Precise analysis of the 228Ra/226Ra isotope ratio for short-lived U-series disequilibria in natural samples by total evaporation thermal ionization mass spectrometry (TE-TIMS)

Abstract

We report a new analytical technique for precise and accurate determination of the ²²⁸Ra/²²⁶Ra ratio by total evaporation thermal ionization mass spectrometry (TE-TIMS). An improved chromatographic separation method employing a new tandem column technique that enables high yield and high purity of Ra is also described. Repeated analysis of Ra standard solution (²²⁸Ra/²²⁶Ra = 1.1) yielded an analytical reproducibility for ²²⁸Ra/²²⁶Ra ranging from 0.2% to 0.9% (2σ) for 170 to 2 fg of Ra. Reproducibility was strongly controlled by counting statistics of ²²⁶Ra⁺ and ²²⁸Ra⁺ ion beam intensities collected by an ion counting detector, implying that in-run precision for Ra isotope analysis of our method can be evaluated using a counting statistics law. Determination of ²²⁶Ra abundances in silicate rock samples were examined by isotope dilution TE-TIMS, and reproducibility was 0.3% (2σ) for JR-2 (²²⁶Ra = 3700 fg g⁻¹) and 0.6% (2σ) for JB-2 (²²⁶Ra = 81 fg g⁻¹); the reproducibility of these measurements are 2–3 times better than previously published conventional TIMS procedures. The ²²⁶Ra/²²⁸Ra ratio in natural samples can be also measured with an analytical uncertainty of ∼1% (2σ). The accuracy of our method was confirmed by measuring (²²⁶Ra/²³⁰Th) and (²²⁸Ra/²³²Th) ratios for JR-2, a sample old enough (1 Ma) to be in ²³⁰Th–²²⁶Ra and ²³²Th–²²⁸Ra radioactive equilibria. Our method is especially effective for samples with a very low abundance of Ra.