Determination of Benzidine in Soil by High Performance Liquid Chromatography with Ultrasonic Extraction

5.00 g of dry soil sample was taken and extracted ultrasonically for 30 min in 10 mL of methanol solution (as extractant) containing 1% (volume fraction) ammonia and 50 mg·L^-1 sodium thiosulfate (to prevent the oxidation of benzidine). Ice was added to keep the temperature below 10℃, and the mixture was settled for 10 min. 2.0 mL of the supernatant was taken and passed through 0.22 μm polytetrafluoroethylene needle filter. Benzidine in the filtrate was directly determined by high performance liquid chromatography with fluorescence detector, with ZORBAX C₁₈ reversed-phase chromatographic column as the stationary phase and a mixture of acetonitrile and 0.01 mol·L^-1 ammonium acetate solution at a volume ratio of 25:75 as the mobile phase. As shown by the results, linear relationship between values of peak area and mass concentration of benzidine was found in the range of 0.50-50.0 μg·L^-1, with detection limit of 0.20 μg·kg^-1. Test for recovery was made on blank soil sample by standard addition method, giving results in the range of 75.9%-99.1%, with RSDs (n=6) of the determined values less than 4.0%.