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The main factors affecting the accurate and precise determination of 236 U using ICP-MS are instrumental background, the isobaric interference of 235 UH 1 molecular ion on 236 U 1 analyte ions, and the presence of 238 U 1 and 235 U 1 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 235 UH 1 formation, D 2 O (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/Dm~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 1 /U 1 is observed in D 2 O as compared with H 2 O as solvent. Optimum results were obtained in ICP-SFMS for a desolvating microconcentric nebulizer system (Aridus) with a minimum hydride formation rate of 9 6 10 27 and a limit for 236 U/ 238 U isotopic ratio measurements of 3-5 6 10 27. 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).

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