Determination of residues of malachite green in finfish by liquid chromatography tandem mass spectrometry

A liquid chromatography tandem mass spectrometry (LC-MS/MS) method for the determination of leuco-malachite green (LMG) in various fish tissues is described. LMG, which is the primary metabolite of the parasiticide and fungicide malachite green (MG), is the targeted analyte to reveal abuse of this veterinary drug in fish. After extraction using McIlvaine buffer and acetonitrile, the extract was purified on an aromatic sulphonic acid solid-phase extraction column. After conversion of LMG into MG by post-column oxidation with PbO2, the effluent was analysed by LC-MS/MS in the multiple reaction monitoring (MRM) mode. The MS-MS trace m/z 329 →m/z 313 was used for quantification of LMG and, for salmon, gave an averaged decision limits (CCα; α 1%) and detection capability (CCβ; β 5%) of 0.11 and 0.18 μg kg−1, respectively, with a measurement each of three consecutive days. The last values were comparable for those for MG. Other traces were used to collect sufficient identification points to establish the identity of this prohibited veterinary drug, which was achieved at CCβ and higher. These values were comparable for other tested species, including pangasius, tilapia, trout and Victoria perch. Recoveries ranged from 66% in trout at 0.4 μg kg−1 to 112% in pangasius at 0.1 μg kg−1. Three out of nineteen samples including pangasius, salmon, shrimps and trout bought in local shops, revealed detectable amounts of residues, i.e. in excess of CCα, and were considered non-compliant. The findings demonstrate the suitability of the presented analytical method to detect residues of malachite green in various aquatic species at relatively low residue levels.