Recent Applications in Capillary Electrochromatography - Chiral Separations by Capillary Electrophoresis - page 481

on resolution, an effect that was only seen when the chiral MIP was used, mean-
ing that the enantiorecognition was not attributed to Tween 20. In [62], an attempt
was made to separate two steroid isomers, 17- -estradiol, which

differed in the position of an OH group. For this purpose, a MIP was synthesized
with

α -estradiol and 17- β

-estradiol as an imprinting molecule. However, only a slight difference in the
retention times of both isomers was achieved. Other studies with MIPs describe
the use of ( S )-naproxen [63], ( S )-ibuprofen [64], (5 S , 11 S ) Tröger's base [65], and
l-tetrahydropalmitine [65] as imprinting molecules for the analysis of their respec-
tive racemates.
Because of the shrinking or swelling that can occur with polymer-based MIPs,
or the cracking and shrinking with silica-based MIPs, a novel approach was used in
[66] for the preparation of silica-based hybrid MIPs. The methodology is based on a
room temperature ionic liquid (RTIL) mediated, nonhydrolytic sol-gel process and a
molecular imprinting technique, producing monoliths that do not require ageing and
drying at high temperatures, and do not display cracking or shrinking. Moreover,
an improved selectivity was seen, as naproxen enantiomers could be resolved with a
resolution of 8.8. Compared with other MIP applications, peaks shapes were accept-
able because the ( S )-naproxen peak (i.e., the retained peak) still exhibited 12,000
plates/m. A MIP was prepared in [67] with zolmitriptan as imprinting molecule
using the above preparation technology.

β

15.3 ENANTIOSEPARATIONS IN PRESSURIZED CAPILLARY
ELECTROCHROMATOGRAPHIC MODE
pCEC is a mode of CEC, where only the inlet vial is pressurized during analysis.
Consequently, a hydrodynamic l ow is superimposed onto the EOF, which generally
results in a faster elution. The l ow proi le is more parabolic than in CEC, and there-
fore the efi ciencies in pCEC are frequently lower than in regular CEC mode. pCEC
experiments are possibly executed on two types of instruments. Either a regular CE
instrument or a modii ed CLC device can be used. In the modii ed CLC device, an
HPLC pump pumps the mobile phase over the capillary column while an electri-
cal i eld is applied simultaneously and this is commercially available. In the latter
instrument, the pressure over the column is kept constant by means of a backpressure
regulator to enable a constant linear velocity inside the CEC column.
A n over view of the applications in the ch i ra l sepa ration i eld is given in Table 15.2.
Although the number of papers dealing with chiral separations in pCEC is lower
than in regular CEC, applications with both particle-based and monolithic columns
were found.

15.3.1 S EPARATIONS ON P ARTICLE -B ASED S TATIONARY P HASES
T he coupl i ng of CEC to elect rospray ion izat ion ( ESI ) mass spect romet r y ( MS) a nd its
application to enantioseparations was described in [68]. Actually, the mode the authors
worked in was pCEC, as only the inlet of the capillary column can be pressurized
when coupling to MS. The CEC-MS experiments were performed on a CE instru-
ment using a Whelk-O1 CSP to separate warfarin and coumachlor enantiomers, the