While PFAS can be released into the environment directly, there is evidence in the literature to suggest that some may also be formed in situ from the conversion of “precursor” compounds that either already exist in the environment as contaminants, or are co-released with the target PFAS.
Common PFAS precursor compounds include fluorotelomer alcohols and fluorotelomer sulfonates. Precursor transformation to PFAS “end products” has important implications for PFAS remediation efforts. By solely focusing on target PFAS removal, without consideration of the total precursor pool, an unanticipated increase in the concentrations of target PFAS may occur over time, resulting in potential future liability.
When testing routine samples for PFAS, Bureau Veritas Laboratories currently reports results for a comprehensive list of known PFAS end products and precursor compounds, in a diverse range of environmental matrices. However, this single test does not measure the potential for PFAS formation due to transformation of precursor compounds over time to the regulated end products. One approach to measure potential PFAS formation in water and soil is to apply the total oxidizable precursors (TOPs) assay.
The TOPs assay requires that samples be collected in duplicate. One part of the duplicate sample is analyzed for PFAS as received, to establish the initial concentrations of the end products of concern (e.g. PFOS, PFOA, etc.). The second part of the duplicate sample is heated with an oxidizing agent under alkaline conditions to transform any precursor compounds into PFAS end products, and is subsequently analyzed for the routine list of PFAS. The difference between the pre- and post-oxidation concentrations of PFAS represents the presence and potential in situ transformation of poly- and perfluorinated precursor compounds.
The results of the TOPs assay provides a good indication of poly- and perfluorinated materials in a sample, not necessarily measured as part of the routine parameter list, that may transform to the more persistent end products, thereby alerting the data user of potential future liability.
Bureau Veritas offers a technical bulletin that details the TOPs assay, sampling requirements, how to interpret the results and some of the limitations of the method that need to be considered when interpreting the results.
 Houtz, E.F. and Sedlak, D.L. (2012), Environ. Sci. Technol., 46, 9342-9349