GC053 Types of Capillary columns

 The tubes can be made of fused silica, glass, or stainless steel, but the fused silica columns are, by far, the most common. One type of column and the more common, is the wall-coated open tubular column (WCOT).

Capillary columns provide the highest resolutions. Internal diameters of 0.10, 0.20, 0.25, 0.32 and 0.53 mm are commercially available. Typical lengths are between 10 to 60 m, although lengths of up to 100 m are commercially available. Long columns result in longer analysis times. Film thickness is between 0.1 and 5.0 µm. Thin films provide fast analysis and resolutions, but low sample capacities. The contrary is true for thicker films, and they are only used for very volatile compounds.

There are also two other types of capillary columns. The support-coated open tubular (SCOT) column and the porous layer open tubular (PLOT) column. SCOT columns contain an adsorbed layer of very small solid support, coated with a liquid phase. Their capacity is increased, compared with WCOT of thin layers. The need for SCOT columns has disappeared with the introduction of cross-linking techniques, which allow to generate stable thick films.

PLOT columns contain a porous layer of a solid adsorbent such as alumina, molecular sieves or Porapak®. They are very well suited for the analysis of light gases and volatile compounds. One example is the separation of oxygen, nitrogen, methane and carbon monoxide on a molecular sieve PLOT column. The share of GC PLOT columns in the market is around 5%.

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