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Sr Isotope Analysis of Picogram-level Samples by Thermal Ionization Mass Spectrometry Using a Highly Sensitive Silicotungstic Acid Emitter Chao-Feng Li, Zhuyin Chu, Xuan-Ce Wang, Jing-Hui Guo, and Simon A. Wilde Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00958 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019
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Analytical Chemistry
Sr Isotope Analysis of Picogram-level Samples by Thermal Ionization Mass Spectrometry Using a Highly Sensitive Silicotungstic Acid Emitter
Chao-Feng Li,*,†, §
Zhu-Yin Chu†, § Xuan-Ce Wang∇, ǁ
Jing-Hui Guo†, § Simon A Wilde‡ †State
Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics,
Chinese Academy of Sciences, Beijing 100029, China §Institutes
of Earth Science, Chinese Academy of Sciences, Beijing 100029, China
Research Centre for Earth System Science, Yunnan Key Laboratory of Earth System
∇
Science, Yunnan University, Kunming 650500, China ǁ
School of Earth and Environmental Sciences, The University of Queensland, Brisbane,
Qld 4072, Australia ‡The
Institute for Geoscience Research (TIGeR), School of Earth & Planetary Sciences,
Curtin University, GPO Box U1987, Perth, WA6845, Australia
*Corresponding
author: Chao-Feng Li
E-mail address:
[email protected] Tel: +86-10-82998583 1
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ABSTRACT: Thermal ionization mass spectrometry (TIMS) has shown excellent analytical precision for Sr isotopic ratio analysis, even for small masses of material (0.5–10 ng). However, because of the sensitivity limit of TIMS, it is still not possible to obtain high precision 87Sr/86Sr isotope ratios for picogram-level sample sizes (30–100 pg) due to the lack of a highly sensitive emitter. This study is the first to employ a highly sensitive silicotungstic acid emitter to measure Sr isotopes at the picogram-level using TIMS. This emitter produces a 3–fold enhancement in the ionization efficiency of Sr and not only significantly reduces the required sample size but also has good external precision. Analyses of the NIST 987 standard yield an external reproducibility (2 RSD, n = 8) better than ± 0.013 % even for 30 pg of Sr. It is possible to yield an internal precision (2 RSE) of ± 0.003 % for 100 pg of sample using the default 1011 Ohm feedback resistors. This method was verified by using a suite of silicate reference materials. Replicate digestions and analyses (n = 8) of the basalt standard BCR-2 (87Sr/86Sr = 0.704998 ± 0.000028, 2 SD) at the 326 ± 30 pg level demonstrates that good external reproducibility is reached on ultra-trace level silicate samples. This method has a wide variety of potential applications for samples containing ultra-low amounts of Sr in geoscience and archaeological studies, such as single grains of mica, sphalerite and pyrite, single mantle melt inclusions, precious extra-terrestrial materials, and human hair to name just a few.
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Analytical Chemistry
INTRODUCTION Strontium has four naturally occurring isotopes, 84Sr, 86Sr, 87Sr and 88Sr, with abundances of 0.5574, 9.8566, 7.0015 and 82.5845 atomic %, respectively1. The abundance of 87Sr is variable due to the addition of radiogenic 87Sr produced by the beta decay of 87Rb with a half-life of 48.8± 0.05 Ga years1. Based on the geochemical characteristics of Sr, the 87Sr/86Sr isotopic ratio is a powerful isotopic indicator that has been applied in geoscience1, environmental science2, food provenance authentication3, and archaeological4-5 studies since the 1960s. In previous investigations4-17, thermal ionization mass spectrometry (TIMS) has shown excellent accuracy and precision in Sr isotopic measurements. Usually, the external precision of 87Sr/86Sr
isotope ratios is 0.002–0.003 % (2 RSD) by TIMS. Hence, TIMS is
internationally recognized as the “gold standard” for determining Sr isotopic compositions. The advantages of the TIMS approach for measuring Sr isotopic composition are well recognized: (1) instrumental memory from sample to sample is generally low (
