Chiral Microemulsion Electrokinetic Chromatography - Chiral Separations by Capillary Electrophoresis - page 245

TABLE 9.3
Examples of Enantioselective Synergy (Benei cial, None,
and Detrimental) for Two- and Three-Chiral-Component
Microemulsions a and Comparison with the Enantioselectivity
Obtained Using the Best Stereochemical Combination
of Microemulsion Components

Oil
Enantiomer

Ethyl acetate
Pseudoephedrine

Dibutyl tartrate
Synephrine

Dibutyl tartrate
N -Methylephedrine

RXX 1.135 RXX 1.038 RXX 1.067

XSX <1.002 XXR <1.002 XXR 1.009

RSX 1.135 RXR 1.038 RXR 1.077

Predicted α

RSX 1.168 RXR 1.038 RXR 1.062

Observed α

Synergy Benei cial 0.033 None 0.000 Detrimental −0.015

RSX 1.168 SXS 1.040 RXS 1.075

Best α

a Surfactant, dodecoxycarbonylvaline; cosurfactant, 2-hexanol.

While increasing the number of chiral microemulsion components can have a
signii cant positive effect on enantioselectivity, particularly if the stereochemical
combination of surfactant, cosurfactant, and/or oil is optimized, the effect on other
chromatographic i gures of merit (efi ciency, retention factor, migration window) is
usually only minimal.

9.4 FUNDAMENTAL STUDIES AND FINDINGS IN CHIRAL MEEKC
To date, only 14 articles on chiral MEEKC have been published. In addition to
these original reports, review articles have also included chiral MEEKC [21,22].
The approaches and major i ndings from the MEEKC publications are presented
in the following sections.

9.4.1 S URFACTANT -B ASED C HIRAL M ICROEMULSIONS
The majority of chiral MEEKC studies have utilized chiral surfactants to achieve
enantioresolution.

9.4.1.1 Dodecoxycarbonylvaline as the Chiral Surfactant
The i rst chiral MEEKC separation to utilize the chiral surfactant DDCV (see
Figure 9.4) was published in 2002 by Pascoe and Foley [11]. The microemulsion con-
tained 1.0% w/v surfactant, 1.2% v/v achiral cosurfactant (1-butanol), and 0.5% v/v
achiral low-interfacial-tension oil (ethyl acetate). A zwitterionic buffer was employed
to lower solution conductivity and allowed for a higher applied voltage. This chiral-
surfactant-based aggregate was used to analyze nine pairs of pharmaceutical enantiom-
ers (one neutral and eight basic); results were compared to those from chiral micellar