Separation selectivity patterns in the capillary electrophoretic separation of anionic enantiomers by octakis-6-sulfato-gamma-cyclodextrin

The capillary electrophoretic separation of anionic enantiomers with multiply-charged, single-isomer, 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 (micro0RCD/micro0SCD), and parameter a, the complexation-induced alteration of the analyte's mobility (micro0SCD/micro0). Function analysis of the model indicates that there are six unique separation selectivity vs. resolving agent concentration patterns: in two of the patterns, separation selectivity asymptotically increases to the limiting value set by parameter b; in two other patterns, separation selectivity passes a local maximum and asymptotically decreases to the limiting value set by parameter b; and in the last two patterns, separation selectivity passes a local maximum, decreases to unity, then, after reversal of the intrinsic migration order, asymptotically increases to the limiting value set by parameter b. Though the patterns with asymptotically increasing selectivities were observed in earlier work, this paper reports the first experimental verification of the existence of the local selectivity maximum during the capillary electrophoretic separation of the enantiomers of several weak acids in high pH background electrolytes with octakis-6-sulfato-gamma-cyclodextrin as the resolving agent.