In the vast majority of cases, open-tubular columns are used for gas-chromatography (GC) separations. Packed columns may be used for separating gases at ambient temperature (or higher), thanks to the large surface area available. Packed columns with small particles can also be used for ultra-fast GC, but the relatively high pressures needed are generally prohibitive of such applications.
Long open-tubular GC columns are almost always prepared from fused-silica. (Again, there is an exception; all-metal columns are used for high-temperature GC applications, such as simulated distillation.) A flexible polyurethane coating on the outside of fused-silica columns makes them hard to break and heat resistant.
Cross-linked polymers are by far the most-common stationary phases. Polysiloxanes are by far the most popular, because they show the highest analyte diffusion coefficients. Polarity can be tuned by varying the substituents on the polysiloxane backbone, from methyl (non-polar) to phenyl- or cyano-groups and phases in between (e.g. 50% methyl, 50% phenyl). Highly polar “wax” columns are still used for polar analytes.
Porous-layer open-tubular (PLOT) columns offer a high surface area, which may be beneficial for the analysis of highly volatile compounds at above-ambient temperatures. As such, they combine some of the advantages of packed columns (retentivity) and of open-tubular columns (permeability).
See Sections 2.1.4 and 2.1.5 for more information on GC columns
