Gas Chromatography:

In the previous units (i.e. Units 4 and 5), it was emphasized in which a reasonable categorization of chromatographic techniques could be proposed on the basis of the nature of the mobile phase. The Liquid chromatography, whereas the mobile phase is liquid, has several diversifications depending upon the shape of support, nature of support and the mechanism responsible for separation. If gas is the mobile phase, the chromatographic methods are called as gas chromatography (GC). It is used for the separation of gaseous or volatile substances. It has just two sub-classifications namely gas-solid chromatography (GSC) and gas- liquid chromatography (GLC). Therefore, in terms of importance and applications, it is not second to any chromatographic method. It may be significant to point out here that the theoretical aspects of chromatographic methods have been developed taking gas chromatography as the model.

Within gas chromatography, the basis for separation is the distribution of solutes between two phases. A single of these phases is a stationary bed of huge surface area (stationary phase) and the other is a gas (mobile phase) that percolates by the stationary phase. If the stationary phase is a solid adsorbent, then it is called as gas- solid chromatography (GSC). Now, the adsorptive properties of the stationary phase are responsible for separating solutes, primarily gases. General solid stationary phases are silica gel, porous polymers, molecular sieves, alumina and charcoal. In case, the stationary phase is liquid, it is known as gas- liquid chromatography (GLC). The liquid is spread (coated) as thin film over an inert support. The primary of separation is the partitioning of the solutes in and out of the liquid film. There is a wide range of the liquid phases with usuable temperatures up to 400ºC. That makes GLC the most versatile and a selective form of chromatography. It is used for analysis of gases, solids and liquids.