Chiral anion exchangers applied to capillary electrochromatography enantioseparation of oppositely charged chiral analytes: investigation of stationary and mobile phase parameters
M. Lammerhofer et al., Chiral anion exchangers applied to capillary electrochromatography enantioseparation of oppositely charged chiral analytes: investigation of stationary and mobile phase parameters, J CHROMAT A, 887(1-2), 2000, pp. 421-437
Weak anion-exchange (WAX) type chiral stationary phases (CSPs) based on ter
t.-butyl carbamoyl quinine as chiral selector (SO) and different types of s
ilica particles (porous and non-porous) as chromatographic support are eval
uated in packed capillary electrochromatography (CEC). Their ability to res
olve the enantiomers of negatively charged chiral analytes, e.g., N-derivat
ized amino acids, in the anion-exchange mode and their electrochromatograph
ic characteristics are described in dependence of several mobile phase para
meters (pH, buffer type and concentration, organic modifier type and concen
tration) and other experimental variables (electric field strength, capilla
ry temperature). The inherent "zwitterionic" surface character of such sili
ca-based WAX type CSPs (positively charged SO and negatively charged residu
al silanols) allows the reversal of the electroosmotic flow (EOF) towards t
he anode at pH values below the isoelectric point (pI) of the modified surf
ace, whereas a cathodic EOF results at pH values above the pi. Since for ne
gatively charged analytes also an electrophoretic transport increment has t
o be considered, which can be either in or against the EOF direction, sever
al distinct modes of elution have been observed under different stationary
phase and mobile phase conditions: (i) co-electrophoretic elution of the ne
gatively charged solutes with the anodic EOF in the negative polarity mode,
(ii) counter-electrophoretic elution with the cathodic EOF in the positive
polarity mode, and (iii) electrophoretically dominated elution in the nega
tive polarity mode with a cathodic EOF directed to the injection end of the
capillary. Useful enantioseparations of chiral acids have been obtained wi
th all three modes. Enantioselectivity values as high as under pressure-dri
ven conditions and theoretical plate numbers up to 120 000 per meter could
be achieved under electrically driven conditions. A repeatability study yie
lded RSD values below 2% for retention times and RSD values in the range of
5-10% for theoretical plate numbers and resolution, thus clearly establish
ing the reliability of the investigated anion-exchange type CEC enantiosepa
ration methods. (C) 2000 Elsevier Science B.V. All rights reserved.