Abstract
We have employed a high‐sensitivity on‐line preconcentration method, cation‐selective exhaustive injection (CSEI) and sweeping MEKC, for the analysis of cocaine, benzoylecgonine, norcocaine, and cocaethylene. We monitored the effects of several of the CSEI‐sweeping‐MEKC parameters – including the pH, the concentrations of SDS and organic modifier, the injection length of the high‐conductivity buffer, and the injection time of the sample – to optimize the separation process. The optimal BGE was 100 mM phosphoric acid (pH 1.8) containing 75 mM SDS with 10% 2‐propanol and 10% tetrahydrofuran as the organic modifier. In addition, electrokinetic injection of the sample at 15 kV for 900 s provided both high separation efficiency and enhanced sweeping sensitivity. The sensitivity enhancements for cocaine, norcocaine, and cocaethylene ranged from 2.06×10^4^ to 3.96×10^4^; for benzoylecgonine it was 1.75×10^3^; the coefficients of determination exceeded 0.9958. The LODs, based on an S/N ratio of 3:1, of sweeping‐MEKC ranged from 33.5 to 52.8 ng/mL; in contrast, when using CSEI‐sweeping‐MEKC the sensitivity increased to range from 29.7 to 236 pg/mL. Under the optimal conditions, we analyzed cocaine in a human urine sample prepared using off‐line SPE to minimize the influence of the matrix. The recovery of the SPE efficiency was satisfactory (ca. 74.9–87.6%). Our experimental results suggest that, under the optimal conditions, the CSEI‐sweeping‐MEKC method can be used to determine cocaine and its metabolites with high sensitivity in human urine.