A SIFT Study of Reactions of Positive and Negative Ions With Polyfluoroalkyl (PFAS) Molecules in Dry and Humid Nitrogen at 393 K.

Abstract

Rationale: Data are required for SIFT-MS analysis of perfluoroalkyl and polyfluoroalkyl substances (PFAS), which are persistent in the environment and cause adverse health effects. Specifically, the rate coefficients and product ion branching ratios of the reactions of H₃O⁺, NO⁺, O₂ ⁺•, O^-•, OH^-, O₂ ^-•, NO₂ ^- and NO₃ ^- with PFAS vapours are needed.

Methods: The dual polarity SIFT-MS instrument (Voice200) was used to generate these eight reagent ions and inject them into the flow tube with N₂ carrier gas at a temperature of 393 K. Vapours of pentafluoropropionic acid, heptafluorobutyric acid, nonafluoro-1-hexanol, perfluoro-2-methyl-2-pentene, perfluorohexanoic acid, perfluoro(2-methyl-3-oxahexanoic) acid, tridecafluoro-1-octanol and nonafluorobutane-1-sulfonic acid were introduced in dry and humid air. Full-scan mass spectra were collected for all reagents at variable PFAS concentrations and analysed numerically.

Results: Rate coefficients were determined for 64 reactions, for which 55 positive and 71 negative product ions were identified. The branching ratios for the primary reaction channels were extracted from the data, and the secondary chemistry with H₂O molecules was qualitatively assessed. The thermochemical data were calculated for the H₃O⁺ reactions using density functional theory (DFT).

Conclusions: An important observation is that secondary reactions with water molecules remove the positive product ions, making them unsuitable for practical SIFT-MS analysis of PFAS vapours. In contrast, most negative reaction product ions are not significantly affected by humidity and are thus preferred for the SIFT-MS analyses of PFAS substances in various gaseous matrices.