Reduction of UH+ formation for U-236/U-238 isotope ratio measurements at ultratrace level in double focusing sector field ICP-MS using D2O as solvent

The main factors affecting the accurate and precise determination of U-236 using ICP-MS are instrumental background, the isobaric interference of U-235 H+ molecular ion on U-236(+) analyte ions, and the presence of U-238(+) and U-235(+) peak tails. An optimized analytical method for attenuating the influence of these factors on uranium isotope ratio measurements at ultratrace level of environmental samples has been developed. In order to reduce (UH+)-U-235 formation, D2O (heavy water) is used as a solvent for the dissolution and dilution of uranium samples. Abundance sensitivity was improved by use of medium mass resolution (m/Deltam = 4450) in comparison with low mass resolution in double-focusing sector field ICP-MS (ICP-SFMS). For solution introduction the performances of several different sample introduction systems (Meinhard, Aridus and ultrasonic nebulizer) were studied. It has been shown, that for all nebulization systems, a diminution in UH+/U+ is observed in D2O as compared with H2O as solvent. Optimum results were obtained in ICP-SFMS for a desolvating microconcentric nebulizer system (Aridus) with a minimum hydride formation rate of 9 X 10(-7) and a limit for U-236/U-238 isotopic ratio measurements of 3 - 5 x 10(-7). A comparison was performed of three commercially available sector field ICP-MS devices, with good agreement found between single collector and multiple collector ICP-MS (MC-ICP-MS)

Uptake of ingested uranium after low 'acute intake'

The uptake of uranium, ingested as a soluble compound, was studied by monitoring the uranium level in urine by inductively coupled plasma mass spectrometry and through measurement of an isotopic tracer. The high sensitivity of this method allows measurement of uranium levels in urine samples from each voiding, therefore more detailed biokinetic studies are possible. To simulate low ((acute intake," five volunteers with "normal" levels (5-15 ng L-1) of uranium in urine ingested a grapefruit drink spiked with 100 mu g of uranium (U-235/U-238 = 0.245%) as uranyl nitrate, and the level of uranium in their urine after ingestion was monitored. Two techniques were applied to estimate the extent of exposure: a) uranium levels above the normal level for each volunteer; and b) the deviation from natural isotopic ratio. Results were normalized relative to the creatinine concentration, which served as an indicator of urine dilution, to reduce effects due to diurnal changes. The results clearly indicate that currently accepted bio-kinetic models overestimate the time between ingestion of dissolved uranium and its excretion in urine, the maximum of which was found to be around 6-10 h. The uptake fraction was in agreement with recent studies, i.e., 0.1-0.5% of the ingested uranium for four of the subjects but above 1.5% for the fifth, and well below the 5% reported in International Commission on Radiation Protection Publication 54. Finally, partial results from the isotope dilution study indicate that uranium absorbed through the intestine interchanges with uranium retained in body organs. The time scale of this process is quite short, and the acute exposure led to a minimum in the isotopic ratio within hours, while recovery back to natural abundance due to low chronic exposure takes several days

Magnesium isotope heterogeneity of the isotopic standard SRM980 and new reference materials for magnesium-isotope-ratio measurements

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Letters to Nature: Magnitudes of sea-level lowstands of the past 500,000 years

Existing techniques for estimating natural fluctuations of sea level and global ice-volume from the recent geological past exploit fossil coral-reef terraces or oxygen-isotope records from benthic foraminifera. Fossil reefs reveal the magnitude of sea-level peaks (highstands) of the past million years, but fail to produce significant values for minima (lowstands) before the Last Glacial Maximum (LGM) about 20,000 years ago, a time at which sea level was about 120 m lower than it is today1, 2, 3, 4. The isotope method provides a continuous sea-level record for the past 140,000 years (ref. 5) (calibrated with fossil-reef data6), but the realistic uncertainty in the sea-level estimates is around 20 m. Here we present improved lowstand estimates—extending the record back to 500,000 years before present—using an independent method based on combining evidence of extreme high-salinity conditions in the glacial Red Sea with a simple hydraulic control model of water flow through the Strait of Bab-el-Mandab, which links the Red Sea to the open ocean. We find that the world can glaciate more intensely than during the LGM by up to an additional 20-m lowering of global sea-level. Such a 20-m difference is equivalent to a change in global ice-volume of the order of today's Greenland and West Antarctic ice-sheets