Proton micro-PIXE control of standard reference materials for PIXE environmental applications

In quality control processes, certified standard reference materials (SRMs) are of great importance. They are necessary for checking the effectiveness and precision of analytical techniques, as well as for comparing results between different laboratories. Owing to the relatively low sample mass analysed by the particle induced X ray emission (PIXE) method, sample inhomogeneity has to be considered in the PIXE analytical technique. The trace element distribution and elemental composition of SRMs must be constant in all portions of the SRM and as close to the certified values as possible, even in the smallest amount of the sample used. In order to study these effects, several powder SRMs and the candidate SRMs used in environmental and biological applications of the PIXE analytical method have been tested for reliability at the microgram scale. Inhomogeneity in each of these samples has been tested in a series of micro-PIXE measurements, using a focused ion beam from the Zagreb proton micro- probe facility. The concentrations of the trace elements in the SRMs should approach the broad beam PIXE values by increasing the scan size. At the same time, the inhomogeneities observed in areal distributions of trace elements in the SRM sample can be related to the scattering of the elemental concentration around the mean value with the increasing of scan size. That is, the elements that have less homogeneous areal distributions, as observed by the microprobe, are found to have more scattered elemental concentrations than ones that have homogeneous areas distributions. (author)