While many studies have characterized the extent of per- and polyfluoroalkyl substances (PFAS) presence at aqueous film-forming foam (AFFF)-impacted sites, the key processes impacting the distribution and fluxes of AFFF-derived PFAS in soil and groundwater remain unclear. The overarching goal of this project is to develop the measurement and application of stable isotope ratios as a diagnostic tool for assessing PFAS sources, transport, transformation, and fate in the environment. As the crucial first step, the project team plans to establish advanced analytical capabilities for compound-specific, multi-element stable isotope analysis of PFAS in the laboratory, using a dedicated, state-of-the-art Orbitrap-based high-resolution mass spectrometer recently installed in the University of Delaware’s Environmental Isotope Science Laboratory. This advanced analytical capability, when successfully developed, is a cost-effective way to provide unique insights into the chemical and physical history of PFAS in complex environments (e.g., manufacturing origins, transformation pathways, and degradation extent). This powerful approach will yield key types of information that are not possible to obtain solely from analyses of PFAS concentrations and may thus become an invaluable tool for site assessment and remediation.

PFAS often contains multiple elements (e.g., carbon [C], nitrogen [N], oxygen [O], sulfur [S]) that can have isotopic substitutions, and thus are good candidates for using compound-specific isotope analysis to identify and elucidate their environmental sources, transport, and transformation processes. The project team will first develop analytical methods for accurately measuring C, N, O, S, and Cl isotopes of individual PFAS in intact molecular ions and selected fragment ions using Orbitrap-based mass spectrometry. It is anticipated that the isotopic abundances of the heavy isotopes of carbon (¹³C ~ 1.1%), nitrogen (¹⁵N ~ 0.4%), oxygen (¹⁸O ~ 0.2%), and sulfur (³⁴S ~ 4.2%) can all be measured precisely by the new Orbitrap Exploris 240 instrument.

Given that environmental samples often contain a mixture of PFAS, after developing the analytical methods for isotope measurements, the project team will establish sample preparation methods to extract and isolate individual analytes from mixtures for precise compound-specific isotope measurements.

This project will establish and demonstrate analytical capabilities that can be used to assess isotope signatures of different PFAS sources and to characterize isotope fractionation during a suite of transformation reactions in multiphase environments at AFFF-impacted sites (e.g., abiotic and microbial degradation and transformation). It is anticipated that a successful outcome of this limited scope effort will allow further exploration in field applications of this advanced analytical capability in a subsequent project. The research is an uncharted area within the broader PFAS community and has the potential to greatly improve the understanding of the physical, chemical, and biological transformations of a large number of PFAS. It will open an innovative approach for applying isotopes in PFAS-related environmental research and public health investigations. (Anticipated Completion - 2025)