Effect of Oxidation of Rhenium Filament on the Formation of Uranium Ion During Determination of Uranium Isotopic Ratios by TIMS

The effect of surface oxidation of rhenium filament on determination uranium isotope was carried out by thermal ionization mass spectrometry (TIMS). The method for the mass spectrometric analysis of uranium was standardized by loading UO₂(NO₃)₂ sample on the vaporization filament and monitoring the intensities of the metal and the oxide peaks. The results showed that the uranium was mainly ionized to U⁺ and UOx⁺(x=1 or 2). The yield of UOx⁺ could increase with the change of vaporization filament current at constant ionization filament current (ion intensity ratio of UOx⁺/U⁺ could get to 1). The extent of oxidation of rhenium filament was higher, the oxide ion intensity was greater. Different degassing conditions, such as cooling duration times before venting were compared. The most efficient measurement to inhibit the oxide layer building up was to store the beads in vacuum condition until the samples were loaded. The higher dry current also could get the filaments to be oxidized. Compared to the chemical state of loading samples such as UO₂Cl₂ and UO₂(NO₃)₂ , relatively small ion intensity was observed when the measurements of inhibiting oxidation of filament were taken. It was emphasized that NO^-₃ had not obvious effect on the formation of oxide ions. Decreasing surface oxidation of rhenium filament or loading the sample with colloidal graphite could decrease the formation of UOx⁺ ions and improve sample utilization during analysis employed U⁺. The yield of UOx⁺ did not affect the results of determination of uranium isotopic ratio using total evaporation mode, but decreasing the formation of UOx⁺ ions could gain a high degree of sample ionization with element ion and reproducible sample fractionation, and then improve the reproducibility of the results. At last, an accurate results could be gained by mass fractionation correction using standards. This study can provide a basis for improving sample utilization during analysis of uranium isotope.