(LA)-MC-ICPMS/MS measurement of Sr radiogenic isotope ratios

Abstract

A prefiltering system made up of a double-Wien filter and a collision/reaction cell is included with the new Neoma MC-ICPMS/MS to reduce spectral interferences and allow high-precision isotope analyses. In the present study, we challenge these capabilities for measuring Sr isotopes. We first test the effect of different combinations of magnetic and electrostatic field values in the double-Wien filter on the transmission of a bandpass window that includes Sr isotopes and their reaction products. We next scrutinize the spectrum of ion products resulting from the introduction of N₂O or SF₆ to remove on-line Rb which interferes with Sr on the 87-mass unit. While N₂O is the preferred reaction gas when using ICPMS/MS, we found that it reacts with Sr to create expected Sr¹⁶O products, but also ¹⁴N- and, most probably, ¹⁸O-based interfering products. By contrast, the introduction of SF₆ generates a more resolved spectrum of F-based products, notably because F is mono-isotopic, but leads to a 50% reduction in signal intensity. Care must, however, be taken to measure ⁸⁸SrF⁺ in a mass range free of the ³²S¹⁹F₃¹⁶O⁺ interference. We then measure the ⁸⁷SrF/⁸⁶SrF ratio first in raw solutions of a suite of geological reference materials without prior Sr extraction and second in sintered in-house materials and natural solid samples using laser ablation. In the solution mode, no RbF⁺ is detectable and the sensitivity is ∼50 V ppm⁻¹ for ⁸⁸SrF⁺. The accurate measurements of the ⁸⁷SrF/⁸⁶SrF ratio of the geological reference materials are achieved with an uncertainty of 70 ppm (±2 SD), which is comparable with that typically obtained using MC-ICPMS with prior Sr extraction. In the laser ablation mode, the performance without SF₆ of the Neoma for measuring Sr isotopes in homogeneous materials is roughly equivalent to that of the Neptune MC-ICPMS. With SF₆, the Neoma MS/MS exhibits a sensitivity of ∼8 mV ppm⁻¹ (based on ⁸⁸SrF) and the measurement of the ⁸⁷SrF/⁸⁶SrF ratio has a short-term external precision of ∼0.0015 (±2 SD). Finally, an analysis of a heterogeneous material (a fossil tooth) reveals more well-resolved spatial variations of the ⁸⁷SrF/⁸⁶SrF ratio than of the ⁸⁷Sr/⁸⁶Sr ratio, likely thanks to the absence of ⁸⁷RbF⁺.