Understanding the Total Oxidizable Precursor Assay: A Comprehensive Review for Water Quality Professionals

Introduction

Standard PFAS analysis measures a fixed list of target compounds — 25 in EPA Method 533, 18 in Method 537.1, 40 in Method 1633. But thousands of PFAS exist, and many of them are precursors: polyfluorinated compounds that are not on those lists but can transform, in the environment or during treatment, into the perfluorinated acids that are regulated. A sample can test low for regulated PFAS while carrying a substantial reservoir of precursors that will become regulated PFAS later.

The Total Oxidizable Precursor (TOP) assay was developed to estimate that hidden reservoir. It deliberately oxidizes a sample to convert precursors into measurable perfluorinated acids, then compares the results before and after oxidation. This article covers how the assay works, what its results mean and do not mean, where it is most useful, and its limitations.

The TOP assay is performed by specialized commercial laboratories rather than in the field, and choosing a laboratory experienced with it is covered in our guide to selecting a PFAS testing lab.

A note on terminology: the TOP assay is sometimes confused with disinfection byproduct formation potential tests, which measure how much trihalomethane or haloacetic acid a water will form when chlorinated. Those are entirely different analyses. The TOP assay concerns PFAS only.

What is the Total Oxidizable Precursor Assay?

Definition and Purpose

The TOP assay estimates the concentration of PFAS precursors in a sample that can be oxidized to perfluoroalkyl carboxylic acids (PFCAs). It was first described in the scientific literature in 2012 and has since been adopted by commercial laboratories as a supplement to targeted PFAS analysis.

Why Precursors Matter

Many PFAS in commercial use are not the fully fluorinated acids that regulations target, but polyfluorinated compounds such as fluorotelomer-based substances and perfluoroalkyl sulfonamides. These precursors can degrade — biologically in soil and groundwater, in wastewater treatment, or through oxidation in water treatment — to form regulated compounds. The consequences are practical:

• Underestimated contamination: Targeted analysis alone can substantially understate the total PFAS burden at a site, particularly where firefighting foam was used.
• Rising concentrations over time: Groundwater concentrations of regulated PFAS can increase as precursors degrade, even after the source is removed.
• Effluent higher than influent: Wastewater plants sometimes discharge higher concentrations of certain PFCAs than they receive, because precursors are converted during biological treatment.
• Treatment surprises: Oxidation processes such as ozonation can convert precursors into regulated PFAS within a treatment plant.

How the TOP Assay Works

Sample Splitting

The sample is divided into two portions. One is analyzed directly by conventional targeted methods to establish baseline PFAS concentrations. The other is oxidized before analysis.

The Oxidation Step

The oxidized portion is treated with persulfate under alkaline conditions and heated. Heat activates the persulfate, and under alkaline conditions the reaction generates hydroxyl radicals in abundance. These radicals attack the non-fluorinated parts of precursor molecules — the hydrogen-containing portions that link the fluorinated chain to the rest of the molecule — and convert the precursors into PFCAs.

Critically, the oxidation does not affect perfluorinated acids themselves. PFCAs such as PFOA and perfluoroalkyl sulfonic acids such as PFOS are already fully oxidized end products and pass through unchanged. That selectivity is what makes the comparison meaningful.

Analysis and Interpretation

Both portions are analyzed by liquid chromatography with tandem mass spectrometry (LC-MS/MS). The increase in PFCAs between the unoxidized and oxidized portions represents the precursors that were converted. Because the length of the PFCA formed reflects the length of the fluorinated chain in the precursor, the pattern of increases also gives some indication of what kinds of precursors were present.

What TOP Results Mean — and Do Not Mean

Interpreting TOP data correctly requires understanding its limits:

• A lower bound, not a total. The assay measures only precursors that oxidize to PFCAs within the target list. Precursors that oxidize to compounds not measured, or that resist oxidation under assay conditions, are not captured. The result is therefore a minimum estimate of precursor load.
• Sulfonamide precursors lose information. Precursors of PFOS and other sulfonates are converted to PFCAs rather than to the sulfonates they would form in the environment. The assay detects them but does not show whether they would ultimately become PFOS.
• Some newer PFAS are not captured. Certain ether-based and other replacement chemistries do not convert predictably to target PFCAs.
• Matrix effects matter. Organic matter consumes oxidant. In high-organic samples such as wastewater, leachate, or soil extracts, the oxidation can be incomplete unless the laboratory adjusts dilution or oxidant dose — and incomplete oxidation understates precursors.
• Not a compliance method. The TOP assay is not an EPA-approved method for regulatory compliance. It is a characterization and investigation tool. Laboratory procedures vary, so results from different laboratories should be compared with caution.

Applications of the TOP Assay

Firefighting Foam Sites

Many fluorotelomer-based firefighting foams contain a large fraction of precursors relative to regulated PFAS. At airports, military bases, and training areas, the TOP assay helps characterize the full PFAS burden and anticipate how groundwater concentrations may evolve.

Treatment System Design

Precursor load affects treatment design in several ways. Precursors can convert to regulated PFAS during oxidative pretreatment or disinfection, and they compete for adsorption sites on treatment media. Knowing the precursor load helps designers anticipate whether regulated PFAS may increase through a treatment process.

Wastewater and Biosolids

At wastewater treatment plants, the TOP assay helps explain why effluent concentrations of some PFCAs exceed influent concentrations, and characterizes the PFAS reservoir in biosolids, which matters for land application decisions.

Source Identification

The pattern of PFCA increases after oxidation can help distinguish between sources — for example, between fluorotelomer-based foams and other inputs — and supports forensic investigations into responsibility for contamination.

Destruction Technology Evaluation

When evaluating PFAS destruction technologies, the TOP assay helps determine whether a process has destroyed precursors or merely converted them into other PFAS.

The TOP Assay Among PFAS Analytical Tools

The TOP assay is one of several approaches for looking beyond targeted compound lists:

• Targeted methods such as EPA Method 533 quantify specific compounds precisely and are required for compliance.
• Total or adsorbable organic fluorine measures all organic fluorine by combustion ion chromatography, capturing PFAS the TOP assay misses but providing no compound-level information.
• Non-targeted high-resolution mass spectrometry can identify unknown PFAS but is primarily a research tool.

How these approaches are used together to screen for PFAS is covered in our guide to PFAS screening methods, and the full range of methods in our overview of PFAS analytical methods.

Practical Considerations

When to Use the TOP Assay

The TOP assay is most valuable where precursors are likely to be significant: firefighting foam sites, industrial sources using fluorotelomer chemistry, wastewater and leachate, and situations where regulated PFAS concentrations are rising or behaving unexpectedly. For routine drinking water compliance it is generally unnecessary.

Working with the Laboratory

Discuss matrix type and expected organic content with the laboratory beforehand, so oxidant dose and dilution can be set appropriately. Ask how the laboratory confirms complete oxidation and which analytes are reported before and after.

Cost

Because the assay requires two analyses plus the oxidation step, it costs more than a standard targeted analysis. It is typically used selectively, on a subset of samples chosen to characterize precursor load, rather than on every sample.

Conclusion

The Total Oxidizable Precursor assay reveals a part of the PFAS picture that targeted analysis cannot see: the precursors that may become regulated PFAS in the future, within a treatment plant, or in the environment. By oxidizing a sample and measuring the resulting increase in perfluorinated carboxylic acids, it provides a practical, if conservative, estimate of that hidden load.

Its results are a lower bound, its procedures are not standardized for compliance, and matrix effects must be managed. Used with those limits in mind, alongside targeted methods and organic fluorine measurements, the TOP assay is a valuable tool for site characterization, treatment design, and understanding why PFAS concentrations sometimes behave in unexpected ways.