Investigating the Unimolecular Breakdown Products and Pyrolysis Products of Short Chain Per/Polyfluorinated Compounds

Abstract

PFAS or per/polyfluoroalkyl substances are synthetic compounds used in a variety of different products such as in clothing, firefighting foam, and non-stick cookware. PFAS are able to be used in such a variety of products due to their hydrophilic and hydrophobic nature as well as their stability. The strong C-F bond found in PFAS makes them resistant to degradation and thus persistent in the environment. PFAS have therefore been found in soil, atmosphere, ground water, surface water, and wastewater etc. Most industries are turning away from using long chain PFAS since they are starting to be banned in multiple countries. Instead, though short-chain PFAS which are not as researched are being used as replacements. The goal of my research was to determine how PFAS break down using collision induced dissociation (CID) mass spectrometry and pyrolysis.

PFAS have been quantified and detected using tandem mass spectrometry in multiple studies from various matrices. The dissociation of short chain PFAS ions using mass spectrometry has not been explored. In order to do this, negative ion mode triple quadrupole mass spectrometry was used to breakdown five short chain PFAS, 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, m/z 149) using CID. Once the products were obtained, density functional theory (DFT), and RRKM kinetic calculations were used to analyse the reactions energetics and rates. 1 loses CO₂ at low lab-frame collision energy. 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. 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). 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 favourable entropy. 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.

One method of removing PFAS from the environment is by burning them at very high temperatures. The problem with this method is that incomplete degradation occurs, and smaller and persistent fluorinated compounds are left over. To investigate this, a Chen-type microreactor was used as well as imaging photoelectron photoion coincidence spectroscopy to explore the pyrolysis of three short-chain PFCs, 2,2,3,3,3-pentafluoropropionic acid (1), 3,3,3-trifluoropropionic acid (2), and 2,2,3,3,3-pentafluoro-1-propanol (3). The products were ionized with VUV synchrotron radiation at the Swiss Light Source and identified by their mass-selected threshold photoelectron spectra (ms-TPES). The results are compared to literature photoelectron spectra and calculated TPES from Franck-Condon simulations performed at the B3LYP/6-311+G(d,p) level of theory. Thermal degradation leads off with CO₂ loss (1,2), HF loss (1,2,3) and formaldehyde loss (3). These result in fluoroethanes, which themselves decompose to form fluoroethenes, and subsequently CF₂. There was evidence for bimolecular processes that form formaldehyde from 1 and 2, and acetylene from 3.