Gas chromatography is a type of chromatography in which the carrier medium is a gas. The technique is used for analyzing or separating a mixture of gases, often derived from a heated liquid mixture. A *carrier" gas (the mobile phase) – often hydrogen – sweeps the mixture around a very long coiled tube containing the stationary phase, which differentially absorbs the components of the mixture. In gas-liquid chromatography (GLC), the stationary phase is a liquid such as a non-volatile oil coating a solid support; in gas-solid chromatography (GSC) it is a solid such as kieselguhr (diatomite). The components of the mixture are detected as they leave the apparatus in turn or, if the chromatograph has been calibrated with known substances, they are identified by the time they take to pass through. This method of chromatography is used to analyze tiny amounts of a sample.