Introduction

The moment of history or dynamics of human and environment are precisely memorized as isotope ratios in variety of materials. Understanding the isotope ratio stem from radioactive isotope can realize an aspect of past condition and transition. Among the isotope ratio, that of strontium (Sr) reflects past aspect and/or a fact basing on radioactive decay or mass discrimination effect, and it widely serves indictors as age determination in geological sciences1, environmental dynamics2,3, forensic sciences4, authenticity judgment5, and determination of geographic origin6. In particular, the artificial nuclide, radioactive strontium (90Sr) records as the memory of nuclear disaster in materials such as Chernobyl or Fukushima Nuclear Power Plant Accidents7.

90Sr is a β-decay only radionuclide (0.55 MeV, 100% yield) and a typical fission product of 235U (5.8% accumulated fission yield) and 239Pu (2.1% accumulated fission yield). Anthropogenic 90Sr is widely spread over earth surface environments with low background radioactivities because of the use and testing of nuclear weapons8. Nuclear accidents are another source of anthropogenic 90Sr emissions into the environment, resulting in local fallout with high activity concentrations with 1.4–80.8 Bq/kg9. 90Sr was one of the major radioactive nuclides released in the Chernobyl10,11,12 and Fukushima9,13,14,15 nuclear reactor accidents. When taken up by animals, including humans, 90Sr has the potential to cause long-term internal exposure16,17. The slight trace of 90Sr is precisely memorized as an isotope ratio between stable isotope of Sr in variety of samples18. Therefore, a precise measurement the isotope ratio of 90Sr and stable Sr is important to determine the understanding of right dynamics of 90Sr in the local environment surrounding the accident site19.

Conventionally, radiometric methods using solid/liquid scintillators or gas ionization detectors were used to detect 90Sr in various environmental samples5,12,13,14,15. The radiometric method is fundamentally a counting experiment of the decaying atoms. Because 90Sr has a half-life of 28.79 years24, the fraction of the decaying atom is significantly small: approximately 1.3 × 109 atoms of 90Sr corresponds to 1 Bq (i.e. decay per second) of activity. Therefore, for precise radiometric 90Sr determination, a relatively large sample size or long analysis time is required12,17. For example, 100 g of soil samples are measured by nitrate precipitation-low background gas-flow counting method (nitrate precipitation-LBC) with 2 weeks9, and 1 L of seawaters are measured by gross beta radiometric counting with iron-barium co-precipitation method14. Because 90Sr is a β-particle emitter, radiometric determination of 90Sr includes complex radiochemical procedures and typically requires 2 weeks or more measurement time17,18. This slow sample processing speed is insufficient for an urgent environmental survey in response to a nuclear emergency27.

An alternative and less time-consuming method to analyze 90Sr is mass spectrometry. Inductively coupled plasma mass spectrometry (ICP-MS) is a new mass spectrometric technique for 90Sr detection27,28,29,30,31. Unlike radiometry, the target of mass spectrometric measurement is all the existing 90Sr atoms in the sample. The large difference in target numbers suggests that mass spectrometric methods are more sensitive for detecting 90Sr. However, measuring a minor isotope is a challenging task in mass spectrometry. Mass spectrometry’s sensitivity to detect trace 90Sr is hindered by high noise signals due to isobaric interferences of 90Zr and molecular ions, as well as peak tailing of the highly abundant 88Sr ions. ICP-MS’s major source of noise signals is 90Zr2,24,25. In environmental samples, the ratio between minor 90Sr and more abundant stable isotopes of Sr or isobaric isotope 90Zr is extremely high (i.e., at least nine orders of magnitude15). ICP ion source efficiently ionizes 90Zr, which remains in the sample solution in trace amounts even after Sr extraction chemistry. The reaction of the ions with O2 gas is used in a dynamic reaction cell (DRC) technique to reduce the 90Zr ion transmission2,24,25. Typically, this technique achieves abundance sensitivities, defined as the intensity ratio between 88Sr peak tail on m/z = 89.908 and 88Sr, on the order of 10−9 2,19. Furthermore, the introduction of a new technique, triple quadrupole ICP-MS (ICP-MS/MS), combined with the O2 gas reaction, effectively reduced the 90Zr ion transmission in the mass spectrometer and lowered the noise signal to 0.1 cps, which correspond to a 90Sr detection limit of 0.11 fg (0.6 mBq)28. With a sample size of 4 µg of Sr, the abundance sensitivity for the 90Sr/88Sr ratio achieved by this technique was 5 × 10−12 28. However, reducing the 90Sr detection limit with the ICP ion source is difficult. The highly efficient ICP ion source mainly produces abundant polyatomic or polyvalent ions from the sample solution’s solvents, Ar gas, and trace impurities in the sample solution, and such ions exist across the entire m/z range. It also emits Ar gas-related ions such as Ar+ and ArO+ at significantly high intensities, which may cause non-spectrum interference of the peak tails. All such interferences are increasing the noise level and limiting the sensitivity of ICP-MS analysis. Even with the ICP-MS/MS technique, the background signal was reported as 0.1–0.2 cps when aspirating a blank solution28, showing that noise signals of sub-cps level are inevitable while using the ICP ion source.

Thermal ionization mass spectrometry (TIMS) is a standard technique for measuring isotope ratios of Sr, such as 87Sr/86Sr and 88Sr/86Sr34. Compared with the ICP ion source, the thermal ionization ion source is energy-limited, and thus ionization is mostly limited to target elements with very few polyatomic ions. In TIMS, Ar-gas-related species, solvent-related species, and polyvalent ions are mostly absent. With less spectrum and non-spectrum interferences, TIMS can reduce noise signal levels on 90Sr and thus increase sensitivity upon 90Sr detection. Recently, several attempts have been made to detect 90Sr using the TIMS technique. Previous studies have failed to detect traces of 90Sr in environmental samples27,28. Kavasi et al. measured 90Sr/88Sr ratios of 90Sr-containing reference materials of wild berry and lake sediment using a sample size of 1000 ng of Sr and reports an abundance sensitivity for the 90Sr/88Sr ratio as 2.1 × 10−10, corresponding to a 90Sr noise signal of 0.77 cps37. However, when the 90Sr/88Sr ratio is lower, their measured 90Sr/88Sr ratio shows a systematic bias toward higher values. To account for such a significant bias, additional empirical “relative bias” errors must be introduced into the analytical uncertainty. This inaccuracy is obvious when the 90Sr/88Sr ratio is lower than 1.2 × 10−9 and is likely to be caused by an inaccurate noise correction scheme for the 90Sr signal. Ito et al. focused on analyzing small-sized samples and constructed an isotope-dilution total-evaporation (ID-TE-) TIMS technique to analyze 90Sr with a sample size of 5–20 ng of Sr38. Their study did not report abundance sensitivity, but it can be estimated as 2 × 10−8 based on the reported analytical conditions of 90Sr noise level of approximately 5 cps and 88Sr target intensity of 4 V38. Among the mass spectrometric 90Sr analysis methods, TIMS is the only method that has been confirmed by an independent IAEA proficiency test39. During the TIMS analysis, the ionization of 90Zr+ is suppressed, whereas Sr is still present and ionizing from the filament38 because of the difference in the evaporation and ionization potentials between the two elements. Therefore, the peak tailing of the highly abundant 88Sr ions is the main source of background signal for 90Sr in TIMS measurements. Peak tail ions can be efficiently reduced using an energy filtering device placed immediately before the detector. However, none of the previous 90Sr analyses used an effective energy filtering device to account for the 88Sr tailing35,36,37,38. Kavasi et al. used a WARP energy filter. However, they did note that the WARP energy filter eliminates low energy ions but has no effect on high-mass side peak tailing, implying that the WARP energy filter does not work for 88Sr peak tail on 90Sr. The ability of TIMS to detect a significantly small amount of 90Sr remains unknown.

In this study, we focused on bringing out the performance of modern TIMS instruments to perform an accurate 90Sr/88Sr measurement of environmental and biological samples with low 90Sr activity using an effective energy filtering technique for 90Sr detection. The Retarding Potential quadrupole (RPQ) lens act as high selectivity filter for ions with disturbed energy or angle40. The use of RPQ lenses for energy filtering coupled with reduction of the multiple noise signal sources resulted in a significantly low and highly stable noise signal for 90Sr compared with the previous studies. With significantly low and stable noise signals, an appropriate noise correction scheme was used in this study to allow accurate measurement of 90Sr/88Sr ratio down to 10−11 level. The measurement of 90Sr-containing reference materials (plants), as well as environmental and biological samples, demonstrates the ability of our technique.

Results and discussion

Noise reduction schemes upon 90Sr detection by energy-filtered TIMS

Peak tailing of 88Sr, which has the largest abundance among Sr isotopes, is the main limiting factor in TIMS detection of 90Sr. The RPQ lenses used in this study eliminate peak tailing from the high-mass and low-mass sides of the 88Sr peak (Fig. 1). The RPQ lens parameters must be fine-tuned to effectively eliminate the 88Sr peak tailing on 90Sr while sacrificing only a small amount of ion transmission rate and peak shape (Figure S1 in supporting information). Finally, after the introduction and fine-tuning of the RPQ lenses, a 1000-fold reduction of the 88Sr peak tail signal was achieved (Figure S2 in supporting information).

Figure 1
figure 1

Mass spectrum in the vicinity of m/z = 88 showing the flat-top peak of 88Sr measured by the center Faraday cup (black lines). The 88Sr peak tails on the low- and high-mass sides is also shown. Peak tails measured by SEM without the RPQ are shown in blue lines and peak tails measured by SEM with the RPQ are shown in red lines.

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Because the 88Sr peak tail signal has been reduced to approximately 0.01 cps on 90Sr, signals from other noise sources must also be controlled at this noise level. The filament material, rather than the sample-derived Zr, is the major source of the isobaric 90Zr signal in TIMS measurement. Typically, filaments were baked out at 4500 mA (approximately 2000 °C) before the use. With this level of bake out, about 1 cps and about 0.1 cps of the 90Zr+ signals were observed from Re and Re-ZR filaments, respectively, during Sr isotope ratio measurement even with the presence of Sr sample on the filament. These were not acceptable levels of noise in this study. Therefore, filaments were baked at higher temperatures at a 5.5 A filament current (approximately 2200 °C) to reduce the filament-derived Zr+ signal. After this high temperature bakeout, the Re and the Re-ZR filaments tend to show 90Zr+ signals on the 0.01 cps order when heated up to 1550 °C without the sample material. Even after this high temperature bakeout, a few Re and Re-ZR filaments still demonstrate higher 90Zr+ emission. Therefore, after the bakeout, all the filaments were inspected for 90Zr intensity to ensure that filaments with higher 90Zr+ emission were not used. The 90Zr intensity threshold at 1550 °C was empirically set to 0.04 cps. Note that the actual 90Zr intensity during Sr isotope measurement is negligible (far lower than 0.04 cps) because Zr+ emission is suppressed during the presence of Sr on the filament.

Ito et al. observed that organic material with m/z 90.0 is a significant isobaric interfering molecule on 90Sr, and this could be perfectly mass separated by slightly shifting the axial m/z from 89.908 to 89.77738. We detected another isobaric interference molecule spectrum at m/z 89.908 with intensities on the order of 0.1 cps or less (Figure S3 in supporting information). The intensity of this noise signal is relatively high at the beginning of the measurement and decreases with time (Figure S4 in supporting information), indicating that this noise signal is related to organics or volatiles. The most plausible molecule for this mass spectrum is 88SrH2+, with hydrogen derived from organics or residual H2O. This noise signal was eliminated by preheating of the sample filament under vacuum before the measurement and by avoiding the use of parafilm/catheter during sample loading.

Abundance sensitivity and detection limit of the 90Sr/88Sr measurement

The 90Sr-free NIST SRM-987 was measured using all these noise reduction schemes to validate the noise level and noise stability on 90Sr. The results of the three analytical sessions with slightly different analytical conditions are summarized in Table 1, Fig. 2, and Figure S5 in supporting information.

Table 1 Results of replicate measurements of NIST SRM 987 in three different mass spectrometric sessions.
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Figure 2
figure 2

Results of repeated 90Sr measurements of 90Sr-free NIST SRM987 in (a) Bremen, (b) Fukushima-1, and (c) Fukushima-2 sessions. White diamonds represent noise uncorrected data of a single measurement. Blue line and blue bands represent the average and 2 SD range of the multiple measurements. The grey area represents the average intensity of dark noise observed during each measurement session.

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The average intensity of the 90Sr noise signal of the three analytical sessions were 0.0118 ± 0.0029 (2SD, n = 4), 0.0168 ± 0.0034 (2SD, n = 13) and 0.0136 ± 0.0029 (2SD, n = 17) for Bremen, Fukushima-1st and Fukushima-2nd sessions, respectively. Note that these noise signals include dark noise counts because they were not corrected. Slightly higher noise intensity of Fukushima-1st compared with the Bremen and Fukushima-2nd reflects the fact that higher SEM operating voltage resulted in higher dark noise level in Fukushima-1st session (Table 1). The average dark noise count during the Fukushima-2nd session was 0.0096 cps, accounting for 70% of the total noise signal. This indicates that our noise reduction scheme has successfully reduced noise signals other than the detector’s intrinsic dark noise to the level of 0.004 cps (Fig. 2). The small variability of the noise signal indicates that the noise signals are well controlled at this level. In this study, the level and stability of the noise signal achieved are superior compared with the noise signals reported by non-energy filtered TIMS technique37 (0.77 ± 0.82 cps, 2SD) and by ICP-MS/MS technique28 (0.1 cps). Noted that measurement with such a low signal requires a long measurement time (1 h in this study) to count the substantial number of ions. The stable nature of the TIMS ion source allows long measurement with stable analytical conditions.

The abundance sensitivity and detection limit of the 90Sr/88Sr ratio can be determined from these data. Finally, the abundance sensitivity (90Sr/88Sr ratio) achieved in the Fukushima-2nd session was 8.3 ± 1.8 × 10−12 (2SD, n = 17; Table 1). A detection limit of a signal is defined by 3σ of the variability of the zero-point (or blank) analysis. Therefore, the detection limit of 90Sr/88Sr ratio finally achieved in the Fukushima-2nd session was estimated to be 2.7 × 10−12 (Table 1 and Figure S5c in supporting information). The fact that the detection limit is lower than the abundance sensitivity demonstrates TIMS’s excellent noise stability and its good control on 90Sr.

90Sr/88Sr measurement of the reference materials and natural samples

Table 2 summarizes the details of the analyzed environmental and biological samples, as well as the 90Sr activity parameters found in the literature (more details are shown in Table S1 in supporting information). As a 90Sr-free sample, a seawater reference material NASS-6 (Atlantic surface water) issued by the National Research Council Canada and a geochemical reference material JCp-1 (modern coral) issued by the Geological Survey of Japan were analyzed. The 90Sr-containing samples analyzed in this study include the International Atomic Energy Agency’s certified reference materials IAEA 156 (clover) and IAEA 330 (spinach). We also analyzed the ash from crayfish and smallmouth bass samples collected in Fukushima prefecture, Japan, after the Fukushima Daiichi Nuclear Power Plant accident. The 90Sr activity of these samples was analyzed using a radiometric method and published elsewhere20.

Table 2 90Sr activity of the environmental and biological samples and results of Sr abundance measurement.
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Table 2 also summarized the measured Sr abundance of the 90Sr containing samples. The decay corrected 90Sr/88Sr reference value of these samples was calculated for each analytical session using this measured Sr abundances together with either the certified 90Sr activity, for IAEA 156 and IAEA 330, or the radiometrically measured 90Sr activity, for Fukushima samples, respectively.

The results of 90Sr/88Sr ratio measurements of the 90Sr containing samples are summarized in Table 3 (more details are shown in Tables S2 and S3 in supporting information), Fig. 3, and Figure S6 in supporting information. The average 90Sr/88Sr ratios of the 90Sr-free samples were − 0.1 ± 2.5 × 10−12 (2SD, n = 5), 0.3 ± 2.9 × 10−12 (2SD, n = 8) and 0.5 ± 3.4 × 10−12 (2SD, n = 5) for NASS-6, JCp-1 (Fukushima-1) and JCp-1 (Fukushima-2), respectively. These results show that zero values of the 90Sr/88Sr ratios were measured without any systematic bias. No effect of sample-derived 90Zr was observed down to 0.004 cps level. These results demonstrate the excellent stability of the noise signal, as well as the robustness of our noise correction scheme.

Table 3 Results of 90Sr/88Sr measurements of the environmental and biological samples.
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Figure 3
figure 3

The measured 90Sr/88Sr ratio of the environmental and biological samples was compared with the decay corrected reference 90Sr/88Sr ratios. The blue circle represents an average of multiple measurements of the sample. Error bars correspond to 2 SD.

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The measured 90Sr/88Sr ratios of the IAEA 330 were 6.7 ± 1.3 × 10−11 (2SD, n = 4) and 5.76 ± 0.43 × 10−11 (2SD, n = 5) for temporally separated Bremen and Fukushima-2nd sessions. Both values agree well with the decay corrected reference 90Sr/88Sr ratios within analytical errors. The measured 90Sr/88Sr ratios of the IAEA 156 were 3.26 ± 0.30 × 10−11 (2SD, n = 6) and it agrees well with the decay corrected reference value within analytical errors. The measured 90Sr/88Sr ratios of the Ash1 (crayfish sample) was 1.38 ± 0.53 × 10−11 (2SD, n = 6) and 1.32 ± 0.42 × 10−11 (2SD, n = 6) for Fukushima-1st and Fukushima-2nd sessions, respectively. The measured 90Sr/88Sr ratios of the Ash2 (smallmouth bass sample) were 1.40 ± 0.39 × 10−11 (2SD, n = 6) and 1.33 ± 0.27 × 10−11 (2SD, n = 6) for Fukushima-1st and Fukushima-2nd sessions, respectively. These measured values agree well with the decay corrected reference 90Sr/88Sr ratios within analytical errors (Table 3). No systematic bias exceeding the analytical uncertainties is observed for these results. All these measurements demonstrate the analytical capability of the energy-filtered TIMS technique, as well as our Sr separation chemistry to measure 90Sr/88Sr ratios of 10−11 range (and possibly in the 10−12 range) in natural environmental and biological samples (Fig. 4).

Figure 4
figure 4

Stable Sr and 90Sr abundances of the analyzed samples (blue circles) were compared with the 90Sr/88Sr detection limit (red line) of this study. The 90Sr/88Sr detection limit of the previous TIMS studies was plotted for comparison. Stable Sr abundance range of selected environmental samples is also plotted: plants, seawater, and river water/groundwater.

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Performance and applicability of 90Sr analysis by energy filtered TIMS

Among the environmental and biological samples analyzed, Sr ion beams run short before 220 cycles in some of the measurements with a sample amount of 100 ng of Sr. Nevertheless, 100 ng measurements mostly show precision indistinguishable with the measurement of larger sample sizes. From these observations, we defined that the minimum amount of sample required for our 90Sr/88Sr measurement as 100 ng of Sr. This is one of the essential parameters to evaluate the performance of TIMS measurement. Given the minimum sample size of 100 ng of Sr, and the 90Sr/88Sr ratio detection limit of 2.7 × 10–12, the detection limit of the absolute 90Sr amount can be estimated as 0.23 ag or 0.0012 mBq. The previous TIMS 90Sr measurements have reported a 90Sr detection limit of 0.17 fg (or 0.88 mBq)37 and 0.029 fg (or 0.15 mBq)38. Our technique has succeeded to lower the 90Sr detection limit of TIMS by three orders of magnitude.

Based on the applicability of this technique to environmental and biological sample measurements, detectable 90Sr activity concentrations in this study are different among samples with different stable Sr abundances (Fig. 4). This is because the TIMS technique measures isotope ratios and thus the detection limit of the 90Sr analysis is primarily determined as 90Sr/88Sr ratio. For example, seawater is one of the typical environmental samples and has a stable Sr abundance of approximately 8 mg/kg. With this energy-filtered TIMS technique, the detection limit of seawater 90Sr activity concentration is estimated as 0.09 Bq/kg. The minimum quantity of the seawater sample required for the measurement is as small as 12.3 µL. River water and groundwater have stable Sr abundances range from several tens to several hundreds of ppb. In this case, the detection limit of 90Sr activity concentration will be as low as 0.1 mBq/kg. A sample size of 10 g will be required to analyze such a dilute sample.

Comparing the capability of 90Sr analysis between TIMS and ICP-MS is not straightforward. The capability of ICP-MS is limited by the detection limit of the absolute amount of 90Sr and the abundance sensitivity defined as the 90Sr/88Sr ratio (Fig. 5). It is not limited by the amount of stable Sr unless the 90Sr/88Sr ratio was interfered with the abundance sensitivity limit. However, the capability of TIMS is limited by the detection limit of 90Sr/88Sr ratio and the minimum amount of sample required for the analysis (Fig. 5). The latter is required to keep the fixed ion beam intensity of 88Sr during the measurement because the performance of the 90Sr/88Sr ratio measurement by TIMS is mainly determined by the ion beam intensity of the most abundant 88Sr. The detection limit of the absolute amount of 90Sr is a derivative of these two parameters for TIMS. Compared with the previous TIMS and ICP-MS/MS techniques, a significant reduction in the 90Sr/88Sr detection limit achieved in this study had widely expanded the applicability of this technique to analyze samples with low 90Sr contents (Fig. 5). Additionally, an order of magnitude reduction of the minimum sample amount compared with the previous TIMS technique allows the analysis of 10 times smaller sample sizes. These features are advantageous in analyzing trace amounts of 90Sr on size-limited environmental samples such as soil exchangeable fraction and eolian dust as well as biological samples with low Sr abundances.

Figure 5
figure 5

The capability of the mass spectrometric 90Sr determination methods is demonstrated using the absolute amount of stable Sr and 90Sr required for a single measurement. Samples within the shaded area are measured by the method. The lowest point of the shaded area corresponds to the detection limit of 90Sr for TIMS.

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Methods

Reagents and standards

For sample preparation, ultrahigh purity grade acids, HNO3, HCl, HClO4, and HF (TAMAURE AA-100; Tama Chemicals Co., Ltd, Japan.), as well as ultra-pure water produced by a Milli-Q Element system (Millipore, USA) were used. The Sr isotopic reference material SRM 987 issued by NIST was used as a 90Sr-free reference standard.

Sample preparation

The IAEA reference material’s certified 90Sr activity values were calculated using their dry weight. Therefore, these materials were weighed and digested as they were. Concentrated HNO3 and HClO4 acids were used to digest IAEA 156 and IAEA 330. The Fukushima samples were analyzed on an ashed sample, which were used in the previous 90Sr activity analysis20. The ashed crayfish and smallmouth bass samples were weighed and digested successively using 2.4 M HCl and conc HNO3. JCp-1 was digested with 5% CH3COOH. Finally, all the samples were dissolved in 3 M HNO3. NASS-6 was mixed with 6 M HNO3 to make a 3 M HNO3 solution.

Sr was separated from the other elements, including Zr, using extraction chromatography with Sr Resin (Eichrom Technologies Inc., USA) using a handmade PTFE column of 0.2 mL volume34. After sample solution loading, the column was rinsed with 2 mL of 6 M HNO3 and 0.5 mL of 3 M HNO3 successively, and Sr was eluted with 2 mL of 0.05 M HNO3. Finally, the separated Sr fraction was reacted with one drop of concentrated HNO3 to decompose resin-derived organics. The total yield of the Sr separation chemistry was approximately 94%34. After the Sr separation chemistry, the Zr/Sr ratio was reduced to 1.2 × 10–5 times the original ratio of the sample.

Quantitative analysis of Sr and the reference 90Sr/88Sr ratio of the samples

The Sr concentrations of the 90Sr containing samples (IAEA 156, IAEA 330, crayfish, and smallmouth bass) were measured using ICP-MS Agilent 7700 (Agilent Technologies, Japan) and a sample of a fraction of the sample solution at Kochi Institute for Core Sample Research, JAMSTEC. The measured Sr concentration was used to calculate the reference 90Sr/88Sr values.

Instrumentation

In this study, two thermal ionization mass spectrometers were used, the Triton™ XT (Thermo Scientific™) at Thermo Fisher Scientific, Bremen, Germany, and the Triton™ Plus (Thermo Scientific™) at Fukushima University, Japan. Both instruments are equipped with eight movable Faraday cups, an axial Faraday cup, and an axial secondary electron multiplier (SEM). The Faraday cups are connected with the standard 1011 Ω amplifiers. A set of RPQ lenses, which is an energy filtering device, is equipped in front of the SEM detector. The configuration of the detectors was summarized in Table S4 in supporting information. The peak position of 88Sr and 90Sr with this collector setting was shown in Figure S7 in supporting information.

This study uses both Re and Re-ZR (rhenium zone-refined) single filaments. High temperature bakeout of the filaments at 5.5 A filament current were performed under high vacuum for 30 min (Bremen) or 60 min (Fukushima). In Fukushima, the baked filaments are further inspected for 90Zr emission. 90Zr intensity of the baked filaments is measured at filament temperature of approximately 1550 °C for 1200 s. Filaments with 90Zr intensities higher than 0.04 cps are disposed of at this point.

90Sr/88Sr ratio measurement by TIMS

The separated sample containing 100–400 ng of Sr was loaded onto a baked filament with a Ta activator. The amount of Sr in the samples was fixed just before the Sr separation. The actual sample amount on the filament is lower than this nominal amount because losses of sample during the Sr separation and sample loading are not included in this number. In Fukushima, dull-red heating during sample loading was avoided because the sample becomes fragile and tends to be partially lost from the filament surface. Maximum Sr ion yield of the analytical condition in Fukushima, estimated by loading 108 ng NIST SRM987 on the filament and keeping the 88Sr signal of 25 V until sample exhaustment, was 2.1% (Table S5 in the supporting information). In both laboratories, Sr isotopes are measured using static multicollection mode with the slightly shifted axial mass settings of 89.8777 to eliminate the isobaric interference of organic molecules on m/z 90.038. In Bremen, filaments were initially heated to 2400 mA at a rate of 300 mA/min while measuring a 5-min baseline for Faraday cup detectors. The filament current increased at 100 mA/min until an 88Sr signal of 20 V was achieved. After a 1-min wait, the filament current was re-adjusted and the measurement was initiated. Ion beam integration of 4.194 s was repeated for approximately 1 h (10 blocks, 86 cycles per block). During the measurement, the filament current was adjusted to keep the 88Sr signal within 20 ± 4 V. In Fukushima, filaments were preheated at 2250 mA for 30 min before the measurement. During the measurement, filaments were heated to 1500 mA at a rate of 1500 A/min and then to 2250 mA at a rate of 214 mA/min. The filament current was increased at 100 mA/min until an 88Sr signal of 25 V was achieved. Ion beam integration of 16.777 s was repeated for approximately 1 h (22 blocks, 10 cycles per block). During the measurement, the inter-block filament current adjustment kept the 88Sr signal at ± 2.5 V. The Faraday cup baseline was measured using a long baseline method utilizing the very stable nature of the Faraday cup noise level of TRITON41,42. A 1 h baseline was measured for every 10 h of the measurement sequence. After the first analytical session in Fukushima, the operating voltage of the SEM detector was adjusted, resulting in slightly different analytical conditions between the two sessions. In both laboratories, FC-SEM yield was calibrated just before the measurement using 84Sr ion beam of ca. 5 × 10−14 A (ca. 3 × 105 cps). Both the RPQ transmission efficiency and the SEM counting efficiency was corrected by this FC-SEM yield. Typical vacuum condition of the analyzer chamber during the measurement were 1.5 × 10−9 mbar and 4.5 × 10−9 mbar for Bremen and Fukushima instruments, respectively. The dark noise of the SEM detector was measured for 1 h along with the Faraday cup baselines. However, all the measurements are not corrected for dark noise. Instead, the average value of multiple 90Sr-free NIST 987 measurements was used to correct overall noise signals on 90Sr. The analytical error of the 90Sr/88Sr ratio was determined as the 2σ internal error, which is two times the standard error of the 90Sr/88Sr ratios of 220 or 860 cycles. In all the measurements, the analytical error of the 90Sr/88Sr ratio was comparable with the √n counting error of the accumulated 90Sr count before noise correction.

Instrumental isotope fractionation is a possible error source of the 90Sr/88Sr ratio measured using mass spectrometry. However, the variability of the average 88Sr/86Sr values caused by the instrumental mass fractionation during the measurement was ± 0.5% (88Sr/86Sr values from 8.31 to 8.41) and was negligible compared with the analytical error of 90Sr/88Sr ratio (> 10%, 2RSD) in this study. Therefore, no correction for instrumental isotope fractionation was made.

Concentration of Zr in NIST 987 and Ta-activator solutions were measured ICP-MS with a dilution ratio of 10,000. For both reagents, Zr concentrations were under detection limit, which limits the Zr concentration of the original solutions as < 1.6 × 102 ppb.

Noise correction scheme for 90Sr signal

The stable nature of the noise signal allows precise noise correction on 90Sr measurement. Our noise correction scheme is as follows. First, the average 90Sr noise signal value was determined using multiple measurements of 90Sr-free NIST SRM-987. Then, the value is subtracted from 90Sr intensities of every measurement cycle in the run. All the data in the measurement were re-calculated off-line using the noise corrected 90Sr intensities, and finally, the noise corrected 90Sr/88Sr ratios are determined. The correction value should be determined every analytical session because the noise intensity level is sensitive to the SEM parameters and analytical conditions. A single correction value is used for all measurements within the analytical session.

Conclusions

This study demonstrates the ability of a TIMS instrument to perform an accurate 90Sr/88Sr ratio measurement of samples with 90Sr/88Sr ratios as low as 2.7 × 10−12. Not only the introduction of effective hardware such as the RPQ lenses but a thorough investigation and elimination of noise signals have resulted in a constantly low noise signal of 90Sr on the 0.001 cps order. The sample throughput of this technique was 40 samples per day for Sr separation chemistry and 21 samples per 2 days for isotope ratio measurement. This is not as fast as the ICP-MS techniques but is significantly faster than radiometric methods. To detect 0.0012 mBq of 90Sr, this energy-filtered TIMS technique requires a sample size of 100 ng of Sr. This sample size corresponds to 12.3 µL of seawater or less than 10 mg of biological samples with Sr abundances larger than 10 mg/kg, which seems easy to handle. The lowered 90Sr detection limit and smaller sample size of this technique are suitable for studying environmental diffusion of radioactive materials, as well as environmental elemental cycling studies using nuclear test origin 90Sr in the earth surface materials.