Quantitation and on-line concentration of enantiomers in open-tubular capillary electrochromatography

The feasibility of open-tubular capillary electrochromatography (OTCEC) with UV detector for quantitation of enantiomers is explored, and a simple on-line sample concentration method to improve detection sensitivity of negatively charged enantiomers more than 1000-fold is described. With a capillary of 25 microm ID, the limits of detection (LODs) for absolute concentration and for enantiomeric ratio are 10(-6) M and 0.6-0.8% (signal-to-noise ratio S/N = 10). Good linearity and reproducibility are observed. The detection sensitivity is enhanced by combination with field-enhanced sample injection (FESI). A water plug is introduced hydrodynamically into the capillary inlet end and then the sample solution prepared with water is introduced with electrokinetic injection. With this concentration technique, the LOD for absolute concentration is reduced to a 10(-9) M level. On the other hand, due to the peak-sharpening effect of FESI, the LOD for enantiomeric ratio for the first-eluted enantiomer is significantly improved, being 0.3%. Effects of the injection conditions, such as length of water plug, buffer concentration, injection voltage, and injection time on the enrichment efficiency are investigated. Online concentration of a racemic compound with two chiral centers is demonstrated.