Enhanced selectivity in CZE multi-chiral selector enantioseparation systems: proposed separation mechanism.

It has been reported many times that the commercial mixtures of chiral selectors (CS), namely highly sulfated beta-CDs (HS-beta-CDs), provide remarkable enantioselectivity in CZE when compared with single-isomer CDs, even single-isomer HS-beta-CDs. This enhanced enantioselectivity of multi-CS enantioseparative CZE is discussed in the light of multi-CS model that we have introduced earlier. It is proposed on a theoretical basis and verified experimentally that the two enantiomers of a chiral analyte under interaction with a mixture of CSs are very likely to differ in their limit mobilities, which is opposite to single-CS systems where the two limit mobilities are likely to be the same. Thus while the enantioseparation is usually controlled by different distribution constants between the two enantiomers and CS used in single-CS systems, an additional, electrophoretic, enantioselective mechanism resulting from different limit mobilities may play a significant role in multi-CS systems. This additional mechanism generally makes the multi-CS systems more selective than the single-CS systems. The possible inequality of limit mobilities is also significant for optimization of separation conditions using mixtures of CSs. A practical example supporting our considerations is shown on enantioseparation of lorazepam in the presence of a commercial mixture of HS-beta-CDs and a single-isomer HS-beta-CD, heptakis(6-O-sulfo)-beta-CD.