Uncovering per- and polyfluoroalkyl substances (PFAS) with nontargeted ion mobility spectrometry-mass spectrometry analyses

Abstract

Because of environmental and health concerns, legacy per- and polyfluoroalkyl substances (PFAS) have been voluntarily phased out, and thousands of emerging PFAS introduced as replacements. Traditional analytical methods target a limited number of mainly legacy PFAS; therefore, many species are not routinely assessed in the environment. Nontargeted approaches using high-resolution mass spectrometry methods have therefore been used to detect and characterize unknown PFAS. However, their ability to elucidate chemical structures relies on generation of informative fragments, and many low concentration species are not fragmented in typical data-dependent acquisition approaches. Here, a data-independent method leveraging ion mobility spectrometry (IMS) and size-dependent fragmentation was developed and applied to characterize aquatic passive samplers deployed near a North Carolina fluorochemical manufacturer. From the study, 11 PFAS structures for various per- and polyfluorinated ether sulfonic acids and multiheaded perfluorinated ether acids were elucidated in addition to 36 known PFAS. Eight of these species were previously unreported in environmental media, and three suspected species were validated.

Fig. 1.. General workflow for nontargeted per- and polyfluoroalkyl substances (PFAS) discovery using liquid chromatography–IMS–collision-induced dissociation–mass spectrometry (LC-IMS-CID-MS).

Feature detection, filtering, and characterization steps are taken to elucidate structures. collision cross section (CCS)– or drift time–related steps are unique to methods using IMS. ppm, parts per million; m/z, mass/charge ratio.

Fig. 2.. Drift time alignment and filtering of fragmentation data.

(A) IMS-collision-induced dissociation–mass spectrometry (IMS-CID-MS) abundance map of 6:2 FTS [M-H]⁻ in plant material extract, demonstrating the utility of drift time filtering for the removal of interfering fragments from co-eluting precursors due to the matrix. All signals outside of the purple horizontal box are removed when drift time filtering is applied. (B) Comparison of Nafion by-product 6 [M-H]⁻ fragmentation spectra without and with drift time filtering where noise from polymer contamination is removed.

Fig. 3.. Elucidated structures of 11 unknowns.

Proposed structures, assigned molecular formulas, identification confidence levels (65), identifiers (CAS RN or PubChem CID), and experimental descriptors [deprotonated m/z and collision cross section (CCS)] are given for each compound. The eight underlined compounds are reported here in environmental media.

Fig. 4.. Perfluoro(2-ethyoxyethane)sulfonic acid (PFEESA) homologous series trends.

In addition to conserved fragmentation patterns, each homolog with an additional CF₂ steadily increased in (A) retention time (RT) and (B) collision cross section (CCS), improving confidence in the previously unreported identifications (PFBESA and PFPeESA).