  gas chromatography (gc), is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. typical uses of gc include testing the purity of a particular substance, or separating the different components of a mixture (the relative amounts of such components can also be determined). in some situations, gc may help in identifying a compound. in preparative chromatography, gc can be used to prepare pure compounds from a mixture. in gas chromatography, the mobile phase (or "moving phase") is a carrier gas, usually an inert gas such as helium or an unreactive gas such as nitrogen. the stationary phase is a microscopic layer of liquid or polymer on an inert solid support, inside a piece of glass or metal tubing called a column (a homage to the fractionating column used in distillation). the instrument used to perform gas chromatography is called a gas chromatograph (or "aerograph", "gas separator"). the gaseous compounds being analyzed interact with the walls of the column, which is coated with a stationary phase. this causes each compound to elute at a different time, known as the retention time of the compound. the comparison of retention times is what gives gc its analytical usefulness. gas chromatography is in principle similar to column chromatography (as well as other forms of chromatography, such as hplc, tlc), but has several notable differences. first, the process of separating the compounds in a mixture is carried out between a liquid stationary phase and a gas mobile phase, whereas in column chromatography the stationary phase is a solid and the mobile phase is a liquid. (hence the full name of the procedure is "gasâliquid chromatography", referring to the mobile and stationary phases, 
Gas_chromatography Laboratory_techniques