Optimizing the total oxidizable precursor (TOP) assay: enhanced PFAS quantification and analytical gap bridging in...

The Total Oxidizable Precursor (TOP) assay was optimized to overcome the limitations of targeted analyses in detecting diverse PFAS compounds. This study evaluated key factors influencing the assay's effectiveness, including temperature, sorption, oxidation, and reagent-to-organic compound ratios, using both spiked solutions and real environmental matrices. The efficiency of solid-phase extraction (SPE) was assessed before and after the assay, revealing that post-oxidation SPE improved sensitivity, particularly at low PFAS concentrations. The modified TOP Assay quantified 56 PFAS and their oxidizable precursors, resulting in up to a 1000% increase in perfluoroalkyl carboxylic acids (PFCA), indicating the presence of unidentified compounds. PFAS precursors oxidation ranged from 78 to 100% in liquid samples and 64 to 100% in solid samples (wastewater, sewage sludge, compost, struvite), confirming the assay's efficacy under optimized conditions (K₂S₂O₈: 240 mM; NaOH: 600 mM; 2.5 mL reaction volume). Optimal conditions were determined based on PFCA formation, precursor transformation rates, and inter-test consistency. This adaptable and versatile protocol enhances PFAS detection across a variety of environmental matrices. Future applications to solid samples could further improve estimates of PFAS contamination in the environment.