The use of synchrotron micro-XRF for characterization of the micro-heterogeneity of heavy metals in low-Z reference materials

Synchrotron μ-XRF (X-ray fluorescence analysis) is a trace-level micro-analytical method that allows one to quantitatively measure the degree of heterogeneity of inorganic trace constituents in a number of reference materials. In the present study, low atomic number materials containing trace levels of heavy metals are concerned. The measurements involve an extensive series of local analyses, performed in identical conditions at different locations on the material. In order to be able to detect low levels of sample heterogeneity, the experimental set-up must permit the collection of spectral data in a highly repeatable manner. In this paper, a procedure for determining the homogeneity of materials on the basis of a series of individual analysis results will be outlined. This includes a discussion on the manner in which a number of the sources of instability that occur during synchrotron μ-XRF experiments can be eliminated. The procedure is employed to measure the degree of micro-heterogeneity of several existing reference materials and to evaluate their suitability for calibration of trace-level micro-analytical methods. After analyzing the micro-heterogeneity of elements in these different reference materials, a minimum sampling mass for homogeneous measurements is calculated.