Total Oxidizable Precursors Assay for PFAS in Human Serum

Per- and polyfluoroalkyl substances (PFAS) are a class of chemicals including over 4700 substances. As a limited number of PFAS is routinely analyzed in human serum, complementary analytical methods are required to characterize the overlooked fraction. A promising tool is the total oxidizable precursors assay (TOPA) to look for precursors by oxidation to perfluoroalkyl acids (PFAA). The TOPA was originally developed for large volumes of water and had to be adapted for 250 μl of human serum. Optimization of the method was performed on serum samples spiked with model precursors. Oxidative conditions similar to previous TOPA methods were not sufficient for complete oxidation of model precursors. Prolonged heating time (24 hours) and higher oxidant amount (95 mg of Na2S2O8 per 225 μl of serum) were needed for complete conversion of the model precursors and accomplishing PFAA yields of 35-100 %. As some precursors are not fully converted to PFAA, the TOPA can only provide semi-quantitative estimates of oxidizable precursors in human serum. However, the TOPA can provide indications about the identity of unknown precursors by evaluating the oxidation products, including PFSA and PFECA. The optimized TOPA for human serum opens for high-throughput screening of human serum for undetected PFAA precursors.