Rapid residue analysis of fluoroquinolones in raw bovine milk by online solid phase extraction followed by liquid chromatography coupled to tandem mass spectrometry

Online solid phase extraction followed by liquid chromatography coupled to tandem mass spectrometry was used for the determination of 15 fluoroquinolones and one quinolone antibiotic in raw bovine and skimmed commercial milk. Milk samples were partially deproteinized and defatted by 15 min centrifugation and then subjected to online solid phase extraction. Chromatographic separation was achieved in less than 15 min. Identification and quantification of the compounds of interest were performed by selected reaction monitoring, using an electrospray ionization source. Relative recoveries were calculated for raw milk (at 0.5, 1 and 1.5 times the MRL, or assuming 30 ng/mL for non permitted compounds) and skimmed milk (at 30 ng/mL) and ranged between 65% and 123% (raw milk) or 78% and 109% (skimmed milk). Matrix interferences were also assessed and yielded significant suppression and enhancement effects, which, at the time of quantification, were corrected by the use of six isotopically labeled standards. Quality parameters of the method were established, and limits of detection of selected antibiotics in raw milk ranged from 0.01 to 1.93 ng/mL, while in skimmed milk from 0.03 to 4.23 ng/mL. The validation of the method has been carried out according to the requirements set by the 2002/657/EC regulation. Finally, the applicability of the method was tested by analysing 28 milk samples.