A packaging contaminant: Isopropylthioxanthone (ITX) in dairy products

A fast, simple and very selective liquid chromatography–mass spectrometry (LC–MS) method for the detection of isopropylthioxanthone (ITX) in dairy products has been developed and validated. After addition of an ITX-d3 as internal standard and a simple extraction from the sample with acetonitrile, the extract was centrifuged and directly injected into the LC–MS system. Chromatographic separation was achieved by means of a Gemini C18 column (100 mm × 2.0 mm i.d. 5 μm) using a gradient of aqueous 20 mM ammonium formiate at pH 4.5 and methanol as the mobile phase, at a flow rate of 0.25 mL min−1. The method was validated according to the guidelines laid down by the Commission Decision 2002/657/EC using the parent ion [M+H]+ (m/z 255) as quantification ion, and the fragment ion (m/z 213) obtained by in-source collision-induced dissociation (IS-CID) as confirmation ion. Absolute and relative recoveries rates were verified at 5, 10, 15 μg kg−1 in yoghurt samples and at 5 μg kg−1 in milk and pudding: mean absolute recoveries were 77% in yoghurt, 50% in pudding and 67% in milk; relative recoveries (after internal standard correction) were always >97% in each matrix. The detection limit (CCα) and the detection capability (CCβ) of method were 6.2 and 7.2 μg kg−1, respectively.