A simple and rapid confirmatory assay for analyzing antibiotic residues of the macrolide class and lincomycin in bovine milk and yoghurt: hot water extraction followed by liquid chromatography/tandem mass spectrometry

A rapid and simple sample preparation procedure for determining residues of antibiotics of the classof macrolides and lincomycin in whole milk and yoghurt by liquid chromatography/tandem massspectrometry (LC/MS/MS) is presented. The method is based on the matrix solid-phase dispersion(MSPD) technique with hot water as extractant. After dispersing samples of milk and yoghurt onsand, target compounds were eluted from the MSPD column by passing through it 5mL of wateracidified with 30 mmol/L formic acid and heated at 708C. After pH adjustment and filtration, a volumeof 200mL of the aqueous extract was directly injected into the LC column. MS data acquisition wasgenerally performed in the multiple reaction monitoring (MRM) mode, selecting two precursor ion toproduct ion transitions for each target compound. Hot water appeared to be an efficient extractingmedium, since absolute recoveries of the analytes in milk and yoghurt were respectively 68–86% and82–96%. The method proved to be robust as matrix effects, even though present, did not affectsignificantly the accuracy of the method, as evidenced by analyzing six different batches of both milkand yoghurt. Using roxithromycin (a macrolide antibiotic not used in veterinary medicine) assurrogate internal standard, the accuracy of the method at three different spike levels of the analytesin milk and yoghurt was 86–107% (RSDs not larger than 10%) and 97–117% (RSDs not larger than13%), respectively. On the basis of a signal-to-noise ratio of 10, we estimated this method can quantifya few ppb of the analytes in milk and yoghurt. These concentrations are well below the tolerancelevels of macrolides and lincomycin in milk set by both the European Union and the US Food andDrug Administration. On analyzing six yoghurt samples, we found evidence for the fact that one ofthe six samples was contaminated with erythromycin B.[...