Precise determination of Os isotope ratios in the 15–4000 pg range using a sparging method using enhanced-sensitivity multiple Faraday collector-inductively coupled plasma-mass spectrometry

Abstract

We have developed a protocol for Os isotope analysis employing a sparging method coupled with an enhanced-sensitivity multiple Faraday collector-inductively coupled plasma-mass spectrometry (MFC-ICP-MS) technique. The enhanced-sensitivity ICP interface with 10¹² Ω high-gain amplifiers allowed for the stable and precise isotopic ratio analysis of Os by sparging in a very wide concentration range of 15–4000 pg. The analytical reproducibilities of Johnson Matthey Chemical (JMC) Os standards at 50, 100, 200, 400, and 2000 pg Os were 0.8, 0.5, 0.2, 0.1, and 0.02% within two standard deviations (2SD), respectively. The low Os (50–200 pg) results compared with those obtained by sparging multiple-ion counter (MIC)-ICP-MS and high Os (400–2000 pg) results rivalled those of desolvating nebulisation MFC-ICP-MS and negative thermal ionisation mass spectrometry (N-TIMS). The analysed geological standards consisting of JCh-1 (chert; ∼15 pg, n = 3), JMS-2 (marine sediment; ∼150 pg, n = 5), UB-N (lherzoritic peridotite; ∼4 ng, n = 4), and JP-1 (harzburgitic peridotite; ∼3 ng, n = 5) showed ¹⁸⁷Os/¹⁸⁸Os = 0.657 ± 0.065, 0.842 ± 0.053, 0.12752 ± 0.00016, and 0.12071 ± 0.00069 (errors are in 2SD), respectively; these results are comparable with those obtained by MIC-ICP-MS and N-TIMS. The results showed that the sparging method coupled with enhanced-sensitivity MFC-ICP-MS is a strong tool for determining Os isotope ratios in natural samples over a wide range of Os concentrations. Simple sample digestion and low procedural blanks using Carius tube digestion alone without any further element separation provide an additional advantage for Os isotope analysis by the method.