A method for age dating of uranium mineral particles by double focusing magnetic sector inductively coupled mass spectrometry (FS-ICPMS) and thermal ionization mass spectrometry (TIMS) was presented. Pb/U ratio in uranium mineral particles was measured by FS-ICPMS without any chemical separation procedure. Pb isotope composition in uranium mineral particles was determined by TIMS associated with micro column anion exchange resin method. Due to simplified chemical procedure and minimum use of reagents, the full procedure blank was reduced to 230 pg Pb, leading to the successful age dating of uranium mineral particles with relative standard deviation less than 2%.
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1μg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。
