Page 45 - Subyek Encyclopedia - Encyclopedia of Separation Science

40 I / CHROMATOGRAPHY/ Derivatization

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CHROMATOGRAPHY

C. F. Poole, Wayne State University, Detroit, MI, separation. Individual compounds are distinguished
USA by their ability to participate in common intermolecu-
lar interactions in the two phases, which can gener-
Copyright ^ 2000 Academic Press
ally be characterized by an equilibrium constant, and
is thus a property predicted from chemical thermo-
Introduction dynamics. Interactions are mainly physical in type or
involve weak chemical bonds, for example dipole}
Chromatography is the most widely used separation dipole, hydrogen bond formation, charge transfer,
technique in chemical laboratories, where it is used in etc., and reversible, since useful separations only re-
analysis, isolation and puriRcation, and it is com- sult if the compound spends some time in both
monly used in the chemical process industry as a com- phases. During transport through or over the station-
ponent of small and large-scale production. In terms ary phase, differential transport phenomena,
of scale, at one extreme minute quantities of less than such as diffusion and Sow anisotropy (complex
a nanogram are separated and identiRed during anal- phenomena discussed later), result in dispersion of
ysis, while at the other, hundreds of kilograms of solute molecules around an average value, such that
material per hour are processed into reRned products. they occupy a Rnite distance along the stationary
It is the versatility of chromatography in its many phase in the direction of migration. The extent of
variants that is behind its ubiquitous status in separ- dispersion restricts the capacity of the chromato-
ation science, coupled with simplicity of approach graphic system to separate and, independent of
and a reasonably well-developed framework in which favourable thermodynamic contributions to the sep-
the different chromatographic techniques operate. aration, there is a Rnite number of dispersed zones
Chromatography is essentially a physical method that can be accommodated in the separation. Conse-
of separation in which the components of a mixture quently, the optimization of a chromatographic sep-
are separated by their distribution between two aration depends on achieving favourable kinetic
phases; one of these phases in the form of a porous features if success is to be obtained.
bed, bulk liquid, layer or Rlm is generally immobile
(stationary phase), while the other is a Suid (mobile
phase) that percolates through or over the stationary The Family of Chromatographic
phase. A separation results from repeated sorp- Techniques
tion/desorption events during the movement of the
sample components along the stationary phase in the A convenient classiRcation of the chromatographic
general direction of mobile-phase migration. Useful techniques can be made in terms of the phases em-
separations require an adequate difference in the ployed for the separation (Figure 1), with a further
strength of the physical interactions for the sample subdivision possible by the distribution process em-
components in the two phases, combined with a fa- ployed. In addition, for practical utility transport
vourable contribution from system transport proper- processes in at least one phase must be reasonably
ties that control sample movement within and fast; for example, solid}solid chromatography, which
between phases. Several key factors are responsible, may occur over geological time spans, is impractical
therefore, or act together, to produce an acceptable in the laboratory because of the slow migration of