Pu-239,Pu-240,Pu-241 fingerprinting of plutonium in western US soils using ICPMS: solution and laser ablation measurements

Sector field inductively coupled plasma mass spectrometry (SF-ICPMS) has been used with analysis of solution samples and laser ablation (LA) of electrodeposited alpha sources to characterize plutonium activities and atom ratios prevalent in the western USA. A large set of surface soils and attic dusts were previously collected from many locations in the states of Nevada, Utah, Arizona, and Colorado; specific samples were analyzed herein to characterize the relative contributions of stratospheric fallout vs. Nevada Test Site (NTS) plutonium. This study illustrates two different ICPMS-based analytical strategies that are successful in fingerprinting Pu in environmental soils and dusts. Two specific datasets have been generated: (1) soils are leached with HNO3-HCl, converted into electrodeposited alpha sources, counted by alpha spectrometry, then re-analyzed using laser ablation SF-ICPMS; (2) samples are completely dissolved by treatment with HNO3-HF-H3BO3, Pu fractions are prepared by extraction chromatography, and analyzed by SF-ICPMS. Optimal laser ablation and ICPMS conditions were determined for the re-analysis of archived alpha spectrometry planchette sources. The best ablation results were obtained using a large spot size (200 mu m), a defocused beam, full repetition rate (20 Hz) and scan rate (200 mu m s(-1)); LA-ICPMS data were collected with a rapid electrostatic sector scanning experiment. Less than 10% of the electroplated surface area is consumed in the LA-ICPMS analysis, which would allow for multiple re-analyses. Excellent agreement was found between Pu239+240 activities determined by LA-ICPMS vs. activity results obtained by alpha spectrometry for the same samples ten years earlier. LA-ICPMS atom ratios for Pu-240/Pu-239 and Pu-241/Pu-239 range from 0.038-0.132 and 0.00034-0.00168, respectively, and plot along a two-component mixing line (Pu-241/Pu-239=0.013 [Pu-240/Pu-239] - 0.0001; r(2) = 0.971) with NTS and global fallout end-members. A rapid total dissolution procedure, followed by extraction chromatography and SF-ICPMS solution Pu analysis, generates excellent agreement with certified Pu239+240 activities for standard reference materials NIST 4350b, NIST 4353, NIST 4357, and IAEA 385. Pu239+240 activities and atom ratios determined by total dissolution reveal isotopic information in agreement with the LA-ICPMS dataset regarding the ubiquitous mixing of NTS and stratospheric fallout Pu sources in the regional environment. For several specific samples, the total dissolution method reveals that Pu is incompletely recovered by simpler HNO3-HCl leaching procedures, since some of the Pu originating from the NTS is contained in refractory siliceous particles.