Determination of 29 Residual Herbicides in Vegetables by HPLC-MS/MS with QuEChERS Extraction

A portion of 5.000 g of the homogenized and pulpified vegetable sample was taken and extracted by shaking for 20 min with 10.0 mL of a mixture of formic acid and acetonitrile (1+99), 2.0 g of NaCl were added to the mixture and the shaking was continued for another 20 min. After centrifuging for 5 min, an aliquot of 1.5 mL of the supernatant was taken and purified by QuEChERS with a powder mixture containing 100 mg each of N-propyl ethylenediamine and octadecyl silane and 10 mg of self-prepared magnetized carbon nanotubes. The mixture was then filtered through 0.22 μm filtering membrane, and the filtrate was used for HPLC-MS/MS analysis. In the HPLC separation, Waters Cortects C₁₈ column was selected as stationary phase and mixtures of CH₃CN (A) and 5 mmol·L^-1 CH₃COONH₄ solution (B) with various ratios were used as mobile phase in the gradient elution. Residual amounts of the 29 herbicides in the eluates were determined by MS/MS under the instrumental conditions. As shown by the results, linear relationships between values of peak area and mass concentration of the 29 herbicides in the range of 5.0-40 μg·L^-1 were obtained, and the detection limits (3S/N) in the range of 0.5-1.5 μg·kg^-1. Test for recovery by standard addition method was made using a blank vegetable sample as matrix, giving results in the range of 83.9%-99.7%, and RSDs (n=6) ranged from 1.3% to 5.6%.