Differential effects of organic modifiers on the enantioseparation of dimetindene maleate with carboxymethyl-beta-cyclodextrin in capillary electrophoresis

Methanol enhances the enantioresolution of dimetindene enantiomers with carboxymethyl-beta-cyclodextrin (CMCD) as chiral selector at a concentration below its optimal value. The same effect was observed with ethanol (EtOH), although less pronounced. On the other hand, the addition of isopropanol (IP) or acetonitrile (ACN) decreases the enantioseparation. To elucidate the underlying mechanisms of these observed effects, other neutral (beta-CD, hydroxypropyl-beta-CD, and trimethyl-beta-CD) as well as chargeable (carboxyethyl-beta-CD and succinyl-beta-CD) CD derivatives were also tested with MeOH as organic modifier. It can be concluded that the increased enantioresolution of dimetindene enantiomers was only noted with CMCD as chiral selector and a short-chain organic modifier containing an alcohol function. The slight deprotonation of CMCD at pH 3.0 was only partly responsible for the high enantioselectivity and the 'favourable' effect of MeOH or EtOH. An important feature that can be concluded from these results is that for this particular analyte approximately the same resolution can be obtained with a lower CMCD concentration and the addition of some MeOH, compared to a MeOH free buffer.