Chiral molecules or enantiomers have identical structures and chemical bonds, but have “non-superimposable” structures. For example, two molecules may be each other’s mirror image. Many – if not most – of the molecules in living systems are chiral. As a result different enantiomers interact very differently with organisms. This has major consequences for pharmaceutical science and it requires quantitative analysis of the enantiomeric excess (the difference in concentrations of two enantiomers) or the enantiomeric purity (the fraction present as the target enantiomer). Analytical separations are usually required for such analyses.
Chiral recognition is required to achieve separation of enantiomers. In principle chiral selectors can be present in the mobile phase or in the stationary phase. The former is unattractive, because the chiral selector is consumed in the analysis an it may interfere with detection methods, such as MS. In capillary electrophoresis no staph exists and a chiral reagent, such as cyclodextrin, may be added to the buffer. In chromatography chiral stationary phases are used. Volatile enantiomers may be separated by gas chromatography (GC), whereas non-volatile enantiomers are separated by liquid chromatography (LC) or, increasingly, by supercritical-fluid chromatography (SFC).
