Abstract
The effects of addition of nitrogen gas with use of three different combinations of sample and skimmer cones on the performance of LA-MC-ICP-MS for in situ B isotope ratio measurements were investigated in detail. Compared to the standard arrangement (H skimmer cone+standard sample cone), sensitivities of B isotopes were improved by a factor of 2.4 and 3.8 with use of X skimmer cone+standard sampler cone and the X skimmer cone+Jet sample cone, respectively. The best within-run precision (2s) of 11B/10B was approximately 50ppm for B4 reference material (B content=31,400μg/g) with use of X skimmer cone+Jet sample cone. It was found that different cone combinations had significant effects on the instrument mass bias, but had little effect on the precisions of B isotope ratios. Addition of nitrogen (2–4ml/min) in the central channel gas did not improve the sensitivity of B. However, significant wider ion axial distribution profile and more stable mass-bias for B isotope ratios were evident. Under selected optimum conditions, good agreements between the measured δ11B values and the reference values were obtained for the international reference minerals (B4, Dravite, Elbaite, IMMRB1 and Schorl), the NIST SRM 610–612 synthetic soda-lime glasses and the MPI-DING glasses (GOR-128-G, GOR-132-G, and StHs6/80). It was found that for low-B containing (11ppm and 31ppm) materials, the precisions (2s) of 11B/10B can be improved by a factor of 2–10 by using the newly designed X-skimmer cone and Jet sample cone, compared to that of using the standard cones. The international reference mineral Danburite (δ11B of −14.92±6.77 2SD, n=32) showed a significant non-uniformity of B isotope ratio distribution. Our first boron isotopic results from standards GSD-1G (δ11B: 11.92±1.03, n=30), GSE-1G (δ11B: 1.63±0.32, n=30), GP-4 (δ11B: −5.85±1.01, n=40) and DD-1 (δ11B: −13.21±0.56, n=35) showed these materials were fairly homogeneous in boron isotope ratios, making them as good candidates as reference materials. The proposed LA-MC-ICPMS method is suitable for the direct determination of B isotope ratio in a variety of geological materials.
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