Examination of the performance exhibited by a single detector double focusing magnetic sector ICP-MS instrument for uranium isotope abundance ratio measurements over almost three orders of magnitude and down to pg g−1 concentration levels

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Abstract
Calibration experiments were performed on a commercially available double focusing magnetic sector single detector ICP-MS instrument to evaluate its potential for accurate isotope abundance ratio measurements down to the 1 pg g−1 isotope concentration level (without involving any preconcentration step). Its performance, for isotope abundance ratios varying over almost three orders of magnitude from ≈1∶1 to ≈1∶500, was checked using the IRMM-072 series of Isotopic Reference Materials. Routine experimental settings were applied and remained unchanged for the entire series of experiments. It is shown that all the certified ratios (certified to within 0.03% expanded combined uncertainty, k = 2) could be matched experimentally within the expanded (k = 2) combined uncertainty of the measurements (from ±0.11 to ±0.33% for 233U/235U, and from ±0.16 to ±0.31% for 233U/238U). For 1∶1 ratios (235U/238U) a 0.05% precision could be reproducibly obtained of which only 33% could be accounted for by Poisson statistics, whereas for 1∶500 ratios a 0.16% precision was obtained which could be completely explained by the same Poisson theory. In addition to the experimental precision other possible sources of uncertainty were investigated. The way isotope abundance ratios are measured (acceleration voltage is varied while the magnetic field is kept constant) is discussed as a potential extra source of mass discrimination. The crucial importance of a proper correction for dead-time effects is demonstrated. Finally, the hypothesis of a contamination with natural uranium that would have biased the results could be ruled out for all samples.

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