Gas Chromatography:

In gas chromatography, the mobile phase is a gas. The gas does not react with the molecules of the analyte. It just carries the sample by the column. Two categories of gas chromatography are encountered

• Gas-solid chromatography (GSC).
• Gas-liquid chromatography (GLC).

In GSC, the retention of the analyte is due to physical adsorption on the column material. In GLC, the separation takes place because of partition between the gaseous mobile phase and the liquid immobilized on the surface of an inert solid. GSC is not as popular as GLC because of some limitations of the former mode. In GSC, the retention time may be unusually large and high temperatures may be required to desorb the species. There might be severe tailing because of non-linear character of adsorption. The active adsorbent might catalyze the adsorbed species and change it to some other compound. As a conclude of all these GSC has a limited utility and gas chromatography more or less refers to gas-liquid partition chromatography. Generally, two types of columns are used in gas chromatography, packed and open tubular or capillary columns. A chromatographic column varies within length from less than 2m to 50m or more. That are constructed of stainless steel, aluminium glass or fused glass or teflon. Open tubular columns are two basic types, wall-coated open tubular and support-coated open tubular. Wall coated columns are capillary tubes coated along with thin layer of stationary phase. Within support coated open tubular columns an inner surface is lined with a thin film of support material. That support material contains the stationary phase. The most commonly used stationary phases are polydimethyl siloaxanes. Organic species bonded to a solid surface are also used as stationary phases.