On-capillary sample preconcentration incorporated in chiral capillary electrophoresis

The on-capillary sample enrichment technique was developed to improve the sensitivity of UV-detection in capillary electrophoresis (CE). The enrichment was achieved in a preconcentration zone (zone length 2 mm) filled with the packing material used for HPLC, which is fixed by an inner frit near to the inlet of the capillary. The feasibility of this technique was demonstrated by chiral separation of a low concentration of verapamil (VER, a calcium channel blocker) enantiomers. The preconcentration procedure involves initial conditioning of the capillary, hydrodynamic sample introduction, washing of adsorbed VER, and its transfer to the capillary. Enriched VER was separated into enantiomers using a chiral selector(25 mM trimethyl-beta-CD) dissolved in the run buffer (pH 2.5). The detection limit was 5 nM, about 200-times lower than that without preconcentration. The detection response indicated good linearity against the concentration of VER over the 10 - 100 nM range for each enantiomer (r>0.999). The present CE system was applied to determine the unbound concentration of VER enantiomers in human plasma after ultrafiltration.