Application note | 000974

TIMS

Measuring 90Sr abundances in environmental samples byThermal Ionization Mass SpectrometryAuthors

Introduction

Shigeyuki Wakaki¹, Jo Aoki², Ryoya

There are four stable isotopes of strontium with different isotopic abundances: 84Sr

Shimode², Katsuhiko Suzuki³, Takashi(0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%). In some environments, such as sites

Miyazaki⁴ Satoshi Sasaki⁵ and Yoshitaka

that have experienced radioactive contamination from nuclear accidents or weapons

Takagai2,6, Jenny Roberts⁷, Hauke

testing, radioactive strontium (90Sr) can be found, with a half-life of 28.8 years. TheVollstaedt⁷

monitoring of 90Sr is important from a human perspective because when contaminated¹ Kochi Institute for Core Sample Research,
Japan Agency for Marine-Earth Sciencefood or water is ingested, 90Sr is concentrated primarily in bones and bone marrow,
resulting in bone cancer, cancer of nearby tissues and leukemia.

and Technology (JAMSTEC), Japan

Typically, the abundance of 90Sr has been measured using radiometric techniques,² Faculty of Symbiotic Systems Science,such as liquid scintillators or gas ionization detectors. The major disadvantage of thisFukushima University, Japantechnique is the total analysis time (5-20 days)¹. In recent years, mass spectrometry³ Submarine Resources Research Center,has provided a means to dramatically reduce analysis time, improving sample through-JAMSTEC, Japan

put2,3. However, the peak tailing from the highly abundant 88Sr has limited the ability of

⁴ Volcanoes and Earth’s Interior Research

conventional mass spectrometry to be able to resolve samples with low radioactivity.

Center, JAMSTEC, Japan

Typical abundance sensitivities for the 90Sr/88Sr ratio (Figure 1) for inductively coupled

⁵ Thermo Fisher Scientific, Tokyo, Japan

plasma mass spectrometry with a dynamic reaction cell (ICP-DRC-MS) was on the⁶ Institute of Environmental Radioactivity,

order of 10 -9 [3], whereas thermal ionization mass spectrometry (TIMS) was on the order

Fukushima University, Japan.

of 10 -10 [4]. However, in their study, Kavasi et al.⁴, used a TIMS with a WARP energy

⁷Thermo Fisher Scientific, Bremen, Germany

filter, which has no effect on peak tailing on the high mass side of the peak. Here, weKeywords
Abundance sensitivity, radioactive
strontium, strontium-90, RPQ,strontium isotopes, TIMS

demonstrate the ability of a different technology – the Retarding

The dried separated sample was re-dissolved in 2% HNO3

Quadrupole Lenses (RPQ) of the Triton Series TIMS – to precisely

solution and loaded in 100-400 ng Sr samples onto rhenium

and accurately measure 90Sr/88Sr ratios of 9.1x10 -12. This enables

zone-refined (Re-ZR) filaments. The Re-ZR had previously

samples with very low activities to be measured. We demonstrate

been degassed at 5.5 A for 30 min under vacuum. This current

the capability of the Triton Series TIMS to measure low activity

was chosen to significantly reduce the 90Zr interference, whichsamples by measuring a range of environmental samples withappears at higher filament temperatures (Figure 2). When thea modern activities ranging between 14.8 and 110 Bq/kg. Data

filament temperature is increased to 1790 °C (ca. 3.5 A filament

shown here is published in Wakaki et al. (2022)⁵.

current), the 90Zr peak can be observed, however at thistemperature Sr is almost entirely consumed. For measurementsof 90Sr, it is recommended to measure at filament temperatures in
the range of 1500-1600 °C.

Figure 1. Schematic to illustrate abundance sensitivity.
Abundance sensitivity = 90Sr/88Sr for a non-radioactive sample (i.e.
contains no 90Sr).

Instrumentation
Two thermal ionization mass spectrometers were used, theTriton™ XT (Thermo Scientific™) at Thermo Fisher Scientific,
Bremen, Germany, and the Triton™ Plus (Thermo Scientific™) at
Fukushima University, Japan. Both instruments were equippedwith a Retarding Potential Quadrupole lens (RPQ) in front of thecentral Secondary Electron Multiplier (SEM) to reduce peak tailingfrom more abundant masses.

Figure 2. Comparison of Re-ZR filaments that have beendegassed at 4.5 A (blue points) and 5.5 A (red points). A nonradioactive sample (SRM® 987) was loaded onto the filaments andmeasured using the conventional measurement mode (at 4 V, 20 V or 40
V 88Sr) for 1 hour. The x-axis shows the final filament temperature at theend of the 1 hr measurement. Note that these data have not been darknoise corrected.

Sample preparation
The IAEA reference material’s certified 90Sr activity values were

Mass spectrometry methods and discussion

calculated using their dry weight. Therefore, these materials wereIn Bremen, filaments were heated to 2400 mA at a rate

weighed and digested as they were. Concentrated HNO₃ andof 300 mA/min. During this time, a 5 minute baseline was

HClO₄ acids were used to digest IAEA 156 and IAEA 330. The

measured. After which, the filament temperature was increased

ashed crayfish and smallmouth bass samples were weighed andto 3000 mA at 100 mA/min to achieve a 20 V signal on 88Sr. After

digested successively using 2.4 M HCl and conc HNO₃. All thea wait of 1 minute and re-adjustment of the filament temperaturesamples were dissolved in 3 M HNO₃. Sr was separated from theto achieve 20 V, the measurement was started. Samples wereother elements, including Zr, using extraction chromatographymeasured using conventional measurement mode at 20 V (88Sr)

with Sr Resin (Eichrom Technologies Inc., USA) using a handmade

for 1 hour (10 blocks, 86 cycles/block, 4.194 s integration time).

PTFE column of 0.2 mL volume⁶. After sample solution loading,A lens focusing was carried out every 3rd block. The filament

the column was rinsed with 2 mL of 6 M HNO₃ and 0.5 mL of 3 M

temperature was modulated to keep the 88Sr signal within

HNO₃ successively, and Sr was eluted with 2 mL of 0.05 M HNO₃.

20 ± 4 V.

Finally, the separated Sr fraction was reacted with one drop ofconcentrated HNO₃ to decompose resin-derived organics. The
total yield of the Sr separation chemistry was approximately 94%⁶.
After the Sr separation chemistry, the Zr/Sr ratio was reduced to1.2 × 10 –5 times the original ratio of the sample.
2In 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 mA/min
and then to 2250 mA at a rate of 214 mA/min.5 The filamentcurrent was increased at 100 mA/min until an 88Sr signalof 25 V was achieved. Samples were measured using
conventional measurement mode at 25 V (88Sr) for 1 hour (22blocks, 10 cycles/block, 16.777 s integration time). During themeasurement, the inter-block filament current adjustment kept
the 88Sr signal at ± 2.5 V.
The collector configuration for both setups is shown in Table 1.
Table 1. Collector configuration for the measurement ofradioactive strontium.
Cup

L3

Isotope86

L2

Detector (R)F (10 Ω)

Sr87
11L1

Sr88

F (10 Ω)11C

Sr90

F (10 Ω)11Sr (89.878 amu)

RPQ-SEM

Results
Abundance sensitivity
Samples of the non-radioactive NIST strontium carbonate
isotopic standard SRM® 987 were measured to determine theabundance sensitivity on 90Sr of the Triton Series TIMS equippedwith an RPQ.
The effect of the RPQ on peak tailing was observed by scanning
across the mass range 88-90.075 amu of a non-radioactive
sample SRM 987 for 2 hours (a total measurement time of 1.5
minutes on each data point) (Figure 3). It is clear that the tailingfrom the 88Sr is effectively removed by 89.878 amu (the axialmass during static measurement). Slightly higher backgrounds

Figure 3. (A) Mass scan of non-radioactive Sr (SRM 987)
integrating the signal in C and L1 for 2 hours. Black vertical line
shows the magnet position during static measurements. (B) Zoom inof same mass scan between 89.5 and 90.075 amu (axial mass) showsabsence of 88Sr peak tailing at 89.878 amu (position of 90Sr signal).

observed in the SEM between 89.1-89.7 are caused by scatteringof ions as the ions are not falling into a cup. Across the range
89.7-90 amu, all ions are collected in cups and the backgroundsignal measured in the SEM is extremely low (< 0.01 cps). By
contrast, the signal on 88Sr is > 11 orders of magnitude greater.
The abundance sensitivity (90Sr/88Sr) achieved by the Triton Series
TIMS equipped with RPQ during static measurements of SRM
987 at 20 V 88Sr was 9.1x10 -12 ± 2.6x10 -12 (2 SD, n=34)(Figure 4). Note that this value is not dark noise corrected.
The limit of detection (LOD) is defined as 3*σ (90Sr/88Sr ratio (darknoise corrected ratio)) = 2.9x10 -12. Using this LOD, it is possible todetermine the minimum activity for environmental samples withvarying concentrations of stable strontium (Figure 5):

Min. 90Sr resolvable =Figure 4. Abundance sensitivity of non-radioactive Sr (SRM987) across three different analytical sessions. Each data point
represents a 1 hour measurement at 20 and 25 V on 88Sr. Error bars
show internal precision (2 SE). Note that these data are not dark noisecorrected).

LOD * [Sr] * 88Sr/SrTOT * 0.001
ArSr

Min. Activity Resolvable =Min. 90Sr resolvable * NA * ln(2)T1/2_Sr

Where “min. 90Sr resolvable” is in mol/kg, LOD is 2.9x10 -12, [Sr]is in mg/kg, 88Sr/SrTOT is 0.8258, ArSr is 87.62 u, “Min. ActivityResolvable” is in Bq/kg, NA is 6.022x1023 mol-1, T1/2_Sr is 28.79 yr x
3.15x10⁷ s/yr.

3

Environmental samples
A number of different radioactive environmental samples ofknown activity were measured to determine the 90Sr/88Sr ratio(Figure 6). These measurements were made on both the Bremen
and Fukushima instruments across 3 analytical sessions (2 yearsapart).
There is good correlation between the measured 90Sr/88Sr of thesamples and the decay-corrected reference value. The variabilityof the 90Sr/88Sr in the IAEA-330 samples might be reflective of thelarger variability in the 90Sr/88Sr of the reference material.
When converting the internal precision of each measurement toRSD, it is clear that IAEA-330 has one of the lowest RSDs of all
the samples measured (Figure 7). For all samples, the internal
precision is very close to the theoretical limit based on counting
statistics, demonstrating the excellent precision obtainable with
the Triton Series TIMS.

Figure 5. Determination of the minimum activity resolvablewith a Triton XT with RPQ (black line) for varying strontium
concentrations. The 90Sr/88Sr detection limit of the previous TIMS
studies⁴ (grey dashed line) and typical environmental samples⁷ (decaycorrected to January 2020) are shown for comparison.

Figure 6. Determination of the 90Sr/88 Sr ratio for a range of environmental samples. The decay corrected reference value(corrected to the measurement date) is shown in grey. Different analytical sessions are separated here due to the difference in the90
Sr/88Sr owing to radioactive decay over time.

4References
1. Hou, X. & Roos, P. Critical comparison of radiometric and mass spectrometric methods
for the determination of radionuclides in environmental, biological and nuclear wastesamples. Analytica Chimica Acta vol. 608 105–139 (2008).
2. Bu, W. et al. The role of mass spectrometry in radioactive contamination assessment
after the Fukushima nuclear accident. Journal of Analytical Atomic Spectrometry vol. 33
519–546 (2018).
3. Feuerstein, J., Boulyga, S. F., Galler, P., Stingeder, G. & Prohaska, T. Determinationof 90Sr in soil samples using inductively coupled plasma mass spectrometry equippedwith dynamic reaction cell (ICP-DRC-MS). Journal of Environmental Radioactivity 99,1764–1769 (2008).
4. Kavasi, N. & Sahoo, S. K. Method for 90Sr Analysis in Environmental Samples Using
Thermal Ionization Mass Spectrometry with Daly Ion-Counting System. AnalyticalChemistry 91, 2964–2969 (2019).
5. Wakaki, S. et al. A part per trillion isotope ratio analysis of 90Sr/88Sr using energy-filteredthermal ionization mass spectrometry. Sci Rep 12, 1151, 1–10 (2022).
6. Wakaki, S., Obata, H., Tazoe, H. & Ishikawa, T. Precise and accurate analysis of deepand surface seawater Sr stable isotopic composition by double-spike thermal ionization
mass spectrometry. Geochemical journal 51, 227–239 (2017).
Figure 7. Cross-plot of relative standard deviation of 90Sr/88 Sr
ratio for each filament against the theoretical limit based on
counting statistics.

7. Ohno, T., Hirono, M., Kakuta, S. & Sakata, S. Determination of strontium 90 inenvironmental samples by triple quadrupole ICP-MS and its application to Fukushimasoil samples. Journal of Analytical Atomic Spectrometry 33, 1081–1085 (2018).

Conclusion
The RPQ feature of the Triton Series TIMS improves theabundance sensitivity by reducing peak tailing on both the highand low mass side of the peak. Replicate measurements of
non-radioactive Sr demonstrate that an abundance sensitivity(90Sr/88Sr) of 9.1x10 -12 can be achieved. This theoretically enables
samples with a low 90Sr activity to be measured with a detection
limit of 2.9x10 -12 (90Sr/88Sr ratio).
The ability of the Triton Series TIMS coupled with the RPQ tomeasure low levels of 90Sr has been demonstrated throughmeasurements of a range of environmental samples. Theseexperiments show that the 90Sr can be accurately and precisely
determined, with precisions close to the limit of counting
statistics.

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