Homogeneity of Components in CRMs

To discuss the development of CRMs for the emerging use of microanalytical techniques, one has to be concerned chiefly with the degree of homogeneity of the components in the material at the designated sample size. Basic indications for the homogeneity properties of a CRM for microanalytical methods and the assessment of these properties can be derived from the general requirements  

The degree of homogeneity of a property in a given material can be determined by repetitive measurements of the property in a number of small units by a method of sufficiently high precision. 

To assess homogeneity, the distribution of chemical constituents in a matrix is at the core of the investigation. This distribution can range from a random temporal and spatial occurrence at atomic or molecular levels over well defined patterns in crystalline structures to clusters of a chemical of microscopic to macroscopic scale. Although many physical and optical methods as well as analytical chemistry methods are used to visualize and quantify such spatial distributions, the determination of chemical homogeneity in a CRM must be treated as part of the uncertainty budget affecting analytical chemistry measurements. 

The statistical nature of the homogeneity problem can be treated theoretically and it is clear that it is a matter of large numbers. Either a large number of individual particles in the investigated unit or a large number of individual analyses will produce consistent mean results, whereas the deviations from the mean (fluctuation of individual results) are an indication of the element distribution in the matrix (homogeneity). 

A homogeneity index or significance coefficienf has been proposed to describe area or spatial homogeneity characteristics of solids based on data evaluation using chemometrical tools, such as analysis of variance, regression models, statistics of stochastic processes (time series analysis) and multivariate data analysis (Singer and 

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