Gas Chromatography GC

 GAS CHROMATOGRAPHY (GC)  

Principle: Here an inert carrier gas (Helium or Nitrogen) acts as the mobile phase. This will 

carry the components of analyte mixture and elutes through the column. The column usually 

contains an immobilized stationary phase. The technique can be categorised depending on the 

type of stationary phase as follow: 

Gas Solid Chromatography (GSC) -  here the stationary phase is a solid which has a large 

surface area at which adsorption of components of the analyte takes place. The separation is 

possible based on the differences in the adsorption power and diffusion of gaseous analyte 

molecules. The application of this method is limited and is mostly used in the separation of 

the low-molecular-weight gaseous species like carbon monoxide, oxygen, nitrogen and lower 

hydrocarbons.

Gas Liquid Chromatography (GLC) - this is the most important and widely used method for

separating and determining the chemical components of volatile organic mixtures. Here the

stationary phase is a liquid that is immobilized on the surface of a solid support by adsorption

or by chemical bonding. The separation of the mixture into individual components is by

distribution ratio (partition) of these anayte components between the gaseous phase and the

immobilized liquid phase. Because of its wide applications most of  the GCs are configured

for the GLC technique.  

Instrumentation :  The instrumentation for GC is different from that of HPLC in that the

injection port, column and detector are to be heated to a pre-specified temperature. Since the

mobile phase here is a gas (carrier gas) the components present in the analyte mixture should

be vaporised, so that it can be effectively carried through the column. The basic

instrumentation for GC includes a carrier gas cylinder with regulator, a flow controller for the

gas, an injection port for introducing the sample, the column, the detector and the recorder.

An outlay is as follow:

In the above illustration the  injection port, column oven and detector are hot zones. The

success of this technique requires the appropriate selection of the column and the temperature

conditions at which the column to be maintained throughout the analysis. Basically the

columns for GC are classified as analytical columns and preparative columns. The analytical

columns are of two types: packed column and open-tubular or capillary column. Both differ

in the way the stationary phases are stacked inside.

In the instrumentation of GC detectors play  unique role. There are a number of detectors,

which vary in design, sensitivity and selectivity. Detectors in GC are designed to generate an

electronic signal when a gas other than the carrier gas elutes from the column. Few examples

and applications of the detectors are:

Thermal Conductivity Detector (TCD) - this operates on the principle that gases eluting from

the column have thermal conductivity different from that of the carrier gas. It is the universal

detector (detects most of the analytes) and is non-destructive and hence used with preparative

GC, but less sensitive than other detectors.

Flame Ionization Detector (FID) -  it is one of the important detectors where the column

effluent is passed into a hydrogen flame and the flammable components are burned. In this

process a fraction of the molecules gets fragmented into charged species as positive and negative.

While positively charged ions are drawn to a collector, negatively charged ions are

attracted to positively charged burner head, this creates an electric circuit and the signal is

amplified. The FID detector is very sensitive, but destroys the sample by burning. It only

detects organic substances that burn and fragment in a hydrogen flame (e.g. hydrocarbons).

Hence its usage is restricted for preparative GC and for inorganic substances which do not

burn.  

Electron Capture Detector (ECD)  -  this is another type of ionization detector which utilises

the beta emissions of a radioactive source, often nickel-63, to cause the ionization of the

carrier gas molecules, thus generating electrons which constitute an electrical current. This

detector is used for environmental and bio-medical applications. It is especially useful for

large halogenated hydrocarbons and hence in the analysis of halogenated pesticide residues

found in environmental and bio-medical samples. It is extremely sensitive. It does not destroy

the sample and thus may be used for the preparative work.

Nitrogen/Phosphorus Detector (NPD) -  the design of the detector is same to that of the FID

detector except that a bead of alkali metal salt is positioned just above the flame. It is also

known as ‘Thermionic Detector’. It is useful for the phosphorus and nitrogen containing

pesticides, the organophosphates and carbamates. The sensitivity for these compounds are

very high since the fragmentation of the other organic compounds are minimized.

Flame Photometric Detector (FPD) -  here a flame photometer is incorporated into the

instrument. The principle is that the sulfur or phosphorus compounds burn in the hydrogen

flame and produce light emitting  species. This detector is  specific for organic compounds

containing sulphur or phosphorus. It is very selective and very sensitive.

Electrolytic Conductivity Detector (ECD Hall) -  this otherwise known as ‘Hall detector’,

converts the eluting gaseous components into ions in liquid solution and then measures the

electrolytic conductivity of the solution in a conductivity cell. The conversion to ions is done

by chemically oxidizing or reducing the components with a “reaction gas” in a small reaction

chamber. This detector is used in the analysis of organic halides and has excellent sensitivity

& selectivity, but is a destructive detector.

The recent developments allow the GC to be  coupled with other analytical techniques like

Infra Red Spectrometry (Gas Chromatography-Infrared Spectrometry, GC-IR)   and    Mass

Spectrometry  (Gas Chromatography- Mass Spectrometry, GC-MS).   These are termed as

‘hyphenated techniques’, and are very efficient for qualitative analysis as very accurate and

precise information like mass or IR spectrum of the individual sample components are readily

obtained as they elute from the GC column. It saves time and reduces the steps involved for a

component to be separated and analysed.

Disadvantages: Samples must be volatile and thermally stable below about 4000

 C. No single

universal detector is available and most commonly used detectors  are non-selective. One

should take much care in the analytical steps  starting from the selection of the column, the

detector and must define the temperatures of all the three ports viz., injection port, column

oven and detector. An improper programming on these will lead to erratic results.