General guidance for the value assignment of reference materials: a proposed scope for the revision of ISO Guide 35

Abstract

The production of reference materials (RMs) is a key activity for the improvement and maintenance of a worldwide coherent measurement system. As detailed in ISO Guide 33, RMs with different characteristics are used in measurements, such as calibration, quality control and method validation, as well as for the assignment of values to other materials. Certified reference materials (CRMs) are also used to maintain or establish metrological traceability to conventional scales, such as the octane number, hardness scales and pH. Furthermore, selected pure substances are used to maintain the international temperature scale. In order to be comparable across borders and over time, measurements need to be traceable to appropriate and stated references. CRMs play a key role in implementing the concept of traceability of measurement results in chemistry, biology and physics among other sciences dealing with materials and/or samples. Laboratories use these CRMs as readily accessible measurement standards to establish traceability of their measurement results to international standards. The property values carried by a CRM can be made traceable to SI units or other internationally agreed references during production. ISO Guide 35 explains how methods can be developed that will lead to well-established property values, which are made traceable to appropriate stated references. For reference material producers (RMPs), there are three ISO Guides that specifically assist with the set-up of a scheme to produce and certify RMs to ensure that the quality of the RMs meets the requirements of the end users. ISO Guide 34 outlines the requirements to be met by an RMP to demonstrate competence. ISO Guide 35 provides general guidance on property assessment issues and explains the concepts for processes such as the assessment of homogeneity, stability and value assignment for certification of RMs. ISO Guide 31 describes the format and contents of certificates for CRMs and accompanying documents for non-certified RMs, respectively. Currently, ISO Guide 35 is in its third edition after it was revised in 2006. The Guide was developed to support best practice in the value assignment for CRMs. But a user of this type of documentation should always consider that it cannot substitute for ‘critical thinking, intellectual honesty and professional skill’ (section 3.4.8. of the Guide to the Expression of Uncertainty in Measurement (GUM), ISO Guide 98-3: 1993). Obviously, the quality of the ‘product’ RM depends as much on these aspects as on the use of general procedures and methods. Therefore, thorough knowledge of the material and its properties and of the measurement methods used during homogeneity testing, stability testing and value assignment of the material, along with a thorough knowledge of the statistical methods, are needed for correct processing and interpretation of experimental data in a typical certification project. It is the combination of these required skills that make the preparation and certification of RMs so complex. The greatest challenge in these projects is to combine these skills to allow for a smooth implementation of the project plan. Since the release of ISO Guide 35 in 2006, ISO Guide 34 was revised and the preparation of ISO Guide 80 was started. At several occasions, ISO Guide 34 refers (also for A. Botha (&) National Metrology Institute of South Africa, Pretoria, South Africa e-mail: abotha@nmisa.org