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 (234U/238U, 235U/238U and 236U/238U) 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 236U/238U ratios were also corrected for the presence of 235U hydrides and tailing of the 238U+ peak. Results 236U/238U ratios were successfully measured in micrometer‐sized particles from the NBS U050 certified standard material with a 236U/238U 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 234U/238U and 235U/238U ratios, while the measurement errors for single particles ranged from −2.7% to +2.1% for 235U/238U and from −17% to +33% for the 234U/238U ratios. Their relative combined standard uncertainties ranged from 3.3% to 32.8% and from 0.4% to 4.0% for 234U/238U and 235U/238U 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 235U/238U ratios and 12% for 234U/238U ratios, and detections limits of a few ag for minor isotopes.