Skinless muscle, liver, gonads, and sediment samples were processed according to routinely performed procedures in the National Marine Fisheries Research Institute laboratory, as previously described (Waszak and Dabrowska, 2009). Sediment organic PP 242
content (Table 1) was determined as loss on ignition (LOI) after burning in a muffle furnace at 560 °C for 12 h. About 10 g of freeze-dried muscle, gonads, and sediment samples were extracted using a fully automated ASE 350 system (Dionex, Sunnyvale, CA, USA) with a mixture of dichloromethane/n-hexane (1:1 v/v). Livers, due to relatively small tissue amount, were not freeze-dried but ground with anhydrous sodium sulfate and extracted on a glass column with the same solvent mixture. Extractable organic matter, which for the purpose of this paper will be referred to as lipid content, was determined gravimetrically on an aliquot of the extract for each sample. The obtained extracts were concentrated by a rotary evaporator to about 2 mL. Then, with an addition of surrogate standards, they were cleaned up on a multilayer column (anhydrous sodium sulfate, deactivated silica, 79% w/w H2SO4-modified silica, deactivated silica, anhydrous sodium sulfate) eluated with 80 mL of hexane. These hexane extracts were concentrated by rotary evaporator and further by a gentle nitrogen stream, solvent exchanged to isooctane, and after an addition of the internal standard, reconstituted to a final volume of 0.5 mL (muscle, gonads, and sediment) or 0.2 mL (liver). The clean up of sediment
extracts involved one additional step i.e., removing of sulfur, which was done by an addition of copper to the hexane extracts before exchanging the solvent to isooctane.