Preventing uranium hydride formation in standard uranium samples for determination of 239Pu by ICP-MS

Abstract

UH⁺ interference poses considerable difficulties for the determination of ²³⁹Pu in samples with high uranium content. A new approach for reducing this interference was developed. Substitution of water with heavy water (D₂O) as a solvent in uranium standard solution eliminated the major source of UH⁺ ions and reduced the UH⁺/U⁺ ratio from a typical value of 1.33 × 10⁻⁵ to a very low value of 2.60 × 10⁻⁷. The method allows the determination of low levels of ²³⁹Pu in samples containing 10⁶ fold excess of uranium. The detection limit for ²³⁹Pu in a sample containing 10 ppm of uranium was determined as 0.58 ± 0.03 ppt. A kinetic isotope effect of ca. 1.4 was observed for the reaction of U⁺ with hydrogen (H) compared with the reaction with deuterium (D). The magnitude of this effect is in agreement with the value calculated based on the collision theory.