Comparison of the fluctuations of the signals measured by ICP-MS after laser ablation of powdered geological materials prepared by four methods.

Sample preparation is a crucial point for quantitative multi-elemental analses by LA-ICP-MS of powdered geological materials. Four different methods are compared in this study with respect to signal stability and intensity as follows: the preparation of glass beads (GlassB) by alkaline fusion method and three grinding and pelletizing methods relying on the use of an organic binder (VanBind, vanillic acid), an adhesive binder (MixGlue, methyl methacrylate) and a sol-gel process for glass formation (SolGel, chemical reaction of tetraethoxysilane), respectively. Sixty elements were analyzed by means of a ns-UV (213 nm) laser ablation system coupled to a single collector sector field ICP-MS with a low or medium mass resolution. Signal stability was found to strongly depend on the sample homogeneity provided by the preparation method. These methods were applied to three geological standard materials (CRM). The following criteria were used to evaluate and compare the methods: (1) proportion of the measurement cycles which are above a given signal intensity threshold (defined here as signal average ± 3 times the standard deviation), (2) signal stability of the analyzed nuclides (internal precision estimated by the relative standard deviations on raw count rates), (3) signal stability of the internal standards added to the samples, (4) external precision estimated by the relative standard deviation over five preparations for each geological CRM. For the majority of the analyzed elements, signals measured for samples prepared with the four methods are reproducible. Specific contamination in one or several elements (Cr, Fe, Co, Ni, Cu, Mo, W, Au and Bi) was observed depending on the sample preparation method. In addition, compared to grinding made with PTFE material, grinding performed with tungsten carbide material was found to produce better homogeneity, especially for the sol-gel and mixing with glue protocols, although some metallic contamination (W and Co) was observed. Thanks to the suppression of grain effects by alkaline melting, the glass bead method systematically provided signal stability and percentage of "over the threshold" close to those of the NIST glasses. This may be explained by the preparation of more homogeneous samples by alkaline melting. Finally, the described methods were found to be reproducible for the majority of the analyzed elements.

Temporal evolution of plutonium concentrations and isotopic ratios in the Ukedo - Takase Rivers draining the Difficult-To-Return zone in Fukushima, Japan (2013-2020).

In 2011, the Fukushima Dai-Ichi Nuclear Power Plant (FDNPP) accident released significant quantities of radionuclides into the environment. Japanese authorities decided to progressively reopen the Difficult-To-Return Zone after the decontamination of priority reconstruction zones. These areas include parts of the initially highly contaminated municipalities located to the north of the FDNPP, including Namie Town, an area drained by the Ukedo and Takase Rivers. Eleven years after the accident, research focused on the spatial distribution of plutonium (Pu) and radiocesium (Cs) isotopes at contrasted individual locations. To complement previous results, the current research was conducted on flood sediment deposits collected at the same locations after major flooding events during eleven fieldwork campaigns organised between 2013 and 2020 at the outlet of the Ukedo and Takase Rivers (n = 22). The results highlighted a global decrease of the Pu and 137Cs contents in sediment with time during the abandonment phase in the region, from 2013 (238.20 fg g-1) to 2020 (4.28 fg g-1). Furthermore, based on the analysis of the 240Pu/239Pu isotopic ratios, the plutonium transiting these rivers (range: 0.166 - 0.220) essentially originated from the global fallout (0.180 ± 0.014 (Kelley et al., 1999)). Sediment showed contrasted properties in the two investigated rivers, which is likely mainly the result of the occurrence of Ogaki Dam on upper sections of the Ukedo River as it strongly impacts the material supply from this river to the Pacific Ocean. A statistical analysis highlighted the strong correlation between Pu activity concentrations and 137Cs activities in both rivers, confirming that both radionuclides are transported with a similar pathway. Despite it was detected early after the accident (2011-2013), the current research demonstrates that plutonium originating from FDNPP is no longer detected in these rivers draining the Difficult-To-Return Zone at the onset of the reopening of the area to its former inhabitants.

Determination of the isotopic composition of single sub-micrometer-sized uranium particles by laser ablation coupled with multi-collector inductively coupled plasma mass spectrometry.

RATIONALE: A multi-collector inductively coupled plasma (MC-ICP) mass spectrometer coupled to a UV ns-laser ablation (LA) system was used to measure uranium isotopic ratios (234 U/238 U, 235 U/238 U and 236 U/238 U) in single uranium particles of various sizes and isotopic compositions, including home-made sub-micrometric natural uranium particles of narrow size distribution (415 ± 60 nm). METHODS: The LA-ICP mass spectrometer was operated in wet plasma conditions thanks to simultaneous injection of the laser aerosol and water vapor through a desolvating nebulizer. The isotopic ratios were corrected for mass bias and gain factors between detectors. The 236 U/238 U ratios were also corrected for the presence of 235 U hydrides and tailing of the 238 U+ peak. RESULTS: 236 U/238 U ratios were successfully measured in micrometer-sized particles from the NBS U050 certified standard material with a 236 U/238 U ratio of ~5 × 10-4 . The analysis of 77 natural uranium sub-µm-sized particles yielded a very good trueness with respect to the expected 234 U/238 U and 235 U/238 U ratios, while the measurement errors for single particles ranged from -2.7% to +2.1% for 235 U/238 U and from -17% to +33% for the 234 U/238 U ratios. Their relative combined standard uncertainties ranged from 3.3% to 32.8% and from 0.4% to 4.0% for 234 U/238 U and 235 U/238 U ratios, respectively. In addition, extremely low detection limits, in the attogram range, were achieved. CONCLUSIONS: This study demonstrates that coupling of a ns-laser ablation system with a MC-ICP mass spectrometer allows measurements of the isotopic composition in natural uranium particles of a few hundreds of nm with very good trueness, average combined standard uncertainties of ~1% for 235 U/238 U ratios and 12% for 234 U/238 U ratios, and detections limits of a few ag for minor isotopes.