Assessment of airborne platinum contamination via ICP-mass spectrometric analysis of tree bark

Tree bark is used as a passive bio-monitor to assess the environmental impact by the use of automotive catalysts containing platinum-group elements. Pt in bark samples collected from UK, Spain, Japan and USA was determined by ICP-MS both conventionally after acid digestion and directly utilising laser ablation. An ultrasonic nebuliser equipped with a membrane desolvator was employed for digest analysis, to improve the sensitivity (limit of detection, 0.8 ng l^-1) and to eliminate the potential spectral interference caused by hafnium oxide. Pt concentrations found in the remote sites were below 9 ng g^-1. Similar concentrations were obtained for Spanish/UK cities with occasionally slightly elevated values. In contrast, the concentrations for bark samples in major Japanese/US cities ranged up to 38 ng g^-1. The data are consistent with the usage of Pt-containing automotive catalysts in the respective sites. Spatial micro-analytical data, acquired on raw bark materials, indicate that Pt occurred in a highly dispersed particulate form and was not readily transformed/solubilised by actions of weathering. Association of Pt with Pd and/or Rh (no correlation with Pb was observed) suggests that the particles detected originated from automotive catalysts.