In situ Sr isotope measurement of small glass samples using multiple-Faraday collector inductively coupled plasma mass spectrometry with 10^12Ω resistor high gain Faraday amplifiers

摘要

An analytical protocol was developed for correcting Kr baseline-induced bias and Rb isobaric overlap factors to analyse Sr isotope ratios for small glass samples using excimer laser ablation (LA) with an Aridus II desolvating nebuliser dual-intake system and multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The combined use of a low-oxide interface setup, along with high-gain Faraday amplifiers with a 1012 Ω resistor, enabled precise determination of Sr isotope ratios from 50-100 μm diameter craters at 10 Hz laser repetition rate. Residual analytical biases of 84Sr/86Sr and 87Sr/86Sr isotope ratios, obtained from Kr baseline suppressions (Kimura et al., 2013, Journal of Analytical Atomic Spectrometry, 28, 945-957), were found to be nonlinear, but the correction method was applicable to 50-200 μm/10 Hz craters. We also found that the 85Rb/87Rb overlap correction factor changed with time with a change in the surface condition of the sampler-skimmer cones. The correction factor of 85Rb/87Rb was thus determined at least once per five unknown measurements using the Aridus solution intake line. We determined 87Sr/86Sr isotope ratios from MkAn anorthite (Sr = 305 ppm, Rb = 0.07 ppm), BHVO-2G, KL2-G, ML3B-G (Sr = 312-396 ppm, Rb = 5.8-9.2 ppm), and BCR-2G (Sr = 337 ppm, Rb = 48.5 ppm) basalt glasses using a 50-100 μm/10 Hz crater. The results agree well with their reference values, determined by thermal ionisation mass spectrometry, even with the high Rb/Sr ratio (0.14) in the BCR-2G glass. The internal/intermediate precisions were ±0.0002 (two-standard deviation: 2SD) for 100 μm craters and ±0.0005 for 50 μm craters. The new instrument settings and analytical protocol improved the precision by a factor of two compared to the previous report using LA-(sector field)-ICP-MS and enables the analysis of sample volumes that are ten times smaller than those used in previous LA-MC-ICP-MS analyses with equal precision.