The total oxidizable precursor (TOP) assay is an analytical method that was recently developed to evaluate the environmental presence of certain per- and polyfluoroalkyl substances (PFAS) which could not be identified using previous laboratory approaches. Specifically, the TOP assay uses chemical oxidation to break down larger, difficult-to-measure polyfluoroalkyl compounds into smaller, more readily identifiable PFAS, namely perfluoroalkyl acids (PFAA). Once the oxidation process is complete, the terminal PFAA are measured by the laboratory and compared to pre-oxidation results for the same sample, giving an indirect estimate of the amount of precursors in the sample.

An inter-laboratory comparison of four commercial laboratories offering the TOP assay was undertaken in 2017/2018. The objectives of the project were to: 1) to assess the variability of TOP assay results provided by four of the commercial laboratories offering this analysis in Canada using real-world groundwater samples obtained from firefighting training areas; 2) to attempt to identify potential sources of any observed variability based on the methodologies employed; and, 3) to determine impacts to site-management.

This presentation will introduce the TOP assay, outline differences in analytical approaches among the four laboratories studied, present the results of the TOP assay and interlaboratory comparisons and identify challenges encountered during the execution of the study.

Lindsay Paterson, Senior Soil Scientist, SLR Consulting (Canada) Ltd.
Lindsay Paterson is a Senior Soil Scientist at SLR Consulting (Canada) Ltd. in British Columbia.

Novel precursors to perfluoroalkyl acids (PFAA) identified from aqueous film-forming foams (AFFF) have increased the number of poly- and per-fluoroalkyl substances (PFAS) to be monitored for environmental samples. One of the challenges associated with quantifying the precursors is the unavailability of chemical standards for most of these compounds. The total oxidizable precursor (TOP) assay is a valuable tool that was developed to estimate the amount of precursors in terms of the equivalence of perfluoroalkyl carboxylates (PFCAs). Despite the success of the TOP assay demonstrated in a few studies, implementation for a variety of environmental samples has seen great data variability, caused by pitfalls associated with unverified oxidation performance for many precursors and matrix interference. Supported by the US Strategic Environmental Research and Development Program (SERDP), we are tasked to improve the robustness of the assay by addressing these issues.

In the first phase, 23 zwitterionic, cationic, and anionic PFAS were successfully integrated into the TOP assay for groundwater. All these AFFF-derived precursors were amenable to TOP conversion, and their oxidative yields were determined. AFFF-impacted groundwater samples from fire-equipment testing sites in Ontario and Newfoundland were tested. Summed PFAS concentrations as high as 5 mg·L-1 were reported before oxidation, and post-oxidation increases of PFCAs up to +2,300% were observed. A significant contribution of increases in individual PFCAs was attributed to precursors such as 6:2 fluorotelomer sulfonamidoalkyl betaine (6:2 FTAB), fluorotelomer sulfonates (6:2 FtS, 8:2 FtS), perfluorooctane sulfonamidoalkyl amine (PFOSAm), and perfluorohexane sulfonamide (FHxSA) at the active firefighting training site. Some of those precursors are not routinely monitored, and the total PFAS concentrations would be underestimated if the TOP assay were not utilized.

In the second phase, the influence of water chemistry (pH, organic carbon content, inorganic ions) is currently being examined. So far, it has been found that the initial solution pH plays a significant role in the oxidation efficiency. For water samples with initial pH between 4.5-5.5, there was poor precursor oxidation (< 50%), while the samples with pH between 6.5-7.5 had near complete precursor oxidation (> 90%). Ongoing work in the lab is focused on the effect of natural organic matter (NOM) and major metal ions. A clean-up procedure that can be implemented to reduce matrix interference before oxidation has also been developed. The findings will be eventually be used to develop standard methods for the PFAS research community.

Eniola Oye-Bamgbose, McGill University
Eniola Oye-Bamgbose is a graduate student working in the research laboratory of Prof. Jinxia Liu at McGill University. He has his bachelor's degree in Chemical engineering and is currently working towards a master's degree in Environmental engineering. His research is focuses on the oxidation of PFAS.

Industries in the United States have phased out production of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) because of potential health risks to humans. As global production of PFOS and PFOA is eliminated, manufacturers have been and continue developing per- and polyfluoroalkylated substances (PFAS) replacement technologies to reformulate or substitute longer chain PFAS (e.g., PFOS and PFOA) with shorter-chain perfluoroalkyl or polyfluorinated substances. These processes result in: fluorotelomer alcohols (FTOH); perfluorobutane sulfonyl fluoride (PBSF)-based derivatives (e.g., perfluorobutane sulfonate (PFBS) as a substitute for PFOS); and polyfluoroethers (e.g., GenX, ADONA and F53B used in the manufacture of fluoropolymers) among others.

While there is a substantial body of knowledge for managing risk from PFOS and PFOA, much less is known about replacement PFAS. Although published toxicological data is limited, studies are suggesting that some of the lower molecular weight PFAS replacement compounds may be more hazardous than their long-chain predecessors. Information on the type and extent of environmental contamination by replacement PFAS is also limited, as most are currently not reported as part of the routine suite of PFAS commonly analysed by commercial analytical laboratories.

Three key PFAS replacements currently being studied are commonly referred to as GenX, ADONA and F53B. PFAS replacements like these tend to have fewer carbon atoms in the chain, but have many similar physical and chemical properties as their predecessors PFOS and PFOA (e.g., they repel oil and water).

Because of increased interest in PFAS replacement compounds and the potential for their regulation in some jurisdictions, methods for the determination of GenX, ADONA and F53B by isotope dilution liquid chromatography coupled with tandem mass spectrometry has been validated.

This presentation will describe: the history and environmental impact of GenX, ADONA and F53B. It will also highlight current science surrounding PFAS measurement as a whole, in support of site remedial activities, including:

• Practical and reasonable approaches to sample collection based on lessons learned over the last five years;
• Expanded PFAS lists reported by the laboratory including PFAS replacements; and,
• Using results from the determination of total oxidizable precursor compounds (TOP Assay) to mitigate potential future site liability.