Tandem Mass Spectrometry of Perfluorocarboxylate Anions: Fragmentation Induced by Reactive Species Formed From Microwave Excited Hydrogen and Water Plasmas

ABSTRACT Rationale Polyfluoroalkyl substances (PFAS) like perfluorooctanoic acid have persistent environmental and physiological effects. This study investigates the degradation of CnF2n+1CO2− (n = 1–7) with neutral radical fragmentation under oxygen attachment dissociation (OAD). Unique fragments absent from collision‐induced dissociation (CID) are observed. Further, potential mechanisms are uncovered by density functional theory (DFT) calculations. Methods From a standard mixture of PFAS, straight‐chain perfluorinated carboxylic acids with carbon chain lengths of one to eight were separated via liquid chromatography and transferred to the gas phase via negative‐mode electrospray ionisation. Each CnF2n+1CO2− of interest was mass selected and fragmented via both CID and OAD in a quadrupole time‐of‐flight mass spectrometer. DFT optimisations of structures were performed at M06/6‐31+g(d), and single point energy calculations were performed at M06‐2X/aug‐cc‐pVTZ for C3F7CO2−. Results Decarboxylation was observed from both CID and OAD, but fluorine ion and hydroxyl addition only occurred with OAD. The DFT calculations suggest that C3F6–• (m/z 150) is most likely formed from by H• attack onto a β‐ C‐F bond, then loss of HF, finally decarboxylation. Further, C3F5O− (m/z 147) likely arises from C3F6–• recombining with OH• to produce energised C3F6OH− ions, followed by α‐ or β‐ elimination of HF to give enolate and/or epoxide‐type products. Conclusions OAD of C3F7CO2− yields unique product ions C3F6–• (m/z 150) and C3F5O− (m/z 147) absent from collision‐induced dissociation. DFT calculations suggest an intricate pathway of H• attack onto a β C‐F bond, then loss of HF, decarboxylation, recombination with OH•, and finally α‐ or β‐ elimination of HF to give the products.