Quantitative Identification of Nonpolar Perfluoroalkyl Substances by Mass Spectrometry

Identifying and quantifying mixtures of compounds with very similar fragmentation patterns in their mass spectra presents a unique and challenging problem. In particular, the mass spectra of most per- and poly-fluoroalkyl substances (PFAS) lack a molecular ion. This complicates their identification, especially when using the absence of chromatographic separation. Here, we focus on linear, nonpolar, short-chain PFAS, which have received less attention than amphipathic PFAS despite their longer environmental lifetimes and greater global warming potentials. We identify and quantify n-C₅F₁₂ and n-C₆F₁₄ in binary mixtures by analyzing small changes in abundances of the main fragment ions following femtosecond tunnel laser ionization, without the need of chromatographic separation. Time-resolved femtosecond ionization mass spectrometry reveals that the metastable cation of both compounds undergoes predissociation within 1-2 ps of ion formation, with yields of C₃F₇⁺ showing evidence of coherent vibrational dynamics. These coherent oscillations are compared to low-level ion-state calculations and supported the idea that the oscillations in the C₃F₇⁺ ion yield are due to vibrations in the C₅F₁₂^+• and C₆F₁₄^+• radical cations and are associated with the predissociation dynamics of the metastable molecular ion. Surprisingly, we find that the fragment ions used for quantifying the mixtures have similar fragmentation dynamics. Conversely, the odd-electron C₂F₄^+• fragment shows different time dependence between the two compounds, yet has negligible difference in the relative ion yield between the two compounds. Our findings indicate that femtosecond laser ionization may be a useful tool for identifying and quantifying mixtures of PFAS without the need of chromatography or high-resolution mass spectrometry.