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.
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