MICROHETEROGENEITY OF TRACE-ELEMENTS IN REFERENCE MATERIALS - DETERMINATION AND STATISTICAL EVALUATION

Solid sampling graphite furnace ZAAS is very efficient for homogeneity testing because of the low sample intake with this method. A number of biological reference materials were investigated for heterogeneity with respect to the laboratory sampling error. It was observed that in some cases the investigated material appears to contain a distinct particle fraction of much higher trace element content (''nuggets'') than in the average particles. It could be shown theoretically and experimentally that in the case of nuggets in a material, statistical evaluation using a Poisson probability function was necessary to obtain realistic values for heterogeneity. The measured homogeneity constants for certain elements in each material give an excellent way of calculating the minimum sample mass necessary for reliable element determination. It is proposed to include this testing method in the certification campaign of new CRMs.