Persulfate-based total oxidizable precursor (TOP) assay approaches for advanced PFAS assessment in the environment – A review

The enormous structural complexity of millions of per- and polyfluorinated alkyl substances (PFASs) poses a major challenge for their chemical analysis. One approach to partially address this challenge is the so-called total oxidizable precursor (TOP) assay. In the past years, however, different laboratories worldwide modified the method in multiple ways leading to a confusingly complex variety of TOP assay approaches. This review aims to provide an overview on this methodological complexity and to identify key parameters and their potential influence on the results and method interpretation. Parameters of all available studies were summarized in a complex table. The amount and concentration of oxidation reagents, the reaction time, temperature and pH value during oxidation, the time of internal standard addition, and the suite of analyzed oxidation products were identified as key factors affecting oxidation efficiency, nature of reaction products, and completeness of precursor assessment. The current complexity of TOP assay approaches demonstrates the need for further method harmonization and standardization, which should include measures to ensure and assess the completeness of precursor oxidation. A further understanding of the chemical reactions during the TOP assay and the inclusion of additional oxidation endpoints were identified as research needs for the future. This review article paves the way for the further implementation of the TOP assay as an important tool to assess the world of PFAS more comprehensively, thereby supporting environmental monitoring, assessment of PFAS contaminations, and the success control of designated PFAS regulations.