Abstract
BackgroundOne of the most critical issues concerning in situ mass spectrometry lies in accounting for elements and molecules that overlap target isotopes of analytical interest in a sample. This study traced the instrumental mass fractionation of Rb and Sr isotopes during laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) to obtain reliable 87Sr/86Sr ratios for high-Rb/Sr samples.FindingsIn the LA-MC-ICPMS analysis, Kr interferences were corrected using Ar and He gas blanks measured without ablating material. Contributions from doubly charged Er and Yb ions were corrected using the intensities of half masses and isotopic compositions reported in the literature. After Kr correction, the calculated 166Er2+ intensity of NIST SRM 610 approached the measured intensity at mass 83, and the 173Yb2+/171Yb2+ ratio agreed with the recommended value within error ranges. Kr- and REE2+-stripped peak intensities were further corrected for Rb interference. Use of the Sr mass bias factor for the calculation of measured 87Rb/85Rb yielded 87Sr/86Sr ratios consistent with the recommended and expected values for low-Rb/Sr materials, such as NIST SRM 616, modern shark teeth, and plagioclase collected from Jeju Island, but failed to account for the 87Rb interference from high-Rb/Sr materials including NIST SRM 610 and SRM 612. We calculated in situ mass bias factor of Rb from the known 87Sr/86Sr ratios of the standards and observed a correlation between Rb and Sr mass fractionation, which allowed inference of the Rb bias from the standard run. Reliable 87Sr/86Sr and 85Rb/86Sr ratios were obtained for SRM 610 and SRM 612 using the inferred mass bias factor of Rb calculated by the standard bracketing method.ConclusionsThis study revealed that Rb and Sr isotopes behave differently during LA-MC-ICPMS and suggests the potential usefulness of the standard bracketing method for measuring the Rb–Sr isotopic compositions of high-Rb/Sr materials.
Highlights
The natural radioactivity of rubidium was demonstrated in 1906 by Campbell and Wood and came into use in the field of isotope geology when mass spectrometry analysis became available in the 1950s
This technique has become a standard methodology for measuring the Sr isotopic composition of Sr-rich minerals, carbonate and phosphate rocks, and biogenic materials, considerable debate remains over isobaric interferences and instrumental mass fractionation (IMF) of Rb and Sr isotopes (Ramos et al 2004; Vroon et al 2008; Müller and Anczkiewicz 2016; Willmes et al 2016)
The in situ data obtained in this study indicate the feasibility of the laser ablation (LA)-MC-ICPMS technique for measuring the Rb–Sr isotopic composition of high-Rb/Sr materials
Summary
(1) After the Kr and REE2+ correction, the 87Sr/86Sr ratios of low-Rb/Sr materials (NIST SRM 616, Jeju plagioclase, and shark teeth) were measured accurately by assuming that Rb and Sr isotopes behave in the same way during LA-MC-ICPMS. (2) Use of the Sr mass bias factor for the correction of Rb interference yielded highly variable and inaccurate 87Sr/86Sr ratios for high-Rb/Sr materials such as NIST SRM 610 and SRM 612. The mass bias factor of Rb was calculated from the recommended 87Sr/86Sr ratios of these SRMs. There were weak to moderate correlations between the mass bias factors of Rb and Sr. Reliable 87Sr/86Sr and 85Rb/86Sr results were obtained using the interpolated mass bias factor of Rb calculated by the standard bracketing method. (3) This study showed that the Rb–Sr isotopic composition of high-Rb/Sr materials can be measured accurately using LA-MC-ICPMS by monitoring the IMF of Rb and Sr isotopes and applying the standard bracketing method. Red dashed lines represent recommended values of 87Sr/86Sr (Woodhead and Hergt 2001) and 85Rb/86Sr (Jochum et al 2011)
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