Evaluation of the Forensic Utility of post-Total Oxidizable Precursor Assay Per- and Polyfluorinated Substances Composition from Aqueous Film Forming Foam

Chemical discrimination between various formulations of aqueous film-forming foam (AFFF) is an important capability for the forensic assessment of per- and polyfluoroalkyl substances (PFAS) impacts on the environment. Identification of AFFF chemical fingerprints has been constrained by the (1) lack of detailed chemical characterizations of AFFF, (2) presence of PFAS compounds in AFFF products that are not detected and/or quantified by current analytical methods, and (3) potential chemical degradation of certain AFFF-containing PFAS towards recalcitrant end-member PFAS following release to the environment. As proof-of-concept for a technique to overcome these limitations, AFFF product samples from various manufacturers and production years were analyzed before and after application of the total oxidizable precursor (TOP) assay, which oxidizes the PFAS in a sample to their terminal end-member products. The pre- and post-TOP assay PFAS results were evaluated to determine if (1) the TOP assay generated stable and reproducible PFAS profiles, and (2) the post-TOP assay PFAS profiles enabled discrimination between legacy perfluorooctanesulfonate (PFOS) AFFF, legacy fluorotelomer (FT) AFFF, and modern FT AFFF. Principal component analysis was used to explore the post-TOP assay PFAS data to identify which variables had the strongest statistical power for AFFF product discrimination. Based on this evaluation, specific chemical ratios of the resulting post-TOP assay PFAS show potential for differentiation among legacy PFOS AFFF, legacy FT AFFF, and modern FT AFFF.