Source Fingerprinting of PFOA via Full- and Intramolecular Stable Isotope Ratios of Carbon Using Orbitrap-IRMS

Remediation approaches for reducing environmental loading of perfluoroalkyl and polyfluoroalkyl substances (PFAS) require the ability to track PFAS origin sources. One common source attribution method for environmental contaminants is stable isotope analysis, with magnetic sector isotope ratio mass spectrometry (IRMS) being the workhorse for making these measurements. However, relatively large sample requirements and the requisite deconstruction of analytes to simple gases (e.g., CO₂) inhibit the application of IRMS to PFAS in environmental contexts, where even trace PFAS concentrations can exhibit toxicological effects, and decomposition of PFAS yields instrument-degrading HF. To circumvent these limitations, we developed an electrospray ionization Orbitrap-IRMS method to determine stable carbon isotope (¹³C) ratios of PFAS with reduced sample quantities, without producing HF, and with the additional capability of performing intramolecular isotope ratio measurements for increased sample discrimination potential. We demonstrate this approach using perfluorooctanoic acid (PFOA) as a model compound. We collected twenty-three samples of neat, commercially available PFOA with different lot numbers from 15 different suppliers and determined that the full molecule stable carbon isotope ratios span ∼25‰ with a measurement precision of 0.7‰. The stable isotope ratios of the decarboxylated fragment of PFOA span ∼28‰ with a measurement precision of 0.2‰. Of the 253 possible pairwise comparisons between the samples, 230 are significantly different based on the combined full and intramolecular isotopic signatures. The isotopic analyses suggest a correlation with the PFOA synthesis method. Our results highlight the potential of Orbitrap-IRMS as a forensic tool for the source attribution of PFAS contaminants.