Chiral Microemulsion Electrokinetic Chromatography - Chiral Separations by Capillary Electrophoresis - page 256

followed by sonication and polymerization. Residual 1-butanol and n -heptane not
encapsulated within the microemulsion polymer (MP) were evaporated and the
polymer was then freeze dried. The MP was dissolved in buffer before use. A smaller
set of enantiomers was used to evaluate MP systems with better separations obtained
with 3.5%, 1.50%, and 0.25% w/w 1-butanol for BNP, BOH, and the barbiturate
derivatives, respectively. Compared to microemulsion formulations containing poly-
meric surfactant, improved resolution was obtained for BNP only although baseline
separation could not be achieved.

9.4.2 C OSURFACTANT -B ASED C HIRAL M ICROEMULSIONS
One chiral MEEKC publication demonstrated the ability of chiral 2-alkanols
(cosurfactant) to resolve enantiomers [13]. Initially, R -(−)-2-butanol (6.6%), R -(−)-
2-pentanol (5.0%), R -(−)-2-hexanol (5.0%), and R -(−)-2-heptanol (3.3%) were evalu-
ated in microemulsion formulations containing SDS (3.5%) and n -octane (0.8%).
The shortest chain cosurfactant, 2-butanol, did not provide any enantioresolution
for the i ve pairs of enantiomers tested. This particular alcohol was deemed to have
too short of an alkyl chain to effectively span the interfacial region of the nanodrop-
let. Separations with 2-pentanol were more successful for three compounds. When
2-hexanol was employed, better resolution was obtained for some of the analytes
but two pairs of enantiomers remained unresolved. Chiral 2-heptanol enabled pro-
pranolol to be separated (unresolved with all other cosurfactants) but reduced the
resolution for enantiomers previously resolved. One of the analytes, N -methyl ephed-
rine, could not be separated with any of the cosurfactants utilized. Structurally, this
analyte lacks the -amino proton present in some of the other compounds thereby

indicating that the chiral recognition mechanism was reliant on hydrogen bonding
with the chiral reagent. Further optimization of the chiral-2-hexanol microemul-
sion was performed by varying cosurfactant concentration, pH, oil phase identity,
and oil concentration. In terms of 2-hexanol concentration, a range of 1%-6% was
studied (Figure 9.12) with an optimal value of 5% established. The pH range used
was 7.2-10.2 and showed the importance of analyte ionization with decreased reso-
lution when the enantiomers were neutral (pH above their p K a ). Ethyl acetate was
substituted for n -octane as the oil phase (same concentration of 0.8%) and resulted
in the same migration order but with decreased resolution. Better approaches for the
evaluation of oil identity would have been to use the same molarity ethyl acetate as
n -octane or to adjust the concentration of the other microemulsion components to
compensate for the lower interfacial tension of ethyl acetate. The last variation in
the microemulsion system composition was the elimination of the oil, resulting in no
resolution. The ability to reverse the elution order by simply switching the cosurfac-
tant stereochemistry was demonstrated for the enantiomers of norephedrine.

β

9.4.3 O IL -B ASED C HIRAL M ICROEMULSIONS
The i rst separation via chiral MEEKC was reported by Aiken and Huie in 1993 and
combined the chiral oil (2 R , 3 R )-di- n -butyl tartrate (0.5% w/w, shown in Figure 9.13)