Digging deep—implementation, standardisation and interpretation of a total oxidisable precursor (TOP) assay within the regulatory context of per- and polyfluoroalkyl substances (PFASs) in soil

Abstract

Over the past decades, thousands of different per- and polyfluoroalkyl substances (PFASs) have been produced and applied in various industrial processes and consumer products. Their structural diversity has reached a level that cannot be covered by classical target screening methods for individual compounds. Large-scale contaminations of soil, however, require the need to adapt new analytical methods that can describe a contamination more comprehensively. While sum parameters such as the total oxidisable precursor (TOP) assay have been developed in the past years, they are not yet applied in the regulatory context of PFASs.In this commentary, we provide an overview on different approaches of the TOP assay as well as its benefits and disadvantages to other sum parameters for PFASs in soil samples. Furthermore, we elaborate its opportunities and its challenges that need to be tackled to implement the TOP assay as a regulatory tool. With several different approaches of the TOP assay being available, a sound and standardised method needs to be agreed upon and more research is necessary to better describe the method. Although the complexity of PFAS contaminations in soil cannot be fully covered by any analytical method alone, the TOP assay can provide valuable data to detect and characterise soil contamination as an inventory for subsequent remediation measures. Therefore, the TOP assay should be implemented as a useful tool both in research and in the regulatory context of PFASs.

PFAS: forever chemicals—persistent, bioaccumulative and mobile. Reviewing the status and the need for their phase out and remediation of contaminated sites

“…This allows extraction losses of unknown polyfluorinated precursor compounds to be avoided. This direct TOP assay (dTOP) gains LOQs in the range of 0.1 to 10 µg/kg for soil samples being more sensitive than the EOF assay [299]. The dTOP assay was also successfully applied to biological samples by Göckener et al [115].…”

Total Oxidizable Precursor (TOP) Assay─Best Practices, Capabilities and Limitations for PFAS Site Investigation and Remediation

“…For example, the perfluoroalkane sulfonamides and perfluoroalkane sulfonamido substances are transformation intermediates from other precursors and can be transformed into PFSAs and PFCAs, and fluorotelomer sulfonates can be transformed into PFCAs . Recent applications of the TOP assay include environmental forensics applications, since precursor identification may be useful for source identification. , More research is needed to support this emerging application.…”

“…45 Recent applications of the TOP assay include environmental forensics applications, since precursor identification may be useful for source identification. 42,53 More research is needed to support this emerging application.…”

“…Therefore, if conducting the TOP assay in-house, it is recommended to consider the various TOP method adaptations that have been described in the literature. 7,35,42,43,47 3.3. Oxidation Efficiency Evaluation with Surrogates.…”

PhotoTOP: PFAS Precursor Characterization by UV/TiO2 Photocatalysis

“…While the TOP assay is conveniently applicable in most laboratories due to the low requirements on equipment and chemicals it also has drawbacks. , During TOP there is a limited amount of oxidant in form of persulfate and hence OH radicals (·OH) available. In the case of high matrix-to-PFAS ratios, persulfate may be consumed by matrix instead of the target PFAS precursors .…”