In gas chromatography (GC) we inject the sample, which may be a gas or a liquid, into a gaseous mobile phase (often called the carrier gas). The mobile phase carries the sample through a packed or capillary column that separates the sample's components based on their ability to partition between the mobile phase and the stationary phase. Because it combines separation with analysis, gas chromatography provides excellent selectivity. By adjusting conditions it usually is possible to design a separation so that analytes elute by themselves, even when the mixture is complex. Additional selectivity is possible by using a detector that does not respond to all analytes. 27.1: Principles of Gas Chromatography In Chapter 26 we covered several important elements of chromatography, including the factors that affect the migration of solutes, the factors that contribute to band broadening, and the factors under our control that we can use to optimize the separation of a mixture. Here we consider two topics that apply to a gas chromatographic separation, both of which are a function of the properties of gases. 27.2: Instruments for Gas Chromatography A typical gas chromatograph includes a supply of compressed gas for the mobile phase; a heated injector to volatilize the sample; a column that holds the stationary phase, and which is placed within an oven whose temperature we can control during the separation; and a detector to monitor the eluent as it comes off the column. 27.3: Gas Chromatographic Columns and Stationary Phases There are two broad classes of chromatographic columns: packed columns and capillary columns. In general, a packed column can handle larger samples and a capillary column can separate more complex mixtures. 27.4: Applications of Gas Chromatography Gas chromatography is widely used for the quantitative analysis of a diverse array of samples in environmental, clinical, pharmaceutical, biochemical, forensic, food science and petrochemical laboratories. It also finds use for qualitative analyses, although these are less common.