Determination of ionophores in raw bovine milk using LC–MS/MS: Application to residue surveillance

Residues of four ionophores (lasalocid, monensin, narasin, and salinomycin) in raw milk samples were extracted with acetonitrile and subsequently determined using liquid chromatography–tandem mass spectrometry. Ionophores could be determined down to 0.1 ng g−1 level, without additional cleanup or concentration of the resulting extract. The analysis of a series of raw milk samples fortified at analyte concentrations ranging from 1 to 20 ng g−1 yielded average accuracies ranging from 60.7% to 118.3% with percent relative standard deviations below 13%. During six months of a surveillance program, 1072 raw milk samples were collected from the transport chain of dairy producers in Alberta and analysed. Monensin was detected in 736 of 1072 samples tested at concentrations ranging from 0.10 to 0.53 ng g−1 which is well below the current Canadian maximum residue limit of 10 ng g−1.