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.

The purity and enantioselectivity of a novel chiral agent, the zwitterionic mono-(6-delta-glutamylamino-6-deoxy)-beta-cyclodextrin (beta-CD-Glu), were studied by capillary electrophoresis. Chiral separation of the enantiomers of chlorthalidone was obtained at pH 2.3, a pH at which beta-CD-Glu is partially protonated. Comparison with the cationic mono-(6-amino-6-deoxy)-beta-cyclodextrin (beta-CD-NH2) enantioselectivity clearly shows that the greater the difference in mobility between the free analyte and the analyte-cyclodextrin complex, the better the resolution. Hydrobenzoin enantiomers were separated at pH 11.2, a pH at which beta-CD-Glu is anionic. Under these conditions, the migration order was opposite to that observed in the presence of beta-CD-NH2 at pH 2.3. When no separation was obtained directly with beta-CD-Glu, a dual cyclodextrin system was developed. Carprofen enantiomers were resolved at pH 2.3 in the presence of a beta-CD-Glu/trimethyl-beta-cyclodextrin (TM-beta-CD) system in which the charged CD confers a non-zero mobility to the analyte, while the neutral CD allows chiral recognition.

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