Sweeping-micelle to solvent stacking for the on-line preconcentration and determination of organic acids in Angelica sinensis by capillary electrophoresis

A novel on-line two-step stacking preconcentration method by sweeping plus micelle to solvent stacking in capillary zone electrophoresis was developed for the simultaneous determination of three organic anions (vanillic acid, ferulic acid and cinnamic acid) in Angelica sinensis. Hexadimethrine bromide was used for electroosmotic flow reversal. The main experimental parameters that affected the separation and sensitivity were investigated and optimized. The best separation was achieved in 50 mM ammonium acetate (pH 12.0) containing 50% methanol (v/v) under a negative voltage of 20 kV. The micellar solution was a mixed solution comprised of 12 mM cetyltrimethyl ammonium bromide and 20 mM ammonium acetate. After injecting a short plug (0.5 psi, 30 s) of micellar solution, the sample was introduced into the capillary at 0.5 psi for 45 s. Under the optimal conditions, the sensitivity enhancement factors obtained by the developed method were between 42 and 77. The intra-day (n = 6) and inter-day (n = 5) precisions of the method expressed as their relative standard deviations were found to be less than 7.2%. The Angelica sinensis sample was pulverized and then refluxed in 95% ethanol and filtered. After an aliquot of the extractant was dried, it was reconstituted in 20 mM ammonium acetate for capillary electrophoresis analysis. The recoveries of the analytes by this method for the analysis of Angelica sinensis were in the range of 94.4% to 108.4%.