Method for detecting Cr isotope in low-content sample by thermal ionization mass spectrometry

Abstract

The embodiment of the invention discloses a method for detecting Cr isotope in a low-content sample by thermal ionization mass spectrometry. The method comprises the following steps: digesting a sample to obtain a to-be-loaded solution; the method comprises the following steps: cleaning and balancing a quartz glass column filled with AG 50W-X12 resin, loading a sample while the quartz glass column is hot, sequentially adding 1N HCl and collecting, eluting matrix elements by the 1N HCl and the 1N HCl and collecting, and combining collected liquids to obtain a to-be-loaded liquid; the method comprises the following steps: cleaning and balancing a PP micro column filled with AG 50W-X12 resin, loading a sample, sequentially adding 1N HF and 1N HCl to elute matrix elements and 6N HCl, and collecting; single rhenium belt sample application is carried out, and a thermal ionization mass spectrometer is adopted for testing. A stable Cr isotope analysis process which is high in recovery rate, low in blank, high in resolution and few in separation step is established on a thermal ionization mass spectrometry (TIMS) platform, and the method has remarkable advantages in high-precision analysis of low-Cr samples and is suitable for high-precision Cr isotope separation and determination of low-Cr-content samples.

Description

Technical Field

[0001] This invention relates to the field of chemical analysis technology, specifically to a method for detecting Cr isotopes in low-content samples using thermal ionization mass spectrometry. Background Technology

[0002] Chromium (Cr) is a multivalent transition metal element, mainly existing in the forms of Cr(III) and Cr(VI). It exhibits significant redox sensitivity and is of great importance in geochemical research. The average abundance of Cr in the Earth's crust is approximately 100 ppm, primarily enriched in ultramafic rocks, while its content is lower in sialic rocks such as granites and sedimentary rocks. Stable isotopes of Cr (…) 50 Cr 52 Cr 53 Cr and 54 Cr is widely used to trace changes in oxygen fugacity and material migration processes from the surface environment to the deep Earth system.

[0003] In low-temperature environments, Cr isotopes undergo significant fractionation primarily through redox reactions. When Cr(VI) is reduced to Cr(III), lighter Cr preferentially migrates, enriching the residual solution with heavier Cr. This process is widely used in reconstructing the oxidation states of paleoatmosphere and water bodies. In high-temperature geological processes such as partial mantle melting, magma crystallization differentiation, and mineral-melt reactions, although Cr isotope fractionation is small, it is sufficient to reflect important geochemical behaviors such as oxygen fugacity in the source region, Cr valence distribution, and mineral partitioning mechanisms. In particular, the coordination behavior of Cr(II) and Cr(III) plays a crucial role in the isotope fractionation of melts and residual minerals.

[0004] In recent years, Cr isotopes have shown broad application potential in tracing oxygen fugacity in magma source regions and identifying deep fluid metasomatic processes. In particular, Cr isotopes provide a unique perspective in the study of Archean tonalite-granodiorite-granodiorite (TTG). TTG is a core component of the Archean continental crust, and its formation involves mechanisms such as partial melting of the mantle, slab melting, or lower crust remelting. It is a key rock type for revealing early continental accretion, tectonic regime changes, and mantle-crust interactions. Due to the complex genesis of TTG and its low Cr content, high-precision Cr isotope analysis helps reconstruct the redox state of its magma source region, thus providing crucial evidence for understanding the evolution of oxygen fugacity in the early Earth's interior, the origin of plate tectonics, and the formation of habitable environments.

[0005] Currently, Cr isotopes are mainly measured using multiple receiver inductively coupled plasma mass spectrometry (MC-ICP-MS) and multiple receiver thermal ionization mass spectrometry (TIMS). The latter, due to its low background, high resolution, and high ion utilization, is more suitable for ultra-high precision analysis and is the preferred technique for high-precision Cr isotope analysis globally. The separation and purification of Cr isotopes mainly employ cation exchange, anion exchange, and their combined methods. Traditional processes typically use AG50W-X8 strongly acidic cation exchange resin and / or AG1-X8 anion exchange resin as a base, combined with multi-stage acid elution to separate Cr from interfering elements such as Fe, Ti, and V. However, existing technologies have the following problems: the anion exchange column method requires oxidizing Cr(III) to Cr(VI) before passing through the column (commonly using ammonium persulfate or potassium permanganate), followed by reduction elution in the column. Oxidants often generate Fe and Mn oxide precipitates, adsorbing trace amounts of Cr(VI) and leading to a decrease in chromium recovery. Simultaneously, the oxidant itself introduces a high process blank. The combined anion and cation exchange method generally suffers from numerous steps, complex operation, and significant sample loss. This method typically requires multiple columns in series, which is not only time-consuming and acid-intensive, but also prone to incomplete valence state conversion and adsorption loss of Cr during multiple valence state transitions and redox processes, resulting in large fluctuations in recovery rate. Currently used cation exchange column methods offer high precision in high-Cr systems, but are limited by resin carrying capacity. When the Cr content in the sample is low, a large sample volume is required to reach the detection limit, easily leading to a relatively high blank, thus making it unsuitable for low-Cr samples.

[0006] The aforementioned problems are particularly prominent in the analysis of TTG and other low-Cr granitic samples. These samples generally have Cr contents below 20 ppm, some even less than 1 ppm. During pretreatment and testing, they are highly susceptible to limitations in resin loading capacity, as well as interference from process blanks and mechanical residues, leading to decreased Cr recovery, unstable isotope signals, and ultimately, difficulty in ensuring measurement accuracy. Therefore, developing high-precision analytical methods suitable for low-Cr content samples and establishing a low-background, high-recovery, and high-resolution separation and testing system on a thermal ionization mass spectrometry platform are crucial foundations for promoting the further application of Cr isotope research in geological and environmental samples. Summary of the Invention

[0007] Therefore, embodiments of the present invention provide a method for detecting Cr isotopes in samples with low content using thermal ionization mass spectrometry.

[0008] To achieve the above objectives, the embodiments of the present invention provide the following technical solutions:

[0009] A method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry, the method comprising the following steps:

[0010] (1) Digest the sample to obtain the sample loading solution;

[0011] (2) After cleaning and equilibration, the quartz glass column filled with AG 50W-X12 resin is loaded with the sample while it is hot. 1N HCl is added and collected in sequence. 1N HCl is used to elute the matrix elements and 1N HCl and collected. The collected liquids are combined to obtain the sample loading solution.

[0012] (3) After cleaning and equilibration, the PP material microcolumn filled with AG 50W-X12 resin was loaded with the sample, and 1NHF and 1N HCl were added sequentially to elute the matrix elements, and 6N HCl was added and collected.

[0013] (4) Single rhenium band spotting was tested using a thermal ionization mass spectrometer.

[0014] To address the environmental characteristics and blank control requirements at different stages of chemical separation, this invention employs a dual-column separation system consisting of a quartz glass column and a polypropylene (PP) column. This system includes two cation exchange separation steps. The first column is a quartz glass column, which possesses excellent high-temperature and strong acid resistance, making it suitable for the initial separation stage with large sample volumes and high acid concentrations. A 1N~6N HCl purification system is used, which can effectively remove major cations such as Fe, Ti, and Ni under high acid conditions, ensuring the quantitative enrichment and valence stability of Cr in the sample. The second column is a PP microcolumn, which features low adsorption and low metal background, allowing safe use in HF-HCl mixed acid systems. The purpose of the second column is to remove residual high-field-strength elements such as Ti, V, and Al after the first column separation, further improving cleanliness and Cr recovery. Through the synergistic effect of HF complexation and HCl elution, Cr is stably eluted in the form of chloride complexes, ultimately obtaining a high-purity Cr fraction. This provides a clean and stable chemical environment for subsequent thermal ionization mass spectrometry determination, which is a key step in improving the recovery rate of Cr in low-content samples and obtaining high-precision isotope measurement results. The separation mechanism of the second column in this invention is as follows: In the initial elution (1N HF) stage, HF forms stable fluorine complexes with high field strength elements (Ti, Al, V, etc.), preventing these elements from being adsorbed by the resin in cationic form, thus allowing them to be preferentially eluted, while Cr... 3+ The fluoride complex did not form a stable fluorine complex and remained on the resin. During the later elution stage (1N~6N HCl), the acidity was gradually increased to allow the Cr to form a stable fluorine complex. 3+ With resin-SO3 - The electrostatic interaction between them is disrupted, and Cr reacts as [Cr(H₂O)₅Cl]. 2+The [CrCl3(H2O)3] complex was quantitatively eluted and collected. This invention utilizes the complexing properties of HF and the competitive adsorption mechanism of HCl to significantly improve the separation of Cr from elements such as Ti, V, and Al. The elution peak of Cr is concentrated in the two columns, achieving a recovery rate of ≥90% in a single operation. Furthermore, this invention uses AG50W-X12 resin with a higher degree of cross-linking. This resin exhibits good structural stability and sample loading capacity under high acid conditions. Studies have shown that compared to AG50W-X8 resin, the method of this invention offers superior separation resolution and repeatability. In summary, the above-mentioned scheme provided by this invention achieves high recovery and low blank, ensuring high purity, high precision, and good repeatability in chromium isotope analysis of low-content samples.

[0015] Further, in step (1), the method for preparing the sample loading solution includes:

[0016] The sample was mixed with HF and HNO3, heated to 130℃ to dissolve, evaporated to dryness at 150℃, HCl and HNO3 were added, heated to 90℃ to dissolve, evaporated to dryness at 130℃, HNO3 was added, the sample was dissolved and evaporated to dryness at 100℃, and then... 50 Cr- 54 The Cr double diluent solution was heated to 100°C, evaporated to dryness, cooled, and then 6N HCl was added. The solution was heated to 130°C for at least 3 hours and then diluted with MQ water to a hydrochloric acid concentration of 1N.

[0017] Studies have found that placing the digested sample in 6N HCl and heating it at 130°C for at least three hours can promote the growth of Cr. 3+ -Cl complexes [mainly Cr(H2O)5Cl] 2+ The formation of [CrCl3(H2O)3] not only stabilizes Cr(III) but also blocks Cr... 3+ →Cr 6+ The conversion pathway causes valence state disorder, which can also improve the exchange efficiency and elution controllability of Cr on the resin, thereby improving the separation peak shape and significantly improving Cr recovery and analytical accuracy.

[0018] Furthermore, based on the Cr content in the sample, 450 μl of [amount of Cr] was added per 1 μg of Cr. 50 Cr- 54 Cr double diluent solution (used to correct for mass discrimination and mass fractionation during experiments). Studies have found that adding Cr in the above proportions... 50 Cr- 54 Cr double diluent solution can dilute the Cr isotopes contained in the sample to... 50 Cr 54Achieving equilibrium through Cr mixing helps improve the accuracy of detection results. After the sample is tested, the true isotope ratio of the sample can be deduced by using a double dilution equation combined with iterative calculation.

[0019] Furthermore, the AG 50W-X12 resin has a particle size of 200-400 mesh.

[0020] Further, in step (2), the rinsing solution used for cleaning includes 6N HCl and MQ water, and the rinsing solution used for equilibration is 1N HCl;

[0021] In step (3), the rinsing solution used for cleaning includes 6N HCl and MQ water, and the rinsing solution used for equilibration is 0.5NHNO3.

[0022] Further, in step (2), the preparation method of the sample solution to be loaded includes: evaporating the collected liquid to dryness, transferring the medium three times with concentrated nitric acid, adding 2N HNO3 and MQ water after complete cooling, heating to dissolve at 130℃, adding 30% H2O2 after complete cooling, and placing the resulting sample solution at room temperature for 24-48 hours. Studies have found that after evaporating the sample purified from a single column to dryness, transferring the medium three times with concentrated nitric acid, then dissolving the evaporated sample in 2N nitric acid and MQ water (volume ratio 250:730), and then heating the nitric acid-mediated sample solution on a hot plate at 130℃ for 2 hours to form different species and Cr... 3+ After balancing, 30% hydrogen peroxide is added, and the sample solution is left to stand at room temperature for 24-48 hours to maximize the promotion of Cr. 3+ The formation of Cr reduces 0 Cr + and Cr 2+ The content of [something] is reduced, thereby increasing the recovery rate.

[0023] Furthermore, before performing step (4), the method further includes: evaporating the collected liquid obtained in step (3) to dryness at 130°C, adding 30% H2O2, evaporating to dryness at 40°C, and repeating this process 2-3 times. Studies have found that using 30% H2O2 as an oxidant can efficiently remove residual organic matter. The principle is that H2O2 decomposes to generate active oxygen (H2O2→H2O+O), which can oxidize the CH bonds in organic compounds, converting organic residues into CO2 and H2O, thereby completely decomposing residual organic matter. This ensures a high and stable signal during instrument testing, guaranteeing the reliability of the detection results.

[0024] Furthermore, the quartz glass column has an inner diameter of 7mm and a resin filling height of 12.8-13.2cm; the PP material micro-column has an inner diameter of 4mm and a resin filling height of 4.8-5cm.

[0025] Furthermore, the samples include basalt, granite, carbonate rock, and gabbro.

[0026] Furthermore, the Cr content in the sample is <20ppm, preferably <5ppm.

[0027] The embodiments of the present invention have the following advantages:

[0028] This invention establishes a stable Cr isotope analysis workflow on a thermal ionization mass spectrometry (TIMS) platform that achieves high recovery, low blank count, high resolution, and fewer separation steps. It demonstrates significant advantages in high-precision analysis of low-Cr samples and is suitable for high-precision Cr isotope separation and determination in samples with low Cr content, especially for Archean tonalite-tungsten granite (TTG) and other geological samples with low Cr content and challenging pretreatment processes. This invention provides a solid technical foundation for Cr isotope studies of granites such as TTG, thereby contributing to the revelation of redox conditions, magma source region characteristics, and mantle-crust material exchange mechanisms in the early continental crust formation, and providing crucial geochemical evidence for reconstructing the early Earth's tectonic regime and oxygen fugacity evolution. Attached Figure Description

[0029] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0030] Figure 1 The elution curve of a column provided by the present invention;

[0031] Figure 2 The rinsing curves of the two columns provided by this invention. Detailed Implementation

[0032] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Main reagents, materials and equipment

[0034] 1) Reagents

[0035] H2O: High-purity water was prepared using a Milli-Q system with a resistivity of approximately 18.2 MΩ / cm.

[0036] Acids: HNO3, HCl, and HF used in the entire experimental procedure were all obtained by purification of BV-III grade reagents through sub-boiling distillation. Acids of different concentrations were obtained by diluting MQ water in different proportions.

[0037] 50 Cr- 54 Cr double diluent: two monoisotope standard solutions 50 Cr spike (high abundance) 50 Cr solution) and 54 Crspike (high abundance) 54 The Cr solution was precisely calibrated using a thermal ionization mass spectrometer (TIMS) and then analyzed according to volume ratios. 50 Cr: 54 Cr = 1:1, mixed thoroughly. 50 Cr spike and 54 Cr spikes were purchased from Oak Ridge National Laboratory (ORNL). The core function of this double diluent is to correct for mass bias and mass fractionation during experiments.

[0038] 2) Resin

[0039] AG 50W-X12 resin: a strong acid cation exchange resin, 200-400 mesh.

[0040] 3) Sample

[0041] The geological standard used was GSP-2 (granite) from the United States Geological Survey (USGS).

[0042] 4) Utensils and Instruments

[0043] All pretreatment procedures were completed in a Class 100 fume hood in a Class 1000 cleanroom at the Scientific Research Institute of China University of Geosciences (Beijing). All PFA dissolution bottles (beakers), pipette tips, and centrifuge tubes used were cleaned and dried in the laboratory.

[0044] All sample analyses were conducted at the Isotope Laboratory of the Scientific Research Institute of China University of Geosciences (Beijing). Trace element concentration analysis was performed using a high-resolution plasma mass spectrometer (Element XR) and an inductively coupled plasma mass spectrometer (ICP-MS), while Cr isotope composition analysis was performed using a thermal ionization mass spectrometer (TIMS).

[0045] Example 1

[0046] This embodiment provides a method for detecting Cr isotopes in low-content samples using thermal ionization mass spectrometry, including the following steps:

[0047] (a) Preparation of sample solution

[0048] 1) Accurately weigh 30mg of sample and pour it into the dissolution bottle, with an error of <5%.

[0049] 2) Add 2 ml of HF and 1 ml of HNO3 to the sample, seal the lid tightly, and heat on a hot plate at 130°C for more than 8 hours. For poorly soluble samples, it is recommended to use sonication to dissolve the sample immediately after adding the acid, then heat and sonicate again to accelerate dissolution.

[0050] 3) After the sample in the dissolution bottle is completely dissolved, adjust the heating plate to 150°C, open the lid, and evaporate the solution to dryness.

[0051] 4) After the solution is evaporated to dryness, add 3 ml of HCl and 1 ml of HNO3 to the dissolution bottle, seal the cap tightly, and heat on a hot plate at 90°C for more than 8 hours to dissolve.

[0052] 5) After dissolving, open the lid and heat the heating plate at 90°C until the color lightens. Continue to use the heating plate to steam at 130°C until it becomes wet salt.

[0053] 6) After the sample is evaporated to dryness, dissolve the sample in 1 ml of concentrated HNO3 (original acid (2D, i.e., evaporated twice on DST to reduce the residual organic matter in the acid) at 130℃. After dissolving, adjust the heating plate to 100℃ to evaporate to dryness.

[0054] 7) Based on the chromium (Cr) content in the sample, add 450 μl of chromium per 1 μg Cr. 50 Cr- 54 The ratio of Cr double diluent was determined, and the diluent was mixed with the sample. The mixed solution was then heated on a 100°C hot plate for more than 12 hours to achieve complete homogeneity.

[0055] 8) Heat the mixed solution at 130℃ to evaporate to dryness, and change the medium to HCl. After complete cooling, use 6N hydrochloric acid to make up to 0.2 ml. Keep the solution at 130℃ for more than 3 hours and wait for column chromatography.

[0056] (II) Separation and purification of Cr

[0057] A two-step chromatographic column separation chemical purification method was adopted.

[0058] One Pillar:

[0059] 1) Add AG 50W-X12 (200-400 mesh) to a quartz glass column with an inner diameter of 7mm, filling to a height of 13cm.

[0060] 2) Washing the resin: Add 20ml of 6N HCl and 10ml of MQ water in sequence.

[0061] 3) Equilibrate the resin: Add 5 ml of 1N HCl.

[0062] 4) Heating and loading: Dilute the 0.2ml heated sample prepared in step (I) with 1ml MQ water to a medium of 1mol / L hydrochloric acid, and load the sample while it is hot.

[0063] 5) Elution and collection of elements: Add 5 ml of 1N HCl and collect, elute matrix elements with 4 ml of 1N HCl and collect with 6 ml of 1N HCl (collect a total of 11 ml).

[0064] Two pillars:

[0065] 1) Add AG 50W-X12 (200-400 mesh) to a PP material micro column with an inner diameter of 4mm, filling to a height of 4.8cm.

[0066] 2) Washing the resin: Add 4 ml of 6N HCl and 3 ml of MQ water in sequence.

[0067] 3) Equilibrium resin: Add 2 ml of 0.5 N HNO3.

[0068] 4) Sample loading: Evaporate the 11 ml sample solution collected from the first column to dryness and transfer it to HNO3 medium. Specifically, after evaporating the sample solution to dryness, transfer it to the medium three times with concentrated nitric acid. After complete cooling, add 250 μl of 2N HNO3 and 730 μl of MQ water. Mix the resulting sample solution on a hot plate at 130℃ for 2 h. After the sample has completely cooled, add 20 μl of 30% H2O2. Let the resulting sample solution stand at room temperature for 36 h to obtain a total of 1 ml of sample for loading onto the second column. The medium is 0.5N HNO3.

[0069] 5) Elution elements: Add 3*2ml of 1N HF and 4*3ml of 1N HCl in sequence.

[0070] 6) Collect the elements: Add 3 ml of 6N HCl and collect.

[0071] The details of the two-step Cr purification scheme using the AG 50W-X12 resin column are shown in Table 1.

[0072] Table 1. Cr isotope purification process

[0073]

[0074] Geological standard GSP-2 was separated and purified according to the above method. The elution curves for column one and column two are shown in the figure. Figure 1 and Figure 2 The dual-column process achieves a recovery rate greater than 85%. The above demonstrates that the method of this invention can effectively separate Cr from matrix elements while maintaining a high Cr recovery rate.

[0075] 7) Cr was separated using the above chemical separation method. After evaporating it to dryness, 100 μl of 30% H2O2 was added, and the mixture was placed on a hot plate and evaporated to dryness at 40°C. This step was repeated 3 times.

[0076] 8) The sample was spotted using a single rhenium (Re) band and finally tested using a thermal ionization mass spectrometer (TIMS).

[0077] Furthermore, to verify the accuracy and applicability of the method of this invention, various international and domestic geological standard materials were selected for testing, including a variety of rock types such as basalt, granite, carbonate rocks, and gabbro. The selected standard materials cover different Cr content ranges and various lithologies to comprehensively evaluate the determination effect of the method of this invention in samples with low, medium, and high Cr content.

[0078] The test results show that the Cr isotope ratios of each standard substance are consistent with the literature reference values ​​within the error range, and the method has good repeatability. This verifies that the method of the present invention has high measurement accuracy and reliability in different types of samples, and is especially suitable for siliceous aluminous rocks with low Cr content. Specific test results are shown in Table 2.

[0079] Table 2 Geological Standards 53 Cr

[0080]

[0081] During the separation test of GSP-2 standard, the recovery rate of the first column was 90%, the recovery rate of the second column was about 95%, and the total recovery rate of the two columns was about 86%.

[0082] This invention monitors the background levels throughout the entire process by adding acid and reagents to the dissolution vessel without placing a sample under identical experimental conditions, maintaining complete consistency with the geological sample flow from dissolution, purification, and testing. The "blank" sample collected after the entire process is evaporated to dryness, dissolved thoroughly in 3% nitric acid, and brought to a final volume. The entire sample after volume adjustment is used for concentration testing without dilution. The signal intensity of chromium in the blank is measured using Element XR. When the ratio of chromium concentration in the blank to that in the sample is less than 0.5%, the influence of the blank on chromium isotope testing is negligible. The total chemical blank in the pretreatment process is less than 1 ng, far lower than the chromium content in the sample, and its impact on the experimental data is negligible.

[0083] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry, characterized in that, The method includes the following steps: (1) Digest the sample to obtain the sample loading solution; (2) After cleaning and equilibration, the quartz glass column filled with AG 50W-X12 resin is loaded with the sample while it is hot. 1N HCl is added and collected in sequence. 1N HCl is used to elute the matrix elements and 1N HCl and collected. The collected liquids are combined to obtain the sample loading solution. (3) After cleaning and equilibration, the PP material microcolumn filled with AG 50W-X12 resin was loaded with the sample, and 1N HF and 1N HCl were added in sequence to elute the matrix elements, and 6N HCl was collected. (4) Single rhenium band spotting was tested using a thermal ionization mass spectrometer.

2. The method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry according to claim 1, characterized in that, In step (1), the preparation method of the sample loading solution includes: The sample was mixed with HF and HNO3, heated to 130℃ to dissolve, evaporated to dryness at 150℃, HCl and HNO3 were added, heated to 90℃ to dissolve, evaporated to dryness at 130℃, HNO3 was added, the sample was dissolved and evaporated to dryness at 100℃, and then... 50 Cr- 54 The Cr double diluent solution was heated to 100°C, evaporated to dryness, cooled, and then 6N HCl was added. The solution was heated to 130°C for at least 3 hours and then diluted with MQ water to a hydrochloric acid concentration of 1N.

3. The method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry according to claim 2, characterized in that, Based on the Cr content in the sample, add 450 μl of Cr per 1 μg of Cr. 50 Cr- 54 Cr double diluent solution.

4. The method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry according to claim 1, characterized in that, The AG 50W-X12 resin has a particle size of 200-400 mesh.

5. The method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry according to claim 1, characterized in that, In step (2), the rinsing solution used for cleaning includes 6N HCl and MQ water, and the rinsing solution used for equilibration is 1N HCl; In step (3), the rinsing solution used for cleaning includes 6N HCl and MQ water, and the rinsing solution used for equilibration is 0.5NHNO3.

6. The method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry according to claim 1, characterized in that, In step (2), the preparation method of the sample loading solution includes: The collected liquid was evaporated to dryness, and the medium was transferred three times with concentrated nitric acid. After complete cooling, 2N HNO3 and MQ water were added, and the solution was heated to 130℃ to dissolve. After complete cooling, 30% H2O2 was added, and the resulting sample solution was left at room temperature for 24-48 hours.

7. The method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry according to claim 1, characterized in that, Before performing step (4), the method further includes: evaporating the collected liquid obtained in step (3) at 130°C, adding 30% H2O2, evaporating at 40°C, and repeating 2-3 times.

8. The method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry according to claim 1, characterized in that, The quartz glass column has an inner diameter of 7mm and a resin filling height of 12.8-13.2cm; the PP material micro-column has an inner diameter of 4mm and a resin filling height of 4.8-5cm.

9. The method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry according to claim 1, characterized in that, The samples included granite, sandstone, oil shale, and gabbro.

10. The method for detecting Cr isotopes in low-content samples by thermal ionization mass spectrometry according to claim 1, characterized in that, The Cr content in the sample is <20ppm, preferably <5ppm.

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