Producers of reference materials guarantee the integrity of the matenal and the certificate validity for a specified time, also known as the shelf life. This expiry date is valid for the unopened material, which is stored under the recommended storage conditions. This does not automatically mean that the user has to discard the unused sample once the she life has expired, but the producer cannot guarantee stability any longer. Users can continue to use the material under their own responsibility, if they have additIonal evidence of stability (e.g. no changes in quality control charts, comparison with other materials), however the material certIficate wIll not be valid.