Stationary Phase
The number of stationary phases suitable for gas chromatography is quite extensive, and choice is
dictated largely by the nature of the sample. A liquid stationary phase should be non-volatile and
thermally stable at the operating temperature of the column, otherwise it will 'bleed' during operation
and cause a drifting baseline on the recorder. In addition it should be chemically stable and inert
towards samples to ensure reliable results. Stationary phases are described as non-polar or polar
according to their structure and separating abilities. Non-polar types include hydrocarbon and silicone
oils and greases. Polar types cover a wide range of polarity and include high molecular weight
polyesters, ethers, carbowaxes, amines, etc. Solid adsorbents used in GSC, e.g. silica and alumina, are
among the most polar of stationary phases.
In general, the most suitable stationary phase for a given sample is that which is chemically similar to it.
Thus, a mixture of saturated hydrocarbons is best separated on a non-polar hydrocarbon-type liquid
such as squalane, Apiezon-L grease or silicone oil DC200. If there is an appreciable difference in
polarity between the sample components and the stationary phase, elution profiles may show tailing or
fronting because of non-linearity of the sorption isotherms. Where a sample contains unknowns or
compounds of varying polarity, a compromise stationary phase must be chosen, usually by trial and
error. The order of elution can be altered by changing the liquid phase. For example, on a non-polar
alkane-type column, t-butyl alcohol (b.pt. 82.6°C) will elute before cyclohexane (b.pt. 80.8°C). If a
more polar liquid phase containing hydroxyl groups is used, the cyclohexane elutes first because of
hydrogen-bonding between the alcohol and the stationary phase.
Stationary phases can be made highly selective by adding compounds to them which have affinities for
certain chemical species. For example, silver nitrate, incorporated into a polar liquid preferentially
retards the elution of alkenes by formation of weak π-complexes. A selection of stationary phases with
their maximum operating temperatures and useful applications is given in Table 4.10.
The amount of stationary phase or 'loading' carried by a solid support affects the efficiency of the
column and the size of sample that can be injected. A high loading, 15–30% by weight, produces a
thick liquid film which impairs efficiency by increasing the mass transfer term C in equation (4.46).
Loadings of 1–10% are to be preferred, although at the lower end of this range the sample size may
have to be restricted to prevent overloading the column. In general lower loadings allow the use of a
lower operating temperature which is an important factor when handling thermally sensitive
compounds. However if the liquid film is too thin, adsorption of the solutes by the solid support may
cause tailing, decomposition or rearrangements.