Enantioselective Chromatography and Related Techniques

A different but experimentally simpler solution is to use a chiral selector (called often in such a case chiral additive) dissolved directly in the mobile phase. The diastereomeric associates thus formed differ in their mobility or adsorption, and in favorable cases they can be separated on an achiral phase. This approach was applied in the 1980s by Pettersson et al. who used native cinchona alkaloids in ion-pairing chromatography [ 51 ]. N owadays, ion-pairing technique is of considerable interest in a number of electrophoretic techniques (see below). 

Chiral separation can be observed when there is a suitable difference between free energies (AG) of diastereomeric associates formed by R- and S-enantiomers of selectand and a chiral selector (SO). The energy differences can be directly attributed to the chiral recognition phenomena, involving complementarity of size, shape, and molecular interaction of SO and SA molecules. These factors are also controlled by experimental conditions such as temperature, mobile phase 

The protonation of cinchona selectors by the acidic analytes may also occur under normal conditions (hydrocarbons + polar nonaqueous modifier) and can be responsible for the strong retention of the analyzed molecules, making such mobile-phase regime less practical for most of the chromatographic applications. 

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Enantioselective chromatography and related techniques are based principally on the reversible formation of diastereomeric associates between both enantiomers of the chiral analyte (selectand, SA) and the chiral selector (SO) that is usually covalently immobilized or coated on a solid support (Figure 13.9). 

Pais, LS., Mata, V.G., Rodrigues, A.E. (2005) Simulated moving bed and related techniques, in Preparative Enantioselective Chromatography, 1st edn (ed. G.B. Cox), Wiley-Blackwell,... 

Gas chromatography-mass spectrometry has also been demonstrated to be a powerful tool in both the qualitative and quantitative analyses of essential oil components. Literature on capillary and enantioselective capillary GC on-line coupled to isotope-ratio MS (IRMS) has been reviewed by Mosandl [1]. These techniques, which are valuable tools in control of authenticity of flavors and essential oils, are discussed in relation to various aspects, such as sample preparation and cleanup, chromatographic behavior of enantiomers, detection systems, and the use of internal isotope standards for quantitation. 

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