Sensitive ²³⁶U/²³⁸U Isotopic Analysis of Trace Uranium in Safeguards Environmental Samples Using Multicollector...

A sensitive analytical technique was developed for determining a 10^-7 level low ²³⁶U/²³⁸U ratio using multicollector inductively coupled plasma mass spectrometry. This analytical technique can determine the subnanogram amount of uranium in safeguards environmental samples. To increase the sensitivity of the ²³⁶U/²³⁸U ratio, (1) the feed solution was concentrated to one-tenth of that in the conventional method and (2) data acquisition was started from the beginning of the solution uptake until all of the solution was exhausted. All signal intensities were recorded. This technique was applied to the IRMM-184 uranium isotopic standard solution with a 1.2410 × 10^{-7 236}U/²³⁸U ratio. The IRMM-184 uranium solution (0.2 mL of 1 ng g^-1 uranium) was prepared, and the ²³⁶U/²³⁸U ratios were determined using our new data collection method. The ²³⁶U/²³⁸U ratios were in agreement with the certified value within the uncertainty (k = 2) evaluated using the Guide to the Expression of Uncertainty in Measurement (GUM) workbench. The uncertainties obtained with this new technique (32% on average) were revised to be 10 times smaller than those obtained with the conventional method using 0.1 ng g^-1 IRMM-184. The tailing effect from the ²³⁸U intense peak was determined using 10 times higher concentrations of NBL CRM U0002 uranium isotopic standard solution (10 ng g^-1 uranium) to improve its uncertainty. Approximately 1-2% of the uncertainties of ²³⁶U/²³⁸U could be further reduced using this improved correction factor.