In gas chromatography (GC) the sample, which may be a gas or liquid, is injected into a stream of an inert gaseous mobile phase (often called the carrier gas). The sample is carried through a packed or capillary column where the sample’s compo- nents separate based on their ability to distribute themselves between the mobile and stationary phases. A schematic diagram of a typical gas chromatograph is shown in Figure 12.16.
The most common mobile phases for GC are He, Ar, and N2, which have the ad- vantage of being chemically inert toward both the sample and the stationary phase. The choice of which carrier gas to use is often determined by the instrument’s de- tector. With packed columns the mobile-phase velocity is usually within the range of 25–150 mL/min, whereas flow rates for capillary columns are 1–25 mL/min. Ac- tual flow rates are determined with a flow meter placed at the column outlet.