Determination of Bromine Stable Isotope Ratios from Saline Solutions by “Wet Plasma” MC-ICPMS Including a Comparison between High- and Low-Resolution Modes, and Three Introduction Systems
Résumé
Abstract
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We describe a novel method for measuring stable bromine isotope compositions in saline solutions such as seawater, brines, and formation waters. Bromine is extracted from the samples by ion exchange chromatography on anion exchange resin AG 1-X4 with NH4NO3 and measured by MC-ICP-MS in wet plasma conditions. Sample introduction through a small spray chamber provided good sensitivity and stability of the Br signal compared to direct injection (d-DIHEN) and desolvation (APEX). NH4NO3 media allowed fast (<3 min) washing of the system. Despite Ar2H+ spectral interference on 81Br+, for the first time low-resolution mode (with appropriate tuning of Ar2H+/81Br+ sensitivity) gave higher precision 81Br/79Br measurements than high-resolution (HR), due to the narrowness of the 81Br+ plateau in HR mode and to slight mass drifting with time. Additionally, 1 μg Br is the lower amount needed for a triplicate determination of δ81Br by MC-ICP-MS, with reproducibility often < ± 0.1‰ (2 SD). Four HBr solutions were prepared by evaporation/condensation in order to obtain in-house reference solutions with 3‰ variations in δ81Br and to assess the reproducibility and accuracy of the method. Long-term (>3 years) reproducibility between ± 0.11 and ± 0.27‰ (2 SD) was obtained for the four HBr solutions, the international standard reference material NIST SRM 977 (δ81BrSMOB = −0.65 ± 1.1‰, 1 SD), and seawaters (synthetic and natural). The accuracy of the MC-ICP-MS method was validated by comparing the δ81Br obtained for these solutions with dual-inlet IRMS measurements on CH3Br gas. Finally, the method was successfully applied to 22 natural samples.