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Gas Chromatography

One of the most common analytical tools for the separation of components of a mixture is known as chromatography. Other separation methods are distillation, crystallization, separating funnel etc. These methods are also good enough but chromatography gives more precise results compare to other methods.

Chromatography can be done in various types like paper chromatography, column chromatography, gas chromatography, gas-liquid and high performance liquid chromatography. In this article we will discuss about gas chromatography.

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What is Gas Chromatography?

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An analytical instrument which can measure the content of various components in a sample through gas chromatograph is called as gas chromatography. In gas chromatography the sample solution is injected in an instrument and transport into separation tube. This separation tube is called as column. The gas stream acts as carrier like He or $N_{2}$ gas.

Here various components of sample mixture are separated inside the column and measure the quantity of the components by detector. The concentration of unknown sample can be measured with the help of a standard sample of known concentration which is also injected into the instrument. The standard sample peak retention time and area are used to calculate the concentration.

Gas Chromatography Theory

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Gas chromatography like gas-liquid chromatography is based on the principal of vaporization of the sample. The vaporized sample is injected onto the head of the chromatographic column and transported through the column by the flow of inert gas or carrier gas like helium or other inter gases. The gas chromatography column contains a liquid stationary phase. The stationary phase adsorbs onto the surface of an inert solid. 

Gas Chromatography Column
                   
The carrier gas that is used in gas chromatography must be inert and should not react with sample or components. Nitrogen, helium, argon, and carbon dioxide mainly used as carrier gas in gas chromatography method. The carrier gas system also contains a molecular sieve which helps to remove water and other impurities from the system.

The sample injection port determines the efficiency of chromatographic method. The sample solution should not be too large and should not be injected too slowly as it may cause band broadening and loss of resolution.  Usually a micro syringe is used to inject sample through a rubber septum. Sample first injected into a flash vaporizer port which is placed at the head of the column. The temperature of sample port must be 50°C higher compare to boiling point of the least volatile component of mixture. In a packed column, the size of sample must be from 1/10th of a microliter up to 20 microliters.

There are two modes of injection of sample in injector. One os split and another is splitless. There is a heated chamber in injector which contains a glass liner. The sample is injected into glass liner through the septum. As the carrier gas enters in the chamber, in split mode of injector gas can leave by three routes. The sample vaporizes and forms a mixture of carrier gas. Mixture passes onto the column and most of it exits through the split.

Gas Chromatography Analysis

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The analysis of sample occurs in the gas chromatography column which can be two types; packed and capillary. The packed columns contain a finely divided, inert, solid support material which is coated with liquid stationary phase. The length of packed columns is 1.5 - 10m with diameter of 2 - 4mm. In case of capillary columns, the internal diameter is a few tenths of a millimeter. Capillary columns can be wall-coated open tubular or support-coated open tubular types. Capillary tube wall-coated columns are coated with liquid stationary phase whereas support-coated columns are lined with a thin layer of support material.

The temperature of columns must be controlled. It must lie within tenths of a degree. The optimum column temperature depends on the boiling point of the sample. If the temperature of column will be slightly above the average boiling point, the sample results in an elution time of 2 - 30 minutes.
Low temperature gives good resolution with high elution times. As the temperature of sample increases, the separation proceeds.

Gas Chromatography Detectors

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There are different types of detectors that can be used in gas chromatography. They have different types of selectivity. A gas chromatography detector which is non-selective can be respond to all compounds. But non-selective detector could not respond to the carrier gas.

However selective detector can be used for a range of compounds which has common physical or chemical property. A specific detector can be used to a single chemical compound only. On the basis of mass and concentration, detectors can be mainly two type concentration dependent detectors and mass flow dependent detectors.

In the concentration dependent detector depends on the concentration of solute and does not destroy the sample. Mass flow dependent detectors destroy the sample and the signal relates to the rate of solute molecules to enter in the detector. The mass flow dependent detector does not depend on the make-up gas.

Advantages of Gas Chromatography

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Gas chromatography is used for the separation of volatile substances from mixture which is very difficult to separate and analyze. Gas chromatography method has several advantages like this method has a high resolution compare to other chromatographic methods. It has high sensitivity with thermal detectors and gives relatively good accuracy and precision. It is one of the fastest method of separation and analysis of sample in which sample can be used with less quantity.
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