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PFAS Total Oxidizable Precursor (TOP) Assay

PFAS Total Oxidizable Precursor (TOP) Assay

Click here to view TestAmerica's White Paper on the TOP Assay entitled "Closing the PFAS Mass Balance: The Total Oxidizable Precursor (TOP) Assay" - Karla Buechler.

The TOP Assay Improves Our Understanding of Risk

Current methodologies for the analysis of per and polyfluoroalkyl substances (PFAS) are designed to measure a discrete list of 14 to 30 compounds.  There are many additional PFAS compounds that are not determined as discrete compounds by existing analytical methods, including Method 537.  Hence, we may be underestimating the PFAS risk potential present in the environment. A new method, the Total Oxidizable Precursor (TOP) assay, can help measure the concentration of non-discrete and difficult to measure PFAS compounds that are not determined by conventional analytical methods.  Assessment of TOP assay data may improve our understanding of potential PFAS environmental risk.


Polyfluorinated compounds are often referred to as “precursors” to the perfluoroalkyl acids (PFAAs), as they biotransform to PFAAs as dead end environmental products.  An example of these environmental biotransformation processes is often seen in biological waste water treatment plants, where significantly more PFOA and PFOS are measured at the outflow than the inflow. The increase is explained by the fact that many PFAS compounds enter the sewage treatment plant uncharacterized and are biotransformed to PFAAs of various chain lengths with PFOS and PFOA often being the only analytes assessed.

How Can We Quantify the Potential for PFAA Production?

Current commercially available analytical methodologies are not capable of quantifying the full suite of PFAS compounds that exist in soil and groundwater. For example, the fluorotelomers and the many other PFAS compounds that biotransform are left undetermined.  In order to fully understand the potential extent of PFAS in the environment, additional laboratory techniques are needed.

TestAmerica Sacramento implemented the TOP assay as a solution to this complex problem.  The TOP assay rapidly converts polyfluorinated PFAA precursors into PFAAs including PFOA, using a hydroxyl radical-based chemical oxidation method.  The TOP assay replicates what micro-organisms in the environment would achieve after many years.  The end result is to provide a range of PFAAs which are detectable by LCMSMS[1]. The TOP assay quantifies the sum of PFAS that could be converted to PFAAs in the environment.  The TOP methodology has revealed that for AFFF-impacted sites, the existing analytical LCMSMS methods are only detecting an estimated 30% to 50% of the total PFAA mass present as PFAA precursors. 

[1] Erika F. Houtz and David L. Sedlak, “Oxidative Conversion as a Means of Detecting Precursors to Perfluoroalkyl Acids in Urban Runoff,” Environmental Science and Technology 46, no. 17 (2012): 9342-49.