Recent Applications in Capillary Electrochromatography - Chiral Separations by Capillary Electrophoresis - page 478

15.2.2 S EPARATIONS ON M ONOLITHIC S TATIONARY P HASES
Various kinds of selectors are used in monolithic CSPs. They are often copolymer-
ized in case of polymeric-based monoliths, or else chemically bonded/coated onto
silica- or polymer-based support. Several overviews concerning the use of mono-
lithic chiral SPs, and of which [40,41] are recent, have already been published. One
concerns the approaches to prepare CD-based monoliths [40]. It is concluded that in
general, gels give higher efi ciencies but in terms of durability, rigid polymers and
silica-based monoliths provide better results. Regarding polymeric monoliths, the
ability of using a wide range of monomers is addressed, enabling the tailoring of
the monoliths easily according to the needs. For both polymeric and silica mono-
liths, however, some skills are required to prepare reproducible SPs. This is also the
main conclusion of another paper [41] in which monoliths with other selectors were
discussed.
Polymeric-based monolithic phases are usually prepared by a one-pot reaction
containing a polymerization mixture, which also includes a charge-providing mono-
mer. The substitution of the charge-providing monomer by another with an opposite
charge can be useful to analyze compounds with electrophoretic mobilities opposite
to the EOF. The chiral selector can also be directly incorporated into the polymeriza-
tion mixture, which presents an advantage of these types of SPs. In [42], negatively
charged vinylsulfonic acid was replaced by positively charged diallydimethylam-
moniumchloride as charge-providing agent in the synthesis of a polymeric monolith.
In this way, the EOF of the SP was superimposed on the electrophoretic mobility of
some hydroxy acids (Table 15.1), which helped to reduce retention times. Analyses
were conducted in ligand-exchange mode with Cu 2+ a s c o u nt e r io n i n t h e m o b i le p h a s e.
The selector was an l-4-hydroxyproline derivative and its concentration was found
to be crucial for enantioselectivity. In combination with the new charge-providing
agent better peak shapes were seen. In [43], N , N -diallyltartaramide was used as an

alternative cross-linker to allyl glycidyl ether, in a preparation procedure for a chiral
monolith using vancomycin as the selector. The new cross-linker could be converted
easily into aldehyde groups containing reaction products through a treatment with
periodate, resulting in cleavage of the cross-linker and the formation of two aldehyde
groups. By this approach, an already existing synthesis procedure could be short-
ened and made more generally applicable, meaning that other selectors can also be
incorporated. The enantioselectivity of the resulting CSP was demonstrated on war-
farin, thalidomide, and bupivacaine. Repeatability experiments provided acceptable
results (RSD between 0.2% and 0.4%) for retention times, selectivity, and retention
factors, and somewhat higher values for resolution and theoretical plate numbers
(RSD between 2.5% and 10.2%).
As already mentioned, varying types of selectors can be used to prepare mono-
lithic SPs. Kato et al. [44] used a BSA encapsulated monolithic phase for the chiral
separation of tryptophan enantiomers. The SP was constructed by a sol-gel method,
which formed a silica matrix that trapped BSA as chiral selector during its formation.
Generally, a poor sample loading capacity was seen, as is often observed with pro-
tein-based CSP [45]. A higher pH provided better separations due to pH dependent
conformational changes of the selector. Injection repeatability and column-to-column

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