Stable Carbon Isotope Analysis of PFOA and PFOS Using Orbitrap Mass Spectrometry

Compound-specific isotope analysis (CSIA) is a powerful tool for pollutant source apportionment. However, conventional isotope ratio mass spectrometry (IRMS) is limited for nonvolatile compounds like perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). Here, we demonstrated high-resolution Orbitrap mass spectrometry for δ13C analysis of these compounds. This technique enables direct isotopologue detection without combustion, providing nM-level sensitivity and molecular structure preservation. δ13C values for PFOA and PFOS reagents agreed well with those from elemental analyzer IRMS (EA-IRMS), with deviations within ±2.0‰ across resolution settings (17,500–140,000). Higher automatic gain control (AGC) targets improved precision; the optimal setting (5 × 106) yielded standard deviations below 1.2‰. Seven PFOA and two PFOS standards, each obtained from different commercial suppliers, were analyzed. The δ13C values measured by Orbitrap showed good agreement with EA-IRMS. In addition, spiked experiments using three different river water samples demonstrated that reliable δ13C measurements of PFOA and PFOS can be achieved in realistic aqueous environmental matrices. This approach expands the applicability of CSIA to nonvolatile PFAS and provides a new isotopic perspective for investigating their environmental sources and fate.