Residue analysis of cortisonics in bovine milk by LC tandem MS

A new method of analysis, based on liquid chromatography-tandem mass spectrometry, was developed for the simultaneous determination of twelve steroidal anti-inflammatory drugs in cow milk. This analytical method was validated according to the European Commission Decision 2002/657/EC. Analytes were separated by ion-suppression reversed-phase liquid chromatography, due to weakly acidic nature of the selected cortisonics. Dexamethasone tetradeuterated was chosen as internal standard. Detection was in positive ion mode, using a high flow electrospray interface (TurboIonSpray). Two Multi Reaction Monitoring (MRM) transitions each analyte were selected. Analytes’ recovery in milk was made up of two steps: 1) sample deproteinization by trifluoroacetic acid; 2) solid phase extraction (SPE) by means of C18 as adsorbent and methanol as eluent. Recoveries were ≥ 70% with relative standard deviations minor than 10%. Matrix effect has become unimportant owing to the efficacy of the extraction procedure in interfering substances removal and to a suitable internal standard. In order to evaluate matrix effect, calibration curves in solvent were compared with calibration curves obtained by blank extracts spiked with analytes. Method limits were evaluated on the quantifier transition (less intense), according to the European Commission Decision 2002/657/EC for decision limit (CCalfa) and detection capability (CCbeta) assessment. Monitoring results on different fresh whole milk brands, have shown in some cases a suspected positivity. Hydrocortisone and Cortisone Acetate were found in these samples at levels between CCalfa and CCbeta. This LC-MS/MS study allowed to observe Triamcinolone transformation during the evaporation step, that could explain low recovery data found in literature.