Uncovering hidden PFAS burden in Canadian fish: A direct Total Oxidizable Precursor (dTOP) assay approach

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants that pose significant risks to ecosystems and human health. Fish is a significant source of human exposure to PFAS that bioaccumulate in food webs. However, total PFAS burden in fish, including contributions from precursor compounds, remains insufficiently characterized due to conventional analytical limitations. This study identified and quantified PFAS in liver samples from northern pike (Esox lucius) collected in the St. Lawrence River, QC, Canada. A direct Total Oxidizable Precursor (dTOP) assay was employed to quantify the total PFAS burden in fish, in which small amounts of liver tissue were fully oxidized rather than their extracts, to mitigate potential losses during extraction. The dTOP assay converts precursors into measurable perfluoroalkyl carboxylates (PFCAs) without requiring an extraction procedure, thereby providing a direct and comprehensive assessment of PFAS contamination. Targeted analysis showed that perfluoroalkyl sulfonates (PFSAs) dominated the PFAS class, with median concentrations an order of magnitude higher than those of PFCAs. Bioaccumulation factors for fish were statistically and linearly associated with chain length for C5-C11 PFCA (R² = 0.72, PFPeA to PFUnDA) and C6-C8 PFSAs (R² = 0.88, PFHxS to PFOS). Fluorotelomer-derived and electrochemical fluorination-derived precursors were also detected at lower concentrations. The dTOP assay revealed substantial amounts of oxidizable precursors in fish liver samples, which had previously been undetectable by targeted analysis. By revealing the substantial contribution of PFAS precursors to total PFAS burdens, this study emphasizes the need for comprehensive monitoring frameworks that account for both terminal and precursor compounds in aquatic biological tissues.