volatiles in foodstuffs by headspace analysis, and in the detection of drugs and their metabolites in
urine, blood and breath samples.
4.3—
Chromatography
Historically, the word chromatography was used by Tswett 2 in 1903 to describe the separation of plant
pigments by percolating a petroleum-ether extract through a glass column packed with powdered
calcium carbonate. Coloured zones were produced by the various pigments migrating through the
column at different rates, the components being isolated by extrusion and sectioning of the calcium
carbonate packing. Modern chromatographic techniques are more complex and are used for a wide
variety of separations frequently involving colourless substances, but the original term is retained.
All the techniques depend upon the same basic principle, i.e. variation in the rate at which different
components of a mixture migrate through a stationary phase under the influence of a mobile phase.
Rates of migration vary because of differences in distribution ratios. Chromatography therefore
resembles Craig counter-current distribution which has been described in the previous section. In the
Craig process, individual equilibrations are performed in a series of separate vessels. If the walls of
these vessels are imagined to be non-existent so that the stationary phase is continuous, and the mobile
phase is allowed to move continuously rather than stepwise, the situation would be closely analogous to
that found in chromatographic separations. In practice the liquid stationary phase is coated onto an inert,
granular or powdered solid support which is either packed into a column or spread on a supporting
sheet in the form of a thin layer. The solid stationary phases used in some chromatographic techniques
have no need of a support if packed into a column but still require a supporting sheet for thin-layer
operation. As the distributing components of a mixture are moved down a column or across a surface by
the mobile phase, they assume a Gaussian concentration profile as they do in the Craig process. In
addition, because both phases are continuous, diffusion and other kinetic effects play a significant role in
determining the width of the profile. This will be discussed in more detail later.
Chromatographic Mechanisms
During a chromatographic separation solute molecules are continually moving back and forth between
the stationary and mobile phases. While they are in the mobile phase, they are carried forward with it
but remain virtually stationary during the time they spend in the stationary phase. The rate of migration
of each solute is therefore determined by the proportion of time it spends in the mobile phase, or in
other words by its distribution ratio.
The process whereby a solute is transferred from a mobile to a stationary phase is called sorption.
Chromatographic techniques are based on four different sorption mechanisms, namely surface
adsorption, partition, ion-