Gas Chromatography

Gas chromatography (GC) is a method of chemical analysis for separating chemicals in a complex sample. This technique uses a flow through narrow tube known as column through which different chemical constituents of a sample in a gas stream (mobile phase; carrier gas) at different rates depending on their various chemical and physical properties and their interaction with a specific column filling called stationary phase. As the sample exit the end of column, they are detected and identified electronically. The function of stationary phase in column is to separate different components, causing each one to exit the column at different times (retention time). Other parameters that can be used to alter the order of time of retention are
1. Carrier flow rate
2. Temperature

In GC analysis, a known volume of gaseous or liquid analyte is injected into the “entrance” (head) of column usually using a micro-syringe. As the carrier gas sweeps the analyte molecules through column, this motion is inhibited by absorption f the analyte molecules either into the column walls or onto packing materials in the column. The rate at which molecules progress along the column depends on strength of adsorption, which in turn depends on type of molecule and the stationary phase materials.

Since, each type of molecule has different rate of progression, the various component of analyte mixtures are separated as they progress along the column and reach end of column at different time (retention time). A detector is used to monitor the outlet stream from the column, thus the time at which each component reaches the outlet and the amount of that component can be determined. Generally substances are identified qualitatively by the order in which emerge (elute) from the column and by the retention time of analyte in column.

Uses of Gas Chromatography
1. Determination of volatile compounds (gases or liquid)
2. Determination of partition coefficients and absorption isotherm
3. Isolating pure components from complex mixtures