A precise and accurate analytical method for determination of osmium isotope ratios at the 1-15 pg level by using a MC-ICP-MS equipped with sparging introduction and high-sensitivity discrete dynode-type ion-counting detectors

Application of the rhenium (Re) and osmium (Os) isotope system is a useful way to date geological materials, trace their circulation, and reconstruct the Earth's surface environment. However, measurements on less than 15 pg of Os have been challenging because of the large contribution of Os from external sources and insufficient ion beam intensity of instruments. Here, we present a measurement procedure we developed for Re and Os isotope analyses that minimizes contamination of Os from external sources and enables measurements on less than 15 pg of Os with high accuracy and precision by using a multiple collector-inductively coupled plasma-mass spectrometer equipped with compact discrete dynode (CDD) ion-counting detectors. The cleaning procedure we developed for laboratory equipment reduced the amount of Os from external sources to 0.002-0.015 pg, one order of magnitude lower than previously reported values. The measurements of Os isotope standard samples containing less than 30 pg of Os by the use of CDD detectors with in-run cross-calibration of differences in counting efficiencies of CDD detectors produced data with much better precision than those produced by using Faraday cup detectors. The measurement results of geological reference materials, JMS-2 and JCh-1, demonstrated that our method using CDD detectors yielded geochemical data with in-run precision that were equivalent to or better than those obtained by conventional procedures. The method enabled reduction of the sample volume and thus enabled high-spatial-resolution analysis with a small amount of the geological sample.