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

Rationale: Polyfluoroalkyl substances (PFAS) like perfluorooctanoic acid have persistent environmental and physiological effects. This study investigates the degradation of C_nF_2n+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 C_nF_2n+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 abstraction 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.