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[MTT44-10] 90Sr/88Sr ratio analysis with extremely low noise: An application of energy filtered-TIMS
Keywords:Strontium-90, mass spectrometry, radionuclide, 90Sr/88Sr ratio
Strontium-90 is a major anthropogenic radionuclide produced by fission of 235U and 239Pu. Because of the potential threat of long-term internal exposure, emission and subsequent diffusion of 90Sr into local environment due to nuclear reactor accidents should be quantitively monitored. Conventional radiometric 90Sr analysis requires relatively large sample size and long analysis time (e.g. 2 weeks), which are not adequate for urgent environmental survey. Recently, rapid and sensitive 90Sr analysis methods were successively developed utilizing various state-of-the-art mass spectrometric techniques, such as ICP-MS (e.g. Takagai et al., 2014), ICP-MS/MS (Ohno et al., 2018), TIMS (Kavasi and Sahoo, 2019), AMS (Sasa et al., 2020) and ID-TE-TIMS (Ito et al., 2020).
In this study, we demonstrate a novel 90Sr analysis method based on isotope ratio analysis using energy filtered-TIMS (Wakaki et al., 2022). Strontium isotope ratios were measured at Fukushima university using Triton Plus (Thermo Scientific). Stable Sr isotopes were collected with faraday cup detectors. 90Sr were detected by a secondary electron multiplier mounted with a set of RPQ lenses, which is an energy filtering device effective for both high and low mass side peak tailing. Approximately 1 h of measurement time with 88Sr ion beam of ca. 2.6 × 10-10 A requires sample amount of >100 ng of Sr.
Typical noise sources of mass spectrometric 90Sr measurement are isobaric interreference of 90Zr and peak tailing of the major 88Sr ion. With multiple noise reduction schemes, including the use of RPQ lenses, we have succeeded to lower the noise level on 90Sr to ca. 0.004cps level. Abundance sensitivity, defined as 90Sr/88Sr, of our method was estimated from the analysis of the 90Sr-free NIST SRM 987 reagent (blank sample) as 8.3 ± 1.8 × 10-12. Detection limit of 90Sr/88Sr ratio, defined as 3 σ of the blank analyses, was estimated as 2.7 × 10-12. With a minimum sample amount of 100ng of Sr, this detection limit of the 90Sr/88Sr ratio corresponds to a 90Sr activity of 0.0012 mBq. The 90Sr detection limit of our method is superior to the previous mass spectrometric 90Sr detection methods by two to three orders of magnitude. The accuracy of this method to measure the 90Sr/88Sr ratio on the 10-11 order was demonstrated by the analysis of biological samples from Fukushima and certified reference materials IAEA 156 and IAEA 330. High sensitivity and short measurement time of this method are suitable for urgent environmental survey and also for studying environmental diffusion of 90Sr using size-limited samples.
References: Ito et al., 2020, Anal. Chem. 92, 16058–16065; Kavasi and Sahoo, 2019, Anal. Chem. 91, 2964–2969; Ohno et al., 2018, J. Anal. At. Spectrom. 33, 1081–1085; Sasa et al., 2020, J. Nucl. Sci. Tec. 58, 72-79; Takagai et al., 2014, Anal. Methods 6, 355–362; Wakaki et al., 2022, Sci. Rep. 12, 1151.
In this study, we demonstrate a novel 90Sr analysis method based on isotope ratio analysis using energy filtered-TIMS (Wakaki et al., 2022). Strontium isotope ratios were measured at Fukushima university using Triton Plus (Thermo Scientific). Stable Sr isotopes were collected with faraday cup detectors. 90Sr were detected by a secondary electron multiplier mounted with a set of RPQ lenses, which is an energy filtering device effective for both high and low mass side peak tailing. Approximately 1 h of measurement time with 88Sr ion beam of ca. 2.6 × 10-10 A requires sample amount of >100 ng of Sr.
Typical noise sources of mass spectrometric 90Sr measurement are isobaric interreference of 90Zr and peak tailing of the major 88Sr ion. With multiple noise reduction schemes, including the use of RPQ lenses, we have succeeded to lower the noise level on 90Sr to ca. 0.004cps level. Abundance sensitivity, defined as 90Sr/88Sr, of our method was estimated from the analysis of the 90Sr-free NIST SRM 987 reagent (blank sample) as 8.3 ± 1.8 × 10-12. Detection limit of 90Sr/88Sr ratio, defined as 3 σ of the blank analyses, was estimated as 2.7 × 10-12. With a minimum sample amount of 100ng of Sr, this detection limit of the 90Sr/88Sr ratio corresponds to a 90Sr activity of 0.0012 mBq. The 90Sr detection limit of our method is superior to the previous mass spectrometric 90Sr detection methods by two to three orders of magnitude. The accuracy of this method to measure the 90Sr/88Sr ratio on the 10-11 order was demonstrated by the analysis of biological samples from Fukushima and certified reference materials IAEA 156 and IAEA 330. High sensitivity and short measurement time of this method are suitable for urgent environmental survey and also for studying environmental diffusion of 90Sr using size-limited samples.
References: Ito et al., 2020, Anal. Chem. 92, 16058–16065; Kavasi and Sahoo, 2019, Anal. Chem. 91, 2964–2969; Ohno et al., 2018, J. Anal. At. Spectrom. 33, 1081–1085; Sasa et al., 2020, J. Nucl. Sci. Tec. 58, 72-79; Takagai et al., 2014, Anal. Methods 6, 355–362; Wakaki et al., 2022, Sci. Rep. 12, 1151.