EXPERIMENTAL-EVIDENCE FOR THE EXISTENCE OF DUOSELECTIVE (TYPE-III) ENANTIOMER SEPARATIONS IN THE CAPILLARY ELECTROPHORETIC ANALYSIS OF CHIRAL WEAK ACIDS

Experimental evidence has been found for the existence of the theoretically predicted duoselective (Type III) enantiomer separations in the capillary electrophoretic (CE) analysis of chiral weak acids. The mobilities of the enantiomers of 3,5-dinitrobenzamido phenylalanine were determined in the pH 3-6 range using constant ionic strength epsilon-amino-n-caproic acid background electrolytes and p-cyclodextrin as resolving agent. The complex formation constants and ionic mobilities of the free and complexed enantiomers were determined using the multiple equilibria-based electrophoretic mobility model. The extended peak resolution equation of CE was then used to calculate the peak resolution surfaces as a function of the pH and the p-cyclodextrin concentration of the background electrolyte, the dimensionless electroosmotic flow coefficient, and the effective portion of the applied potential. Though the observed peak resolution values were lower than predicted by the theory due to the presence of electromigration dispersion, the predicted and observed loci of the extrema of the resolution surface agreed well. The desionoselective/ionoselective/duoselective (DID) separation selectivity model and the peak resolution model permit the rational optimization of the CE separation of weak acid enantiomers.