HPLC determination of ethoxyquin and its major oxidation products in fresh and stored fish meals and fish feeds

A new method was developed to determine 1,2‐dihydro‐6‐ethoxy‐2,2,4‐trimethylquinoline (EQ, ethoxyquin), 2,6‐dihydro‐2,2,4‐trimethyl‐6‐quinolone (QI) and 1,8′‐di(1,2‐dihydro‐6‐ethoxy‐2,2,4‐trimethylquinoline) (DM) in fish meals or fish feeds, QI and DM being the oxidation products of EQ. The sample was first extracted with hexane. After the removal of hexane the three analytes were extracted from the resulting oil with acetonitrile and determined by C 18 reverse phase high‐performance liquid chromatography with a UV detector set at λ = 280 nm. The mobile phase was acetonitrile–0.01  M ammonium acetate (80:20 v/v). The recoveries for EQ, QI and DM from the samples spiked at different levels varied in the ranges 90–100 per cent, 75–85 per cent and 90–100 per cent respectively. At room temperature, QI and DM were the major oxidation products of EQ in stored fish meals and fish feeds. Loss of EQ from fish meals is faster than that from feeds, resulting in relatively higher accumulations of QI and DM in the fish meals. Both QI and DM, especially the former, were not stable during storage of either and could be further oxidised to unidentified compounds. The residue levels of these two compounds were thus unpredictable during storage intervals. When the storage temperature was increased to 50 °C, EQ disappeared more rapidly, but neither QI nor DM accumulated. © 2000 Society of Chemical Industry