PGE (PLATINUM GROUP ELEMENTS) COSMOCHEMISTRY FOR METALLIC METEORITIES

采用电感耦合等离子体（ICP）作为离子源的质谱仪（ICPMS）是目前公认的一种用于元素和同位素分析的通用分析技术。然而，icp - pms一直是最常用的敏感元素分析工具，用于同位素分析的例子并不多。这主要是因为目前基于icpms （ICP-QMS）的四极杆分析仪的同位素丰度比精度不足以满足地质测年和核工业的需求。由于探测器性能低、背景贡献大和/或质量分辨效应大，严重制约了icp - qms2同位素测量的精密度和准确度。近年来介绍了多收集器icp质谱法（MC-ICPMS）测量同位素比值的方法。多个收集器允许同时监测每种同位素，从而消除信号不稳定性，从而限制分析精度。此外，使用法拉第杯作为离子检测器的模拟检测器模式与大多数ICP-QMS仪器的检测系统（高增益脉冲计数模式）相比，可以提供更好的动态范围和分析精度。MC-ICPMS的相对标准偏差一般为0.005-0.01% （s-sigma）。MC-ICPMS的分析精密度和准确度与热电离质谱（TIMS）相当。然而，为了进一步利用MC-ICPMS进行同位素地球科学研究，迫切需要提高仪器的元素灵敏度。最近，Matsumoto等人（1993）描述了一种能够直接检测和数字化图像的新型离子探测器。该二维离子探测器由25万像素组成，是一种堆叠式光电二极管型放大MOS智能成像仪（AMI），首次应用于二次离子质谱（SIMS）。AMI的工作原理是基于对具有千电子伏特能量的离子轰击在硅光电二极管中产生的电子的存储和探测。该系统应用于MC-ICPMS仪器，具有良好的线性度和4个数量级的动态范围，可直接检测二次离子。MC-ICPMS分析中显示的独特特征清楚地表明，AMI离子检测器使我们能够检测多种同位素，从而提高同位素测量的分析精度。此外，通过ICPMS测量发现AMI探测器的灵敏度比法拉第探测器高至少5倍。这表明AMI离子探测器适用于最强大的高增益离子探测器，用于同位素的多次采集。少

Mass spectrometer using inductively coupled plasma (ICP) as an ion source (ICPMS) is now accepted as a versatile analytical technique for elemental and isotopic analysis. However ICPMS had been most frequently utilised as a tool for the sensitive elemental analysis, and examples for isotopic anaylsis are not many. This is mainly because that the precision of isotopic abundance ratio achieved by present quadrupole analyzer based-ICPMS (ICP-QMS) is not always sufficient for the purchases of geological dating and nuclear industries. It has been suggested that the precision and accuracy of the isotopic measurement achieved by the ICP-QMSare severely restricted from the low performance of detector, high background contribution and/or large mass discrimination effect. Recently the measurement of isotopic ratios by multiple collector-ICP massspectrometry (MC-ICPMS) has been described. Multiple collectors allow eachisotope to be monitored simultaneously, and thus removing signal instability as … More a limitation on analytical precision. Furthermore, the analogue detector mode using Faraday cups as ion detector can offer better dynamic rangs and analytical precision compared with the detection system applied to most ICP-QMS instrument (high gain pulse counting mode). Relative standard deviations of 0.005-0.01% (s-sigma) could be typically achieved by the MC-ICPMS.Levels of analytical precision and accuracy achieved by the MC-ICPMS was comparable to those exhibited by thermal ionisation mass spectrometry (TIMS). However, enhancement of elemental sensitivity of the instrument is strongly required for the further progress of isotope geosciences using a MC-ICPMS.Recently, novel ion detector capable of directly detecting and digitizing on images were described by Matsumoto et al. (1993). This two dimensional ion detector which composed of 250,000 pixels, is a stacked photodiode type Amplified MOS Intelligent Imager (AMI) and was first applied to the secondary ion mass spectrometry (SIMS). Operational principles of AMI was based on the storage and detection of electrons in silicon photodiode generated by ion bombardments having kiloelectronvolt energy. The system applied to a MC-ICPMS instrument succeeded in directly detecting secondary ions with an excellent linearity and a wide dynamic range of four orders of magnitude for each pixel. The unique features shown in the MC-ICPMS analysis clearly demonstrate the AMI ion detector enables us to detect multiple isotopes which can improve the analytical precision of the isotope measurement. Moreover, sensitivity of AMI detector demonstrated by the ICPMS measurement were found to be at least 5 times higher than those of the Faraday detectors. This indicates that the AMI ion detector is suitable to most powerful high gain ion detector for the multiple collection of isotopes. Less