Evaluation of enantioselective nonaqueous ion-pair capillary electrophoresis as screening assay in the development of new ion exchange type chiral stationary phases

This study evaluates how enantioselectivity values for the separation of chiral N-derivatized amino acids achieved in non-aqueous ion-pair CE with cinchona alkaloid derived chiral counter-ions are related quantitatively to those obtained in enantioselective anion-exchange chromatography with the same selectors immobilized on silica supports. Three different setups of CE experiments, viz. a conventional total filling technique (TFT), a countercurrent technique (CCT), and a partial filling technique (PFT), were investigated and were tested in terms of the goodness of fit of correlations between apparent electrophoretic and chromatographic a values. Both selectands and selectors have been varied. Correlation coefficients higher than 0.95 are commonly obtained with all three CE techniques, yet even better fits are found for TFT-CE and CCT-CE. The common premise for the good correlations is the equal molecular recognition mechanism in ion-pair and ion-exchange modes, both involving ionic interactions as driving force for selectand-selector association, and the fact that separation selectivity in both techniques is primarily determined by the difference of the binding constants of (R) and (S) enantiomers with the chiral selector, i.e. the intrinsic or thermodynamic enantioselectivity. Factors related to the separation technique, which also influence apparent enantioselectivity and possibly contribute to a deterioration of the fit, are discussed. The good correlations prove that enantioselective ion-pair CE may be used as screening method in the development and optimization of ion-exchange type chiral stationary phases.