Intrinsic selectivity in capillary electrophoresis for chiral separations with dual cyclodextrin systems

Defined as the ratio of the affinity factors of the analytes for a complexing agent, the intrinsic selectivity is representative of the very nature of the complexing agent. When more than one complexing agent are present in the background electrolyte, it is possible to define several intrinsic selectivities according to whether complexing agents are considered separately or all together. A theoretical model with respect to selectivity is presented for separations that involve two complexing agents, using the concept of apparent constant for complex formation. When only independent complexation occurs (absence of mixed complexes), then the intrinsic selectivity of a complexing agent X in the presence of a complexing agent Y can be easily related to the intrinsic selectivity of each complexing agent and to complex formation constants. Dual systems of cyclodextrins (CDs), implementing the cationic mono(6-amino-6-deoxy)-β-cyclodextrin (β-CD-NH(2)) and a neutral CD (trimethyl-β-CD (TM-β-CD) or dimethyl-β-CD (DM-β-CD)), were studied to illustrate this model and to offer an alternative to the separation of neutral enantiomers when β-CD-NH(2) shows no or insufficient stereoselectivity. With a dual β-CD-NH(2)/TM-β-CD system at pH 2.3, arylpropionic acid enantiomers were baseline resolved and benzoin derivatives were partially resolved. For the arylpropionic acids, β-CD-NH(2), which is not stereoselective, confers on them a nonzero mobility, while TM-β-CD allows the chiral recognition. A study of the respective influence of ΤM-β-CD and β-CD-NH(2) concentrations was performed to determine the optimal conditions with respect to resolution. This theoretical approach allowed characterization of the intrinsic selectivity of neutral CDs for pairs of neutral enantiomers and therefore identification of the potential of neutral chiral agents for neutral enantiomers.