U- 236/U-238 Analysis Of Femtograms Of U-236 By Mc-Icpms

A new analytical method has been developed to determine atomic U- 236/U-238 ratios in samples with only femtograms of( 236)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-238 tail at 236 atomic mass unit is reduced from 10(-6) to 10(-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( 236)U/U-238 atomic ratio is as low as 2 X 10(-10). Measurements on reference materials with U- 236/U-238 ratios of 10(-7)-10(-9), including the IRMM-075 series and the ETH Zurich in-house standard ZUTRI, demonstrate the accuracy of our MC-ICPMS technique. The analytical precisions (2 sigma) are +/- 4% for 5 fg of U- 236 at a U- 236/U-238 of 1 X 10(-8) and +/- 8% for 2 fg of( 236)U at a( 236)U/U-238 of 4 10(-9) 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- 236/U-238 in various small natural samples, such as marine carbonates and seawater.