Structural isomers of polyfluorinated di- and tri-alkylated phosphate ester surfactants present in industrial blends and in microwave popcorn bags

Introduction In this study, we provide strategies for detecting and quantifying the structural isomers of polyfluorinated di- and tri-alkyl surfactants (PFAS) by mass spectrometry (MS). We specifically investigate polyfluorinated dialkylated phosphate ester surfactants (x:2/y:2 diPAPS, (F(CF2) x CH2CH2O-P(O)(O)−-OCH2CH2(CF2) y F)) and their thioether analogues (x:2/y:2 S-diPAPS, F(CF2) x CH2CH2SCH2-C[CH2O)2P(O)(O)−]-CH2SCH2CH2(CF2) y F), which are used for industrial applications, such as oil- and water-repellent coatings on paper and board. DiPAPS have been found in human blood and are metabolised to the persistent perfluoroalkyl carboxylic acids (PFCA) in rats. Materials and methods A microwave popcorn bag extract was analysed by ultrahigh-pressure liquid chromatography coupled to a negative electrospray ionisation-quadrupole time-of-flight MS. Results and discussion The extract contained S-diPAPS, diPAPS and trialkylated (triPAPS) impurities. TriPAPS were also present in industrial and synthetic diPAPS standards, and were verified with an 8:2/8:2/8:2 triPAPS standard. The eight elemental compositions (m/z’s) of diPAPS in the extract represent 19 precursor ion structures, and the six S-diPAPS m/z’s represent at least 13 structures. The diPAPS had [M-H]− precursor ions of m/z 789, 889,…1,489 and the S-diPAPS of m/z 921, 1,021,…1,421, corresponding to fluorinated chains from C6–18. Each m/z appeared as one to three chromatographic peaks of structural isomers, where, e.g. m/z 1,189 was present as 10:2/10:2, 8:2/12:2 and 6:2/14:2 diPAPS. The isomers formed different products ions, thus only half of the m/z 1,189 diPAPS concentration was measured with one precursor ion > product ion transition. Conclusion In general, knowledge about structural isomers of poly-alkylated PFAS is needed for the estimation of types and amounts of perfluorinated degradation products, such as PFCA from diPAPS.