High-precision uranium isotopic analysis for environmental forensics using MC-ICPMS: Demonstration studies at the Hanford Site, Washington
- Author(s): Christensen, John N.
- Dresel, Evan P.
- Conrad, Mark E.
- DePaolo, Donald J.
- et al.
The contrasts in isotopic composition between natural and anthropogenic uranium and the wide variation in the composition of different processed uranium sources, promotes the measurement of uranium isotopic composition as a fingerprint and tracer of uranium contamination in the environment. Previous studies mainly have focused on the use of only one of the isotope ratios of U, e.g., 234U/238U, 238U/235U or 236U/238U. We measure all three of these ratios in environmental samples in order to better distinguish, characterize, identify and apportion U sources. For our U isotopic measurements, we employ an IsoProbe (GV Instr. Inc.) multiple-collector ICP source magnetic sector mass spectrometer. U isotopic compositions are measured simultaneously using a combination of Faraday cups (for 235U and 238U) and a Daly photomultiplier ion counting system (for 234U and 236U in two separate analyses). U is separated from samples (e.g., vadose zone pore water, groundwater, rock/soil samples) prior to introduction to the MC-ICPMS via a desolvation system. At 7E-11 amps of 238U ions, a single analysis of a 20ppb U solution uses ~10ng of sample U. For correction of instrumental mass fractionation, we use bracketing analyses of a natural secular equilibrium U standard. This allows us to avoid the use of a double 233U-236Uspike for mass fractionation correction that would compromise our ability to measure 236U/238U. We also use the standard analyses for Daly/Faraday gain and for peak-tail correction of the 236U analyses. Typical precision for 238U/235U is ?0.05% 2s, while for 234U/238U it is ?0.15% 2s. Precision for 236U/238U is ?0.15% 2s down to the 10-7 range where precision degrades by a factor of ten. The limit for 236U/238U measurement is about 2x10-8, only ~five times higher than accelerator MS. For 1ppb U, this represents 5x107 atoms 236U per liter water.We will present three ongoing studies at the Hanford site as a demonstration of our techniques: (1) investigation of groundwater and vadose zone contamination in the B-BX-BY WMA (Christensen et al. (2004) Env. Sci. Tech., in press) (2) signatures of vadose zone contamination in waste cribs near U-Plant and (3) investigation of U contamination of the Columbia River.