Promotion of metrological traceability through the provision of a metrologically traceable proficiency testing reference value for the mass fraction of benzoic acid in fish sauce

Kim S.H., Cho H., Heo S.W., Hwang E.

The Asia Pacific Metrology Program (APMP) and the Asia Pacific Laboratory Accreditation Cooperation (APLAC) joint Proficiency Testing (PT) programs for toxic elements such as cadmium (Cd) and lead (Pb) or nutritional elements such as iron (Fe) and zinc (Zn) in food were organized by the Korea Research Institute of Standards and Science (KRISS) with the aim of enhancing the quality of measurement and metrological traceability in various economies of the Asia Pacific region by evaluating the performance with rigorous evaluation. Three APMP-APLAC joint PT programs for elemental analyses were carried out by KRISS sequentially, where candidate certified reference materials (CRMs) were used as the PT materials and metrologically traceable certified reference values (RVs) were used as the PT assigned values for the evaluation of participants' results, which allows reliable evaluation of participant performance. This article describes the operation of the PTs and the overall performance of the participating laboratories. The effectiveness of these joint PT programs and trends in PT performance assessment are also discussed. These PT programs confirm the significant importance of using the metrologically traceable RVs instead of the consensus values from participants as the PT assigned value for reliable assessment. The lack of understanding of the concept of coverage factor, degree of freedom, standard uncertainty, and expanded uncertainty was revealed by some participants in these PT programs. Interpreting the zeta-scores or En scores, which are derived by using measurement uncertainties, in conjunction with the z-scores is highly meaningful for assessing participants' ability in measurement capabilities and measurement uncertainty evaluation. Assessment of participants' performance considering measurement uncertainties helps the participants to check how reasonable their measurement uncertainty estimation was. The results of PTs also demonstrated that these PT programs are useful for improving the measurement capability of the laboratories, whereas more capability-building in uncertainty evaluation is required for further improvement.

Wong Y., Fung A.W., Yip Y., Sin D.W.

This paper reviews some proficiency testing (PT) programmes operated under the auspices of Asia Pacific Metrology Programme and Asia Pacific Laboratory Accreditation Cooperation Joint Proficiency Testing Working Group. The assigned values of these joint PTs are traceable to the SI. Provision of PTs based on reference values (PTRVs) with metrological traceability enables meaningful performance evaluation and assessment of metrological comparability of the participants’ results. Special focus will be given to those joint PTs related to analysis of inorganic elements in various sample matrices organized or co-organized by Government Laboratory of Hong Kong, China. Comparison based on different evaluation criteria (i.e. consensus value vs reference value) is carried out. Comprehensive evaluation and review of some performance indicators revealed several crucial findings such as the performance of different analytical conditions, recent analytical trends and areas of improvements. The organization of a series of these joint PTs could be regarded as a strong support of the concept that PTRV with metrological traceable reference values is an effective way to promote the concept of metrological traceability to the field laboratories and provide a meaningful performance evaluation tool for assessment of PT performance.

Cheow P.S., Teo T.L., Fortune T.J., Ebarvia B.S., Hashim S.N., Styarini D., Wang J., Gui E.M., Lu T., Mungmeechai T., Tangtrirat P., Dacuya A., Veranga A.R., Amandy G., Li H.L., et. al.

This paper describes a collaborative effort by five metrology institutes to organise a proficiency testing (PT) programme which aims to evaluate the performance and improve the measurement capabilities of food testing laboratories on additives in food sauce in the Association of Southeast Asian Nations (ASEAN). It was the first joint PT programme organised by the metrology institutes under the auspice of the ASEAN Reference Material Network (ARMN). Five common additives, namely: benzoic acid, sorbic acid, methyl paraben, n-propyl paraben and saccharin in tomato sauce, were chosen as the measurands of the PT programme. Thirty-nine laboratories from five economies participated in this PT programme through the ARMN network. Metrologically traceable assigned values used for performance evaluation of the additives were jointly determined by three metrology institutes using isotope dilution mass spectrometry (IDMS). Issues discussed in the paper include determination of assigned values, performance evaluation, participating laboratories’ analytical methods and evaluation of measurement uncertainties. The overall performance of participating laboratories was considered to be satisfactory as 88 % of the participating laboratories achieved satisfactory z-scores (or z’-scores). The joint ARMN accuracy-based PT programme enables a broader understanding of the comparability of measurement capabilities of the food testing laboratories, which play an important role in supporting trade-related industries among the close trading economies in the Southeast Asian region.

Cheow P.S., Teo T.L., Lu T., Gui E.M., Lee T.K.

This paper describes a metrological approach to evaluate the measurement capability of laboratories participating in a then Asia Pacific Laboratory Accreditation Cooperation [APLAC, now Asia Pacific Accreditation Cooperation] proficiency testing (PT) programme on mass fraction of acesulfame potassium and sucralose in cake mix flour. The programme was jointly organised by the Chemical Metrology Laboratory of the Health Sciences Authority, Singapore, and the Singapore Accreditation Council. Through the extensive APLAC network of accreditation bodies, 41 nominated laboratories from 32 economies registered, of which 40 laboratories submitted their results. Reference values of artificial sweeteners in the programme for performance evaluation were obtained from a high-accuracy liquid chromatography isotope dilution mass spectrometry (LC-IDMS) method. One of the unique features of this PT programme was the provision of pure substance certified reference materials (CRMs) of acesulfame potassium and sucralose to the participating laboratories as calibration standards to ensure comparability and metrological traceability of results. The participating laboratories’ performance, based on z-scores, were evaluated using the assigned reference value determined from IDMS. Five and one of the laboratories’ results would be incorrectly evaluated using consensus value calculated from the laboratories’ results for performance evaluation and using commercial standard as calibrant, respectively. This PT programme demonstrates the importance of the use of metrologically traceable reference values as the assigned values for appropriate evaluation of accuracy and reliable assessment. The provision of CRMs also heightened the testing laboratories’ awareness on the importance of using CRMs as calibrants for accurate measurements.

Kaarls R., Mackay L., Samuel A., Sin D.W., Mok C., Wong Y., Yip Y.

There is an increasing need for laboratory capacity building through the organisation of proficiency testing programmes with metrologically traceable reference values in the Asia-Pacific region. Stakeholders such as the regional metrology organisation, metrology institutes, and accreditation bodies are increasingly aware that consensus values of proficiency testing programmes could sometimes be significantly biased if the participating laboratories do not have clear understanding of their measurement capabilities and measurement uncertainties. In this article, we provide a review of the effectiveness of the use of reference values obtained from international comparisons among metrology institutes for performance evaluation in associated proficiency testing programmes. Important issues are discussed including determination of assigned values, the main methods employed by national metrology institutes/designated institutes and field analytical laboratories for analysis, estimation of measurement uncertainty, and guidance on analytical challenges presented by these studies. Application of measures such as appropriate matrix reference materials, particularly from incurred sources, for assessing extraction efficiencies and other measurement procedural issues, and proper knowledge of the uncertainty budgets are essential tools leading to effective and reliable measurements.

Teo T.L., Gui E.M., Lu T., Cheow P.S., Giannikopoulou P., Kakoulides E., Lampi E., Choi S., Yip Y., Chan P., Hui S., Wollinger W., Carvalho L.J., Garrido B.C., Rego E.C., et. al.

The supplementary comparison APMP.QM-S8: determination of mass fraction of benzoic acid, methyl paraben and n-butyl paraben in soy sauce was coordinated by the Health Sciences Authority, Singapore under the auspices of the Organic Analysis Working Group (OAWG) of the Comité Consultatif pour la Quantité de Matière (CCQM). Ten national metrology institutes (NMIs) or designated institutes (DIs) participated in the comparison. All the institutes participated in the comparison for benzoic acid, while six NMIs/DIs participated in the comparison for methyl paraben and n-butyl paraben. The comparison was designed to enable participating institutes to demonstrate their measurement capabilities in the determination of common preservatives in soy sauce, using procedure(s) that required simple sample preparation and selective detection in the mass fraction range of 50 to 1000 mg/kg. The demonstrated capabilities can be extended to include other polar food preservatives (e.g. sorbic acid, propionic acid and other alkyl benzoates) in water, aqueous-based beverages (e.g. fruit juices, tea extracts, sodas, sports drinks, etc) and aqueous-based condiments (e.g. vinegar, fish sauce, etc). Liquid--liquid extraction and/or dilution were applied, followed by instrumental analyses using LC-MS/MS, LC-MS, GC-MS (with or without derivatisation) or HPLC-DAD. Isotope dilution mass spectrometry was used for quantification, except in the case of a participating institute, where external calibration method was used for quantification of all three measurands. The assigned Supplementary Comparison Reference Values (SCRVs) were the medians of ten results for benzoic acid, six results for methyl paraben and six results for n-butyl paraben. Benzoic acid was assigned a SCRV of 154.55 mg/kg with a combined standard uncertainty of 0.94 mg/kg, methyl paraben was assigned a SCRV of 100.95 mg/kg with a combined standard uncertainty of 0.40 mg/kg, and n-butyl paraben was assigned a SCRV of 99.05 mg/kg with a combined standard uncertainty of 1.36 mg/kg. The k-factors for the estimation of the expanded uncertainties of the SCRVs were 2.26, 2.57 and 2.57, respectively. The degree of equivalence (with the SCRV) and its uncertainty were calculated for each result. All the participating institutes (except in one case for benzoic acid) were able to demonstrate or confirm their capabilities in the determination of polar food preservatives in water or aqueous-based beverages/condiments. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Kim S.H., Heo S.W., Cho H., Yim Y., Lim Y., Lee K., Hwang E.

The Asia Pacific Metrology Program (APMP) and Asia Pacific Laboratory Accreditation Cooperation (APLAC) joint proficiency testing (PT) program was established to improve the ability of quantitative analysis and enhance the quality of the PT program through a better regional linkage between the national metrology institutes (NMIs) and the accreditation bodies (ABs) in the Asia-Pacific region. The first APMP-APLAC joint PT was carried out to quantify hazardous elements Cd and Pb in kimchi cabbage powder, where a candidate certified reference material (CRM) was used as the PT material. A total of 83 laboratories from 37 countries registered, of which 73 laboratories submitted their results. Certified reference values (RVs) provided by Korea Research Institute of Standards and Science (KRISS) were used to evaluate the z -scores ( z ) of participants' results. This program confirms significant importance of the use of metrologically traceable RVs as PT assigned values for rigorous evaluation and reliable assessment.

Yarita T., Otake T., Aoyagi Y., Numata M., Takatsu A.

Proficiency testing was organized by the National Metrology Institute of Japan (NMIJ) as a measure of analytical competency in the quantification of pesticide residues in husked wheat powder. Seventy-one participants submitted analytical concentrations of the target pesticides (diazinon, fenitrothion, malathion, and etofenprox) along with details of the analytical method employed. Two types of assigned values were obtained for each target pesticide, i.e., the participants' analytical results and the results obtained by NMIJ based on isotope-dilution mass spectrometry (IDMS). The former values were lower than the latter due to the incomplete recovery yield of the target pesticides during the pretreatment process. The discrepancy between the two assigned values was particularly pronounced for malathion because of the longer duration of water-soaking used for the test samples. Two corresponding types of z-scores were then calculated to evaluate the analytical performance of the participants, where the z1-score indicates the performance of a participant relative to all participants, and the z2-score indicates the relative deviation of the analytical results of the participant from the IDMS value.

Sin D.W., Wong Y.

This paper compares reference values assigned by isotope dilution mass spectrometry with participants’ consensus values in a small-scale proficiency test (PT) on the determination of residues of organochlorine pesticides (α-endosulfan, β-endosulfan and endosulfan sulfate) in a tea sample. The PT was specifically organized to benchmark the technical capability among the local participating laboratories in Hong Kong providing pesticide testing service to their customers and to identify possible areas for further improvement. Participant’s performance, based on z-score, was evaluated using consensus values and standard deviation that was estimated from the Horwitz function. Two out of the seven participating laboratories were identified to produce either questionable or unsatisfactory results (in terms of z-scores), showing that the performance of the majority was satisfactory. However, the number of questionable or unsatisfactory results increased to three if the consensus values were replaced by assigned reference values derived from gas spectrometry-isotope dilution mass spectrometry with higher metrological traceability than those used in obtaining the consensus values. Though the said PT program was fit for its intended purposes at that point of time, revisiting the program explicitly demonstrates the significant importance of assigned reference values in achieving reliable assessment in reality. Unless the bias of consensus values is known with metrological traceability, PT scheme providers should be cautious in assessing the competence of participating laboratories using consensus values particularly in small-scale inter-comparison programs. The importance of pre-wetting dry tea samples to improve recovery of pesticide residues is also noted.

Cheng S., Mesquida C., Gui E.M., Cheow P.S., Lee T.K., Teo T.L.

A proficiency testing (PT) programme on the determination of benzoic acid in orange juice was organised by the Chemical Metrology Laboratory, Health Sciences Authority, Singapore, for the local food testing laboratories. The PT programme used a metrologically traceable assigned value determined by isotope dilution mass spectrometry to assess the performance of the participating laboratories. This paper discusses the reasons and approach to applying metrological principles to determine the assigned value of a PT programme. The procedure undertaken by the Laboratory to prepare a batch of PT sample, conduct homogeneity and stability testings, as well as value assignment, is presented. The discussions also include the estimation of the measurement uncertainties arising from the characterisation, testing for homogeneity and testing for stability of the PT sample. The Coordinator of the PT programme used the assigned value and its associated measurement uncertainty, together with those of the participating laboratories to assess their performances. Statistical tools applied in the performance evaluations are also presented.

Araujo P., Frøyland L.

An approach to the estimation of possible different sources of variation found in proficiency testing experiments is described. Four errors namely, technique, analyst, laboratory and geographical location are considered and calculated by using a rational experimental design based on hierarchical classification. The treatment of the confidence of the design over different experimental arrangements is explored and visualised by calculating a function that depends only on the design and not on the experimental response. An illustrative example based on simulated data is used to show how the theory could be applied in practice.

Mackay L.G., Taylor C.P., Myors R.B., Hearn R., King B.

The selection of an appropriate isotope dilution mass spectrometry technique for the practical application of this potentially primary method of analysis is highly important. The NARL approach for the application of the exact matching double isotope dilution mass spectrometry technique developed by Henrion is presented. NARL's approach utilises exact matching to minimise the effect of measurement biases within the method but also includes the thorough examination of all other biasing factors. The approach has been successfully tested in international CCQM intercomparisons with other national metrology institutes.

De Bièvre P., Peiser H.S.

A current interest in chemistry concerns traceability of analytical measurements to the International System of Units (SI) and the estimation of their uncertainties in accordance with principles of metrology, that is, measurement science. “Primary methods of measurement” achieve traceability to SI directly without intermediate reference standards or materials and without significant empirical correction factors. Isotope-dilution mass spectrometry should be regarded as such a method. It has the potential of smallest presently achievable uncertainties for analytical measurements directly or for the certification of reference materials including those with abnormal isotopic composition. A simple explanation of the method including its basic equations is given. Full uncertainty estimation is emphasized in terms of these equations. The wider use of concepts of metrology in chemistry is discussed.