Experimental verification of a predicted, hitherto unseen separation selectivity pattern in the nonaqueous capillary electrophoretic separation of weak base enantiomers by octakis (2,3-diacetyl-6-sulfato)-gamma-cyclodextrin.

Abstract

The capillary electrophoretic separation of cationic enantiomers with single-isomer multivalent anionic resolving agents was reexamined with the help of the charged resolving agent migration model. Three general model parameters were identified that influence the shape of the separation selectivity and enantiomer mobility difference curves: parameter b, the binding selectivity (K(RCD)/K(SCD)), parameter s, the size selectivity (mu0(RCD)/mu0(SCD)), and parameter a, the complexation-induced alteration of the analyte's mobility (mu0(RCD)/mu0). In addition to the previously observed discontinuity in separation selectivity that occurs as mu(eff) of the less mobile enantiomer changes from cationic to anionic, a new feature, a separation selectivity maximum was predicted to occur in the resolving agent concentration range where both enantiomers migrate cationically provided that (i) K(RCD)/K(SCD) <1 and mu0(RCD)/mu0(SCD) >1 and (K(RCD)mu0(RCD))/(K(SCD)mu0(SCD)) > 1, or (ii) K(RCD)/K(SCD) >1 and mu0(RCD)/mu0(SCD) <1 and (K(RCD)mu0(RCD))/(K(SCD)mu0(SCD)) <1. This hitherto unseen separation selectivity pattern was experimentally verified during the nonaqueous capillary electrophoretic separation of the enantiomers of four weak base analytes in acidic methanol background electrolytes with octakis(2,3-diacetyl-6-sulfato)-gamma-cyclodextrin (ODAS-gammaCD) as resolving agent.