Despite their environmental importance, many per- and polyfluoroalkyl substances (PFAS) cannot be identified via targeted liquid chromatography-mass spectrometry because relatively few analytical reference standards exist for this chemical class. Nontarget analyses performed by means of high-resolution mass spectrometry (HRMS) are increasingly common for the discovery and identification of PFAS in environmental and biological samples. As PFAS are more frequently identified via HRMS, the certainty of these identifications must be communicated through a reliable and harmonized framework. We propose a confidence scale, derived from the widely used “Schymanski Scale,” specifically for PFAS identified by nontarget HRMS. Levels of certainty correspond to the strength of evidence commonly used for PFAS identification in suspect or nontarget analysis, ranging from level 1 to level 5. Sublevels are used to clarify types of diagnostic evidence and distinguish between possible PFAS isomers. For example, Level 1a “Confirmed by Reference Standard” and level 1b “Indistinguishable from Reference Standard” are separately defined because certain PFAS have isomers that are virtually indistinguishable based on MS/MS fragmentation. Similarly, level 5 “Exact Masses of Interest,” are further characterized by suspect screening or data filtering, two common forms of feature prioritization. This confidence scale is consistent with general criteria for communicating confidence in the identification of small organic molecules by HRMS (e.g., through a match to library MS/MS) but incorporates the specific conventions used in PFAS classification and analysis (e.g., detection of homologous series). Our scale clarifies the level of certainty in PFAS identification and facilitates more efficient identification.

The fate of per- and polyfluoroalkyl substances (PFAS) in surface waters depends on their individual chemical structures. PFAS are surfactants and can partition to the surface microlayer at the air-water interface and to foams that form naturally in lakes and rivers. Determining their chemical partitioning is important for identifying exposure levels for people, birds, and aquatic mammals, and for informing remediation efforts. This work focuses on determining enrichment factors of PFAS in the surface microlayer and foams in freshwaters. Sample sites include rivers and lakes in Wisconsin, including Lake Michigan. Water column, surface microlayer, and naturally forming foam samples were analyzed for 36 PFAS compounds, as well as background water chemistry and dissolved organic matter measurements using ultrahigh-resolution mass spectrometry. PFAS concentrations range widely, with elevated concentrations in waters impacted by historical use of aqueous film forming foams (AFFF). PFAS enrichment to the surface microlayer increases with increasing chain length and is greater for perfluorosulfonic acids compared to perfluorocarboxylic acids. Enrichment factors of >1,000 are observed for foams compared to water column measurements and follow the same chemical trends as the surface microlayer enrichment factors. Collectively, this work demonstrates that chemical structure plays a key role in the partitioning behavior of PFAS in freshwaters.

As per- and polyfluoroalkyl substances (PFAS) pose a great threat to public health, the U.S. Environmental Protection Agency established the draft Method 1633 to detect PFAS in various environmental media based on solid-phase extraction and liquid chromatography-tandem mass spectrometry. Though precise and sensitive, the operational complexity and high cost of the standard method hinder the regular monitoring of PFAS. Raman spectroscopy is a promising complementary tool because of its fingerprinting ability for trace analysis, low operational cost, and fitness for field-deployable applications. For the practical application of Raman spectroscopy, a well-established Raman library is a prerequisite due to the current lack of comprehensive and reproducible spectral data on PFAS. We propose a simple method to concentrate PFAS in organic solvents and to establish a Raman library for PFAS. We drop-coated PFAS solutions onto aluminum foil and dried them to concentrate and secure solid PFAS. The different functional groups and chain lengths affected the evaporation and crystallization behavior of PFAS. Raman maps were then collected from the sessile drops using a 532 nm laser and a confocal Raman spectrometer. Regardless of the chemical structure of PFAS, they shared common Raman peaks at around 300, 380, 524, 570, and 724 cm^-1, while having varying peak-to-peak ratios. To differentiate PFAS congeners and alkyl acids, principal component analysis was performed on wavenumbers between 200 and 1,000 cm^-1. This research created a novel reproducible Raman spectral library of PFAS that will be a foundation for simple and facile PFAS screening using Raman spectroscopy.

Per- and polyfluoroalkyl substances (PFASs) are emerging chemical contaminants of global concern. There is interest in developing forensics tools for source allocation of PFASs measured in the environment, although PFAS fingerprints associated with specific source types are poorly defined. Our objective was to define PFAS fingerprints that are unique to specific source types. We collected 92 samples from six source types including AFFF-impacted groundwater (AFFF-GW), landfill leachate (LL), biosolids leachate (BL), municipal WWTP effluent (WWTP), wastewater from the pulp and paper industry (PP), and wastewater from the power generation industry (PG) and quantified 50 target and 188 suspect PFASs in each sample. We used principal component analysis and hierarchical clustering analysis to determine whether the source types exhibit characteristic PFAS fingerprints. We used support vector classification, logistic regression, and random forest classifiers to identify the PFASs that are most diagnostic of each source type. We found that the most diagnostic PFASs for AFFF-GW include five target PFASs (PFHxA, FHxSA, PFPeS, PFHxS, FBSA) and one suspect PFAS (3H-PFENA). The most diagnostic PFASs for LL include two target PFASs (5:3 FTCA, 6:2 FTCA) and three suspect PFASs (4:2 FTThA, MeFPrSAA, MeFBSAA). We also found the BL samples are best classified with LL samples and EtFOSAA and MeFOSAA assist in that classification. WWTP, PP, and PG samples contained few target PFASs, but the insecticide fipronil was a robust indicator of WWTP samples. Our results define PFAS fingerprints for multiple source types and can be used as a basis for PFAS source allocation.

The proliferation of per- and polyfluoroalkyl substances (PFAS) in the firefighting-foam industry has raised concerns about their potential environmental and health impacts, leading to a surge in research on fluorine-free alternatives. In January 2023, the new military testing specification (MIL-PRF-32725) for fluorine-free foam (F3) was released, which limits the use of foams within the Department of Defense that do not meet this specification. This talk provides a critical analysis of the present state of the proposed PFAS alternatives in the foam sector, with an emphasis on the various fluorine-free options that have been developed to date. A nuanced perspective of the challenges and opportunities of more sustainable replacements is explored by examining the performance, cost, and regulatory considerations associated with these fluorine-free alternatives. Ultimately, this evaluation shows that the transition to fluorine-free replacements is likely to be complex and multifaceted, requiring careful consideration of the trade-offs involved. Yet, the ongoing work will provide valuable insights for future research on alternatives to foam and enhancing the safety and sustainability of fire suppression systems.