Sampling adequacy and measurement significance in ion beam analysis

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Abstract

The many methods of ion beam analysis are all characterized by the sampling or interrogation of only a small volume of the material. Some materials may well be extremely homogeneous and spot-sampling measurements will be meaningful but this is not always true even for man-made materials and certainly only exceptionally valid for natural materials. This presents the problem to the ion beam analyst of how to sample so that his results bear a meaningful relationship to the actual problem being addressed. In this work this problem is statistically quantified in terms of a Poisson model and discussed in relation to ion beam analysis. The treatment shows that the confidence interval for an analysis of a sample, where the element of interest is concentrated in a small number of mineral grains, is large and asymmetric. As a result estimates based on such measurements will be biased to low values. Examples of the application of the method to the determination of gold and of platinum in geological samples are described. It is found that the practical limit of detection can be severely limited by the statistics of sampling and a simple criterion is presented in terms of which this limit of detection can be evaluated.

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    Istituto Nazionale di Fisica Nucleare, Genova, Italy.

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