Recent Applications in Capillary Electrochromatography - Chiral Separations by Capillary Electrophoresis - page 486

electrolyte, retention times were decreasing as a function of pH due to a higher EOF
velocity, followed by an increase due to a reduction in the electrophoretic mobili-
ties as the analytes become less charged at higher pH. Using phosphate, no signii -
cant changes in elution time were seen because the electrophoretic mobilities were
higher than the EOF. However, phosphate provided more and better enantiosepara-
tions. Another application on a modii ed CLC system describes the separation of
α -amidophosphonates on perphenylcarbamoylated- β -CD based CSP [74].

15.3.2 S EPARATIONS ON M ONOLITHIC S TATIONARY P HASES
CD-modii ed silica particles (Chiradex) were packed inside a capillary and immobi-
lized using a sol-gel solution as presented in [75]. The particles are glued together and
are retained in the capillary without the need for frits. The resulting SP is often referred
to as a particle-glued monolith in the literature. Separations were conducted in pCEC
mode and compared with CLC separations. The results indicated that with shorter
analysis times, higher theoretical plate numbers (and hence higher Rs) can be achieved
using pCEC mode due to the superposition of the EOF onto the pressure-driven l ow.
The highest selectivity was obtained when the EOF and the pressure-driven l ow direc-
tions were the same and when the analyte is uncharged, eliminating its electrophoretic
mobility. Of course, analysis times become longer in this situation.
In [76], CSPs created from silica particles with either a teicoplanin aglycone selec-
tor or a ristocetin A selector, embedded in an achiral continuous bed were studied,
similar to the approach presented in [60] where a ligand-exchange CSP was used.
There was no need to pressurize both vials during analysis, thus it could be con-
ducted on any CE instrument. The highest possible concentration of silica particles
in the polymeric matrix was found to be 25%. These CSPs were tested using sev-
eral analytes, such as aliphatic and aromatic acids, and dipeptides (Table 15.2). The
phases showed acceptable enantioselectivity toward the selected analytes. Moreover,
the EOF direction can be favored in the direction of the electrophoretic mobility of
the analyzed substance by means of the used charge-providing agent in the poly-
merization mixture. In [77], particle-loaded monoliths were prepared with teico-
planin aglycone bonded particles. The achiral monolith was prepared using a ring
opening metastasis reaction. This reaction is much faster than the regular polymer-
ization of, for example, methacrylamide monoliths and provides SPs within 30 min.
The selected analytes were 12 glycyl-dipeptides, which at least could be partially
resolved. Compared with the particle-based monoliths from [76], these SPs showed
a similar separation behavior, separation power, and robustness against high electri-
cal i elds and back pressures. It indicates that the achiral backbone does not have a
signii cant inl uence on the chiral performance of the SP.

15.4 ENANTIOSEPARATIONS IN OPEN-TUBULAR
CAPILLARY ELECTROCHROMATOGRAPHY
In OT-CEC, a layer of SP is coated or bonded to the capillary wall. Because the SP
does not completely i ll the capillary, shortcomings such as a low sample loading
capacity and an instability of the SP over time are often observed. Nevertheless,