Dendroanalysis (i.e., the determination of the element contents in tree rings) is widely used in studies of environment and climate change. Uranium concentration in annual growth rings of oak (Quercus Petraea) trees was measured using both Laser Ablation (LA) and solution mode High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS). Direct uranium analysis of solid samples was performed using a New Wave UP213 nano-second ND:YAG laser ablation device coupled to an Element-XR HR-ICP-MS. Three tree samples taken for the present study were located upstream and downstream of a uranium mine site at Rophin (Puy de Dôme, France), with a mining operation period between 1949 and 1958. About thirty tons of uranium were extracted and thirty thousand tons of wastes were stored afterwards. Five dated growth rings years 1900, 1940, 1952, 1958 and 1990 chosen for the study cover the mining history of the site. The aim of the study is to assess the use of oak tree rings as biomonitors of past and recent uranium contamination from mining areas in order to contribute to a better understanding of the transfer of uranium from mining activities and thus its impact on the environment. In parallel, performances of both techniques (laser ablation vs solution mode) for uranium determination in tree ring samples will be evaluate in order to identify the best analytical tool for the purpose of the study.
Using the LA-HR-ICP-MS technique, two-dimensional (2D) mapping of uranium has been developed to study its spatial distribution at the surface of tree rings. Pressed pellets of the standard reference material NIST1570a spinach leaves and doped cellulose powder at concentrations ranging from0 to 400 ng.g−1 were used as uranium standards for quantification. The UP213 laser was operated in pre-ablation mode before analysis to overcome sample contamination during handling. 2D maps of uranium concentrations on one mm2 surface of tree rings were carried out in two hours with the following laser ablation parameters: ablation line scan, 5 μm.min−1 scan speed, 10Hz laser frequency, 80μm spot diameter and 50% output energy. Time resolved uranium signals were normalized to 13C internal standard signals. For HR ICP-MS solution analysis, tree rings were cut and acid digested by oven with the mixture 15.5 M concentred nitric acid/30% hydrogen peroxide. Uranium data show the discrepancy of both analytical techniques explained by the sample heterogeneity. However, higher concentrations of uranium are observed for downstream trees in growth rings covering the mine operation period.
Keywords: HR-ICP-MS, laser ablation, oak tree rings, uranium, 2D elemental mapping, mining activities.