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Assessment of Interferences in Cu Isotope Ratio Measurements Using Multiple-Collector Inductively Coupled Plasma Source Mass Spectrometry

CAI Jun-jun, ZHU Xiang-Kun, TANG Suo-han, LI Shi-zhen, HE Xue-xian   

  1. Laboratory of Isotope Geology, Ministry of Land and Resources ; Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road. , Beijing 100037, China
  • Received:2006-09-20 Revised:2006-09-20 Online:2006-09-20 Published:2006-09-20

Abstract: Multiple-collector inductively coupled plasma source mass spectrometry (MC-ICP-MS) is a relatively new technique for high precision copper isotopic ratio measurements. However, potential interferences may exist in the measurements of ^65Cu/^63Cu ratios. Assessment of the interferences and effects of Cu concentration on the analyses of Cu isotope compositions using MC-ICPMS is presented here. The problem is first investigated by measuring the abundances of Na in the samples to be able to conclude that the isobaric interference of these polyatomic ions is negligible in this study. Secondly, by assuming that there is no interfering signal at mass 63, the anticipated effect of interference at mass 63 on 665 Cu measurement is modeled. The modeling result shows that even the interfering signal is very small, its effect on the ε^65 Cu value can still be between sample and standard is large. Thirdly, by comparison significant when the difference in Cu concentrations of the modeling results and actual measurements, it has been shown that the isobaric interference on mass 63 is negligible. Fourthly, the results of actual measurements show that the true 65 Cu value can be obtained if the Cu concentration of a sample solution is within the range of 0.5 - 4.0 times of that of the standard. Fifthly, the possibility of matrix effects on measured ^65Cu/^63Cu ratios has been investigated. Of primary concern in this study are the effects of Fe and Co contents on the measured ^65 CU/^63 Cu ratio. This has been examined using synthetic mixtures with Fe:Cu and Co:Cu molar ratios ranging from 0.1 to 100, and 0.5 to 10, respectively. Results of these experiments show that there is no clear dependence of ^65Cu/^63Cu on Fe/Cu 〈 100, Co/Cu 〈 7 respectively under the working conditions. Finally, the results of repeated measurements over a period of ten months show that the ε^65 Cu value of the Romil Cu solution obtained from these analyses is 3.5 ±1 (2sd) relative to NBS standard