Sensitive 236U/238U Isotopic Analysis of Trace Uranium in Safeguards Environmental Samples Using Multicollector Inductively Coupled Plasma Mass Spectrometry.

Abstract

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 236U/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.