Table 4.10 Some stationary phases used in gas chromatography(b)—
Capillary (Open Tubular) Columns
This type of column has become the most widely used because of its superior resolving power for
complex mixtures compared to that of a packed column. Capillary columns are typically 5–50 metres
long and between 0.1 and 0.60 mm internal diameter. A thin film (0.1– 5 μm thick) of the liquid
stationary phase is coated or bonded onto the inner wall of the tube (hence the alternative description of
wall-coated open tubular or WCOT columns) which is made of high-purity fused silica (quartz).
Keeping the total metallic impurities in the silica to around 1 ppm or less renders it extremely inert
thereby minimizing peak tailing and the possibility of thermal decomposition of chromatographed
substances. The exterior of the tube is coated with a layer of a polyimide or aluminium as a protection
against cracking or scratching.
The unrestricted flow of carrier gas through the centre of capillary columns results in a much smaller
pressure drop per metre than for packed columns. They can therefore be made very much longer and
will generate many more theoretical plates, i.e. up to about 150,000 plates per 25 metres compared with
a few thousand for a 2-metre packed column. A narrow bore and thin layer of stationary phase are
essential to promote rapid mass