Abstract: A method has been developed for the simultaneous determination of 128 pesticide residues in milk by liquid chromatography-tandem electrospray mass spectrometry (LC-ESI-MS/MS). A total of 10 mL milk was extracted with 20 mL acetonitrile (plus 4 g magnesium sulfate and 1 g NaCl) for two times. The concentrated supernatant was cleaned-up by a C18 solid phase extraction cartridge to remove the lipophilic compounds. The eluate was concentrated to about 0.5 mL in 45 ℃ water bath with nitrogen. The 1 mL mixture of acetonitrile and water (3:2, v/v) was added into the sample and mixed thoroughly for 30 s. The sample solution was filtered by 0.2 μm millipore filters before LC-ESI-MS/MS determination. This method was evaluated at the two spiked levels of 2 times and 8 times of limits of the detection (LODs) in 5 parallel experiments, respectively. The results showed that the average spiked recoveries of 128 pesticides at the low spiked level (0.14 μg/L~0.62 mg/L) were from 60.4% to 118.4% with relative standard deviations (RSDs) of 2.1%~24.3%, and the average spiked recoveries at the high spiked level (0.56 μg/L~2.48 mg/L) were from 64.4% to 118.5% with the RSDs of 1.3%~24.1%. The method showed a good linear relationship with the correlation coefficients over 0.99. The LODs were from 0.07 μg/L to 0.31 mg/L. This method is selective, accurate and easy to operate.

Key words: milk , multi-pesticide residues, liquid chromatography-tandem mass spectrometry (LC-MS/MS)