A method for determining lithium isotopes in coal by secondary ion mass spectrometry

Abstract

The application discloses a method for determining lithium isotopes in coal by secondary ion mass spectrometry, and relates to the technical field of coal-based metal isotope determination; the application identifies the occurrence phase of lithium in coal before SIMS testing, and performs SIMS analysis in a single occurrence phase, so that the problem of poor lithium isotope testing accuracy caused by mixed occurrence phases is effectively avoided; in the SIMS testing, a reference material similar to the lithium occurrence phase in the coal sample to be tested in chemical composition and structure is used, a suitable correction strategy is proposed, the differences in secondary ion amounts and isotope fractionation under different matrix conditions are corrected, and the interference of the matrix effect is reduced; the occurrence phase of lithium is set to match the SIMS testing parameter combination, so that the secondary ion signal is more stable, and the testing repeatability is improved; the method disclosed by the application is helpful to realize stable and accurate in-situ determination of lithium isotopes in different occurrence phases in coal.

Description

Technical Field

[0001] This invention relates to the field of coal-based metal isotope determination technology, specifically a method for determining lithium isotopes in coal using secondary ion mass spectrometry. It is applicable to the in-situ accurate detection of lithium isotopes in different occurrence phases of coal samples and can be widely used in coalfield geological exploration, evaluation of key coal-based metal resources, geochemical process tracing and other related fields. It can also be extended to the determination of lithium isotopes in other low-lithium, heterogeneous and complex materials in micro-area regions. Background Technology

[0002] Lithium (Li), as one of the key metals, has the characteristics of low density and excellent electrochemical performance, and is widely used in many fields such as new energy, new materials, aerospace, and medicine. With the rapid development of the global new energy industry, the demand for lithium resources continues to rise, and finding new sources of lithium resources has become a key research direction.

[0003] Traditionally, salt lake brines and hard-rock lithium deposits were considered the main sources of lithium resources. However, in recent years, Chinese geologists have discovered anomalous lithium enrichment in several coalfields in North China, Southwest China, and Northwest China. Coal and coal-associated minerals are increasingly being regarded as potential lithium resource reserves. The lithium content in some high-lithium coal seams or interbedded rock exceeds the crustal Clarke value, and some enriched strata, after gray-based conversion, reach relevant resource evaluation indicators, demonstrating promising development and utilization prospects.

[0004] Coal, as a complex organic-inorganic composite system, is not only an important energy carrier, but its lithium content and isotopic composition also contain rich geochemical information, revealing the sedimentary environment, material source, mineralization regularity, and subsequent alteration processes of coalfields. This provides significant guidance for the genetic study and exploration evaluation of coal-series lithium resources. However, the occurrence state of lithium in coal is extremely complex. It can be adsorbed on the surface of organic matter or mineral particles, enter the octahedral sites of clay minerals, undergo isomorphous substitution with cations such as Al, Mg, and Fe, or be encapsulated within organic matter or mineral particles. This complex occurrence mode leads to significant micro-regional heterogeneity in lithium distribution within coal, with considerable differences in lithium content and isotopic composition among different micro-components and mineral phases.

[0005] Currently, the methods for testing lithium isotopes in coal are mainly divided into two categories: one is the overall testing method, which mainly uses multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS); the other is the in-situ micro-area testing method, which mainly includes secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).

[0006] Among these methods, MC-ICP-MS is currently the mainstream method for lithium isotope testing in coal, offering high accuracy. However, this method has significant limitations: First, the sample pretreatment process is complex, requiring crushing, grinding, and acid digestion of the coal sample. This process disrupts the in-situ occurrence of lithium in different micro-components and minerals within the coal, making it impossible to achieve in-situ micro-area lithium isotope determination and establish a correlation between lithium isotope data and specific components in the coal. Second, reagents such as nitric acid and hydrofluoric acid used in the acid digestion process, as well as experimental instruments, can easily introduce exogenous lithium contamination, leading to deviations in the measurement results. Furthermore, the high organic matter content and complex matrix composition in coal may affect the accuracy of lithium isotope determination. Therefore, the traditional MC-ICP-MS solution method has certain limitations in revealing the migration and occurrence mechanisms of lithium isotopes in different components of coal.

[0007] To compensate for the shortcomings of overall testing methods, in-situ micro-area analysis techniques have been gradually applied to lithium isotope testing of geological samples. Among them, secondary ion mass spectrometry (SIMS) has become an ideal technique for lithium isotope micro-area determination in coal due to its advantages such as submicron spatial resolution, high sensitivity (detecting lithium content from ppm to ppb), no need for complex pretreatment, and the ability to perform in-situ non-destructive testing. Currently, the technology of using SIMS for lithium isotope testing of geological samples (such as minerals and rocks) is relatively mature, with a testing accuracy of about 1‰, which can meet the needs of isotope tracing.

[0008] However, coal samples differ significantly from conventional geological samples (such as pure minerals and rocks). They are characterized by high porosity, high organic matter content, uneven elemental distribution, and complex organic-inorganic interfaces. Directly applying existing SIMS testing technology to coal samples presents significant compatibility issues: First, the porous structure of coal samples leads to uneven primary ion beam sputtering, resulting in unstable secondary ion yields and affecting test repeatability; second, organic matter is easily decomposed under primary ion beam bombardment, causing fluctuations in secondary ion yields and decreased signal stability, which is detrimental to lithium ions (…). 6 Li + , 7 Li + The following issues are identified: 1) interference from the signal, leading to a decrease in the signal-to-noise ratio; 2) significant matrix effects at the organic-inorganic interface of the coal sample, which may cause differences in instrument mass fractionation; 3) the standard substances used in existing SIMS tests are mostly glass, minerals or other homogeneous inorganic standards, which differ greatly from the matrix composition of the coal sample (organic-inorganic composite system), making it difficult to effectively correct the matrix effect and further reducing the accuracy of the test.

[0009] In summary, existing methods for testing lithium isotopes in coal either fail to achieve in-situ micro-area determination or suffer from insufficient accuracy and stability, making it difficult to meet the demand for precise determination of lithium isotopes in different occurrence phases of coal. This limits the exploration and evaluation of coal-based lithium resources and the in-depth development of lithium isotope geochemistry research. Therefore, developing a method adapted to the characteristics of coal samples and capable of achieving in-situ micro-area determination of lithium isotopes in coal has significant practical significance and application value. Summary of the Invention

[0010] This invention overcomes the shortcomings of existing technologies and proposes a method for determining lithium isotopes in coal using secondary ion mass spectrometry. It achieves accurate and in-situ determination of lithium isotope composition at the micro-area scale of coal samples. By establishing group testing of micro-areas with different host phases in coal, external standard material correction, and IMF correction methods, it reduces isotope fractionation errors caused by different matrices and solves the problem of poor accuracy in lithium isotope testing due to host phase mixing.

[0011] This invention is achieved through the following technical solution:

[0012] A method for determining lithium isotopes in coal using secondary ion mass spectrometry, characterized by comprising the following steps:

[0013] Step S1, Sample preparation and micro-region localization: Prepare a coal sample target that meets the requirements of SIMS testing, and identify and localize micro-regions of different lithium-containing phases in the coal;

[0014] Step S2, SIMS test condition settings: A secondary ion mass spectrometer is used to optimize the detection parameters for targeted sputtering of a micro-area of ​​the coal sample;

[0015] Step S3, alternating measurement of external standard material and sample: The sample is calibrated using external standard material to obtain the results at each point. 7 Li / 6 Li ratio and ensure data consistency;

[0016] Step S4: Instrument mass fractionation calculation, sample calibration, and δ 7 Li conversion: using measured external standard substances and samples 6 Li, 7 The Li signal is used to calculate the instrument mass fractionation factor and correct the isotope ratio.

[0017] Step S5, Data Quality Control and Result Output: Calculate and output δ from the test data. 7 Li.

[0018] Furthermore, the preparation of the coal sample target in step S1 includes the following steps:

[0019] 1) Select coal samples for crushing and grinding, and use a stereomicroscope to select particles with no obvious impurities and intact structure, with a single particle size of less than 1 mm;

[0020] 2) Select NIST SRM series standards. After fixing the crushed coal sample particles and the standards required for subsequent SIMS analysis to the bottom of the epoxy resin mold with double-sided tape, the target is made.

[0021] Furthermore, step S1 involves identifying and locating micro-regions of different lithium-bearing phases in coal, including:

[0022] The sample target was preprocessed and analyzed using the TESCAN Integrated Mineral Analyzer. Data was acquired in DotMapping mode. The current and BSE signal intensities were calibrated using the platinum Faraday cup automatic program, and the EDS signal was calibrated using Mn standard. The organic and inorganic phases were identified using the Panorama, Field, and Grain Viewer modules of TIMA software, and the suitable SIMS analysis region was determined and located, and the micro-region coordinates were marked.

[0023] Furthermore, the detection parameters of the secondary ion mass spectrometer in step S2 include:

[0024] SIMS test conditions were set as follows: an oxygen negative ion source was selected as the primary ion source, and the Köhler illumination mode was used. Secondary ions were extracted at 10 kV through an immersion lens, and then transported to the mass spectrometer via a lens assembly and deflector. Automatic centering of the secondary ion beam was achieved by automatically adjusting the deflector. 7 Li + The signal is used as a reference peak for peak position calibration and signal optimization; 10 22 The Ω Faraday cup detection system alternately measures ions in ion counting mode. 7 Li + and 6 Li + Ion beam intensity.

[0025] Furthermore, the alternating determination of external standard substances and samples in step S3 includes:

[0026] Step S31: Place the pretreated coal sample target into the sample chamber of the SIMS instrument, evacuate to a vacuum, and use a primary ion source to perform point sputtering on the labeled micro-area; perform alternating measurements on the standard sample and the sample, with the test sequence being "standard sample - a set of sample points - standard sample", so that each set of sample points corresponds to the adjacent standard sample data for drift correction and IMF calculation; after completing a preset number of micro-area analyses, perform an instrument quality calibration to reduce the deviation caused by instrument drift;

[0027] Step S32: Repeat the sampling for each micro-region several times, and record the data within each cycle.6 Li + and 7 Li + The cumulative count is used as the signal strength. During the test, data is simultaneously collected. 12 C + and 30 Si + The secondary ion signal is used as a phase discrimination parameter for the test micro-region.

[0028] Furthermore, the lithium isotopic composition is δ 7 Li(‰) represents, and is defined as:

[0029] ;

[0030] Where m represents the measured sample, and the standard sample is L-SVEC purified LiCO3 with an absolute isotopic abundance ratio of [value missing]. 6 Li / 7 Li = 0.08215 ± 0.00023, the equivalent ratio is approximately 7 Li / 6 Li = 12.173. This method uses R... ref =( 7 Li / 6 Li) ref =12.173 as δ 7 Base parameters required for Li conversion;

[0031] The Instrument Mass Fractionation (IMF) factor is defined as follows:

[0032] ;

[0033] Among them, R std,average It is obtained during the testing of each set of standard samples. 7 Li / 6 The average value of Li, δ 7 Li std For the standard sample, the known δ 7 Li value;

[0034] Define the ratio R after sample calculation sam :

[0035] ;

[0036] Among them, R sam, measured It is a sample 7 Li / 6 The measured ratio of Li, R sam It is a correction to the measured ratio of the sample;

[0037] The corrected ratio is converted into the sample's δ. 7Li value:

[0038] .

[0039] Furthermore, step S5, data quality control and result output, includes the following steps:

[0040] Calculate the phase discrimination parameter M of the test micro-region:

[0041] ;

[0042] The test micro-regions were grouped according to the M value: micro-regions with M≥1 were enriched with organic matter, and micro-regions with M<1 were enriched with minerals.

[0043] Furthermore, statistical analysis was performed on micro-region data from different matrix types, and δ values ​​were output independently. 7 Li value; select standards matching organic matter and minerals respectively as primary calibration standards and calculate IMF, and select appropriate standards as monitoring standards to check the stability of the calibration results. δ for each microregion of the sample. 7 Li outputs its corresponding internal precision and records the drift and correction records within each batch of tests.

[0044] The beneficial effects of this invention compared to the prior art are as follows:

[0045] 1. This invention achieves precise in-situ micro-area determination of lithium isotopes in coal, overcoming the limitations of traditional overall testing methods: This invention uses the TESCAN Integrated Mineral Analyzer (TIMA) to accurately identify and locate the host phases of coal samples, and performs SIMS testing within a single host phase (organic phase or inorganic mineral phase), avoiding isotope signal shifts caused by the mixing of different host phases. It can obtain the lithium isotope composition of organic matter and mineral phases in coal separately, clearly reflecting the lithium isotope differences between different components, providing more refined and accurate data support for studying the migration and fractionation mechanisms of lithium in coal, and filling the technical gap in in-situ micro-area determination of lithium isotopes in coal.

[0046] 2. This invention effectively reduces the influence of matrix effects and interference factors, improving test accuracy: This invention uses pre-calibrated NIST SRM series standards as external and monitoring standards. It groups and calibrates according to the phase type of different test micro-regions. The instrument mass fractionation (IMF) factor is calculated by alternating measurements of standards and samples, establishing a calibration method that effectively reduces the influence of differences in secondary ion yield and isotope fractionation under different matrix conditions. Simultaneously, by optimizing the sample preparation process to remove surface contamination and selecting uniform micro-regions without obvious pores and cracks as SIMS test micro-regions, test accuracy is further improved. Experimental verification shows that this method can improve the accuracy of isotope test results while preserving the in-situ spatial location information of the sample and the corresponding phase type.

[0047] 3. This invention improves the stability and repeatability of the test, ensuring data reliability: Targeting the characteristics of coal samples, this invention sets a matching combination of SIMS test parameters, making the secondary ion signal more stable; through an alternating test sequence of "standard sample - a set of sample points - standard sample," drift correction is performed on each set of sample points; instrument calibration is performed after completing a preset number of micro-area analyses, reducing the impact of instrument drift; each micro-area is repeatedly sampled for several cycles, and the cumulative count is used as the signal strength, reducing random errors; simultaneously, it introduces... 12 C + / 30 Si + As a phase discrimination parameter for testing micro-regions, grouping and independent analysis of micro-region data of different matrix types further improves the repeatability and reliability of the data. Experiments show that the internal accuracy of single-point analysis can reach 1-2‰ (2SE), which meets the requirements of in-situ micro-region analysis of lithium isotopes in coal.

[0048] 4. Wide applicability, strong practicality, and good promotional value: This invention does not depend on a specific coal rank or a single coal type, and is applicable to coal samples of different ranks such as lignite, bituminous coal, and anthracite, as well as coal samples with different organic matter content and different mineral compositions. At the same time, the technical concept of this method can be extended to the determination of lithium isotopes in other low-lithium, heterogeneous, and complex materials, providing a new technical path for the application of secondary ion mass spectrometry in related fields, and has broad application prospects.

[0049] 5. Simple and efficient operation, no complicated pretreatment required: The sample preparation process of this invention is relatively simple, without the need for complicated acid digestion, avoiding exogenous contamination and sample damage; the SIMS testing process can be automated, with an analysis time of about 36 minutes for each micro-region, resulting in high testing efficiency, suitable for batch sample testing, and able to meet the testing needs in actual scientific research and production. Detailed Implementation

[0050] To make the technical problem to be solved, the technical solution, and the beneficial effects of the present invention clearer, the present invention will be further described in detail with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. The technical solution of the present invention will be described in detail below with reference to embodiments, but the scope of protection is not limited thereto.

[0051] This embodiment uses typical high-lithium coal from North China as test samples, including No. 8 coal (XJ-8) from Xinjing Coal Mine in Qinshui Coalfield and No. 11 coal (DLT-11) from Donglu Open-pit Coal Mine in Ningwu Coalfield. Both samples were collected from the Taiyuan Formation coal seam. XJ-8 is anthracite with a vitrinite random reflectance of 2.57%–2.59% and a total lithium content of 235–313 μg / g; DLT-11 is bituminous coal with a vitrinite random reflectance of 0.66%–0.74% and a total lithium content of 190–253 μg / g. The method of this invention was used to perform micro-area determination of lithium isotopes in their inorganic mineral phases. The specific steps are as follows:

[0052] Step S1: Sample preparation and micro-area localization

[0053] The core objective of this step is to prepare a coal sample target that meets the requirements of SIMS testing, remove surface contaminants, and accurately identify and locate micro-regions of different lithium-containing phases in the coal, laying the foundation for subsequent SIMS testing. This step is specifically divided into the following two sub-steps:

[0054] Step S11: Coal sample pretreatment and target preparation

[0055] 1. Coal sample selection and crushing: Select fresh, unweathered coal samples without obvious impurities. Manually crush the coal samples to a particle size of less than 1 mm. Then, use a stereomicroscope to select coal sample particles that are free of impurities and have an intact structure to ensure that the internal structure of the particles is intact and to avoid cracks generated during the crushing process that may affect subsequent tests.

[0056] 2. Standard and coal sample fixation: NIST SRM 610 and NIST SRM 612 were selected as standard samples (their known δ values ​​are...). 7 With Li values ​​of 32.5‰ and 31.2‰ respectively, the selected coal sample particles were placed at the bottom of the epoxy resin mold to ensure that the coal sample particles and the standard sample were evenly distributed and to avoid mutual interference during the test. Then, epoxy resin and curing agent were poured into the mold, the mold was gently shaken to remove air bubbles, and the mold was placed at room temperature for 24 hours to cure, thus obtaining the coal sample target blank.

[0057] 3. Target surface polishing: Take out the solidified coal sample target blank and polish the target surface step by step on a polishing machine using a 1μm single crystal diamond suspension. During the polishing process, continuously add Milli-Q ultrapure water to keep the target surface moist and avoid powder generated during polishing adhering to the target surface. Polish until the target surface is flat, smooth, without scratches or depressions. Observe under a microscope to ensure that the internal structure of the coal sample particles is fully exposed and the flatness meets the SIMS test requirements (surface roughness Ra≤0.1μm).

[0058] 4. Cleaning and Drying: Place the polished coal sample target into an ultrasonic cleaner, add a neutral detergent solution, and clean for 30 minutes to remove residual polishing powder and impurities from the target surface; then rinse three times with Milli-Q ultrapure water to thoroughly remove detergent residue; place the cleaned coal sample target on a hot plate to dry, removing residual moisture and surface contamination from the target surface; after drying, place it in a desiccator for later use to prevent the surface from getting damp or contaminated.

[0059] Step S12: Phase recognition and micro-area localization

[0060] 1. TIMA test condition settings: Place the dried coal sample target into the TESCAN Integrated Mineral Analyzer (TIMA) and set it to high vacuum mode (vacuum degree ≤ 1×10⁻⁶). -5 The acceleration voltage was 25 kV, the probe current was 8.87 mA, the beam size was 74.35 nm, and the focal length was 15 mm. Data was acquired in Dot Mapping mode with 2000 X-ray counts per point to ensure the accuracy of data acquisition. The current and BSE (backscattered electron) signal intensity were calibrated using a platinum Faraday cup automatic program, and the EDS (energy dispersive spectroscopy) signal was calibrated using a Mn standard. After calibration, the sample was tested.

[0061] 2. Phase Identification: Using the Panorama, Field, and GrainViewer modules of TIMA software (Version 2.6.0), the acquired X-ray spectra and BSE images were analyzed to identify the organic and inorganic mineral phases in the coal sample. The organic phase appeared dark gray in the BSE image, and the EDS spectrum mainly showed characteristic peaks for C, with no obvious characteristic peaks for other mineral elements (Si, Al, Fe, etc.). The inorganic mineral phase (mainly clay minerals, with kaolinite being the most abundant) appeared grayish-white, and the EDS spectrum showed characteristic peaks for elements such as Si, Al, and O. Through software analysis, the distribution areas of the organic and clay mineral phases were clearly identified, excluding non-target mineral phase areas such as quartz and pyrite.

[0062] 3. Micro-area localization and marking: Within the identified kaolinite region, micro-areas with diameters of 20-50 μm were selected as SIMS analysis areas. The micro-areas were required to have a uniform internal structure, be free of pores and impurities, and avoid cross-phase regions. The coordinates (X and Y axis coordinates) of each micro-area were recorded using TIMA software and marked in the software to ensure that subsequent SIMS testing could accurately locate the target micro-area. In this embodiment, multiple kaolinite micro-areas were selected for testing.

[0063] Step S2: SIMS Test Condition Setting

[0064] This step optimizes the SIMS test parameters based on the characteristics of different host phases in the coal sample to ensure stable secondary ion signals, high signal-to-noise ratio, and reduce interference factors. It is specifically divided into the following two sub-steps:

[0065] Step S21: Setting up the primary ion source and secondary ion transport conditions

[0066] 1. Ion source selection and parameter settings: Select an oxygen negative ion source (O2O3). - As a primary ion source, the oxygen ion source has the advantages of enhancing the yield of secondary lithium ions and reducing isotope fractionation, making it suitable for testing low lithium content in coal samples. The accelerating voltage is set to 13kV and the primary ion beam intensity is 20nA to ensure that the primary ion beam has sufficient energy for target surface ablation, while avoiding excessive beam intensity that could lead to decomposition of organic matter in the coal sample and damage to the target surface.

[0067] 2. Beam spot and illumination mode settings: Use the Köhler illumination mode with a 200μm aperture and adjust the beam spot shape to an elliptical beam spot of 20μm×30μm. The beam spot size matches the micro-area size located in step S12 to ensure that the primary ion beam can completely cover the target micro-area and avoid interference from adjacent areas due to an excessively large beam spot or insufficient signal strength due to an excessively small beam spot.

[0068] 3. Secondary ion transport setup: Secondary ions are extracted at 10 kV through immersion lenses and transported to the mass spectrometer via lens assemblies (LT1, LT3) and deflectors (DTFA, DTCA); the ions are then transported using the instrument's built-in automatic centering program. 7 Li + Using the signal as a reference peak, the deflector parameters are adjusted to achieve automatic alignment of the secondary ion beam, ensuring that the secondary ions can be accurately transmitted to the detection system and reducing signal loss.

[0069] Step S22: Setting up test conditions for quality analysis instruments

[0070] 1. Slit and Aperture Parameter Settings: Set the energy slit width to 50 eV to filter unstable secondary ions and improve signal stability; set the inlet slit width to 400 μm, the contrast aperture diameter to 400 μm, the field aperture to 5 × 5 mm, and the outlet slit to 900 μm. By adjusting these parameters, ensure the mass spectrometer's mass resolution is not less than 1300 (10% peak height) to guarantee stable Li₂ emission. + Secondary ion peak shape and mass window, and reduction of adjacent mass peaks and energy-instantaneous ion pairs. 6 Li + and 7 Li + The impact of signal measurement.

[0071] 2. Detection mode setting: Use 10 22 The Ω Faraday cup detection system alternately measures ions in ion counting mode. 7 Li + and 6 Li + Ion beam intensity ensures rapid and accurate acquisition of the signal intensity of both types of ions.

[0072] Step S3: Alternate determination of external standard material and sample.

[0073] This step involves alternating measurements of standard and sample samples to obtain the lithium isotope ratio of the standard samples. This provides a basis for subsequent instrument mass fractionation (IMF) calculations and sample calibration, while also reducing the impact of instrument drift. Specifically, it consists of the following two sub-steps:

[0074] Step S31: Test sequence and instrument calibration

[0075] 1. Sample target loading and vacuum treatment: Place the coal sample target prepared in step S1 into the sample chamber of the SIMS instrument, close the sample chamber, and evacuate to a vacuum level of 1×10⁻⁶. -7 To avoid interference from air molecules on the secondary ion signal, the vacuum treatment time should be no less than 30 minutes.

[0076] 2. Alternating test sequence: The alternating test sequence of "standard sample - a set of sample points - standard sample" is adopted, that is, first test the NISTSRM 610 standard sample, then test a set of coal sample micro-areas, and then test the NIST SRM 610 standard sample again, so that each set of sample points can obtain the data of the adjacent standard samples before and after, which are used for subsequent drift correction and IMF calculation; in this embodiment, the test sequence is: NISTSRM 610 → a set of kaolinite micro-areas → NIST SRM 610, and the test of all micro-areas is completed in sequence.

[0077] 3. Instrument Calibration: After each set of micro-area analyses is completed, an instrument calibration is performed. The calibration includes primary ion beam intensity, secondary ion transport efficiency, mass resolution, etc. During the calibration process, NIST SRM 612 is used as a calibration standard to ensure that the instrument is in a stable working state and to reduce test deviations caused by instrument drift.

[0078] Step S32: Signal Acquisition and Recording

[0079] 1. Micro-area cyclic acquisition: Each micro-area is repeatedly acquired for 20 cycles, with a total analysis time of approximately 36 minutes. Each micro-area test includes three stages: pre-sputtering (approximately 120 seconds), automatic alignment (approximately 120 seconds), and signal integration time over multiple cycles (approximately 1920 seconds). Pre-sputtering aims to remove the contamination layer (approximately 10 nm thick) from the micro-area surface, ensuring that the analysis focuses on the lithium isotope composition within the coal sample. Automatic alignment corrects for secondary ion beam offset, ensuring accurate signal acquisition. Signal integration time is used to accumulate signal data. 6 Li + and 7 Li + Ion counting improves signal strength and detection accuracy.

[0080] 2. Signal recording: Record the signal within each loop. 6 Li + and 7 Li + The cumulative count is used as the signal strength for that cycle, and the average value of 20 cycles is taken as the final signal strength of the micro-region; simultaneously, during the test, data is collected. 12 C + and 30 Si + The secondary ion signals are recorded and their cumulative counts are used to calculate the phase discrimination parameters of the micro-regions in subsequent tests. In addition, the test time, instrument parameters, standard sample test data and other information of each micro-region are recorded to facilitate subsequent data traceability and quality control.

[0081] Step S4: Instrument Mass Fractionation (IMF) Calculation, Sample Calibration, and δ 7 Li conversion

[0082] This step calculates the IMF factor using standard sample data, corrects the measured lithium isotope ratios of the samples, and finally converts them into the sample's δ¹⁸O value. 7 The Li value ensures the accuracy of the test results, and is specifically divided into the following three sub-steps:

[0083] Step S41: δ 7 Li definition and reference ratio parameter determination

[0084] Lithium isotope composition is δ 7Li(‰) is represented by the following formula:

[0085] ;

[0086] Where m represents the measured sample, and the standard sample is L-SVEC purified LiCO3 with an absolute isotopic abundance ratio of [value missing]. 6 Li / 7 Li = 0.08215 ± 0.00023, the equivalent ratio is approximately 7 Li / 6 Li = 12.173. This method uses R... ref =( 7 Li / 6 Li) ref =12.173 as δ 7 The required baseline parameters for Li conversion; ensuring the consistency and comparability of conversion results.

[0087] Step S42: IMF Calculation and Sample Ratio Correction

[0088] 1. Instrument Mass Fractionation (IMF) Factor Calculation: The IMF factor is used to correct for isotope fractionation errors caused by the instrument itself. Its calculation formula is as follows:

[0089]

[0090] Among them, R std,average It is obtained during the testing of each set of standard samples. 7 Li / 6 The average value of Li, δ 7 Li std For the standard sample, the known δ 7 Li value (where δ) 7 Li std, 610 =32.5‰, δ 7 Li std, 612 =31.2‰). For example, if the standard samples before and after a certain group of samples are NIST SRM 610, its The standard sample obtained from the test of this group of test micro-areas 7 Li / 6 The average Li value is used to calculate the IMF factor for that sample point using the formula.

[0091] 2. Sample ratio correction: Defines the calculated ratio of the samples. This is used to correct the measured ratio of the test micro-region, and its calculation formula is as follows:

[0092]

[0093] Among them, R sam, measured It is a test micro-area7 Li / 6 The measured ratio of Li, R sam It is a correction to the measured ratio of the test micro-region. Substituting into the formula, the corrected sample ratio is obtained. .

[0094] Step S43: δ 7 Li conversion

[0095] Corrected sample ratio Substitute into the following formula to convert to the δ of the sample. 7 Li value:

[0096]

[0097] in, The final lithium isotope composition (unit: ‰) for each microregion of the sample is calculated separately to obtain its corresponding δ. 7 Li value.

[0098] Step S5: Data Quality Control and Result Output

[0099] This step verifies the reliability of the test data through a series of quality control measures, groups and analyzes micro-area data of different matrix types, and finally outputs accurate and reliable test results. It consists of the following three sub-steps:

[0100] Step S51: Matrix identification and grouping

[0101] Calculate the phase discrimination parameter M for each test micro-region to distinguish the matrix type (organic phase or mineral phase) of the micro-region. The calculation formula is as follows:

[0102]

[0103] in, 12 C + and 30 Si + These are the samples collected in step S32. 12 C + and 30 Si + The secondary ion cumulative count was performed; the test micro-regions were grouped according to the M value: when M≥1, the micro-region was enriched with organic matter and belonged to the organic phase micro-region; when M<1, the micro-region was enriched with minerals (mainly clay minerals) and belonged to the mineral phase micro-region; in this embodiment, the M value of most kaolinite micro-regions was between 0.0001 and 0.05, and the grouping results were consistent with the occurrence identified in step S12.

[0104] Step S52: Data quality control verification

[0105] 1. Standard Sample Quality Control: A standard sample matching the mineral (NIST SRM 610) was selected as the primary calibration standard, and its IMF factor was calculated. Simultaneously, NIST SRM 612 was selected as the monitoring standard, and the measured δ values ​​of the monitoring standard were used for quality control. 7 The Li value is compared with known values ​​to verify the stability of the correction results; in this embodiment, the measured δ of the monitoring standard NIST SRM 612 is used. 7 The Li value is 31.0‰-31.4‰, and the deviation from the known value of 31.2‰ is no more than 0.2‰, indicating that the correction result is stable and reliable.

[0106] 2. Repeatability control: Calculate the standard error (2SE) of 20 cycles of test data for each micro-region. The internal precision (2SE) of single-point analysis is required to be 1-2‰. If the 2SE of a micro-region is greater than 2‰, it is considered invalid data and the micro-region needs to be retested. In this embodiment, the 2SE of all micro-regions is between 0.2‰ and 2‰, which meets the repeatability requirements.

[0107] 3. Drift quality control: Record the instrument drift in each batch of tests, calculate the drift amount of the standard sample test data, and require the drift amount to be ≤0.5‰ / h. If the drift amount exceeds the threshold, the instrument needs to be recalibrated and the test needs to be repeated. In this embodiment, the instrument drift amount is 0.3‰ / h, which meets the quality control requirements.

[0108] Step S53: Output the result

[0109] Statistical analysis was performed on the micro-area data that passed quality control, and the δ values ​​of the mineral phase micro-areas were output. 7 Li value: δ of kaolinite micro-regions in XJ-8 7 The Li values ​​range from -10.9‰ to 2.4‰, and the δ¹⁸O values ​​of the kaolinite microregions in DLT-11 are... 7 The Li value ranges from -4.9‰ to 13.3‰; simultaneously, the δ value of each microregion is output. 7 The Li value and its corresponding internal precision (represented by 2SE, where 2SE ≤ 2‰), along with drift records, calibration records, standard test data, and other information within each batch of tests, form a complete test report for subsequent research.

[0110] Supplementary explanation: This embodiment only uses the mineral phases of No. 8 coal from Xinjing Coal Mine in Qinshui Basin, a typical lithium-rich coalfield in North China, and No. 11 coal from Donglu Open-pit Coal Mine in Ningwu Coalfield as examples to illustrate the specific implementation of the present invention. For other coal ranks and coal samples from other regions, it is only necessary to adjust the sample preparation parameters (such as polishing precision and cleaning time) and SIMS test parameters (such as beam spot size and ion beam intensity) appropriately according to the specific characteristics of the coal sample (such as organic matter content, mineral composition, and lithium content) to achieve accurate measurement, all of which fall within the protection scope of the present invention.

[0111] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for determining lithium isotopes in coal using secondary ion mass spectrometry, characterized in that, Includes the following steps: Step S1, Sample preparation and micro-region localization: Prepare a coal sample target that meets the requirements of SIMS testing, and identify and localize micro-regions of different lithium-containing phases in the coal; Step S2, SIMS test condition settings: A secondary ion mass spectrometer is used to optimize the detection parameters for targeted sputtering of a micro-area of ​​the coal sample; Step S3, alternating measurement of external standard material and sample: The sample is calibrated using external standard material to obtain the results at each point. 7 Li / 6 Li ratio and ensure data consistency; Step S4: Instrument mass fractionation calculation, sample calibration, and δ 7 Li conversion: using measured external standard substances and samples 6 Li, 7 The Li signal is used to calculate the instrument mass fractionation factor and correct the isotope ratio. Step S5, Data Quality Control and Result Output: Calculate and output δ from the test data. 7 Li.

2. The method for determining lithium isotopes in coal using secondary ion mass spectrometry according to claim 1, characterized in that, The preparation of the coal sample target in step S1 includes the following steps: 1) Select coal samples for crushing and grinding, and use a stereomicroscope to select particles that are free of impurities and have a complete structure, with a single particle size of less than 1 mm; 2) Select NIST SRM series standards. After fixing the crushed coal sample particles and the standards required for subsequent SIMS analysis to the bottom of the epoxy resin mold with double-sided tape, the target is made.

3. The method for determining lithium isotopes in coal using secondary ion mass spectrometry according to claim 2, characterized in that, Step S1 involves identifying and locating micro-regions of different lithium-bearing phases in coal, including: The sample target was preprocessed and analyzed using the TESCAN Integrated Mineral Analyzer. Data was acquired in DotMapping mode. The current and BSE signal intensities were calibrated using the platinum Faraday cup automatic program, and the EDS signal was calibrated using Mn standard. The organic and inorganic phases were identified using the Panorama, Field, and Grain Viewer modules of TIMA software, and the suitable SIMS analysis region was determined and located, and the micro-region coordinates were marked.

4. The method for determining lithium isotopes in coal using secondary ion mass spectrometry according to claim 1, characterized in that, The detection parameters of the secondary ion mass spectrometer in step S2 include: SIMS test conditions were set as follows: an oxygen negative ion source was selected as the primary ion source, and the Köhler illumination mode was used. Secondary ions were extracted at 10 kV through an immersion lens, and then transported to the mass spectrometer via a lens assembly and deflector. Automatic centering of the secondary ion beam was achieved by automatically adjusting the deflector. 7 Li + The signal is used as a reference peak for peak position calibration and signal optimization; 10 22 The Ω Faraday cup detection system alternately measures ions in ion counting mode. 7 Li + and 6 Li + Ion beam intensity.

5. The method for determining lithium isotopes in coal using secondary ion mass spectrometry according to claim 1, characterized in that, Step S3, which involves alternating measurements of external standard substances and samples, includes: Step S31: Place the pretreated coal sample target into the sample chamber of the SIMS instrument, evacuate to a vacuum, and then perform point sputtering on the labeled micro-area using a primary ion source; perform alternating measurements on the standard sample and the sample, with the test sequence being "standard sample - a set of sample points - standard sample", so that each set of sample points corresponds to the adjacent standard sample data for drift correction and IMF calculation; after completing a preset number of micro-area analyses, perform an instrument quality calibration to reduce the deviation caused by instrument drift; Step S32: Repeat the sampling for each micro-region several times, and record the data within each cycle. 6 Li + and 7 Li + The cumulative count is used as the signal strength. During the test, data is simultaneously collected. 12 C + and 30 Si + The secondary ion signal is used as a phase discrimination parameter for the test micro-region.

6. The method for determining lithium isotopes in coal using secondary ion mass spectrometry according to claim 1, characterized in that, Lithium isotope composition is δ 7 Li(‰) represents, and is defined as: ； Where m represents the measured sample, and the standard sample is L-SVEC purified LiCO3 with an absolute isotopic abundance ratio of [value missing]. 6 Li / 7 Li = 0.08215 ± 0.00023, the equivalent ratio is approximately 7 Li / 6 Li = 12.

173. This method uses R... ref =( 7 Li / 6 Li) ref =12.173 as δ 7 Base parameters required for Li conversion; The Instrument Mass Fractionation (IMF) factor is defined as follows: ； Among them, R std,average It is obtained during the testing of each set of standard samples. 7 Li / 6 The average value of Li, δ 7 Li std For the standard sample, the known δ 7 Li value; Define the ratio R after sample calculation sam : ； Among them, R sam, measured It is a sample 7 Li / 6 The measured ratio of Li, R sam It is a correction to the measured ratio of the sample; The corrected ratio is converted into the sample's δ. 7 Li value: 。 7. The method for determining lithium isotopes in coal using secondary ion mass spectrometry according to claim 1, characterized in that, Step S5, data quality control and result output, includes the following steps: Calculate the discrimination parameter M of the phase in the test micro-region: ； The test micro-regions were grouped according to the M value: micro-regions with M≥1 were enriched with organic matter, and micro-regions with M<1 were enriched with minerals.

8. The method for determining lithium isotopes in coal using secondary ion mass spectrometry according to claim 7, characterized in that, Statistical analysis was performed on micro-area data from different matrix types, and δ values ​​were output independently. 7 Li value; select standards matching organic matter and minerals respectively as primary calibration standards and calculate IMF, and select appropriate standards as monitoring standards to check the stability of the calibration results. δ for each microregion of the sample. 7 Li outputs its corresponding internal precision and records the drift and correction records within each batch of tests.

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