Precise and accurate analysis of deep and surface seawater Sr stable isotopic composition by double-spike thermal ionization mass spectrometry

抄録

This paper describes an analytical technique for highly precise and accurate determination of radiogenic and stable Sr isotopic composition using double-spike thermal ionization mass spectrometry. Repeated analyses of the in-house isotopic reference Wako-9999 against NIST SRM-987 gave an average δ⁸⁸Sr of +0.32‰, with a long-term 2 SD external reproducibility of ±0.02‰ (n = 15). Detailed evaluation of the Sr isotope fractionation behavior during a column chromatographic Sr separation process using Sr spec resin showed systematic variation of δ⁸⁸Sr in the eluate, from +1.05‰ to –0.64‰, as the Sr elution progressed. The Sr isotope fractionation factor between the Sr spec resin and 0.05M HNO₃ was estimated as 0.999947 ± 0.000001. During the chemical separation procedure, a very small amount of Sr with a highly fractionated isotopic composition was found to be lost in the sample loading and purification stage prior to Sr collection. This may cause a small but significant systematic mass bias in high-precision non-double-spike analyses. The analysis of 11 seawater samples from the Pacific and Atlantic oceans, including four deep seawater samples taken below the carbonate compensation depth (CCD) in the North Pacific Ocean, gave consistent results, with an average δ⁸⁸Sr of 0.407 ± 0.012‰ (2 SD). All analyzed data agreed with literature values, demonstrating the accuracy of the stable Sr analysis in this study. The δ⁸⁸Sr values of deep seawater, which were consistent with those of surface water, showed that seawater stable Sr isotopic composition is homogeneous to depths below the CCD.