HR: 1330h
AN: V42D-0378    [PDF]
TI: Improving the Sensitivity of Uranium Isotope Ratio Measurements
AU: Friedrich, J
EM: fried@mpch-mainz.mpg.de
AF: Max-Planck-Institut f\"{u}r Chemie, Postfach 3060, Mainz, 55020 Germany
AU: * Snow, J
EM: jesnow@mpch-mainz.mpg.de
AF: Max-Planck-Institut f\"{u}r Chemie, Postfach 3060, Mainz, 55020 Germany
AB: Accurate and precise measurements of natural and anthropogenic 235/238 U isotope ratios are important for a range of investigations where the amount of sample is extremely restricted and/or the analyte is only present in ultra-trace quantities. Examples include biological, cosmochemical, environmental, geological, and radiological studies. We have developed and validated a novel method using our Nu Instruments Nu Plasma Multi Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) and a 233U, 236U mixed double spike for the measurement of 235U/238U ratios. Our multi-dynamic technique employs the installed quadrupole zoom optics and fixed positioning of the ion counting detectors such that rather than the commonly used mass dispersion of 1 or 2, we utilize a mass dispersion of 1.5. Using this configuration, we can simultaneously monitor the 235U and 238U ion beams in the first cycle followed by a second cycle that simultaneously monitors the 233U and 236U beams. This innovative approach allows us to correct for the considerable, but consistent, instrumental mass fractionation and ion-counter amplification bias within each sequence. Since we were hesitant to use a U500 (235U, 238U equal atom) solution for spike calibration because of possible enriched U laboratory and instrumentation contamination, we used a U960 (terrestrial 235U/238U) solution for isotopic calibration of the spike. This standardization corrected for the small amounts of 235U and 238U in the spike solution by using a U960 standard solution. With a mean intraday 2-sigma precision of 1.5 permil and an overall 2-sigma precision of 2.25 permil using individual sample sizes of 350 pg (8.8 x10 E11 atoms), we are confident our technique will be useful for identifying U isotopic anomalies present in many sample types.
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1094 Instruments and techniques
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting