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.