Chiral anion exchangers applied to capillary electrochromatography enantioseparation of oppositely charged chiral analytes: investigation of stationary and mobile phase parameters

Citation
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
Citations number
32
Categorie Soggetti
Chemistry & Analysis","Spectroscopy /Instrumentation/Analytical Sciences
Journal title
Volume
887
Issue
1-2
Year of publication
2000
Pages
421 - 437
Database
ISI
SICI code
Abstract
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.