Total oxidizable precursor assay: Applications and limitations. A study on the occurrence of perfluoroalkyl and polyfluoroalkyl substances (PFASs) in Chinese Wastewater Treatment Plants

Abstract: Total oxidizable precursor (TOP) assay is an in-lab method which oxidatively converts precursor compounds of poly- and perfluoroalkyl substances (PFASs) into measurable perfluorinated alkyl acids (PFAAs). The method is a very strong tool to semi-quantify PFASs that would otherwise be missed in conventional targeted analysis using liquid chromatography - tandem mass spectrometry (LC-MS/MS). However, there are still challenges for it to be a fully quantitative tool. This report aims to increase the knowledge in using measures to improve the quality assurance of the assay, as well as investigate potential limitations of applying TOP assay outside the scope of what it was originally developed for. Furthermore, the objective was also to apply an evaluated TOP assay method on sample extracts from influent and effluent water from wastewater treatment plants (WWTPs) in China to investigate the occurrence of both legacy and novel PFASs. To achieve improved quality assurance of the method, the use of isotopically labelled standards in the TOP assay were investigated. 13C8-FOSA was used as a model compound to measure the oxidation efficiency in the assay; the results showed that the molar yield of 13C8-FOSA to 13C8-PFOA was 99% ± 2% (n=12) for the WWTP samples. This indicated it to be a good tool to monitor the oxidation, but more research is needed to understand the fate and reaction rate of other precursors. Oxidation of sample extracts were performed without evaporating solvent before oxidation to minimize loss of volatile precursors. This procedure was shown to be possible but require additional oxidation agent and base and may lead to different product pattern after oxidation. Further, performing TOP assay on sample extracts will introduce bias in which types of precursors are extracted and may lead to an underestimation of total PFAAs precursor present, depending on the extraction method or solvent type used. Additionally, the stability of two novel compound groups in the assay were investigated: 1) The Pre 2002 and Post 2002 formulas of Scotchgard™ fluorinated side-chain polymers were both degraded in the assay. For the Pre 2002 formula the primary degradation product was PFOA and for the Post 2002 formula, PFBA was the degradation product. Additionally, both the Pre 2002 and Post 2002 formulas were shown to undergo hydrolysis in high pH conditions, producing perfluoroalkyl sulfonic acids (PFSAs). 2) Per- and polyfluorinated ether acids (PFEAs) were also investigated in the TOP assay, which indicated that perfluorinated ether acids are stable in high pH oxidative conditions but may degrade in low pH oxidative conditions, while polyfluorinated ether acids may be degraded under oxidative conditions in both high and low pH. The analysis of influent and effluent water from Chinese WWTPs was done by screening for 14 legacy PFASs, 16 novel PFASs, and performing TOP assay on sample extracts as well as analysis of extractable organic fluorine for mass balance analysis. Perfluorobutanoic acids (PFBA) had the highest detection rate among the legacy PFASs, and hexafluoropropylene oxide dimer acid (HFPO-DA) was the most prevalent novel PFASs detected. TOP assay on the anionic fraction lead to varying degree increase in PFAS concentration (0-15x) while performing the oxidation on the neutral/cationic fraction did not lead to any observable increase in PFASs. Even with TOP assay, the proportion of extractable organic fluorine still unidentified was 11% to 98%.