Procedures to certify and assign values

The certification of reference materials follows strict rules which are described in special ISO Guide 35 [73] the certified value should be an accurate estimate of the true value with a reliable estimate of the uncertainty compatible with the end use requirements . Depending on the type of property value to be certified and the type of CRM, there may be differences in the approach applied. The certification of primary calibrants such as pure compounds and calibration solutions (see Section 3.3) relies on the identification, purity and stoichiometry assessment and on gravimetric methods. Matrix CRMs cannot be certified on the basis of direct gravimetric methods since samples have to be analysed after a total transformation or removal of the matrix in this case, there are three possible approaches  

In all cases, only laboratories of the highest and proven quality should be involved. 

The certification of a property value in a material leads to a certified value which is typically the mean of several determinations or the result of a metrologically valid preparation procedure, e.g. weighing. The confidence intervals or uncertainty limits of this mean value should also be determined. These two basic analytical parameters should be included in a certificate of 

Some information which is useful for the user of the CRM cannot be given in a simple certificate. Therefore, some CRM producers, e.g. BCR, make the material available together with a certification report which details the information given in the certificate. In particular, such reports highlight the difficulties encountered in the certification and typical errors which may arise using the material with some analytical procedures. 

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