JP7849685B2 - Method for simultaneous analysis of multiple alpha-emitting radionuclides - Google Patents

Description

This invention relates to a method for analyzing alpha-emitting radionuclides, such as thorium (Th), neptunium (Np), americium (Am), curium (Cm), plutonium (Pu), and uranium (U), and more particularly to a method for simultaneously analyzing multiple radionuclides using a high-frequency inductively coupled plasma mass spectrometer.

Radioactive materials undergo decay, emitting radiation such as alpha, beta, and gamma rays, in an attempt to become more stable atomic nuclei. While the quantification of alpha-emitting radionuclides can be performed by measuring alpha rays using ZnS(Ag) scintillation counters, gas flow counters, and silicon semiconductor detectors, it was not possible to simultaneously analyze nuclides with similar alpha-ray energies. To quantify each nuclide individually, various separation and enrichment operations were generally required, resulting in a complex process.

Furthermore, while generally known inductively coupled plasma mass spectrometry methods allow for the quantitative determination of individual nuclides with different masses without requiring prior separation, they still couldn't separate and quantify isobaric nuclides such as 238U and 238Pu, or 241Am and 241Pu. Additionally, because the sample is radioactive, it's preferable to introduce it into the instrument at a low concentration, around ppt. However, low concentrations increase the influence of interference from multiple nuclides and elements due to contamination, resulting in reduced measurement accuracy.

This invention has been made in view of the aforementioned problems of the prior art, and aims to provide a method for simultaneously analyzing multiple radionuclides without requiring complicated steps.

To achieve the above objective, the inventors diligently investigated measurement methods that utilize the advantages of high-frequency inductively coupled plasma mass spectrometers (ICP-MS, ICP-MS/MS).

《High frequency inductively coupled plasma mass spectrometer: ICP-MS》
Generally, liquid samples are introduced into a nebulizer in the sample introduction section and aerosolized. This aerosol is then introduced into a plasma ion source. In the plasma ion source, argon gas is introduced into the center of a high-frequency coil to which high-frequency energy is applied. The powerful high-frequency energy causes argon atoms to collide with each other, generating a high-energy argon plasma. The aerosolized sample is instantly decomposed in the plasma (temperature 6000-10000K), and the elements to be measured are atomized and then ionized. These ions pass through a pair of orifices called a sampling cone and a skimmer cone (some devices have a hyperskimmer cone), with pores of about 1 mm, and are introduced into the ion lens section. The ion lens section focuses the ions, separates ions from neutral matter, and blocks photons entering from the plasma. A collision-reaction cell either eliminates interference by introducing an unreactive gas (He) into the cell and causing it to collide with interfering ions, utilizing the kinetic energy difference with the element to be analyzed (collision mode), or by introducing a highly reactive gas (oxygen, methane, ammonia, etc.) into the cell and reacting it with the interfering ions to eliminate interference (reaction mode). A quadrupole mass filter consists of four parallel rod-shaped electrodes, and a quadrupole electric field is formed by applying a voltage that is a superposition of a DC voltage and a high-frequency AC voltage, with the polarity of the opposing electrodes being the same. When ions are introduced into the quadrupole electric field along the rod-shaped electrodes with a low acceleration voltage, the ions move while oscillating in the vertical and horizontal directions. However, if the voltage is changed while keeping the DC/AC voltage ratio constant, at a certain moment only ions with a specific mass number will oscillate stably, pass through the quadrupole, and reach a detector equipped with a secondary electron multiplier tube. On the other hand, ions with other mass numbers will have a large amplitude, diverge, collide with the electrodes, and disappear. Secondary electron multiplier tubes are detectors capable of detecting weak ion flows with high precision. They utilize the property that secondary electrons are emitted when ions collide with a metal surface or a specially processed ceramic surface. These emitted secondary electrons are accelerated by an electric field and further amplified exponentially through repeated collisions. Ultimately, the secondary electrons are amplified hundreds of thousands to over ten million times, allowing them to be extracted as large signal currents per mass.

ICP-MS (ICP-MS/MS) offers many advantages, including: 1) miniaturization, 2) high-speed scanning, 3) relatively low cost due to widespread adoption, 4) no special national qualifications required, and 5) ease of operation. However, ICP-MS (ICP-MS/MS) has a limitation in its mass resolution; while it can separate ions with a single mass difference, it cannot separate isobaric ions.

《Key Features of the Invention》
Therefore, in order to overcome the above-mentioned disadvantages, the present invention features "coarse separation of isobars using a column" and "precise separation of isobars by reaction with gas" as its key characteristics.
(1) Crude separation of isobars using a column Separation of plutonium and uranium, which are alpha-emitting radionuclides that often have isobars and are present in excess. The sample solution containing the alpha-emitting radionuclides is introduced into a column or filter packed with solid-phase extraction resin, and these are separated. As the solid-phase extraction resin, for example, DGA resin (manufactured by EICHROM TECHNOLOGIES) or TRU resin (manufactured by EICHROM TECHNOLOGIES) can be used, but it is not particularly limited as long as it can achieve the above objective. For example, when using DGA resin, the sample to be measured is prepared in a nitric acid solution (4M HNO3 + 0.1% H2O2 or 4M HNO3 + 0.1M NaNO2). Then, when an alpha-emitting radionuclide sample solution prepared in a 4M HNO3 + 0.1% H2O2 nitric acid solution is introduced into a column or filter packed with DGA resin, uranium, plutonium, americium, curium, etc. are adsorbed onto the DGA resin. Subsequently, when a low-grade nitric acid solution (0.2M HNO3 + 0.1% H2O2; if 4M HNO3 + 0.1M NaNO2 is used as the nitric acid solution, 0.1M NaNO2 is used instead of 0.1% H2O2; the same applies hereafter) is passed through, uranium elutes from the DGA resin. After that, for example, when a 0.5M HCl + 0.1% H2O2 solution is passed through, americium and curium, etc., elute from the DGA resin, and when a 0.5M HCl + 1% ascorbic acid solution is passed through, plutonium elutes from the DGA resin. Furthermore, when using TRU resin as another solid-phase extraction resin, for example, americium and plutonium can be separated by using 4M HCl.
(2) Precise separation of isobars by reaction with gas By passing high-purity oxygen and ammonia gas through the collision reaction cell of an ICP-MS (ICP-MS/MS), the interference can be eliminated by converting elements that are highly reactive with these gases into other masses. For example, americium and plutonium can be converted to AmO and plutonium to PuO2 by adjusting the amount of oxygen gas within a predetermined range. Also, by adjusting the amount of oxygen gas within a predetermined range, plutonium can be converted to PuO and uranium to UO2, making it possible to separate and measure the isobars of plutonium and uranium.
In addition, by adjusting the amount of ammonia gas within a predetermined range, it is possible to separate and measure isobaric plutonium and uranium by converting only uranium into an ammonia adduct (such as UN2H4).

《Method for Analyzing Alpha-Particle Emitting Radionuclides According to the Present Invention》
The analytical method of the present invention, possessing the above important features, is characterized by introducing a sample solution into a column or filter packed with a solid-phase extraction resin, adsorbing alpha-emitting radionuclides contained in the sample solution onto the solid-phase extraction resin, then separately eluting elements, particularly those having isobars, into an eluent, atomizing the eluent and introducing it into a plasma ion source for ionization, introducing oxygen or ammonia gas into the resulting ion group to convert the mass of ions of elements highly reactive with these gases, and then measuring a specific signal that has passed through a quadrupole mass filter. Depending on the target of measurement, only the column may be used, or only the ion mass conversion using oxygen or ammonia gas may be used, but by using both, for example, even with low-concentration samples on the order of ppt, it is possible to reliably eliminate interference from other elements and radionuclides and quantify with high accuracy without complex pretreatment. Furthermore, this analytical method can be automatically controlled using appropriate software. In addition, the sample solution can be concentrated to an appropriate concentration before being introduced into the column or filter.

According to the present invention, alpha-emitting radionuclides can be quantified individually without requiring complicated pretreatment by separating substances using a column packed with solid-phase extraction resin, precisely separating isobars using a collision-reaction cell, and separating ions other than those with a specific mass number using a quadrupole mass filter.

Figure 1 shows a schematic configuration of an apparatus suitable for carrying out the analytical method of the present invention. Figure 2 shows an example of separating a mixed standard solution containing alpha-emitting radionuclides using a column. Figure 3 shows the changes in plutonium and americium due to oxidation reactions in a collision reaction cell. Figure 4 shows the changes in plutonium and americium due to the reaction with ammonia gas in a collision reaction cell. Figure 5 shows the results of simultaneously measuring 10 alpha-emitting radionuclides using the method of the present invention.

The embodiments of the present invention will be described below with reference to the accompanying drawings. It goes without saying that the present invention is not limited to the embodiments described below, and various modifications and alterations are possible.

《Analyzer》
Figure 1 shows a schematic configuration of an apparatus suitable for carrying out the analytical method of the present invention. In Figure 1, 1 is an autosampler (automatic preparative device) that stores sample containers filled with sample solutions obtained by eluting the sample to be analyzed in a suitable dissolving solution, 2 is a column, and 3 is a high-frequency inductively coupled plasma mass spectrometer, which includes a nebulizer (coaxial nebulizer or ultrasonic nebulizer) 4, a plasma ion source 5, an interface cone 6, an ion lens 7, a collision reaction cell 8 equipped with a gas supply means 8a, a quadrupole mass filter 9, and a detector 10.

"column"
In Figure 1, the sample solution filled in the sample container stored in the autosampler (automatic sorting device) 1 is supplied to column 2. Alpha-emitting radionuclides and the like are adsorbed onto the solid-phase extraction resin (not shown) packed in column 2.

《Adsorption process》
The autosampler (automatic sample preparation device) 1 draws up a sample solution filled in a specific sample container and supplies it to column 2 using a pump (not shown), allowing the alpha-emitting radionuclide to be adsorbed onto the solid-phase extraction resin packed in column 2. After supplying a certain amount of sample solution to column 2, the probe of the autosampler 1 moves to the position containing the eluent. The sample solution after passing through column 2 may be directly introduced into an inductively coupled plasma mass spectrometer (ICP-MS, ICP-MS/MS), or it may be drained through a drain (not shown), and is not particularly limited.

《Elution process》
In the process of eluting alpha-emitting radionuclides adsorbed on solid-phase extraction resin packed in column 2, the eluent is supplied from a pump and delivered to column 2. In column 2, the alpha-emitting radionuclides adsorbed on the solid-phase extraction resin are eluted according to the supplied eluent and introduced into an inductively coupled plasma mass spectrometer (ICP-MS). For introduction, a coaxial nebulizer or an ultrasonic nebulizer (USN) is used.

《Separation of alpha-emitting radionuclides》
Figure 2 shows an example of separating a mixed solution containing alpha-emitting radionuclides using a column configured as described above. The composition of this mixed solution is a nitric acid (4M HNO3 + 0.1M NaNO2) solution containing 235U, 238U, 241Am, 244Cm, 245Cm, 246Cm, 240Pu, 242Pu, 232Th, and 237Np. 220 mg of solid-phase extraction resin (DGA resin) was packed into the column. In this example, the mixed solution was passed through the column, the alpha-emitting radionuclides were adsorbed onto the solid-phase extraction resin, and then the eluent was passed through the column. The eluent after passing through the column was then sequentially introduced into an inductively coupled plasma mass spectrometer (ICP-MS) to quantify the alpha-emitting radionuclides contained in the eluent after passing through the column. In Figure 2, the horizontal axis represents time, and the correspondence with each step in the table above the graph is shown at the top of the graph. The vertical axis represents the detection intensity obtained by inductively coupled plasma mass spectrometry (ICP-MS). Since high-purity oxygen was introduced into the ICP-MS collision reaction cell during detection, all alpha-emitting radionuclides in the mixed solution were detected as oxides. As shown in Figure 2, alpha-emitting radionuclides could be separated by using a solid-phase extraction resin.

《Isostracal interference removal by collision reaction cells》
As shown in Figure 2, while uranium can be separated from the alpha-emitting radionuclides, americium and plutonium (241Am and 241Pu), which have isobars, remain in the solution without being separated. If this solution is introduced into a high-frequency inductively coupled plasma mass spectrometer, isobars with the same mass number cannot be separated.
Therefore, in Figure 1, by supplying gas (oxygen or ammonia gas) from the gas supply means 8a into the collision reaction cell 8, the differences in reactivity between these gases and each alpha-emitting nuclide, for example, by setting the amount of oxygen gas within a predetermined range, americium is converted to AmO and plutonium to PuO2. Also, by setting the amount of oxygen gas within a predetermined range, only plutonium is converted to PuO, and U is converted to UO2, making it possible to separate and measure the isobaric plutonium and uranium.

Figure 3 shows the difference in products produced by the oxidation reaction of americium and plutonium in a collision reaction cell, based on the oxygen gas flow rate. The horizontal axis represents the oxygen gas flow rate introduced into the collision reaction cell, and the vertical axis represents the detection intensity obtained by an inductively coupled plasma mass spectrometer (ICP-MS). It can be seen that when the oxygen flow rate is 2 mL/min or higher, americium is converted to AmO, and plutonium is converted to PuO2 instead of PuO. The oxygen flow rate is preferably between 2.0 mL/min and 3.0 mL/min. As shown in Figure 3, if the oxygen flow rate is too high, the detection intensity of both AmO and PuO2 decreases, so 3.0 mL/min or less is preferable. Therefore, it can be seen that by setting the oxygen flow rate between 2 mL/min and 3.0 mL/min, 241Pu and 241Am can be separated and analyzed.

Figure 4 shows the difference in products resulting from the reaction of uranium and plutonium with ammonia gas in a collision reaction cell, based on the ammonia gas flow rate. The horizontal axis represents the ammonia gas flow rate introduced into the collision reaction cell, and the vertical axis represents the detection intensity obtained by an inductively coupled plasma mass spectrometer (ICP-MS). From Figure 4, it can be seen that when the ammonia gas flow rate is 1.0 ml/min or higher, uranium is converted to an ammonia adduct (e.g., UN2H4), while plutonium remains approximately as Pu. From the above, it can be seen that by setting the ammonia gas flow rate to 1.0 ml/min or more and 2.0 ml/min or less, 238Pu and 238U can be separated and analyzed. Furthermore, since the detection intensity of plutonium decreases as the ammonia gas flow rate increases, the introduced ammonia gas flow rate is more preferably 1.0 ml/min or more and 1.5 ml/min or less. Furthermore, for quantitative analysis, a calibration curve is typically created using the method employed with inductively coupled plasma mass spectrometers (ICP-MS, ICP-MS/MS), allowing for quantitative analysis from the detected values.

Figure 5 shows the results of simultaneous measurement of 10 alpha-emitting radionuclides using the method of the present invention. DGA resin was used as the solid-phase extraction resin.
First, in step 1, the column packed with DGA resin was washed with 8 ml of washing solution (4 M HNO3 + 0.1% H2O2 solution). Then, in step 2, a sample solution containing Th, Np, Am, Cm, Pu, and U (5 ml of 4 M HNO3 + 0.1% H2O2 solution, 20 ppt of Th, 20 ppt of 241 Am, 5.36 ml of 244 Cm, 1 ppt of 242 Pu, 20 ppt of U, 237 Np originated from the 241 Am sample, and 240 Pu, 245 Cm, and 246 Cm originated from the 244 Cm sample) was introduced into the column, and the above alpha-emitting radionuclides were adsorbed onto the DGA resin. Subsequently, in step 3, the column was washed with 8 ml of the above washing solution. Next, in step 4, 45 ml of uranium eluent (0.2 M HNO3 + 0.1% H2O2) was introduced into the column to elute the uranium. The resulting eluent was then introduced into a high-frequency inductively coupled plasma mass spectrometer (ICP-MS/MS; NexION Perkin Elmer) at a solution flow rate of 300 μl/min, and the detection intensity shown as step 4 in Figure 5 was obtained. The solution flow rate was kept constant until the end of step 7, and oxygen gas was introduced into the collision reaction cell at a rate of 2.0 ml/min from the start to the end of detection. After confirming that the uranium elution was approximately complete by checking the detection intensity, in the next step 5, 17 ml of americium and curium eluent (0.5 M HCl + 0.1% H2O2) was introduced into the column to elute the americium and curium. The detection intensity shown as step 5 in Figure 5 was obtained by directly introducing the eluent into an inductively coupled plasma mass spectrometer (ICP-MS/MS). Confirmation of the detection intensity confirmed that the elution of americium and curium was approximately complete. As the next step, step 6, 26 ml of plutonium eluent (0.5 M HCl + 1% ascorbic acid) was introduced into the column to elute plutonium. This eluent was then introduced into an inductively coupled plasma mass spectrometer (ICP-MS/MS), and oxygen gas was introduced into the collision reaction cell at a rate of 2.0 ml/min. The detection intensity shown as step 6 in Figure 5 was obtained. Finally, as step 7, 16 ml of thorium and neptunium eluent (0.01 M ammonium oxalate) was introduced into the column to elute thorium and neptunium. The eluent was directly introduced into an inductively coupled plasma mass spectrometer (ICP-MS/MS), and oxygen gas was introduced into the collision reaction cell at a rate of 2.0 ml/min. The detection intensity shown as step 7 in Figure 5 was obtained in this manner. Thus, the method of the present invention eliminates interference from other elements and nuclides such as rhenium trioxide (mass number 235) and lead dioxide (mass number 239), and allows for the simultaneous measurement of 10 alpha-emitting nuclides with higher accuracy, even at low concentrations of a few ppt, without the need for complicated pretreatment.

Based on the above, by adjusting the type of gas introduced into the collision reaction cell of the present invention and the flow rate of that gas, alpha-emitting radionuclides can be separated and quantified without complicated procedures, even in the case of isobars.

The method of the present invention can quantitatively analyze multiple alpha-emitting radionuclides simultaneously without requiring complicated steps, making it applicable to various environmental analysis institutions and businesses.

1. Autosampler (Automatic Sampler)
2. α-nuclide enrichment and separation column or filter 3. High-frequency inductively coupled plasma mass spectrometer 4. Coaxial nebulizer or ultrasonic nebulizer 5. Plasma ion source 6. Interface cone 7. Ion lens 8a. High-purity oxygen supply means 8. Collision/reaction cell 9. Quadrupole mass filter 10. Detector

Claims (3)

A method for analyzing a sample containing multiple alpha-emitting radionuclides,
A solution of the sample is introduced into a column equipped with a solid-phase extraction resin that adsorbs at least one of the aforementioned plurality of alpha-emitting radionuclides.
The above-mentioned solid-phase extraction resin is used to adsorb at least one of the multiple alpha-emitting radionuclides contained in the sample solution.
A step of separating at least one of the plurality of alpha-emitting radionuclides by eluting the alpha-emitting radionuclides adsorbed on the solid-phase extraction resin into an eluent,
The process includes the steps of atomizing the eluent and introducing it into a plasma ion source for ionization, introducing oxygen gas into the ion group to convert the mass of at least one ion of an alpha-emitting nuclide contained in the ion group, and then measuring the signal of a specific mass that has passed through a quadrupole mass filter.
The eluent after the separation step contains plutonium and americium.
A method for analyzing a sample containing multiple alpha-emitting radionuclides, characterized in that the oxygen gas introduction flow rate is 2 ml/min or more and 3 ml/min or less. A method for analyzing a sample containing multiple alpha-emitting radionuclides,
A solution of the sample is introduced into a column equipped with a solid-phase extraction resin that adsorbs at least one of the aforementioned plurality of alpha-emitting radionuclides.
The above-mentioned solid-phase extraction resin is used to adsorb at least one of the multiple alpha-emitting radionuclides contained in the sample solution.
A step of separating at least one of the plurality of alpha-emitting radionuclides by eluting the alpha-emitting radionuclides adsorbed on the solid-phase extraction resin into an eluent,
The process includes the steps of atomizing the eluent and introducing it into a plasma ion source for ionization, introducing ammonia gas into the ion group to convert the mass of at least one ion of an alpha-emitting nuclide contained in the ion group, and then measuring the signal of a specific mass that has passed through a quadrupole mass filter.
The eluent after the separation step contains plutonium and uranium.
A method for analyzing a sample containing multiple alpha-emitting radionuclides, characterized in that the flow rate of the ammonia gas introduced is 1 ml/min or more and 2 ml/min or less. In the separation step, a plurality of alpha-emitting radionuclides are adsorbed onto the solid-phase extraction resin.
In the separation step, the first nuclide is eluted using an eluent that elutes the first nuclide among the plurality of alpha-emitting nuclides.
The measurement process is performed using the obtained eluent.
After measuring the first nuclide, in the separation step, the second nuclide is eluted using an eluent that elutes a second nuclide different from the first nuclide.
A method for analyzing a sample containing a plurality of alpha-emitting radionuclides according to claim 1 or 2, wherein the measurement step is performed using the obtained eluent.