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Despite the fact that thousands of perfluoroalkyl and polyfluoroalkyl substances (PFASs) are in widespread commercial use, analytical methods capable of quantifying them in environmental samples are available for less than 100 compounds. The total oxidizable precursor (TOP) assay provides a means of quantifying unknown PFASs by oxidizing PFASs into perfluoroalkyl carboxylates (PFCAs), which are readily measured by liquid chromatography–tandem mass spectrometry. To improve the reproducibility of the TOP assay, 13C8 perfluorooctane sulfonamide was added to samples prior to oxidation as an efficiency indicator and its conversion to 13C8 perfluorooctanoic acid was quantified. The recovery of precursors was also improved by including the analysis of ultrashort PFCAs, namely trifluoroacetic acid and perfluoropropyl carboxylic acid, which revealed the presence of ultrashort precursors. A method for distinguishing between branched and linear PFCAs was also integrated into the assay to enhance its ability to differentiate between PFASs produced by electrochemical fluorination and those produced by fluorotelomerization. In addition to its potential application in forensic studies, the improved TOP assay reduces uncertainties associated with incomplete oxidation of precursors or formation of ultrashort PFCAs that are not detected by standard analysis methods.
Tsou et al. (Mon,) studied this question.