The dispersion of the results, provided as CV% (robust standard deviation of the results / robust mean of the results (in %)), allows to describe how strong or not is the consensus met (see Figure 1).
It is to be noted that a large majority of untrue results were so by overestimation (87.5%). This suggests a positive bias for these analyses.
Figure 4. Spiking performance (2). Figure 1. CV% over time.
The dispersion of most of these molecules shows the difficulty to analyse this kind of matrix. Though, several high CV can be attributed to a smaller number of participants at the time, in the case of the PFDS for instance. A slight decrease in the dispersion of PFDA, PFHpA and PFOA between December 2018 and December 2021 can be observed.
At the considered concentration (from approximately 0.040 to 0.250 µg.l -1) and despite a limited number of data, there seems to be no correlation between the assigned value and the CV for these molecules. Below is the example of PFHxS and PFOS (see Figure 2).
Figure 2. CV% against concentration.
Some information can also be obtained for the spiking performed on the samples. The consensus value (or assigned value) obtained in the tests can actually be compared with the theoretical spiking value (see Figure 3 and Figure 4).
Figure 5. Mean performance of the participants (Dec-16 to Dec-21).
Figure 6. Performance of the participants for PFHxS and PFOS (Dec-16 to Dec-21) Conclusion Figure 3. Spiking performance (1).
Overall, the spiking values are well recovered when the molecules are found and can be divided into 3 main categories: • Average recovery rate (absolute) below 10%: PFHpA, PFOA and PFDA • Average recovery rate (absolute) between 10% and 20%: PFHxS, PFOS and PFDS • Average recovery rate (absolute) above 20%: PFHxA
For PFHxA, it seems that an occasional natural contamination of the matrix is plausible to justify the highest recovery rate. All of the molecule are added from the same solution and no systematic overestimation of this molecule is observable. Neither is a general overestimation of the molecules. In particular, for December 2020, the assigned value represents +48% of the spiking, suggesting pre-existing PFHxA in the matrix.
There seems to be no major stability or analytical issues considering that most of the molecules are found every time.
In the case of PFDS, the lack of participants played an important part in the absence of assigned values for several tests.
Overall, the performance observed on Figure 5 is satisfactory as no mean % of untrue is above 20% despite 3 individual tests reaching 30%. Error bars represent the standard error of the mean. There was no specific evolution over the years (see Figure 6 below for two examples), the performance has stayed quite stable over time if we exclude the very start of the PTS. Only 2 laboratories out of the 31 considered over all the campaigns have a global percentage of untrue results above 30%. These analyses seemed to be already well handled by a majority of participants from the very beginning of the PTS.
The surveillance of perfluorinated compounds in water has been developing over the years. In order to meet this new demand, a dedicated proficiency testing scheme allows the participants to have a better control of their routine analyses and to potentially evaluate themselves on new molecules of interest. This kind of test is very useful to assess the performance of laboratories and detect bias or non-compliant results; thus, act as a warning signal for the implementation of corrective and/or curative actions in the laboratories. Participation in several proficiency tests per year is of considerable importance, particularly to detect drift or bias in the results, through the use of control charts. Proficiency tests are an essential tool for the quality management of laboratories and for the continuous improvement of their analytical performance.
The number of results is now sufficient after a few years of testing and grants the possibility to get robust data: laboratories performance, spiking recovery, stability and participation. In the case of perfluorinated compounds, the participants have shown their ability to conduct these analyses in a satisfactory manner despite a challenging matrix. There was no significant evolution over time in the performance by looking at the percentage of untrue results, though the dispersion has slightly improved for PFDA, PFHpA and PFDA.
The need for such testing might grow as the regulation tends to be stricter for these molecules in fresh water.
References
1. Valeria DULIO et Sandrine ANDRES – Recommendations of the CEP to the MEDDE for the selection of relevant substances to be monitored in aquatic environments for the second cycle of the DCE (2016-2021) – Report AQUAREF 2013 – 102 p.
2. Stahl, T., Mattern, D. & Brunn, H. Toxicology of perfluorinated compounds. Environ Sci Eur 23, 38 (2011). 3. Approval of the French Ministry of Environment:
http://www.labeau.ecologie.gouv.fr/
4. International standard: ISO/IEC 17025:2017 - General requirements for the competence of testing and calibration laboratories.
5. International standard: ISO 13528:2015 - Statistical methods for use in proficiency testing by interlaboratory comparisons
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