4. Conclusions and outlook

MRs and MRCs play a major role in the quality assurance and quality control systems of testing laboratories. Their use is recommended in accreditation standards [16] . However, there are a number of limitations, which make it impossible to ensure the availability of RMs and CRMs for all types of measurement. The major limiting factors are :

• the stability of the matrix or substance to be determined ;

• material homogeneity ;

• representativeness based on the variety of samples analyzed by laboratories, in terms of matrix composition, substance concentration levels, etc.

As we have seen, stability is the most critical factor for a CRM. Stabilization and homogeneity conditions may...