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ISO 33405:2024 ( formerly ISO Guide 35) – Reference materials -- Approaches for characterization and assessment of homogeneity and stability

The production of reference materials (RMs) is a key activity for the improvement and maintenance of a worldwide coherent measurement system. 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. General requirements for all types of RM production are set out in ISO 17034. These general requirements include the assessment of the homogeneity and stability of the material, the characterisation of the material using a metrologically valid approach and the value assignment for the certified values with their associated uncertainties. Technical detail is, however, left entirely to other guides, of which ISO Guide 35 is the most general and comprehensive. 

With respect to the assessment of homogeneity and stability, the new 2024 1st  edition of the ISO 33405 standard include guidance on the need for experimental studies when substantial prior experience is available; a larger range of homogeneity study designs; improved guidance on the use of accelerated stability studies; additional, and more flexible, guidance on within-unit homogeneity assessment; greater attention to post-certification monitoring; and new provisions for end-user storage and repeated sampling from individual RM units.

The characterisation-clause elaborates on the different approaches that is provided in a NOTE to ISO 17034. A clear distinction is made between the use of a single method in a single laboratory approach when the method can be validated with a certified reference material (CRM) of the same kind as the candidate reference material compared to when a similar CRM is not available.  For the approach where more than one method is used in one or more laboratories the concept of the importance of the number of independent data sets is explained in more details.  A lot more technical detail is also provided for the use of an interlaboratory comparison for the certification study.

Link to online paper

Paper citation:

Ellison, S.L.R. & Botha, A., “Principles for the assessment of homogeneity and stability in the new ISO Guide 35:2017” Accred Qual Assur (2018) 23: 47.