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They are usually made of the highest purity fused silica obtained by the combustion of tetrachlorosilane (SiCl₄) in an oxygen‐rich atmosphere. The internal diameter of the tube used for these columns varies from 100 to 530 μm. The technology is particularly delicate in order to obtain perfectly cylindrical columns with a thickness of 50 μm and lengths going up to 100 m (Figure 2.7). These columns have a polyimide outer coating, which is a thermally stable polymer (T_max = 370°C), to make them less fragile, both chemically and mechanically. They can be wound into coils around a lightweight metal circular support. Some manufacturers offer columns made from a metal capillary (aluminium, nickel or steel) which tolerates high operating temperatures on the order of 450°C, providing that the stationary phase is stable enough. The internal surface of the column is usually treated to promote good bonding for the stationary phase. This could be a chemical treatment or the deposit of a thin layer of alumina or silica gel.

The regular thickness of the stationary phase can vary between 0.05 and 5 μm. It is either simply deposited or better yet grafted with covalent bonds, possibly followed by a polymerization with cross‐linking on the wall. This deposit is obtained by evaporating a solution or by polymerization in situ in contact with the wall. These are WCOT (wall‐coated open tubular) or PLOT (porous layer open tubular) columns, depending upon the nature of the stationary phase employed. Columns are particularly stable and can be rinsed periodically with solvents, which enable them to recover their initial performance levels.

To compare or anticipate the behaviour of capillary columns, it is useful to know the phase ratio β = V_M / V_S. By designating ID as the internal diameter of the column and d_f as the thickness of the deposited film, an approximate calculation leads to:

(2.1)

The column’s capacity is related to the phase ratio but also, for each solute, to its retention time, since k is inversely proportional to it. The phase ratio β, accessible from the physical characteristics of the column, and k (retention factor) from the chromatogram, help us calculate the partition coefficient K of a solute, whose value is generally quite high (1,000 for example) owing to the nature of the mobile phase (gas).

WCOT columns are well suited to mass spectrometry detection and are thus used often. To deposit a film of known thickness, a method consists in filling the column with a solution of stationary phase of known concentration (e.g. 0.2% in ether), so that the desired thickness is obtained after solvent evaporation. This layer can then be cross‐linked by a peroxide or by γ irradiation. The process is similar to the application of paint on a surface that has been pretreated to obtain good adherence.