A confirmatory analysis of malachite green residues in rainbow trout with liquid chromatography–electrospray tandem mass spectrometry

A quantitative liquid chromatography–tandem mass spectrometric (LC–MS/MS) method has been developed for the determination of malachite green (MG) and its metabolite leucomalachite green (LMG) in fish. Residues were extracted with an acetonitrile–acetate buffer and purified using the automated solid-phase extraction (ASPEC). Residues were analyzed with a reversed-phase LC–MS/MS using a positive-ion electrospray ionisation (ESI). Isotope-labelled leucomalachite green (LMG-D5) was used as an internal standard for the quantification of LMG residues. The related dye, brilliant green (BG) was used as an instrumental standard. Identification and quantification of analytes were based on the ion transitions monitored by multiple reaction monitoring (MRM). The decision limit (CCα) for MG and LMG was 0.13 and 0.16 μg kg −1. The respective detection capabilities (CCβ) were 0.22 and 0.27 μg kg −1. The absolute recovery (repeatability SD r) was in the range of 58–65% (7.8–11.2%) for MG and 59–68% (9.7–16.9%) for LMG. LMG was quantified also based on the internal standard, giving a recovery (repeatability SD r) of 103–110% (4.8–9.3%). The method was further evaluated by analyzing a total of 34 fish residue monitoring samples, of which eight samples were found to be non-compliant containing low residues of LMG.