Features of a Good CRM

The number of CRMs available on the market is, and probably always will be, smaller than the actual need of the scientific community at large. This does not mean, however, that the analyst in pursuit of quality assurance can take any CRM that is at that moment on a shelf in the laboratory. Five general criteria can be formulated to characterize basic features of a CRM [15, 35] that is fit for a given type of analysis  

Contemporary methods of trace analysis such as NAA, ICP-OES, and ICP-MS make possible simultaneous determination of several elements. Determination of only one or two elements is relatively rarely, requested. Therefore the CRM certified for many trace elements helps to save time and money. 

Some workers have also used K/ to characterize the homogeneity of materials [51, 53]. 

It can be shown [35,41] that for a fictitious case of a two-component mixture in which the component of interest (trace element), A, is present only in grains containing 100 % of this component, the sampling constant is given by  

As can be seen, the chances that a material appears inhomogeneous with respect to a trace component are greater with a lower concentration of this component and greater particle size of the material [35, 50]. Therefore, CRM producers should always state the minimum mass of material that should be taken for analysis. Modem instmmental methods now allow analysis of very small samples however, one should keep in mind that errors due to inhomogeneity can dramatically increase with a decrease in the effective mass being analyzed, and that CRMs intended for microanalysis are scarce or nonexistent [42, 43]. 

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