Abstract
Reference material certificates prepared in accordance with ISO 17034:2016 must include a statement of metrological traceability for each certified value. Users of these materials are required by accreditation bodies and other organisations to request the provision of such statements and to ensure that the stated traceability is fit for the intended use of the material. However, ISO 17034:2016 says nothing about the format to be adopted for such statements. Several possibilities are described in guidance documents published by the ISO Committee for Reference Materials (ISO/REMCO) but these sometimes lack clarity and consistency. One reason for this is the absence of a clear consensus on the appropriate content of such statements. The UK Reference Materials Working Group has addressed this situation with an extensive discussion of the issues. This paper summarises the Group’s conclusions on the purpose, drafting and interpretation of traceability statements for certified values of reference materials, including matrix materials, pure calibrants and calibration solutions. It will be of interest to producers and users of reference materials and also accreditation bodies which accredit facilities to the conformity assessment standards ISO 17034:2016 and ISO/IEC 17025:2017. The document includes a brief overview of four aspects of traceability which impact the preparation of traceability statements: the basis of metrological traceability; the reason for adopting traceable measurements; the type of information on traceable values required by users of reference materials; and how to present information on traceability in the form of a traceability statement.
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Notes
Calibration is defined in the VIM [9] as an operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.
Metrological traceability is defined in the VIM [9] as a property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.
The requirement for traceable calibration data extends to all quantities which significantly contribute to the uncertainty of the certified value, e.g. assay of lead in blood by atomic absorption spectrometry may require traceable data for the balance calibration and the volumetric glassware as well as the lead solution used for calibration of the spectrometer.
Where characterisation depends on values from multiple laboratories, it is important to provide additional information about who was responsible for selecting and for utilising their data as well as the statistical or other methodology used in applying the data to the certification. Further information to be provided about the participating laboratories may include, for example, their accreditation status, performance with QC samples provided by the producer, in PT schemes or previous certification trials, etc.
Where additional information is available from the CRM producer, an appropriate reference to its availability may be included on the certificate.
The term primary standard (or primary measurement standard) is described in the VIM (see bibliography) as a measurement standard established using a primary reference measurement procedure or created as an artefact, chosen by convention. A widely accepted meaning with respect to chemical reference materials is a calibration standard which has been certified for the measurand without recourse to calibration using another standard certified for that measurand. In the example given for lead, the primary standard of pure lead could, for example, be assessed for purity by measuring and correcting for the amount of each impurity present in the sample or by assaying the sample for the amount of lead using an EDTA titration. In many cases such primary standards will have been prepared and certified by the CRM producer (e.g. a national measurement institute) but the example is not limited to this situation.
The term higher-order CRM is used to describe a material for which the uncertainty of the certified value of the measurand is smaller than the uncertainty target for the measurand in the material being certified. Higher-order CRMs include, for example, elemental solutions purchased from a commercial supplier for calibration of ICP-OES instruments or matrix CRMs such as the metal alloys widely used for calibration of XRF instruments. In each case, these calibration materials will themselves need to comply with the requirement for metrologically traceable values. The use of solid matrix calibration materials for techniques such as XRF requires particular care to ensure that the composition and preparation of the material closely matches that of the sample. Detailed information on the certification of these materials should be provided to allow users to make a reliable assessment of their suitability.
References
ISO 17034 (2016) General requirements for the competence of reference material producers. ISO, Geneva
ISO Guide 30 (2015) Reference materials—selected terms and definitions. ISO, Geneva
ISO Guide 31 (2015) Reference materials—contents of certificates, labels and accompanying documentation. ISO, Geneva
ISO Guide 33 (2015) Reference materials—good practice in using reference materials. ISO, Geneva
ISO Guide 35 (2017) Reference materials—guidance for characterization and assessment of homogeneity and stability. ISO, Geneva
ISO/TR 16476 (2016) Reference materials—establishing and expressing metrological traceability of quantity values assigned to reference materials. ISO, Geneva
ISO Guide 80 (2014) Guidance for the in-house preparation of quality control materials (QCMs). ISO, Geneva
ISO/IEC 17025 (2017) General requirements for the competence of testing and calibration laboratories. ISO, Geneva
JCGM 200 (2012) International vocabulary of metrology—basic and general concepts and associated terms (known as the VIM), 3rd edn. BIPM, Sevres
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Eurachem (2019) Traceability in chemical measurement, 2nd edn. https://www.eurachem.org/index.php/publications/guides/trc
Koeber R, Linsinger TPJ, Emons H (2011) An approach for more precise statements of metrological traceability on reference material certificates. Accred Qual Assur 15:255–262
Acknowledgements
This paper was prepared with the collaboration and approval of the UK Reference Materials Working Group. The author is grateful to the UKRMWG members and in particular to those who provided the examples of traceability statements: Kevin Cleaver, John Hammond, Gill Holcombe, Peter Jenks and Richard Meeres.
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Sargent, M. The provision and use of traceability statements for reference materials. Accred Qual Assur 25, 367–372 (2020). https://doi.org/10.1007/s00769-020-01450-8
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DOI: https://doi.org/10.1007/s00769-020-01450-8