4 —
Separation Techniques
If it were possible to identify or quantitatively determine any element or compound by simple
measurement no matter what its concentration or the complexity of the matrix, separation techniques
would be of no value to the analytical chemist. Most procedures fall short of this ideal because of
interference with the required measurement by other constituents of the sample. Many techniques for
separating and concentrating the species of interest have thus been devised. Such techniques are aimed
at exploiting differences in physico-chemical properties between the various components of a mixture.
Volatility, solubility, charge, molecular size, shape and polarity are the most useful in this respect. A
change of phase, as occurs during distillation, or the formation of a new phase, as in precipitation, can
provide a simple means of isolating a desired component. Usually, however, more complex separation
procedures are required for multi-component samples. Most depend on the selective transfer of
materials between two immiscible phases. The most widely used techniques and the phase systems
associated with them are summarized in Table 4.1.
Table 4.1 Classification of separation techniques
Technique Phase system
solvent extraction liquid–liquid
solid phase extraction liquid–solid
gas chromatography gas–liquid
gas–solid
liquid chromatography liquid–liquidliquid–solid
thin-layer chromatography liquid–solid
liquid–liquid
ion-exchange and liquid–solid
size exclusion chromatography liquid–liquid
supercritical fluid chromatography supercritical fluid–liquid or solid
electrophoresis liquid
capillary electrochromatography liquid–solid