Total Oxidisable Precursor Assay

Traditional PFAS analysis only targets the key analytes and therefore may or may not greatly underestimate the presence of PFAS in the environment. The Total Oxidisable Precursor Assay and the determination Total Organic Fluorine have been suggested as means of exposing the presence of these underlying cryptic PFAS.

Total Oxidisable Precursor Assay

The Total Oxidisable Precursor Assay (TOP assay or TOPA) is a standardised pre-treatment of water samples or sample extracts (soil and water) designed to expose underlying PFAS not amenable to standard analysis.

Water samples, sample extracts (soil or water) or diluted foam products are incubated with potassium persulfate (60 mM) and sodium hydroxide (0.125 M) at 85°C for 6 hours. Samples are neutralised and then run for the full suite of PFAS compounds [3,4].

Note that this is an empirical test and comparable results can only be achieved by precisely following the conditions of the test.

Under the conditions of the assay it is expected that fluortelomer sulfonates are broken down to shorter chain carboxylates by cleavage of the non-fluorinated portion of the molecule. Perfluorinated carboxylates and sulfonates are stated to remain intact under the conditions of the assay.

International Policy on the Management of Firefighting Foam

While policy is still in the formative stages in many countries globally, some countries have developed guidance to address the potential presence of unknown PFAS chemicals. Australia (Queensland Department of Environment and Heritage) recognises the potential contribution of cryptic PFAS to the environment. A policy has been developed that provides guidelines for using the TOPA for the assessment of soils, waters and foam products [5]. 

ALS Analysis and Reporting

ALS developed and has provided TOP pre-treatment and subsequent analysis in Australia for clients for over a year and now brings this method to Canada in support of this sector.  ALS UK was also the lead laboratory working on developing the Arcadis version of the TOP Assay in 2015. ALS has also performed internal proficiency across the ALS PFAS centers of excellence in Australia, Europe, Canada and the U.S. to ensure quality and consistency remain at the highest levels for this procedure.

ALS will perform the usual sample preparation used for waters (including low level waters), diluted foam products and soils followed by oxidation of the sample under the standard conditions for the TOP assay.

Please note that only the full analytical suite is available for this assay in order to account for the full range of products produced post-oxidation.

Oxidation of AFFF Products

Over the last 18 months, ALS has performed numerous oxidation trials in order to obtain a deeper knowledge of the Total Oxidisable Precursor Assay. This includes the impact of concentration plus the differences between first generation and modern foams.

A number of foam products were subject to the TOP alkaline persulfate digest and analysed by LCMSMS. Three products were examined: 3M light water™, Ansulite™ and Aer-O-Water™ representing the first being the classic pre-2000 AFFF and the latter two examples of more recent short-chain foams.
Note that only the Aer-O-Water product was in its original packaging whilst the other products had been supplied as subsamples of uncertain origin.

Figure 1 indicates significant growth in total PFAS following oxidation for Ansulite and Aer-O-Water. Growth of perfluorocarboxylic acids in 3m Light Water may be attributable to unspecified “fluoroalkyl amide derivatives” described in a 1996 MSDS from 3M in Australia. The outcomes of these digests are consistent with results reported in the literature on this subject [6].

In Figure 2 the concentration of the AFFF can affect both qualitative and quantitative outcome of the oxidation process. Oxidation at high dilutions leads to full conversion of the material to carboxylic acids. At higher concentrations, this conversion is less than complete which indicated exhaustion of the oxidant. At a 500-fold dilution, 6:2-FTS is a large component of the composition which is consistent with the presence of fluorotelomer sulfonamido betaines in some modern-day products [2].

Oxidation of Field Samples

ALS test work included taking samples that recorded reasonably high results in normal testing and subjected these to the TOP assay.
The pre-oxidation composition of samples indicate use of both PFOS and fluortelomer containing foams in the one location (PFOS in light blue and 6:2-FTS in pale orange).
From the composition of un-oxidised samples it can be seen that contamination originates from a mixed source – both PFOS-containing and more recent fluorotelomer foams. 

Note that post-oxidation there is not much growth in total fluorinated compounds but that the fluorotelomers undergo oxidation to carboxylic acids. Also, in the case of samples 2 and 3 PFOS is seen to reduce slightly which is contrary to the assumptions behind the assay.