高性能、低本底的TaF_(5)发射剂溶液制备与微量样品Sr同位素测量

A Preparation Method of the TaF_(5)Emitter Solution with High Ionization Efficiency and Low Sr Blank for the Sr Isotopic Measurement of Micro-samples

热电离质谱计为精确测量地质样品的同位素组成提供了前提,然而低的离子化效率常常难以获得低含量样品或微量样品的高精度同位素组成,严重影响测量精度和准确度。发射剂可以有效降低元素的电离电位,提高热电离质谱计的离子化效率,但未纯化的发射剂不可避免引入本底,影响测量结果的准确度。鉴于此,通过制备高效、低本底TaF_(5)发射剂溶液,提高Sr元素的电离效率,为微量样品Sr同位素组成精确测量提供技术支撑。制备的发射剂溶液经过纯化后,通过测量不同Sr用量的标准Sr溶液(NIST SRM 987)和标准岩石粉末(BCR-2和BHVO-2玄武岩)同位素组成验证其效果和本底。实验结果表明:纯化的TaF_(5)发射剂溶液具有极低的Sr本底(平均值约为0.02 pg·μL^(-1)),仅需使用1~2μL发射剂就能有效提高Sr离子化效率,且在测量过程中保持稳定的离子信号强度;微量的标准Sr溶液和标准岩石粉末87 Sr/86 Sr值的测量结果均落在推荐值范围内,满足纳克/亚纳克级样品Sr同位素测量的精度和准确度要求。这一高效、低本底TaF_(5)发射剂溶液可应用于亚纳克级超微量样品Sr同位素组成测量,能够进一步拓展Sr同位素地球化学应用前景。

Thermal ionization mass spectrometers can provide the precondition for high-precision measurement of isotopic composition of geological samples,but it is normally difficult to obtain accurate isotopic composition of the samples having low elemental contents or very small volume or quantity,due to low ionization efficiency of elements.Therefore,efficiently improving the ionization efficiency is one of the key ways to measure isotopic composition of the micro-samples.The emitter can effectively decrease the ionization potential of elements and improve the ionization efficiency during the measurement using the thermal ionization mass spectrometer(TIMS),but unpurified emitter will inevitably introduce more blank and reduce the accuracy of measurement results.A preparation method to produce TaF_(5)emitter solution for the Sr isotopic measurements on micro-samples was provided by the TIMS technique.This emitter solution is characterized by very low Sr blank and high efficiency to aid Sr ionization on the basis of the measurements on the standard Sr solution(NIST SRM 987)and the standard rock powders(BCR-2 and BHVO-2 basalts).The experimental results show that TaF_(5)emitter solution after the purification has a very low Sr blank with a mean value of about 0.02 pg·μL^(-1).Only 1-2μL of the emitter solution used for each sample can effectively improve Sr ionization efficiency,and maintain high and stable signal intensity of Sr isotopes during the measurements.The 87 Sr/86 Sr values measured on the micro-samples of the standard Sr solution and standard rock powders fall in the ranges of their reference values,meeting requirements in the precision and accuracy for the measurement of Sr isotope for nanogram even sub-nanogram samples.This high-efficiency and low-blank TaF_(5)emitter solution can apply to the measurements of Sr isotopic composition of low-content samples,ensuring more applications of the Sr isotope geochemistry.