Buffer effects in the desionoselective/ionoselective/duoselective separation selectivity model-assisted optimization of the capillary electrophoretic separation of enantiomers I. Low-pH phosphate buffers

Abstract

The complexation of aromatic permanently charged anions by β-cyclodextrin in capillary electrophoresis has been studied using phosphate background electrolytes in which the pH was varied from 2.0 to 3.6. The complex formation constants and ionic mobilities proved independent of the pH, indicating that the electrophoretically determined parameters of the multiple equilibria-based separation selectivity model (DID model) are meaningful and should be suitable for the optimization of enantiomer separations. Therefore, the effective mobilities of the enantiomers of α-methoxy-α-trifluoromethyl phenylacetic acid, suspected to belong to the families of ionoselective or duoselective separations, were measured as a function of pH and β-cyclodextrin concentration in the 2 < pH < 4 and 0 < c CD < 15 m M ranges in phosphoric acid-based background electrolytes. The effective mobilities were fitted to the DID model to extract the respective acid dissociation and complex formation constants and ionic mobilities. Peak resolution for the duoselective separation was calculated with these parameters as a function of pH and the β-cyclodextrin concentration of the background electrolyte, as well as the dimensionless electroosmotic flow coefficient. The shapes of the predicted and observed peak resolution curves agreed well, demonstrating that the DID model can be used for the optimization of the CE separations of enantiomers.