Mass fractionation of Rb and Sr isotopes during laser ablation-multicollector-ICPMS: in situ observation and correction

Background One 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 87 Sr/ 86 Sr ratios for high-Rb/Sr samples. Findings In 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 166 Er 2+ intensity of NIST SRM 610 approached the measured intensity at mass 83, and the 173 Yb 2+ / 171 Yb 2+ ratio agreed with the recommended value within error ranges. Kr- and REE 2+ -stripped peak intensities were further corrected for Rb interference. Use of the Sr mass bias factor for the calculation of measured 87 Rb/ 85 Rb yielded 87 Sr/ 86 Sr 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 87 Rb 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr and 85 Rb/ 86 Sr ratios were obtained for SRM 610 and SRM 612 using the inferred mass bias factor of Rb calculated by the standard bracketing method. Conclusions This 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.