Differences between Bligh and Dyer and Soxhlet extractions of PCBs and lipids from fat and lean fish muscle: Implications for data evaluation

Both Bligh and Dyer extraction using chloroform/ methanol and Soxhlet extraction using hexane/acetone were employed for extracting total lipid and polychlorinated biphenyls (PCBs) from muscle tissue in four species of fish: herring (Clupea harengus), salmon (Salmo salar), cod (Gadus morhua) and Northern pike (Esox lucius), all caught in the southern part of the Baltic Sea. The Bligh and Dyer method allowed a greater amount of total lipid but a lesser amount of total PCBs to be extracted than the Soxhlet method with hexane/acetone did. For all the fish except cod, the sample wet weight PCB concentrations differed significantly for the two extraction methods. When the sample PCB was normalized to extracted total lipid, the differences in the yield of the two methods increased due to differences in total lipid yield. This was most pronounced for the lean fish (cod and pike). When the two methods were compared in terms of efficiency in the extraction of different PCBs, a relationship of this to PCB congener lipophilicity, expressed in terms of Kow, could be shown. Differences between methods and variability within the data provided from the first method were highest for the PCB congeners in the lipophilicity regions of log Kow < 6.5 and > 7.5. The study indicated that comparability is better for PCB analysis data from triacylglycerol-rich samples (herring and salmon) than from lean samples (cod and pike). The results suggest that in order to avoid comparisons of data from different samples being misleading, PCB pattern and/or single congener data should be obtained using the same extraction technique.