Gas Chromatography - opengeology.in

Gas Chromatography is a chromatographic technique used to separate volatile compounds and those which can vaporized by heat. If a mixture of compounds is analysed using GC system, each compound can be separated and quantified. It is useful and important because of following –

High resolution, accuracy, precision
High sensitivity
Quick analysis
Small sample (µl – µgm)

Components of gas chromatography

This is the overall main components of a gas chromatography system.

(Philadelphia College of Pharmacy, 2012)

Now let us discuss them one by one…

Carrier gas in a high pressure cylinder : Carrier gas are those which carries our sample. He, N2, Argon are used as carrier gasses.

Helium is preferred for thermal conductivity detectors because of its high thermal conductivity relative to that of most organic vapors.
Flow rate is adjusted by means of a needle valve mounted on the base of the flow meter and controlled by capillary restrictors.
The operating efficiency of the gas chromatograph is directly dependent on the maintenance of constant gas flow.

Sample injection system : Typical sample volumes range from 0.1 to 0.2ml

Liquid samples are injected by a micro syringe with a needle inserted through a self-scaling, silicon-rubber septum into a heated metal block by a resistance heater.
Gaseous samples are injected by a gas-tight syringe or through a by-pass loop and valves.

The separation column : It is the heart of gas chromatography that is made up of metals that is coiled into an open spiral or a flat pancake shape. Columns can be Packed column (glass or stainless steel) or Capillary column (fused quartz).

Column is placed in a chamber so that uniform temperature can be maintained.
Several size of columns are used depending on the requirements.
Column has two phases – Stationary phase of silicon grees / vax and a mobile phase which moves with inert gas.

Temperature programming : Rising column temperature decrease retention times. Fixed temperature separation = isothermal separation

Liquid phase : An infinite variety of liquid phases are available limited only by their volatility, thermal stability and ability to wet the support. NO single phase will serve for all separation problems at all temperatures.

Non Polar : These materials separate the components in order of their boiling points.
Specific purpose phases : Relying on a chemical reaction with solute to achieve separation. AgNO3 in glycol separates unsaturated hydrocarbons.

Detector : Detectors sense the arrival of the separated components and provide a signal.

These are either concentration dependent or mass dependent.
The detector should have adequate sensitivity and fast response time.
The detector should be close to the column exit and the correct temperature to prevent decomposition.
Thermal conductivity detector: It is universal detector, Thermal conductivity of gas decreases with analyte present.
Flame Ionisation detector : Excellent for organic molecules, non responsive to H2O, CO2, SO2, NOx (as shown in image)

Recorder : The recorder should bw connected with a series of good quality resistances connected the input to attenuate the large signals.

Procedure of the Gas Chromatography

A small amount of sample is injected with the help of syringe quickly. The injection must be a point in time and entire sample must go at the same time.
The temperature of the injection port must be 20-40o degree so that sample could evaporate quickly.
The vaporized sample that will mix with the inert gas mobile phase to be carried to the gas chromatography column.

Components which are in the mixture are separated based on their abilities to adsorb on or bind to, the stationary phase. It depends on Boiling point of compound, Solubility in the liquid phase, Temperature of the column. “as shown in image”
A component that adsorbs most strongly (Let’s assume ‘A’) to the stationary phase will spent most time in column and will have the longest retention time.
A component that adsorbs the least strongly (Let’s assume ‘B’) to the stationary phase will spend the least time in the column and will have shortest retention time.

The components of the mixture reach the detector at different times due to differences in the time they are retained in the column.
The component that is retained the shortest in the column (B) is detected first. The component that is retained the longest time in the column is detected last (A).

Limitations :

Unknown samples can not be detected, we must have reference peaks. (like methane peak comes at 5min. so we can take reference).
Analyst must be thermally stable.
Analyst must be relatively volatile.
Not possible to operate at biological conditions.

Possible interview Questions :

What is makeup gas in GC ?

What is zero air in GC ?

On what basis molecules separates ?

What if molecules have same boiling point ?

What is split ratio in GC ?

Callibration used in GC ?

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