Rapid separation of pharmaceutical enantiomers using electrokinetic chromatography with a novel chiral microemulsion

Abstract

A novel oil-in-water microemulsion incorporating the chiral surfactant dodecoxycarbonylvaline (DDCV) was used to achieve the rapid enantiomeric separation of pharmaceutical drugs by electrokinetic chromatography (EKC). Incorporation of DDCV into a microemulsion resulted in an elution range more than double that provided the micellar form of the surfactant aggregate. Interestingly, for the same compounds the enantioselectivity provided by the chiral DDCV microemulsions ranged from 1.06–1.30 for the neutral and cationic drugs, which was slightly higher than that provided by chiral DDCV micelles. The use of a low surface tension oil (ethyl acetate) permitted a much lower concentration of chiral surfactant to be employed; this, together with the use of a zwitterionic buffer (ACES) resulted in a very low conductivity microemulsion that allowed a higher separation voltage to be utilized, resulting in rapid enantiomeric separations (<8 min.). Mobility matching of the buffer cation(s) was used to improve peak shape and efficiencies. In our limited survey of the phase diagram, the optimum composition of the microemulsion buffer was 1.0% (w/v) DDCV (30 mM), 0.5% (v/v) ethyl acetate, 1.2% (v/v) 1-butanol and 50 mM ACES buffer at pH 7.