Gas chromatography (GC), is a type of chromatography in which the mobile phase is a carrier gas, usually an inert gas such as helium or nitrogen and the stationary phase is a microscopic layer of liquid on an inert solid support. The stationary phase lines the inside of a very long very thin tube known as a column.

A gas chromatograph is a chemical analysis instrument for separating chemicals in a sample. A gas chromatograph uses a thin capillary fiber known as the column, through which different chemicals pass at different rates depending on various chemical and physical properties. As the chemicals exit the end of the column, they are detected and identified electronically. The function of the column is to separate different components, causing each one to exit the column at a different time.

In a GC analysis, a known volume of gaseous or liquid analyte is injected into the entrance of the column, usually using a microsyringe. Although the carrier gas sweeps the analyte molecules through the column, this motion is inhibited by the absorption of the analyte molecules either onto the column walls or onto packing materials in the column. The rate at which the molecules progress along the column depends on the strength of absorption, which in turn depends on the type of molecule and on the column materials. Since each type of molecule has a different rate of progression, the various components of the analyte mixture are separated as they progress along the column and reach the end of the column at different times. 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 by the order in which they emerge from the column and by the residence time of the analyte in the column.

Two types of columns are used in GC:

Capillary columns have a very small internal diameter, on the order of a few tenths of millimeters. The column walls are coated with the active materials. Most capillary columns are made of fused-silica with a polyimide outer coating. These columns are flexible, so a very long column can be wound into a small coil

Packed columns contain a finely divided, inert, solid support material (eg. diatomaceous earth) coated with a liquid or solid stationary phase. The nature of the coating material determines what type of materials will be most strongly adsorbed. Thus numerous columns are available that are designed to separate specific types of compounds. Most packed columns are 1.5 - 10 m in length and have an internal diameter of 2 - 4 mm. The outer tubing is usually made of stainless steel or glass.

Because molecular adsorption and the rate of progression along the column depend on the temperature, the column temperature is carefully controlled to within a few tenths of a degree for precise work. Reducing the temperature produces the greatest level of separation, but can result in very long elution times. For some cases temperature is ramped either continuously or in steps to provide the desired separation. This is referred to as a temperature program. Electronic pressure control can also be used to modify flow rate during the analysis, aiding in faster run times while keeping acceptable levels of separation.

Additionally, choice of carrier gas is important, with hydrogen being the most efficient and providing the best separation. However, helium has a larger range of flowrates that are comparable to hydrogen in efficiency, with the added advantage that helium is non-flammable, and works with a greater number of detectors. Therefore, helium is the most common carrier gas used. Detectors A number of detectors are used in gas chromatography. The most common are the flame ionization detector (FID) and the thermal conductivity detector (TCD). Both are sensitive to a wide range of components, and both work over a wide range of concentrations. While TCDs are essentially universal and can be used to detect any component other than the carrier gas, FIDs are sensitive primarily to hydrocarbons, and are more sensitive to them than TCD. Both detectors are also quite robust.