In situ analysis of titanium isotope ratios in stardust using LA-CC-MC-ICPMS/MS.

Abstract

Presolar grains are nanometre-scale dust grains that exhibit large isotope excursions that illustrate the stellar isotopic input into the Solar System. Further, it is thought that they were differentially incorporated into meteorite parent bodies and thus can be used to trace planetary genetics and construction. In situ mapping of the distribution of presolar grains in the matrix of primitive meteorites therefore provides a key means to achieve this goal. However, in situ methods complicate isotopic measurements, such as those of Ti, due to their large isobaric interferences. To enable such measurements a prototype a collision cell, multicollector inductively-coupled plasma mass spectrometer with a pre-cell mass filter (CC-MC-ICPMS/MS) was developed and called Proteus. In this study we show that, when coupled to a laser ablation system, Proteus has the capability to measure, in situ, large Ti isotope excursions such as those expected in presolar grains (>200‰). Within the collision cell we introduced O₂ gas and react Ti⁺ to TiO⁺ and perform the multi-collector isotope ratio measurement on the TiO⁺ species. The presence of isobaric interferences from Ca⁺, V⁺, and Cr⁺ are greatly reduced due to their lower ion reaction efficiency with O₂ gas. The measurement of TiO⁺ using the pre-cell mass filter ensures that these ions are measured in a cleared region of the mass-spectrum where a Ni⁺, Cu⁺, and Zn⁺ ions would otherwise be present as interferences. Using this technique, complex rock samples with high Ca/Ti and Cr/Ti, for example BIR-1G, give the same mass-independent isotopic Ti ratios as essentially pure Ti-minerals, e.g. brookite. By reducing isobaric interferences from in situ measurements we can detect the large isotopic excursions in presolar grains without the added impediment of non-solar interference corrections for isobaric interferences.