Electrokinetic supercharging in nonaqueous capillary electrophoresis for online preconcentration and determination of tamoxifen and its metabolites in human plasma

An online preconcentration method, namely electrokinetic supercharging (EKS), was evaluated for the determination of tamoxifen and its metabolites in human plasma in nonaqueous capillary electrophoresis with ultraviolet detection (NACE-UV). This method was comprehensively optimized in terms of the leading electrolyte (LE) and terminating electrolyte (TE) injection lengths, as well as electrokinetic sample injection time. The optimized EKS conditions employed were as follows: hydrodynamic injection (HI) of 10mM potassium chloride as LE at 150mbar for 36s (4% of total capillary volume). The sample was injected at 10kV for 300s, followed by HI of 10mM pimozide as TE at 150mbar for 36s (4% of total capillary volume). Separation was performed in 7.5mM deoxycholic acid sodium salt, 15mM acetic acid and 1mM 18-crown-6 in 100% methanol at +25kV with UV detection at 205nm. Under optimized conditions, the sensitivity was enhanced between 160- and 600-fold when compared with our previously developed method based on HI at 150mbar for 12s. The detection limit of the method for tamoxifen and its metabolites were 0.05-0.25ng/mL, with RSDs between 2.1% and 3.5%. Recoveries in spiked human plasma were 95.6%-99.7%. A comparison was also made between the proposed EKS approach and the standard field-amplified sample injection (FASI) technique. EKS proved to be 3-5 times more sensitive than the FASI. The new EKS method was applied to the analysis of tamoxifen and its metabolites in plasma samples from breast cancer patients after liquid-liquid extraction.