Monitoring Over-Oxidation and Under-Oxidation in the TOP Assay Using Isotopic Standards

Per- and Polyfluoroalkyl Substances (PFAS) in the Environment
Oral Presentation

Prepared by J. Thorn, H. Kruelle, T. Barnhart
Eurofins Environment Testing (USA), 2425 New Holland Pike, Lancaster, PA, 17601, United States


Contact Information: [email protected]; 781-710-9664

ABSTRACT

The Total Oxidizable Precursor (TOP) Assay reveals "hidden" PFAS precursor burdens in environmental samples – compounds that typically evade detection by published analytical methods but may transform into perfluoroalkyl acids (PFAAs) in the environment through biological, chemical, and photochemical processes. While this transformation occurs more slowly than under controlled laboratory TOP assay conditions, recent evidence suggests that certain in-situ remediation treatments for legacy pollutants may accelerate precursor conversion at co-contaminated sites. Beyond quantification, the TOP assay offers valuable forensic capabilities for identifying precursor presence, differentiating manufacturing processes, and tracing contamination sources.

However, the TOP assay faces limitations in real-world applications. Incomplete oxidation in complex environmental matrices and the generation of unmeasurable or unidentified transformation products can compromise the reliability of the post-oxidation data. While these challenges cannot be entirely eliminated, strategic quality control measures can effectively monitor and quantify their impact.

This study introduces the use of isotopically labeled standards designed to track TOP assay performance across diverse matrices. The first standard, a labeled perfluoroalkyl carboxylic acid (PFCA), monitors process integrity by detecting inadvertent over-oxidation of terminal PFAAs. The second standard, a labeled perfluoroalkyl sulfonamide (FASA), verifies complete oxidation efficiency. The third standard—the labeled oxidation product of the FASA—confirms expected transformation pathways. Together, these standards provide comprehensive process validation from oxidation initiation through completion.

Method validation employed sequential analysis using both pre-oxidation and post-oxidation protocols. Process blanks confirmed baseline performance and established system suitability criteria. Laboratory-fortified samples containing known, measurable precursors verified complete conversion and confirmed transformation products. Real-world validation using AFFF-impacted samples with documented precursor profiles demonstrated the system's effectiveness in complex environmental matrices. Results from both EPA Method 1633A and an internally developed ultrashort-chain PFAS method will be presented, including observations on over-oxidation and under-oxidation trends as well as matrix influences. These quality control standards enable laboratories to distinguish between analytical limitations and true precursor concentrations, providing defensible data interpretation for environmental investigations and forensic applications.