Analysis of Strychnos alkaloids in traditional Chinese medicines with improved sensitivity by sweeping micellar electrokinetic chromatography

The application of an on-line sweeping preconcentration method in micellar electrokinetic chromatography (MEKC) for the determination of Strychnos alkaloids, namely strychnine and brucine, has been investigated in this work. After experimental optimizations, the best separation was achieved in 50 mmol l−1 H3PO4 (pH 2.0) containing 100 mmol l−1 SDS and acetonitrile in a ratio of 4:1 (v/v), with an applied voltage of −20 kV at 20 °C. The sample matrix consisted of 100 mmol l−1 H3PO4 (pH 2.0), and sample introduction was performed at 0.5 psi for 270 s, with photodiode array detection at 203 nm. Compared with the conventional MEKC injection method, up to 100-fold improvement in concentration sensitivity was achieved in terms of peak height by using this sweeping injection technique. In the method, the compound berberine was used as the internal standard for the improvement of the experimental reproducibility. The calibration curve was linear over a range of 0.5–15 μg ml−1 for both strychnine and brucine, with a correlation coefficient of 0.998 and 0.997, respectively. The detection limits (S/N = 3:1) for strychnine and brucine were 0.05 and 0.07 μg ml−1, respectively. The sweeping-MEKC method has been successfully applied to the analysis of strychnine and brucine in Strychnos nux-vomica L. and its Chinese medicinal preparations.