Improved in situ technique for the analysis of Os isotope ratio in sulfide using laser ablation-multiple ion counters inductively coupled plasma mass spectrometry

A new technique is developed for in situ measurement of 187Os /188Os in sulfide using LA-MC-ICP-MS, with the improvement of the spatial resolution and quantification limits. In this study, an all-ion counters (all-IC) configuration is adopted for static concurrent measurement of 187Os+187Re and 188Os along with 185Re. To cross-calibrate the detection efficiencies of the different ion counters, an Au standard solution is introduced to the instrument and its signal is measured in each ion counter by a dynamic peak jumping mode. Then, the signal intensities of IC1, IC2 and IC3 are all normalized to that of IC0. The sample-standard bracketing (SSB) method is adopted not only for the Re interference and Os mass fractionation correction but also for the monitoring of detection efficiency during continuous measurement. In detail, the in-house sulfide standard CR-1P (only spiking Re) is specifically employed for correcting interference 187Re on 187Os and monitoring detection efficiency variation in IC0 and IC1. The mass fractionation of Os and detection efficiency variation of IC1 and IC2 are calibrated against another in-house sulfide standard CO-1P (only spiking Os). Even though the 188Os intensity decreases approximately 20 times, our improved method still provides accurate and precise results of 187Os/188Os ratio, whereas published methods that employ Faraday detectors for the signal collection do not work. Furthermore, the threshold of reliable interference correction of 187Re on 187Os is estimated by analysing synthetic sulfide with different Re/Os ratios. The analytical results demonstrate that the relative deviations (RD) of the calculated 187Os/188Os ratio will be limited to within 0.8%, 1%, 3% and 5%, when 187Re/188Os is lower than 0.055, 0.09, 0.42 and 0.75 (i.e., Re/Os: 0.012, 0.019, 0.089 and 0.159), respectively. In general, this new method significantly improves the spatial resolution and quantification limits of in situ measurements, which is beneficial for analysing sulfide in the depleted subcontinental lithospheric mantle (SCLM).