bration and changes in operating temperature are achieved by circulating the oven air with a fan.
(a)—
Packed Columns
These rarely exceed 2–3 metres in length with an internal diameter of 2–3 mm and are made of stainless
steel or glass, the latter being less reactive to thermally sensitive compounds and facilitating visual
inspection of the packed bed. For GLC they are completely filled with an inert porous granular solid
support which is coated with a thin film of a liquid or semi-liquid stationary phase (vide infra). GSC
columns are filled with a solid stationary phase which has adsorbent properties. Packed columns do not
have the very high resolving power of capillary columns but, compared to the latter, they are cheap,
robust and have a high sample capacity which allows the use of a simpler injection system. Their
popularity has diminished steadily in recent years as capillary columns have improved, but for some
applications they may still be preferred.
Solid Support
The function of a solid support is to hold the liquid phase used for packed column GLC immobile
during the separation process. It should be inert, easily packed and have a large surface area. Calcined
diatomaceous earth and firebrick, both mainly silica, are commonly used, being marketed under various
trade names such as Celite, Chromosorb and Stermachol. The materials must be rendered chemically
inert before coating with stationary phase because trace metal impurities and surface silanol (Si—OH)
groups produce surface active sites which promote undesirable adsorption effects. Adsorption causes
tailing (p. 82) and may result in catalysed decomposition or rearrangements of the solutes passing
through the column. Pretreatment consists of acid- or alkali-washing to remove the trace metals, and
silanizing to convert the Si—OH groups to silyl ethers, e.g. Si—O—Si(CH 3 ) 3. Dimethyl dichlorosilane
or hexamethyldisilazane are frequently used for this purpose. If highly polar compounds are to be
separated, silanized glass beads or granular PTFE are supports less likely to cause tailing. Because they
are non-porous, they can support a maximum of only about 3% of stationary phase, and the size of
sample that can be chromatographed is smaller than with the silaceous solid supports.
The particle size of a solid support is critical in striking a compromise between column efficiency and
speed of separation. Both the multiple path term A and the mass transfer term (C of equation (4.46) (p.
89)) are reduced by reducing particle size thus leading to increased efficiency. However, as particle size
is reduced, the pressure drop across the column must be increased if a reasonable flow rate is to be
maintained. The optimum particle sizes for 1/8 in columns are 80/100 or 100/120 mesh and for 1/4 in
columns 40/60 or 60/80 mesh.