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Total Oxidisable Precursor (TOP) Assay

As a result the growth of the synthetic chemical industry, over the last 50 years, many thousands of new fluorinated molecules have become almost ubiquitous in the environment.   A multitude of poly- and perfluoroalkyl substances (PFAS), valued for their ability to repel both water and oils, have been used for decades for diverse applications such as water repellent “outdoor” fabrics, stain resistant carpets, non-stick frying pans and firefighting foams. 

PFOS (perfluorooctane sulphonate) and PFOA (perfluorooctanoate) are the best know examples of PFAS. Their potential impact on human health has already been recognized internationally as they are extremely persistent, bioaccumulative and toxic.

As a result, PFOS is restricted under the Stockholm Convention and classed as a persistent organic pollutant (POP), with PFOA being actively considered for inclusion. However, more recently, regulators’ in differing countries interest and concern has expanded to take in a much wider number of the PFAS now present in the environment such as perfluorinated or polyfluorinated alkyl substances compounds (PFAS) found in aqueous film forming foam (AFFF) and multiple other products.  These perfluorinated or polyfluorinated compounds  are sometimes termed “precursors” as they can biotransform to form more simple perfluorooctanoic acids such as PFOA.

As a result, there are significant analytical challenges to overcome when considering how to assess soil and groundwater contaminated with PFAS as there are multiple analytes to consider, not just PFOS and PFOA.


Routine analysis of these compounds is undertaken by Liquid Chromatography - Triple Quadrupole Mass Spectrometry (LC-QQQ). This analytical technique is very selective and sensitive, allowing for compounds which have been calibrated for to be detected at low concentrations, typically around 1 ng/l or less in water and 1 ug/Kg in soil.

A key issue here for risk assessors and remediation practitioners is that routine methodology can only reliably report known, calibrated compounds - whereas, many AFFF and other formulations, contain a multitude of PFAS compounds.

 These unknown ‘precursor’ compounds are sometimes referred to ‘PFAS dark matter’. These compounds can be in a number of different ionic states and they biotransform over time in the environment to become simple perfluorooctanoic acids, which do not degrade or transform further.

Trying to analyse for all the potential PFAS compounds present in a sample is not practically possible.

The Total Oxidisable Precursor (TOP) assay has been developed using simple hydroxyl radical chemistry to transform the PFAS precursors in a sample to measurable Perfluorinated carboxylic acid compounds which can be measured. 

This gives an indication of the PFAS dark matter in the samples which will biotransform over time to the Perfluorinated carboxylic acids.

The TOP assay provides information analogous to TPHCWG, in that the carbon length of the PFAS pre-cursors are provided, although specific substance information is not. This helps to understand the likely toxicity and bioaccumulation effects of the PFAS substances present.

As part of the validation for this method a series of AFFF were analysed pre and post oxidation. The results are presented below.

SYNLAB and Arcadis have worked collaboratively to bring this exciting product from academic research to full commercialisation to help fill some of the gaps in knowledge that regulators and remediation practitioners have in relation to PFAS impacted sites.