239, 240, 241Pu 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 HNO 3 -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 HNO 3 -HF-H 3 BO 3 , 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 μm), a defocused beam, full repetition rate (20 Hz) and scan rate (200 μm 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 239+240 Pu activities determined by LA-ICPMS vs. activity results obtained by alpha spectrometry for the same samples ten years earlier. LA-ICPMS atom ratios for 240 Pu/ 239 Pu and 241 Pu/ 239 Pu range from 0.038–0.132 and 0.00034–0.00168, respectively, and plot along a two-component mixing line ( 241 Pu/ 239 Pu = 0.013 [ 240 Pu/ 239 Pu] – 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 239+240 Pu activities for standard reference materials NIST 4350b, NIST 4353, NIST 4357, and IAEA 385. 239+240 Pu 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.