A method for preparing a three silicon isotope standard sample for celestial samples

Abstract

The application discloses a preparation method of a three silicon isotope standard sample for celestial body samples, and belongs to the field of geochemical stable isotopes. The method comprises the following steps: preparing two basic materials according to the average composition of the most representative silicate rocks on the earth, and grinding and mixing the two basic materials after decarburization; adding a silicon-29 diluent to one of the basic materials, repeatedly sintering and grinding to obtain material A; mixing part of the material A into the other basic material, and again sintering and grinding to obtain material B; and obtaining the three silicon isotope standard sample through sample dissolution and chemical purification of the material B. Through MC-ICP-MS testing and calculation of silicon isotope anomalies of the standard sample, it is found that the prepared standard sample is uniform in composition, and the non-mass-related fractionation signal is clear, so that the prepared standard sample can be used for correcting the mass discrimination effect in the analysis process of a to-be-tested sample, accurately identifying silicon isotope anomalies in celestial body samples, improving the accuracy and comparability of analysis results, and providing a reliable experimental reference for planetary science and geochemical research.

Description

Technical Field

[0001] This invention belongs to the field of stable isotopes in geochemistry, specifically relating to a method for preparing silicon isotope standards for celestial samples. It is applicable to the determination of silicon isotope composition anomalies in celestial samples such as meteorites, lunar or Martian samples, and is of great significance to planetary science and geochemical research. Background Technology

[0002] Silicon has three stable isotopes, namely 28 Si、 29 Si、 30 Si, where 28 Si is the dominant element (natural abundance is about 92.22%). 29 Si and 30 Si is approximately 4.69% and 3.09%, respectively. Silicon isotopic composition is typically expressed in terms of δ¹⁸O. 30 Si, δ 29 Si represents the percentage deviation of a sample from the international standard reference NBS-28 (NIST SRM8546), specifically parts per thousand (ppm). 30 Si / 28 Si、 29 Si / 28 Si ratio (δ) 30 Si=[( 30 Si / 28 Si) 样品 / ( 30 Si / 28 Si) NBS-28 -1]×1000 (‰), δ 29 (The same applies to Si). In the analysis of astronomical samples, it is necessary to correct for mass discrimination effects during the analysis process using reference materials, while simultaneously identifying and quantifying any non-mass-dependent silicon isotope fractionation signals that may exist in the sample. However, current technologies lack homogeneous tri-silicon isotope standards with clear and stable non-mass-dependent fractionation signals, resulting in insufficient accuracy and comparability of silicon isotope analysis results for astronomical samples, thus hindering in-depth research in planetary science and geochemistry. Currently used mass spectrometry analysis requires specific nebulizers, fog chambers, sample cones, and other components, combined with high-resolution modes to reduce the influence of interfering ion clusters, but the lack of suitable standards limits the correction effect and fails to meet the requirements of high-precision analysis. Summary of the Invention

[0003] The purpose of this invention is to provide a method for preparing tri-silicon isotope standards for astronomical samples. The aim is to prepare tri-silicon isotope standards with uniform composition and clear non-mass-dependent fractionation signals, serving as a benchmark reference material for tri-silicon isotope analysis of astronomical samples. This method effectively corrects the mass discrimination effect during the analysis process, accurately determines and quantifies the non-mass-dependent silicon isotope fractionation signals in the astronomical samples to be tested, improves the accuracy and comparability of analytical results, and provides a reliable experimental reference for related research in planetary science and geochemistry.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is to provide a method for preparing silicon isotope standards for astronomical samples, comprising the following steps:

[0005] (1) Raw material preparation: Based on the average chemical composition and content of silicate rocks, analytically pure oxide and carbonate powders that match the chemical composition of silicate rocks are mixed in proportion to prepare two portions of basic materials (about 10 g) and placed in a platinum crucible that has been cleaned with hydrofluoric acid.

[0006] (2) Decarburization and mixing: Place two platinum crucibles containing the base material into a muffle furnace for decarburization treatment, and then transfer them to an alcohol-cleaned corundum crucible to grind them into powder so that the base material is fully mixed.

[0007] (3) Preparation of material A: Since the required dilution dose is small, in order to minimize the loss in the experiment, silicon-29 diluent is added quantitatively to one of the decarburized and mixed base materials. It is first ground in a corundum crucible to avoid splashing, then transferred to a furnace for sintering, taken out and quenched (in glass state) and ground into powder. The sintering-grinding is repeated at least 3 times to obtain powdered material A.

[0008] (4) Preparation of material B: Take a portion of material A quantitatively (the amount taken out must meet the requirement that the target isotope abnormal signal is higher than the detection accuracy of MC-ICP-MS), mix it into another portion of decarburized and mixed base material, reduce the concentration of diluent, repeat the sintering-grinding of step (3) at least 3 times, so that the diluent is fully mixed in the material and the composition is uniform, and obtain material B.

[0009] (5) Chemical purification: After dissolving material B, chemical purification is carried out to obtain trisilicon isotope standard samples;

[0010] (6) Detection and analysis: The silicon isotope composition of the three silicon isotope standards was tested using MC-ICP-MS (multiple receiver inductively coupled plasma mass spectrometry). Based on the measured stable isotope data, the isotope composition and homogeneity were analyzed, and the silicon isotope anomaly value (Δ') was calculated. 29Mass-independent fractionation (MIF) of silicon isotopes and silicon isotopes.

[0011] As a further description of the above technical solution, the silicate rocks mentioned in step (1) include basalt, andesite, and rhyolite, which are the most representative silicate rocks on Earth. Their chemical composition is highly similar to the matrix composition of most celestial silicate samples, and there are gradient differences (such as SiO2 content: basalt < andesite < rhyolite), which can cover the composition range of different types of celestial silicate samples, making the standard sample more applicable and meeting the diverse needs of celestial sample analysis; the analytically pure oxides include SiO2, TiO2, and Al2O3, and the carbonates include MgCO3, CaCO3, Na2CO3, and K2CO3.

[0012] As a further description of the above technical solution, the SiO2 is derived from NBS-28 (NISTSRM8546) quartz sand with a purity >99%.

[0013] As a further description of the above technical solution, the specific process of decarburization in step (2) is as follows: first, heat to 950-1000 ℃ at a heating rate of 1.5-1.8 ℃ / min, hold for 500-600 min, and then cool down to room temperature at a cooling rate of 1.5-1.8 ℃ / min; the grinding is carried out in an alcohol-cleaned corundum crucible.

[0014] As a further description of the above technical solution, the amount of silicon-29 diluent added in step (3) is 2 × 10⁻⁶ of one part of the base material mass. -4 ~4×10 -4 The sintering temperature control process is as follows: first, heat to 45-55 °C, then heat to 950-1000 °C at a rate of 9-10 °C / min, then heat to 1350-1450 °C at a rate of 5-6 °C / min, then heat to 1500-1600 °C at a rate of 2-3 °C / min, hold for 500-600 min, and finally cool down to 45-55 °C at a rate of 3-4 °C / min.

[0015] As a further description of the above technical solution, the sample dissolution in step (5) adopts the alkaline dissolution method. The specific operation is as follows: weigh material B and NaOH and place them in a silver crucible. Heat at 700-720 °C for 10-15 minutes. After cooling to room temperature, transfer to a sample dissolution tank, add secondary ultrapure water, shake well, and let stand at room temperature for 12-16 h. Finally, add secondary purified nitric acid for acidification and let stand at 60-80 °C for 3-5 h to obtain the sample mother liquor to be chemically purified.

[0016] As a further description of the above technical solution, the chemical purification in step (5) is carried out by column purification using a G50W-X12 cation exchange resin column. The column purification process is as follows: first, the resin is eluted 3 to 4 times with 6 mol / L secondary purification nitric acid to remove matrix elements. Then, the resin is washed with ultrapure water until the pH value is neutral. Then, the sample mother liquor is loaded onto the chromatographic column. After elution with ultrapure water, the eluent is collected. If the eluent still contains matrix elements, the above column purification process is repeated at least once to completely remove the matrix elements. The eluent with completely removed matrix elements collected is the trisilicon isotope standard.

[0017] As a further description of the above technical solution, the resin particles of the G50W-X12 cation exchange resin chromatographic column have a particle size of 200-400 mesh.

[0018] As a further description of the above technical solution, the parameters for the MC-ICP-MS test in step (6) are set as follows: RF power 1000~1200 W, cooling argon flow rate 15~17 L / min, auxiliary argon flow rate 0.7~1.0 L / min, atomizing argon flow rate 0.8~1.2 L / min, extraction voltage -1800~-2200 V. 28 Si sensitivity 5 V·ppm -1 The solution extraction rate was 40–60 μL / min; a PFA nebulizer (50 μL / min, ESI), a quartz dual-channel cyclone nebulizer (ESI), an H-type extraction cone, and a Ni Jet sample cone were used; three Faraday cups, L3, C, and H3, were used to collect samples. 28 Si + , 29 Si + , 29 Si + To reduce the impact of interfering ion clusters in high-resolution mode.

[0019] As a further description of the above technical solution, in step (6), the silicon isotope anomaly is determined according to the formula Δ' 29 Si= (δ' 29 Si-0.5178×δ' 30 The calculation is performed using Si)×1000 (unit: ppm), where δ' 29 Si and δ' 30 Si represents the apparent obtained directly from MC-ICP-MS testing, without final quality discrimination correction. 29 Si isotopic composition deviation and apparent 30 Si isotopic composition deviation.

[0020] When performing silicon isotope analysis on unknown samples (such as meteorites, rocks / soil returned from the moon or Mars, or other silicate systems such as terrestrial rocks and minerals), the tri-silicon isotope standard of this invention can be used to correct for mass discrimination effect by employing the sample-standard interpolation method.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] The trisilicon isotope standard prepared by the method of this invention has uniform composition, and its δ¹⁸O₂ content was measured. 30 The Si value varies within a range of less than 0.01‰, δ 29 The Si value variation range is less than 0.04‰, and the non-mass-dependent fractionation signal is clear and repeatable, meeting the benchmark reference requirements for astronomical sample analysis. It can effectively correct the mass discrimination effect in the analysis of three silicon isotopes, accurately identify and quantify the non-mass-dependent silicon isotope fractionation signal in the astronomical sample to be tested, and significantly improve the accuracy and comparability of the analysis results. At the same time, the preparation method of this invention is highly controllable. By precisely controlling the amount of silicon-29 diluent added and the melting and sintering conditions, the performance of the standard sample can be ensured to be stable, and the preparation process is repeatable, which is convenient for those skilled in the art to operate and implement. This standard sample is also applicable to the analysis of meteorite, lunar or Martian samples and other silicate systems, and can provide a reliable experimental reference for the study of non-equilibrium silicon isotope processes in the fields of planetary science and geochemistry, and promote the in-depth development of related research. Detailed Implementation

[0023] The claims of the present invention will be further described in detail below with reference to specific embodiments, but this does not constitute any limitation on the present invention. Any limited modifications made by any person within the scope of protection of the claims of the present invention are still within the scope of protection of the claims of the present invention.

[0024] The present invention will be further described in detail below with reference to the preparation of silicon isotope standards for basalt.

[0025] A method for preparing trisilicon isotope standards for astronomical samples includes the following steps:

[0026] (1) Raw material preparation: Weigh 5.66 g, 0.21 g, 1.81 g, 0.77 g, 1.09 g, 0.33 g, and 0.13 g of SiO2, TiO2, Al2O3, MgCO3, CaCO3, Na2CO3, and K2CO3 respectively, and prepare two portions of about 10 g of basic materials, which are placed in a platinum crucible that has been soaked in hydrofluoric acid for 2 days and cleaned; among them, SiO2 is selected as NBS-28 (NIST SRM8546) quartz sand with a purity of >99%.

[0027] (2) Decarburization and mixing: Two platinum crucibles containing the base material were placed in a muffle furnace for decarburization. The decarburization program was set as follows: first, the temperature was heated to 1000 ℃ at a rate of 1.6 ℃ / min for about 600 minutes, and then held for 600 minutes. After that, the temperature was lowered to room temperature at a rate of 1.6 ℃ / min for about 600 minutes. After that, the crucibles were taken out and transferred to a corundum crucible that had been cleaned with alcohol and ground into powder.

[0028] (3) Add 3 mg of silicon-29 diluent (a standard reagent with known isotopic abundance) to one portion of the decarburized and mixed base material, and carefully grind it in a corundum crucible to avoid splashing. Then transfer it to a furnace for sintering. The temperature control process for sintering is as follows: first heat up to 50°C, then heat to 1000°C at a rate of 9.5°C / min for 100 min, then heat to 1400°C at a rate of 5°C / min for 80 min, then heat to 1550°C at a rate of 2°C / min for 75 min, hold for 600 min, and then cool down to 50°C at a rate of 3°C / min over 500 min. After the sintered material is taken out, quench it to a glassy state, grind it into powder, and repeat the sintering-grinding process 3 times to ensure that the diluent and base material are fully mixed to obtain powdered material A.

[0029] (4) Preparation of material B: Take about 0.1466 g of material A and mix it into another portion of base material to reduce the concentration of diluent. Repeat the sintering-grinding process of step (3) at least 3 times to ensure that the diluent is fully mixed in the material and the composition is uniform, so as to obtain powdered material B.

[0030] (5) Dissolution and purification: Material B is dissolved using the alkaline dissolution method. The specific process is as follows: Weigh material B and an appropriate amount of NaOH into a silver crucible using an analytical balance. Place the silver crucible in a muffle furnace and heat it at 720℃ for 10 min. After heating, remove it and place it on an asbestos plate to cool to room temperature. Rinse the outer wall of the crucible with secondary ultrapure water from a wash bottle. Then place the crucible into a clean, labeled dissolution vessel. Add 15 ml of secondary ultrapure water to the dissolution vessel containing the crucible and shake it evenly. Then leave it at room temperature overnight (about 16 h). The next day, add 0.21 ml of secondary purified nitric acid (superior grade or higher nitric acid purified by two sub-boiling distillations before use) to the dissolution vessel. Shake it evenly and leave it at 70℃ for 3 h to obtain the sample mother liquor to be purified, which can be used for subsequent chemical purification.

[0031] Chemical purification was performed using a G50W-X12 cation exchange resin (200-400 mesh) column. The column purification procedure was as follows: First, the column was eluted sequentially with 2 ml, 3 ml, and 2 ml of 6 mol / L purified nitric acid to remove the resin matrix elements. Then, it was eluted with 2 ml, 2 ml, and 3 ml of ultrapure water, respectively. The pH of the eluent was checked and found to be neutral (if the pH was still acidic, elution with ultrapure water was continued until the pH was neutral). Then, 3 ml of sample stock solution was loaded and eluted with 2 ml and 3 ml of ultrapure water, respectively. The eluent was collected. If matrix elements were still present in the eluent, the column purification procedure was repeated at least once until no matrix elements were present in the eluent. The final collected eluent with completely removed matrix elements was the trisilicon isotope standard.

[0032] The above-mentioned sample dissolution and chemical purification were all carried out in a clean laboratory of Class 1000 or higher, and the containers used were poly(perfluoroethylene) propylene (PFA) plastic containers with screw caps. They were then washed sequentially with 10% by mass of superior pure nitric acid, hydrochloric acid, and nitric acid, thereby reducing the background value of the entire process and reducing experimental errors.

[0033] (6) Detection and analysis: The Si isotopic composition of the obtained three silicon isotope standards was determined by multi-receiver inductively coupled plasma mass spectrometry (MC-ICP-MS). At the same time, the petrological standards of Si (BHVO-2, AGV-2) were measured to verify the accuracy of the instrument. The test used a PFA material nebulizer (50 μL / min, ESI), a quartz dual-channel cyclone nebulizer (ESI), an H-truncation cone, and a Ni Jet sample cone. The L3, C, and H3 Faraday cups were used to collect samples. 28 Si + , 29 Si + , 30 Si + The test was conducted in high-resolution mode to minimize interfering ion clusters. 12 C 16 O + , 14 N2 + , 14 N 16 O + The parameters were set as follows: RF power ~1200W, cooling argon 16 L / min, auxiliary argon 0.8 L / min, nebulizing argon 1.0 L / min, extraction voltage -2000 V. 28 Si sensitivity 5 V·ppm⁻¹, solution extraction rate 50 μL / min.

[0034] Three parallel tests were conducted, and the results are shown in Table 1. Table 1 shows that within the error range, the silicon isotope composition of the samples is uniform, and δ0.05 30 The Si value varies within a range of less than 0.01‰, with an average value of 0.00‰; δ 29 The Si value varies within a range of less than 0.04‰, with an average value of 0.14‰. The average value of the non-mass-dependent fractionation (MIF) is 0.28‰. The calculated silicon isotope anomaly (Δ') 29 Si= (δ' 29 Si-0.5178×δ' 30 The average concentration (Si) × 1000, ppm) is 140 ppm, with an error (2SD) of approximately 30 ppm. This indicates that by precisely controlling the amount of silicon-29 isotope added and the melting and sintering conditions, the prepared glass standard maintains good homogeneity in overall chemical composition and physical structure. At the same time, it exhibits clear and repeatable non-mass-related isotope deviation characteristics in the tri-silicon isotope system, which is higher than the instrument detection accuracy. It can be used as a standard material for studying silicon isotope anomalies in astronomical samples.

[0035] Table 1. Silicon isotope composition, non-mass-related fractionation, and silicon isotope anomalies from multiple sampling tests of three silicon isotope standards.

[0036]

[0037] It should be noted that in actual operation, the raw material ratio can be adjusted according to the target standard sample (andesite, rhyolite properties), and the dissolution and purification steps can be appropriately increased or decreased according to the sample properties.

[0038] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions will not cause the essence of the corresponding technical solutions to deviate from the protection scope of the present invention.

Claims

1. A method for preparing tri-silicon isotope standards for astronomical samples, characterized in that, Includes the following steps: (1) Raw material preparation: Based on the average chemical composition and content of silicate rocks, analytically pure oxide and carbonate powders that match the chemical composition of silicate rocks are mixed in proportion to prepare two basic materials respectively; (2) Decarbonization and mixing: After decarbonization, the base material is ground into a uniform powder. (3) Preparation of material A: Add silicon-29 diluent to one portion of the decarburized and mixed base material in a quantitative manner, and repeat sintering-grinding at least 3 times to obtain powdered material A; (4) Preparation of material B: Take a portion of material A in quantitative form and mix it into another portion of decarburized and mixed base material. Repeat the sintering-grinding process at least 3 times to obtain material B. (5) Chemical purification: After dissolving material B, chemical purification is carried out to obtain trisilicon isotope standard samples; (6) Detection and analysis: The silicon isotope composition of the three silicon isotope standards was tested using MC-ICP-MS, the isotope composition and homogeneity were analyzed, and the silicon isotope anomaly value and non-mass-related fractionation signal of silicon isotope were calculated.

2. The method for preparing silicon isotope standards for astronomical samples according to claim 1, characterized in that: The silicate rocks mentioned in step (1) include basalt, andesite and rhyolite; the analytically pure oxides include SiO2, TiO2 and Al2O3, and the carbonates include MgCO3, CaCO3, Na2CO3 and K2CO3.

3. The method for preparing silicon isotope standards for astronomical samples according to claim 2, characterized in that: The SiO2 is derived from NBS-28 quartz sand with a purity of >99%.

4. The method for preparing silicon isotope standards for astronomical samples according to claim 1, characterized in that: The specific decarburization process in step (2) is as follows: first, heat to 950-1000 ℃ at a heating rate of 1.5-1.8 ℃ / min, hold for 500-600 min, and then cool down to room temperature at a cooling rate of 1.5-1.8 ℃ / min; the grinding is carried out in an alcohol-cleaned corundum crucible.

5. The method for preparing silicon isotope standards for astronomical samples according to claim 1, characterized in that: The amount of silicon-29 diluent added in step (3) is 2 × 10⁻⁶ of one part of the base material mass. -4 ~4×10 -4 The sintering temperature control process is as follows: first, heat to 45-55 °C, then heat to 950-1000 °C at a rate of 9-10 °C / min, then heat to 1350-1450 °C at a rate of 5-6 °C / min, then heat to 1500-1600 °C at a rate of 2-3 °C / min, hold for 500-600 min, and finally cool down to 45-55 °C at a rate of 3-4 °C / min.

6. The method for preparing silicon isotope standards for astronomical samples according to claim 1, characterized in that: The sample dissolution method described in step (5) is an alkaline dissolution method. The specific operation is as follows: weigh material B and NaOH and place them in a silver crucible. Heat at 700-720 °C for 10-15 minutes. After cooling to room temperature, transfer to a sample dissolution vessel, add secondary ultrapure water, shake well, and let stand at room temperature for 12-16 h. Finally, add secondary purified nitric acid for acidification and let stand at 60-80 °C for 3-5 h to obtain the sample mother liquor to be chemically purified.

7. The method for preparing silicon isotope standards for astronomical samples according to claim 6, characterized in that: The chemical purification in step (5) is carried out using a G50W-X12 cation exchange resin column. The column purification process is as follows: first, the resin is eluted 3 to 4 times with 6 mol / L nitric acid to remove matrix elements. Then, the resin is washed with ultrapure water until the pH value is neutral. Then, the sample mother liquor is loaded onto the chromatographic column and eluted with ultrapure water twice. The eluent is then collected, which is the trisilicon isotope standard.

8. The method for preparing silicon isotope standards for astronomical samples according to claim 7, characterized in that: The resin particles of the G50W-X12 cation exchange resin column have a particle size of 200-400 mesh.

9. The method for preparing silicon isotope standards for astronomical samples according to claim 1, characterized in that: The parameters for the MC-ICP-MS test in step (6) are set as follows: RF power 1000–1200 W, cooling argon flow rate 15–17 L / min, auxiliary argon flow rate 0.7–1.0 L / min, nebulizing argon flow rate 0.8–1.2 L / min, and extraction voltage -1800–-2200 V. 28 Si sensitivity 5 V·ppm -1 The solution extraction rate is 40–60 μL / min.

10. The method for preparing silicon isotope standards for astronomical samples according to claim 1, characterized in that: In step (6), the silicon isotope anomaly value is calculated according to the formula Δ' 29 Si=(δ' 29 Si-0.5178×δ' 30 The calculation is performed using Si)×1000.

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