Chiral Resolution and Chromatographic Behaviour of Enantiomers

Resolution (Rs) is defined as the separation of two peaks in terms of their average peak width at half height (I) or at a base width of 4a (II) [10]. 

In the case of enantiomeric pairs the term chiral resolution (cRs) is used. 

As outlined in Table 6.15,100 % separation occurs, if chiral resolution cRs = 2.50 in practice cRs 1.50 (99.73% separation) is defined as a baseline resolution ]10]. Thus, optimal chiral resolution (cRs 1.50) should be achieved using a suitable chiral stationary phase in enantioselective capillary gaschromatography (enantio-cGC). 

Although separation factor a has the greatest impact on peak resolution, an accurate determination of high enantiomeric excess (ee-values) as shown in Fig. 6.23 ]11] is only possible, in the case of highly resolved enantiomer separation. 

At the present state of knowledge, the mechanisms of GC enantiomer separation have not been elucidated. Unusual chromatographic behaviour and reversal of the elution order of enantiomers have been observed. Consequently, the usefulness of a given chiral stationary phase as well as the order of elution of separated enantiomers cannot be predicted. References of definite chirality are essential to identify the separated isomers, no matter whether directly stereoanalyzed with chiral stationary phases or via derivatized stereoisomers [12-19]. 

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