Microhomogeneity of candidate reference materials: Comparison of solid sampling Zeeman-AAS with INAA

Homogeneity of certified reference materials (CRMs) is a crucial property especially when minimum sample mass of a certain material used for analysis is considered. Particularly in microanalytical techniques CRMs are frequently used to calibrate quantitative analysis, therefore reliable information on the confidence interval of reference values with regard to milligram or sub-milligram sample amounts is urgently needed. Relative element specific homogeneity factors H-e were determined in order to characterize a single cell algae candidate reference material issued by the IAEA, Vienna, for a number of elements. Solid sampling Zeeman AAS and micro-INAA were applied to determine H-e values in two batches of monocellular algae with considerably different concentrations of trace elements. It could be shown that the extremely low and narrow particle size distribution of the materials offers the opportunity to use such natural matrix materials for quantitative analysis with microanalytical techniques, such as PIXE, laser ablation MS, or solid sampling AAS, where milligram amounts or less of the material are analyzed. Such homogeneity factors in selected reference materials could be used to attach a precisely determined mass related confidence interval to the certified concentrations, reflecting the properties of the material. If a mass related homogeneity factor reflecting the material's true heterogeneity could be applied, systematic bias of analytical procedures together with intermethod bias can be traced.