U/U Analysis of Femtograms of U by MC-ICPMS.

A new analytical method has been developed to determine atomic U/U ratios in samples with only femtograms of U using a secondary electron multiplier (SEM) on a multicollector high-resolution inductively coupled plasma mass spectrometer (MC-ICPMS). The abundance sensitivity of the U tail at 236 atomic mass unit is reduced from 10 to 10 with the deployment of a retarding potential quadrupole lens. This method features the reduction of polyatomic interferences from hydride, nitride, lead, and plutonium and the evaluation of nonlinear SEM behavior. The instrument sensitivity is 1-2%, and the estimated methodological detection limit of the U/U atomic ratio is as low as 2 × 10. Measurements on reference materials with U/U ratios of 10-10, including the IRMM-075 series and the ETH Zurich in-house standard ZUTRI, demonstrate the accuracy of our MC-ICPMS technique. The analytical precisions (2σ) are ±4% for 5 fg of U at a U/U of 1 × 10 and ±8% for 2 fg of U at a U/U of 4 × 10 level. Compared to state-of-the-art accelerator mass spectrometry techniques and triple quadrupole-based ICPMS, our detection limit is not as low, but the required sample size is 3-40 times lower, and the throughput is as high as 3-4 samples per hour. The new MC-ICPMS-SEM technique is sensitive enough for determining U/U in various small natural samples, such as marine carbonates and seawater.