The homogenized sample is diluted with water. After adding copper(II) chloride solution and 20 mg of ascorbic acid, dilute the sample to the volume with water. After thorough mixing and microfiltration over a 0.45 microns filter, the filtrate is ready for injection into the HPLC system and detected at 254 nm. The 125 mm x 4 mm i.d. chromatographic column is packed with Hypersil ODS 5 microns, eluted with a mobile phase of water-methanol (80:20, V/V) containing 20 mmol/L tetrabutyl ammonium bromide, 0.03 mol/L sodium acetate buffer (pH 4) at a flow rate of 0.8 mL/min. The injection volume is 20 microL. The response value was linear between 10 mg/L-400 mg/L. The recovery was 95.5%-98.9%. The RSD was 0.82%-1.32%.