Article Cite This: Anal. Chem. XXXX, XXX, XXX−XXX
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Method for 90Sr Analysis in Environmental Samples Using Thermal Ionization Mass Spectrometry with Daly Ion-Counting System Norbert Kavasi and Sarata Kumar Sahoo* Fukushima Project Headquarters, National Institutes for Quantum and Radiological Science and Technology (QST) 4-9-1, Anagawa, Inage-ku, Chiba, 263-8555, Japan
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ABSTRACT: In this work, a new 90Sr analysis method was developed using the Isotopx Ltd., Phoenix X62 thermal ionization mass spectrometer (TIMS). Excellent ion beam sensitivity was demonstrated with the detection of 1 mBq (0.2 fg) 90Sr on a Daly ion-counting system. The abundance sensitivity for the 90Sr/88Sr ratio was 2.1 × 10−10, and this could ensure measurement of 100 Bq·kg−1 (19 fg·g−1) 90 Sr in an environmental sample with 100 μg·g−1 stable strontium concentration. For analytical method validation, 90Sr was determined in two certified reference materials, for example, wild berry (IRMM-426) and freshwater lake sediment (NIST-4354), for the first time in the history of TIMS. This mass spectrometry method is faster than conventional radiometric techniques; however, interference from 90Zr and peak tailing on the higher mass side from 88Sr must be considered for a reliable 90Sr determination.
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the detection limit to the ultratrace level (fg range).2,4 However, ICP-MS methods must consider (1) isobaric interference of 90Zr on 90Sr, which cannot be removed by mass resolution but requires complete chemical separation of Zr from Sr; (2) multiple polyatomic interferences generated by the reactions of gases in the plasma (e.g., oxides, hydrides); and (3) peak tailing disturbance from stable 88Sr which is the highest abundant Sr isotope. In environmental samples, both 90Zr and 88Sr are present at high concentrations. For example, 90Zr concentration is around 34 μg·g−1 and 88Sr is around 80 μg·g−1 in IAEA-375 reference soil material, while the actual 90Sr concentration is around 11 fg·g−1 (58 Bq·kg−1, decay corrected to November 15, 2017).5 For practical purposes, a decontamination factor of 3.1 × 1010 for 90Zr removal (accepting just a 10% contribution) and abundance sensitivity of 1.4 × 10−10 on the higher mass side of 88 Sr are required. To achieve a reliable 90Sr determination in environmental samples with low natural quantities of stable Sr (plants, water, etc.), instrumental abundance sensitivity on the order of 10−9 or higher is a minimum requirement.6 Collision and reaction cell plasma source mass spectrometers, for example, ICP-DRC-MS and ICP-QQQ-MS are the most commonly used mass spectrometry instruments for 90Sr determination.3,5,7−9 They have been applied to remove 90Zr, which overlaps 90Sr by using O2 reaction in the collision/ reaction cell and forming ZrO. Under this condition, 90Sr
trontium (Sr) comprises four stable isotopes with different isotopic abundances: 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%), and 88Sr (82.58%). In the environment, 90Sr also exists as a radioactive Sr isotope (T1/2 = 28.8 y) with an anthropogenic origin. 90Sr is a pure beta emitter produced in nuclear reactors and a potential environmental contaminant because of nuclear accidents and weapon tests.1,2 Radiometric measurement techniques, using solid or liquid scintillators, gas ionization detectors, and so on, are the common methods for 90Sr analysis. In such radiometric methods, the radionuclide of interest is analyzed by measurement of its characteristic radiation via decay.2 Direct counting of the radionuclide with a mass spectrometer is also possible. Generally, mass spectrometric methods are suitable to measure radionuclides with long half-lives (>100 y) with low specific activity (e.g., specific activity of 238U is 1.2 × 104 Bq·g−1) when the low radioactivity is associated with high numbers of atoms, such as U and Pu isotopes, and so on. The main advantage of mass spectrometry methods over radiometric methods is the short analysis time, for example, 20−30 min, which allows large sample throughput. In a nuclear accident situation, it is critical to support decision makers quickly with reliable information regarding the degree of radioactive contamination. This demand has opened a new trend in mass spectrometry, motivating the study and improvement of the abilities of mass spectrometry instruments for rapid radionuclide determinations of 90Sr.3 90 Sr analyses using inductively coupled plasma mass spectrometry (ICP-MS) has reduced analysis time significantly and the enhancements in detection sensitivity have improved © XXXX American Chemical Society
Received: November 9, 2018 Accepted: January 15, 2019
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DOI: 10.1021/acs.analchem.8b05184 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry Table 1. Measurements of Mixtures of NIST-SRM-987 Strontium Standard with Different Amounts of 90Sr sample ID 100 50 25 5 2.5 1 0.5
stable Sr (μg·g−1) 503 507 505 506 506 508 497
90
Sr mBq(fg) 90(17.6) 46(9.0) 23(4.5) 4.6(0.9) 2.2(0.4) 1.0(0.2) 0.5(0.1)
90
Sr mBq(fg) loaded 180(35.2) 92(18.1) 46(9.0) 9.2(1.8) 4.4(0.9) 2(0.4) 1(0.2)
90
Sr/88Sr calcd
4.54 2.31 1.16 2.30 1.13 4.93 2.55
× × × × × × ×
−8
10 10−8 10−8 10−9 10−9 10−10 10−10
90
Sr/88Sr measured mean 4.51 2.29 1.15 2.30 1.18 5.22 2.90
× × × × × × ×
−8
10 10−8 10−8 10−9 10−9 10−10 10−10
90
Sr mean count (cps) 120 69 40 9 5 2 1
RSDa (%)
Nb
0.12 0.50 0.96 1.76 11.9 7.3 18.8
7 8 8 8 7 8 8
a
Relative standard deviation. bNumber of measurements.
a contaminated Fukushima soil sample which requires an understanding of the peak tailing from 88Sr. We analyzed certified reference materials (wild berry IRMM-426 and lake sediment NIST-4354) for analytical method validation of environmental samples.
oxidation is not supported energetically and thus highly effective 90Zr removal can be achieved.7,8,10 Accelerator mass spectrometry (AMS) and resonant laser ionization mass spectrometry (RIMS) instruments have also been used for 90 Sr determination. In spite of the recent improvements for AMS to discriminate masking isobaric interfering elements and its inherently excellent abundance sensitivity (range 10−13− 10−16),11−13 widespread application of AMS instruments is not common owing to their complicated operational procedure and very high purchase and operating expenses. According to the literature RIMS has been rarely used for 90 Sr measurement in spite of its excellent abundance sensitivity performance (around 7 × 10−11). The application of RIMS for 90 Sr determination in natural samples is limited because it is not widely accessible to the mass spectrometry market.2,14 Measurement of stable strontium isotopes by thermal ionization mass spectrometry (TIMS) is a standard analytical technique. The ion detection efficiency for TIMS, particularly with activators on the filament, can be several percent, while the controlled ionization process maintains excellent ion beam stability that can result in a highly precise and accurate single element isotope analysis for environmental samples.15 Strontium has an ionization potential (5.7 eV) that is lower than that of Zr (6.6 eV) and favors production of Sr+ ions prior to Zr+ during the ionization process. However, TIMS like ICPMS must have a high mass abundance sensitivity to be quantified and allow measurement of 90Sr in natural and environmental samples. 90 Sr analysis in biological and environmental samples with TIMS has been attempted recently but a successful achievement was not reported.16,17 An attempt was made to analyze 90 Sr in animal bone sample (IAEA-A-12) containing 90Sr.16 In this sample the estimated 90Sr/88Sr ratio was 4.2 × 10−11 and the TIMS instrument (Triton-T1, Thermo Fisher Scientific, Germany) with Faraday cup detectors could not detect the 90Sr ion beam. However, there were some proposals to realize 90Sr measurements, such as using a more sensitive detector system, for example, secondary electron multipliers (SEM) to detect 90 Sr, measuring 90Sr/87Sr or 90Sr/84Sr to obtain a stronger 90Sr ion beam and applying retarding potential quadrupole (RPQ) energy filter to improve the abundance sensitivity.16 Another group attempted to measure 90Sr in environmental samples (grass, moss, bark, and tree leaf) collected from the exclusion zone around the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) and was not successful.17 In our work we present the first measurements of 90Sr at fg level using a newly installed Phoenix X62 (Isotopx Ltd., U.K.) TIMS mounted with a Daly detector system. We measured synthetic gravimetric standards to determine the detection limit of 90Sr in the presence of stable Sr, and we also measured
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EXPERIMENTAL SECTION Sample Preparation. Synthetic gravimetric calibration standard solutions (Table 1) were prepared by mixing stable strontium carbonate isotope standards NIST-SRM-987 (National Institute of Standards and Technology, U.S.A.) with 90Sr (SrCl2) standard solution (Japanese Radioisotope Association, Japan) and Milli-Q2 purified water (>18 MΩ cm at 25 °C). The certified reference material samples (around 1 g of wild berry and 0.5 g of lake sediment) were decomposed in a microwave digestion system (Milestone, Italy) using analytical grade Tamapure AA-100 reagents (6 mL of HNO3 (68%) and 1 mL of H2O2 (35%) for wild berry; 6 mL of HNO3 (68%), 1 mL of HClO4 (70%), and 2 mL of HF (38%) for lake sediment samples). Strontium separation was carried out using 0.5 mL Sr extraction chromatography resin of 100−150 μm particle size (Eichrom Technologies, Inc., U.S.A.) that was slurry filled into a polypropylene gravity column (Muromac, Japan; size, 42 mm in length and 5 mm in diameter). The Sr resin columns were washed with 10 mL of H2O and preconditioned with 5 mL of 8 M HNO3 with a ∼0.2 mL min−1 flow rate. Following decomposition, the reference material solutions were dried up and dissolved in 5 mL of 8 M HNO3 and loaded into the prepared columns. After rinsing the column with 3 mL of 8 M HNO3 and 3 mL of 3 M HNO3, Sr was stripped with 0.05 M HNO3. The strontium separation procedure is shown in Figure S1. Strontium separation was repeated twice to eliminate the possibility of 90Zr interference. After separation, the samples were loaded onto single degassed zone refined grade five (99.999%) rhenium filaments together with one microliter of a TaF5 activator. Typically, sample loads of 1 μg Sr were used. Instrumentation. A Phoenix X62 (Isotopx Ltd., U.K.) TIMS was used in this study. The instrument has a 27 cm radius magnet with extended ion optics giving a 610 mm mass dispersion. The mass resolution (10% valley) is 500. The source vacuum was maintained at less than 2 × 10−8 mbar using a large 700 L·s−1 capacity turbo molecular pump backed by an oil-free scroll pump. A cold trap filled with liquid nitrogen was also used. The analyzer vacuum was maintained at less than 1 × 10−9 mbar using a 40 L·s−1 ion pump and a 300 L·s−1 turbomolecular pump in combination. The TIMS is equipped with nine Faraday collectors to measure the major Sr isotopes and an ion-counting Daly conversion dynode with a photomultiplier tube to measure the B
DOI: 10.1021/acs.analchem.8b05184 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
Figure 1. Multicollector assembly in the analysis housing of the Phoenix62 TIMS and Faraday cup detector set up for 90Sr analysis.
ultratrace 90Sr (Figure 1). This Daly detector system is positioned behind a wide aperture retarding potential (WARP) filter. The WARP is an energy filter that prevents low energy ions, produced by inelastic collisions within the analyzer vacuum, from entering the ion-counting Daly detector. The low mass abundance sensitivity was measured at 237U with respect to 238U (237U/238U) is 2 × 10−9. However, the WARP filter has no effect on high mass peak tailing, for example, 89 Sr/88Sr, since the high mass peak tail is produced by high energy ions from the ion source. The ability of a TIMS to resolve 90Sr from the high mass peak tail of 88Sr is due to the small energy spread of ions generated from the Re filament and TaF5 activator. In this method, the 86Sr ion signal of 8 × 10−11 amp (∼5 × 108 cps) was measured on a Faraday cup instead of the 88Sr ion signal while simultaneously measuring 90Sr on the ion-counting Daly detector (Figure 1). The 90Sr/88Sr ratio was obtained multiplying 90Sr/86Sr with 86Sr/88Sr = 0.1194 (i.e., the natural ratio). The 88Sr ion intensity was maintained at approximately 4.2 × 109 cps more than 1 h. Baselines were measured at ±0.5 amu on either side of the 90Sr on the ion counter and 86Sr on the Faraday cup for 60 s. The baseline at each point was then averaged and subtracted from the peak. Peak integrations were for 10 s and 12 integrations per block of data. Samples were not measured to exhaustion. The relative gain of the Daly detector with respect to the Faraday cup was 95 ± 1%. However, it was not measured for each sample as the gain is not a major source of error. Stable Strontium Measurement. The stable strontium concentration of the samples was measured with an ICP-MS Agilent-8800 (Agilent Technologies, U.S.A.). The detailed strontium analysis method is described elsewhere.18
Figure 2. Correlation between the 90Sr/88Sr ratio measured by TIMS and the calculated 90Sr/88Sr ratio for NIST-SRM-987 mixed with 90Sr gravimetric standards.
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Figure 3. Correlation between the90Sr/88Sr ratio measured by TIMS and the 90Sr activity on filament, for NIST-SRM-987 mixed with 90Sr gravimetric standards.
RESULTS AND DISCUSSION Gravimetric Standards. Synthetic gravimetric strontium solutions (∼500 μg·g−1 × 2 μL) were deposited onto the filament, giving a stable Sr concentration of 1 μg with varied concentrations of 90Sr on the filament, that is, from 180 mBq (35 fg) to 1 mBq (0.2 fg) 90Sr. Concentration of 1 Bq 90Sr contains 30 ng·g−1 stable Sr, which is negligible compared to the NIST-SRM-987 Sr standard concentration. The results are shown in Table 1 and Figures 2 and 3. Measured 90Sr/88Sr
varies between 4.5 × 10−8 to 2.9 × 10−10. There is excellent agreement between the calculated 90Sr/88Sr and measured 90 Sr/88Sr (Figure 2), as well as gravimetric 90Sr and measured 90 Sr deduced from the isotope ratio (Figure 3). The lowest amount of 90Sr (1 mBq or 0.2 fg) measured had an averaged C
DOI: 10.1021/acs.analchem.8b05184 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
Figure 4. Peak tail of 88Sr in the 90Sr peak measured by TIMS in a Fukushima soil sample (90Sr/88Sr ratio is 1.44 × 10−9).
baseline correction was measured as a blank and sample ID 25 (Table 1) was measured as a reference. The average ion beam intensity at m/z = 90 was 0.77 cps with 0.41 standard deviation. For DL calculation the classical approach was applied that is based on the standard deviation of the ion beam signals of the blank:20
ion beam intensity of 1.2 cps (n = 8). Dark noise generated from the detector was less than 0.1 cps, but the peak tailing is larger thus creating a discrepancy for the detection limit based on dark noise. Consequently, the detection limit estimation should be based on the abundance sensitivity or peak tailing analysis. In this case the detection limit quantification is difficult as it depends on the size of the ion beam generated thus the signal to concentration (or activity) functional relationship cannot be produced. During measurements of the gravimetric standards, the 86Sr beam was maintained at a similar intensity while 90Sr beam intensity was decreased gradually, as its activity concentration was reduced in the standard solutions. The fewer counts on the Daly ion-counting system resulted in higher measurement errors as can be seen in Table 1. From the 90Sr/88Sr ratio of 1.18 × 10−9 the relative standard deviation (RSD) increased greatly to 10% or even higher, reaching 20% at the ratio of 2.9 × 10−10 (1 mBq or 0.2 fg 90Sr). Since larger 86Sr ion beam detection is not possible on the installed Faraday cup detector (1011 Ω resistor), the 90Sr signal intensity cannot be expected to be over 10 cps at 10−9 or lower values of the 90Sr/88Sr ratio. Consequently, the precision of 90Sr determination is very far from the precision of conventional isotope ratio measurements, for example, 0.003% RSD for 87Sr/86Sr ratio. However, this precision is acceptable if it is compared to the precision of radiometric methods applied to determine 90Sr. To assess the accuracy of the 90Sr determination with TIMS the relative bias was calculated for each 90Sr/86Sr ratio measurement and is presented in Figure S2. With 90Sr/88Sr ratios of 2.3 × 10−9 (9 mBq of 90Sr) the relative bias was below 1% and increased from the ratio of 1.1 × 10−9 (4 mBq of 90Sr) up to 12%, as the detected 90Sr intensity was below 10 cps. Considering the accuracy of radiometric measurement where radiochemistry is necessary for sample preparation, the maximum acceptable relative bias is around 20−35%; thus, the accuracy of this TIMS measurement is excellent.19 Detection Limit. For detection limit (DL) estimation, 1000 ng NIST-SRM-987 Sr standard without 90Sr and no
ji C − C B zyz DL = k·σB·jjj 1 z j I1 − IB zz (1) k { where k is a confidence factor (3), σB is standard deviation of blank measurement series (0.41 cps), C1 is the 90Sr activity loaded on the filament (46 mBq), CB is the 90Sr activity of the blank (0 mBq), I1 is the 90Sr intensity loaded on the filament (65.4 cps), and IB is the blank intensity at m/z = 90 (0.77 cps). Considering the above-mentioned limitations, the estimated DL is 0.88 mBq (0.17 fg), which corresponds to 90Sr/88Sr isotope ratio of 2.1 × 10−10 using the equation from Figure 3. Abundance Sensitivity Test on a Fukushima Soil Sample. A highly contaminated soil sample was collected from the exclusion zone around the FDNPP to check the abundance sensitivity performance of the TIMS instrument on an actual environmental sample. The 137Cs activity of the sample was 4600 ± 100 Bq·g−1 (decay corrected to March 11, 2011) while the stable Sr concentration was 170 ± 3 μg·g−1. Since radiocesium isotopes (134Cs, 137Cs) are beta emitters, they have to be efficiently separated from 90Sr, if radiometric analysis is applied. This is a challenging task in the case of environmental Fukushima samples since radiocesium significantly dominates 90Sr.21 Figure 4 shows a mass scan obtained using the ion-counting Daly detector with the high mass peak tail from 88Sr underneath the 90Sr peak. The 86Sr on the Faraday cup detector is shown for reference and is coincidental with the 90 Sr peak. The 90Sr peak intensity is approximately 7 cps. The measured isotopic ratio of 90Sr/88Sr is 1.44 × 10−9 which is equivalent to the 1060 ± 10 Bq·kg−1 90Sr activity concentration. The peak tail from 88Sr is still clearly present beneath the 90Sr peak but its intensity is typically
