Handling and Use Errors

As mentioned before, RM producers go to great lengths, as are required by the ISO Guide 31 (1996) to prove homogeneity and stability, and to establish the best sample size and storage conditions for an optimal shelf life. This information is normally provided in the RM certificate. Nevertheless, users must pay particular attention to a number of procedures in the use of RMs to avoid invalid results. 

The stability of individual analytes within a matrix material is often quite variable. A good example is shown by NIST SRM 968b fat soluble vitamins and cholesterol in serum. The material must be shipped and stored at -8o°C. The SRM is certified for a range of vitamins, most of which are quite stable at -20°C, or even +4°C, but the beta-carotene and other components are not. It is therefore essential to ensure the material, if the carotene components are of interest, is shipped and stored correctly. 

Manufacturers of CRMs go to considerable lengths to ensure that the CRMs are stable, therefore attention must be paid to specified storage conditions and shelf life. 

It must not be forgotten that in most cases the shelf life stated by the producer refers to an unopened unit, and that once opened shelf life is often not guaranteed. This applies especially to CRMs packed under a protective atmosphere and stored at reduced temperatures see Sections 2.1, 3.1, and 7.1.a.5 for further consideration of these issues. 

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