Plutonium and americium determination in mosses by Laser Ablation ICP-MS combined with Isotopes Dilution Technique

Environmental contamination by artificial radionuclides and the evaluation of their sources requires precise isotopic analysis and accurate determination of transuranium elements, above all plutonium and americium. This can be achieved by alpha spectrometry or by inductively coupled plasma mass spectrometry (ICP-MS) after chemical separation of actinides. In the present work, laser ablation (LA) ICP-MS was combined with isotope dilution for the ultratrace level determination of 239Pu, 240Pu and 241Am on the surface of targets after electroplating. 242Pu and 243Am were used as isotope spikes. An overall detection efficiency of better than 10-3 counts per atom was achieved for 238U in LA-ICP-MS. The limits of quantification (LOQ, 10σ) for 232Th, U, 237Np and Pu isotopes were comparable with or better than the detection limits of alpha spectrometry for long-lived radionuclides. Thus, the estimated detection limit for Pu was as low as 3.6fgg-1 in soil after chemical separation. Comparison of LA-ICP-SFMS with alpha spectrometry yielded good agreement of Pu concentrations in moss samples at the level of 5×10-14 to 2.5×10-13gg-1, but poorer coincidence was observed for 241Am because of very low concentrations (about 2×10-14gg-1). The 240Pu/239Pu isotope ratio was almost constant within experimental errors for all samples analyzed with a weighted average value of 0.212±0.003. The obtained results (global fallout after nuclear weapon tests) confirm once again that mosses are particularly suited for use as bioindicators of environmental contamination.