Abstract: A method to determine 9 penicillin residues in milk and milk powder by high performance liquid chromatography- tandem mass spectrometry (HPLC-MS/MS) was established. Samples were extracted by acetonitrile-water, cleaned by solid-phase extraction, detected by HPLC-MS/MS and quantified by external standard method. The detection limit was 1 μg/kg for ampicillin and nafcillin, 2 μg/kg for amoxicillin, piperacillin, penicillin G, penicillin V and cloxacillin, 4 μg/kg for oxacillin and dicloxacillin in milk, and was 8μg/kg for ampicillin and nafcillin, 16 μg/kg for amoxicillin, piperacillin, penicillin G, penicillin V and cloxacillin, 32 μg/kg for oxacillin and dicloxacillin in milk powder, respectively. The linearity was satisfactory with a correlation coefficient ＞ 0.9997 at concentrations ranging from 1.0 to 50 ng/mL. Spike average recoveries for the 9 penicillins in milk was in the range of 77.58% to 97.77% with a RSD of 4.24% to 16.59% (n = 10), and Spike average recoveries in milk power in the range of 85.95% to 96.78% with a RSD of 2.71% to 13.98% (n = 10). In conclusion, the established method is convenient, rapid, accurate and sensitive so that it may be applied to determinate penicillin residues in milk and milk powder.

Key words: high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS), penicillin, bovine milk, milk powder