Unravel\nthe in-Source Fragmentation Patterns of Per-\nand Polyfluoroalkyl Substances during Analysis by LC-ESI-HRMS

In-source fragmentation\n(ISF) was inevitable during electrospray\nionization (ESI) of per- and polyfluoroalkyl substances (PFAS) when\nanalyzed by liquid chromatography coupled with mass spectrometry (LC-MS),\nresulting in reduced response of molecular ions and misannotation\nof MS features. Herein, we analyzed 82 PFAS across 12 classes to systematically\nidentify the structures with ISF potentials and reveal the fragmentation\npathways. We found up to 100% ISF for 38 PFAS in six classes, which\nall contain the carboxylate (CO<sub>2</sub><sup>–</sup>) headgroup,\nincluding perfluoro(di)carboxylates (PF(di)CA), omega H/Cl substituted\nPFCA (ωH/Cl-PFCA), fluorotelomer carboxylates, and perfluoroalkyl\nether carboxylates (PFECA). Seven ISF pathways were identified, including\ndirect cleavage of C–CO<sub>2</sub><sup>–</sup>, C–O,\nand C–C bonds and eliminations of HF/CO<sub>2</sub>HF through\ncyclic transition states by the mechanisms of β-elimination,\nMcLafferty rearrangement, or H···F bridging. We found\nthat the loss of CO<sub>2</sub> is a prerequisite for most other pathways,\nexplaining the absence of ISF for PFAS without a CO<sub>2</sub><sup>–</sup> headgroup. The elevated bond dissociation energy of\nC–CO<sub>2</sub><sup>–</sup> explained the reduced ISF\nfor long-chain PFCA and ωH-PFCA. Raising the MS vaporizer and\nion transfer tube temperatures significantly aggravated the ISF of\nmost PFAS. These findings provide valuable references to inform the\nstructural identification of PFAS and their degradation products.