Residues of Ethoxyquin and Ethoxyquin Dimer in Ocean-Farmed Salmonids Determined by High-Pressure Liquid Chromatography

The occurrence of residues of ethoxyquin (EQ) and ethoxyquin dimer (DM) from fish feeds in the liver and muscle of farmed salmon and steelhead was studied. For quantitative analysis of DM, the tissue was partially hydrolyzed with 50% NaOH, and then the lipid and antioxidant residues were extracted with hexane. After solvent removal and recovery into acetonitrile, DM was determined by reversed‐phase high‐pressure liquid chromatography (HPLC) with a fluorescence detector, with the excitation wavelength set at 360 nm and the emission wavelength at 440 nm. The mobile phase was acetonitrile:0.01M ammonium acetate (80:20, v:v). The EQ levels were very low, but the DM levels were higher. The recoveries from the spiked samples were 88 ± 4% for DM. Sample site determinations indicated that the DM residue level could be associated with depot fat except in the liver, where the tissue content of DM was 60% to 70% less. Similar results appear satisfactory for ethoxyquin extracted from fish oils.