

sol-gel germania-CN/PDMS coated microextraction capillary

Analyte

Peak area repeatability

(n=4)

tR Repeatability

(n=4)

Class Name

Mean peak

area

(aribitary

unit)

R.S.D. (%) Mean tR

(min)

R.S.D.

(%)

Ketoens

Phenols

Acids

Alcohols

Valerophenone

Hexanophenone

Heptanophenone

2,3-dichlorophenol

2,4,6-trichlorophenol

Pentachlorophenol

Decanoic acid

Undecanoic acid

Nonanol

Decanol

Undecanol

3.5

9.6

24.5

7.9

10.1

20.1

3.0

8.0

4.4

15.3

29.7

6.89

3.75

3.50

3.18

1.84

2.21

4.26

4.51

3.52

1.81

1.61

8.40

9.10

9.77

7.52

8.76

8.89

8.65

9.29

6.70

7.49

8.23

0.07

0.06

0.06

0.20

0.06

0.19

0.07

0.05

0.54

0.28

0.19





350 ºC

146

Table 4.3 GC-FID detection limit data (n=4) for chlorophenols, alcohols, free fatty acids

and ketones extracted from aqueous samples by CME using sol-gel germania-CN/PDMS

coated microextraction capillary

Analyte Class Name Detection Limit (ng/L)

Ketoens

Phenols

Acids

Alcohols

Valerophenone

Hexanophenone

Heptanophenone

2,3-dichlorophenol

2,4,6-trichlorophenol

Pentachlorophenol

Decanoic acid

Undecanoic acid

Nonanol

Decanol

Undecanol

185.7

30.6

25.4

226.6

187.5

105.0

227.9

250.9

387.9

115.8

20.0

Extraction conditions: 10 cm x 0.25 mm i.d. microextraction capillary, 40 min

extraction time; Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS open tubular GC

column; splitless desorption of the extracted analytes in the GC injector, 30 ºC -300 ºC, 5

min; GC oven temperature rose from 30 to 300 ºC at rate of 20 ºC/min; helium carrier gas;

FID 350 ºC

147


CME-GC experiments conducted on sol-gel germania-CN/PDMS coated capillaries using

a chlorophenol (pentachlorophenol), an alcohol (undecanol), a free fatty acid (undecanoic

acid) and a ketone (heptanophenone) as test solutes

Analytes Mean peak area (arbitrary

unit)

R.S.D. (%)

Hepanophenone

Undecanol

Undecanoic acid

Pentachlorophenol

23.9

29.1

5.8

11.9

1.4

2.4

1.5

5.8





350 ºC

148

Being polar compounds, alcohols demonstrate higher affinity for water and are

usually difficult to extract from an aqueous matrix. In the present work, this class of

analytes were extracted from aqueous sample using sol-gel germania-CN/PDMS coated

capillary (Figure 4.2) that did not require any analyte derivatization, pH adjustment, or

salting out effect. From the Figure 4.2, germania-CN/PDMS showed excellent extraction

ability to alcohols. The introduction of PDMS helped incorporate CPTES into the

germania-based sol-gel material because of the excellent network forming properties of

PDMS.

On the other hand, CPTES provided cyano polar functional groups into the system,

which possess stronger affinity to polar compounds. Compared with our previous

germania-PDMS [32], the introduction of small polar precursor such as CPTES provides

sol-gel Germania coatings capability of extracting polar analytes from aqueous media.

Extraction of free fatty acids and their analysis by GC is always difficult due to

the presence of highly polar carboxyl group in the structure of these molecules and the

propensity of these molecules toward adsorption. Therefore these molecules are often

derivatized to methyl esters and the generated esters are subsequently analyzed, which is

time-consuming. Besides, quantitative derivatization of fatty acids in trace concentrations

may be problematic. All these drawbacks of traditional approaches for the analysis of

fatty acids can be avoided by using cyanopropyl-based sol-gel Germania coatings. In

this work, sol-gel germania-CPTES coatings were used to extract free fatty acids from an

aqueous medium (Figure 4.3).

149

Figure 4.2 CME-GC analysis of a mixture of alcohols using a sol-gel

germania-CN/PDMS coated microextraction capillary. Extraction conditions: 10 cm x

0.25 mm i.d. microextraction capillary, extraction time, 40 min (Gravity fed at room


column; splitless analyte desorption by rapidly increasing the GC injector temperature

from 30 ºC to 300 ºC, 5 min; oven temperature programmed from 30 ºC to 300 ºC at a

rate of 20 ºC/min; helium carrier gas; FID 350 ºC Peaks: (1) Nonanol (2) Decanol (3)

Undecanol

150

Chlorophenols (CPs) are one of the major classes of environmental pollutants

constitutes an important group of priority toxic pollutants listed by EPA, with possible

carcinogenic properties. Chlorophenols (CPs) have been widely used as preservatives,

pesticides, antiseptics, and disinfectants [59], and they are often found in waters [60],

soils, and sediments [61]. CPs are highly polar compounds with significant affinity

toward water. In our study, four CPs were extracted from water samples using germania

based CN-PDMS coated capillaries. Low detection limits of ng/L level were achieved.

Figure 4.4 shows an example of excellent the ability of a sol-gel germania-CN/PDMS

coated CME capillary to accomplish preconcentration of some chlorophenols.

Ketones are relatively less polar compared with above classes of compounds, and

present health and environmental concerns. They are formed as by-products in the

drinking water disinfection processes. Many of these by-products have been shown to be

carcinogens or carcinogen suspects. However, it often requires their derivatization prior

to their extraction or chromatographic analysis. In sol gel germania based CME-GC

experiments (Figure 4.5), no derivatization step was necessary either for extraction or GC

analysis of ketone samples.

151

Figure 4.3 CME-GC analysis of a mixture of acids using a sol-gel germania-CN/PDMS

coated microextraction capillary. Extraction conditions: 10 cm x 0.25 mm i.d.

microextraction capillary, extraction time, 40 min (Gravity fed at room temperature).

Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary column;

splitless analyte desorption by rapidly increasing the GC injector temperature from 30 ºC

to 300 ºC, 5 min; oven temperature programmed from 30 ºC to 300 ºC at a rate of 20

ºC/min; helium carrier gas; FID 350 ºC Peaks: (1) Decanoic acid (2) Undecanoic acid (3)

Dodecanoic acid

152

Figure 4.4 CME-GC analysis of a mixture of chlorophenols using a sol-gel






rate of 20 ºC/min; helium carrier gas; FID 350 ºC Peaks: (1) 2,4,6-trichlorophenol (2)

Pentachlorophenol

153

Figure 4.5 CME-GC analysis of a mixture of ketones using a sol-gel






rate of 20 ºC/min; helium carrier gas; FID 350 ºC Peaks: (1) Valerophenone (2)

Hexanophenone (3) Heptanophenone

154

4.3.4 Thermal stability of sol-gel germania coatings in CME

Sol-gel germania-CN/PDMS coating showed excellent thermal stability. After

coating and conditioning, the conditioning temperature was stepwise increased from 250º

C to 350ºC with an interval of 20 ºC. At each temperature, the capillary was conditioned

for one hour. The experimental peak area data obtained after conditioning the capillary at

each temperature were compared. In Figure 4.6 present, the extraction performance of the

sol-gel germania-CN/PDMS coated capillary was studied for nonanol (an alcohol) in the

course of stepwise conditioning of the capillary. The results suggest that sol-gel

germania-CN/PDMS coating retain its excellent extraction capability after being

conditioned at 330ºC.

155

Figure 4.6 Effect of conditioning temperature on the performance of sol–gel

germania-CN/PDMS microextraction capillary in the extraction of nonanol. Extraction

conditions: 10 cm x 0.25 mm i.d. microextraction capillary, extraction time, 40 min

(Gravity fed at room temperature). Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS

coated GC capillary column; splitless analyte desorption by rapidly increasing the GC

injector temperature from 30 ºC to 300 ºC, 5 min; oven temperature programmed from 30

ºC to 300 ºC at a rate of 20 ºC/min; helium carrier gas; FID 350 ºC Peaks.

156

4.4 Conclusion

A cyanopropyl-based sol-gel germania organic-inorganic hybrid coating was

developed for the preconcentration of polar analytes from aqueous by capillary

microextraction. The developed coating was prepared by using two different precursors

TEOG (germania component) and CPTES (silica component) and a small amount of

PDMS. PDMS is a non polar polymer. The sol-gel geamania-PDMS capillary reported in

previous work only showed excellent affinities for nonpolar and moderately polar

compounds. However, the sol-gel germania-CN/PDMS capillary was effective in

extracting polar analytes such as alcohols, fatty acids, and phenols, because presence of

high polar cyanopropyl ligands. Good run-to-run and capillary-to-capillary

reproducibility (peak area RSD) was achieved for most polar analytes (below 7 %) and

ng/L level detection limits were achieved in CME-GC-FID for most analytes, using the

developed sol-gel germania hybrid coatings for capillary microextraction. The developed

sol-gel germania-CPTES/PDMS capillary also showed excellent thermal stability, due to

the chemical anchoring of such polar coatings onto the fused silica surface.

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APPENDICES

161

Appendix A

162

Appendix A (Continued)

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Appendix B

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Appendix B (Continued)

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ABOUT THE AUTHOR

Li Fang was born in Bengbu, Anhui Province, China. She obtained her Bachelor’s

degree in Chemistry Education from Anhui Normal University in 2000. She continued

her studies at University of Science and Technology of China (Hefei, Anhui) and received

her Master of Science degree in Analytical Chemistry in 2003. Later, she came to United

States and joined the Department of Chemistry, University of South Florida to pursue a

Doctorate Degree. The work presented here was conducted under the guidance of Dr.

Abdul Malik. She has three publications in international journals.

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