Rapid separation of Sr from carbonates for accurate 87Sr/86Sr isotope analysis by TIMS

Abstract

The strontium isotopes (⁸⁷Sr/⁸⁶Sr) in carbonates can serve as a fundamental geochemical indicator holding an indispensable status in both modern and paleo-geochemical investigations. Though thermal ionization mass spectrometry (TIMS) is widely used as the most accurate technique for ⁸⁷Sr/⁸⁶Sr isotope analysis, an ion-exchange resin chromatography Sr separation step is generally required to eliminate interferences and meet the stringent matrix requirements before sample introduction. However, this step often renders sample preparation tedious and time-consuming, which leads to low sample throughput and high experimental cost. In this study, we proposed a new method for separating Sr from complex carbonate matrices without ion-exchange chromatography. Our new separation scheme employing a fuming nitric acid protocol relies on the significantly lower solubility of Ba and Sr in concentrated inorganic acids compared to matrix elements (e.g. Ca, Mg, Rb, etc.). The whole separation is composed of three successive steps after sample dissolution, with first a coprecipitation of trace Sr with Ba, then a separation of Sr and Ba from matrix elements in 15 M HNO₃ (total 2 mL), and finally a separation of Sr from Ba using 12 M HCl (1.5 mL). The separation can be achieved in less than two hours and the robustness of the method is further assessed by analyzing seven different carbonate reference materials, with Sr concentrations spanning from 116 to 12 000 μg g⁻¹. The Sr isotope ratios measured using our new protocol show excellent agreement with previously reported values, validating the new Sr purification. Our study clearly shows the potential usefulness of this rapid, simple, and efficient method for elaborating Sr geochemistry in carbonates.