Effect of surfactant concentration and buffer selection on chromatographic figures of merit in chiral microemulsion electrokinetic chromatography

The enantiomeric resolution of 15 different pharmaceutical compounds was explored using chiral microemulsion electrokinetic chromatography (MEEKC). The microemulsion employed was comprised of the chiral surfactant dodecoxycarbonylvaline (DDCV), 1-butanol, and ethyl acetate, at an initial composition of 1% w/v:1.2% v/v:0.5% v/v, respectively. The effect of varying the background buffer composition, voltage, and ultimately the surfactant concentration and/or aggregate phase ratio were examined. Changing from a zwitterionic buffer ((2-[2-amino-2-oxoethyl)amino]ethanesulfonic acid, ACES) to the same concentration of phosphate buffer improved the efficiency and decreased overall analysis time, but also resulted in a decrease in chiral resolution. Furthermore, using phosphate buffer while simultaneously increasing the percent DDCV from 1 to 4% increased the efficiencies from a range of 34 000 to 59 000 N/m to a range of 160 000 to 400 000 N/m. While the enantioselectivities did not change significantly, the improvement in efficiencies, elution range, and retention factors provided an increase in both resolution and the number of enantiomers that were separated. Using an optimized microemulsion comprised of phosphate buffer and 4% DDCV, chiral separation was achieved for all 11 pairs of enantiomers, with a resolution ranging from 0.90 to 4.71. Moreover, the average resolution doubled in going from nonoptimized to optimized conditions for five of the eleven compounds. Finally, a comparison was made of the effect of increasing only the surfactant concentration by a factor of 4 versus increasing the overall composition (or phase ratio) by a factor of 4. Ultimately, the microemulsion containing 4% DDCV provided a larger elution range, greater resolution, and more optimal retention than that provided by the 4 x phase increase.