GAS CHROMATOGRAPHY AND ITS

INSTRUMENTATIONPrepared by:-ARGHA SEN

M.PHARM( PHARMACEUTICAL ANALYSIS)BENGAL SCHOOL OF TECHNOLOGY

IntroductionThe suggestion that separation of components of a

mixture in the gaseous state could be achieved using a gaseous mobile phase was first Martin and Synge in 1941.

The first description of instrumentation and application was made by James and Martin in 1952.

Gas chromatography is a technique used for separation of volatile substances, or substances that can be made volatile, from one another in a gaseous mixture at high temperatures.

Types of GCGas Solid Chromatography(GSC) The stationary phase, in this case, is a solid. It is the affinity

of solutes towards adsorption onto the stationary phase which determines, in part, the retention time. The mobile phase is, of course, a suitable carrier gas. This gas chromatographic technique is most useful for the separation and analysis of gases like CH4, CO2, CO, ... etc.

Gas Liquid Chromatography(GLC) The stationary phase is a liquid with very low volatility

while the mobile phase is a suitable carrier gas. GLC is the most widely used technique for separation of volatile species.

Instrumentation

Carrier GasA carrier gas should have the following properties:Highly pure (> 99.9%): inert: higher density: compatible with the detector: cheap and available.The carrier gas pressure ranges from 10-50 psi. Depending on the column dimensions, flow rates from 1-150 mL/min are reported. Conventional analytical columns usually use flow rates in the range from 20-50 mL/min while capillary columns use flow rates from 1-5 mL/min. Commonly used gases include nitrogen, helium, argon, and carbon dioxide.

Sample Injection SystemsSeptum type injectors are the most common. These are

composed of a glass tube where vaporization of the sample takes place.

The sample is introduced into the injector through a self-sealing silicone rubber septum.

The carrier gas flows through the injector carrying vaporized solutes.

The temperature of the injector should be adjusted so that flash vaporization of all solutes occurs. If the temperature of the injector is not high enough (at least 50 degrees above highest boiling component), band broadening will take place.

Carrier Gas

Syringe

Vaporization Chamber

To Column

Septum

Automatic SamplerSample vials are glass, throw away type with vapor-

tight septum caps.The sampler flushes the syringe with new sample to

remove traces of previous sample.Pumps new sample to wet the syringe to remove any

bubbles, takes in a precisely measured sample and pumps in to the gas chromatograph.

Purge and Trap samplingVolatile organic samples can be purged from the sample and

trapped on Tenax GC contained in an 11 cm tube.Tenax-GC is a porous polymer based on 2,6-diphenyl-p-

phynelene oxide.Trappe samples can be easily stored and sent to another site for

analysis.Desorption fro Tenax occurs with helium flow at 300o C.The desorped volatiles are then collected in a precolumn cooled

by dry ice.The per column is then connected to the GC column , the dry

ice is removed and the analysis is started at room temperature.

Head Space Technique. It involves analysis of volatile components in a complex and viscous mixture containing high proportion of non-volatile components.

For quantitative analysis calibration of the volatiles in the vapour is necessary. To reach this state the sample is placed in a

glass vial and thermostatted. When equilibrium is achieved, an aliquot of the gas phase above the sample is rapidly transferred onto the GC column.

All the devices that are commonly used for gas sampling may be applied to headspace analysis, including gas-tight syringes and gas-sampling valves.

Principle of headspace sampling by either direct on-column sampling or by pressure/loop-filling with previous pressurization of the headspace vial.

Principle of cryogenic headspace trapping with splitless on-column headspace sampling.

Head space Gas Chromatography

Columns The column in chromatography is undoubtedly the heart of the

technique. A column can either be a packed or open tubular.

PACKED COLUMNS These columns are fabricated from glass, stainless steel, copper, or other

suitable tubes. Stainless steel is the most widely used because it is most inert and easy to

work with. The column diameters currently in use are ordinarily 1/16" to 1/4" 0.D.

Columns exceeding 1/8" are usually used for preparative work while the 1/8" or narrower columns have excellent working properties and yield excellent results in the analytical range.

Column length can be from few feet for packed columns to more than 100 ft for capillary columns.

ColumnsOPEN TUBULAR/CAPILLARY COLUMNS

Open tubular or capillary columns are finding broad applications. These are mainly of two types:

• Wall-coated open tubular (WCOT) <1 mm thick liquid coating on inside of silica tube

• Support-coated open tubular (SCOT) 30 mm thick coating of liquid coated support on inside of silica tube.

The most frequently used capillary column, nowadays, is the fused silica open tubular column (FSOT), which is a WCOT column.

The external surface of the fused silica columns is coated with a polyimide film to increase their strength.

Stainless steel packed column

Capillary columns

Support materials and Stationary Phases

The solid support should ideally have large surface area (at least 1 m2/g), has a good mechanical stability, thermally stable, inert surface in order to simplify retention behavior and prevent solute adsorption, has a particle size in the range from 100-400 mm.Examples:- diatomaceous earth, glass beads with suitable mesh size( 80-100 mesh, 100-120 mesh). A liquid stationary phase should be inert to the analyte, less volatile, and

thermally stable. In general, the polarity of the stationary phase should match that of the sample

constituents ("like" dissolves "like"). Most stationary phases are based on polydimethylsiloxane or polyethylene glycol (PEG) backbones:

Support materials and Stationary Phases

The polarity of the stationary phase can be changed by derivatization with different functional groups such as a phenyl group. Bleeding of the column is cured by bonding the stationary phase to the column; or crosslinking the stationary phase.

Liquid Stationary Phases should have the following characteristics:

• Low volatility.• High decomposition

temperature (thermally stable).

• Chemically inert (reversible interactions with solvent).

• Chemically attached to support (to prevent bleeding).

• Appropriate k' and a for good resolution.

Temperature programmingGas chromatographs are usually capable of performing what is known as

temperature programming gas chromatography (TPGC).TPGC is a very important procedure, which is used for the attainment of

excellent looking chromatograms in the least time possible. Isothermal - Keep oven at one temp thru run. Not very useful. Possibly

useful for series of very similar compounds differing by boiling points such as alcohols ( MeOH, EtOH, n-PrOH, i-PrOH, BuOH, i-BuOH)

Gradient - temp profile: 40 deg hold for 10 min then 10deg/min to 240 deg and hold there for 20 min. Advantages: 1- resolution and 2- analysis time.

0

40

80

120

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240

0 10 20 30 40 50 60

Time (min)

Tem

p (

deg

C)

Detection Systems Several detectors are

available for use in GC. Each detector has its own characteristics and features as well as drawbacks. Properties of an ideal detector include:

1. High sensitivity.2. Minimum drift.3. Wide dynamic range.4. Operational temperatures up

to 400 oC.5. Fast response time.6. Same response factor for all

solutes.7. Good reliability (no fooling).8. Nondestructive9. Responds to all solutes

(universal ).

Types of Gas Chromatography Detectors

Non-selective Responds to all compounds present in carrier gas stream except the carrier

gas itself

Selective Responds to range of compounds with a common physical or chemical

characteristic

Specific Responds to a single specific compound onlyDetectors can also be grouped into concentration or mass flow detectors

Concentration DependentThe response of such Gas Chromatography detectors is proportional to the

concentration of the solute in the detector such as TCD. Dilution of sample with makeup gas will lower detector response.

Mass Flow DependentSignal is dependent on the rate at which solute molecules enter the detector

such as FID. Response of such detectors is not affected by makeup gas flow rate changes.

Common GC Detectors

Flame Ionization Detector (FID)

Mass sensitive detector   Response depends on conducting

power of ions or electrons produced on burning of organic compounds in the flame 

 Selective detector but sample detected must be combustible 

 Large linear dynamic range (107)  No response to inorganic and

permanent gases such as CO, CO2, NH3, CS2, N2, etc.

It is the most widely used detector in Gas Chromatography 

Thermal Conductivity Detector (TCD)

Non-destructive universal detector 

 Response depends on the thermal conductivity difference between the carrier gas and the eluted components 

 Wide dynamic range (107 – % to ppm levels) 

 Responds also to inorganic gases such as CO, CO2, NH3, CS2, N2, etc.

Sample is not wasted Easy to operate

Electron Capture Detector (ECD)

The ECD ionizes the carrier gas by means of a radioactive source. The potential across two electrodes is adjusted to collect all the ions and a steady saturation current, is therefore, recorded.

Electrons from β-source ionize carrier molecules capture electrons and decrease current ; Simple and reliable ; Sensitive (10-15 g/s) to electronegative groups (halogens, peroxides) ;Largely non-destructive ; Insensitive to amines, alcohols and hydrocarbons ; Limited dynamic range (102).

Applications Qualitative Analysis . Quantitative Analysis. Separation of fatty acids derived from fixed oils Miscellaneous-analysis of foods like carbohydrates, proteins, lipids, vitamins,

steroids, drug and pesticides residues, trace elements

Pollutants like formaldehyde, carbon monoxide, benzen, DDT etc

Dairy product analysis- rancidity

Separation and identification of volatile materials, plastics, natural and synthetic polymers, paints, and microbiological samples

Inorganic compound analysis Residual solvent analysis.

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