LC-MS/MS Determination of Residual Amounts of Methomyl and Oxamyl in Onion

The sample (5.000 0 g) was mixed with 10 mL of acetonitrile and 5 g of anhydrous sodium sulfate, and the solution was extracted by vertical oscillation for 30 min. The extract (2 mL) was purified with 14 mg of grapheme by dispersive solid phase extraction (DSPE). The purified solution was then separated by LC using Poroshell 120 EC-C₁₈ column as stationary phase and a mixture of methanol and water in various ratios as mobile phase in gradient elution. MRM was adopted in MS analysis, and external standard method was used for quantification. Linear relationships between values of peak area and mass concentration of methomyl and oxamyl were found in the range of 5.00-200 μg·L^-1, with detection limits (3S/N) of 3.0, 0.2 μg·kg^-1, respectively. Test for recovery was made by standard addition at 3 concentration levels, giving results in the range of 66.5%-94.0%, with RSDs (n=6) ranged from 7.4% to 11%.