F AC 31 08 [a] Gas Chromatogrphy

7/27/2019 F AC 31 08 [a] Gas Chromatogrphy

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Chapter 31

Dong-Sun Lee/ CAT-Lab / SWU 2012-Fall version



Gas Chromatography (GC)

Introduction

Gas chromatography is a chromatographic technique that can be used to

separate volatile organic compounds.

Two types of GC are encountered: gas-solid chromatography(GSC) and

gas-liquid chromatography(GLC). GLC is finds widespread use in all

fields of science, where its name is usually shortened to GC.

A gas chromatograph consists of a flowing mobile phase, an injection port, a

separation column containing the stationary phase, and a detector.

GSC is based on a solid stationary phase on which retention of analytes is the

consequence of physical adsorption. GLC is based on partitioning behavior of the analyte between the mobile gas phase and the liquid stationary phase in

the column.



Characteristics of GLC

1. Sensitivity : mg ~ pg (10 – 3 ~ 10 – 9 g)

2. Versatility : from rare gases to liquids and solids in solution

with 800~1000 MW

3. Speed of analysis : typically 5 ~ 30 min ,

complex 3 ~ 30 mixture separation

4. Reproducibility : qualitativeAccuracy : quantitative



GC-14A gas chromatograph (Shimadzu)with an integrator.

HP 5890 gas chromatograph (Hewlett

Packard) with an integrator

GC-MS : GC-Q plus ion-trap GC-MSn ( Thermoquest - Finnigan ) Xcalibur software



Fuel

gas

Carrier

gas

Regulator

Trap Make-up gas

Splitvent

Injector Detector

Column oven

Column

Fuel

gas

Electrometer

Recorder Integrator

Computer

Valve/ Gage

Schematic diagram of a gas chromatograph system.

Basic components of Gas Chromatograph

Carrier gas supply

Sample introduction inlet

Column and controlled-temperature oven

Detector & oven

Recorder



Carrier gas system in gas chromatograph

The purpose of the carrier is to transport the sample through the column to the detector.

The selection of the proper carrier gas is very important because it affects both column and

detector performance. The detector that is employed usually dictates the carrier to be used.

From a column performance point of view a gas having a small diffusion coefficient is

desirable (high molecular weight, e.g., N2, CO2, Ar) for low carrier velocities while large

diffusion coefficients (low molecular weight, e.g., H2, He) are best at high carrier velocities.

The viscosity dictates the driving pressure. For high-speed analysis, the ratio of viscosity to

diffusion coefficient should be as small as possible. H2 would be the best choice, followed

by helium.

The purity of the carrier should be at least 99.995% for best results. Impurities such as air

or water can cause sample decomposition and column and detector deterioration. In

temperature programmed runs, impurities in the carrier gas such as water can be retained at

low temperatures but are then eluted at higher temperatures impairing the baseline. Manyinstrument problems have been traced to contaminated carrier gases.

The carrier must also be inert to the components of the sample and the column.



Properties of common carrier gases

Gas molecular weight Thermal conductivity Viscosity

× 105 at 100oC × 10 – 6 at 100oC

(g-cal/sec-cm- oC ) (P)

Ar 39.95 5.087 270.2 *

CO2 44.01 5.06 197.2

He 4.00 39.85 234.1

H2 2.016 49.94 104.6**

N2 28.01 7.18 212.0

O2 32.00 7.427 248.5***

* at 99.6oC ** at 100.5oC **** at 99.74oC



Using the correct carrier and detector gases are an important factor in installing

a new GC. The five gases commonly used as carrier gas and detector fuels in

capillary gas chromatography are helium, hydrogen, nitrogen, argon-methane,

and air. The types of gases necessary are partly determined by the detectionsystem used. Factors to consider for each individual gas are discussed below.

Carrier Gas Choice

Carrier gases that exhibit a broad minimum on a van Deemter profile are

essential in obtaining optimum performance. Volumetric flow through a

capillary column is affected by temperature. When temperature programming

from ambient to 300oC, the flow rate can decrease by 40 percent. A carrier gas

that retains high efficiency over a wide range of flow rates and temperatures is

essential in obtaining good resolution throughout a temperature programmed run.Figure 1 shows the van Deemter profile for hydrogen, helium, and nitrogen

carrier gases.



Van Deemter curves for GC of n-C17H36

at 175oC, using N2, He, or H2 in a 0.25

mm diameter × 25 m long wall coated

column with OV-101 stationary phase



Hydrogen is the fastest carrier gas (uopt), with an optimum linear velocity of

40cm/sec, and exhibits the flattest van Deemter profile. Helium is the next

best choice, with an optimum linear velocity of uopt = 20cm/sec. Nitrogen's

performance is inferior with capillary columns because of its slow linear velocity, uopt = 12cm/sec. Argon-methane has a slower optimum linear

velocity than nitrogen and is not recommended for use as a carrier gas with

capillary columns. Air is not recommended as a carrier gas because it can

cause stationary phase oxidation.

With hydrogen and helium as carrier gases, the minimum H.E.T.P. valuescan be maintained over a broader range of linear velocities than with nitrogen,

and high linear velocities can be used without sacrificing efficiency. Nitrogen

is beneficial only when analyzing highly volatile gases under narrow

temperature ranges where increasing stationary phase interaction is desirable.

Otherwise, the use of N2 results in longer analysis times and a loss of resolution for compounds analyzed on a wide temperature range.

http://www.restekcorp.com/gcsetup/gcsetup3.htm



Exert Caution when Using Hydrogen as a Carrier Gas

Hydrogen is explosive when concentrations exceed 4% in air . Proper safety precautions

should be utilized to prevent an explosion within the column oven. Most gas

chromatographs are designed with spring loaded doors, perforated or corrugated metalcolumn ovens, and back pressure/flow controlled pneumatics to minimize the hazards

when using hydrogen carrier gas. Additional precautions include:

• Frequently checking for leaks using an electronic leak detector.

• Using electronic sensors that shut down the carrier gas flow in the event of pressure loss.

• Minimizing the amount of carrier gas that could be expelled in the column oven if a leak

were to occur by installing a flow controller (needle valve) prior to the carrier inlet

bulkhead fitting to throttle the flow of gas (for head pressure controlled systems only) as

shown Fig. 2.

• Fully open the flow controller (needle valve) and obtain the proper column head pressure,

split vent flow, and septum purge flow rates. Decrease the needle valve flow rate until the

head pressure gauge begins to drop (throttle point). Next, increase the flow controller

(needle valve) setting so that the right amount of flow is available to the system. Should a

leak occur, the flow controller will throttle the flow, preventing a large amount of

hydrogen from entering the oven.



Make-up and Detector Fuel Gases

Gas added to the stream after the column is called makeup gas. Choosing the correct

make-up and detector gases will depend on both the detector and application. Most GC

detectors operate best with a total gas flow of approximately 30ml/min. to ensure high

sensitivity and excellent peak symmetry. Refer to your GC manual for optimum flowrates on different instruments. Carrier gas flows for capillary columns range from 0.5 to

10ml/min. which are well below the range where most detectors exhibit optimal

performance. To minimize detector dead volume, make-up gas is often added at the

exit end of the column to increase the total flow entering the detector. Make-up gas

helps to efficiently sweep detector dead volume thereby enhancing detector

sensitivity.

Make-up gas can be added directly to the hydrogen flame gas for flame ionization

detectors (FID), nitrogen phosphorous detectors (NPD), and flame photometric detectors

(FPD) or added to the column effluent by an adaptor fitting. However, GCs such as

Perkin-Elmer and Fisons do not require make-up gas.

Combustion type detectors (FID, NPD, FPD) use three gases: make-up, hydrogen (fuelgas), and air (combustion/oxidizing gas). For non-combustion detectors, such as the

thermal conductivity detector (TCD), electron capture (ECD), and photo ionization

detector (PID), only carrier and make-up gases are required. In the case of the

electrolytic conductivity detector (ELCD), the make-up gas is hydrogen, as a reaction gas

in the halogen and nitrogen mode or air in the sulfur mode. Table I shows recommended

gases for various detectors.



Carrier gases and detector fuel gases for use with various GC detectors

TCD ECD FID NPD FPD ELCD

PID

Carrier Gases He O O O O O O O

H2 O - O - O O O

N2 O O O O O - O

Combustion/

Reaction Gases

H2 - - O O O O -

Air - - O O O - -

Make-up Gases N2 O O O O O - O

He O - O O O - O

ArCH2 - O - - - - -

http://www.restekcorp.com/gcsetup/gcsetup3.htm



Effect of impurities

- Impurities such as hydrocarbon, oxygen, water

contribute to unwanted noise levels, excessive baseline drift .

- Molecular sieve --- Moisture trap, Oxygen trap, Chemical filter

Effect of water on column efficiency

- Carrier gas dryness is very important !! (use anhydrous sodium sulfate)

- Water can and usually does react with some portion of the column.

This results in loss of resolution and tends to produce asymmetric

or tailing peaks.

Unwanted components or ghost peaks may also appear.

Another effect is a net loss of sensitivity.



Gas purifiers

The trap will remove any water vapor or oils that may have been introduced inthe filling process since a number of gases are water pumped. The contaminants

removed by the trap could otherwise interact with the column packing material

to produce spurious peaks. In addition the contaminants can cause increased

detector noise and drift.

The traps should be reconditioned (about twice a year ) by heating to 300 oC

for 4~8 hr with a stream of gas passing through it or in a vacuum oven.



Carrier gas purifiers

GC1 2 3 4

Gas

cylinder

1. Hydrocarbon trap

2. Moisture trap

3. Oxygen trap

4. Indicating oxygen trap



Gas purifier recommendation for GC applications

Capillary column GC Carrier Hydrocarbon, Moisture, Oxygen

with any detector Make-up None - all detector but

ECD moisture & oxygen

Air for FID Hydrocarbon

H2

for FID None

ELCD reaction gas Hydrocarbon

Packed column GC Carrier Hydrocarbon, Moisture, Oxygen

with FID or TCD

Packed column GC Carrier Hydrocarbon, Moisture, Oxygen

with ECD, FPD, NPD,

MSD



Flow requirements

1. Stable

2. Reproducible

3. Convenient

The more constant the flow rates,

the more precise and accurate the results.

Flow controller ( Pressure controller )

--- to maintain precise and accurate flow rates



Effect of decreased flow rate or lower temperature

- All peaks have shifted to longer retention times

- Apparent loss of peak height- The base of each peak is wider ,

however, individual peak area remain constant.

Effect of increased flow rate

- Sample components are squeezed toward

the injection point

- Cause two components to elute together,

appearing as single peak



Regulations of carrier gas

Carrier cylinder bottled at about 2500 psi(150-160 atm)

Two stage pressure regulator :

- first stage : high inlet pressure

- second stage : low outlet pressure

( set at 40~100psi)

Gas generators



Gas flow rate control

A 1 % change in carrier gas flow rate will cause a 1% change in retention time.

For all these reasons it is important to keep the flow of the carrier gas constant.

1. Control of carrier gas inlet pressure

2. Control of carrier gas flow rate

In isothermal operation the means of regulation is immaterial because both

means provide constant inlet pressure as well as constant flow rate. In

temperature programmed runs, however, the situation is quite different. If onemaintains the inlet pressure constant the flow rate will change. Therefore, with

temperature programming of the column, the flow rate must be controlled.

Pressure controllers

1. The second stage regulator on the cylinder

2. A pressure regulator mounted in the GC

3. A needle valve(variable restrictor) mounted in the GC

4. A fixed restrictor mounted in the GC



Flow measurement

1) Rotometer

The column flow rate is typically indicated by a rotometer .

( Calibrate equilibrium position indicating the flow )

Rotometer is operated by the volume of gas passing a ball in a tapered cell.

2) Bubble meter

3) Electronic flow sensor



Relationships between inside diameter, column length, mesh size, and carrier

gas flow for packed column

Inside diameter Mesh size for Mesh size for Carrier flow

mm length up to 3m length over 3m N2, ml/min He or H2, ml/min

2 100~120 80~100 8~15 15~30

3 100~120 80~100 15~30 30~60

4 80~100 60~80 30~60 60~100

John A. Dean, Analytical Chemistry Handbook, McGraw-Hill, 1995, p. 4.31 .



Comparison of 1/8-in(0.316 cm) packed, wide bore, and WCOT columns

1/8 in packed Wide bore WCOT

Inside diameter, mm 2.2 0.53 0.25

Film thickness, m 5 1~5 0.25

Phase volume ratio() 15~30 130~250 250

Column length, m 1~2 15~30 15~60

Flow rate, ml /min 20 5 1

Effective plates(Neff ) per meter 2000 1200 3000

Effective plate height (Heff ),mm 0.5 0.6 0.3

Typical sample size 15 g 50 ng

John A. Dean, Analytical Chemistry Handbook, McGraw-Hill, 1995, p. 4.31 .



Recommended ranges of gas flow rates

Detector Gases Range Capillary Packed

FID Carrier 2 ml/min 40 ml/min

Hydrogen 30~50 ml/min 35 ml/min 40 ml/min

Air 300~600 ml/min 350 ml/min 500 ml/min

Make-up(N2) 10~60 ml/min 30 ml/min not used

NPD Hydrogen 2~4 ml/min

Air 40~80 ml/min

Make-up(N2, He) 10~20 ml/min not necessary

PID Make-up 5~10 ml/min

Sheath 30~40 ml/min

FPD Carrier 1~3 ml/min 30~50 ml/min

Hydrogen 85~100 ml/min 100~120 ml/min

Air 100~120 ml/min 110~135 ml/min



Inlet requirements

1. Temperature controlled

2. Low volume (total swept by carrier )

3. Inert construction

Column Overload

If too large an sample were allowed to enter small bore(capillary) columns

column overload and a loss of resolving power would like occur.



Liner

All liners help protect the vaporized sample form contacting the metal wall of the inlet as

sample flows onto the column. Deactivated glass wool may be used as an aid for sample

vaporization, to minimize discrimination based on boiling point, and to provide a surface on

which non0volatiles can be trapped. The simplest liner is a straight tube, which gives all-

around good performance at low cost. Single-taper liners improve on a straight tube byminimizing sample vapor contact with metal at the bottom of the injection port, although

they are somewhat more expensive. Liners are deactivated borosilicate glass, except quartz

where noted. Liners are guaranteed inert for phenols, organic acids and bases.

Why is Glass Wool Added to an Injector Liner? - General GC

The Glass wool serves three major purposes.

The Glass wool will prevent the small pieces of septa from reaching the column.

The presence of Glass wool will help the injected sample stay in the liner a little

longer which will help the sample to vaporize and mix more thoroughly with the

carrier gas.

If positioned properly it will wipe the outer surface of the syringe needle andim rove the recision of the li uid in ection.



Inlet configuration

1. Direct column inlet --- 1/8 " OD or larger column

sampling syringe is actually inserted into the end of the column

needle guide / cap / spring / septum

mounting holes / carrier gas in / inlet body column

Swagelock ferrules / Swagelock nut

2. Splitter inlet --- open tubular column or less than 1/8" OD column

Because of the limited capacity for sample of these small bore columns and

the difficulty of injecting extremely small volume samples, a large portion of

the injected sample is vented to atmosphere by the inlet.

septum / preheated carrier gas / mixing tube / restrictor

buffer volume / tapered needle / gold gasket / column fitting



Injection port for split injection into an open tubular column. The glass liner is slowly

contaminated by nonvolatile and decomposed samples and must be replaced periodically.

For splitless injection, the glass liner is a atraight tube with no mixing chamber. For dirty

samples, split injection is used and a packing material can be replaced inside the liner to

adsorb undesirable components of the sample.



Common injection techniques

1) Hot flash vaporization

Direct

Cold-trap

Split

Splitless

2) Direct cold : on column

Split or on-column

Split

1) Simple

2) High column efficiency3) Column may be protected

On-column

1) Best accuracy

2) Thermolabile compounds

3) Trace analysis



Representative injection conditions for split, splitless, and on-column injection into an

open tubular column.



Direct injector1) Good sensitivity

2) Low column efficiency

3) Best for thick films, widebore column ( 0.53 mm )

Hot on-column injectors1) Reduced column efficiency

2) Best with thick films, widebore columns

3) Nonvolatiles may damage column4) Cold on-column injector may be used with 0.1 to 0.53 mm i.d. columns

Advantage of on-column1) Best reproducibility : Quantitative results

2) No split, no loss of high boilers3) "Cold" on-column injection available



Advantage of splitless

1) High sensitivity ( 95 % of sample on column )2) Solvent effect produces narrow sample bands

3) Same hardware as split injection

Disadvantage of splitless

1) Slow sample transfer to column

2) Must dilute sample with volatile solvent

3) Time consuming : must cool column

4) Poor for thermolabile compounds



Split and splitless injections of a solution containing 1 vol % methyl isobutyl ketone

(bp 118 oC) and 1 vol % p-xylene (bp 138 oC) in dichloromethane (bp 40 oC) on a BP-

10 moderately polar cyanopropyl phenyl ,ethyl silicone open tubular column(0.22

mm I.d., 10 m long, 0.25 m, column temperature =75 oC).

C i j i



Common injection methods

Syringe injection

Valve injection

Sampling Syringe

0 ~ 1 μL --- the sample is totally

confined to the needle

0 ~ 5 / 0 ~ 10 μLneedle / barrel / plunger

Gas sampling valve

Sample inSample loop --- 1/4 or 10 mL loop size

compatible with needs

Sample vent

Carrier gas to column



Solvent effect

t-1 t-2

time t-1 --- just after injection, solvent and sample are condensed in a

long plug at the front of the column.

The column temperature must be cold enough to condense the

solvent.

time t-2 --- after some time, the column temperature has been raised,

most of the solvent has evaporated, and the solvent effect

has left the sample molecules concentrated in a narrow band.

As the column is further heated, the remaining solvent andsample molecules are rapidly vaporized

--- resulting in high column efficiency and narrow peak.



Syringe for solid phase microextraction.

Sampling by SPME and desorption of analyte from the coated fiber into a gas

chromatograph.



Purge and trap apparatus for extractingvolatile substances from a liquid or solid

by flowing gas.



GC column

Parts of Column

1) Tubing material

Stainless steel--- reactive ( steroids, amines, free acids )

Glass ------------ can be made inert, difficult handling

Fused silica ---- flexible

most inertmost popular

high resolution

2) Stationary phase

Solid support --- carefully sized granular

Liquid phase --- active portion of the column

Porous polymers

Adsorbents



Important column parameters

1) Inside diameter

2) Length

3) Film thickness

4) Stationary phase composition

5) Flow rate



Column diameter

i.d. Resolution Speed Capacity Ease

100 micrometer +++ +++ + +

(narrow bore )

250, 320 ++ ++ ++ ++

(mid bore)530 + ++ +++ +++

(wide bore)

Column length

N œ L R œ L1/2 t R œ L

Col mn



24-foot 1/8" packed column

wound on 6" coil 6-foot 1/4" packed column

wound on 5" coil

60-meter 0.53mm metal

wide bore

capillary column wound on

3.5" coil

15-meter 0.53mm fused silica

wide bore capillary column

wound on 7" cage30-meter .25mm metal narrow

bore capillary column wound

on 3.5" coilhttp://www.srigc.com/catalog/columns.htm

ColumnGlass wool --- both ends of the column

1- ½” (inlet side)

1/4” (detector side)



Fused silica surface made by the reaction of SiCl

4and water vapor

in a flame

- SiO2 contains 0.1 % – OH groups

- Very inert

- Uniform chemical surface

Fused silica - High tensile strength

- Flexible

- Sheath of polyimide

- Very inert



Fused Silica Capillary Columns A fused silica capillary column is comprised of three major parts (Figure 1).

Polyimide is used to coat the exterior of fused silica tubing. The polyimide protects

the fused silica tubing from breakage and imparts the amber-brown color of columns.

The stationary phase is a polymer that is evenly coated onto the inner wall of thetubing. The predominant stationary phases are silicon based polymers

(polysiloxanes), polyethylene glycols (PEG, Carbowax) and solid adsorbents.

F igure 1.

Capillary columns have to be properly

installed to maximize their performance

and lifetime. You can obtain enhanced

column performance and lifetime by

following these recommended installation

guidelines. More detailed installation,

operational and troubleshootinginformation can be found in the following

references



WCOT = Wall Coated Open Tubular

invented and patented by Dr Marcel Golay

Tubing - Fused silica

- Glass

- Stainless steel

Liquid phase coating

WCOT - - - High resolution

Film thickness : 0.5 to 5.0 micrometer

i.d. : 0.10, 0.25, 0.32, 0.53 mm

Length : 10 to 60 m

Open tubular GC column

O ti l id li f t b l GC l



Operational guideline for open tubular GC columns

WCOT

narrow intermediate wide bore

Column inner diameter, mm 0.25 0.32 0.53

Maximum sample volume, l 0.5 1 1

Maximum amount for

one component, ng 2~50 3~75 5~100

Effective plates(Neff ) per meter 3000~5000 2500~4000 1500~2500

Trennzahl(separation) number

per 25 m 40 35 25

Optimum flow for N2, ml /min * 0.5~1 0.8~1.5 2~4

Optimum flow for He, ml /min ** 1~2 1~2.5 5~10

Optimum flow for H2, ml /min *** 2~4 3~7 8~15

* Optimum velocity is 10 to 15 cm/s for each column

** Optimum velocity is 25 cm/s for each column

*** Optimum velocity is 35 cm/s for each column

Oth t f ill l



Other types of capillary columns

SCOT = Support Coated Open Tubular

Solid support : Celite

Liquid phase

Not available fused silica tubing

PLOT = Porous Layer Open Tubular

Porous adsorbent : alumina or molecular sieve

* Molecular sieve --- efficient for H2, Ne, Ar, O2, N2, CO, CH4.

Porous carbon stationary phase ( 2 m

thick) on inside wall of fused silica

open tubular column.



Capillary column vs Packed column

Capillary Packed

Length 60 m 2 m

Theoretical plates( N/m) 3000-5000 2000

Total plates

length×( N/m)

180000-300000 4000

Advantages of capillary column

1) Not packed : long lengths : high resolution

2) Thin film ; efficient, fast

3) Used silica --- inert surface, better results

Disadvantages of capillary column

1) More expensive

2) Limited liquid phase

Requires very small samples

3) Dedicated instrumentation ---

capillary inlets, septum purge



(Left) Gas chromatogram of alcohol mixture at 40o

C using packed column ( 2mmI.D., 76 cm long containing 20 % Carbowax 20 M on a Gas-Chrom R support and

FID.

(Right) Chromatogram of vapors from headspace of beer can, obtained with 0.25 mm

diameter, 30 m long porous carbon column oerated at 30 oC for 2 min and then

ramped up to 160 oC at 20 oC/min.



Component Separation with the Column

< The process of separation >

A series of partitions : Dynamic In-and Out (or Stop-and-Go)

All differential migration process.

The most volatile components usually pass through the column

first, the least volatile or highest boiling emerges last.

Mobile phase( Driving force)

Stationary phase ( Resistive force)

Analytes



Capillary Column Installation Steps

1. Check traps, carrier gas, septum, liner 2. Place the nut and ferrule on the column and carefully cut the column end

3. Install the column into the injector

4. Turn on the carrier gas

5. Install the column into the detector

6. Inspect for leaks7. Confirm carrier gas flow and proper column installation

8. Condition the column

9. Accurately set the carrier gas velocity

10. Bleed test

11. Run test mixClick here for a complete listing of tools available from J&W, including magnifiers and cutting tools.

http://www.jandw.com/gccolumn.htm#Fused Silica Capillary Columns



Recommended Installation Tools and Supplies

1. Cutting tool such as a diamond or carbide tipped pencil, sapphire tipped pencil or ceramic cleaving wedge

2. Magnifier (10-20X)

3. Ruler

4. Wrench

5. Ferrules

6. Vial of solvent7. Clean syringe

8. Supply of an appropriate non-retained compound

9. Column test mixture

10. Flow meter

11. Other supplies: septa, clean injector liners, liner ferrules/O-rings, etc.

C diti i f th C l



Conditioning of the Column

Once the column has been checked for proper installation and the absence of

leaks, it is ready for conditioning. Heat the column to its isothermal, upper

temperature limit (temperature limits listed below) or a temperature 10-20 oC

above the highest operating temperature of your particular method. Do not exceedthe upper limit or column damage will result. Heat the column rapidly - slow

temperature programming is not necessary. After the column has reached the

conditioning temperature, plot the baseline. Keep the baseline on scale so that it

can be observed. The baseline should be elevated at first then start to drop after 5-

10 minutes at the conditioning temperature. The baseline will continue to drop for 30-90 minutes then stabilize at a constant value. If the baseline does not stabilize

after 2-3 hours or does not start to significantly decrease after 15-20 minutes,

either a leak is present or a contamination problem exists. In either case,

immediately cool the oven down below 40 oC and resolve the problem. Continued

conditioning will result in column damage or the inability to obtain a stable

baseline. Excessive conditioning of the column may result in a shortened lifetime.

In general, polar stationary phases and thick film columns usually require longer

times to stabilize than less polar and thinner film columns. GS PLOT columns

require a different conditioning procedure than liquid stationary phase columns.



Temperature Limits

The temperature limits define the range over which the column can be safelyused. If the oven is operated below the lower temperature limit, poor

separation and peak shape problems will be evident, but no column damage

will occur.

Upper temperature limits are usually given as two numbers. The first or

lower temperature of the two is the isothermal limit. The column can bemaintained at this temperature for indefinite periods of time. The second or

higher temperature is the program limit. The column can be heated to this

temperature for short periods of time (<10 minutes). Exceeding the upper

temperature limits will significantly reduce column lifetime.



Liquid Phase(=Stationary phase) Classes

1. Non-polar phase gives boiling point order separation

2. Selective phase separates components that have close b.p. and

small structural differences

3. Polar phase depends on internal functional groups to separate

compounds that have reactive – OH, – NH2 or other polar radicals

4. Each stationary phase retains solutes in its own class best

Raising percentage of stationary phase leads to

1) Greater capacity for solute

2) Longer retention time

3) Increased HETP



Column Temperature

Dimethylpolysiloxane

DB-1; 0.1, 0.25 and 1.0 m -60 to 325/350 oC

DB-1; 3.0 and 5.0 m -60 to 280/300

o

CDB-1; Megabore 0.1 m -60 to 360 oC

DB-1; Megabore 1.5 m -60 to 300/320 oC

DB-1; Megabore 3.0 and 5.0 m -60 to 260/280 oC

DB-1ht -60 to 400 oC

(5%-Phenyl) Methylpolysiloxane

DB-5 -60 to 325/350 oC

DB-5; Megabore -60 to 300/320 oC

DB-5; Megabore 5.0 m -60 to 260/280 oC

DB-5ht -60 to 400 oC

DB-5ms -60 to 325/350 oC

DB-5ms; Megabore -60 to 300/320 oC

DB-5.625 -60 to 325/350 oC

Polyethylene Glycol

DB-WAX 20 to 250/260 oC

DB-WAX; Megabore 20 to 230/240 oC

Base Deactivated Polyethylene Glycol

CAM 60 to 220/240 oC

Carbowax 20M

Carbowax 60 to 220/240 oC



-Cyclodextrin for chiral column.

Column oven and temperature control



Column oven and temperature control

Oven size : column sits in an oven

Inner volume : laboratory GC ; about 22-25 L

process GC ; about 40 L

Isothermal

The oven temperature is kept constant during the entire analysis

Practical temperature < 250oC

Maximum temperature < 400oC

Temperature programming

The oven temperature is varied during the analysis

Linear

Non linear

High temperature GC (HTGC)

Working limit : conventional GC --- 330oC

HTGC --- 450oC

Masses of analysed substrates: 600-1000





With homologues, the retention time increases exponentially with the number

of carbon.

As retention time increases, width increase and the height decreases, making

detection impossible after a few peaks have eluted.

Since solubility of gas in a liquid decreases as temperatures goes up, we can

reduce the retention of a compound by increasing column temperature.

General steps to create a program assuming that the separation is possible

1) Determine initial temperature and time based on best possible separation

offirst few peaks

2) Report for the last few peaks to find the best final temperature and time

3) Experiment with various ramps to account for the rest of the components




Factors to consider :

Variations in solubility of solutes

Changes in volatility of solutes

Stability of solutesFlow rate changes

Stability of stationary phase

Must stay within Tmin/Tmax of column

Other factors are found experimentally

A temperature program



A temperature program

Ex.

40 oC(5 min) – 10oC/min – 250oC(10 min)

A: initial temperature and holding time

B: ramp (oC/min)

C: final temperature and holding time

Some GCs will allow for a more complex program.

A

B

C

Raising column temperature

1) Decrease retention time

2) Decrease resolution

3) Sharpens peaks



Comparison of isothermal and programmed temperature chromatography. Each sample contains linear

alkanes run on a 1 6 mm × 6 m column containing 3% Apiezon L (liquid phase) on a 100/200 mesh

F AC 31 08 [a] Gas Chromatogrphy

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