Effect of Microemulsion Composition on Chiral Separations Using Microemulsion Electrokinetic Chromatography
Adeline B. Kojtari and Joe P. Foley. Drexel University, Philadelphia, PA
Electrokinetic chromatography (EKC) has been proven to be a useful tool in order to separate charged, neutral, and chiral species with short run times, high efficiencies and excellent resolution. EKC differs from capillary electrophoresis (CE) in that a pseudostationary phase (PSP) is employed in order to separate analytes, providing selectivity that is not achievable in CE. Water-in-oil microemulsions can be employed as a PSP, and are particularly useful for the separation of achiral and chiral analytes that have limited water solubility. Microemulsions are spherical aggregates comprised of a surfactant, co-surfactant, and an oil, and can be prepared with one or more chiral constituents in order to separate enantiomers of pharmaceutical and other compounds. Numerous combinations of one-, two-, and three-chiral-component microemulsions have been previously prepared in our group, using dodecoxycarbonylvaline (DDCV; R, S, or racemic), racemic 2-hexanol, and dibutyl tartrate (D, L, or racemic) or diethyl tartrate (D, L, or racemic), with effective chromatographic figures of merit. A two-chiral component system has previously shown to be most effective, where a chiral surfactant and oil will be used as the PSP (racemic co-surfactant). However, when microemulsions formulated with equimolar concentrations of dibutyl tartrate and diethyl tartrate (DET) are compared (lower volume percentage of DET), the efficiency observed with DET is significantly lower. This study will investigate new formulations of DDCV-2-hexanol-DET microemulsions with the objective of improving the efficiency without compromising other chromatographic figures of merit such as enantioselectivity, elution range, and of course resolution. The first formulation to be tested will employ an equal volume percentage of diethyl tartrate as previously formulated with dibutyl tartrate rather than an equimolar concentration. Results will be reported for six pairs of pharmaceutical enantiomers: ephedrine, pseudoephedrine, N-methylephedrine, metoprolol, synephrine, and atenolol.