Influence of methanol on the enantioresolution of antihistamines with carboxymethyl-beta-cyclodextrin in capillary electrophoresis.

According to the model of Wren and Rowe, the separation between two enantiomers in capillary electrophoresis (CE) decreases if an organic modifier is added to the run buffer containing a neutral cyclodextrin (CD) in a concentration below its optimal value in a solvent-free system. In previous work, however, it was observed that the addition of methanol to the background electrolyte (BGE) containing not charged carboxymethyl-beta-CD in a concentration below its optimal value, increased the enantioresolution of dimetindene maleate. The enantioresolution decreased when other organic modifiers (ethanol, isopropanol or acetonitrile) were added and/or when other neutral (beta-CD, hydroxypropyl-beta-CD) or chargeable (carboxyethyl-beta- and succinyl-beta-CD) CDs were used. In this CE study further attempts are made to elucidate the observed phenomena through investigating other basic drugs. The effect of organic modifier and CD concentration on the enantioseparation was studied by means of central composite designs. It is shown that obtaining this increase in enantioresolution depends upon the type of CD, the type of organic modifier, and the structure of the analytes. It was also observed that small differences in the structure of the analytes or the CD could have an influence on the enantioresolution. The addition of methanol also resulted in different effects on the resolution of closely related analytes.