Total oxidizable precursors assay for human serum: a tool for holistic per- and polyfluoroalkyl substances (PFAS) exposure assessment
Authors/Creators
- 1. Norwegian Institute for Air Research (NILU)
Description
Presentation at SETAC Europe 32d Annual Meeting, Copenhagen (May 2022)
Abstract:
Per- and polyfluoroalkyl substances (PFAS) are a class of industrial chemicals that have been detected worldwide in the environment, in wildlife and in humans. Even if over 4700 PFAS are registered in the market, only few of them are commonly analyzed in human samples and some fluorine mass-balance studies are showing that currently measured PFAS might not be enough to describe the full extent of PFAS human exposure. Among the underlooked PFAS in human samples, precursors are of concern for human health because these compounds can be metabolized to perfluoroalkyl acids (PFAA) that are already known to be toxic.
To look for both known and unknown precursors in human serum an adaptation of the total oxidizable precursors assay (TOPA) was developed. In the TOP assay precursors are converted to easy to measure PFAA by oxidation with heat activated persulfate under alkaline conditions. The total oxidizable precursors are then quantified by measuring the PFAA produced by oxidation.
The TOPA was introduced as a complementary tool for water analysis and had to be modified to be applied to small volumes of human serum samples. Modifications to the method were tested on reference serum samples spiked with model precursors. To fully convert the model substances the heating at 85°C had to be extended and a large excess of oxidant was needed. However, even when precursors were fully converted, the yield of identified oxidation products was not always 100%, ranging from 33 to 100%. As there can be precursors that are not at all or not fully converted to PFAA, care must be taken in quantifying the total amount of precursors present using the sum of PFAA produced by oxidation.
Unequivocal information on the structure of the oxidizable part vanishes, but the distribution of products is a good indication to that, as well as to the structure of the original perfluorinated part. In general, the products are perfluorocarboxylic acids with the same or shorter perfluorinated chain, but for some precursors PFOS and perfluoroethercarboxylic acids can also be observed as oxidation products.
The final method can be used to determine well known PFAS and oxidizable precursors using only one small serum aliquot. The TOP assay is a valuable tool to estimate the contribution of precursors present in human serum directly linked to human exposure, indicating their structure and type without requiring additional sample amounts, instrumentations or standards.