Ionization emitter for lead isotope analysis of trace samples, preparation method and application thereof

Abstract

The present invention provides an ionization emitter for lead isotope analysis of trace samples, its preparation and application. Among them: the ionization emitter is a β-phase silicon nitride emitter, specifically composed of a β-phase silicon nitride suspension and phosphoric acid solution as an auxiliary material for the emitter. The application involves a method for testing lead isotopes in trace samples, using a β-phase silicon nitride suspension as the ionization emitter to enhance the thermal ionization efficiency of lead samples, while using phosphoric acid solution to assist ionization, and using a high-purity tantalum filament as the sample carrier to test lead isotopes. The method of the present invention has the advantages of low cost and simple operation. Compared with the traditional method based on rhenium strip and silica gel spotting, the β-phase silicon nitride emitter of the present invention can obtain good sensitivity on the tantalum strip, and good lead isotope test accuracy can be obtained for sample amounts of 5 - 530 ng, which can meet the analysis needs of most geological samples and can reduce the test cost of thermal ionization mass spectrometry consumables by about 70%.

Description

Technical Field

[0001] The present invention belongs to the technical field of analytical chemistry, and particularly relates to an ionization emitter for analyzing lead isotopes in trace samples, a preparation method thereof, and an application thereof in thermal ionization mass spectrometry analysis. Background Art

[0002] Lead has four natural isotopes ( 204 Pb, 206 Pb, 207 Pb and 208 Pb). Among them, 204 Pb is a stable isotope, 206 Pb and 207 Pb are respectively the radiogenic decay daughters of 238 U and 235 U, 208 Pb is the radiogenic decay daughter of 232 Th. The natural abundances of lead isotopes show a large variation range in different rock minerals. For example, the abundance variation ranges of 204 Pb, 206 Pb, 207 Pb and 208 Pb are generally 1.04 - 1.65%, 20.84 - 27.48%, 17.62 - 22.1% and 51.28 - 56.21% respectively, and their abundances are controlled by the U / Pb and Th / Pb ratios in the sample and the production age of the sample. Based on the above three decay systems, by measuring the Pb isotope ratios and U / Pb and Th / Pb ratios of uranium- and thorium-rich minerals (such as zircon, monazite, sphene, perovskite, etc.) in rocks, precise dating of ancient geological bodies can be carried out. Since Pb is often associated and enriched in polymetallic sulfide deposits in nature, the Pb isotope ratio is usually a sensitive tracer for studying the genesis of deposits and indicating geochemical processes. Since the 1960s, Pb isotopes have been widely used in the research of geochemistry, petrology, and ore depositology. In the 21st century, the Pb isotope ratio tracing technology has also been extended to applied research in environmental science, food science, and scientific and technological archaeology, such as tracing the sources of water and air pollution, food pollution sources, and bronze ware archaeology.

[0003] In summary, Pb isotopes have important and wide application prospects in the research of earth science, environmental science, scientific and technological archaeology, etc. The prerequisite for the above applications is to obtain high-precision Pb isotope ratio data, which generally requires the determination of 206 Pb / 204 Pb, 207 Pb / 204 Pb and 208 Pb / 204For Pb, the single internal analysis accuracy generally needs to be better than 0.03% (2RSE), and the external accuracy of long-term measurement generally needs to be better than 0.2% (2RSD). Since the 1960s, thermal ionization mass spectrometry (TIMS) has been the preferred analytical instrument for Pb isotope testing due to its excellent test accuracy, high sensitivity and low memory effect. High sensitivity is the prerequisite for high-precision testing. The emitter is the core material related to the test sensitivity of TIMS. In the past half century, analysts have developed a variety of emitters to continuously improve the sensitivity of Pb. The main emitters that have been published are the following six: ①, borax; ②, silica gel + borax; ③ silica gel + perrhenic acid; ④ silica gel + zirconium; ⑤ silica gel + phosphoric acid + germanium + rhenium; ⑥, silica gel + phosphoric acid. The above six published emitters are all tested with Re (rhenium) ribbon as the filament material. Among them, the excitation methods of emitters ①, ②, ③, and ④ require a large amount of sample (>1000ng), and have been eliminated. Emitter ⑤ requires a mixed solution of germanium and rhenium doped on the basis of silica gel, and the technology is relatively complicated and is rarely used. Emitter ⑥ only needs to coat silica gel and phosphoric acid on the surface of Re filament to achieve high-precision determination of Pb samples, and only 0.5 to 200ng Pb is required each time.

[0004] However, the technology based on silica gel and phosphoric acid as emitters must use Re filaments as sample carriers. Re filaments are relatively expensive and are the main consumables cost consumption for TIMS testing. If the traditional silica gel and phosphoric acid emitters are used to spot samples on cheap Ta (tantalum) filament materials, stable and high-intensity Pb signals cannot be obtained. The Pb signal will show violent fluctuations, without emission plateaus, and the sensitivity is also very poor. Therefore, it is impossible to obtain high-precision Pb isotope data on the Ta band using traditional silica gel emitters.

[0005] Therefore, how to reduce the testing cost while maintaining a high sensitivity to meet the analysis requirements of most geological samples and provide an ionizing emitter for lead isotope analysis of trace samples is a technical problem that needs to be solved urgently. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide an ionizing emitter suitable for trace sample and high-precision lead isotope analysis, a preparation method thereof and an application thereof in thermal ionization mass spectrometry analysis, so as to reduce the existing lead isotope thermal ionization mass spectrometry test cost and ensure the test accuracy and sensitivity.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides an ionization emitter for lead isotope analysis of trace samples, and the ionization emitter is a β-phase silicon nitride emitter;

[0009] The β-phase silicon nitride emitter specifically uses a β-phase silicon nitride suspension and is composed of a phosphoric acid solution as an emitter auxiliary material.

[0010] It should be noted that solid β-phase silicon nitride powder generally cannot be directly coated with samples. Therefore, it is generally spotted on a sample carrier (in the present invention, a high-purity tantalum filament is used) in the form of a suspension. Adding dilute phosphoric acid serves two purposes. On the one hand, it acts as an adhesive, enabling the phospho-β-phase silicon nitride emitter to be better coated and firmly fixed on the surface of the sample carrier. On the other hand, it can also assist ionization and moderately enhance the ionization efficiency of the sample.

[0011] Furthermore,

[0012] The β-phase silicon nitride suspension is prepared by the following method: The high-purity β-phase silicon nitride powder is successively washed with dilute hydrochloric acid and high-purity deionized water at least once each to reduce the sample spotting background; generally, washing once each is sufficient.

[0013] Then, the treated β-phase silicon nitride powder is added to deionized water to prepare a β-phase silicon nitride suspension with a preset concentration.

[0014] Furthermore,

[0015] The concentration of the β-phase silicon nitride suspension is converted according to the dose of the β-phase silicon nitride emitter required for each test and the spotting volume of the β-phase silicon nitride suspension. Specifically, the dose of the high-purity β-phase silicon nitride powder required for each test is 10 ± 1 micrograms, and the spotting volume of the β-phase silicon nitride suspension is 0.9 - 1.1 μL. Therefore, the concentration of the β-phase silicon nitride suspension is 10 mg / mL.

[0016] In the present invention, the concentration of the β-phase silicon nitride suspension mainly depends on the dose of the β-phase silicon nitride emitter required for each test. Generally, the dose of the high-purity β-phase silicon nitride powder required for each test is 10 ± 1 micrograms, and the maximum should not exceed 15 micrograms. Otherwise, it will significantly affect the testing of lead samples. For example, there are the following problems: 1. Sample detachment; 2. Contamination of the ion source; 3. Unstable signal emission; The preferred emitter dose in the present invention is 10 ± 1 micrograms.

[0017] In addition, if the spotting volume of the β-phase silicon nitride suspension is too large for each test, the sample will dry slowly, and there is a risk of sample diffusion. Therefore, the spotting volume of the β-phase silicon nitride suspension for each test is generally controlled at 1 μL, and the maximum should not exceed 1.1 μL. Therefore, the corresponding concentration of the β-phase silicon nitride suspension is generally 10 mg / mL, and is specifically converted according to the dose of the β-phase silicon nitride emitter required for each test and the spotting volume of the β-phase silicon nitride suspension.

[0018] Preferably, the purity of the high-purity β-phase silicon nitride powder is greater than 99.99%.

[0019] Preferably, the particle size of the high-purity β-phase silicon nitride powder is <2 μm.

[0020] Preferably, the concentration of the phosphoric acid solution is 0.3 - 0.5 mol / L.

[0021] In a second aspect, the present invention also provides a method for preparing an ionization emitter for the analysis of lead isotopes in the above-mentioned trace samples. The emitter is a β-phase silicon nitride emitter, and the β-phase silicon nitride emitter is specifically composed of a β-phase silicon nitride suspension and a phosphoric acid solution as an auxiliary material for the emitter; during preparation, first prepare the β-phase silicon nitride suspension, and then prepare a dilute phosphoric acid solution with the required concentration, specifically as follows:

[0022] (1) The preparation method of the β-phase silicon nitride suspension is as follows:

[0023] S1. Pretreatment of β-phase silicon nitride, specifically as follows:

[0024] S11. First, weigh the high-purity β-phase silicon nitride powder and place it in a plastic centrifuge tube. Add dilute hydrochloric acid in proportion, seal and shake the centrifuge tube, and shake for 2 - 3 minutes. Use dilute hydrochloric acid to wash the β-phase silicon nitride powder to reduce the sample background. Then, centrifuge for 10 minutes with a centrifuge and take out the upper dilute hydrochloric acid solution;

[0025] S12. Then add high-purity water to wash the β-phase silicon nitride powder again, seal and shake the centrifuge tube for 2 - 3 minutes again. Then, load it into the centrifuge and centrifuge for 10 minutes, and suck out the upper clear liquid again;

[0026] S13. Repeat steps S11 and S12, and cross-wash the high-purity β-phase silicon nitride powder with dilute hydrochloric acid and high-purity deionized water at least once each; the final precipitate phase is the pretreated β-phase silicon nitride. Generally, cross-washing with dilute hydrochloric acid and high-purity deionized water once each is sufficient for the pretreatment of β-phase silicon nitride. At this time, there is no need to repeat steps S11 and S12.

[0027] S2. Weigh the β-phase silicon nitride pretreated in step S12, add deionized water, and prepare a β-phase silicon nitride suspension with a preset concentration; the concentration of the β-phase silicon nitride suspension is converted according to the dose of the β-phase silicon nitride emitter required for each test and the spotting amount of the β-phase silicon nitride suspension. Specifically, the dose of the high-purity β-phase silicon nitride powder required for each test is 10 ± 1 μg, and the spotting amount of the β-phase silicon nitride suspension is 0.9 - 1.1 μL. Therefore, the concentration of the β-phase silicon nitride suspension is approximately 10 mg / mL;

[0028] The containers in the above pretreatment process are generally ordinary polyethylene centrifuge tubes.

[0029] (2) Preparation of phosphoric acid solution

[0030] Weigh concentrated phosphoric acid solution and add deionized water in proportion to prepare a phosphoric acid solution with a concentration of 0.3 - 0.5 mol / L, thus obtaining it.

[0031] Generally, a saturated concentrated phosphoric acid with a concentration of 14.63 mol / L is used to add deionized water to prepare a phosphoric acid solution with a concentration of about 0.4 mol / L.

[0032] Furthermore, in the above method,

[0033] In step S11, the dosage of dilute hydrochloric acid for cleaning is calculated based on 1 ml per (10 ± 0.1 mg) of high-purity β-phase silicon nitride powder;

[0034] In step S12, the dosage of high-purity water for cleaning is calculated based on 1 ml per (10 ± 0.1 mg) of high-purity β-phase silicon nitride powder.

[0035] In the third aspect, the present invention also provides a method for testing lead isotopes in trace samples. The lead isotopes are tested and analyzed by using the above-mentioned ionization emitter for analyzing lead isotopes in trace samples or the ionization emitter for analyzing lead isotopes in trace samples prepared by the above-mentioned preparation method; specifically, it is a method for testing trace lead isotopes by using a high-purity β-phase silicon nitride suspension as a high-sensitivity emitter to enhance the ionization efficiency of lead samples, simultaneously using a phosphoric acid solution to assist ionization, and using a high-purity tantalum filament as a sample carrier.

[0036] Furthermore, the method for testing lead isotopes in trace samples specifically includes the following steps:

[0037] (1) Take an appropriate amount of the ionization emitter composed of β-phase silicon nitride suspension and phosphoric acid solution and coat it on the surface of a high-purity tantalum filament. After the emitter is evaporated to dryness, spot the lead sample on the surface of the filament. Adjust the current to 1.2 amperes to evaporate the lead sample, then continue to increase the filament current until the filament turns bright red and maintain it for 2 - 3 seconds, and then return the current to zero;

[0038] (2) Load the high-purity tantalum filament holder carrying the lead sample into a thermal ionization mass spectrometer, and use the thermal ionization mass spectrometer for testing to obtain high-precision lead isotope analysis data.

[0039] It should be noted that during spotting, the β-phase silicon nitride suspension must be loaded / coated on the surface of the high-purity tantalum filament. The β-phase silicon nitride suspension is preferably freshly prepared and can be stored for at most 7 days. Otherwise, the β-phase silicon nitride may undergo a slow hydrolysis chemical reaction and turn into a silica colloid, thus affecting the test sensitivity.

[0040] Further, when the emitter is actually loaded on the surface of the tantalum filament, the prepared phosphoric acid solution is first spotted on the filament. After the phosphoric acid is evaporated to dryness, the β-phase silicon nitride suspension is coated on the phosphoric acid coating.

[0041] Specifically, the coating process of the emitter in step (1) is as follows:

[0042] Take 1 - 1.5 μL of phosphoric acid solution with a concentration of 0.3 - 0.5 mol / L (preferably 0.4 mol / L) and coat it on the surface of the high-purity tantalum filament. Adjust the filament current to evaporate the phosphoric acid solution to dryness first, and then take 0.9 - 1.1 μL of β-phase silicon nitride suspension with a certain concentration and cover it on the dried phosphoric acid coating. After the β-phase silicon nitride suspension is evaporated to dryness, spot the lead sample on the surface of the filament.

[0043] Specifically, the concentration of the β-phase silicon nitride suspension is converted according to the dose of β-phase silicon nitride emitter required for each test and the spotting volume of the β-phase silicon nitride suspension. Specifically, the dose of high-purity β-phase silicon nitride powder required for each test is 10 ± 1 μg, and the spotting volume of the β-phase silicon nitride suspension is 0.9 - 1.1 μL. Therefore, the corresponding concentration of the β-phase silicon nitride suspension is about 10 mg / mL; specifically, it is converted according to the dose of β-phase silicon nitride emitter required for each test and the spotting volume of the β-phase silicon nitride suspension. Excessive phosphoric acid solution will not further improve the test sensitivity of Pb, but it is easy to contaminate the ion lens group. Generally, the spotting volume for each test is 1 μL, and the maximum does not exceed 1.5 μL.

[0044] Preferably, the spotting volumes of the β-phase silicon nitride suspension and the phosphoric acid solution used in the test are both 1 μL. Correspondingly, the concentration of the β-phase silicon nitride suspension is preferably 10 mg / mL.

[0045] Further,

[0046] When using the thermal ionization mass spectrometer for testing in step (2), the filament temperature is 1030 - 1150 °C.

[0047] Further,

[0048] In step (1), the dosage of the lead sample is 5 - 530 ng. The lead sample is an international standard sample or an actual rock sample; for an actual rock sample, a minimum spotting volume of only about 60 ng can obtain high-precision lead isotope analysis data.

[0049] The test principle of the present invention is as follows: According to the Langmuir-Kingdom empirical formula, the higher the work function on the surface of the metal ribbon, the higher the ionization efficiency of positive ions. The present invention uses a high-purity (purity higher than 99.9%) tantalum filament as the sample carrier. When spotting samples, a β-phase silicon nitride suspension provided by the present invention is added as a high-sensitivity ionization emitter. At the same time, a phosphoric acid solution is used as an auxiliary material for the emitter to assist ionization, which can significantly improve the ionization efficiency of lead, indirectly increase the surface work function of the tantalum filament, thereby enhancing the ionization efficiency and analysis sensitivity of lead, and further reducing the amount of lead samples used.

[0050] The beneficial effects of the present invention are as follows:

[0051] 1. There are six traditional ionization emitters: ① borax; ② silica gel + borax; ③ silica gel + perrhenic acid; ④ silica gel + zirconium; ⑤ silica gel + phosphoric acid + germanium + rhenium; ⑥ silica gel + phosphoric acid. All of them need to use a high-purity rhenium filament as the sample carrier. The high price of the rhenium filament results in high costs for thermal ionization mass spectrometry testing.

[0052] Compared with the traditional ionization emitter for lead isotope analysis, the present invention provides a new β-phase silicon nitride emitter to replace the traditional silica gel-based emitter. This emitter is composed of a β-phase silicon nitride suspension and uses a phosphoric acid solution as an auxiliary material for the emitter. This emitter can provide high-sensitivity Pb testing on a low-cost high-purity tantalum filament; using a Ta ribbon as the sample carrier to test trace lead isotopes can reduce the filament testing cost by 70%. In addition, the present invention also has high sensitivity for actual rock samples, and only 60 ng of sample volume is required to obtain high-precision Pb isotope analysis data.

[0053] 2. The spotting background of the β-phase silicon nitride-phosphoric acid emitter provided by the present invention is low, only 3.0 pg of Pb each time, and it does not cause contamination to samples with a small sample volume.

[0054] 3. The test method for lead isotopes provided by the present invention uses a tantalum filament to replace the rhenium filament, significantly reducing the testing cost (reducing the testing cost by about 70%). At the same time, it has good sensitivity, can meet the actual sample analysis requirements of 5 - 530 ng, can meet the analysis needs of most rock samples, and has strong application prospects. Specific Embodiments

[0055] To better elaborate on the content of the present invention, the following specific examples are used to further verify the present invention. It should be noted here that the examples are only to describe the present invention more directly. They are only a part of the present invention and cannot constitute any limitation to the present invention.

[0056] In the following examples, the sources of the selected raw materials are:

[0057] Premium β-phase silicon nitride powder (purity: 99.9%, particle size: 1 μm, Shanghai Yaoge Alloy Materials Co., Ltd.);

[0058] Ta filament material (H.Cross Company, USA, purity 99.98%, specification: 0.75mm×0.020mm;

[0059] MOS-grade hydrochloric acid, nitric acid, hydrofluoric acid (purified by sub-boiling distillation once, Sinopharm Chemical Reagent Co., Ltd.);

[0060] Ultra-pure water (Millipore Corporation, USA, Millipore Simplicity ultra-pure water system, conductivity of the produced water: 18.2 MΩ / cm);

[0061] Lead isotope standard NIST 981 (National Institute of Standards and Technology, USA);

[0062] Rock reference samples JA-3 (andesite), JR-2 (rhyolite), JB-3 (basalt), JG-1a (granite) are all from the Geological Survey of Japan;

[0063] Rock reference samples BCR-2 (basalt), BIR-1 (basalt), W-2 (dolerite) are all from the Geological Survey of the United States;

[0064] AG1×8 anion resin (particle size: 200 - 400 mesh, Bio-Rad Company, USA).

[0065] Example 1

[0066] 1. Preparation of emitter

[0067] 1) Weigh 15 ± 0.1 mg of β-phase silicon nitride powder into a centrifuge tube, add 1.5 mL of 1 mol / L hydrochloric acid, close the centrifuge tube and shake the centrifuge tube repeatedly for 2 - 3 minutes to wash the β-phase silicon nitride and reduce the sample background. Place the centrifuge tube in a centrifuge and centrifuge for 10 minutes, then use a pipette to take out the upper layer solution.

[0068] 2) Then add 1.5 mL of high-purity deionized water, close the centrifuge tube again and shake the centrifuge tube for 2 - 3 minutes, place it in a centrifuge and centrifuge for 10 minutes, and use a pipette to suck out the upper layer solution again;

[0069] 3) The obtained precipitate phase is the pretreated β-phase silicon nitride powder.

[0070] 4) Add the β-phase silicon nitride powder obtained in step 3) to 1.5 mL of high-purity deionized water to prepare a β-phase silicon nitride suspension with a concentration of 10 mg / mL for use as a lead isotope emitter, and set it aside.

[0071] 2. Preparation of phosphoric acid solution

[0072] Saturated phosphoric acid (14.63 mol / L) was added to deionized water to dilute and prepare a 0.4 mol / L phosphoric acid solution, which was used as a propellant auxiliary material and was set aside.

[0073] 3. Sample loading test

[0074] The sample loading detection method is as follows:

[0075] 1) Apply 1μL of 0.4mol / L phosphoric acid solution to the surface of high-purity tantalum filament, adjust the filament current to evaporate the phosphoric acid solution first, then take 1μL of 10mg / mL β-phase silicon nitride suspension to cover the evaporated phosphoric acid coating, and after the β-phase silicon nitride suspension is evaporated, then spot 20ng of the international standard NIST 981 on the surface of the tantalum filament, adjust the current to 1.2 amperes to evaporate the lead sample, and then slowly increase the filament current until the filament turns bright red and maintains for 2 to 3 seconds, and immediately return the current to zero.

[0076] 2) Load the sample into a Triton Plus thermal ionization mass spectrometer, and use the Triton Plus thermal ionization mass spectrometer to test the international standard NIST 981 sample. During the test, the filament temperature is 1050-1150 degrees.

[0077] 3) Wait 208 After the Pb signal stabilized, the test program was run, 160 sets of data were collected, and the Pb isotope detection results were recorded. The data collection time was about 13 minutes. The test results are shown in Table 2.

[0078] Example 2

[0079] Example 2 is substantially the same as Example 1, except that this example uses 10 ng of NIST 981 spot sample to test the sensitivity and test accuracy of the emitter. The test results are shown in Table 3.

[0080] Example 3

[0081] Example 3 is similar to Example 1, except that 5 ng of NIST 981 spotting amount is used to test the sensitivity and test accuracy of the emitter. The test results are shown in Table 4.

[0082] Example 4

[0083] Different from Examples 1, 2, and 3, Example 4 uses actual rock samples. Weigh 50 ± 0.1 mg of BCR-2 basalt sample powder into a PFA sample dissolver, add traditional mixed acid for sample dissolution (2 ml hydrofluoric acid + 0.2 ml nitric acid), seal the sample dissolver, place it on a hot plate at 180 degrees and heat for 4 days. Then evaporate the sample to dryness, extract the sample with 1 mL of dilute hydrobromic acid (0.7 mol / L), and purify the sample through traditional anion resin technology. The detailed description of the sample dissolution and preparation technology is the same as the published technology by Li et al (Int. J. Environ. Res. Public Health 2019, 16: 4772). After obtaining a high-purity Pb sample, dissolve the sample with 3 μL of 3 mol / L hydrochloric acid, and then take 1.5 μL of the sample solution and spot it on a Ta filament. The specific spotting and mass spectrometry tests are the same as those in Example 1. Calibration is carried out using the external standard method, the fractionation coefficient of Pb is 0.11% / amu, and the test results are shown in Table 5.

[0084] Example 5

[0085] Example 5 is roughly the same as Example 4, except that in this example, 50 ± 0.1 mg of BIR-1 basalt is selected. Half of the purified Pb sample is taken for testing. Calibration is carried out using the external standard method, the fractionation coefficient of Pb is 0.11% / amu, and the test results are shown in Table 6.

[0086] Example 6

[0087] Example 6 is roughly the same as Example 4, except that in this example, 50 ± 0.1 mg of W-2 diabase is selected. Half of the purified Pb sample is taken for testing. Calibration is carried out using the external standard method, the fractionation coefficient of Pb is 0.11% / amu, and the test results are shown in Table 7.

[0088] Example 7

[0089] Example 7 is roughly the same as Example 4, except that in this example, 50 ± 0.1 mg of JB-3 basalt is selected. Half of the purified Pb sample is taken for testing. Calibration is carried out using the external standard method, the fractionation coefficient of Pb is 0.11% / amu, and the test results are shown in Table 8.

[0090] Example 8

[0091] Example 8 is roughly the same as Example 4, except that in this example, 50 ± 0.1 mg of JA-3 andesite is selected. Half of the purified Pb sample is taken for testing. Calibration is carried out using the external standard method, the fractionation coefficient of Pb is 0.11% / amu, and the test results are shown in Table 9.

[0092] Example 9

[0093] Example 8 is substantially the same as Example 4, except that in this example, 50 ± 0.1 mg of JR-2 rhyolite is selected. Half of the purified Pb sample is taken for testing, and the external standard method is used for calibration. The fractionation coefficient of Pb is 0.11% / amu. The test results are shown in Table 10.

[0094] Example 10

[0095] Example 8 is substantially the same as Example 4, except that in this example, 50 ± 0.1 mg of JG-1a granite is selected. Half of the purified Pb sample is taken for testing, and the external standard method is used for calibration. The fractionation coefficient of Pb is 0.11% / amu. The test results are shown in Table 11.

[0096] The concentrations of the phosphoric acid solution, the concentration of the β-phase silicon nitride suspension, and the Pb sample spotting amounts in the above examples are as shown in Table 1 below:

[0097] Table 1 Data on the concentrations of the phosphoric acid solution, the concentration of the β-phase silicon nitride suspension, and the Pb spotting amounts in Examples 1 to 10

[0098]

[0099] The test results of the above Examples 1 to 10 are as shown in Tables 2 to 11 respectively:

[0100] Table 2 Analysis results of the 20 ng international standard sample NIST 981 in Example 1

[0101]

[0102] Table 3 Analysis results of the 10 ng international standard sample NIST 981 in Example 2

[0103]

[0104] Table 4 Analysis results of the 5 ng international standard sample NIST981 in Example 3

[0105]

[0106] Table 5 Analysis results of the international rock standard sample BCR-2 in Example 4

[0107]

[0108] Note: After the sample was dissolved, half of it was applied to the filament. The sample application amount was estimated based on the Pb element concentration (10.92 ppm) and the column recovery rate (82%). The Pb element concentration was cited from the International journal of environmental research and public health; the literature value of the Pb isotope ratio was cited from Chemical Geology. 2004, 211, 275–303;

[0109] Table 6 Analysis Results of International Rock Standard Sample BIR-1 in Example 5

[0110]

[0111] Note: After the sample was dissolved, half of it was applied to the filament. The sample application amount was estimated based on the Pb element concentration (3.0 ppm) and the column recovery rate (82%). The Pb element concentration was cited from Geostandards Newsletter, 2001, 25, 87–125; the literature value of the Pb isotope ratio was cited from Chemical Geology. 2004, 211, 275–303;

[0112] Table 7 Analysis Results of International Rock Standard Sample W-2 in Example 6

[0113]

[0114] Note: After the sample was dissolved, half of it was applied to the filament. The sample application amount was estimated based on the Pb element concentration (6.5 ppm) and the column recovery rate (82%). The Pb element concentration was cited from Geostandards Newsletter, 2001, 25, 87–125; the literature value of the Pb isotope ratio was cited from Acta. Geol. Sin, 2003, 77, 44–58;

[0115] Table 8 Analysis Results of International Rock Standard Sample JB-3 in Example 7

[0116]

[0117] Note: After the sample was dissolved, half of it was applied to the filament. The sample application amount was estimated based on the Pb element concentration (5.04 ppm) and the column recovery rate (82%). The Pb element concentration was cited from Geostandards Newsletter, 2001, 25, 87–125; the literature value of the Pb isotope ratio was cited from J. Anal. At. Spectrom, 2016, 31, 1150–1159;

[0118] Table 9 Analysis Results of International Rock Standard Sample JA-3 in Example 8

[0119]

[0120] Note: After the sample is dissolved, half of it is applied to the filament. The sample application amount is estimated based on the Pb element concentration (5.04 ppm) and the column recovery rate (82%). The Pb element concentration is cited from the literature Geostandards Newsletter, 2001, 25, 87–125; the literature value of the Pb isotope ratio is cited from Geochem. J, 2006, 40, 121–133;

[0121] Table 10 Analysis Results of International Rock Standard Sample JR-2 in Example 9

[0122]

[0123] Note: After the sample is dissolved, half of it is applied to the filament. The sample application amount is estimated based on the Pb element concentration (21.3 ppm) and the column recovery rate (82%). The Pb element concentration is cited from the literature Geostandards Newsletter, 2001, 25, 87–125; the literature value of the Pb isotope ratio is cited from Geochem. J, 2006, 40, 121–133;

[0124] Table 11 Analysis Results of International Rock Standard Sample JG-1a in Example 10

[0125]

[0126] Note: After the sample is dissolved, half of it is applied to the filament. The sample application amount is estimated based on the Pb element concentration (25.8 ppm) and the column recovery rate (82%). The Pb element concentration is cited from the literature Geostandards Newsletter, 2001, 25, 87–125; the literature value of the Pb isotope ratio is cited from J. Anal. At. Spectrom, 2016, 31, 1150–1159;

[0127] Table 12 Signal Intensity and Emission Duration of β-Si3N4 Suspension for Different Sample Amounts of Lead

[0128]

[0129] Table 13 Compilation of Published Results of TIMS Testing of NIST981

[0130]

[0131] Remarks: Literature sources: Catanzaro et al., J.Res. 1968, NBS72A. No.3, 261-267; Nohda et al., Geochem.J. 1999, 33, 133-139; Kani et al., J.Mass.Spectrom.Soc.Jpn, 2002, 50, 199-203

[0132] Based on the above results, Tables 2 to 4 list the multiple analysis results of different sample amounts (20 ng, 10 ng, 5 ng) of the international standard sample NIST 981 using 10 μg of β-phase silicon nitride suspension respectively. The test results show that for all international standard samples in the range of 5 - 20 ng, 206 Pb / 204 Pb, 206 Pb / 204 Pb and 206 Pb / 204 Pb, the internal analysis precision is better than 0.03% (2RSE), and the reproducibility of its external precision is consistent within the error range with the average reference value of NBS981 reported in the literature (Table 13). Even for a sample amount of 5 ng, the internal precision of the Pb ratio of all samples is better than ±0.03% (2RSE). For multiple analyses of 206 Pb / 204 Pb, 206 Pb / 204 Pb and 206 Pb / 204 Pb, the external precision is better than ±0.10% (2RSD), ±0.14% (2RSD), ±0.20% (2RSD) respectively. The accuracy obtained by the emitter used in this study is consistent within the error range with that of the traditional silica gel emitter. Since Ta tape can be used to replace Re tape as the filament material for Pb testing, the analysis cost of mass spectrometry testing is reduced by about 70%.

[0133] In order to further verify whether the β-phase silicon nitride emitter and its testing method provided by the present invention can meet the application research of actual samples, the present invention also tested through the actual rock samples in Examples 4 to 10 above. From the result data in Tables 5 to 11 above, it can be seen that using the β-phase silicon nitride emitter has good testing precision for actual geological samples. During the data testing of actual rock samples, 208 the intensity of Pb is generally controlled at 2500 - 3500 mV. For all rock samples, 206 Pb / 204 Pb, 206 Pb / 204 Pb and 206 Pb / 204The internal analysis precision of Pb is better than 0.02% (2RSE). The test results are in agreement with the literature values within the error range, indicating that the β-phase silicon nitride emitter of the present invention has high accuracy and can fully meet the needs of Pb isotope analysis of actual rock samples. Table 1 lists the actual sample loading amounts (60 - 530 ng) of each actual rock sample. The Pb content of the vast majority of actual rock samples is generally greater than 2 ppm. For conventional Pb isotope analysis, to eliminate the heterogeneity of the samples, usually a sample digestion amount of 50 - 100 mg can obtain at least 200 ng of Pb. In order to strictly evaluate the sensitivity of the emitter, only half of the prepared samples were coated on the filament in this test. The sample loading amounts of the actual samples all correspond to a rock consumption of 25 mg. During the actual test process, the ion current intensity of Pb can reach above 2.5 V and the emission is stable. Therefore, the sensitivity of the β-phase silicon nitride emitter provided by the present invention can meet the analysis requirements of the vast majority of geological samples.

[0134] Table 12 above lists the emission duration and signal intensity for different sample loading amounts of the Pb standard NBS981. 208 Pb has the highest isotope abundance in the Pb isotope system. Therefore, 208 the emission intensity of Pb is used as the direct scale for sensitivity evaluation. The results in Table 12 above show that for the test method provided by the present invention, even for a 5 ng lead sample, 208 the intensity of Pb can reach 650 - 900 mV, and in this plateau region 208 the Pb signal can be stably emitted for more than 18 minutes. The actual sample collection time only needs 13 minutes (4 s integration, 160 sets of data collection amount) to obtain an analysis precision better than 0.03% (RSE). This also shows that the β-phase silicon nitride emitter provided by the present invention has extremely high sensitivity and high accuracy for Pb isotope analysis.

[0135] It should be noted that those of ordinary skill in the art should understand that any modification or equivalent replacement of the technical solution of the present invention without departing from the purpose and scope of the technical solution of the present invention shall be covered by the scope of the claims of the present invention.

Claims

1. An ionization emitter for lead isotope analysis of trace samples, characterized in that the ionization emitter is a β-phase silicon nitride emitter; the β-phase silicon nitride emitter specifically uses a β-phase silicon nitride suspension and is composed of a phosphoric acid solution as an emitter auxiliary material.

2. The ionization emitter for lead isotope analysis of trace samples according to claim 1, characterized in that the β-phase silicon nitride suspension is prepared by the following method: First, cross-clean the high-purity β-phase silicon nitride powder with dilute hydrochloric acid and high-purity deionized water at least once each to reduce the sample background; then add the treated high-purity β-phase silicon nitride powder to high-purity deionized water to prepare a β-phase silicon nitride suspension with a preset concentration.

3. The ionization emitter for lead isotope analysis of trace samples according to claim 2, characterized in that the concentration of the β-phase silicon nitride suspension is converted according to the dose of β-phase silicon nitride emitter required for each test and the spotting amount of the β-phase silicon nitride suspension. The dose of high-purity β-phase silicon nitride powder required for each test is 10 ± 1 μg, and the spotting amount of the β-phase silicon nitride suspension is 0.9 - 1.1 μL. Therefore, the concentration of the β-phase silicon nitride suspension is 10 mg / mL.

4. The ionization emitter for lead isotope analysis of trace samples according to claim 2 or 3, characterized in that the purity of the high-purity β-phase silicon nitride powder is greater than 99.9%; the particle size of the high-purity β-phase silicon nitride powder is <2 μm.

5. A preparation method of an ionization emitter for lead isotope analysis of trace samples, characterized in that, The ionization emitter is a β-phase silicon nitride emitter. The β-phase silicon nitride emitter specifically uses a β-phase silicon nitride suspension and is composed of a phosphoric acid solution as an emitter auxiliary material; (1) The preparation method of the β-phase silicon nitride suspension is as follows: S1. β-phase silicon nitride pretreatment, specifically as follows: S11. First, weigh the high-purity β-phase silicon nitride powder and place it in a centrifuge tube. Add dilute hydrochloric acid in proportion, seal and shake the centrifuge tube for 2 - 3 minutes to wash the high-purity β-phase silicon nitride powder with dilute hydrochloric acid to reduce the sample background; then, load it into a centrifuge and centrifuge for 10 minutes, and take out the upper dilute hydrochloric acid solution; S12. Then add high-purity deionized water, seal and shake the centrifuge tube again for 2 - 3 minutes, load it into a centrifuge and centrifuge for 10 minutes, and suck out and discard the upper clear liquid again; S13. Repeat steps S11 and S12, cross-clean the high-purity β-phase silicon nitride powder with dilute hydrochloric acid and high-purity deionized water at least once each. The precipitated phase obtained is the pretreated β-phase silicon nitride; S2. Weigh the β-phase silicon nitride pretreated in step S13, add deionized water to prepare a β-phase silicon nitride suspension with a preset concentration; the concentration of the β-phase silicon nitride suspension is converted according to the dose of β-phase silicon nitride emitter required for each test and the spotting amount of the β-phase silicon nitride suspension. The dose of high-purity β-phase silicon nitride powder required for each test is 10 ± 1 μg, and the spotting amount of the β-phase silicon nitride suspension is 0.9 - 1.1 μL. Therefore, the concentration of the β-phase silicon nitride suspension is 10 mg / mL; (2) Preparation of phosphoric acid solution Weigh the concentrated phosphoric acid solution, add deionized water in proportion, and prepare a phosphoric acid solution with a concentration of 0.3 - 0.5 mol / L, that is, obtain it.

6. The preparation method according to claim 5, wherein: In step S11, the concentration of the dilute hydrochloric acid used for cleaning is 0.9 - 1.1 mol / L, and its dosage is calculated based on 1 ml per 10 ± 0.1 mg of high-purity β-phase silicon nitride powder; In step S12, the dosage of the high-purity deionized water used for cleaning is calculated based on 1 ml per 10 ± 0.1 mg of high-purity β-phase silicon nitride powder.

7. A method for testing lead isotopes of trace samples, characterized in that, The lead isotope test and analysis are carried out using the ionization emitter for trace sample lead isotope analysis described in any one of claims 1 - 4 or the ionization emitter for trace sample lead isotope analysis prepared by the preparation method described in any one of claims 5 - 6; specifically, a high-purity β-phase silicon nitride suspension is used as a high-sensitivity emitter to enhance the ionization efficiency of the lead sample. At the same time, a phosphoric acid solution is used to assist ionization, and a high-purity tantalum filament is used as a sample carrier to test trace lead isotopes. The specific steps are as follows: (1) Apply an ionization emitter composed of an appropriate amount of high-purity β-phase silicon nitride suspension and phosphoric acid solution onto the surface of the high-purity tantalum filament. After the emitter is evaporated to dryness, spot the lead sample onto the filament surface. Adjust the current to 1.2 amperes to evaporate the lead sample, and then continue to increase the filament current until the filament turns bright red and maintain for 2 - 3 seconds, then return the current to zero; (2) Load the high-purity tantalum filament carrying the lead sample into a thermal ionization mass spectrometer, and use the thermal ionization mass spectrometer for testing to obtain high-precision lead isotope analysis data; the filament temperature during testing is 1050 - 1150 °C.

8. The test method for trace sample lead isotopes according to claim 7, wherein: The coating process of the emitter in step (1) is specifically as follows: Take 1 - 1.5 μL of a phosphoric acid solution with a concentration of 0.3 - 0.5 mol / L and apply it onto the surface of the high-purity tantalum filament. Adjust the filament current to evaporate the phosphoric acid solution first, and then take 0.9 - 1.1 μL of a β-phase silicon nitride suspension with a preset concentration and cover it on the evaporated phosphoric acid coating. After the β-phase silicon nitride suspension is evaporated to dryness, then spot the lead sample onto the filament surface.

9. The test method for trace sample lead isotopes according to claim 7, wherein: In step (1), the dosage of the lead sample is 5 - 530 ng; the lead sample is an international standard sample or an actual rock sample.

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