Elemental micro patterns in tree rings - a feasibility study using scanning proton microprobe analysis

The feasibility of applying particle-induced X-ray emission spectroscopy (PIXE), using a scanning proton microprobe (SPM), to trace-element analysis in wood and bark samples is demonstrated. Detection limits of the order of 1-10 ppm are reached for several elements. Elemental X-ray images of the analyzed sections show the patterns of several elements; e.g. Cl, K, Ca, Mn, Fe, Cu and Zn. Some of these patterns can be related to the annual tree-ring structure. It is observed that the variation in elements that have environmental influence can be rather large within a single tree-ring, thus illustrating the possible difficulties of using tree-ring sections as a pollution monitor. The variations in elemental concentrations when crossing from bark to wood are also shown to be smooth for some elements but rather abrupt for others.