Factors Influencing Cyclodextrin-Mediated Chiral Separations - Chiral Separations by Capillary Electrophoresis - page 97

It is worth noting that Lin et al. also observed an unusual temperature effect on
enantioselectivity for three basic analytes using randomly sulfate-substituted- -CD

under reversed-polarity mode [18]. They demonstrated that the temperature differ-
ently affects the mobility of both enantiomers. Indeed, the mobility differences of
the i rst-migrating (+)-enantiomers between 40°C and 25°C are higher than those of
the (−)-enantiomers above a well-dei ned CD concentration depending on the stud-
ied analyte.
In order to explain the temperature effect on enantioselectivity for four basic ana-
lytes, Peng et al. have studied the thermodynamic process during the chiral separation
using sulfated-

β

-CD in the reversed polarity mode [19]. An increase in temperature
was unfavorable to enantioselectivity for ephedrine and norephedrine, but favorable
for synephrine and epinephrine. The differences in enthalpy and entropy were calcu-
lated in order to determine if the driving force of the enantioseparation mechanism
is either enthalpy or entropy. For ephedrine and norephedrine, the enantioseparations
were an enthalpy-driven process, but an entropy-driven process for synephrine and
epinephrine. Therefore, the authors concluded that these differences in the driving
force explain the temperature effect on enantioselectivity. In addition, they related
both temperature effects to the structure of the analytes and more precisely, to the
presence of one (or two) hydroxyl group(s) on the phenyl ring.

β

4.7 DUAL SYSTEMS
Several types of dual systems including at least one CD can be applied in the
separation of enantiomers. In this section, applications dealing with the addition
of an achiral additive (calixarene or SDS) to a CD as well as dual CD mixtures are
presented.

4.7.1 A CHIRAL A DDITIVE -CD D UAL S YSTEMS
Hashem et al. studied the effect of achiral p -sulfonatocalixarenes on the enantiosep-
arations of propranolol and brompheniramine using a complete i lling technique
(CFT) of HP-

-CD [20].
The PFT implies that the capillary is i lled with the BGE containing the selector
only up to the detection window. The use of the PFT is very advantageous when the
selector exhibits a strong UV-absorbance, as it is the case for calixarenes. In both
techniques (CFT and PFT), the BGE used during the separation process in the inlet
and outlet ports of the capillary did not contain the selector. The effect of two pH
conditions (2.50 and 4.65) as well as the addition of an organic solvent (MeOH and
ACN) was evaluated. The application of HP-

β -CD combined with a partial FT (PFT) of calixarene/HP- β

β -CD (CFT) followed by calixarene/

HP- -CD (PFT) at pH 4.65 was found to give the best enantioselectivity although

the second enantiomer peak always showed a strong tailing. The authors proposed
several assumptions to explain the results but investigations with techniques such as
NMR should be undertaken in order to elucidate the molecular interactions between
the analyte, the CD derivative, and the calixarene.
Huang et al. reported the resolution of the enantiomers of four poorly water-soluble
organophosphorus pesticides in CE using nonaqueous (MeOH/ACN) and aqueous-
organic (MeOH/H 2 O/ACN) systems [21]. Sodium cholate, which is a chiral selector,

β