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

► A rapid method was developed for the determination of ionophore antibiotics in raw milk samples using LC-MS/MS. ► The method permits the determination of four ionophores at concentrations as low as 0.1 ng g−1 with no cleanup. ► The method was successfully employed in a surveillance study of over 1000 raw milk samples. ► Monensin was detected in 69% of samples tested at concentrations ranging from 0.10 to 0.53 ng g−1, well below the current Canadian maximum residue limit of 10 ng g−1.