In situ Sr isotope measurement of small glass samples using multiple-Faraday collector inductively coupled plasma mass spectrometry with 1012 Ω resistor high gain Faraday amplifiers

Abstract

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.