Understanding TOP Assay: A Comprehensive Guide for Water Treatment Professionals

Introduction

The TOP assay — short for Total Oxidizable Precursor assay — is the analysis most often recommended when standard PFAS results do not add up. A site where firefighting foam was used shows modest PFOA and PFOS levels; groundwater concentrations climb years after the source was removed; a wastewater plant discharges more of certain PFAS than it receives. In each case, the missing piece is usually precursors: PFAS that routine analysis does not measure but that convert into regulated compounds over time.

This article is a practical guide to using the assay: when to order it, what to tell the laboratory, how to read the report, and what conclusions the data supports. The chemistry behind the method is covered separately in our explanation of the total oxidizable precursor assay.

What the TOP Assay Measures

In brief: the laboratory splits the sample. One portion is analyzed for target PFAS in the usual way. The other is oxidized with heated alkaline persulfate, which converts precursors into perfluoroalkyl carboxylic acids (PFCAs), and then analyzed the same way. The increase in PFCAs between the two portions is the precursor signal.

Two features of the chemistry shape everything about interpretation:

• Perfluorinated acids do not change. PFOA, PFOS, and other terminal compounds pass through oxidation unaltered, which is what makes the before-and-after comparison meaningful.
• Everything becomes a carboxylic acid. Precursors that would have become PFOS in the environment are converted to PFCAs in the assay. The assay detects them, but the result does not tell you what they would have formed naturally.

When to Order a TOP Assay

Strong Candidates

• Firefighting foam sites. Fluorotelomer-based foams contain a large precursor fraction. At airports, military installations, and fire training areas, targeted analysis alone can substantially understate the PFAS present.
• Unexplained concentration trends. Where monitoring shows regulated PFAS rising without a new source, precursor conversion is a likely explanation.
• Wastewater and biosolids. Where effluent exceeds influent for certain PFCAs, or where biosolids are being evaluated for land application.
• Industrial sources using fluorotelomer chemistry. Including paper and textile coating operations.
• Treatment evaluation. Where an oxidative process — ozonation, for example — might convert precursors into regulated compounds within the plant.

Poor Candidates

• Routine drinking water compliance. Compliance is determined by approved targeted methods. The TOP assay has no role in it.
• Clean groundwater with no known source. If targeted analysis finds nothing and there is no plausible precursor source, the assay usually adds cost without insight.

Commissioning the Analysis

What to Tell the Laboratory

The TOP assay is not standardized in the way approved compliance methods are, so laboratory procedures differ. Establish the following before sampling:

• Matrix and expected organic content. Organic matter consumes oxidant. High-organic samples — wastewater, leachate, soil extracts — may need dilution or increased oxidant dose, and the laboratory needs to plan for it.
• The analyte list reported before and after oxidation. It should be the same list in both, and it should include short-chain PFCAs, which is where much of the increase appears.
• How the laboratory confirms complete oxidation. Ask what checks are run and what happens if oxidation was incomplete.
• Reporting format. Confirm you will receive both data sets and the calculated differences, not just a summary figure.

Sampling

Sampling requirements are the same as for any PFAS analysis: fluoropolymer-free materials, polypropylene or HDPE containers, no waterproof field gear or treated papers, and field blanks. Because the assay requires two analyses, confirm sample volume requirements with the laboratory, as they are larger than for standard analysis.

Cost and Turnaround

The assay costs more than standard targeted analysis — two analyses plus an oxidation step — and takes longer. It is typically run on a subset of samples chosen to characterize precursor load, with standard analysis on the remainder.

Reading the Report

The Basic Comparison

A TOP report presents pre-oxidation and post-oxidation concentrations for each analyte. Work through it in this order:

1. Check that perfluorinated sulfonates are unchanged. PFOS and other sulfonates should be similar before and after. A large change suggests an analytical problem worth querying.
2. Calculate the PFCA increase. Sum the increases across the carboxylic acids. This is the precursor signal.
3. Look at which chain lengths increased. The pattern indicates the fluorinated chain lengths of the precursors present, which is useful for source identification.
4. Compare the increase with the original total. Where the precursor signal is comparable to or larger than the measured PFAS, targeted analysis alone was giving a substantially incomplete picture.

What the Numbers Support

Reasonable conclusions from TOP data:

• A precursor reservoir is present, and its approximate magnitude.
• Regulated PFAS concentrations may increase over time as precursors degrade.
• Treatment design should account for PFAS beyond those currently measured.
• The precursor pattern is consistent, or inconsistent, with a suspected source type.

What They Do Not Support

• A total PFAS figure. The result is a lower bound. Precursors that do not convert to measured PFCAs are invisible to the assay.
• A prediction of future PFOS. Sulfonate precursors appear as carboxylic acids in the assay, so it cannot forecast how much PFOS will ultimately form.
• A compliance determination. The assay is not an approved method for any regulatory limit.
• Direct comparison between laboratories. Because procedures vary, results from different laboratories should be compared cautiously, and ideally a single laboratory should be used throughout a project.

Warning Signs in a Report

• Sulfonate concentrations that change significantly after oxidation.
• A negligible increase in a sample where a fluorotelomer source is known — which may indicate incomplete oxidation rather than absence of precursors.
• Post-oxidation results reported without the corresponding pre-oxidation data.

Using TOP Data in Decisions

Site Investigation

Precursor data changes how the extent of contamination is understood and can shift conclusions about plume behavior and remediation timelines. It also supports source attribution, where the pattern of precursors helps distinguish between contributing sources.

Treatment Design

Precursors compete for adsorption sites and can convert into regulated compounds during treatment. Where the precursor load is significant, designs based on targeted PFAS alone may prove undersized.

Choosing Among Screening Approaches

The TOP assay is one of several ways to look beyond targeted lists. Organic fluorine methods capture more PFAS but give no compound information; non-targeted mass spectrometry identifies unknowns but rarely quantifies them. The trade-offs are set out in our guide to PFAS screening methods, and the approved targeted methods against which all of this is measured in our coverage of EPA Method 533.

Conclusion

The TOP assay answers a specific question: how much PFAS is present that standard analysis is not measuring, and that could become regulated PFAS later. Used where precursors are plausible — foam sites, industrial sources, wastewater — it frequently changes the picture substantially.

Getting value from it depends on commissioning it properly: agreeing matrix handling and analyte lists with the laboratory beforehand, insisting on both data sets in the report, and interpreting the result as a lower bound rather than a total. Read that way, the assay is among the more informative analyses available for understanding PFAS contamination.