Gas Chromatography

Gas chromatography (GC) separates mixtures of volatile compounds. The mixture must vaporise without decomposing.The mobile phase is an unreactive carrier gas, such as helium or nitrogen. The stationary phase is usually a liquid with a high boiling temperature, coated as a thin film on a solid support or on the inside wall of a long, thin, coiled column. The column sits inside an oven so that the components stay in the gas phase.A small sample is injected into the heated inlet, where it vaporises and is carried into the column by the gas. Each component separates according to its solubility in the liquid stationary phase and its boiling temperature. A component that is more soluble in the stationary liquid, or less volatile, spends longer in it and has a longer retention time. As each component leaves the column, a detector records a peak.Components are identified by comparing retention times with known compounds under the same conditions (same column, carrier gas flow rate and oven temperature). The peak areas show relative amounts. Raising the oven temperature makes all components move through faster, so all retention times become shorter.GC is often linked to a mass spectrometer (GC-MS): each component is separated by GC and then identified from its mass spectrum. Uses include forensic analysis and drug testing.Worked example: a known sample of ethanol has a retention time of 2.8 min on a GC column. A blood sample run under the same conditions gives a peak at 2.8 min, so it contains ethanol; the peak area can then be compared with standards to find the concentration.