TOP Assay is an incredible tool to support PFAS detection in water, soil, and other matrices. Per- and polyfluoroalkyl substances, or PFAS, have become a major environmental and public health concern. This is due to their persistence in nature and potential health risks. As the spotlight on PFAS grows, so does the need for reliable methods to assess their presence. Among the available techniques, the Total Oxidizable Precursor (TOP) assay has emerged as a powerful tool in PFAS analysis. Let’s dive into the details of the method and when it can be a better choice than traditional targeted analysis.
The Origin of the Method
The work of Houtz and Sedlak[1] derived the method. It offers more in-depth insight into the risks associated with environmental PFAS presence. Targeted analyses, like method 1633, measure specific PFAS compounds like perfluorooctanoic acid (PFOA) or perfluorooctanesulfonic acid (PFOS). These compounds are the most studied and most regulated. But, PFAS are not limited to these compounds. In reality, there are thousands of PFAS compounds. The “precursors” make up most of the other PFAS. Through environmental processes, precursors become the regulated PFAS compounds.
TOP addresses this challenge by measuring the potential PFAS contamination. Meaning you are not only seeing the existing PFAS but precursors that become regulated PFAS as well. The technique converts PFAS precursors into their measurable forms (PFOA / PFOS) through oxidation. This reveals a more complete picture of PFAS contamination in a sample. By accounting for both present and potential PFAS, this method protects against future risks.
How It Works
Samples undergo a chemical oxidation process that transforms PFAS precursors. Here’s a simplified breakdown of the steps involved:
- Sample Collection: TOP requires the collection of 2 samples. 1 for pre-oxidation testing and the other for post-oxidation testing. Both samples should be 500 ml.
- Pre-Oxidation Analysis (Optional): The first sample undergoes a targeted analysis using liquid chromatography-mass spectrometry (LC-MS). This is the pre-oxidation results.
- Oxidation: The second sample undergoes a radical-based oxidation reaction. This involves the use of strong oxidants like sodium persulfate or UV-activated persulfate with heat. This converts PFAS precursors into stable end-products like PFOA or PFOS.
- Analysis: The oxidized sample is then analyzed using liquid chromatography-mass spectrometry (LC-MS). This analysis is your post-oxidation results and includes the oxidized PFAS that were not detectable before.
This process includes both detectable PFAS and those that could form over time. Measuring PFAS after oxidation reveals a more comprehensive contamination profile.
Applications
Non-targeted methods succeed when PFAS contamination is not characterized by targeted analysis alone. Common applications include:
AFFF-Impacted Sites
The TOP assay is powerful in sites affected by aqueous film-forming foams (AFFF), used in firefighting. AFFF contains many PFAS precursors. Over time, they will degrade into regulated compounds. This protects the health and safety of firefighters and the environment.
Environmental Site Assessments:
Used to assess contamination at sites with known or suspected PFAS use. This includes industrial sites, airports, and military bases. Precursor analysis helps provide a more accurate baseline for contamination. This provides a comprehensive understanding of current and future litigation.
Wastewater Treatment Plants:
Non-targeted analysis identifies the total PFAS load in wastewater, including potential breakdown products. Your team can then check treatment efficacy and understand PFAS sources.
Why Choose the TOP Assay over Targeted Analysis?
Targeted analysis focuses on identifying specific PFAS compounds. Non-targeted analysis provides a broader assessment by capturing the total potential contamination. Here are some reasons why this assay can be better in certain cases:
1. Comprehensive Detection of Precursors
Targeted analysis misses precursor compounds. Even the best, most innovative targeted methods can only see ~90 analytes. You cannot understand the full scope of contamination through such a small lens. Precursors can degrade into toxic, regulated PFAS over time. Non-targeted analysis provides a proactive approach. By identifying the “hidden” PFAS, it offers a complete characterization of contamination at PFAS sites.
2. More Accurate Risk Assessment
When left unaddressed, precursors degrade into regulated PFAS compounds. This can lead to high levels of contamination left untreated during clean-up. These untreated PFAS will transform and cause contamination issues in the future.
By accounting for precursors, the assay offers a more complete risk profile. This is particularly important for remediation planning. Understanding the total PFAS load can guide decision-making on containment and cleanup strategies.
3. Cost-Effective Long-Term Monitoring
For ongoing monitoring efforts, non-targeted analysis allows a snapshot of potential future contamination. This can reduce the frequency or need for many targeted analyses over time. It helps environmental consultants, regulatory bodies, and remediation professionals gain insights. These insights streamline monitoring and compliance efforts.
4. Improved Decision-Making for Remediation
Knowing your site is one of the most important parts of a remediation project. A broader PFAS profile aids in making more informed remediation decisions. For example, at AFFF-contaminated sites, the assay shows current and future contamination. This allows site managers to address the future contamination as well.
Limitations
While TOP is powerful, it does have some limitations.
Limited Selectivity[2]: It doesn’t identify individual precursor compounds. The oxidation process converts all precursors to end products. This makes it less specific than targeted analysis and not useful for forensics. Additionally, it can only provide information on precursors that oxidize into detectable PFAS. Some may remain undetected if they don’t convert into measurable forms.
Volume Requirements: TOP analysis requires 1L of sample to be collected. For certain sites, sample volume can be a major constraint. When considering PFAS analysis methods for your site, you will need to understand how much water you have available. Other methods can provide PFAS data using volumes as low as 250ml.
Volatilization of PFAS: The conventional heat based method can lose some PFAS compounds to volatilization. Alternative methods like UV-activated persulfate[3] have been developed to address this issue.
PFAS Analysis is Evolving, are you Ready?
TOP assay is a valuable laboratory analysis method in PFAS assessment. Without it, you could be driving blind into a field of PFAS landmines. Understanding total PFAS contamination beyond regulated compounds provides data for long term solutions.
PFAS analysis is most powerful with a combination of targeted and non-targeted methods. FREDsense is pushing the boundaries of PFAS analysis in the lab and with PFAS field testing. Providing both targeted and non-targeted analysis.
References
- 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. https://doi.org/10.1021/es302274g
- Patch, D., O’Connor, N., Ahmed, E., Houtz, E., Bentel, M., Ross, I., Scott, J., Koch, I., & Weber, K. (2024). Advancing pfas characterization: Development and optimization of a UV-H2O2-top assay for improved PFCA chain length preservation and organic matter tolerance. Science of The Total Environment, 946, 174079. https://doi.org/10.1016/j.scitotenv.2024.174079
- Rehman, A. U., Crimi, M., & Andreescu, S. (2023). Current and emerging analytical techniques for the determination of pfas in environmental samples. Trends in Environmental Analytical Chemistry, 37. https://doi.org/10.1016/j.teac.2023.e00198