Gas chromatography columns are of two designs: packed or capillary. Packed columns are typically a glass or stainless steel coil (typically 1-5 m total length and 5 mm inner diameter) that is filled with the stationary phase, or a packing coated with the stationary phase. Capillary columns are a thin fused-silica (purified silicate glass) capillary (typically 10-100 m in length and 250 µm inner diameter) that has the stationary phase coated on the inner surface. Capillary columns provide much higher separation efficiency than packed columns but are more easily overloaded by too much sample.
Picture of a packed GC column, Picture of capillary GC column
The most common stationary phases in gas-chromatography columns are polysiloxanes, which contain various substituent groups to change the polarity of the phase. The nonpolar end of the spectrum is polydimethyl siloxane, which can be made more polar by increasing the percentage of phenyl groups on the polymer. For very polar analytes, polyethylene glycol (a.k.a. carbowax) is commonly used as the stationary phase. After the polymer coats the column wall or packing material, it is often cross-linked to increase the thermal stability of the stationary phase and prevent it from gradually bleeding out of the column.
Small gaseous species can be separated by gas-solid chromatography. Gas-solid chromatography uses packed columns containing high-surface-area inorganic or polymer packing. The gaseous species are separated by their size, and retention due to adsorption on the packing material.