No abstract

In order to accurately evaluate the performances of any electrolyte medium, a clear concept of selectivity in capillary electrophoresis and related electroseparation techniques is proposed. Selectivity is defined as the ratio of the affinity factors of both analytes for a separating agent (phase, pseudophase, or complexing agent present in the background electrolyte). When in the presence of a complexing agent and if only 1:1 complexation occurs, selectivity corresponds to the ratio of the apparent binding constants and is independent of the concentration of the complexing agent. This concept is illustrated through the separations of neutral and anionic enantiomers in the presence of a cationic cyclodextrin, the mono(6-amino-6-deoxy)-β-cyclodextrin, as a chiral complexing agent. The values obtained for different pairs of enantiomers are discussed with regard to the functional groups that distinguish them. When the analytes have the same mobilities in free solution and in their complexed form, then the resolution equation developed in micellar electrokinetic chromatography may be applied and optimum conditions (affinity factors, chiral agent concentration) can be predicted.

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.