Determination of δ88/86Sr Using Matrix Correction by MC-ICP-MS

Abstract:

Stable Sr isotopic compositions (δ^88/86Sr) in marine carbonates potentially provide key information on paleoseawater temperature (Rüggeberg et al. 2008). Traditional methods for δ^88/86Sr determination by ⁸⁷Sr-⁸⁴Sr double-spike TIMS or MC-ICP-MS require chemical purification of Sr before spectrometric measurements because of matrix effects. Recent studies suggested that the matrix-matching method, in which matrix-matched standard solutions were used to bracket untreated water samples, gave precise and accurate results for sulfur isotopic ratios by MC-ICP-MS (Lin et al., 2014). The obvious advantage of this method is that there is no need for chemical purification, thus eliminating the possibility of isotope fractionation during the ion chromatography and expediting sample throughput. In this study, we applied the matrix-matching method to δ^88/86Sr determination by MC-ICP-MS. NIST 987 Sr solution and a purified seawater sample (collected from the South China Sea) were selected for this study. Given that major matrices in carbonate come form Ca²⁺, NIST 987 and SW solutions containing 40 ppm Ca²⁺ were prepared by adding high-purity Ca solution. All solutions used contained 200 ppb Sr and the ⁸⁸Sr/⁸⁶Sr ratios were measured using a Neptune MC-ICP-MS. The purified SW was first determined using SSB method, in which pure NIST 987 was used as standard to bracket SW and yielded δ^88/86Sr value of 0.366 ± 0.008‰ (2SE, n = 10). The δ^88/86Sr values of Ca-bearing SW were then measured by using pure NIST 987 solution as the working standard to investigate matrix effects. The determined δ^88/86Sr value (0.039 ± 0.021‰; 2SE, n = 10) deviated obviously from the reference value. Finally, the matrix-matched NIST 987 was applied as the working standard to bracket the Ca-bearing SW, and the measured δ^88/86Sr value is 0.351 ± 0.009‰ (2SE, n = 10), consistent with the reference value within uncertainties. The consistent δ^88/86Sr values and comparable external precision suggested that the matrix-matching method is applicable to δ^88/86Sr determination when Ca²⁺ is the major matrix inference. However, its application to natural carbonate samples with more complicated matrices needs further exploration. [1] Lin et al. 2014, Rapid Commun. Mass. Sp. [2] Rüggeberg et al. 2008. Earth. Planet. Sc. Lett.