Per- and polyfluoroalkyl substances (PFASs) are man-made fluorinated compounds that havebeen detected in humans, the environment, and wildlife. The strong C-F bonds contribute tothe high thermal stability of some PFASs and make them extremely resistant to degradationunder normal environmental conditions. The commonly studied classes of PFASs areperfluoroalkyl carboxylic acids and perfluoroalkyl sulfonic acids. Though the phase-out ofperfluorooctane sulfonic acid and perfluorooctanoic acid for some time, they were the mostfrequent compounds measured and detected in biota.In this study, the total oxidisable precursor (TOP) assay, which is a method used to indirectlymeasure and estimate PFAS precursors, was evaluated. Different parameters were studied tounderstand their effects on oxidation of precursor compounds following TOP assay method(e.g., oxidation rate, pH of the oxidation solution and different washing steps during solidphaseextraction)The results from this experiment showed that the target precursors were oxidised and part ofthe proportion of the compounds formed ranged from 37 to 100% (molar ratio). Whenperforming TOP assay, it was shown that some precursors oxidised before 0.5 h while otherprecursors were oxidised after 4 hours. However, it was still unknown what products wereformed when some precursors were oxidised e.g., 7:3 fluorotelomer carboxylic acid (7:3-FTCA) and 10:2 fluorotelomer unsaturated carboxylic acid (10:2- FTUCA) because of animbalance of precursors to degradation products (molar to molar ratio).When the washing steps for solid-phase extraction were evaluated for potential losses ofprecursors, it could be seen that the ammonium acetate and the 20% methanol in Milli-Q waterled to the loss of some precursors. These results showed precautions were needed wheninterpreting the results using TOP assay.