PGE (PLATINUM GROUP ELEMENTS) COSMOCHEMISTRY FOR METALLIC METEORITIES

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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