Evaluation of uncertainty of reference materials

Certification of reference materials is far more than just characterisation of a selected homogeneous batch of material. From the perspective of the I SO Guide on the Expression of Uncertainty in Measurement (GUM) all uncertai nty sources relevant to the user of an individual certified reference mater ial (CRM) sample at a moment in time should be part of the CRM uncertainty. This not only includes the full uncertainty of the batch characterisation (rather than the statistical variation), but also all uncertainties related to possible between-bottle variation, instability upon long-term storage a nd instability during transport to the customer.