CN108414607B

CN108414607B - Detection method for measuring ultra-trace radioactivity background of polymer

Info

Publication number: CN108414607B
Application number: CN201810023562.6A
Authority: CN
Inventors: 刘洪涛
Original assignee: Sun Yat Sen University
Current assignee: Sun Yat Sen University
Priority date: 2018-01-10
Filing date: 2018-01-10
Publication date: 2021-01-01
Anticipated expiration: 2038-01-10
Also published as: CN108414607A

Abstract

The invention belongs to the field of analytical chemistry, and relates to a detection method for measuring ultra-trace radioactivity background in a polymer. The invention adopts a pretreatment mode of a dry ashing method, establishes an analysis method for measuring the ultra-trace radioactivity background by ICP-MS, and realizes the analysis of ultra-trace elements in organic glass. The detection limit of the detection method can reach below 1pg/g, and in addition, the method also has the advantages of simple operation, rapidness and accuracy.

Description

Detection method for measuring ultra-trace radioactivity background of polymer

Technical Field

The invention belongs to the field of analytical chemistry, and relates to a detection method for measuring ultra-trace radioactivity background in a polymer.

Background

(1) Background of the study of background measurement of radioactivity

Particle physics is a widely recognized science of researchers as one of the most important subjects in advanced scientific research. The mesoparticle research plays an important role in international particle physics research, is a new physics which is unique experimental evidence up to now and surpasses a particle physics standard model, and provides sufficient evidence for solving the origin and evolution of a microscopic material structure and a macroscopic universe. Therefore, the study of Zhongwei is the subject of research by modern researchers. After 2012's experiments in the gulf of central asia of china found a new oscillation of the central microson, the second central microson experiment plan was started to be constructed and implemented at the end of 2014, the best site of the experiment workstation was located at the gate of the river, and the best precision, the lowest background and the largest scale central microson the world were tried to be constructed, so as to thoroughly reveal the mystery of the central microson mass sequence.

Currently, the experimental station of the jiangmen's medicolegal project to construct a large experimental facility in 700 meters underground, including a plexiglass central detector containing 20kt of liquid scintillator and a small number of supporting devices. The organic glass sphere is supported by more than 500 stainless steel support nodes, and the outer layer is provided with nearly 20000 photomultiplier tubes for observing weak light signals generated by interaction of the neutrons and the liquid scintillator medium. Because natural radioactive elements are commonly present in the daily environment of our life, the atomic nuclei of the natural radioactive elements are unstable and often radiate in the energy forms of alpha rays, beta rays, gamma rays and the like, and meanwhile, a large number of cosmic rays exist in the environment, optical signals generated by the natural radioactive elements and the cosmic rays far exceed meson signals, and serious background interference is caused for the detection of the meson signals. Therefore, high demands are made on the surrounding environment of the neutron detector: 1) the device is built in the deep underground, and a large amount of cosmic rays are blocked by a rock stratum; 2) the middle part uses ultrapure water as a shielding layer, and the decay rays of natural radioactive elements in the environment are blocked by water.

In addition, in the fields of nuclear physics, particle physics and celestial particle physics, a great deal of experimental results show that the detector is most severely interfered by the radioactive background of the device materials for almost all experiments, such as U-238, Th-232, K-40, Co-60 and the like. The radioactive isotopes have alpha and beta decay in a long period of time, and generate light signals such as gamma rays, and the radioactive decay naturally becomes a background case, so that the sensitivity and the working efficiency of the neutron detector are restricted, and the expected physical result of an experiment is influenced. Obviously, the complex detector device is built by materials such as organic glass, stainless steel connecting pieces, photomultiplier tubes, cables and the like. The radioactive isotopes U-238, Th-232, K-40 and Co-60 in the materials are commonly present in the environment, so that the radioactive isotopes can be introduced in the production, processing, transportation, detector assembly and other links of the materials. At present, effective control of background of detector materials faces huge challenges, and meanwhile, measuring the radioactive background of various materials of the detector and screening qualified materials are a great subject.

Based on the research background, the radioactive background of the materials used by the underground laboratory infrastructure and the neutron experiment detector system needs to be measured and evaluated, reference opinions are provided for material selection of the neutron detector, and the most suitable material is found to serve as the neutron detection device.

(2) Current state of the art radioactive element measurement method

Since the discovery of radioactivity in the 90 s of the 19 th century, radioactive elements have important research value in numerous disciplines, such as: life sciences, environmental sciences, geologic chronology, nuclear applications, and the like. At present, there are many methods for measuring radioactive elements, and two major techniques are in the mainstream: 1) conventional radioactivity measurement analysis techniques; 2) non-radioactive measurement analysis technique: mainly based on mass spectrometry.

Conventional radiometric analysis techniques have been frequently used, and mainly include the following three aspects: alpha count, beta count, gamma count. For the measurement of ultra trace radioactive background in materials studied by the subject, the above methods all have common drawbacks: 1) the measurement of the ultra-trace radioactivity background is limited by the sensitivity and detection limit of the instrument; 2) the analysis period is longer; 3) the background noise is large; 4) the sample consumption is large; 5) the counting rate is low. Therefore, the method is not ideal for quickly and accurately measuring the ultra trace elements.

There are many methods based on mass spectrometry, which mainly include: spark Source Mass Spectrometry (SSMS), Laser Ionization Mass Spectrometry (LIMS), Glow Discharge Mass Spectrometry (GDMS), Accelerator Mass Spectrometry (AMS), surface Thermal Ionization Mass Spectrometry (TIMS), Secondary Ion Mass Spectrometry (SIMS), Resonance Ionization Mass Spectrometry (RIMS), inductively coupled plasma mass spectrometry (ICP-MS), and the like. The ICP-MS has the characteristics of simple operation mode, a sample introduction system and multi-element rapid analysis, and is popular with analysis testers in actual analysis work. ICP-MS is used as a part of an inorganic mass spectrometry technology, and the analysis capability of ion counting is achieved, so that the radioactive element analysis method is expanded. Generally, the radioactivity measurement results in a sample radioactivity, which reflects the number of atoms the radionuclide decays per unit time, denoted as A, in Becker (Bq). Indeed, the radiometric method measures the products of the nuclear rearrangement process, not the radionuclide itself. This method is distinct from mass spectrometric measurement of radionuclide concentrations, however, since nuclear rearrangement occurs in a predictable manner, the correlation between the two methods can be established by the following equations (1-1 or 1-2).

In the formula: m-element mass, g or g.L^-1

A-Activity of radioactivity (decay Rate) Bq or Bq. L^-1

M-molar mass, g.mol^-1

N_AAvogastron constant

Lambda-decay constant, s^-1

t_1/2Half-life, s

ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) is an Inductively Coupled Plasma-Mass Spectrometry, ICP is an excellent light source found in the early 70 th 20 th century, and ICP is combined with the advantages of high sensitivity and less interference of a Mass spectrometer by the strong advantage of an ion source in the 80 th century to become a trace ultra-trace inorganic element analysis technology which is generally accepted as the most advantageous. The ICP-MS technology is well established as can be seen from the number of emerging academic papers, and the technology is developed more and more mature. The research application in geological science is rapidly expanded to a plurality of discipline fields such as environment, metallurgy, petroleum, biology, medicine, semiconductor, nuclear material and the like. Although the ICP-MS technology is mature, research on analysis performance of instruments is never stopped, and the ICP-MS technology still has great application prospect and development space.

Basic principle of ICP-MS analysis of radioactivity: the sample is introduced in the form of solution, is carried into an atomization system by carrier gas (argon) for atomization, enters an axial channel of a central rectangular tube in the form of aerosol, is fully evaporated, dissociated, atomized and ionized under the atmosphere of high temperature and inert gas, ions are extracted and focused by a lens system and then enter a mass analyzer, and the mass analyzer is separated according to different mass-to-charge ratios of the ions. The ion signal is received by an electron multiplier tube, amplified and detected. The presence or absence of the element can be qualitatively detected based on the characteristic mass of the ion, and the ion current intensity of the element is proportional to the concentration of the element, which determines the mass of the element in the sample. The mass (m) of the target isotope is calculated through the natural isotope abundance, and the mass (m) is knownDecay constant (λ) or half-life (t) of each radionuclide_1/2) In the case of (2), the desired activity (A) is converted by the formula (1-1) or (1-2).

ICP-MS analysis advantages: ICP-MS is widely used for measurement of radioactive elements, and is not distinguished from instrument-specific analytical properties:

1) high sensitivity, low background and low detection limit. With the research and improvement of modern instrument analysis technology, the superior analysis performance of the instrument is created. Such as: the improvement of the thermal plasma high sensitivity mode, the ion lens focusing system and the electron multiplier not only greatly improves the sensitivity of the instrument, but also effectively reduces the background noise. It is due to the improved sensitivity and background that the detection limit of the instrument is also greatly reduced. Therefore, the method is particularly suitable for trace ultra trace element analysis.

2) The ability to simultaneously analyze multiple elements. The quadrupole rod mass analyzer provides peak jump scanning and multi-channel scanning modes, can scan and acquire data of a plurality of elements in the same sample, greatly improves the analysis efficiency and shortens the analysis period.

3) Relatively simple sample pretreatment. ICP-MS adopts a solution-form sample introduction mode, and the sample is subjected to simple acid dissolution, digestion or melting and other treatment.

4) The sample injection volume is small. The improvement of the peristaltic pump, the high atomization efficiency, the rapid analysis of the detector and the like enable the amount of samples required for completing one-time sample analysis to be small, namely about a few milliliters.

5) The linear dynamic range is wide. Modern instruments employ dual-mode discontinuous dynode electron multipliers to simultaneously determine the low-concentration and high-concentration element contents of the same sample in a pulse and analog signal mode. The dynamic linear range of accurate detection can reach 8-9 orders of magnitude, and great convenience is brought to actual analysis and test.

6) The interference is less. ICP-MS has a relatively simple mass spectrum, producing only one or a few isotopic singly charged ion peaks per element. There is still a small amount of doubly charged ions and simple polyatomic ion interference. In addition to adding interference correction formula and quality monitoring measures in software design, modern instruments also adopt advanced design in instrument hardware, such as adopting technologies of cold plasma, shielded torch, collision/reaction tank and the like.

Compared with the radioactivity measurement method, the mass spectrometry technology has the advantages. TABLE 1 Mass Spectrum vs. alpha Spectrum for radionuclides²²⁶Ra and^239,240measurements of Pu were compared. As is evident from the data in the table: the mass spectrometry technology has significant advantages in performance such as detection limit, sample analysis duration, sample usage and the like.

TABLE 1 Mass Spectroscopy and alpha Spectroscopy vs. radionuclides²²⁶Ra and^239,240analytical Performance of Pu^[1]

(3) Research status about ICP-MS measuring radioactive elements

Throughout the measurement methods of radioactive elements mentioned at home and abroad, inductively coupled plasma mass spectrometry (ICP-MS) is distinguished from many methods by the advantages of high sensitivity, low detection limit, rapid multi-element simultaneous analysis and the like. In the past, the measurement of radioactive elements is mostly trace analysis, and the traditional radioactivity measurement technology can achieve the aim. However, the background of radioactivity of materials around a neutron detector, for example, is beyond the measurement range of the conventional radioactivity measurement technology, and the conventional radioactivity measurement method is difficult to meet the requirements from the technical point of view. In addition, the traditional radioactive measurement method has long analysis period, large sample consumption and relatively complex operation, and the traditional method has many defects from the viewpoint of economic benefit. Therefore, ICP-MS can be an important aid and complement in long half-life radioelement measurements. The method can not only ensure the accuracy and reliability of the experimental result, but also be different from the conventional method, and simplify the analysis process. Especially for trace ultra trace element analysis, the problems of pollution and loss in the sample processing process can be greatly reduced. The measurement research of ICP-MS on the radioactive background of the material can be established, the problems in the aspects of technology and economy can be effectively solved, and the method has important significance for the analysis and measurement work of actual samples.

In the prior art, ICP-MS has been used for measuring the radioactivity background, for example, the laboratory of the inventor uses the existing ICP-MS instrument to complete U, Th measurement of ng/g level in the iron-based amorphous strip material, and satisfactory results are obtained. However, the ICP-MS measurement methods in the prior art still cannot meet the detection requirements for the background case in the neutron detector experiment, the detection of radioactive elements still stays at ng/g, and how to span the detection level of metal elements such as U, Th from ng/g to pg/g is a troublesome problem.

Disclosure of Invention

The invention aims to provide a method for measuring radioactive background in organic materials by adopting ICP-MS. The invention also aims to provide a method for measuring the radioactive background, which is simple and convenient to operate, extremely high in sensitivity and stable, and can be used for detecting the ultra-trace radioactive background in organic matters, particularly organic polymers. Powerful reference data are provided for research of the neutron detector, experimental data obtained by the method are uploaded to a database to share results, and repeated work can be avoided.

The above object of the present invention is achieved by the following technical means:

the invention provides a detection method for measuring ultra-trace radioactive background in a polymer, which comprises the following steps:

(1) putting the polymer into a quartz crucible for coking treatment;

(2) the coked polymer is incinerated together with the crucible until no obvious residue is left;

(3) cooling to room temperature after ashing, adding HNO-containing solution into the crucible₃And aqueous HCl;

(4) heating the quartz crucible until the solution boils slightly;

(5) after cooling, it was measured by ICP-MS.

Wherein the radioactive background is a non-volatile radionuclide; preferably, said radiationThe background of sexual activity is²³⁸U、²³²Th、⁶⁰Co、⁴⁰One or more of K; more preferably, the radioactivity is locally²³⁸U、²³²Th、⁶⁰One or more of Co; more preferably, the radioactive background is²³⁸U and/or²³²Th. Wherein,²³⁸U、²³²Th、⁴⁰k is more radioactive and is the subject of major interest in the field,⁶⁰co is less radioactive, and⁴⁰k is affected by allomones in ICP-MS analysis⁴⁰Interference of Ar.

Wherein the polymer is selected from organic polymers. Organic polymers that can be treated with a char ash are suitable treatment targets for this invention. As illustrative examples, such organic polymers as biomedical polyesters, polyvinylidene fluoride (PVDF), polymethyl methacrylate, and the like. In an exemplary embodiment of the invention, the polymer is selected from the group consisting of polymethylmethacrylate (colloquially referred to as plexiglass).

Specifically, in the method, in the step (1), the sampling amount of the polymer is 10-25 g; preferably 18-22 g; more preferably 20 g. In a preferred embodiment, the sampling is performed by a quartering method.

In the step (1), coking treatment is carried out on an electric furnace; further, the temperature of coking treatment is 200-400 ℃; in a preferable embodiment, the temperature of the coking treatment is 250-350 ℃; in a more preferred embodiment, the coking process is carried out at a temperature of 300 ℃. The organic glass is melted rapidly due to the over-high temperature, because the air contains O₂Easy to catch fire; the temperature is too low, the reaction takes long time, and the coking is not facilitated. As a mode, the polymer can be placed in a quartz crucible, and then the crucible is placed on an electric furnace for coking, the open coking mode is convenient for adding the substance to be analyzed at any time, the coking reaction speed can be controlled by adjusting the temperature, and whether the reaction is complete or not can be observed visually.

In the step (2), the ashing temperature is 450-600 ℃; the ashing time is not less than 30 min; as a preferred embodiment, the ashing temperature is 500 ℃; the ashing time was 30 min.

In the step (3), HNO is contained₃And aqueous HCl, HNO₃The volume fraction of (A) is 1-10%; preferably, it is 2%; the volume fraction of HCL is 0-1%; preferably, it is 0.5%.

Preferably, step 3, comprises HNO₃And the volume of aqueous HCl is 1-2 mL.

It is noted that for HNO containing₃And volume of aqueous HCl containing HNO₃And the volume of the aqueous solution of HCl is strictly controlled, the volume cannot be 1mL lower than the volume of an ICP-MS test sample injection, the sample concentration is reduced due to too large volume, the detection limit of an instrument cannot be reached, the measurement result is unreliable, and therefore the volume is controlled to be between 1 and 2 mL.

The polymer mass is set according to the expected method detection limit, according to the formula

SD-standard deviation of 11 blank solution measurements, among others; m-sample size

Sd (u) 0.633pg, sd (th) 1.39 pg; the method detection limit of 1pg/g is expected, and therefore the sampling amount m is not less than 4.18g, and to achieve accurate quantification of 1pg/g, the method quantification limit LOQ is 3.3LOD, and therefore the sampling amount m is not less than 13.9g, and further, a larger sampling amount not only increases the time required for pretreatment but also consumes the sample, and therefore, in general, the sampling amount of 20g is preferred. Therefore, more preferably, the sample size is 20 g.

It is also noted that, as a preferred embodiment, HNO in step (3)₃And HCl are separately purified prior to use to remove metal ions and solid particulates from the acid, thereby reducing reagent blank. The experiment of the invention proves that the acid reagent used in the experimental process brings a large blank if the acid reagent is not purified. The purification method can be a reagent purification method commonly used in laboratories, such as the respective purification of HNO by using a sub-boiling distillation method₃And HCl; as a more preferred embodiment, HNO₃And HCl are separately purified by 3 sub-boiling distillations.

In the step (3), when the acid solution is added, the crucible wall is uniformly soaked along the crucible wall, so that the analyte is dissolved in the acid solution.

In the step (4), the crucible added with the acid solution is placed on an electric hot plate and heated to be slightly boiled, so that an analyte is further fully dissolved in the acid solution, and further the heating temperature is 100-200 ℃; preferably, the heating temperature is 150 ℃. And (4) after heating is finished, waiting for measurement by an ICP-MS instrument.

In the step (5), the parameters of ICP-MS are preferably matched with the substrate and the pretreatment method of the invention, and in the invention, as a preferred embodiment, the parameters of the ICP-MS instrument used for measuring U, Th are; RF power: 1400-1700W; flow rate of carrier gas: 0.8-1.0L/min; flow rate of auxiliary gas: 0.7-0.9L/min; cooling air flow rate: 13.0-15.0L/min; CCT flow rate: 4.0-6.0 mL/min; the measurement mode is one of KED or STD. In a preferred embodiment, the KED mode is selected.

More preferably, the parameters of the ICP-MS are RF power: 1548.6W; flow rate of carrier gas: 0.930L/min; flow rate of auxiliary gas: 0.795L/min; cooling air flow rate: 13.88L/min; CCT flow rate: 4.809 mL/min.

For trace radioactive background analysis, the blank value of the environment in which the experiment is carried out is increased, and the experiment proves that the blank value of the environment in a ten thousand grade clean room is slightly better than that of the common laboratory, and the difference is larger for analysis of Th. As a preferred embodiment, the entire detection method of the present invention is performed in a ten thousand clean room.

In addition, vessel contamination is a non-negligible factor, and particularly in the aspect of ultra-low content measurement, vessel materials often have certain influence on elements to be measured, such as adsorption loss or container contamination. In the experiment, for the detection of metals, in order to reduce such contamination, acid bubbles are generally used, followed by washing. In the invention, the samples to be tested, such as organic glass, quartz crucible and other containers which can possibly be used as basic samples to be tested, are soaked in acid before the experiment and then washed by ultrapure water.

Arnqquist I J et al have already been adoptedDry ashing and ICP-MS were used to measure U and Th in some polymers, however, in this method, dry ashing treatment of samples using ultra-low background electroformed copper foil as a container was required^[2]. However, this also requires removal of copper through a resin column for ICP-MS analysis. The copper foil has a certain blank value, and the comparative example 1 proves that the resin column passing also generates a larger blank value, so that the problems of high whole-sequence blank, high detection limit, loss of substances to be detected and the like are caused by more steps, and the influence can be ignored for the substances to be analyzed with larger content, but the method is extremely unfavorable for the analysis of the extremely trace radioactive background. Furthermore, in the method of Arnquist I J, which is²³⁸U、²³²The detection line of Th can only reach more than 1pg/g, but can not reach the measurement requirement of less than 1pg/g in the neutron detector material. In the present invention, the detection limit of the method U, Th is less than 1pg/g, even less than 0.3 pg/g. The lower detection limit of U compared with Th is as low as 0.1pg/g (see Table 3), which indicates that the method of the present invention is more advantageous for the detection of U, and the relative standard deviation of U in the actual sample analysis is lower than Th (Table 4).

The invention adopts a simpler processing method in a breakthrough way, but obtains a lower detection limit, which benefits from ingenious design among all the steps of the invention and ingenious balance on the selection and the dosage of various materials.

For example, the invention adopts a high-purity quartz crucible as a vessel to carry out coking treatment on a sample and then uses acid solution which is purified for many times for redissolving, and directly uses a specific dosage of HNO₃And aqueous HCl solution into the crucible and then heated, which are specifically studied and skillfully designed and selected, so that the present invention achieves a very outstanding detection sensitivity and operational simplicity.

The method has the beneficial effects that:

the sample pretreatment has the following advantages:

1) the invention has the advantages that the detection limit of nonvolatile metal in the polymer is reduced to 0.X pg/g, which is a great breakthrough for detecting the radioactive background, because the detection method in the prior art is difficult to meet the measurement requirement below 1pg/g, and for such as a neutron detector, an extremely sensitive detection method is urgently needed for instruments with strict radioactive background requirements. The method of the invention reduces the detection limit of the radioactive background to 0.X pg/g level, screens out the detector research and development material meeting the physical requirements by carrying out the measurement of the radioactive background of the material, can obviously reduce the background case in the detector experiment, achieves the purpose of improving the signal-to-noise ratio of the neutron detector, realizes the effective detection of the neutron, makes great contribution to the understanding of the microcosmic particle physical laws for human beings, and also makes great contribution to the cosmology, the celestial physics and even the geophysical physics;

2) the method is simple, rapid and accurate, adopts the conventional dry ashing method, and only needs 2 hours in the whole analysis process;

3) the feeding of the sample amount is not limited, and the sample can be supplemented in time as required when the electric furnace is opened for heating;

4) less acid is used in the digestion process, so the reagent blank and other pollution possibility are reduced;

5) the method and the idea of the invention can provide an idea for carrying out measurement on other types of materials in the detector, such as copper fasteners, stainless steel water tanks, support frames and the like.

Drawings

FIG. 1 technical scheme;

FIG. 2 is a flow diagram of the extraction in comparative example 1;

FIG. 3 comparative example 1 with HCl followed by H₂C₂O₄The recovery rate of elution;

FIG. 4 digestion ramp-up procedure in comparative example 1, where procedure A: keeping at 190 deg.C for 15 min; procedure B: keeping at 210 deg.C for 15 min; procedure C: keeping the temperature at 210 ℃ for 60 min;

FIG. 5 is a graph showing the change of the hydrolysis solution after water addition in comparative example 1, wherein (a) before water addition: is clear and transparent; (b) adding water: white turbidity (procedure A, B); (c) adding water: clear and transparent (procedure C);

FIG. 6 HNO in comparative example 1₃Effect of multiple purifications on U, Th blank values;

FIG. 7 is a graph showing the temperature, pressure and power changes in the process of comparative example 2;

FIG. 82% HNO₃+ 0.5% HCl as elution solvent before injection, elution effect on U, Th.

Detailed Description

The technical solutions of the present invention are further illustrated by the following specific examples, which do not represent limitations to the scope of the present invention. Insubstantial modifications and adaptations of the present invention by others of the concepts fall within the scope of the invention.

Example 1 one of the pretreatment methods: dry ashing process

The dry ashing method is generally used for destroying organic matters in a sample, and generally comprises the steps of firstly placing a crucible on an electric hot plate for heating, decomposing and carbonizing, then transferring the crucible to a muffle furnace for ashing, and finally dissolving residues, so that the dry ashing method is widely applied to biological tissues, foods and environmental samples.

In the experiment, the dry ashing method is selected for carrying out sample pretreatment on the organic glass, because the organic glass is a high polymer formed by polymerizing organic monomers and contains a large amount of organic matrixes; secondly, the elements U, Th researched by the experiment are all elements which are difficult to volatilize and cannot be lost due to volatilization; furthermore, the method can increase the sampling amount and reduce the reagent pollution, so the method can be used for the research of the experiment. In the experiment, the dry ashing method is investigated through blank value and blank labeling experiments, and the dry ashing method is evaluated from the aspects of linearity, method detection limit, method quantitative limit, precision, accuracy and the like.

The experimental steps are as follows:

(1) preparation of the experiment

1) Sub-boiling distillation method for respectively purifying HNO₃And HCl;

2) sample pretreatment;

organic glass samples (15cm x 2cm x 3mm) are in the form of strips, and in order to fully digest the sample, it is desirable to obtain a sample with a small volume to increase the contact area for the reaction, and to knock the sample into a granular form. Soaking in 30% nitric acid for 10min, cleaning mechanical impurities on surface, rinsing with ultrapure water for three times, and air drying.

(2) And (3) dry ashing treatment: weighing the empty crucible with the mass m1, accurately weighing 20g of sample (accurate to 0.0001g) into a quartz crucible, slowly heating on an electric furnace (300 ℃) until the sample is gradually carbonized into a coke cluster shape, taking down and moving to a muffle furnace at 500 ℃ for dry ashing for 30 min. No obvious residue is left after dry ashing, and the mixture is taken out and cooled to room temperature. 2mL of 2% HNO was added dropwise along the crucible wall₃+ 0.5% HCl solution after wetting the crucible walls, heat on a hot plate (150 ℃ C.), remove the solution immediately after it boils slightly, and weigh m2 after cooling. The solution in the crucible (Δ m — m2-m1) was transferred to a centrifuge tube and measured by ICP-MS. The optimized parameters for ICP-MS are as follows in Table 2. Under the optimized pretreatment method and detection conditions, experiments prove that the method disclosed by the invention obtains a very low detection limit and ideal recovery rate and precision in the field.

TABLE 2 operating parameters of ICP-MS instrument

Results and discussion

(1) Linear, methodological detection limit, and quantitative limit

Preparation of a standard working curve: 50 μ L of U, Th standard stock solution (100 μ g/mL) was accurately removed with 2% HNO₃+ 0.5% HCl solution to prepare 100 mug/L primary intermediate mixed standard stock solution; ② accurately transferring 500 mu L of U, Th primary intermediate mixed standard stock solution (100 mu g/L), and using 2% HNO₃+ 0.5% HCl solution to prepare 1 microgram/L secondary intermediate mixed standard stock solution; ③ accurately transferring 1 mu g/L of secondary intermediate mixed standard stock solution and using 2 percent of HNO₃And + 0.5% HCl solution is diluted step by step to prepare a series of standard solutions with the concentrations of 1ng/L, 2ng/L, 5ng/L, 10ng/L, 20ng/L, 50ng/L and 100 ng/L.

As shown in table 3, when the dry ashing method is analyzed, the linear correlation coefficient r of U, Th is greater than 0.999 within the specified working curve range, and a good linear relationship is shown, so that the measurement requirements can be satisfied. The instrument detection limit of U, Th measurement in organic glass is 0.1ng/L grade, and ultra trace analysis can be carried out. U, Th, the detection limit of the method is 0.10pg/g and 0.21pg/g respectively, the quantitative limit of the method is 0.32pg/g and 0.70pg/g respectively, the method has low blank value and large sampling amount, so the detection limit and the quantitative limit of the method can reach very low level, and the measurement requirement of U, Th in organic glass within 1pg/g in the experiment is completely met.

TABLE 3U, Th Linear relationships and method detection limits, quantitation limits (dry ashing method)

(2) Accuracy and precision

To confirm the contamination or loss of U, Th element in the dry ash process, a blank spiking recovery experiment was performed. 3 crucibles were taken and each portion was loaded with U, Th standards corresponding to the sample content, which resulted in 3 portions of blank spiking solution as in the sample treatment process. Similarly, a blank solution was prepared according to the above method and the blank spiked recovery results are shown in Table 4.

TABLE 4 recovery and precision (dry ashing method)

As shown in Table 4, the recovery of both element U, Th was slightly higher, indicating that both elements were easily contaminated, with U recovery of 110% -131% and Th recovery of 111% -130%, both within the allowable range of the recovery limit. And the relative standard deviation RSD of the three parallel samples of the two elements is within 10 percent, and the precision is better. Therefore, the dry ashing method satisfies the actual measurement requirements.

(3) Sample spiking recovery and sample analysis

6 parts and 20g of organic glass samples (accurate to 0.0001g) are weighed in parallel to evaluate the sample results; meanwhile, 3 groups of different scaling quantities are designed to carry out sample scaling experiments, 9 parts and 20g of organic glass (accurate to 0.0001g) are weighed in parallel, each 3 parts is taken as one group, and each group of samples is sequentially added with low, medium and high U, Th standards (the ratio of the low, medium and high scaling quantities to the method quantitative limit is controlled to be 1, 10 and 100); similarly, a process blank solution was prepared. The U, Th content was measured in the samples as shown in Table 5. The recovery of the elements at different scalar values was calculated and the results are shown in table 6.

As can be seen from Table 5, the results of the dry ashing method are all higher than the detection limit of the method, and the dry ashing method can realize accurate measurement of ultra trace elements in the experiment and has obvious advantages near the quantitative limit of the method. The content of U, Th in the organic glass is respectively 0.39pg/g and 0.77pg/g through calculation, the relative standard deviation RSD of U, Th in the determination results of 6 parallel samples is respectively 23.3 percent and 50.5 percent, and the measurement precision is better for the measurement of the ultra-low content elements. It is obvious that the precision of Th is less than U, which may be caused by non-uniform sample, and the collection method of sample, such as quartering sampling, can be standardized.

TABLE 5 analysis results of organic glass samples

As can be seen from Table 6, the recovery rates were better in the sample spiking recovery experiments in which the ratios of the low, medium and high spiking amounts to the method quantitation limit were controlled to 1, 10 or 100. Around the limit of quantitation, the recovery rates of U, Th were 97.7% and 65.8%, respectively, and the accuracy of the results was better; the relative standard deviation RSD of the measurement results of U, Th parallel samples is 24.1 percent and 35.9 percent respectively, and the measurement precision is better. In the standard recovery experiments near higher limit of quantitation (10 times and 100 times), the recovery rates of U, Th are 95.1% -98.0% and 84.7% -91.0%, respectively, the method has good recovery rates, and the relative standard deviation RSD of the measurement results of 3 parallel samples of two elements is within 10%, and the test precision is also good. For ultra trace element analysis, the accuracy and precision of the method are within acceptable ranges.

TABLE 6 results of sample spiking recovery

Note: in parenthesis, an addition amount is indicated.

The method is summarized as follows:

the pretreatment method of the dry ashing method is established to realize U, Th measurement by ICP-MS: the treatment capacity of the organic glass by adopting the dry ashing method can reach 20g, the acid consumption in the whole process is only 2mL, and the whole treatment process is about 2 h. The results show that: in the designed concentration range, the linearity of the U, Th working curve is good, the detection limit of the method is 0.10pg/g and 0.21pg/g respectively, the quantification limit of the method is 0.32pg/g and 0.70pg/g respectively, and the method accords with the measurement requirement of ultra trace analysis. The relative standard deviation RSD of U, Th in the results of the parallel measurement of the three standard samples is 8.6 percent and 8.7 percent respectively, the test precision is good, and the recovery rates are 110 to 131 percent and 111 to 130 percent respectively. 6 samples are weighed in parallel for measurement, and the results show that the U, Th content in the organic glass (TC 0#) is 0.39 +/-0.16 (pg/g) and 0.77 +/-0.32 (pg/g), so that the accurate quantification of the ultra-trace U, Th in the organic glass is realized. Finally, the conversion is carried out by the formulas (1-1) and (1-2)²³⁸U、²³²The radioactivity of Th was 4.9. + -. 2.0 (. mu.Bq/kg) and 7.2. + -. 1.3 (. mu.Bq/kg), respectively.

Example 2 detection conditions

The optimized parameters for ICP-MS are shown in Table 2. The optimized parameters are more suitable for the accurate determination of the target object after the local detection and the matching pretreatment method. So that the detection accuracy can be further improved. The comparison of the modes in the detection conditions is schematically illustrated. For example, the spiked solution (C61.35 ppt) was measured 3 times in both STD and KED modes and the results are shown in table 7.

TABLE 7 comparison of accuracy and precision of U, Th detection under different detection modes

For detection U, Th, a more pronounced effect in terms of accuracy can be achieved with the KED mode and specific detection conditions. Especially for the detection of U, the accuracy can reach 99.35 percent which is very rare.

Elution solvent investigation

In the ICP-MS U, Th measurement process, 2% HNO is used to avoid the influence of the residue of the previous sample on the measurement of the next sample and reduce the test time₃+ 0.5% HCl was examined as the elution solvent and U, Th elution experiments are shown in FIG. 8. U, Th, all fall rapidly in a short time, approaching 0. Therefore, 2% HNO was used in this experiment₃+ 0.5% HCl as elution solvent.

Comparative example 1 second pretreatment method: microwave digestion-solid phase extraction method

Solid Phase Extraction (SPE) has been developed in recent years as an enrichment method, and has been paid more and more attention to people, and its position in sample pretreatment is also increasingly prominent. For the two radioactive elements U, Th studied in the present invention, resins with selective adsorption for them have become commercial products, and there are a lot of literature reports on the application of extraction techniques in radiochemistry. The technology can process large-volume samples and can efficiently separate and enrich target substances. ICP-MS is adopted for detection in the experiment, and in order to meet the sample introduction mode in a solution form, a solid organic glass sample needs to be digested. Because the organic glass is an organic high-molecular polymer material, the aim of complete digestion is difficult to achieve by a common dissolution method and a wet digestion method. Therefore, an appropriate, effective and complete digestion method is sought. With the increasing popularization of the ICP-MS technology, the microwave digestion is widely applied to sample pretreatment in recent years, and the microwave digestion is used for quickly dissolving a sample under the conditions of higher temperature and pressure by utilizing microwave heating.

The inventor tries to process the sample by microwave digestion in experiments, at present, a laboratory has a CEM MARS6 high-flux microwave digestion instrument, 40 digestion tanks can be operated simultaneously, because each digestion tank has limited sample carrying amount, the high flux can process the samples in batches, and then the digested solutions are mixed together, thereby realizing the purpose of increasing the sample processing amount. And then, selectively enriching the digested solution by using a Solid Phase Extraction (SPE) method to improve the concentration of the target element (U, Th), thereby realizing accurate detection.

The method explores the contents of the following four aspects:

the extraction process can be divided into four steps: cleaning, adsorbing, purifying and eluting, wherein the selection of proper adsorption and elution conditions is the key point of the solid-phase extraction research.

The experimental steps are as follows: selecting UTEVA resin with selective adsorption to U, Th, making into SPE column, and extracting with 3M HNO as shown in FIG. 2₃A known concentration of U, Th standard of 1. mu.g/L was prepared and the optimal extraction conditions were determined by recovery experiments.

Results and discussion

1) Adsorption conditions

3M HNO was selected for this experiment₃As a solution medium, the concentration C of the solution after extraction of 1. mu.g/L of the mixed standard solution through the SPE column was measured_{Detection of}The percentage of U, Th adsorbed by the UTEVA resin was calculated for a 1. mu.g/L mixed standard solution. The results are shown in Table 8: the adsorption of the UTEVA resin to U, Th is close to 100%, so that 3M HNO is adopted₃As the adsorption conditions in this experiment, U, Th was effectively adsorbed.

TABLE 8U, Th concentration in solution after UTEVA resin

2) Elution conditions

The use of 0.02M HCl followed by 0.03M H was examined₂C₂O₄The effect of elution. As a result, as shown in FIG. 3, U was completely eluted with only 0.02M HCl, and the recovery rate reached 98%. The elution of HCl to Th is only 81% and the rest 13% needs oxalic acid to be eluted. Therefore, the following conclusions are drawn: for Th at a level of 1. mu.g/L, the elution effect of HCl at a low concentration is not very good and further elution of oxalic acid is required. In this experiment, the remaining Th was eluted due to the formation of a complex between oxalic acid and Th. However, the introduction of oxalic acid caused a significant increase in the U, Th blank value (see Table 9) due to the purity factor of the oxalic acid reagent, which produced a large background interference with the experimental measurements. Comprehensively considering: oxalic acid, while it may improve the recovery of Th, introduces a reagent blank, which is contrary to the low concentration measurements of this experiment. Therefore, 0.02M HCl was chosen as the eluent for U and Th.

TABLE 9 use of HCl followed by H₂C₂O₄Blank value introduced by elution

The sample amount of each digestion tank and the corresponding microwave digestion program realize the complete digestion of the organic glass by optimizing the temperature, time and power in the digestion program.

The experimental steps are as follows:

1) sample pretreatment

2) Sample digestion

The sample amount of the digestion tank is related to the sample type, and the use criteria of the instrument require that: the organic sample should be limited to 1 g/container. Organic glass is a polymer of methyl methacrylate monomer, is difficult to digest, and is digested by a microwave digestion method. The operation is as follows: weighing about 0.1g of organic glass in a polytetrafluoroethylene digestion tank, adding 10mL of concentrated nitric acid, and placing for pre-digestion; secondly, no obvious reaction exists in the pre-digestion process, the cover is screwed down and placed into a microwave digestion instrument, and different microwave digestion programs are set for digestion; taking out the digestion tank after digestion is finished, opening the tank, and indicating that the sample is completely digested as the solution is clear and transparent; fourthly, the mixture is put into an acid dispelling instrument (180 ℃) to be heated and dispelled acid.

Results and discussion

The microwave digestion instrument works in a temperature control mode, organic glass is mainly organic components, and violent reaction possibly occurs in the digestion process, so that the instrument Classic mode is adopted in the experiment, a gradient program is manually set, and the following use principle is followed: the temperature climbing rate is not more than 10 ℃/min, the power is determined according to the number of the cans, and the maximum working temperature of the high-pressure can is 210 ℃ (the tolerance temperature is 260 ℃). Three sets of gradient temperature program were set up in order to slow down the severity of the digestion reaction. As shown in FIG. 4, procedure A, B, C, the digestion solution is shown in FIG. 5 after digestion is complete.

As analyzed in conjunction with fig. 4 and 5, the solutions after digestion were all clear and transparent (a) by the three temperature program A, B, C, but during the transfer with water, the white turbidity appeared in the program A, B digestion solution (b) and the clear and transparent program C digestion solution (C), and it was evident that the organic glass was not digested completely under the program A, B conditions, because the temperature and holding time set in the program A, B was not sufficient to allow the sample to be completely digested. Procedure C the digestion solution appeared clear and transparent (C) by increasing the temperature and increasing the time, so procedure C (table 10) was selected as the digestion procedure for this experiment.

TABLE 10 microwave digestion procedure

And (3) establishing a microwave digestion-solid phase extraction analysis method, and evaluating the effectiveness of the method on the experimental measurement from blank values, method detection limits and quantitative limits.

Experimental procedure

Weighing 6 parts of organic glass sample (accurate to 0.0001g) of about 0.1g, adding 10mL of concentrated nitric acid into each part of the organic glass sample in 6 digestion tanks respectively, digesting the sample according to the digestion program explored by the content of the second part, dispelling acid after digestion, evaporating to be nearly dry, and then using 5mL of 3M HNO₃And (3) rinsing the digestion tank for multiple times, combining and collecting 6 parts of rinsed solution, extracting by using an SPE (UTEVA) column, and extracting according to the extraction conditions of the SPE column researched in the first part. And finally, acid-expelling the eluent, and fixing the volume to 1mL to obtain a sample solution, and waiting for the machine test. Similarly, parallel sample solutions and process blank solutions were prepared and analyzed as described above.

Results and discussion

TABLE 11U, Th Linear relationships and method detection limits, quantitation limits (microwave digestion-solid phase extraction)

Note: the method detection limit and the quantitative limit in the experiment are calculated according to the sampling amount of 0.6000g and the constant volume of 1 mL.

As analyzed by the data in Table 11, the linear correlation coefficient r of U, Th is greater than 0.999 in the designed concentration range of the microwave digestion-solid phase extraction method, which shows that the instrument has good linear response to U, Th in the concentration range. The detection limit of the instrument U, Th in the method can reach 0.1ng/L, which indicates that the instrument can theoretically realize ng/L level measurement, but the detection limit and the quantitative limit of the method are as high as dozens or hundreds of pg/g, and the accurate quantification of the U, Th content in the organic glass about 1pg/g cannot be met.

Three factors (vessel, environment and reagents) affecting the blank value were examined and optimized.

The sample pretreatment method is further considered, and the reason that the experimental result cannot reach the expected target is analyzed. For the ultra trace element analysis of the experiment, the blank value is a key factor influencing the measurement result and depends on the process and the method of the sample pretreatment to a great extent. The researched extraction and sample digestion processes belong to sample pretreatment, and the ICP-MS has high sensitivity, so that the problems of blank and pollution of environment, vessels and reagents in the sample preparation process are particularly obvious. Blank values introduced during sample pretreatment are measured based on established digestion and extraction. According to the measurement result, factors causing high blank value are analyzed specifically, mainly from three aspects of environment, vessels and reagents, and are improved in an appropriate mode. Therefore, this subsection separately examines the impact of reagents, environment, and vessels on blank values.

1) Reagent

HNO₃Is not only a digestion solution but also an adsorption medium, and is also a reagent with the largest dosage in the experimental process. The method can remove metal ions and solid particles in the nitric acid liquid by adopting a sub-boiling distillation method, thereby reducing the reagent blank, and is a reagent purification method commonly used in a laboratory. Similarly, hydrochloric acid can be further purified by sub-boiling distillation, but because the amount of hydrochloric acid used is small and the hydrochloric acid used is already of OPTIMA grade, the effect on the blank value is negligible. The focus of this subsection is to investigate the effect of nitric acid on the white value after multiple purifications. The results are shown in FIG. 6. As can be seen from the figure, after 3 times of purification, the blank value of U, Th has a significant decline trend, which indicates that the reagent blank can be reduced to a certain extent by the sub-boiling distillation method, and the method has great significance for the measurement of ultra-low content elements.

2) Environment(s)

The natural radioactive elements measured by the experiment have ubiquitous characteristics and are often distributed in experimental environments such as atmosphere and dust. Because the sample pretreatment process is not completely closed, the laboratory environment will also have a certain effect on the blank value. In this subsection, blank values of two different laboratory treatments are compared, and the result is shown in table 12, and the blank value obtained in a ten thousand grade clean room is slightly better than that of a common laboratory, so that sample treatment can be completed in a laboratory (for example, a hundred grade clean room) with higher cleanliness requirement, and the experimental measurement result is effectively improved.

TABLE 12 comparison of blank values in different experimental environments

3) Vessel

Vessel contamination is a non-negligible factor, and particularly in the aspect of ultra-low content measurement, vessel materials often have certain influence on elements to be measured, such as adsorption loss or container contamination. Therefore, in this experiment, it is necessary to examine the vessel as well. Table 13 compares the U, Th concentrations in the solution before and after digestion, and it can be seen that the U concentration in the solution after digestion is increased by one order of magnitude and the Th concentration is also increased by a small amount. Obviously, the pollution caused by the polytetrafluoroethylene digestion tank used in the experiment in the digestion process causes large system errors in the measurement result, and finally causes the failure of the whole measurement. Adopts 30 percent of HNO₃Cleaning methods such as soaking overnight, empty tank digestion and the like attempt to improve the problem of pollution introduced by the digestion tank, but no obvious effect is seen. The reason for this may be that the polytetrafluoroethylene polymer material is porous and coated with some impurities to cause continuous precipitation during high temperature and high pressure reaction.

TABLE 13 Change in the value of U, Th blank in the solution before and after digestion

The method is summarized as follows:

the microwave digestion-solid phase extraction pretreatment method is established to realize U, Th measurement by ICP-MS: the solid phase extraction technology can increase the concentration of elements, thereby improving the detection limit of the method. UTEVA resin was selected as the U, Th-enriched extraction material, adsorption and elution conditions were optimized, and determined as 3M HNO, respectively₃And 0.02M HCl. A microwave digestion program is designed for carrying out organic glass digestion, and finally the purpose of completely digesting the sample can be achieved by adding 0.1g of sample into 10mL of concentrated nitric acid and keeping the mixture at 210 ℃ for 1 h. In the present method, the detection limits of U, Th are 10 respectivelySince U, Th was not detected in plexiglass, 9pg/g and 13.4 pg/g. And (3) investigating the influence of factors such as environment, vessels and reagents on the blank value, wherein the pollution introduced by the material (polytetrafluoroethylene) of the digestion tank is a key factor for limiting the experimental measurement.

Comparative example 2 third pretreatment method: super microwave method

Super microwave (ultrawave) is a great technical innovation in microwave digestion in recent years, and is favored by many users since the first super microwave enters china in 2008. The optimal microwave digestion instrument in the world breaks through various limitations of conventional microwave digestion, and has the following advantages in the aspect of sample pretreatment: 1) the amount of sample treated was greatly increased: the maximum sample amount can reach 25g through a single digestion tank, or a large batch of samples (77 samples) are digested simultaneously; 2) can digest samples which are difficult to digest by other digestion methods under the conditions of high temperature and high pressure (260 ℃ and 200 bar); 3) the digestion of the same batch can process different types of samples; 4) the amount of acid used is not limited, and is small; 5) the digestion tube does not need to be pressure-resistant, and common test tubes and colorimetric tubes can also be used; 6) the reaction is carried out at a high pressure N₂The process is carried out in the atmosphere, and the phenomena of liquid boiling, element loss and contamination are avoided.

The experiment selects the super microwave to carry out sample pretreatment, and compared with the comparative proportion of 1, the sampling amount of a super microwave single tank is one order of magnitude higher, so that large sampling amount can be realized, the analyte concentration is improved, the number of required digestion tanks is small, and the blank value introduced by a container is reduced. Meanwhile, the acid consumption is reduced, and the reagent blank is reduced. The super microwave method has excellent analysis performance, is expected to solve the blank value problem in the practical measurement of the comparative example 1, and finally realizes the accurate detection of U, Th. Table 14 lists the analytical properties of ordinary microwave and super microwave.

TABLE 14 comparison of common microwave and Supermicrowave analytical Properties

For the organic glass sample of the experimental study, the digestion treatment is carried out by adopting super microwave, and the maximum sampling amount, the acid consumption and the digestion procedure (temperature, pressure and time) of the organic glass sample are further researched. Because the limit of the super microwave to the digestion tubes (temperature resistance, pressure resistance and mechanical strength) is less, the digestion tanks made of high-purity quartz materials are selected for analysis and investigation besides the polytetrafluoroethylene material digestion tanks in the section considering that the blank interference introduced by the polytetrafluoroethylene material digestion tanks is larger. Meanwhile, the applicability of the super microwave method to U, Th measurement in organic glass is evaluated from blank values, method detection limits and quantification limits.

The method explores the contents of the following three aspects:

for the organic glass sample of the experimental study, the digestion treatment is carried out by adopting super microwave, and the maximum sampling amount, the acid consumption and the digestion procedure (temperature, pressure and time) are further explored.

And (4) adopting super microwave to perform digestion. The operation steps are as follows:

1) weighing about 5.0000g of sample in a polytetrafluoroethylene digestion tank, adding 35mL of concentrated nitric acid and 20mL of ultrapure water, and standing for pre-digestion;

2) no obvious reaction occurs in the process of pre-digestion, a cover is covered and put into a high-pressure reaction cavity, and N is filled into the cavity₂The initial pressure was shown as 40bar and the digestion program in table 15 was set up for digestion.

3) And taking out the digestion tank after digestion, opening the cover, removing the nitrogen oxide gas, and indicating that the solution is clear and transparent that the sample is completely digested. The temperature, pressure and power curves during the reaction are shown in FIG. 7.

4) Heating the digested solution in acid-expelling instrument (180 deg.C), evaporating to near dryness, and adding 2% HNO₃+ 0.5% HCl to 1mL to be tested.

TABLE 15 digestion procedure for Supermicrowave

And (3) establishing a super microwave analysis method, and evaluating the applicability of the super microwave method to U, Th measurement in organic glass from blank values, method detection limits and quantitative limits.

Experimental procedure

Weighing 2 parts of organic glass sample (accurately weighed and accurate to 0.0001g) of about 5g each, adding 35mL of concentrated nitric acid and 20mL of ultrapure water into each of 2 digestion tanks, digesting the sample according to '3.4.2 sample digestion', mixing 2 parts of digested solutions together, driving up to acid, evaporating to be nearly dry, and then using 2% of HNO₃+ 0.5% HCl to 1mL to be tested. Similarly, parallel sample solutions and process blank solutions were prepared and analyzed as described above.

Results and discussion

When the super microwave treatment is carried out, U, Th has a working curve as shown in Table 16, and linear correlation coefficients r are all larger than 0.999, which meet the measurement requirements. The instrument detection limits of U, Th obtained by the method are 0.154ng/L and 0.370ng/L respectively, the method detection limits are 4.97pg/g and 8.88pg/g respectively, and the method quantification limits are 16.5pg/g and 29.6pg/g respectively. Compared with a microwave digestion-solid phase extraction method, the detection limit and the quantitative limit of the super microwave method both have obvious reduction tendency, and the main reason is that the sample amount is increased from 0.6g to 10g and is increased by a majority of orders of magnitude, so the reduction tendency is obvious. However, the method still fails to meet the measurement requirements of the experiment.

TABLE 16U, Th Linear relationship and method detection limits, quantitation limits (Supermicrowave method)

Note: the method detection limit and the quantitative limit in the experiment are calculated according to the sampling amount of 10g and the constant volume of 1 mL.

Because the limit of the super microwave on the digestion tube (temperature resistance, pressure resistance and mechanical strength) is less, the method selects the digestion tank made of high-purity quartz for analysis investigation besides the digestion tank made of polytetrafluoroethylene materials in consideration of larger blank interference introduced by the digestion tank made of polytetrafluoroethylene materials.

The result measured by using the super microwave method shows that the problem of pollution introduced by the polytetrafluoroethylene digestion tank is a main factor causing blank value increase, so that the detection limit of the method is deteriorated. Because the material requirement of the digestion tank by the super microwave is less, the blank value of the digestion tank made of high-purity quartz is considered and considered, and the result is shown in table 17, the blank values of the polytetrafluoroethylene tank and the quartz tank are not obviously different, so that the blank value of the quartz tank cannot be effectively improved.

TABLE 17 Effect of digestion tank materials on blank values

The method is summarized as follows:

the pretreatment method for establishing the super microwave realizes U, Th measurement by ICP-MS: the super microwave overcomes the limitation of the traditional microwave on the sample amount and the temperature, and saves the acid consumption and the analysis time. After optimizing digestion conditions, 35mLHNO was added to 5g of the sample₃And 20mLH₂And O, the temperature can reach 255 ℃, and the sample can be completely digested in less than 1 h. In the present method, the detection limit of U, Th was 4.97pg/g and 8.88pg/g, respectively, and although the detection limit was significantly reduced, the expected target was not achieved. The blank value of the method is inspected, and the digestion tank (polytetrafluoroethylene) is still a limited factor of the method. The blank value of the quartz digestion tank is inspected, and the blank value is not obviously improved.

To summarize:

with the intensive research on unknown fields, the developed and designed electronic devices are continuously broken through and innovated in performance, including material radioactive background measurement. The radioactive background of the material is often a key factor for limiting the sensitivity and the working efficiency of the device, so the measurement work on the radioactive background is increasingly highlighted in the future research. Because the traditional radioactivity measurement technology has poor analysis performance such as low sensitivity, large background disturbance, long sample treatment period, large sample dosage and the like, a satisfactory result cannot be obtained in actual measurement, the ICP-MS method is adopted to measure the radioactivity background of the material, so that the defects can be overcome, and the simple, quick and accurate analysis can be realized.

In the experimental process, the inventor carries out a great deal of analysis work on three pretreatment methods and a plurality of experimental parameters and experimental conditions of each pretreatment method, and as a result, in the three methods, only the dry ashing method meets the measurement requirement of the ultra-trace radioactive background by virtue of the maximum sampling amount and the minimum blank value. In the research process, the inventor finds that a polytetrafluoroethylene digestion tank brings a large blank value in a microwave digestion-solid phase extraction method and a super microwave method, the blank value cannot be effectively improved in the super microwave method even if a high-purity quartz digestion tank is adopted, and a quartz crucible has little influence on the blank value in a dry ashing method. This is probably because the super-microwaving is carried out under a high temperature and high pressure condition in a concentrated acid, and the solution and the container are sufficiently reacted at this time, and the background precipitation of the container is increased, and the blank is increased. In addition, compared with a microwave digestion-solid phase extraction method and a super microwave method, the dry ashing method has superior analysis performance such as simple operation, time saving, accuracy, good precision and the like, is particularly suitable for measuring ultra-trace elements in the organic glass in the experiment, and provides substantial reference opinions for ultra-low background measurement work. Table 18 shows a comparison of the three pretreatment methods, wherein the pretreatment method sensitivity of the dry ashing method is as low as below 1pg/g, with an extremely significant breakthrough development for detection of the radioactive background of the material, especially in the material of the mesogen detector, and the analysis time length is the shortest of the three pretreatment methods, only 2 h.

For the measurement of ultra trace elements, particularly for the analysis method with the sensitivity lower than 1pg/g, the selection of the analysis method, such as the selection of pretreatment and the selection of an analysis instrument; in addition, each step in the analysis method may have a great influence on the analysis result, such as the amount of the sample, the choice of the elution solvent, the choice of the container, the analysis environment, and the like. Therefore, a reasonable choice of pre-treatment and analysis instruments is one of the key analytical methods, after which, for each treatment step, it is necessary to design and adapt it particularly reasonably to achieve accurate ultra trace analysis, and these detailed steps are also decisive for the success of ultra trace analysis.

TABLE 18 comparison of the three pretreatment methods

Reference documents:

1.Lariviere D,Taylor V F,Evans R D,et al.Radionuclide determination in environmental samples by inductively coupled plasma mass spectrometry[J].Spectrochimica Acta Part B:Atomic Spectroscopy,2006,61(8):877-904.

2.Arnquist I J,Hoppe E J,Bliss M,et al.Mass Spectrometric Determination of Uranium and Thorium in High Radiopurity Polymers Using Ultra Low Background Electroformed Copper Crucibles for Dry Ashing[J].Analytical Chemistry,2017,89(5):3101-3107.

Claims

1. a detection method for measuring an ultra trace radioactive background in a polymer is characterized by comprising the following steps:

(1) putting the polymer into a quartz crucible for coking treatment;

(4) heating the quartz crucible until the solution boils slightly;

(5) measuring by ICP-MS after cooling;

the radioactivity background is²³⁸U、²³²Th、⁶⁰Co、⁴⁰K is one or more of K.

2. The method of claim 1, wherein said radioactive background is²³⁸U、²³²Th、⁶⁰One or more of Co.

3. The method of claim 1, wherein said radioactive background is²³⁸U and/or²³²Th。

4. The method of claim 1, wherein said polymer is selected from the group consisting of organic polymers.

5. The method of claim 4, wherein the organic polymer is selected from any one of biomedical polyester, polyvinylidene fluoride, and polymethyl methacrylate.

6. The method of claim 1, wherein said polymer is selected from the group consisting of polymethylmethacrylate.

7. The method of claim 1, wherein in step (1), the sampled amount of the polymer is 10 to 25 g.

8. The method of claim 1, wherein the sample size of the polymer in step (1) is 18 to 22 g.

9. The process of claim 1, wherein in step (1), a sample of polymer is taken at 20 g.

10. The method of claim 1, wherein in step (1), the sampling is performed by quartering.

11. The method of claim 1, wherein in step (1), the coking process is carried out on an electric furnace.

12. The method according to claim 1, wherein the coking treatment temperature in the step (1) is 200 to 400 ℃.

13. The method according to claim 1, wherein the coking treatment temperature in the step (1) is 250 to 350 ℃.

14. The process of claim 1, wherein in step (1), the coking process is carried out at a temperature of 300 ℃.

15. The method according to claim 1, wherein in the step (1), the crucible is placed on an electric furnace to perform a coking treatment.

16. The method according to claim 1, wherein in the step (2), the ashing temperature is 450 to 600 ℃ and the ashing time is not less than 30 min.

17. The method according to claim 1, wherein in the step (2), the ashing temperature is 500 ℃; the ashing time was 30 min.

18. The method of claim 1, wherein step (3) is performed with HNO-containing solution₃And an aqueous solution of HCl is added along the quartz crucible wall to uniformly wet the crucible wall.

19. The method according to claim 1, wherein in the step (3), HNO is contained₃And aqueous HCl, HNO₃The volume fraction of (A) is 1-10%.

20. The method according to claim 1, wherein in the step (3), HNO is contained₃And aqueous HCl, HNO₃Is 2% by volume.

21. The method according to claim 1, wherein in the step (3), HNO is contained₃And HCl in the aqueous solution, wherein the volume fraction of HCl is 0-1%.

22. The method according to claim 1, wherein in the step (3), HNO is contained₃And HCl in water, the volume fraction of HCl being 0.5%.

23. The method according to claim 1, wherein in the step (3), HNO is contained₃And the volume of aqueous HCl is 1-2 mL.

24. The process according to claim 1, characterized in that HNO in step (3)₃And HCl were purified separately prior to use.

25. The process of claim 24, wherein the HNO is purified by sub-boiling distillation₃And HCl.

26. The method of claim 24, wherein HNO is₃And HCl were purified 3 times, respectively.

27. The method according to claim 1, wherein the heating temperature in the step (4) is 100 to 200 ℃.

28. The method according to claim 1, wherein the heating temperature in step (4) is 150 ℃.

29. The method of claim 1, wherein in step (4), the heating is performed on an electric hot plate.

30. The method of claim 1, wherein the parameters of ICP-MS are RF power: 1400-1700W; flow rate of carrier gas: 0.8-1.0L/min; flow rate of auxiliary gas: 0.7-0.9L/min; cooling air flow rate: 13.0-15.0L/min; CCT flow rate: 4.0-6.0 mL/min; the measurement mode is one of KED or STD.

31. The method of claim 1, wherein the parameters of ICP-MS are RF power: 1548.6W; flow rate of carrier gas: 0.930L/min; flow rate of auxiliary gas: 0.795L/min; cooling air flow rate: 13.88L/min; CCT flow rate: 4.809 mL/min.

32. The method according to claim 1, wherein the measurement mode of ICP-MS is KED.

33. The method of any one of claims 1 to 32, wherein the entire experimental process is carried out in a ten thousand clean room.