On the Unimolecular Breakdown Products of Short Deprotonated Per- and Poly-Fluorinated Acids and Alcohols

The strong C–F bond found in per- and poly-fluorinated alkyl substances (PFAS) makes them resistant to degradation and thus persistent in the environment. One of the most common methods for quantifying PFAS in environmental matrices is to use tandem mass spectrometry. However, the dissociation of ions made by deprotonating PFAS alcohols and acids has only been qualitatively explored. In this study, we investigated the breakdown of deprotonated 2,2,3,3,3-pentafluoropropionic acid (1, m/z 163), 3,3,3-trifluoropropionic acid (2, m/z 127), 2,2,3,3,3-pentafluoro-1-propanol (3, m/z 149), 3,3,3-trifluoro-1-propanol (4, m/z 113), and trifluoromethanesulfonic acid (5) by energy-resolved collision-induced dissociation (CID) tandem mass spectrometry and density functional theory (M06/6-311+G(d,p)). Ion 1 loses CO₂ at low lab-frame collision energy. Ion 2 also loses CO₂ to form the 1,1,1-trifluoroethane ion (m/z 83) and 1,2-difluoroethylene to form FCO₂⁻ (m/z 63). RRKM calculations for the two reactions show that m/z 83 has a higher entropy of activation driving its formation. Ion 3 undergoes the loss of CH₂O to form the pentafluoroethyl anion (m/z 119) and the loss of HF to form CF₂CF₂COH⁻ (m/z 129). Ion 4 produced four fragment ions with two primary reactions making CF₃CHCH⁻ (m/z 95) + H₂O and CF₂CHCOH₂⁻ (m/z 93) + HF, which go on to dissociate further to produce CF₃⁻ (m/z 69) + HCCH + H₂O and CF₂CH⁻ (m/z 63) + CH₂O + HF. At low collision energy, m/z 95 dominates due to a lower energy transition state, but as internal energy increases, m/z 93 takes over as its transition state has a more favorable entropy. Ion 5 produced FSO₃⁻ (m/z 99), SO₃⁻ (m/z 80), and CF₃⁻ (m/z 69). SO₃⁻ was the most abundant fragment due to its higher electron affinity.