The above results suggest a series of similarities and differences between SO4 -• and HO• for the oxidation of legacy and emerging "PFAS precursors" and their potential value for PFAS sample analyses. First of all, both radicals are destructive to the reactive PFAS structures. The defluorination from NBP-H, 6:2 FTSA, and 6:3 FTCA by HO• reached 25, 54, and 52%, respectively. Because SO4 -• can also oxidize the C-H bond and further degrade PFCAs, the molecular defluorination of NBP-H reached 48-88% with varying S2O8 2-doses. Therefore, quantifying "organic fluorine" after oxidative sample treatment with either SO4 -• or HO• will substantially underestimate the amount of C-F bonds in the original sample. It is also critical to note that different types of PFAS can allow very different levels of defluorination (e.g., NBP-COOH versus NBP-H by SO4 -•; CnF2n+1-CH2CH2-COO -versus HC2F2n-COO -by HO• ). In addition, if the original unknown PFAS molecule does not contain a C-H bond or -COO -, it would escape from target analysis. For these reasons, the TOP assay might be less accurate than combustion ion chromatography for total F quantitation, but it can be very informative for qualitative analysis and may play a critical role in identifying original PFAS structures. Second, experimental results have shown that the majority of PFAS structures cannot be fully mineralized by either SO4 -• or HO•. If large excess of SO4 -• could deplete key -COO - containing products, using different oxidant doses may alter the product presence or dominance (Figure ). This feature is unique for SO4 -• oxidation. For comparison, the oxidation of NBP-H, 6:3 FTCA, and 6:2 FTSA with HO• resulted in rather consistent product spectra regardless of oxidant dose (Figure ). It is also noteworthy that SO4 -• or HO• can have very different product dominance for the legacy fluorotelomers (Figure ). The complementary use of both radicals will lead to novel information toward deducing the structures of original "PFAS precursors".