Improving Measurement Reliability of the PFAS Top Assay

The work described in this report was conducted to fulfil the inaugural Australasian Land and Groundwater Association (ALGA) Research and Development Grant. The project was an inter-laboratory assessment of the per- and poly-fluoroalkyl substances (PFAS) total oxidisable precursor (TOP) assay conducted by the project team – National Measurement Institute (NMI), Australian Laboratory Services (ALS), Eurofins and Ventia Utility Services Pty Ltd (Ventia).

The PFAS total oxidisable precursor (TOP) assay was first developed in 2012 as a method for identifying per- and poly-fluorinated non-target PFAS, thereby providing a better understanding of the extent of overall PFAS contamination present within a sample.

The method for the study involved preparation of four spiked water samples by NMI and analysis of the samples by NMI, Australian Laboratory Services, Eurofins. The four spiked water samples were:

• S1 - milliQ water spiked with Tridol foam (40,000 x dilution) and PFOSA.
• S2 - milliQ water spiked with spiked with 8:2 monoPAP, PFDA and PFOS.
• S3 - milliQ water spiked with Tridol foam (40,000 x dilution), PFOSA, PFDA and PFHxS.
• S4 - diluted liquid from a worm farm (Total Organic Carbon content of 120 mg/L) spiked with Tridol foam (40,000 x dilution) and PFOSA, PFDA and PFHxS. ALS and Eurofins did not know the contents of the samples, pre-analysis. All three laboratories analysed the samples pre- and post-TOP assay. All laboratories based their TOP Assay method on Houtz and Sedlak (2012) with modifications.  In all cases, extra doses of oxidant and/or extended oxidation times were used. All laboratories reported that these modifications were required to sufficiently oxidise the samples to meet the NEMP ratio test for aqueous samples (sum of [PFAA precursors] divided by sum of [Total PFAS] <5%).

For PFAS results post-oxidation, a high variability within and between participants’ results was observed. Due to the limited amount of data and the fact that each laboratory used different methodology for oxidation and analysis no significant trend was observed.

Application of the TOP assay did not fully convert the precursors to PFCAs for Laboratories 1 and 3. A test for acceptability of oxidation (per HEPA (2018) NEMP) is presented and all results passed these criteria except for S3 for Laboratory 1.

Laboratory 2 reported 6:2 FtS below the level of reporting (LOR) post-oxidation indicating complete conversion of the PFAA precursor. Noting that Laboratory 2 diluted the sample prior to oxidation reducing the organic load and perhaps improving the efficiency of the oxidation process. Sample S2, spiked with 8:2 monoPAP (a fluorotelomer precursor), show reasonable consensus post-oxidation results for PFCAs. The data suggests the majority of 8:2 monoPAP has oxidised under the TOP assay conditions to several PFCAs as was seen in the post-TOPA results.

HEPA (2018) NEMP defines a successful oxidation as the ratio of the sum of concentrations of PFAA precursors to the sum of total PFAS as less than 5%. Using their 6 times dosage of oxidant in a single incubation period (cycle), Laboratory 1 passed these criteria except for a marginal exceedance for sample S3.  Laboratory 2 diluted samples prior to oxidation and employed three oxidation cycles over three nights to achieve quality objectives.  Laboratory 3 used 6 times the dosage of oxidant and two cycles for samples S1 & S2 then increased dosage for samples S3 & S4. All laboratories reported that these modifications were required to meet the NEMP ratio test (sum of PFAA precursors to sum of PFAS). Applying the Houtz and Sedlak (2012) method without modification will have insufficient oxidant for samples with high organic content. It has been reported that samples with high organic Questions from the session:

content and/or high concentrations of PFAA precursors can consume oxidant facilitating the need for extra dosages.

In addition to the results presented above six sequential oxidant doses vs a single upfront six-times oxidant dose were investigated. There was no material difference in performance between sequential dosing and a single six-times upfront dose.  One observation that is interesting to note is the increase in PFOS across the sequential doses.  It is suggested that increasing dosage may result in an elevated alkaline environment, initiating hydrolysis of PFOSA to PFOS.  This observation is consistent with the PFOS results originally reported by the three labs.  Both labs 1 and 3 who performed higher overall dosages reported higher PFOS concentrations.  Lab 2, with a lower final (3x) dosage reported lower PFOS, and at a level consistent with the 3rd dose from the successive trials.  The results of this trial suggest either successive small doses or a single large dose are valid approaches to achieve effective oxidation of challenging matrices. Also, that high dosages may create alkaline conditions sufficient to convert precursors to PFSAs via hydrolysis rather than the expected PFCAs.  Where a significant increase in PFSAs is observed from pre- to post-TOP, sample dilution may be a considered approach to achieving equivalent oxidation at a lower dose and avoiding alkaline hydrolytic conditions, noting potential for the need to raise limits of reporting.

The results reported were used to assess laboratories’ accuracy in the measurement of PFAS before and after application of the TOP assay. The laboratories managed to comply with current NEMP parameters (with some minor exceptions) however, all laboratories were required to modify the original Houtz and Sedlak (2012) approach.  In all cases, extra cycles and/or increased reagent dosage was required to meet NEMP targets. A consensus method is not provided here, rather, advice to laboratories on how best to develop methodology and apply to environmental samples (as presented in 4.2).

The results indicated that fulfilment of quality objectives require increased oxidant dosage and/or extra oxidative cycles. The advice to laboratories developing a routine TOP assay method:

• Choose a method that will comply with NEMP requirements for as many sample types as possible.  Increased dosages and multiple cycles are recommended.
• If samples do not comply with the NEMP ratio test post oxidation treatment, then further oxidative treatment is required.  Another option is to dilute the sample prior to oxidation to try and reduce organic load.  Dilution can result in raising of the Limit of reporting to an extent where the results lack analytical meaning.
• Take note of the concentrations of sulfonates pre- and post-oxidation.  In this study, PFOS & PFHxS were spiked into samples as monitoring compounds.  The sulfonates should have similar concentrations pre-oxidation compared to post-oxidation (as required under NEMP QA for equivalence of sulfonate concentrations).
• Assess total PFAA after each oxidation cycle No change in PFAA concentrations between cycles (within measurement uncertainty) is a reasonable indicator that the oxidation process is complete and that there are no significant PFAA precursors remaining.
• Assuming the sample does not contain >C8 PFAA precursors then C10 and >C10 acids should also have similar concentrations pre-oxidation versus post-oxidation.

Questions from the audience:

Gavin S: Can you give an explanation on how the EOF test works?  Extractable Organic Fluorine...

For a solid sample, extract the sample with solvent as per normal LC-MSMS analysis.  Use a portion of normal soil/biosolid extract and dry down in autosampler “boat” for combustion IC.  For water, either a portion of conventional solid phase extraction extract or adsorb onto activated carbon per AOX.

Emily S: You touched on organic content. Is there any value for organic content where TOPA is not appropriate? Or would this just be related to dilutions required?

We use a balance of dilution and multiple dosage until we achieve the desired performance for the assay.  In general, three to six times the published dosage covers most contingencies.  Too much dilution can raise the LOR to an impractical level.

Paul N: Is there a risk of producing more <C4 PFAAs if too many/too strong reagents are used in the TOPA?

As long as high pH is maintained, only hydroxyl radicals will be produced.  We have demonstrated that repeated or multiple dosages (hydroxide and persulfate) leave all perfluorcarboxylic acids intact. 

Danielle C: My question relates to the slide with the lab differences between S2, S3 and S4. I’m interested in the consistent and variance between S1 results between labs. i.e it seems to have a higher variance. Danielle C: consistency*

Given only three labs were involved, it is hard to infer too much about the interlab variability for each sample.  If NMI does arrange a larger study, this type of information might be achievable.

Gavin S: Hi Karl, this study covered TOPA of water samples, what about soil/biosolids/biota?

Have to walk before we run.  Certainly, TOPA study with more complex matrices will be valuable.

Andrew T: Did any lab report QA/QC relating to the TOP assay? Specifically any data that showed the performance of the digest e.g isotope reactions after digest.

The study was performed in the manner of a proficiency trial in which only the relevant sections on the result sheet were filled in.  No question on QA/QC.  My understanding is that all labs used the NEMP criterion or a variation thereof (for ALS the sum of residual fluorotelomers divided by the sum of carboxylates ~5% or less) as a measure of complete digestion.

Phil S: Karl. Thanks for a very clear presentation

Dr B: Will there be an equivalent study to develop the TOF (Total Organic Fluorine) assay and to get its LOR down to similar to that for PFAS?

The technique accessible to labs is combustion ion chromatography.  Detections of around 5 to 10 ppb are at the low end for this technique so that lower levels need sample enrichment.  Inorganic fluorine background can be an issue in these circumstances. I’m not aware of any plans for a study.  Ideally further work on TOF, EOF and AOF would be valuable.

Kali M: Is there an overall indication that increased organic matter in sample matrix affects accuracy of TOPA results (can result in overstating results?)

Rather than doing multiple dosing on an individual sample, ALS will repeat the analysis at a higher dosage (ultimately at a level sufficient to get rid of competing organic carbon) and this does not affect accuracy of the tests since the PFAA endpoints are very stable.  As part of QC, the lab would also monitory pH at the end of the process, if this remains high, this is an indication that chain shortening has not occurred.  In addition, ALS will spike with labelled PFOS and PFOA surrogates which are expected to survive the oxidation intact.

Emily S: Are there any plans to include % PFCA and PFAAs as part of the lab QC reporting

ALS is considering offering PFCA and PFSA sums as part of this assay as we believe they are more meaningful than sum of PFAS.  The ratio of the sum of fluorotelomer sulfonates to PFCAs is probably the best rule of thumb indicator of the completeness of oxidation.  This is because they are current target analytes for most labs and also because these can be produced by hydrolysis from “precursors” after the exhaustion of oxidant”.

Acknowledgments

Ventia Utility Services Pty Ltd (Ventia), in conjunction with the National Measurement Institute, Australian Laboratory Services and Eurofins Environment Testing Australia were awarded the inaugural Australasian Land and Groundwater Association (ALGA) Research and Development Grant to complete this project.

Members of the project include:

National Measurement Institute

Australian Laboratory Services

Eurofins Environment Testing Australia (Eurofins)

Ventia

Steering Committee Members include:

Ventia, NMI, ALS and Eurofins would like to thank ALGA for their foresight in creating the grant and their contribution to development of the contaminated land industry in the Australia and New Zealand region.

Ventia would also like to thank the project and steering committee members for their hard work, openness and collaborative attitude in successfully completing this much needed project.

April 2019 event report by Michal Grigor
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