Reference Material Homogeneity and Stability

A reference material is only useful if the unit a customer opens represents the whole lot. Homogeneity studies test whether property values differ between units (between-unit) and, where relevant, within a unit at the stated minimum sample size. Stability studies test whether values change over time: short-term studies cover transport conditions, and long-term studies cover storage until expiry and support post-certification monitoring.

For certified reference materials, both studies feed the certified value's uncertainty alongside characterisation. ISO 33405:2024, which replaced ISO Guide 35, describes approaches to designing and evaluating these studies; ISO 17034:2016 requires producers to assess them. The statistical evaluation and the uncertainty contributions are the producer's expert work under its accredited procedures.

The studies depend on production records. Units are numbered in fill order so samples can be drawn across the run; the sampling plan, the units selected, storage conditions and the measurement data must all trace back to the lot. Isochronous stability designs, where units are moved from reference conditions at planned times and measured together, depend on knowing exactly where each unit was stored and when.

A worked example: a lot of 2,000 ampoules is filled and numbered; 15 units spread across the fill are drawn for homogeneity; the stability plan reserves units at reference and elevated temperatures; post-certification monitoring later shows drift, so the producer reviews the data, decides whether to shorten the expiry and notifies customers who hold affected units.

Related concepts: certified reference material, stability programme and cannabinoid homogeneity (a product-testing use of the same word). Sources: ISO 33405:2024, ISO 17034:2016.