Radioisotope quantitative method

Abstract

To provide a quantitative method that can accurately obtain a content of a radioisotope.SOLUTION: A radioisotope quantitative method is configured to: obtain a radioisotope from a mass analysis result of a target sample, and an existence ratio of a first stable isotope and a second stable isotope; obtain an existence ratio of the first stable isotope and the second stable isotope in a mixed sample from a mass analysis result of the mixed sample of a spike sample and the target sample; obtain a relation between a content of the first stable isotope and signal intensity corresponding to a radioisotope from a mass analysis result of a plurality of reference samples different in a content of the stable isotope; obtain a content of the first stable isotope in the target sample from the existence ratio of the first stable isotope and the second stable isotope in each of the mixed sample and the target sample, and each content of an existence ratio of the first stable isotope and the second stable isotope in the spike sample; and obtain a content of a radioisotope element in the target sample from the content of the first stable isotope, an existence ratio of the radioisotope and the first stable isotope in the target sample, and the relation.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for quantifying one specific radioisotope in a sample containing multiple different isotopes. [Background technology]

[0002] It is an isotope of Sr (strontium). 90 Sr (strontium 90) is 235 It is a radioactive isotope produced by nuclear fission of U (uranium 235). 90 Sr is a radioisotope that decays by beta decay with a half-life of 28.8 years. 90 Y (yttrium 90), and then 90 Y decays to a stable isotope with a half-life of 64 hours. 90 It becomes Zr (zirconium 90). 90 Sr emits radioactive beta rays during beta decay, and has chemical properties similar to those of Ca, meaning that it tends to accumulate in the bones and teeth of humans and animals. 90 Exposure to Sr involves the ingestion of 90 Internal exposure to Sr and exposure to external 90 This includes both external exposure to Sr and 90 In order to investigate the effects of internal and external exposure to Sr, it is used in biological samples of humans and animals, and in environmental samples such as soil. 90 Attempts are being made to quantify Sr.

[0003] Sr. 90 In addition to Sr, the stable isotope 84 Sr., 86 Sr., 87 Sr and 88 There is Sr. Surface ionization mass spectrometry (TIMS) is a method for determining the specific isotope ratio of an element with multiple isotopes, and surface ionization mass spectrometry using isotope dilution (ID-TIMS) is a method for determining the content of an element with multiple isotopes.

[0004] The isotope dilution method is a method in which a spike sample, the abundance ratio and content (weight) of which are both known and containing multiple isotopes in a different abundance ratio from that of a target sample, is mixed with a target sample in which the contents of the multiple isotopes are unknown but the abundance ratio is known, and the abundance ratio of the multiple isotopes in the target sample is calculated from the abundance ratio of the multiple isotopes in the mixed sample and the abundance ratio of the multiple isotopes in the target sample.

[0005] In TIMS, the sample is baked onto the surface of a filament, and then an electric current is passed through the filament in the TIMS to heat it, thereby ionizing the isotopes in the sample. The ions generated are separated according to their mass-to-charge ratio (m / z). A detector is placed to detect each of the separated ions, and the abundance ratio of the isotopes in the sample can be determined by simultaneously detecting the intensities of the isotope ions separated into each m / z using the detector (see Non-Patent Document 1).

[0006] Therefore, when calculating the amount of radioisotope to be measured using ID-TIMS, the amount of stable isotope contained in the sample is calculated using the isotope dilution method and simultaneously measured using a separate detector. 90 The amount of radioisotope in the target sample is calculated from the ratio of Sr intensities. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Chihiro Ito, Ryoya Shimode, Takashi Miyazaki, Shigeyuki Wakaki, Katsuhiko Suzuki and Yoshitaka Takagai, "Isotope Dilution-Total Evaporation-Thermal Ionization Mass Spectrometric Direct Determination of Radioactive Strontium-90 in Microdrop Samples", Analytical Chemistry, (USA), American Chemical Society, November 10, 2020, Vol. 92, pp. 16058-16065 [Non-Patent Document 2] Shigeyuki Wakaki, Jo Aoki, Ryoya Shimode, Katsuhiko Suzuki, Takashi Miyazaki, Jenny Roberts, Hauke ​​Vollstaedt, Satoshi Sasaki and Yoshitaka Takagai, "A part per trillion isotope ratio analysis of 90Sr / 88Sr using energy-filtered thermal ionization mass spectrometry", Scientific Reports, (UK), Nature Research, January 21, 2022, Vol. 12, pp. 1-10 Summary of the Invention [Problem to be solved by the invention]

[0008] TIMS has multiple detectors to simultaneously detect ions separated by m / z, and the ions separated by m / z are incident on multiple detectors arranged at the focusing position. However, some of the ions may end up incident on another detector arranged next to the detector they should be incident on. When another isotope ion is incident on a detector corresponding to an isotope with a very low content, such as a radioisotope, even if the number is small, the rate of change of the number of the radioisotope ions is large, so the correct isotope ratio of the radioisotope and stable isotope cannot be obtained. As a result, the content of the radioisotope is calculated incorrectly, which reduces the measurement accuracy of the content.

[0009] An object of the present invention is to provide a method for quantifying radioisotope content with high accuracy. [Means for solving the problem]

[0010] The method for quantifying a radioisotope according to the present invention, which has been made to solve the above problems, comprises: A target sample analyzing step of determining an abundance ratio of a radioisotope of a predetermined element in the target sample and a first stable isotope and a second stable isotope, which are stable isotopes of the predetermined element, from a result of analyzing the target sample by mass spectrometry; A mixed sample analysis step of preparing a mixed sample by mixing a spiked sample containing the first stable isotope and the second stable isotope, the spiked sample having a known content of each of the stable isotopes, with the target sample, and determining the abundance ratio of the first stable isotope and the second stable isotope in the mixed sample from the result of analyzing the mixed sample by mass spectrometry; a reference sample analyzing step of preparing a plurality of reference samples which are samples containing the first stable isotope but not the radioisotope and have different amounts of the stable isotope, and determining a relationship between the amount of the first stable isotope and a signal intensity corresponding to the radioisotope from the results of analyzing the plurality of reference samples by mass spectrometry; and a radioisotope content calculation step of determining the content of the first stable isotope in the target sample from the abundance ratio of the first stable isotope to the second stable isotope in each of the mixed sample and the target sample, and the contents of the first and second stable isotopes in the spiked sample, and determining the content of the radioisotope element in the target sample from the content of the first stable isotope, the abundance ratio of the radioisotope to the first stable isotope in the target sample, and the relationship.

[0011] The present invention was made as a result of the inventor's discovery that when a sample contains a radioisotope and a stable isotope of a predetermined element, the predetermined stable isotope affects the mass spectrometry result of the radioisotope (signal intensity of the mass-to-charge ratio m / z corresponding to the radioisotope), and that this effect is proportional to the content of the predetermined stable isotope. In other words, the method for quantifying a radioisotope of the present invention is characterized in that the relationship between the content of a predetermined stable isotope of a predetermined element in a sample, the mass spectrometry result of the stable isotope, and the mass spectrometry result of the radioisotope of the element is previously determined, and the mass spectrometry result of the radioisotope of the element is corrected using this relationship.

[0012] Thermal ionization mass spectrometry (TIMS) can be used to analyze the target sample, the mixed sample, and the reference sample. By using TIMS, it is possible to analyze Li (lithium), B (boron), Mg (magnesium), S (sulfur), Cl (chlorine), K (potassium), Ca (calcium), Ti (titanium), V (vanadium), Cr (chromium), Fe (iron), Ni (nickel), Cu (copper), Zn (zinc), Ga (gallium), Ge (germanium), Se (selenium), Br (bromine), Rb (rubidium), Sr (strontium), Zr (zirconium), Mo (molybdenum), Ru (ruthenium), Pd (palladium), Ag (silver), Cd (cadmium), In (indium), Sn (tin), Sb (arsenic), and the like. It can quantify the following radioactive isotopes: Antimony, Te (Tellurium), Ba (Barium), La (Lanthanum), Ce (Cerium), Nd (Neodymium), Sm (Samarium), Eu (Europium), Gd (Gadolinium), Dy (Dysprosium), Er (Erbium), Yb (Ytterbium), Lu (Lutetium), Hf (Hafnium), Ta (Tantalum), W (Tungsten), Re (Rhenium), Os (Osmium), Ir (Iridium), Pt (Platinum), Tl (Thallium), Pb (Lead), Ra (Radon), Th (Thorium), Pa (Protactinium), and U (Uranium).

[0013] In mass spectrometry, a Faraday cup or a secondary electron multiplier can be used as a detector for detecting radioisotopes and stable isotopes. It is also preferable to provide a filter for preventing other isotopes with a mass number similar to that of the radioisotope being detected from entering the detector for detecting the radioisotope being measured. Examples of such filters include energy filters (WARP: wide aperture retardation potential filter, RPQ lens: retarding potential quadrupole lens) used in the radioisotope quantification method described in Non-Patent Document 2. By providing such a filter, for example, it is possible to prevent the radioisotope being detected from entering the detector. 90In the case of Sr 90 It is possible to reduce interference caused by Zr. 90 Sr can be quantified more accurately. Effect of the Invention

[0014] The method according to the present invention allows the content of radioisotopes to be determined accurately. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 shows experimental results showing the effect of reducing the m / z 90 background (BGN) using an RPQ lens. [Diagram 2] 1 is a graph showing experimental results illustrating the effect of a RPQ lens in reducing the signal intensity of Zr at m / z 90. [Diagram 3] A graph showing the results of investigating the effect of natural Sr on m / z 90 noise. [Figure 4] A graph showing the relationship between the amount of natural Sr (ng) and the background equivalent amount of 90Sr (ag) and background equivalent radioactivity (μBq). [Diagram 5] Graph showing the results of a 90Sr addition and recovery test using ID-RPQ-TIMS. [Figure 6] A graph showing the relationship between the lower detection limit of 90Sr when quantifying 90Sr using ID-RPQ-TIMS and the amount of natural Sr in a sample. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The method for quantifying a radioisotope according to the present invention comprises the steps of: A target sample analyzing step of determining an abundance ratio of a radioisotope of a predetermined element in the target sample and a first stable isotope and a second stable isotope, which are stable isotopes of the predetermined element, from a result of analyzing the target sample by mass spectrometry; A mixed sample analysis step of preparing a mixed sample by mixing a spiked sample containing the first stable isotope and the second stable isotope, the spiked sample having a known content of each of the stable isotopes, with the target sample, and determining the abundance ratio of the first stable isotope and the second stable isotope in the mixed sample from the result of analyzing the mixed sample by mass spectrometry; a reference sample analyzing step of preparing a plurality of reference samples which are samples containing the first stable isotope but not the radioisotope and have different amounts of the stable isotope, and determining a relationship between the amount of the first stable isotope and a signal intensity corresponding to the radioisotope from the results of analyzing the plurality of reference samples by mass spectrometry; and a radioisotope content calculation step of determining the content of the first stable isotope in the target sample from the abundance ratio of the first stable isotope to the second stable isotope in each of the mixed sample and the target sample, and the contents of the first and second stable isotopes in the spiked sample, and determining the content of the radioisotope element in the target sample from the content of the first stable isotope, the abundance ratio of the radioisotope to the first stable isotope in the target sample, and the relationship.

[0017] In the target sample analysis step, the abundance ratios of a radioisotope, a first stable isotope, and a second stable isotope of a predetermined element contained in the target sample are obtained from the results of analyzing the target sample by mass spectrometry. Here, the more naturally occurring a stable isotope is, the more likely it is to affect the mass spectrometry results of the radioisotope. Also, the closer the mass number of a stable isotope is to the radioisotope to be measured, the more likely it is to affect the mass spectrometry results of the radioisotope. Therefore, it is preferable to select the first and second stable isotopes based on the amount naturally occurring and the difference between the mass number of the radioisotope to be measured. For example, if the radioisotope to be measured is 90 In the case of Sr, the stable isotopes of Sr include 84 Sr, 86 Sr, 87 Sr, 88 There is Sr, 84 Sr is 90 The difference in mass number with Sr is large, and 84 Sr is86 Sr, 88 The amount of strontium found in nature is very small compared to Sr. 87 Sr is 87 Since they are produced by the radioactive decay of Rb, they do not show a constant abundance in environmental samples. 87 Sr is 86 Sr, 88 The amount of strontium present in nature is very small compared to that of strontium. 86 Sr and 88 It is preferable that one of the Sr is a first stable isotope and the other is a second stable isotope. 84 Sr, 86 Sr, 88 Any two of Sr may be the first and second stable isotopes. In short, when an element has three or more naturally occurring stable isotopes, the abundance ratios of the three or more stable isotopes are obtained in advance, and the first and second stable isotopes are appropriately selected from the obtained ratios. In this case, the spike sample should also contain three or more stable isotopes.

[0018] In the mixed sample analysis process, a target sample is first mixed with a spiked sample to prepare a mixed sample. The spiked sample contains a first and a second stable isotope with known contents, and the ratio of the contents is the abundance ratio of the first and the second stable isotope. Next, the mixed sample is analyzed by mass spectrometry to determine the abundance ratio of the first and the second stable isotope contained in the mixed sample.

[0019] In the reference sample analysis step, first, a plurality of reference samples are prepared, which are samples that do not contain the radioisotope but contain the first stable isotope, and have different amounts of the stable isotope. The lower and upper limits of the amount of the first stable isotope in the plurality of reference samples are preferably determined based on the amount of naturally occurring stable isotope. In this case, the reference samples may include a reference sample in which the amount of the first stable isotope is 0 (zero). Next, the relationship between the amount of the first stable isotope and the signal intensity of the mass-to-charge ratio m / z corresponding to the radioisotope is obtained from the results of analyzing the plurality of reference samples by mass spectrometry. Examples of the "relationship" include a graph (calibration curve) or table showing the relationship between the amount of the first stable isotope and the signal intensity of the mass-to-charge ratio m / z corresponding to the radioisotope, a formula showing the signal intensity of the mass-to-charge ratio m / z corresponding to the radioisotope, and the like, in which the amount of the first stable isotope is a variable.

[0020] In the radioisotope content calculation step, the content of the first stable isotope in the target sample is calculated from the abundance ratio of the first stable isotope and the second stable isotope in each of the mixed sample and the target sample obtained in the previous steps, and the respective contents of the first and second stable isotopes in the spiked sample. In addition, in the target sample analysis step, the result of analyzing the target sample by mass spectrometry is corrected using the relationship, and the abundance ratio of the radioisotope and the first stable isotope in the target sample is calculated. The content of the radioisotope element in the target sample is calculated from the abundance ratio of the radioisotope and the first stable isotope in the target sample thus calculated, and the content of the first stable isotope. Except for the fact that the result of analyzing the target sample by mass spectrometry in the target sample analysis step is corrected using the relationship, the content of the radioisotope in the radioisotope content calculation step is the same as the process using the isotope dilution method.

[0021] The isotope dilution method is a method in which a spiked sample containing multiple isotopes whose abundance ratio and content are both known is prepared, and the spiked sample is mixed with a target sample containing the multiple isotopes whose abundance ratio is known but whose content (or concentration) is unknown to obtain a mixed sample, and the abundance ratio of the multiple isotopes in the mixed sample is measured to determine the content of the multiple isotopes in the target sample from the results.

[0022] Since the present invention is a method for quantifying a radioisotope, the target sample contains a stable isotope of the same element as the radioisotope to be quantified. For example, if the radioisotope to be quantified is strontium (Sr), 90 In the case of Sr, the first and second stable isotopes are selected from the stable isotopes of Sr contained in the target sample. For example, the first and second stable isotopes are 88 Sr, 86 Sr in the target sample 86 Sr concentration (%) is δ 86n , in spiked samples 86 Sr concentration (%) is δ 86s Then, the weight of the target sample is W n (g) is expressed by the following equation (1).

number

[0023] In formula (1), C s is the Sr concentration in the spiked sample (μg / g), W s is the amount of spiked sample (g), N W is the apparent atomic weight of Sr in the target sample (g / mol, 87.68 in Table 1 in this example), S w is the apparent atomic weight of Sr in the spiked sample (g / mol, 85.95 in Table 2 in the example), R 88 / 86n , R 88 / 86s , R 88 / 86m are the target sample, spike sample, and mixed sample, respectively. 88 Sr and 86 Represents the abundance ratio of Sr. Naturally occurring radioisotope 90 When Sr is the target of quantification, N Wis determined from the atomic weights and isotopic ratios of each naturally occurring Sr isotope. 88 / 86m can be determined by analyzing the mixed sample by surface ionization mass spectrometry (TIMS). 88 / 86n When is unknown, it can be determined by analyzing the target sample with TIMS.

[0024] In the target sample 90 Mass W of Sr 90 (g) is the weight of the target sample W n (g) can be calculated by substituting it into the following equation (2).

number

[0025] In formula (2), 90 Sr w teeth 90 Mass number of Sr, R 90 / 86m is the amount of the target sample 90 Sr and 86 The other symbols are as explained in formula (1).

[0026] In the method for quantifying radioisotopes according to the present invention, the amount of radioisotopes contained in the target sample is calculated from the results of analyzing the target sample and the mixed sample with the TIMS device and the above formulas (1) and (2). 90 When determining the amount of Sr, the stable isotope in the target sample 88 Sr-induced 90 Correction is performed to eliminate noise at the mass-to-charge ratio of Sr m / z 90. 88 Sr-induced 90 An example carried out by the present inventors to investigate noise at the mass-to-charge ratio m / z 90 of Sr will be described.

[0027] [Example] [1] Equipment [1-1]TIMS device The TIMS instrument (Triton NMR spectrometer, Thermo Fisher Scientific, Bremen, Germany) was used. TM Plus TIMS) was used. 90 A secondary electron multiplier (SEM) is used as the detector for Sr, and a stable isotope ( 88 Sr, 87 Sr, 86 Sr, 84 The detectors for Sr were all Faraday cups (FC) (L1 to L4). All FCs were 10 11 Connected to an Ω amplifier. 90 The atomic mass of Sr is 89.907721 amu, but 90 Zr and 88 SrH2 + In order to avoid the influence of the RPQ lens, a RPQ lens was installed in front of the SEM and the mass-to-charge ratio detected by the SEM was adjusted to m / z = 89.8927 (hereafter referred to as m / z 90). Hereafter, the TIMS with a RPQ lens installed in front of the SEM is also referred to as RPQ-TIMS.

[0028] A rhenium (Re) filament was used as the filament to which the sample was applied. Before use, the Re filament was baked at 2200°C as a pretreatment to remove surface impurities (organic matter, zirconium (Zr), etc.). The pretreated Re filament was measured at 1550°C for 20 minutes using the above-mentioned TIMS device, and those with a background noise of m / z90 of 0.03 cps or less were used for the actual analysis. In a previous experiment conducted by the inventor, it was found that when the temperature of the Re filament during analysis with the TIMS device exceeded 1640°C, 90 Because background noise at m / z 90 due to Zr, which has an atomic mass similar to that of Sr, was observed, the temperature of the Re filament during the measurement was measured with an optical pyrometer (Thermo Fisher Scientific, Inc.), and Sr isotopes were measured at temperatures below 1640 °C.

[0029] [1-2] Verification equipment To verify the effect of the method for quantifying radioactive materials according to the present invention, an ICP-MS apparatus (NexION300S (PerkinElmer, Inc., Waltham, MA, USA)) and a low background radioactivity automatic measurement apparatus (Oxalate Precipitation-LBC (LBC-4202B), Hitachi-Aloka Medical Co., Ltd., Tokyo, Japan) were used. The low background radioactivity automatic measurement apparatus was used with a Geiger-Muller tube and 1750 V was applied. Measurements were performed using a 10% methane-90% argon mixed gas (PR gas). Furthermore, an inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima 7300 DV, Perkin Elmer) was used to calculate the recovery rate of natural Sr.

[0030] [1-3] Other equipment To prepare the samples, we used an IQ MILL-2070 (Frontier Laboratories Ltd., Koriyama, Japan) with agate balls of 15 mm diameter, a Titan MPS microwave digester (PerkinElmer), and an ELGA ultrapure water supply system (18.2 MΩ cm, ELGA VEOLIA, High Wycombe, UK).

[0031] [2] Sample preparation [2-1] Reagents Ultra-high purity strontium carbonate (≥99.99 w / w%, hereafter referred to as “natural Sr”) was obtained from Fujifilm Wako Pure Chemical Industries, Ltd. (Osaka, Japan). · 86 Sr-enriched strontium carbonate (hereinafter referred to as " 86 (Sr spike) obtained from Oak Ridge National Laboratory (Oak Ridge, TN, USA) · 90 Sr radioactive standard solution (3.7 kBq; radioactive purity >99%) was obtained from the Japan Radioisotope Association (Tokyo, Japan). Tantalum chloride (99.99%, trace metals basis): obtained from Sigma-Aldrich Co. LLC (St. Louis, MO, USA). Ultrapure-100 ultra-high purity hydrofluoric acid (HF, purity 46.0-51.0%): obtained from Kanto Chemical Co., Ltd. (Tokyo, Japan). EL Phosphoric Acid 86 (high-purity reagent grade for electrical equipment, H3PO4: 85.0-86%): obtained from Kanto Chemical Co., Ltd. (Tokyo, Japan). ·TAMAPURE-AA ultra-high purity grade concentrated nitric acid (68 w / w%) was obtained from Tama Chemicals Co., Ltd. (Kanagawa, Japan). Ultrapure water: Ultrapure water was prepared using an ultrapure water production system (product name: DIRECT-Q UV 3 Ultrapure water supply system ("DIRECT-Q" is a registered trademark), Merck Millipore SAS, Molsheim, France).

[0032] [2-2] 86 Preparation of Sr spiking solution A Sr standard solution (1000 mg / L; natural Sr solution) was prepared by dissolving 0.1000 g of baked natural Sr (SrCO3) in 100.00 mL of 0.10 M nitric acid (HNO3). 86 Dissolve 5.0 mg of Sr spike in 100.0 mL of 0.10 M HNO3. 86 A Sr spike stock solution (50 mg / L) was prepared. 86 The Sr spike stock solution was diluted with 0.10 M HNO3 to the appropriate ratio. 86 Sr spike solution was used. 86 Sr spiked solution (0.10 mg / L) was used for the measurements.

[0033] In addition, radioactive isotopes 90 To achieve highly accurate measurement of the Sr content, 86 The isotope ratio of the Sr spike stock solution was determined based on the results of analysis using a TIMS device using a diluted solution of 1 mg / L of Sr standard solution diluted with ultrapure water. 84 Sr(0.56%), 86 Sr(9.86%), 87 Sr(7.0%), 88Sr(82.58%) has four isotopes. 87 Sr is a radioactive substance 87 Since it is produced by decay from Rb, its abundance varies based on geological factors. Therefore, in the calculation of the isotope dilution method in this example, 84 Sr(0.60%), 86 Sr(10.60%), 88 Sr (88.79%) was used as the isotope abundance ratio. Sr standard solution and 86 The isotope ratio and apparent atomic weight of the Sr spike stock solution are shown in Tables 1 and 2 below.

[0034] [Table 1]

[0035] [Table 2]

[0036] [2-3] Preparation of tantalum activator 6 mL of ultrapure water was dropped onto 0.4 g of tantalum chloride (TaCl5) powder placed in a Teflon (registered trademark) container, and a white dispersion was obtained using an ultrasonic cleaner for 10 minutes. Then, 0.40 mL of HF (stock solution), 0.40 mL of H3PO4 (stock solution), and 4.00 mL of HNO3 (stock solution) were added to the dispersion in order to completely dissolve the TaCl5 powder and obtain a transparent solution. Ultrapure water (9.2 mL) was added to this solution and used as a tantalum activator (Ta-act, Ta concentration: 2 w / v%).

[0037] [2-4] 90 Preparation of Sr standard solution 90 The Sr radioactive standard solution was diluted with 8M nitric acid (HNO3) to 0.5 mBq / mL. 90 A dilute Sr solution was prepared. 9020.00 mL of the diluted Sr solution was passed through a commercially available linked Sr resin cartridge column (particle size: 50-100 μm, 2 mL cartridge type, Eichrom Technologies, Lisle, USA) to chemically remove other elements (especially Zr). The Sr resin column was washed with 20.00 mL of ultrapure water and 20.00 mL of 8 M HNO3 in sequence. 90 After the Sr dilution, the cartridge column was washed with 36 mL of 8 M HNO3 and 36 mL of 3 M HNO3, and then 10.00 mL of 0.015 M HNO3 was used to remove strontium from the column. 90 Sr was dissolved. 90 The Sr eluate was adjusted to 1.0 mBq / mL with 0.015M HNO3. 90 This was used as the Sr standard solution. 90 The radioactivity of Sr is the amount of decay from the inspection date to the measurement date. 90 The concentration was calculated from the half-life of Sr and adjusted to the correct concentration. The Zr concentrations of all reagents used in the following experiments were measured by ICP-MS, and all were confirmed to be below the detection limit (16.0 pg / mL).

[0038] [3] Sample collection and pretreatment [3-1] Human tears and saliva Tear and saliva samples were collected from one subject. 86 Sr spike solution 100.0 μL ( 86 Sr spike 10 ng) was mixed in a Teflon container to prepare a mixed solution (Note: 90 To verify that Sr quantification is possible 90 To conduct the Sr addition and recovery test, 86 Sr spike solution should contain 1.0 mBq / mL of 90 500.00 μL of Sr standard solution was added.) Next, the mixed solution was dried on a hot plate (120°C, 2 hours), and then the dried material was dissolved in 1.00 μL of 0.10 M HNO3 to prepare a measurement sample.

[0039] [3-2]Eyelashes Eyelash samples were collected from one subject. Eyelashes (1.0 mg) were heated once in a Teflon container (250°C, 30 min) and cooled to room temperature (25°C). Then, 1 mL of hydrogen peroxide (30.0-35.5 w / w%; Fujifilm Wako Pure Chemical Industries, Ltd.; Osaka, Japan) and 1 mL of concentrated nitric acid (68 w / w%) were added to the Teflon container to prepare a mixed solution. The mixed solution was dried on a hot plate (120°C, 2 h), and the dried product was dissolved in 1.00 μL of 0.10 M HNO3. The HNO3 solution was completely dried to obtain a yellow powder. This yellow powder was dissolved in 1.00 μL of 0.10 M HNO3 to obtain 1 μL of the solution and 0.10 mg / L 86 The measurement sample was prepared by mixing 100.0 μL of the Sr spike solution in a Teflon container (note that 90 To verify that Sr quantification is possible 90 To conduct the Sr addition and recovery test, 86 Sr spike solution should contain 1.0 mBq / mL of 90 500.00 μL of Sr standard solution was added.

[0040] [3-3] Teeth Wild boar teeth were collected from wild boars that had been collected and slaughtered for pest control in Katsurao Village, Fukushima Prefecture (2022), Nihonmatsu City (2019), and Tomioka Town (2019). The mouse teeth were collected from a wild mouse collected in Otama Town, Fukushima Prefecture, Japan (2021). This mouse was provided for the preparation of a skull specimen for an academic study separate from the present invention. Human teeth (wisdom teeth) were collected from one subject, and artificial teeth were prepared from bioceramics "hydroxyapatite" (φ13 mm × 2 mm, purity 99% or higher, HOYA Technosurgical Corporation, Tokyo, Japan) as a substitute material for medical research and used as the standard specimen.

[0041] The boar and human teeth were sterilized in an autoclave at 121°C for 30 minutes, and the mouse teeth at 70°C for 1 hour. They were then dried and stored in a desiccator. All teeth were treated as follows to prepare the measurement samples for measurement with the TIMS device.

[0042] First, the teeth were crushed to a particle size of less than 2 mm. Approximately 20 mg of the crushed teeth were then crushed in a ball mill using a polycarbonate container and agate balls (diameter 15 mm) to obtain fine particles (particle size: <1 mm), and then further crushed in an agate mortar to obtain a powder. The tooth powder (10.0 mg) was dissolved in a microwave decomposition apparatus using a mixture of 3 mL of HNO3 (60%) and 3 mL of HCl (35%) at 220°C for 35 minutes, and then heated the solution at 100°C in a fume hood to completely dry it. The dried sample was dissolved in 10 mL of 8.0 M HNO3.

[0043] This solution was then loaded onto a connected Sr resin cartridge column (" 90 The tooth samples were passed through a 0.10 mg / L elution column (see "Preparation of Sr standard solution"), and then the cartridge column was washed with 8 M HNO36 mL and 3 M HNO36 mL. Sr was eluted with 0.015 M HNO310.00 mL. 86 1 mL of the solution containing 100.0 μL of the Sr spike solution was mixed in a Teflon container. The mixed solution was dried on a hot plate (120 °C, 2 h) and dissolved in 1.00 μL of 0.10 M HNO3 to obtain a measurement sample.

[0044] [4] Analysis procedure In the method for quantifying radioisotopes in this example (hereinafter referred to as ID-RPQ-TIMS), a pretreated Re filament was heated at 800 mA while a mixed solution ( 90 Sr and 86 A 1.00 μL sample containing Sr spikes was applied onto the filament, and then 1.00 μL of Ta-act was applied onto the Re filament while maintaining the temperature of the Re filament. Next, the sample ( 90 Sr,86 The samples (including Sr spikes and Ta-act) were baked out at 1500 mA for 30 seconds. After baking out, the Re filament with the sample attached was used for analysis with the RPQ-TIMS instrument. Analysis with the RPQ-TIMS instrument was performed with 3×10 -7 The test was performed under vacuum pressure of 1000 mbar or less. The sample attached to the Re filament was heated at a rate of 1500 mA / min. 86 Heating was performed at a rate of 100 mA / min until the Sr intensity reached 6V. 86 Data acquisition was started when the intensity of Sr reached 6 V. The temperature of the Re filament was once increased to 1640°C and then maintained below 1640°C.

[0045] [5] Verification by radioactivity measurement method (oxalic acid precipitation - LBC) Wild boar tooth samples (30 g) collected from Katsurao Village and Tomioka Town were used for the 90 The samples were pretreated according to the oxalic acid precipitation-LBC method, which is a radioactive analysis method for Sr. A portion of the pretreated tooth samples was analyzed by both oxalic acid precipitation-LBC and ID-RPQ-TIMS, and the results were compared. Radioactive analysis by oxalic acid precipitation-LBC required 20 days to measure one sample. 90 The Sr recovery rate was determined by measuring the natural Sr (carrier) concentration in the solution before and after precipitation using ICP-AES.

[0046] <Experimental Results> [1] Reduction of background noise (BGN) by RPQ lenses As mentioned above, 90 Attributable to substances other than Sr 90 In order to reduce the Sr mass-to-charge ratio (m / z 90) noise, 90 A RPQ lens was installed in front of the secondary electron multiplier, which is the detector for detecting Sr. To investigate the effect of the RPQ lens in reducing the m / z 90 noise, a sample (0.10 mg / L) was injected into the TIMS instrument (RPQ-TIMS) equipped with the RPQ lens. 86 The m / Z 90 intensity and 86Sr measured intensity and m / Z 90 intensity when the sample was analyzed using a TIMS instrument without a RPQ lens. 86 The measured Sr intensity was compared. The results are shown in Figure 1. The analysis was performed by applying the sample to a baked Re filament. 86 The measurement intensity of Sr was set to 6 V. 90 Sr is not included.

[0047] In Figure 1, from top to bottom, the marks indicated by triangles, circles, diamonds, and circles are based on RPQ-TIMS. 86 The graph shows the measured intensity of Sr, the measured intensity of m / z 90 by TIMS, the Re filament temperature during measurement by RPQ-TIMS, and the intensity of m / z 90 by RPQ-TIMS. The horizontal axis of FIG. 1 represents time (min), and the vertical axis represents intensity or temperature. 86 The measured intensity of Sr and the filament temperature of Re showed almost the same results as those obtained by RPQ-TIMS.

[0048] As can be seen from Figure 1, the signal intensity of m / z 90 was higher without the RPQ lens than with the RPQ lens. This result indicates that the background noise (BGN) of m / z 90 can be reduced by using the RPQ lens. Furthermore, when the temperature of the Re filament reached 1640°C or higher (after about 68 minutes), the signal intensity of m / z 90 increased regardless of the presence or absence of the RPQ lens. This is probably due to the presence of impurities in the Re filament and reagents that are difficult to remove. 90 Derived from Zr 90 This was presumably due to interference from Sr isobaric ions. From the above, it was found that when measuring Sr isotopes, it is preferable to keep the temperature of the Re filament below 1640°C.

[0049] [2] RPQ lens 90 Reduction effect of Zr signal intensity (m / z 90) RPQ lens 90To investigate the effect of reducing the signal intensity of Zr (m / z 90), samples were prepared by mixing various amounts of natural Sr with Zr, and the signal intensity of m / z 90 was measured when each sample was analyzed using RPQ-TIMS. The analytical conditions were the same as those in [1]. The temperature of the Re filament was maintained at approximately 1600°C. The results are shown in Figure 2. The horizontal axis of Figure 2 shows the Zr concentration (mg / L) in the sample, and the vertical axis shows the signal intensity of m / z 90. The dotted line in Figure 2 shows the signal intensity level of m / z 90 without the RPQ lens in Figure 1, and the straight line shows the signal intensity level of m / z 90 with the RPQ lens in Figure 1. Figure 2 shows that if the Zr concentration in the sample is below the sub-mg / L level, it does not affect the signal intensity of m / z 90. On the other hand, the signal intensity of m / z 90 is 90 It increased in proportion to the amount of Zr. 90 It was found that in order to quantify Sr with high accuracy, it is necessary to remove as much Zr as possible from the Re filament and from the sample.

[0050] [3] Effect of natural Sr on m / z 90 noise Even if Zr is removed from the sample or the Re filament, the m / z 90 BGN occurs. Moreover, the main factor affecting the m / z 90 BGN is the natural Sr in the sample (especially the 88 The m / z 90 BGN size varies depending on the amount of natural Sr. Therefore, to investigate the relationship between the amount of natural Sr and the size of the m / z 90 BGN, samples with natural Sr amounts of 0ng, 50ng, 100ng, and 300ng (1μL of sample with natural Sr concentration adjusted to 0mg / L-300mg / L) were repeatedly analyzed for half a year using RPQ-TIMS. The analysis was carried out as follows: 86 A mixture of 10 ng of Sr spike and 0-300 ng of natural Sr was applied to a Re filament together with 1 μL of Ta-act, and the mixture was measured by RPQ-TIMS. The other analytical conditions were the same as in [1].

[0051] In Fig. 3, the horizontal axis represents the measurement date and time, the left vertical axis represents the signal intensity of m / z 90, and the right vertical axis represents the signal intensity of m / z 90 (cps). 90The background equivalent weight (BEW) of Sr (ag: attogram) is shown. In Fig. 3, the circle indicates the signal intensity at m / z 90, and the diamond indicates 90 The BEW value of Sr is shown. Table 3 shows the amount of natural Sr (ng), the signal intensity at m / z 90 (cps), 90 Figure 4 shows the relationship between the BEW (ag) of Sr and background radioactivity (μBq) and the amount of natural Sr (ng). 90 1 is a graph showing the relationship between the BEW (ag) of Sr and background equivalent radioactivity (μBq). [Table 3]

[0052] As can be seen from Figure 3, when samples with the same amount of natural Sr were measured by RPQ-TIMS, the BGN at m / z 90 showed a small fluctuation and was stable even on different measurement days (relative standard deviation (RSD): 9.64-11.46%). This is because the RPQ lens was used. 88 This is probably because we were able to match all conditions except for the amount of Sr. 90 The BEW of Sr is a natural 88 The RSD of the BEW measurement was dependent on the amount of Sr in the natural environment. 88 The amounts of Sr were similar in different samples.

[0053] Figure 4 shows the amount of natural Sr in the sample and the corresponding BGN at m / z 90. 90 The relationship between the amount of Sr and the amount of strontium is shown in Fig. 4. As shown in Fig. 4, the relationship between the two is linear. 90 If the amount of Sr (ag: attogram) is y, y is expressed by the following equation (10), where the amount of natural Sr is the variable x. y = 0.046x + 0.2864 (10) It can be found using.

[0054] Therefore, the amount of natural Sr in the sample was determined by TIMS (ID-RPQ-TIMS) using the isotope dilution method of the present invention, and the amount of natural Sr corresponding to the BGN at m / z 90 was calculated from the determined amount and the above formula (10). 90 Calculate the amount of Sr and 90 By subtracting from the Sr measurement, 90 The exact value of Sr can be determined. Table 4 below shows the natural Sr and 90 The measured value of Sr, the measured value of natural Sr and the BEW calculated from the above formula (10), and the corrected 90 The amount of Sr (quantitative value) is shown.

[0055] [Table 4]

[0056] The experimental conditions are as follows. Filament used: Baked Re filament Measurement intensity: 86 Sr 6V Analytical sample: 0.1 mg / L 86 Sr spike 100 μL, 0-3 mg / L natural Sr 100 μL, 0.0098 ag / μL 90 Sr 100 μL, Ta-act 1 μL The amount of natural Sr was set so that the Sr concentration in 1 μL of sample was 0 to 300 mg / L.

[0057] As can be seen from Table 4, the measured values ​​of natural Sr almost matched the amounts added. 90 The value obtained by subtracting the correction value from the measured value of Sr almost coincides with the amount of added Sr, which is a minute amount at the attogram level. 90 It was found that Sr could be quantified with high precision.

[0058] Figure 5 shows the results of the ID-RPQ-TIMS 90 The horizontal axis of Figure 5 shows the results of a Sr addition and recovery test. 90 The amount of Sr (ag), the left vertical axis is 90The content (ag) calculated from the Sr measurement results, and the right vertical axis represents radioactivity (μBq). The experimental conditions are as follows. Filament used: Baked Re filament Measurement intensity: 86 Sr 6V Analytical sample: 0.1 mg / L 86 Sr spike 100 μL, 0.0098~0.196 ag / μL 90 Sr 100 μL, Ta-act 1 μL

[0059] As shown in Figure 5, the addition 90 The amount of Sr, 90 The measured amount of Sr has a high linearity (y=1.008x-0.0239; R 2 =0.9999), and quantification was successfully achieved at the ag (μBq) level in 1 μL (or 1 mg).

[0060] Figure 6 shows the results of the ID-RPQ-TIMS 90 When quantifying Sr 90 6 is a graph showing the relationship between the detection limit of Sr and the amount of natural Sr in a sample. In FIG. 6, the horizontal axis is the amount of natural Sr (ng) in 1 μL of sample, and the left vertical axis is 90 The lower detection limit of Sr mass (ag) is shown on the right vertical axis, and the lower detection limit of radioactivity (μBq) is shown on the right vertical axis. The lower detection limit varies depending on the amount of natural Sr in the sample, so it was calculated as follows. The BEW was calculated from the amount of natural Sr in the sample calculated by Equation (10) and ID, and the BEW variance (2SD) was calculated for each amount of natural Sr. 90 Sr content and 90 According to the relational equation for the amount of Sr measured (shown in FIG. 5), the measured value by ID-RPQ-TIMS tends to be slightly higher than the quantitative value, so the value calculated above by correcting downward the BEW variation was set as the detection limit. From the value when the amount of natural Sr is 0 (zero) in FIG. 6, by using the quantitative method of the present invention, 90 The detection limit for Sr was achieved at 0.091ag / μL (0.466μBq / μL).

[0061] [4] Analysis results of various samples Next, samples taken from wild boars, humans, and mice were used to determine the 90 The results of the Sr analysis are explained below. [4-1] Tooth samples 90 Sr analysis results Included in boar tooth measurement samples 90 Sr was quantified by ID-RPQ-TIMS. To verify the quantitative performance of ID-RPQ-TIMS, 245.5 mL of the boar tooth measurement sample was taken for verification, and the remaining 4.5 mL was used for ID-RPQ-TIMS analysis. 90 The oxalic acid precipitation-LBC method was used to verify the quantitative value of Sr, and the ICP-AES method was used to verify the quantitative value of natural Sr. Note that the oxalic acid precipitation-LBC method is a radioactive analysis method that requires a large amount of sample, so it can be used only when a sufficient amount of sample can be collected, such as boar teeth.

[0062] The analysis conditions for ID-RPQ-TIMS are as follows. [Analysis conditions] Filament used: Baked Re filament Measurement intensity: 86 Sr 6V Analytical sample: 0.1 mg / L 86 Sr spike 100 μL, boar tooth measurement sample 1 mg, Ta-act 1 μL

[0063] In wild boar tooth samples 90 The measurement results of the Sr quantitative values ​​are shown in Table 5. [Table 5]

[0064] As can be seen from Table 5, the results of ID-RPQ-TIMS and the verification method 90 The quantitative values ​​of Sr and natural Sr were in good agreement.

[0065] Next, the ID-RPQ-TIMS was used to analyze tooth samples taken from wild boars, mice, and humans. 90 To verify that Sr quantification is possible, 90 A Sr addition and recovery test was conducted. 90 We prepared samples with and without Sr added (0.98 ag; 5 μBq). 90 Sr was quantified. An artificial human tooth was used as a blank sample. The wild boar teeth used in this study were taken from a different individual from the wild boar teeth shown in Table 5. The results are shown in Table 6.

[0066] [Table 6]

[0067] In Table 6, the top row of each tooth sample is 90 The bottom row shows the sample without Sr addition. 90 Sr-added sample 90 The quantitative value of Sr is shown. Therefore, the value obtained by subtracting the value in the upper row from the value in the lower row is 90 As can be seen from Table 6, the amount of Sr added to each tooth sample 90 The measured amount of Sr added was almost the same as the actual amount added.

[0068] Table 6 shows the quantitative values ​​of natural Sr in each tooth sample measured by ID-RPQ-TIMS and ICP-MS. These quantitative values ​​are almost the same, and ID-RPQ-TIMS can accurately quantify natural Sr. From this, it can be concluded that the ID-RPQ-TIMS of the present invention can quantitatively determine both natural Sr and 90 This is a useful method for simultaneous determination of Sr.

[0069] [4-2] Samples other than teeth 90 Sr analysis results To demonstrate the effectiveness of ID-RPQ-TIMS for investigating internal radiation exposure in an emergency using micro-scale biological samples, we carried out spike-recovery tests on human tears, saliva, and eyelash samples. 90 Sr was added at 0.98 ag (5 μBq). Other test methods were the same as those for the tooth sample. 90 The results of simultaneous quantitative determination of Sr and natural Sr are shown in Table 7.

[0070] [Table 7]

[0071] As can be seen from Table 7, in all the tears, saliva, and eyelash samples, 90 The quantitative value of Sr was almost consistent with the amount of Sr added. 90 It was found that Sr could be quantified at the attogram level.

Claims

1. a target sample analyzing step of determining an abundance ratio of a radioactive isotope of a predetermined element in the target sample and two stable isotopes of the predetermined element, that is, a first stable isotope and a second stable isotope, from the results of analyzing the target sample by mass spectrometry; a mixed sample analysis step of preparing a mixed sample by mixing a spiked sample containing the first stable isotope and the second stable isotope, the spiked sample having known contents of the first stable isotope and the second stable isotope, with the target sample, and determining the abundance ratio of the first stable isotope to the second stable isotope in the mixed sample from the results of analyzing the mixed sample by mass spectrometry; a reference sample analyzing step of preparing a plurality of reference samples which are samples containing the first stable isotope but not the radioactive isotope, and which have different amounts of the stable isotope, and determining the relationship between the amount of the first stable isotope and the signal intensity corresponding to the radioactive isotope from the results of analyzing the plurality of reference samples by mass spectrometry; a radioisotope content calculation step of determining the content of the first stable isotope in the target sample from the abundance ratio of the first stable isotope to the second stable isotope in the mixed sample and the target sample, and the contents of the first and second stable isotopes in the spiked sample, and determining the content of the radioisotope in the target sample from the content of the first stable isotope, the abundance ratio of the radioisotope to the first stable isotope in the target sample, and the relationship.

2. 2. The method for quantifying a radioisotope according to claim 1, The method for quantifying a radioisotope, wherein the mass spectrometry is surface ionization mass spectrometry.

3. 3. The method for quantifying a radioisotope according to claim 2, A method for quantifying a radioisotope, wherein the surface ionization mass spectrometry uses a secondary electron multiplier as a detector for detecting each of the radioisotope and the stable isotope.

4. 4. The method for quantifying a radioisotope according to claim 3, A method for quantifying a radioisotope, wherein a detector for detecting the radioisotope is provided with an energy filter that blocks the incidence of stable isotopes.

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