Atomic Absorption Spectroscopy With A Resistively Heated ......Atomic Absorption Spectroscopy With A...

Atomic Absorption Spectroscopy With A

Resistively Heated Carbon Furnace

by

MAHMOUD CHAMSAZ, M.Sc., D.I.C.

A thesis submitted for the degree of Doctor of

Philosophy of the University of London.

May, 1978.

Department of Chemistry, Imperial College of Science and Technology, South. Kensington, London S.W.7.

To my wife

Summary

This thesis describes the development and application

of a resistively heated graphite furnace for trace element

analysis by atomic absorption spectrometry (AAS).

The first chapter is concerned with the history of

AAS and the development of non-flame atom cells. The

second chapter describes the theory of atomic absorption.

First the theoretical aspects of absorption spectroscopy

are briefly discussed, and then the theory of atomization

processes together with the interferences encountered in

non-flame atom cells will be considered.

The construction and subsequent development of a

continuously-operated graphite furnace and the associated

nebulizer and desolvation system is described in chapter

three.

The last chapter is concerned with trace element

analysis. Three different methods of sampling are

examined. The first is continuous sample introduction

for the determination of Cd, Ca, Co, Pb, Mg and Zn. The

second method describes the determination of difficult

to atomize elements (Mo and Al) using discrete sample

introduction. The last section investigates the

advantages of a deposition technique and these are illus-

, trated by Cd, Mo, Mg and Ca.

III

ACKNOWLEDGEMENTS

I would like.to thank sincerely my supervisor,

Professor T.S. West, for the opportunity to gain research

experience whilst working under his guidance.

I would also like to thank Dr. R.A. Chalmers and

Dr. I.L. Marr for allowing me to work in their laboratory

and for their help during my period in Aberdeen.

I am particularly indebted to my immediate supervisor,

Dr. B.L. Sharp, for his encouragement, useful discussion

and great concern throughout my research programme.

The research work presented in this thesis was

carried out in the Chemistry Department of the Imperial

College of Science and Technology and the University of

Aberdeen from October, 1974 to May, 1978. It is entirely

original except where due reference is made. No part of

this work has previously been submitted for any degree.

IV

Chapter I

CONTENTS

Page Introduction

1.1. History 1

1.2. Atom Cells 2

1.2.1. Furnaces 3

1.2.1.a Low Temperature Furnaces 3

1.2.1.b Graphite Furnaces 4

1.2.2. Filaments and Open Cells 12

1.2.2.a Wire Filaments 12

1.2.2.b Graphite Filaments 15

1.2.2.c Sample - Boats 16

1.2.3. Cathodic Sputtering Cells 17

1.2.4. Plasmas 20

1.2.4.1. Non-flame-like Plasmas 20

1.2.4.1a Arcs 20

1.2.4.1b Sparks 21

1.2.4.2. Flame-like Plasmas 22

1.2.5. Lasers 23

1.3. Conclusion 24

Chapter II Theory of Atomic Absorption Spectrometry

2.1. Principles 26 2.2. The Intensity of a Spectral

Line 26 2.3. The Width of Spectral Lines 27

2.4. Variation of Atomic Absorption with Concentration

2.4a The Absorption Coefficient, K 28

2.4b The Total Absorption Factor, AT 28

V

4c The Absorbance, A 29

2.5. Theoretical Aspects of Atomization Process 29

2.5.1. = Thermodynamic Approach 29

2.5.2. Kinetic Approach 33

2.5.2a Atomization under Increasing Temperature 34

2.5.2b Atomization under Isothermal Conditions 39

2.6. Interferences with Electrothermal Atomizers 42

2.6.1. Physical Interferences 43

2.6.1a Sample Introduction Inter- ferences 45

2.6.1b Memory Effects 45

2.6.2. Spectral Interferences 45

2.6.2a Line Overlap Effects 45

2.6.2b Emission Radiation Effects 46

2.6.2c Scattering Effects 46

2.6.2d Molecular Absorption Interferences 46

2.6.3. Chemical Interferences 49

2.6.3a Anion/Cation Interferences 49

2.6.3b Pyrolysis Losses 49

2.6.3c Condensation 49

2.6.3d Carbide Formation 50

2.6.3e Nitride Formation 51

2.6.3f Ionization 52

Chapter III Instrumentation

3.1. Graphite Furnace Atomizer 52 System

3.2. Power Supply 53

3.3. Temperature Measurements 55

V I

•

3.4.

3.5.

3.6.

3.7.

Purge Gas

Stopped Gas Flow

Pyrolysis Treatment

Nebulizers

Page

55

58

59

62

3.8. Sample Introduction System 65

3.9. Spectral Light Source 72

3.10. Detection System 73

Chapter IV Determination of Elements by Graphite Furnace AAS

4.1. Continuous Sample Introduction 'Method 75

4.1.1. Optimization of Parameters 76

4.1.1.a Optimization of Monochromator Slit-width 76

4.1.1.b Optimization of HCL Current 79

4.1.1.c Optimization of Nebulizer Gas Flow Rate 79

Optimization of Temperature of the Spray Chamber 79

4.1.1.e Optimization of Purge Gas Flow Rate 82

4.1.1.f Optimization of Atomization Temperature 82

4.1.2. Sensitivity, Detection limit and Reproducibility 88

4.1.3. Interference Effects Study 89

4.1.4. Results and Discussion 89

4.1.4.1 Cadmium 89

4.1.4.1a Introduction 89

Analysis 90

4.1.4.2 Calcium 94

VII

VIII

Page


4.1..4.2b Analysis 94

4.1.4.3 Cobalt 98


4.1.4.3b Analysis 99

4.1.4.4 Lead 104



4.1.4.5 Magnesium 105

4.1.4.5a -Introduction 105

4.1.4.5b Analysis' 109

4.1.4.6 Zinc 112



4.2. Discrete Sample Introduction Method 118

4.2.1. Optimization of Parameters 118

4.2.1.a Optimization of Atomization Temperature 118

4.2.1.b Optimization of Purge Gas Flow Rate. 120


4.2.2.1 Molydenum 120


4.2.2.1b Analysis 120.

4.2.2.2 Aluminium 125



4.3. Aerosol Deposition Method 128

3.1. Optimization of Parameters 128

4.3.1.a Optimization of Deposition Time 129

4.3.1.1) Optimization of Deposition Temperature 129


Conclusion 140

References 142

IX

CHAPTER 1

INTRODUCTION

1

1.1. History

Isaac Newton is regarded as the founder of the science

of spectroscopy, following his analysis of the continuous

solar spectrum in 1666. Later in 1802, Wollaston dis-

covered several dark lines in the solar spectrum by passing

the sun light through a slit. These lines were also

observed by Fraunhofer in 1817. In 1823 Fraunhofer built

the first transmission grating and measured the exact

wavelengths of these lines. The basic principal of atomic

absorption was published by Kirchoff in 1860 (1). Using

the atomic lines discovered by Fraunhofer, he was able to

deduce the presence of certain elements in the solar

atmosphere. Working with Bunsen, the use of spectral lines

for determining alkaline metals in a flame was reported (2).

The first analytical use of atomic absorption was reported

by Woodson (3) in 1939 who determined mercury in the

atmosphere. The first paper to realize the general

practical of an atomic absorption spectrometry (AAS) as an

analytical technique was published independently by Walsh

.(4) and Alkemade and Milatz (5) in 1955. Alkemade

constructed an apparatus in which the emission of an

element introduced into a flame was employed as a source

of resonance radiation. The analyte solution was

nebulized into a second flame and the decrease in the

radiation intensity from the first flame was measured.

Walsh showed the advantages of AAS over emission

spectrometry (ES) and proposed the absorbance method of

recording the signal. He proposed the design of an

apparatus for performing analysis and later in 1957 he and

his colleagues published their first results on the

experimental developments of the technique (6). Since

then AAS has been extensively used. and has provided a

sensitive and selective technique for the analysis of

trace elements. The sensitivity and precision of AAS

is limited by the characteristics of the excitation

source and the atom production technique.

1.2. Atom Cells

This thesis is concerned with a graphite furnace

atomizer and its application to AAS. It is therefore

relevant to discuss the development of the different

types of atom cells used in atomic spectrometry.

Flames have been widely used as atomizers for the

production of atoms in both AAS and atomic flu3rescence

spectrometry (AFS). They do, however, suffer from some

disadvantages. Flame cells are not suitable for the

atomization of solid samples and the volume of sample

needed for the analysis of solutions is large. Flame

background absorption and emission at the resonance line

cause considerable noise and lower the precision of the

technique, and finally, the precise control of the

chemical environment is not possible in flame cells. It

is those limitations which have led to the development of

non-flame cells. Winefordner (7) in a study of typical

graphite cells has shown that the concentration of the

atoms can be at least 500 times higher than that in a

flame, giving rise to an increase of 10-100 times in

sensitivity for both atomic absorption and atomic emission.

9

3

In non-flame cells the chemical environment is controlled

by the use of .an inert gas which promotes the formation of

free atoms. Careful design of non-flame cells is

necessary to limit both emission and background absorption.

Many types of non-flame cells have been developed,

these may be categorised as follows: Furnaces, Filaments,

Cathodic sputtering cells, Plasmas and Lasers.

1.2.1. Furnaces

Furnaces•yield high analytical sensitivity and a

number of different types have been described.

1.2.1a Low Temperature Furnaces

The maximum temperature which can be achieved with

these devices is around 1500°C. As a result, memory effects

may be significant and because of the low working temperature,

chemical interferences are more severe than in flames.

However, the high sensitivity obtainable to some extent

offsets these disadvantages. Mislan (8) has demonstrated

the use of a 36cm silica tube of 2.5cm i.d. as an atom cell.

The tube can be heated to a maximum temperature of 1250°C

by a wire wound resistance furnace. An indirect nebulizer

spray chamber was used to transfer the sample solutions to

the absorption tube. Cd was investigated and excellent

detection limits were obtained. Choong and Long-Seng (9)

have used an 80cm long silica tube heated to 1200°C for the

study of absorption spectra of silver vapour. Hudson (10)

has employed a stainless-steel absorption cell heated by a

resistance wire for AA measurements on sodium vapour.

Tomkins and Ercoli (11) have reported the use of a tantalum

4

tube as an absorption cell. The tube could be operated

to a temperature of 1400°C and enabled them to study the

spectra of barium, calcium, thallium and radium. Black

et al (12) have described the construction of a platinum

tube furnace for use in AFS. The sample is nebulized

into the platinum tube which is heated electrically up to

a maximum temperature of 1600°C. They studied the atomic

fluorescence of Cd, Zn, Hg and Fe and showed that'this type

of furnace can be effectively used for studies on volatile

elements.

1.2.1b Graphite Furnaces

The use of high temperature graphite furnaces enables

the efficient atomization of a wide range of elements to

be achieved. King (13) in 1905 was the first to use an

electrothermal atomizer, in this case for observing the

emission spectra of elements. The arc-heated furnace

consisted of a carbon tube 16mm.o.d. and 5mm i.d.

held inside a carbon block. A hole was drilled at the

centre of the tube for admission of a carbon electrode.

An arc was formed between the furnace wall and the carbon

electrode giving rise to temperatures of 2200°C. Later

in 1908 King (14) constructed a resistively heated furnace,

The furnace was constructed around a graphite tube 15-20cm

long, 16mm o.d. and 4mm i.d. and is shown schematically in

Fig. 1.1. The tube was held at the centre of a brass

cylinder 40cm in length and 10cm in diameter. The furnace

was heated by a 200 amp. current from a 5KW transformer at

25V. During the study of the emission spectrum of

f

a. Carbon tube

b. Brass cylinder

c. Supports for the carbon tube

d. Quartz window

e. Copper leads

f. Hydrogen gas inlet

Fig. 1.1. Schematic diagram of a resistively heated carbon furnace designed by King.

5

6

titanium, working temperatures of 2500°C were recorded

for the inside of the tube (15,16). The furnace was

later modified and was used for the study of the emission

and absorption spectrum of iron (17) and determination of

oscillator strengths (18). The King furnace was improved

and employed for vacuum work by Paul (19) and by Codling

and co-workers (20,21) and has been the standard device

for the production of absorption spectra of metal vapours.

L'Vov (22-27) was the first to -propose a pulsed

method of atomizing samples in a graphite furnace. The

graphite furnace used in his early work was similar to

the arc atomizer of King. It consisted of a graphite tube

10cm long, lOmm external diameter and 3mm internal diameter.

A carbon electrode was used for sample introduction and an

auxiliary electrode for arcing. The furnace was pre-

heated by a 10KW transformer prior to introduction of the

sample electrode. A d.c. arc. was automatically formed

for 3-4 seconds between the sample electrode and the

auxiliary electrode as soon as the sample electrode was

moved into the furnace. The graphite tube was lined with

tantalum foil in order to reduce the loss of atomic

vapour by diffusion through the porous carbon. The whole

assembly was contained under an aluminium cover evacuated

and filled with argon to the desired pressure. Because of

the inefficiency of heating the furnace by the d.c. arc,

L'Vov and Lebedev (28) modified the furnace to enable

resistive heating of the electrode. The modified furnace

is schematically shown in Fig. 1.2. The absorption tube

was a graphite cylinder 3 - 5cm in length

a

a. Electrode with the sample

b. Graphite tube

c. Graphite contacts.

Fig. 1.2. Electrothermal atomizer designed by Lvov.

7

8

with 2.5-5mm internal and 6mm external diameter. The

furnace was heated by an a.c. current from a 4KW trans-

former at 10V to a maximum temperature of 3000°C. .Samples

in the form of solid or liquid were placed on an auxiliary

carbon-rod electrode 6mm in diameter heated by an a.c.

current from a 1KW transformer. The atomization system

was enclosed and puryed with an inert gas. The rod was

heated for 2-3 seconds after the tube had reached its

required temperature and the atomic absorption signal was

then recorded. The graphite tubes were coated with a layer

of Pyrolytic graphite which resulted in low gas permeability,

high heat conductivity and resistance to oxidation. Using

liquid samples of 2-5 /21 L'Vov (29) obtained atomic

absorption sensitivities for 37 elements ranging between

10-10 -14 to 10-14 g. L'Vov and Khartsyzov (30) studied the

application of the graphite furnace to the determination of

elements having their resonance lines below 190nm. Employ-

ing an argon-purged graphite furnace with lithium fluoride

lenses and windows, high frequency discharge lamp sources

and a vacuum monochromator, they were able to measure the

atomic absorption. of sulphur, iodine and phosphorus with

sensitivities of the order of lx10-1 0

g, 3x10-11

g and

3x10-12

g respectively. This furnace has also been used

for the determination of oscillator strengths and the

Lorentz widths of atomic lines (31,32).

In 1965 Massmann (33,34) simplified the L'Vov furnace

and constructed a graphite furnace for AAS. Tie furnace

consisted of a resistively heated graphite tube and was

mounted as shown in Fig.l.3. The tube was 5.5cm long,

a. Graphite tube

b. Steel holders

c. Sample inlet port

d. Mounting holder

e. Insulation

Fig. 1.3. Schematic diagram of a resistively heated graphite furnace designed by Massmann.

(side view).

9

10

6.5mm i.d and 1.5mm wall thickness. A hole, 2mm in

diameter, was drilled through the tube wall at the centre

to enable the introduction of liquid samples (5-200p1).

The graphite tube was water-cooled and operated in an

atmosphere of argon to prevent it from being oxidized.

The tube required 400 A at 101.7 to heat it to its maximum

temperature of 2500°C. Later Massmann (35) aevised a

cell for atomic fluorescence measurements. A cap-

shaped graphite cuvette, 4cm long, 5.5mm i.d. and 1.5mm

thickness was employed which could hold sample solutions

of 5 to 50p1. The cup was held virtically between two

stainless steel electrodes in an enclosed chamber. The

solution was placed into the cuvette and its open top and

the emission was viewed through a slit cut into the wall.

Solid samples up to lmg could also be used without any

background absorption effects. Massmann (36) has

reported atomic absorption and atomic fluorescence results

for a number of elements with good sensitivities.

Manning and Fernandez (37) have constructed a graphite

furnace similar to that of Massmann. The tube, 5cm long

and 9.5mm internal diameter, was heated by a 400 A current

at 0-10V to temperatures of 2500-2600°C and could accomodate

solid samples. Employing a three stage heating cycle,

they were able to perform the direct determination of

copper and strontium in milk. This analysis is difficult

by flame methods, without sample pre-treatment. The use-

fulness of the Massmann type furnace has been demonstrated

in the analysis of biological material (38-41) and water

(42-45).

11

Woodriff and Co-workers (46-50) have constructed and

used a graphite tube furnace for AAS. The tube was 15cm

long, 9mm outside diameter and 7mm inside diameter and

supported inside a graphite shield tube 29cm long and

12mm inside diameter with 4mm wall thickness. The

graphite tube was heated by a current from an arc welder

to temperatures up to 3000°C. Liquid samples were

nebulized and introduced to the furnace through the side arm

by a stream of argon. A modified version of the furnace

was adapted for discrete sampling of liquids and solids.

The samples were placed in a graphite cup and introduced

through the side arm. This furnace has not been widely

used in commercial atomic absorption spectrometers because

of its large size.

Morrison and Talmi (51,52) have constructed an

inductively heated graphite furnace for the direct analysis

of solids or evaporated solutions by. AAS and AES. The

furnace, a graphite crucible of 22mm o.d., 16mm i.d. and

11cm long, was heated to temperatures of 2500°C by

consuming 4.5KW of power at 3 MHZ. The induction furnace

was used to vaporize and atomize the sample while excitation

of the atoms was achieved by the helium plasma produced by

the RF field. The absolute sensitivities for a number

- of elements were inferior to those given by Woodriff (47),

Massmann (35), West (53) and L'Vov (54) by 1-4 order of

magnitude. However, the larger samples analysed in the

RF furnace compensates for this disadvantage whenever

dealing with real samples.

Headridge and Smith (55) have employed an induction

furnace for the determination of cadmium in solutions and

zinc-base metals. The graphite tube, 15mm i.d., 38mm

o.d. and 7.5cm long was heated to 1900°C by a six turn

induction coil coupled to a 6KW induction generator. The

same authors have reported an improved furnace design

yielding greater sensitivity (56). The furnace consisted

of a vertically mounted graphite tube, 12cm long, 38mm

o.d. and 13mm i.d., closed at the lower end and having

two hollow side arms at right angles to the tube to

provide a light path. The graphite was surrounded by

a quartz jacket and heated by the induction coil to a

temperature of 2400°C. The furnace was flushed with

argon to avoid oxidation of the graphite. Cadmium was

determined in solution and sinc-based alloys with sen-

sitivities of 0.62 ng and 0.4 ng respectively.

1.2.2. Filaments and Open Cells

1.2.2a Wire Filaments

Bunsen (57) in 1859 described the use of a platinum

loop for the sampling of various metals. Metal powders

were placed on the platinum loop and introduced into a

Bunsen flame; the emitted light was dispersed and the

spectra of the metals were recorded. Several techniques

have been developed recently which use an electrically

heated filament for the vaporization of, solution samples.

With these devices the signals are transient and are

obtained as the atomic vapour passes trhough the absorption

or fluorescence light path.

12

Brandenburger (58) and Brandenburger and Bader (59-61)

have reported the use of a copper filament for the

determination of mercury and a platinum filament for the

determination of cad,m.ium, zinc, lead, thallium, copper,

silver and gold. The wires were coated with the element

of interest and on heating, the elements were vaporized

into the atomic absorption light path and measured. They

have claimed the method to be 100 to 10,000 times more

sensitive than with a flame atomizer.

Bratzel, Dagnall and Winefordner (62) have employed

a filament technique similar to that of Brandenburger and

Bader for atomic fluorescence studies of volatile elements.

A platinum loop was used to vaporize the sample solutions.

The vaporized element was then swept into the excitation

light path and its atomic fluorescence was measured. Using

1 Al samples, detection limits of 10-7, 10-8

and 10-15g

were reported for gallium, mercury and cadmium respectively.

West et al. (63) have reported the construction of a

tungsten filament atom reservoir for measuring trace amounts

of zinc, lead, copper and silver by AAS. They employed a

cylindrical tungsten filament 6cm long and 2.2mm in diameter

with its centre ground to locate 1 Al of sample solutions.

They compared this method with a carbon filament atom

reservoir and reported improved sensitivities. The

increased sensitivities may have been due to less pene-

tration of the solution into the filament. A study of

interference effects showed that the tungsten filament

removed or greatly reduced the interferences. This was

attributed to the greater ability of tungsten in transfer-

. ing heat to the atoms thus minimizing condensation which is

13

responsible for many interference effects.

Tantalum and Molybdenum filaments have been employed

for the atomic absorption analysis of copper, nickel and

cobalt (64). In contrast to tantalum, molybdenum did not

react with these elements and gave reproducible results

with high sensitivities, equal or superior to those

obtained with the Massmann furnace (44). However,

nitrate ions at concentrations higher than 0.01M reacted

with Mo and caused permanent damage to the filament. For

the elements studied, the molybdenum filament showed a

substantial decrease in the observed interference effects

when compared with a graphite filament.

Newton and Davis (66) have described an electrically

heated tungsten-alloy wire loop atomizer. The solution

could be introduced either directly with a 5plpipet, or by

submerging the metal wire into the solution for a certain

time, or by electrodeposition. Using different methods of

sampling, detection limits were reported for 19 elements

and compared with results obtained using flame and graphite

atomizers. Newton.et al.(67) have also employed a tungsten-

rhodium wire loop for the determination of cadmium and

lead by AAS. The loop was soaked in the solution for a

certain period of time and the metal ions were pre-

concentrated onto the surface. The loop was then

electrically heated and the atomic absorption signal

measured. Using this technique, detection limits of

- 4x10 14g and 2x10

-11g were reported for cadmium and lead

respectively. This method of sampling gave up to 100

times increase in sensitivity for some elements over the

14

15

standard flameless wire loop technique.

Williams and Piepmier (68) have constructed and

operated a tungsten filament atomizer for the determination

of calcium, chromium, copper, iron, magnesium, manganese

and tin. The filament, obtained from a light bulb, was

heated by a 6V power supply at 4 A and sheathed with a

flow of argon. Using 3/11 sample solutions, sensitivities

comparable with those of conventional methods were

obtained.

Mounce, Dagnall, Sharp and West (69) have reported

the construction of a tantalum loop atomizer for the atomic

fluorescence determination of bismuth in a non-dispersive

system. The tantalum wire, 0.25mm in diameter, was

sheathed in argon. The flow rate of the argon was found

to have a profound effect on the sensitivity of the method.

A detection limit of 3.2x10-11g was obtained with a re-

producibility of 6.5%. The important features of metal

filament atomizers are low background emission in the U.V.,

a non-reactive evaporation surface and a low power require-

ment.

1.2.2b Graphite Filaments

West and Williams (70) have reported the construction

of a simple and yet sensitive atom cell for use in both AAS

and AFS. It consisted of a graphite filament, 20mm long

and 2mm in diameter, heated up to 2500°C by passing a current

of about 100 A at 5V. The two filament supporting elect-

rodes were water cooled and the whole assembly was housed

in a chamber purged with argon. The detection limit for

16

magnesium and silver was 10-10 g for AAS and 10-16 and

3x10-11

g respectively for AFS. These results represented

gains in sensitivity of 3-7 orders of magnitude over

flame methods. Alder and West (71) modified the original

atom cell in which an open filament with inert gas shield-

ing was used. Employing this device; atomic fluorescence

(72-75) and atomic absorption (76-81) measurements on a

range of elements were reported.

Amos (82) has described a furnace termed the "Mini-

Massmann" which was similar to that of the West and

Williams filament cell with two modifications. A trans-

verse hole, 1.5mm in diameter, was drilled into the rod

which acted as a cavity and smaller liquid samples (0.5

to 1/41) could be accommodated. The argon or nitrogen

shielding gas was replaced by hydrogen which on ignition

formed a diffusion flame. This device has been used for

the determination of various elements in blood and lub-

ricating oils (83,84).

Winefordner el al.(85) have constructed an electrically

heated graphite filament similar to that of West and

Williams. The rod had a cavity for locating the sample.

They have reported atomic absorption determinations of

arsenic, chromium, copper, iron, nickel, lead, silver, gold

and maanesium in aqueous solutions, blood and lubricating

1.2.2c Sample Boats

Donega and Burgess (86) have described a device which

used sample boats cut to 50mm x 6mm size from lmm tantalum

17.

or tungsten foil, or 5mm graphite sheet. The boats were

capable of holding 50 to 100 )l of solution and were heated,

by a variac at 12V and 30-50 A to 2200°C in less than 0.1

second. The boats were supported by two copper rods and

enclosed in an inert atmosphere maintained eta pressure of

between 1 and 300 torr.

Hwang et al.(87) in a more thorough study of the

tantalum ribbon atomizer, have reported the sensitivities

for 36 elements. Suzuki et al.(88) .have employed a

similar cell for the trace analysis of aluminium, chromium,

copper, iron, magnesium and manganese. The metal strip

was flushed by an inert gas and powered by a low voltage

(0-110V) transformer at 20 A. Nickel, tungsten, tantalum

and platinum were investigated as the strip material.

Small signals were obtained with platinum and both platinum

and nickel could not be used at high temperatures. Tantalum

and tungsten showed similar results but tantalum was

preferred because it was easier to fabricate. Continuum

emission from the tantalum above 400 nm, limited its

application to those elements having their resonance lines

at longer wavelengths. The advantage of this type of

atomizer is the low power required for electrothermal

heating.

.1.2.3. Cathodic Sputtering Cells

The sputtering process is a very convenient and

efficient method of producing an atomic vapour directly

from a metanic sample. Paschen (89) in 1916 developed the

hollow-cathode tube as a spectral line source, and in 1959

Russell and Walsh (90) suggested its use both as a line

18

source and an atom cell in AAS. Later in 1960, Walsh et

al.(91,92) constructed a hollow-cathode tube and a sputt-

ering chamber for atomization of metallic samples. The

metal sample in the form of a cylindrical hollow-cathode,

4cm long and 12mm i.d. was fitted into a spring clip in

the lid of the stainless steel sputtering chamber. The

tube was then sealed and filled with argon to lmm Hg

pressure and the discharge initiated. The chamber which

was fitted with silica windows was then placed in the

light path of'an atomic absorption spectrometer. The cell

was used for the determination of silver in copper (92).

The technique is restricted to metal samples as the cathode

has to be prepared from the sample itself.

Goleb and Brody (93) modified the Walsh cell and

produced a device in which the cathode was made of aluminium.

Sample solutions were evaporated onto the inside wall of the

cathode. Using this technique, they were able to detect

microgram quantities of refractory and non-refractory

elements.

Ivanov et al.(94) have described the construction of

a graphite hollow-cathode sputtering cell. The sample was

evaporated onto a fine molybdenum wire which was then placed

in the central axis of the hollow-cathode. Copper and

calcium were investigated with this device. A pulsed dis-

charge has been used to produce atomic vapours in a neon-

filled hollow-cathode lamp (95). Copper and magnesium were

determined in this way without any interference from the

emission spectra.

Gandrud and Skogerboe (96) have reported the use of

19

a hollow-cathode discharge cell to detect nanogram amounts

of silver, arsenic, calcium, cadmium, mercury, antimony,

selenium and zinc. Sample solutions were evaporated onto

graphite or aluminium discs which were then placed on a

brass rod cathode. The cell was run at 3 torr of argon,

and 50 mA discharge current. Kirkbright and Wilson (97)

modified this device and were able to detect u.03,pg of

iodine using the 183 nm line.

Massmann (98) has constructed a hot hollow-cathode

cell for the analysis of solid samples by atomic absorpt-

ion. It consisted of a graphite tube, 30mm long, 7mm i.d.

and 9.3mm o.d., supported on a small cylindrical graphite

electrode. The cathode assembly was mounted on a molybdenum

rod 2mm in diameter. The water-cooled, earthed, brass base

plate of the assembly housing acted as the anode. The

cell was fitted with silica windows and purged with argon.

A discharge was operated at 1-10 torr and powers up to 1KW.

Detection limits were reported for the determination by AAS

of silver, antimony, zinc, copper, cadmium, magnesium,

manganese and chromium in 30mg samples of relatively

volatile matrices.

Hot hollow-cathode cells have been found to have

advantages over cool cathodes. With cool cathodes, the

ccalLinuum radiation from the cathode and also the spectrum .

emitted by the sample interferes seriously with the

absorption measurements. However, hot hollow-cathode cells

can only be employed for the determination of relatively

low volatility elements as they can not be heated above

.2000°C. Because of the low pressure inside the hollow-

cathode, the residence time of the atomic vapour in the

absorption volume is short and therefore the method is

not. suitable for detection of small amounts of elements.

1.2.4. Plasmas

A plasma is defined as a mass of vapour or gas in

which a significant fraction of the atoms or molecules

are ionized. A classification of plasma types has been

given by Sharp (99). He divided plasmas into two types,

flame-like and non-flame-like plasmas, and these are brief-

ly' discussed in this section.

1.2.4.1 Non-flame-like Plasmas.

In this type of plasma, the discharge is confined to

a column joining the current-carrying electrodes. Arcs

and sparks are examples of this type of plasma.

1.2.4.1aArcs

The arc is a sensitive source capable of detecting

elements below the limits of detection of a spark. An

arc is an electric discharge which has to be initiated

either be momentary' mechanical connection across the

electrode gap or by means of an auxiliary spark. The

necessary components for a d.c. arc are a direct current

power supply, a variable resistor and a discharge gap.

The d.c. source has to be capable of furnishing of voltage

from 50 to 300V at 1 to 30 A. When a substance is placed

into the arc, the high temperature of the arc (3000 to

8000°K) causes volatilization and excitation of the atoms.

High voltage a.c. arcs have also been used which employ a

potential difference of 100V or more. A.C. arcs have been

20

21

found to be steadiex and more reproducible than the d.c arcs.

Belydev (100,101) have employed a d.c. arc for the

determination of cadmium and silver in graphite matrices.

A graphite electrode, heated by a 6 A arc, was used to

raise the temperature and hence to atomize the elements.

The technique was ,recommended for high and average

volatility elements and the detection limits were 10 to

100 times better than by flame emission. Kantor and

Erdey (102) have used a time-resolved a.c. arc atomizer

and high intensity spectral lamps for AAS. Marinkovic

and Vickers (103) coupled a conventional nebulizer-spray

system to an arc and used the system for the determination

of aluminium, boron, magnesium, vanadium and tungsten and

achieved sensitivities comparable to flame AAS.

1.2.4.1b Sparks

An a.c.spark is capable of producing much higher

excitation energies than the a.c. arc with less heating

effect. The circuitry for sparks is simple and very

similar to that of an a.c. arc. The spark is produced

by connecting a high-voltage transformer (10-50KV) across

the two electrodes. The spark is more reproducible and

more stable than arc. Because of the lower heating effect,

it is well suited for the analysis of low melting materials

and can be readily adapted for the analysis of solutions.

Robinson (104,105) has applied a spark initiated in a

flame and in a nebulized analyte solution in an attempt to

produce free aluminium atoms for AAS. The later system

was found to be effective and aluminium was determined

with a sensitivity of 3 PPm at 394.4 nm.

22

1.2.42 Flame-like Plasmas

This type is characterised by existing with a signific-

ant portion of the discharge external to the main core into

which power is coupled. There are two main types of

flame-like plasma; the d.c. arc plasma and the high

frequency plasma.

A plasma torch is a device designed to heat gases to

very high temperatures by taking the advantages of the high

conductivity of an ionized gas. The conventional d.c.

plasma torch requires electrodes for carrying energy to

the gas and is characterised by operating at high current

(100-1000 A) and low voltage (10-100V). Employing an

electrode coupled plasma torch atomizer, Friend and

Diefenderfer (106) have reported the determination of two

refractory elements, aluminium and lanthanum, by AAS.

Their results show that the formation of refractory oxides

is eliminated when a plasma system is employed. The d.c.

plasma torch has not been extensively utilized in AAS.

The inductivity-coupled radio frequency plasma torch

(ICP) was first described by Reed (107-109) in 1961. In

a typical radio frequency plasma torch developed by Fassel

(110), argon is introduced tangentially into the annular

tube gap between the two outer quartz producing a low-pressure

region at the end of the inner tube. Argon is also

introduced into the inner tube producing a low-velocity

laminar flow on which the plasma operates. A third

injector tube is added for the introduction of'sample into

the centre of the plasma. The induction coil, a water-

cooled copper pipe, is coupled to a RF generator giving

23

2 to 30KW output at 5 to 40 MHZ. A graphite rod is

introduced into the coil which is. conductively heated

and produces thermal electrons ( a tesla spark may also

be used). The free electrons are accelerated by the

magnetic field causing a further breakdown in the gas.

The neutral argon is then heated to temperatures of up

to 10,000°K on collision with the energetic charged

particles. The usefulness of the ICP as a practical

source for analytical spectrometry has been demon-

strated by Fassel et al.(111) and Greenfield (112).

Although usually used in emission mode, the determination

of aluminium, calcium, magnesium, neodymium, rhenium,

titanium, tungsten, vanadium and ytterbium by AAS with

good sensitivities have been reported. ICP's have

also been applied to AAS by several other workers (113-115).

The high temperatures available with ICPrs minimises the

chemical interferences caused by the formation of stable

refractory compounds. The much greater energy available

in the plasma makes it possible to vaporize refractory

solid samples directly. The background radiation is

also markedly less than with flame.

1.2.5. Lasers

The development of lasers in recent years has made

, it possible to directly atomize solid samples for atomic

absorption analysis (116,117). The technique of atom-

izing samples by a pulsed laser beam has been described

in detail by Karyakin et al.(118-120). A parallel beam

of light generated by a laser is focussed onto the sample.

The intense heat produced by the pulsed energy (a few

24

joules to a few tens of joules) raises the temperature

to 5000-10,000°C and on evaporization, emission or

absorption is observed in the atomic vapour.

Hagenah et al.(121) have described the effectiveness

of a laser pulse as an atomization source and have reported

the most detailed experimental results on the use of lasers

in AAS. They were able to determine copper, silver,

calcium in graphite with sensitivities at the RPM level.

1.3. Conclusion

Electrothermal atomizers have provided a considerable

improvement in sensitivity for many elements by AAS. This

is due to the transfer of the complete sample aliquot to

the atomizer thereby overcoming the inefficiency of a

nebulization system and avoiding the large dilution of

atomic population which occurs in the flame cell. Graphite

furnaces have been found to be superior to filaments.

Filaments are more susceptable to interferences as there is

no energy available above the filament to be imparted to

the atoms leaving the heated surface. In furnaces, the

atomic vapour is confined within the tube extending the

residence time of the free atoms and hence improving the

sensitivity. They also produce a slow rise time signal

amenable to modulated a.c. amplification and the use of

- strip chart recorders. In filaments, the transient nature

of the signals requires faster electronics and a rapid

response recorder. The power consumption for furnaces

is larger than that of filaments. However, furnaces

allow larger sample solutions or solids to be analysed.

The discrete sampling technique can only be applied with

small sample aliquots, and often this results in poor

reproducibility. When furnaces are used with continuous

sampling, reproducibility is much better but the sensit-

ivity is considerably decreased due to the inefficiency

of the nebulization process. However, for many elements,

these sensitivities are better than those obtained by the

flame technique. In addition, techniques such as "depos-

ition" can be employed to improVe sensitivities (see

chapter 1V).

25

CHAPTER II

THEORY OF ATOMIC ABSORPTION SPECTROMETRY

26

2.1. Principles

Atomic absorption spectrometry is defined as the

absorption of radiant energy from an external light source

by atoms. By receiving energy from radiation of a

particular wavelength, atoms become excited and reach a

higher energy level. The transitions between low and

high energy levels produce the atomic spectrum of, the

element. For most elements, the characteristic

absorption wavelength is the resonance line which results

from transitions from the ground state to the lowest excited

state. The resonance line is the strongest in the

absorption spectrum because it involves the lowest energy.

Further information concerning the theory of the resonance

radiation and excited atoms may be obtained in the book by

Mitchelland Zemansky (122). '

2.2. The Intensity of a Spectral Line •

The intensity of a spectral line produced by emitting

or absorbing atoms depends on two factors. Firstly, with

a given number of atoms in each energy level depends on the

intrinsic properties of the atom; the greater the probab-

ility of a transition, the more intense is the corres-

ponding spectral line. Secondly, the number of transitions

depends on the number of atoms available at the initial

energy level from which the transition occurs. The

relative population of an atom's level is an important

factor in the determination of the relative intensity of the

spectral lines: The Maxwell-Boltzmann law can be easily

adapted for the calculation of relative population of the

27

quantized levels. Walsh (4) has shown that the number of

atoms in the first excited state is only a small fraction

of the number of atoms in the ground state. The ratio

of atoms in the excited state to the ground state becomes

appreciable only at high temperatures and for transitions

of long wavelengths. The fraction of atoms in the higher

excited states is even less, hence the number of absorbing

atoms in the ground state can be considered to be equal

to the total number of free atoms. As a result, the

temperature rarely affects this equality unless there are

excited states just above the ground state. An under-

standing of the relative population of atomic states is

necessary in the selection of appropriate absorption

lines.

2.3. The Width of Spectral Lines

Even the sharpest spectral line has a finite width and

this has an important consequence in the application of both

AAS and AFS. The width of an absorption line is governed

by the following factors:

a) Natural broadening due to the finite lifetime of

an atom in the excited state (--10-8

sec.) which ranges

between 10-5 - 10-4 nm.

b) Doppler broadening due to the random thermal

motion of atoms relative to the observer which is of

the order of 10-3

- 10-2 nm for the spectral lines of

most elements.

c) Lorentz broadening due to the collision of emitt-

ing or absorbing species with foreign atoms or mole-

cules.

28

d) Holtsmark or resonance broadening due to the

collision of like atoms.

e) Stark broadening due to the external electric

fields or charged particles.

f) Self absorption broadening which is important

only for lines having their lower level at or near

the ground state. Self absorption happens as a

result of absorption of radiation by the present

atoms. Self reversal is a particular case of self

absorption in the presence of a temperature gradiant.

2.4. Variation of Atomic Absorption with Concentration

The relationship between atomic absorption and atomic

concentration has been fully published (122-124) and will

be breifly discussed here. The theory of AAS can be best

described in terms of three quantities; the absorption

coefficient, the total absorption factor and the absorbance.

2,4a The Absorption Coefficient, Kv

The absorption coefficient, K , is defined by

o -K L I = I (a ./) (2-1)

whereIci andIare the incident and transmitted intensities

of radiation of frequency 1) passing through an absorption

cell of having a path length of L. The dependence of Kv on

frequency is often represented by the Voigt function (125)

which includes both Doppler and Lorentz broadening.

2.4b The Total Absorption Factor, AT

The total absorption factor is defined in terms of the

total radiation energy passing through the cell and is

given by:

29

I -I AI AT = 9 = • : Io

I

(2-2)

where AI x 100 is defined as the percentage of absorption I

2.4c The Absorption, A

Absorbance, A, is defined by the following expression:

I A =.log

A is related to AT

by :

= log ( 1 ) 1-A

T

(2-3)

(2-4)

Absorbance can easily be calculated, assuming K is p

constant, and provides a linear relationship with atomic

concentration over a wide concentration range.

2.5. 'Theoretical As ects of Atomization Process

This thesis is concerned with a continuously operated

graphite furnace, and is thus relevant to discuss briefly

the theoretical background to the atomization process.

Most of the work on furnace atomic absorption has been

carried out on practical analysis and only a few authors

have investigated the process of atom formation. Two

approaches have been proposed to study the mechanism.of

atomization and these are discussed in this section.

2.5.1. Thermodynamic Approach

In order to understand the thermodynamic approach,

the various reactions occuring in an atomizer must be

considered. It has been found that with carbon furnace

0

and can be determined experimentally.

30

atomizers, oxy-anion salts of metal solutions are prefered

to those of the halide. This is because the halide

solutions tend to give a higher degree of molecular

volatilization and so the degree of atomization is decreas-

ed. ' Most oxy-anion salts are decomposed to the metal

oxide on heating.

Maessen and Posma (126) have assumed that atomization

occurs by dissociation of the metal oxide. The dissociat-

ion of a metal oxide, MO, can be shown by the following

equation:

MO M 0 2 2

The free energy of this system, AG, is given by

-AG = RT Ln K

(2-5)

(2-6)

where R is the gas constant, T the temperature and K the

equilibrium constant. The degree of dissociation of the

metal oxide,v, is related to partial pressure of oxygen by:

K 104-. = p

KP + 4P(02)

(2-7)

By knowing AG, K and hence pk.can be calculated. In carbon

atomizers, however, there are two more reactions to consider:

C + 202 CO (2-8)

CO + 202 CO2 (2-9)

,These reactions control the partial pressure of oxygen and,

hence, the degree of dissociation of the meatal oxide. From

a knowledge of partial pressure of oxygen they predicted that

the order of atomization for cobalt, lead, gold and zinc

would be Zn4:Pb4Au4Co.

The most extensive studies into the mechanism of

atomization have been carried out by Campbell and Ottaway

(127). These authors have suggested that in the carbon

furnace atomizer, carbon plays a role in the direct

reduction of metal compounds to produce atoms. One good

example is magnesium sulphate which is converted to

magnesium oxide at 890°C and sublimes without decomposition

to gaseous molecules of magnesium oxide at 2770°C.

Magnesium, however, can be determined with very good

sensitivities using a carbon furnace at temperatures of

about 1550°C. This indicates that carbon is effective in

the direct reduction of metal compounds to atoms.

A possible mechanism of reduction of metal oxides to

atoms by graphite is shown in equation (2-1o).

MO(s) + C(0- ---0=C0(g) + M(g) (2-10)

The energy released in this reaction.would seem to be

sufficient for the production of metal atoms in the gaseous

state. Assuming that the reduction of metal oxides by

carbon is rapid, Campbell and Ottaway have proposed that the

temperature at which free metal atoms are formed can be

determined. This can be achieved by calculating the

temperature at which the free energy, AGo, for the react-

,ion (2-10) becomes zero. They have calculated the lowest

temperatures for the production of atoms for 27 elements.

Table (2-1) shows the thermodynamic reduction temperatures

and appearance temperatures together with corresponding

melting and boiling points. The appearance temperature is

defined as the lowest temperature at which a substantial

31

Table 2.1. Thermodynamic Reduction Temperatures and • Appearance Temperatures for Various Elements Together with Melting and Boiling Points for Comparison

Oxide Element m.p.,K Element b.p.,K Temperature Appearance at which AG° temperature becomes negative, K of element,

Pb0 600 2042 1000-1100 1,000

Al203 932 2720 2400-2500 2,300

Cu20 1356 2855 1800-1900 1,730

Fe304 1812 3160 1700-1800 1,750

Na20 371 1163 1200-1300 1,230

Ni0 1728 3110 1700-1800 1,800

Cr2 03 2 76 2915 1800-1900 1,800

ZnO 693 1.181 1200 • 1;100

Co0 1768 3150 1800 1,720

Sc203 1673 2750 2400-2500 2,450

Cd0 594 1038 800 850

V203 2190 3650 2400-2500 2,350

Si.02 1683 2950 2300 2,300

Ba0 983 1910 2300 2,200

TiO2 1 1950 3550 2400-2500 2,420

Li02 454 1604 1900-2000 2,100

Mn304 1517 2314 1600-1700 1,600

Ag20 1234 2450 1200-1300 1,150

K

Table 2.1. continued

Oxide Element m.p.tK Element b.p.,K Temperature Appearance

at which AG° temperature becomes' negative, K of element, K

Sr0 1043 1640 2300 2,100

Hg0 234 629 Below room temp.

B203 2300 4200 AG°react still+ve

Nb0 2770 5200 3000

Sn02 505 2960 1350 1,800

K20 336 1039 1100 1,550

Sb203 903 1910 1100-1200 1,550

Ca0 1123 1765 2400-2500 1,800

Mg0 923 1390 2100-2200 1,550

34

population of atoms is produced in.the carbon furnace.

These workers have found good agreement between

experimentally determined appearance temperatures and the

ones predicted by thermodynamic calculations for 22

elements out of 27 elements.

Aggett and Sprott (128) have compared the appearance

temperatures of various elements in both graphite and

tantalum atomizers. Comparison of these temperatures

indicates whether or not oxide reduction by carbon is

the cause of atom formation. For the majority of elements,

the minimum atomization temperature from tantalum was

within 100°C of the atomization temperature from carbon.

Of the 15 elements studied, only the minimum vaporization

temperature of cobalt, iron and tin from carbon were signif-

icantly lower than that from tantalum. It was suggested

that for these four elements, the free atoms are formed

through the direct carbon reduction of the metal oxides.

To check this conclusion, the most likely reaction between

the metal oxide and graphite was postulated and the free

energy for that reaction was calculated at the temperature

when atoms were first observed from - carbon furnaces. For

aluminium, cadmium, calcium, magnesium, manganese and zinc,

the free energies were positive and the most likely •

mechanism of atom formation would be the dissociation of

metal oxides. For cobalt, iron, nickel and tin, however,

the free energies were negative indicating that reduction

of the oxide by carbon is thermodynamically feasible. It

was concluded that for these elements, carbon acts in the

reduction of metal oxides to free atoms.

35

Czobik and Matousek (129) have also investigated the

mechanism of atom formatibn in the carbon furncace. They

have reported the effect of various anions as acids•on the

atomization temperature of silver, cadmium, copper, nickel,

lead, tin and zinc. Of the anions investigated, only

phosphate affected the atomization temperatures. Addition

of phosphate resulted in an increased atomization temperat-

ure for the elements Cd, Zn, Ag and Pb which atomized at

a lower temperature than tin and had no effect for elements

Cu,Ni and Cr which atomized at higher temperatures than

tin. Two mechanisms of atom formation were then suggested.

The first mechanism involves reduction ofmetal oxides by

carbon and is only applicable to compounds which can form

oxides at temperatures lower than those required for

reduction process to occur e.g., Cu, Ni, Cr. The second

mechanism is thermal dissociation of metal compounds and is

applicable to compounds of higher thermal stability which

decompose at temperatures higher than those required for the

reduction process e.g.,Ag, Cd, Pb, Zn.

The thermodynamic approach has been sucessful in

explaining some of the observed effects in electrothermal

atomizers, however, it does not explain why the elements

Al, Ca, Cr, Mo, Ti and V form thermodynamically stable

carbides at and below the temperatures at which the free

energy for reaction (2-10) is zero. Neither does it

provide any indication of atomization rates or prediction

of absorption peak shapes.

2.5.2. Kinetic Approach

The models which have been proposed for studying the

kinetics of atomization can be classified into two groups,

(a) atomization under increasing temperature and (b) atom-

ization under isothermal conditions.

2.5.2a Atomization Under Increasing Temperature

L,Vov (130) has proposed a general kinetic approach

for the production of metal atoms in a graphite cuvette in

which the sample is vaporized under constantly increasing

temperature conditions. This method can also he applied

to rod and filament atomizers as well as easily atomized

elements at high temperatures in a furnace.

The rate equation for the change in the number of

atoms in a graphite furnace can be written as:

• dN _ dt -I' ) n2(t)

(2-11)

Where dN is the rate of change in the number of atoms, N, dt

present in the gaseous state in the atomizer at time t,

nI(t) the number of atoms entering the system at time t and

n2(t) the number of atoms escaping from it at time t.

Because it is assumed that the evaporation of the substance

occurs at a constantly increasing temperature, atom

formation is a linear function of time and is given by:

n (t) = At

(2-12)

Where A is a constant. The integration of n1(t) from

time 0 to t i , the atomization time, would be equal to the

total number of atoms in the sample, No, thus:

I n i (t) dt = No (2-13)

or n .(t) = (2-14) t1-

36

Assuming that atoms are removed from the furnace by

diffusion, then:

37

n ) = 2(t • t2 (2-15)

Where t2 is the residence time of atoms in the system. By

substituting equation (2-14) and (2-15) into equation (2-11)

the following equation is obtained:

dN =2Not dt t 2 t2 (2-16)

Solution of equation (2-15) describes the number of atoms,

N, in the system at time t.

For t < t1

0

e t12

t12 t2

Or

At time t =

vaporized,

t92

= 2No - 1 e

-t

(2-17)

(2-18)

t2 )

completely

- t

t- 2 t2 (

tl, when the sample has been

Nt reaches its maximum value:

2 2 1 - 1 + :- 2No e t2 Nt=t t, 1 t2 1

For t>ti equation (2-15) must be modified to account for

the fact that the sample introduction step has been completed

and only simple diffusional decay follows, hence:

dN =-N dt t

2

2 t - 1

(2-19)

N =

ft

2No t2 t2

2 t to dt = 2No

-t

1

+

(2-18)

(2-20)

(2-21)

into equation

-t tl-t

t2

_ t2

can

equation

t1 t7 t2

(2-22)e

to give the number

2

be used

t

Solving this equation yields:

38

Equation (2-17) and(2-22)

of atoms, N, in the system at time t.

Torsi and Tessari (131-134) have developed a kinetic

modal for determination of atomization energies using a

carbon filament atomizer. They have obtained an

atomization energy of 405 KJ mol-1

(95 Kcal mol-1

) for

the production of Ni atoms, which suggests that the atomic

species are produced as a result of sublimation of the

metal.

Johnston, Sharp, West and Dagnall (135) have

employed a similar approach to examine the nature of atom

production from a non-mechanistic viewpoint. They have

assumed that the vaporization of solid analyte and, hence,

the equilibrium concentration of gaseous analyte species

are governed by a simple Boltzmann factor. They have

N

t dN

Ntl

Nt = Nt e 1

Substituting for Nt from

(2-21) gives:

2

N = 2No t2

t1 2

associated various energies with the vaporization process

which are then characteristic of the analyte. These

energies are related to either the heat of vaporization

of the metal or to the metal oxide bond dissociation

energy, which ever is larger.

2.5.2b Atomization Under Isothermal Conditions

Fuller (136), has measured the variation in atomic

concentration in a graphite furnace under isothermal

conditions. The rate equation for the change in the

amount of atoms can be written as:

dN dt• ni(t) -n2(t) (2-23)

The notations are the same as used earlier. The rate

of formation of atoms was found to follow first order

kinetics and is given by:

nl(t) = Kl(No-Nt) (2-24)

Where. No is the initial amount of element, Nt the amount

of element atomized up to time t and K1 the rate constant

for the atomization process.

Integration of equation (2-24) yields:

= N0(1-e -;.K1t

(2-25)

Substituting for Nt into equation (2-24) gives (2-26)

-K1t ni(t) = KiNoe

(2-26)

The rate of loss of atoms from the furnace is

dependent on the amount of atoms present in the furnace at

time t and is given by:

39

n2(t) = K2N

(2-27)

Where K2 is the rate constant for the removal of atoms

from the furnace and is proportional to the flow rate of

inert gas through the furnace. Substituting equations

(2-26) and (2-27) into equation (2-23) produces equation

(2-28) which on intenration gives the concentration of

atoms at any time.

-K1t

dN dt

=K1Noe K2N

(2-28)

-K1t -K2t

K1 N No e e ) (2-29) K2-K1

Fuller's approach is applicable to furnace atomizers,

particularly at low temperature and for elements which

are difficult to atomize.

Kinetic measurements can be used to postulate the

mechanism of atom formation. There are four possible

mechanisms which fit simple kinetics for the formation

of atoms in a graphite furnace.

1) MO(s/l) Fast 1 M(s/l) + 2 02

M(s/l) Sl M(9) ow

2) --- MO(s/1)+ C Fast--0- M( §/1) + CO

M(s/l) ----o- Slow M (g)

3) MO(s/1) ------a- Slow M(s/1)

2. + 0

M(s/l) Fast M(g)

41

4) MO(s/1)+ C Slow

m (s/l) + CO

Fast M(s/1) (g)

Fuller (136) has employed the kinetic approach to

investigate the mechanism of atom formation for copper.

Kinetic measurements were carried out under the same

conditions in the presence of carbon and in the absence

of carbon but with the presence of a different reducing

agent. If the results are identical, then the most

probable mechanism would be 1,2 or 3, if, however,

atomization does not occur or it'occurs but at different

rates, then mechanism 2 and 4 would be most probable.

Experiments were carried out with plain carbon tube and

tantalum-lined tubes. The results showed that the product-

ion rate of copper atoms was considerably greater with the

tantalum liner present. From the results it was concluded

that atomization normally occurs through reduction by

carbon and that this process is the most probable rate

controlling step in the production of copper atoms. The

mechanism of atom formation for copper was therefore

concluded to be as follows:

- Slow 0+(s.--•- ?Cu CO 2 - (s/l)

Fast Cu (g) (s/l)

Kinetic theories of atomization can be used to

investigate the method of signal measurement. The common

methods of measuring signals in AAS are the measurement of

At the time t = t1, when the sample is completely atomized,

the number of atoms reaches its peak value, Npeak' Using t1

equation (2-17) and assuming then t2

42

an equilibrium absorbance in the flame technique (4) and

peak absorbance in the non-flame technique (137). An

alternative method has been described by Massmann (138).

The theoretical advantage of this method has been shown by

L'Vov (130).

Considering equation (2-17) as the value of t increases

the magnitude of number of atoms tends towards an equilib-

rium va.lueN , given by: equil t2

= Nequil N — o t1 (2-30)

= Npeak No (2-31)

In this case the peak value corresponds to the total

number of atoms in the sample, No.

The integration method is based on measuring the

integral value Q of atoms during the entire period of

production of atoms. The relationship between Q and

the number of atoms, N, is given by:

t3 QN Ndt

(2-32)

0

where t3 is the length of time during which the signal is

recorded. By substituting equations (2-17) and (2-22)

into equation (2-32), and after integration, provided that

t3 - t1 4 t2 then QN

= No t2 (2-33)

The ratio of the magnitude of the signal to the total

number of atoms can be used as a measure of absolute

sensitivity. For the equilibrium method of signaL

measurement

t2 Nequil t2 for — < 1 No

t1 t,

(2-34)

Thus it can be seen that rapid introduction of sample

into the system and also long residence time results in

higher sensitivity.

For the peak method:

N

peak - t1

44

particularly advantageous.

2.6. Interferences with Electrothermal Atomizers

Electrothermal atomizers exhibit considerably more

susceptibility to interferences than the equivalent flame

techniques. Many interference effects have been reported

but only a few authors have explained the cause of these

interferences.

Employing a carbon rod atomizer, Amos et al.(139)

have studied the interference effects encountered in

determination of lead and compared their results with carbon

tube atomization. It was shown that chemical and spectral

interferences were decreased when the carbon tube atomizer

was used. A decrease in the carrier gas flow rate also

resulted in a slight decrease in the chemical interferences

due to the increased residence time of the atoms in the

reducing environment. It was concluded that the short

residence time of the atoms produced by a carbon rod was

responsible for the higher level of interference.

Robinson (140.) has reported the construction of a

carbon hollow T atomizer. The atomization takes place in

the stem of the atomizer and the absorption measurement is

carried out along the cross piece. This design yielded

greater atomization efficiency together with complete

destruction of the solvent resulting in reduction of chemical

interferences and removal of molecular absorption.

The addition of salts (141) and acids (142-145) has

been used to overcome some of the interferences observed

in graphite atomizers. The integration method of signal

measurement has been found to be effective in removing

vaporization interferences (146-148).

The interferences encountered in furnace atomizers

may be classified into three types:-

physical, spectral and chemical.

2.6.1. Physical Interferences

2.6.1a Sample Introduction Interferences

Because of the temperature gradient existing along

most atomizers, sample size and positioning of the sample

is of great importance when discrete sampling is used.

In the case of continuous sample introduction, the physical

process which occurs during nebulization have an important

effect on the sensitivity of determination. Signal

integration can be used to reduce some of variations

resulting from variable sampling.

2.6.1b Memory Effects

Incomplete atomization of an element causes

accumulation of that element on the atomizer with sub-

sequent enhancement in the analytical signals. This type

of interference occurs with elements which form stable

refractory oxides. The use of higher atomization temper-

ature or longer atomization time can minimize this inter-

ference. Unlike furnaces, memory effects are rarely

- observed with filaments.

2.6.2. aectral Interferences

2.6.2a Line Overlap Effects

This kind of spectral interference is very rare in AAS.

Fassel et al. (149) have reported that spectral line inter-

45

ferences may occur when there is a significant overlap of

the Primary Source emission line profile with the

absorption line profiles of any interfering species in

the atom cell. These interferences are usually avoided

by selecting an alternative interference free absorption

line.

2.6.2b Emission Radiation Effects

These are produced by emission from the heated

atomizer and other elements in the sample. Light

emission from the carbon follows ah approximately black-

body curve and therefore the effect is more serious when

measurements are made in the visible region of the spectrum.

This interfering emission can be largely eliminated by

using a modulated source lamp and a lock in amplifier

detector. The signals from atom cell are not modulated

and are therefore rejected by the measurement system.

2.6.2c Scattering Effects

Large concentrations of matrix vaporized during the

atomization stage, can lead to scattering of the incident

light beam. Light scattering was first demonstrated by

Willis (150) and thought to be the cause of light losses

observed in AAS. Later, Koirtyohann and Pickett (151)

demonstrated that molecular absorption rather than

scattering was responsible at least in part for the light

losses reported by Willis (150).

2.6.2d Molecular Absorption Interferences

This type of interference is caused by molecular

species vaporized during the atomization stage. Like

46

47

scattering, molecular absorption results in false absorbance

signals which must be corrected. There are three methods

of background correction; the use of a continuum source,

the adjacent line method and use of the Zeeman effect.

The use of a continuum source was first described by

Koirtyohann and Pickett (152). This involves measuring

the non-atomic absorption signal at an adjacent wavelength

using a deuterium arc. Hydrogen and deuterium hollow

cathode lamps have also been employed as continuum sources.

This technique, however, can not be used above 350 nm due

to the lack of intensity of the source. The adjacent

line technique invloves measuring the absorbance at a

non-absorbing line which is adjacent to the analyte

resonance line. The Zeeman effect employs a magnetic

field to split the spectral lines from the light source to

produce non-absorbing lines outside the atomic absorption

profile. It has recently been employed (153) to correct

molecular absorption inEaectrothermal atomizers.

Alkali halides are the most serious cause of molecular

absorption interferences (154,155). Employing a graphite

furnace, Wilson and Kirkbright (156) have reported the

determination of iodine from iodine - containing salts at

206.1 nm. When they used a deuterium lamp to correct for

_non-specific absorption, the iodine absorption signal was

not observed. A further study by Kirkbright et al.(157)

indicated that the absorption signal observed at 206.1 nm was

due to molecular potassium iodide vapour produced in the

graphite furnace. Solutions of sodium and potassium

sulfate, nitrate, chloride, bromide and iodide were also

48

investigated. For sulphates and nitrates of sodium and

potassium, no absorption was observed in the range 190 nm

to 360 nm. It was then suggested that on heating,. these

salts are decomposed to the corresponding metal oxides.

This is then followed by the reduction of the metal

oxides by carbon as demonstrated by Ottaway et al. (127).

Yasudu and Kakiyama (158) have described,the absorption

spectra observed for the halides, sulfates and nitrates

of the transition metals. The resu.lts showed that the

gaseous metal halides were formed and vaporized in the

atomizer at temperatures of 300-500°C. Metal sulfates

and nitrates were decomposed to the metal oxides on heating.

This was confirmed by observing the absorption spectrum of

sulfur dioxide and nitric oxide when metal sulfates and

nitrates were heated to about 300-400 and 100-150°C

respectively.

Ediger (159) has reported a method of interference

modification using a chemical agent. For the determin-

ation of cadmium in the presence of 0.15mg sodium chloride,

the non-atomic absorption signal was so large that even

with background correction the cadmium signal was not

observed. Addition of ammonium nitrate to the furnace

reduced the non-atomic absorption signal considerably.

- This was due to coversion of the sodium chloride to a

more volatile compound. Employing this technique, Ediger

et al.(160) were able to determine copper in sea water with

a graphite furnace. The detection limit was found to

be 10-3

PPm compared, with 0.03 PPm obtained by direct

injection of sea water (43).

2.6.3. Chemical Interferences

2.6.3a Anion/Cation Interferences

As mentioned earlier, the presence of oxyanions does

not seem to interfere with elemental determinations. With

halides, on the other hand, a loss of element can occur

through vaporization of the molecular halide. Losses of

lead as volatile PbC12 and Pb Cl have been reported (161,

162) in determination of lead in the presence of chloride.

It was found that treating sample with hydrogen and ashing

at temperatures of about 730°C removed the chloride inter-

ference. Cation interferences are more complex and have

not been generally explained.

2.6.3b Pyrolysis' Losses

When using discrete sampling, a pyrolysis stage is

often employed prior to atomization. As a result, the

sample matrix is broken down or vaporized giving rise to

reduced interference effects. Fuller (163) has reported

losses of copper and nickel during the pre-atomization

stage at temperatures above 600°C. It was suggested

that metal oxides are formed on heating which are then

reduced to the corresponding metals by carbon from the

furnace. The metals are then lost through evaporation.

Losses of mercury (164), lead, cadmium, berylium and

vanadium (165) have also been reported.

2.6.3c Condensation

After atomization of an element, the atoms cool

quickly by entering the cooler gas layer above the atom-

izer and condense rapidly. In the presence of other

49

50

elements which are atomized or vaporized at similar

temperatures,.the analyte element becomes occluded with the

interfering elements on condensation. This type of inter-

ference is of importance mainly with filament atomizers

because of the steep temperature gradient above the

. atomizer surface. West et al.(73,74) have demonstrated

that many inter-element interferences happen as a result

of a gas-phase interaction. It was suggested that

absorption measurements under limited field viewing

condition should minimize the interference effects (76).

A horizontal slit was placed across the primary source

which shielded the region where most inter-element effects

would likely occur. Jackson and West (78) reported that

the only interference observed with limited field viewing

was from elements of similar volatility to the analyte

atoms.

2.6.3d Carbide Formation

Losses of atomic vapour may occur as a result of

the formation of involatile carbides between the analyte

and the incandenscent graphite. Pyrolytic coating of

the graphite tube has been found to reduce carbide formation

considerably (166) and hence enhances the sensitivities for

many elements. Sturgeon and Chakrabarti (167) in a

study of the mechanism of atom loss in graphite furnace

showed that the pyrolytic coating considerably reduced

the diffusion losses of atoms through the graphite walls.

Mo and V were investigated and the results showed that the

rate of loss of molybdenum and vanadium atomic vapour from

51

uncoated tubes was increased by 21% and 12% respectively, •

over that obtained with coated tubes.

Although carbide formation has been found to be

disadvantageous, it can be employed to passivate tubes and

therefore prevent the analyte forming a carbide. Fisher

et.al.(168) have reported a 10 fold signal enhancement for

the determination of beryllium after the carbon was treated

with zirconium. The zirconium carbide on the surface of

the carbon prevented formation of beryllium carbide and

hence improved its sensitivity. These authors have also

reported enhancement factors of 2.6, 1.8 and 2.5 for

manganese, chromium and aluminium respectively when the

furnace was treated with lanthanum.

2.6.3e Nitride Formation

If nitrogen is used as the purge gas, some elements

may form thermally stable nitrides (169) and hence the

sensitivity is reduced. L'Vov (170) has demonstrated

that a loss of Al can occur by reaction with nitrogen

in the presece of incandescent graphite to form thermally

stable aluminium nitride.

Sturgeon et.al.(171) have reported a 60 per cent

reduction in aluminium sensitivity when nitrogen was

substituted for argon as the sheath gas. Atomization

of aluminium in a tantalum of tungsten lined atomizer,

however, produced equal sensitivities for both argon and

nitrogen gases. Substituting pyrolytic graphite for

standard graphite decreased the above difference by 25 per cent.

It was suggested that carbon plays a role in the formation

of the nitride as shown in equation (2-37)

Al203(s)+ 3C0 (2-37) + 3C

(s) + N2(g) 2A1N(s)

Aluminium nitride is stable up to 2500°K.

2.6.3f Ionization

Ottaway and Shaw (172) have investigated the

possibility of ionization interferences in carbon furnace

AAS and AES. Barium was chosen as a test element and

signals observed for atomic absorption and emission and

also ion absorption. Addition of caesium to barium

suppressed the ionic absorption signal. However, in

contrast to flame atomization (173), the suppression of

ionization did not produce any increase in the atomic

absorption signal. The reason was thought to be that

the ion population is negligible compared with the atom

population. From the results obtained they conclude

that ionization interferences would not be significant in

carbon furnace spectrometry.

52

CHAPTER III

INSTRUMENTATION

53

3.1. Graphite Furnace Atomizer System

The graphite furnace used in this study is a

modification of the device described by West and Williams

(70). A schematic diagram of the furnace is shown in

Fig.3.1. The whole assembly is mounted on a stainless

steel base and housed in a glass chamber fitted with a

vertical stem at the top centre for sample introduction and

another one at the side as the exhaust outlet. The space

between the electrodes is enclosed. by two insulating

syndanio plates at the side and another at the top with a

hole at the centre for sample introduction. These plates

were found to disintegrate gradually at high temperatures

and so stainless- steel plates were used to line and

protect their inner surface from high temperatures.

The graphite tubes 20mm long and 5mm internal dia-

meter were machined from Ringsdorff high purity graphite

rods. A hole of 3.5mm in diameter was drilled at the

centre of the tube to provide introduction of the

sample aerosol into the tube. In order to obtain good

electrical contact between the electrodes and graphite

tube, graphite rings were machined to hold the tube.

The electrodes used for this system were rather bulky

and caused condensation of water on their surfaces. These

electrodes were later replaced by two L-shaped electrodes

which were much thinner than the early designs and showed

no condensation.

3.2. Power Supply

When operating a graphite furnace continuously, a

54

b

a

C

e

a. Graphite. tube

b. Stainless steel electrodes

c. Stainless steel base

d. Water inlet

e. Water outlet

f. Argon inlet

Fig.3.l. Graphite furnace system used for this study (side view).

55

large power is usually needed for heating. In this study

the power was supplied by wiring three transformers (12V

and 100 A) in parallel which could provide a maximum

current of 300 A at 12V. Two copper strips 75cm long

and 2.5cm wide with a thickness of 3mm were used for

transferring the electrical power to the electrodes. The

voltage supplied to the furnace could be varr:;ed from 0 to

12V by a variac transformer (270V and 20A) connected to a

30A single phase a.c. power supply.

3.3. Temperature Measurements

An optical pyrometer was employed to measure the

temperature of the graphite furnace for this study. This

was achieved by focussing the radiation from the inner wall

of the tube at the viewing lens of the optical pyrometer.

At voltages less than 4V, however, there was no significant

radiation from the tube and a thermocouple had to be used.

The thermocouple was a chrome-alumel type and was inserted

inside the tube and in contact with it during temperature

measurements. The maximum measurable temperature with

this type of thermobouple was 1,000°C and so measurements

were carried out up to 4.5 volts. The results are shown

in Fig.3.2. The data were obtained while nebulizing

distilled water at an uptake rate of 1.9 1.min-1.

3.4. Purge Gas

An inert gas was continuously flushed through the

graphite furnace in order to maintain a non-oxidizing

environment and hence increase the lifetime of the graphite

tubes. The three gases commonly used for this purpose are

3200

2800

2400

2000 -

1600 -

1200

800

400

2 4 6 8 10 12

Voltage, V

Fig.3.2. Temperature vs. voltage curves for the furnace. (a) Thermocouple measurements, (b) optical pyrometer measurements.

U 0

Tem

pera

ture

,

56

.57

argon, nitrogen and hydrogen. It has been xeported that

argon yields the maximum sensitivity for the determination

of lead (87). Argon has the lowest specific heat and

thermal conductivity when compared with the other gases.

It seems that the thermal properties of the purge gas

have a significant effect on sensitivity for many elements.

Manning et al.(37) ih investigating the choice of gases

for analytical measurements reported a three times reduct-

ion in aluminium sensitivity with nitrogen as the purge

gas. Other workers (166) have also reported a signif-

icant reduction in aluminium sensitivity by substituting

nitrogen for argon. This may be due to formation of

involatile aluminium nitride.

Amos et al.(139) have reported the use of an argon-

hydrogen mixture which yielded a reduction of interference

effects together with improved sensitivity for many

elements. On heating, the hydrogen ignites spontaneously

and burns as alargon-hydrogen-air entrained diffusion flame.

It has been shown that the temperature of the flame around

the graphite does not exceed 500°C. It was concluded

therefore that the flame temperature does not contribute

significantly to the atomization process. The enhance-

ment was attributed to the highly reducing environment

caused by the presence of hydrogen which promotes higher

atomization efficiency. A 20-fold improvement in alumin-

ium sensitivity was obtained when hydrogen was introduced

into argon in the ratio 2 argon to 1 hydrogen.

It was mentioned earlier that oxygen must be excluded

from the graphite atomizer to avoid oxidation of the graphite.

58

Some workers (174,175), however, have added oxygen

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