Uncertainties of the results - Interlaboratory Proficiency Test 15/2020. VOC compounds in water

All but one participant reported the expanded uncertainties (k=2) with their results for at least some of their results (Table 4, Appendix 9). The range of the reported uncertainties varied between the measurands and the sample types.

Several approaches were used for evaluating the measurement uncertainty (Appendix 14). The most used approach was based on using the data obtained from method validation. The approaches based on the internal quality control (IQC) data from synthetic and routine sample replicates as well as the IQC data with proficiency test results were also used. One participant used MUkit measurement uncertainty software for the estimation of their uncertainties [6, 7].

The free software is available on the webpage: www.syke.fi/envical/en. Generally, the used approach for estimating measurement uncertainty did not make definite impact on the uncertainty estimates.

Within the optimal measuring range, the expanded measurement uncertainty (k=2) should typically be 20 – 40%. Close to the limit of quantification the relative measurement uncertainty is higher.

Table 4. The ranges of the reported expanded uncertainties by participants as percent (k=2, U

%).

Measurand Synthetic sample

A1V, % Surface water sample

N2V, % Soil sample

M3V, %

1,2,4-Trichlorobenzene 15–30 15–40 30–44

1,2-Dichlorobenzene 15–37 15–37 20-39

1,2-Dichloroethane 15–38 15–40 29–39

1,4-Dichlorobenzene 15–32 15–40 29–40

Benzene 15–37 15–37 30–40

Chloroform 15–34 15–40 27–39

cis-1,2-Dichloroethene 15–30 15–40 20–43

Dichloromethane 20–31 20–35 20–42

ETBE 10–30 10–35 30–50

Ethylbenzene 20–43 20–43 20–39

m/p-Xylene 15–38 15–40 20–39

MTBE 10–31 10–35 30–39

o-Xylene 20–33 20–35 20–39

Styrene 15–41 15–41 20–45

TAME 15–33 15–35 15–39

Tetrachloroethene 20–43 20–43 20–40

Tetrachloromethane 20–30 20–40 30–40

Toluene 17–37 17–37 20–39

trans-1,2-Dichloroethene 25–33 20–33 25–39

Trichloroethene 15–37 15–40 28–41

4 Evaluation of the results

The performance evaluation of the participants was based on the z and E

scores, which were calculated using the assigned values and the standard deviation for proficiency assessment (Appendix 7). The z and E

scores were interpreted as follows:

Criteria Performance

 z   2 Satisfactory

2 <  z  < 3 Questionable

| z   3 Unsatisfactory –1.0 < E

< 1.0 Satisfactory E

 –1.0 or E

 1.0 Unsatisfactory

In total, 82 % of the results were satisfactory when total deviation of 15–35 % from the assigned value was accepted. Further, 80 % of the results evaluated based on E

scores were satisfactory.

Altogether 80 % of participants used accredited analytical methods at least for a part of the measurements. The summary of the performance evaluation and comparison to the previous PT is presented in Table 5. In the previous similar PT (VOC 11/2016) the performance was satisfactory for 81 % of the participant results [5].

Table 5. Summary of the performance evaluation in the proficiency test VOC 15/2020.

Sample /

Measurand 2×s

% Satisfactory

results, % Assessment A1V

Aromatic VOC

compounds 15–20 79

In the PT VOC 11/2016 80 % of the results were satisfactory when deviation of 15 % was accepted [5].

A1V

Chlorinated VOC compounds

15–25 75 In the PT VOC 11/2016 78 % of the results were satisfactory [5].

A1V

Oxygenates 20 78 In the PT VOC 11/2016 72 % of the results were satisfactory when deviation of 15 % was accepted [5].

N2V Aromatic VOC compounds

25–30 82 Only approximate performance evaluation for styrene and toluene as the criteria for the reliability of the assigned value and/or the s

were not met. In the PT VOC 11/2016 83 % of the results were satisfactory when deviation of 30 % was accepted [5].

N2V

Only approximate performance evaluation for 1,2-dichlorobenzene, 1,2-dichloroethane, 1,4-dichlorobenzene, dichloromethane, and trichloroethene as the criteria for the reliability of the assigned value and/or the s

were not met. Also in the PT VOC 11/2016 80 % of the results were satisfactory when deviation of 25-35 % was accepted [5]. The results for cis-1,2-dichloroethene, tetrachloromethane, and trans-1,2-dichloroethene were evaluated with E

scores.

N2V

Oxygenates 25–30

73 E

scores:

60 Only approximate performance evaluation for MTBE as the criteria for the reliability of the assigned value and the s

were not met. In the PT VOC 11/2016 82 % of the results were satisfactory when deviation of 25-30 % was accepted [5]. The results of ETBE were evaluated with E

scores.

S3V Aromatic VOC

compounds 25–30 94

Good performance and for measurands ethylbenzene, o-xylene and toluene the performance was excellent (100 %). In the PT VOC 11/2016 74 % of the results were satisfactory when deviation of 30-35 % was accepted [5].

S3V Chlorinated

VOC compounds 25–30 91

Good overall performance. For 1,2,4-trichlorobenzene, 1,2-dichlorobenzene, 1,2-dichloro-ethane, chloroform, and cis-1,2-dichloroethene the performance was excellent (100 %).

Only approximate performance evaluation for 1,4-dichlorobenzene and trichloroethene as the criterion for the reliability of the assigned value was not met. In the PT VOC 11/2016 85 % of the results were satisfactory when deviation of 30-35 % was accepted [5].

S3V

Oxygenates 30–35 79 Only approximate performance evaluation as the criteria for the reliability of the assigned

value and/or the s

were not met. Also, in the PT VOC 11/2016 79 % of the results were

satisfactory [5].

5 Summary

Proftest SYKE carried out the proficiency test (PT) for analyses of volatile organic compounds water and soil in November 2020 (VOC 15/2020). A synthetic sample, a surface water sample and a soil sample were delivered to the participants. In total there were 10 participants in this proficiency test.

Either the calculated concentration (synthetic sample) or the median of the reported results was used as the assigned value for the measurands. The expanded uncertainty of the assigned value was estimated at the 95 % confidence level and it was 2.2–9.2 % for the calculated assigned values and 3.0–19.6 % for the assigned values based on the median of the reported results.

The performance of the participants was evaluated by using z and E

scores. In this PT 82 % of the results evaluated with z scores were satisfactory when deviation of 15–35 % from the assigned value was accepted. Further, 80 % of the results evaluated with E

scores were satisfactory. The performance of the participants was on the same level as in the previous similar PT in 2016 when 81 % of the results were satisfactory.

6 Summary in Finnish

Proftest SYKE järjesti marraskuussa 2020 pätevyyskokeen laboratorioille, jotka määrittävät haihtuvia orgaanisia yhdisteitä (VOC) vedestä ja maasta (VOC 15/2020). Osallistujille toimitettiin synteettinen näyte, pintavesinäyte sekä maanäyte. Pätevyyskokeeseen osallistui yhteensä 10 laboratoriota.

Testisuureiden vertailuarvona käytettiin laskennallisia pitoisuuksia (synteettinen näyte) tai osallistujien tulosten mediaania. Vertailuarvon laajennettu epävarmuus 95 %:n luottamusvälillä oli 2,2–9,2 % käytettäessä vertailuarvona laskennallista arvoa ja 3,0–19,6 % käytettäessä vertailuarvona osallistujatulosten mediaania.

Tuloksia arvioitiin z- ja E

-arvojen avulla. Koko tulosaineistossa hyväksyttäviä tuloksia oli

z-arvojen perusteella 82 %, kun vertailuarvosta sallittiin 15–35 %:n poikkeama. Tuloksista, jotka

arvioitiin E

-arvoilla, oli hyväksyttäviä 80 %. Hyväksyttävien tulosten määrä oli samalla tasolla

kuin edellisessä vastaavassa vertailussa vuonna 2016, jolloin hyväksyttäviä tuloksia oli 81 %.

REFERENCES

1. SFS-EN ISO 17043, 2010. Conformity assessment – General requirements for Proficiency Testing.

2. ISO 13528, 2015. Statistical methods for use in proficiency testing by interlaboratory comparisons.

3. Thompson, M., Ellison, S. L. R., Wood, R., 2006. The International Harmonized Protocol for the Proficiency Testing of Analytical Chemistry laboratories (IUPAC Technical report). Pure Appl. Chem. 78: 145-196, www.iupac.org.

4. Proftest SYKE Guide for laboratories: www.syke.fi/proftest/en → Current proficiency tests www.syke.fi/download/noname/%7B3FFB2F05-9363-4208-9265-1E2CE936D48C%7D/39886 . 5. Koivikko, R., Nuutinen, J., Tervonen, K., Lanteri, S., Kutramoinen, H., Väisänen, R. and

Ilmakunnas, M. 2017. Laboratorioiden välinen pätevyyskoe 11/2016. Haihtuvat orgaaniset yhdisteet vedestä ja maasta. Suomen ympäristökeskuksen raportteja 5/2017.

http://hdl.handle.net/10138/177657.

6. Näykki, T., Virtanen, A. and Leito, I., 2012. Software support for the Nordtest method of measurement uncertainty evaluation. Accred. Qual. Assur. 17: 603-612. MUkit website:

www.syke.fi/envical.

7. Magnusson B., Näykki T., Hovind H., Krysell M., Sahlin E., 2017. Handbook for Calculation of Measurement Uncertainty in Environmental Laboratories. Nordtest Report TR 537 (ed. 4).

(http://www.nordtest.info)

In document Interlaboratory Proficiency Test 15/2020. VOC compounds in water and soil (sivua 18-22)