Determination of residues of malachite green in aquatic animals.

Residues of malachite green (MG) were extracted from homogenized animal tissues with a mixture of McIlvaine buffer (pH 3.0)-acetonitrile, and purified over an aromatic sulfonic acid solid-phase extraction column followed by HPLC or LC-ESI-MS-MS analysis. Ascorbic acid and N,N,N',N'-tetramethyl-1,4-phenylenediamine dihydrochloride were added to reduce de-methylation of the dye. Responses were recorded at 620 nm (HPLC) or by multiple-reaction-monitoring (LC-MS-MS) after post-column oxidation using PbO(2). MG and its primary metabolite leuco-malachite green (LMG) were successfully determined at 2.5-2000 microg/kg in catfish, eel, rainbow trout, salmon, tropical prawns and turbot, with a limit of detection at 1 microg/kg (HPLC) and 0.2 microg/kg (LC-MS-MS) for both MG and LMG. Recoveries for LMG were between 86+/-15% (prawn) and 105+/-14% (eel). Freeze-thawing cycles, and storage at 4 degrees C and -20 degrees C affected the recovery of both MG and LMG. Analyses of eel, trout and (processed) salmon field samples collected at local retailers, fish-market and -shops demonstrated trace levels of MG-residues.