热电离质谱法测定铀同位素过程中铼灯丝氧化对目标铀离子形成的影响

采用热电离质谱法(thermal ionization mass spectrometry, TIMS)测定铀同位素过程中，质量分馏效应会引起同位素比值测量值偏离真值，一般采用外标校正法对质量分馏效应进行校正，这要求测量过程中标准物质和样品产生一致的质量分馏行为。因此，除考虑点样的一致性外，测量过程中产生单一的目标离子也非常重要。本研究采用热电离质谱技术考察了铼灯丝氧化对铀同位素测定的影响。将1 μg铀以硝酸盐溶液的形式点于铼样品带上，通过监测铀及其氧化物的离子流强度，发现铀主要存在U⁺和UOx⁺(x=1或2)的电离形态，并且灯丝表面氧化程度越高，UOx⁺的产率越高（UOx⁺/U⁺可达到1）。实验结果表明，灯丝去气过程过早使灯丝暴露于大气或点样过程中，以及使用较高的样品蒸干电流都会加剧灯丝的氧化。控制灯丝表面氧化或点样过程中加入石墨，可有效降低UOx⁺的产率，提高目标离子的电离效率。全蒸发测量结果表明，UOx⁺离子流强度大小对测量结果无明显影响，但通过降低UOx⁺的离子流强度，可提高测量结果的重现性，进而提高测量过程中分馏行为的一致性。该研究可为提高铀同位素测定过程中样品的利用率提供理论依据。

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⁺ andUOx⁺(x=1 or 2). The yield ofUOx⁺ could increase with the change of vaporization filament current at constant ionization filament current (ion intensity ratio ofUOx⁺/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 ofUOx⁺ ions and improve sample utilization during analysis employed U⁺. The yield ofUOx⁺ did not affect the results of determination of uranium isotopic ratio using total evaporation mode, but decreasing the formation ofUOx⁺ 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.