Page 31 - Tandem Techniques

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specific tandem techniques as they will differ significantly with each type of instrument employed.

Gas Chromatography Columns

There are basically two types of chromatographic column, the packed column and the capillary column.
Both types are usually formed into coils or loops to make them sufficiently compact to fit inside small
ovens. The packed column is usually 3 or 4 mm I.D. and 3 to 6 ft long, although, if designed correctly,
and if the apparatus can cope with a sufficiently high inlet pressure, columns up to 100 ft long can be
prepared [2]. The column case itself is usually made of glass but sometimes of metal and is packed with
a diatomaceous earth such as celite coated with the stationary phase. The celite is ground and screened
to about 100 mm particle diameter and de-activated by treatment with a suitable silane such as
hexachlorodisilazane. The packing is then coated with stationary phase by wetting it with a solution of
the stationary phase in a volatile solvent and evaporating the solvent in a rotary evaporator. The loading
of stationary phase can vary (usually between 5% and 10% w/w), the higher the load, the larger the size
of the sample that can be placed on the column. This can be advantageous when the associated
instrument lacks a comparable sensitivity, e.g. the IR spectrometer. The column is packed by adding the
packing and tapping until a maximum packing density is achieved.

Figure 1.2
The Septum Injector