A chromatographic column provides a location for physically retaining the stationary phase. The column’s construction also influences the amount of samplethat can be analyzed, the efficiency of the separation, the number of analytes that can be easily separated, and the amount of time required for the separation.
There are two types of column used in gas chromatography – packed and capillary.
Packed columns are the routine work horses of gas chromatography, being cheaper and easier to use and often giving adequate performance.
Capillary columns generally give far superior resolution and although more expensive are becoming widely used, especially for complex mixtures.
Both types of column are made from non-adsorbent and chemically inert materials. Stainless steel and glass are the usual materials for packed columns and quartz or fused silica for capillary columns.
Packed Columns
A packed column is constructed from glass, stainless steel, copper or aluminum and is typically 2-6 m in length, with an internal diameter of 2-4 mm. The column is filled with a particulate solid support, with particle diameters ranging from 38-44 μm to 250-354 μm.
The most widely used particulate support is diatomaceous earth. These particles are porous, with surface areas of 0.5 -7.3 m2/g, which provides ample contact between the mobile phase and stationary phase.
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| Fig. 1: A packed column in a G.C. oven |
Packed columns can handle larger amounts of sample than capillary columns. Samples of 0.1 -10 μl are routinely analyzed with a packed column.
Capillary Columns
These consist of silicon dioxide, alkali glass or borate glass. Their id varies between 30 and 500 μm. Column lengths are usually between 1 and 100 m. There are three types of capillary column:
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| Fig. 2: A gc capillary column |
Packed capillary columns in which the packing is an adsorbent (e.g. silica gel). These are used for adsorption chromatography only, not partition chromatography, and are suitable for strongly polar components.
Thin film capillary columns, in which the liquid phase is applied to the inner walls of the column in the form of a thin film.
Thin coating capillary columns, which consist of a thin, finely divided coating of carrier material which is then itself coated with the liquid phase.
Capillary columns provide a significant improvement in separation efficiency compared to packed columns. The pressure needed to move the mobile phase through a packed column limits its length. The absence of packing material allows a capillary column to be longer than a packed column allowing for more efficient separations.
However, due to their smaller diameter capillary columns require smaller samples – less than 10-2 μl.
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