Multi-element ion-exchange chromatography and high-precision MC-ICP-MS isotope analysis of Mg and Ti from sub-mm-sized meteorite inclusions

摘要

The analytical improvement of new generation mass spectrometers has reached a level required for high-precision isotope analysis of very small and unique natural samples. The multi-element isotopic signatures of meteorite inclusions can potentially provide detailed insight into the origin of our solar system. As such, in-line separation and isotope analysis of multiple elements from such unique samples is highly desirable, but rarely accommodated by current chromatographic purification procedures necessary for accurate isotope analysis using multiple collector inductively coupled plasma source mass spectrometry (MC-ICP-MS). Here, we report a new multi-element ion chromatographic purification procedure, specifically developed for the separation of very small amounts of Mg (~5 μg) and Ti (0.7-1 ng) from individual samples. Highly isotopically anomalous sub-mm sized materials, such as Ca-AI-rich inclusions in primitive meteorites, can be routinely analysed for their mass-independent μ（~26)Mg~*, μ~（45）Ti~*,μ~（48）Ti~*, μ~(49)Ti~* and μ~(50)Ti~* isotope compositions with an estimated external reproducibility of 4.1, 11, 8.8, 18 and 12 ppm (2sd), respectively. These procedures consume significantly less material than previous methods and as such represents a 6- to 10-fold improvement in precision. The method ensures >99.9% Mg and >97% Ti recovery, thereby allowing for the accurate determination of mass-dependent compositions with a precision better than ~60 ppm amu~(-1) for both Mg and Ti. It can further be adapted to accommodate the in-line separation of e.g. Ca, Cr, Fe, Ni, W, Mo, Ru, V, Zr and Hf from the same sample matrices. Doping tests on a synthetic Ti standard demonstrate that isobaric interference can be adequately corrected for provided that atomic ratios are Ca/Ti < 0.07, V/Ti < 0.04 and Cr/Ti < 0.005 and Zr~(2+), Mo~(2+) and Ru~(2+) have no influence on Ti measurements when atomic ratios are Zr/Ti < 0.002, Mo/Ti < 0.01 and Ru/Ti < 0.001, which is ensured by the chromatographic procedures. Magnesium and titanium isotope data for terrestrial geostandards and isotopically anomalous chondrite meteorites (OC, CR, CV, and Cl) and two CV CAIs are in excellent agreement with literature values, demonstrating the accuracy of our methods. The two CAIs plot on an extension of the mass-independent correlation line between μ~（46）Ti~* and μ~（50）Ti~* defined by inner solar system materials, suggesting residual nucleosynthetic effects in CAI.