The TOP Assay, or Total Oxidizable Precursor Assay, is a laboratory method developed in 2012 for the detection and estimation of oxidizable precursor compounds of per- and polyfluoroalkyl substances (PFAS) in environmental and biological samples. The method was first published by Houtz and Sedlak in Environmental Science & Technology in 2012.
Principle
The TOP Assay uses a heat- and alkaline-activated persulfate oxidation process to convert unknown PFAS precursor compounds into stable perfluoroalkyl acids (PFAAs), specifically perfluoroalkyl carboxylic acids (PFCAs), which can be measured using standard targeted analytical methods such as liquid chromatography-tandem mass spectrometry (LC-MS/MS). The concentration of PFAS precursors in a sample is estimated by comparing the PFAA concentrations in a sample before and after oxidation. An increase in PFAA concentration after the TOP Assay indicates the presence of oxidizable precursors.
Procedure
Duplicate samples are collected and analyzed: one sample undergoes the TOP Assay pretreatment (oxidation with potassium or sodium persulfate under alkaline conditions at elevated temperature), while the other remains untreated. Both samples are then analyzed by targeted PFAS analysis. The difference in PFAA concentrations between the treated and untreated samples provides an estimate of the concentration of oxidizable PFAS precursors.
Applications
The TOP Assay is applied to a range of matrices, including surface water, groundwater, soil, sediment, wastewater, textiles, and biological samples such as blood serum. It is used in environmental site investigations, particularly at sites contaminated with aqueous film-forming foams (AFFFs) used in firefighting, and for assessing PFAS treatment performance in remediation systems. Australia is one of the few countries that integrates the TOP Assay into regulatory PFAS management.
Limitations
Several technical challenges have been identified with the TOP Assay. These include: incomplete oxidation of some precursors due to matrix interference from co-contaminants or natural organic matter; the production of ultrashort-chain PFAAs (e.g., trifluoroacetic acid, perfluoropropionic acid) that are not included in standard targeted analytical methods; the presence of ether-containing PFAS that may be resistant to oxidation or produce non-targeted intermediates; and significant interlaboratory variability due to the absence of a standardized method. No published, standardized reference method or consensus guidelines currently exist for the TOP Assay.
References
- Ateia, M., Chiang, D., Cashman, M., & Acheson, C. (2023). Total Oxidizable Precursor (TOP) Assay—Best Practices, Capabilities and Limitations for PFAS Site Investigation and Remediation. Environmental Science & Technology Letters, 10(4), 292–301.
- Houtz, E. F., & Sedlak, D. L. (2012). Oxidative Conversion as a Means of Detecting Precursors to Perfluoroalkyl Acids in Urban Runoff. Environmental Science & Technology, 46(17), 9342–9349.
