Mobile phases are generally inert gases such as helium, argon, or nitrogen. The injection port consists of a rubber septum through which a syringe needle is inserted to inject the sample. The injection port is maintained at a higher temperture than the boiling point of the least volatile component in the sample mixture. Since the partitioning behavior is dependant on temperture, the separation column is usually contained in a thermostat-controlled oven. Separating components with a wide range of boiling points is accomplished by starting at a low oven temperture and increasing the temperture over time to elute the high-boiling point components.
Most columns contain a liquid stationary phase on a solid support.
Separation of low-molecular weight gases is accomplished with solid adsorbents. Capillary columns provide much higher separation efficiency than packed columns but are more easily overloaded by too much sample. The most common stationary phases in gas-chromatography columns are polysiloxanes, which contain various substituent groups to change the polarity of the stationary phase. The most nonpolar stationary phase consist of polydimethyl siloxane. More polar stationary phases increase the percentage of phenyl groups on the polymer.