A high-precision analysis method for lithium and its isotopes in complex environmental media

Abstract

This invention provides a high-precision analytical method for lithium and its isotopes in complex environmental media. The method includes: preparing test solutions using differentiated pretreatment processes for different types of environmental samples; analyzing the lithium and other metal element content in the obtained solutions using inductively coupled plasma mass spectrometry (ICP-MS), employing a conventional analysis mode for lithium analysis and using rhodium as an online internal standard for calibration; performing lithium isotope separation and purification on the obtained solutions, including a cation exchange chromatography step, determining the optimal lithium collection range through elution curves, and obtaining purified lithium samples; and using a multi-receiver ICP-MS combined with a standard-sample cross-validation method to perform high-precision determination of the lithium isotope ratios in the purified lithium samples. This invention establishes a systematic analytical framework covering various types of environmental media such as water, soil, sediments, plants, and hair, featuring high sensitivity, good precision, and strong matrix tolerance, and can be widely applied in environmental science, geochemistry, and pollution source tracing.

Description

Technical Field

[0001] This invention belongs to the field of analytical chemistry technology, specifically relating to a high-precision analytical method for lithium and its isotopes in complex environmental media. Background Technology

[0002] Trace analysis of lithium and lithium isotope fingerprinting are core tools for elucidating the sources, migration pathways, and fate of lithium in the environment. With the explosive growth of the lithium-ion battery industry, the environmental release risks of lithium as a critical strategic resource are becoming increasingly prominent. Accurately determining the lithium content and isotopic composition in various environmental media has become a fundamental prerequisite for conducting lithium pollution characterization studies and health risk assessments. However, environmental sample matrices are complex and diverse, encompassing water, soil, sediments, biological samples, and human tissues, with lithium content ranging from trace levels to high concentrations. Furthermore, the high concentrations of coexisting matrix elements (such as sodium, calcium, and aluminum) pose significant challenges to the sensitivity, accuracy, and interference resistance of analytical methods.

[0003] Existing analytical methods are mostly developed for single sample types, lacking a systematic, end-to-end high-precision analytical technology framework applicable to multiple environmental media. For example, for the digestion of soil and sediment samples, traditional methods often employ open-top hot plate digestion or high-pressure closed digestion vessel digestion. The former consumes large amounts of acid, is time-consuming, and easily introduces contamination, while the latter is cumbersome and requires high sample weighing accuracy. In lithium isotope analysis, how to achieve efficient purification and accurate determination of lithium in samples with low lithium content and high matrix remains a bottleneck restricting further research. Some existing purification methods, such as liquid-liquid extraction based on specific extractants, are often developed only for specific sample types, with complex procedures and poor universality.

[0004] The inventors of this application discovered in their research that existing analytical methods lack a systematic technical solution that can cover various environmental media such as water, soil, sediment, plants, and hair, integrating differentiated sample pretreatment, accurate determination of elemental content, and high-precision isotope analysis. In particular, the purification step before isotope analysis lacks effective quality control standards for recovery rate and residual interfering ions, making it difficult to guarantee the accuracy and reproducibility of the measurement results. Summary of the Invention

[0005] This application provides a high-precision analysis method for lithium and its isotopes in complex environmental media. The purpose is to establish a high-precision analysis method system for lithium element and lithium isotopes covering multiple types of environmental media (river water, sewage, tap water, sediment, soil, plants, hair, etc.), and to solve the problems of lack of systematic and full-process analysis methods in the prior art, as well as the difficulty in purifying lithium isotopes and insufficient measurement accuracy in samples with low lithium content and high matrix.

[0006] To solve the above-mentioned technical problems, the technical solution proposed in this application is as follows:

[0007] This invention provides a high-precision analysis method for lithium and its isotopes in complex environmental media, characterized by comprising the following steps: S1: Different pretreatment processes are used to prepare the test solution for different types of environmental samples. S2: The lithium and other metal elements in the test solution obtained in step S1 were analyzed by inductively coupled plasma mass spectrometry (ICP-MS); the lithium element was analyzed in No gas mode and rhodium (Rh) was used as an online internal standard element for calibration. S3: The test solution obtained in step S1 is subjected to lithium isotope separation and purification, including cation exchange chromatography separation step, and the optimal lithium collection range is determined by the elution curve to obtain the purified lithium sample. S4: The lithium isotope ratio of the lithium sample purified in step S3 was determined with high precision using a multi-receiver inductively coupled plasma mass spectrometer (MC-ICP-MS) combined with the standard-sample cross method.

[0008] Furthermore, the environmental samples to be tested mentioned in step S1 include water samples, soil and sediment samples, and biological samples; the differentiated pretreatment process specifically includes: For water samples, after filtration through a 0.45 μm filter membrane, the solution was acidified with nitric acid to a 2.5% HNO3 matrix. For soil and sediment samples, after drying and grinding, a mixed acid system of anti-aqua regia-hydrofluoric acid, which is composed of nitric acid, hydrochloric acid and hydrofluoric acid in a volume ratio of 3:1:1, is used for digestion in a super microwave digester. The acid is removed at 170℃ until it is nearly dry, and then the volume is adjusted with 2.5% HNO3. For plant samples, after drying and pulverizing, they were digested in a super microwave digester with concentrated nitric acid, and the acid was removed at 170℃ until nearly dry, and then the volume was adjusted with 2.5% HNO3. For hair samples, after alternating ultrasonic cleaning, drying, and pulverization with acetone-water, a mixture of concentrated nitric acid and hydrogen peroxide was used for digestion in a super microwave digester. The acid was removed at 170°C until nearly dry, and then the volume was adjusted with 2.5% HNO3.

[0009] Furthermore, the digestion program of the super microwave digester is a programmed temperature control; for soil and sediment samples, the digestion program includes: heating to 150°C for 10 min and holding for 10 min, then heating to 220°C for 5 min and holding for 20 min; for plant samples, the digestion program includes: heating to 150°C for 10 min and holding for 5 min, then heating to 220°C for 10 min and holding for 20 min; for hair samples, the digestion program includes: heating to 160°C for 10 min and holding for 30 min, then heating to 180°C for 10 min and holding for 30 min; and for soil and sediment samples, after removing acid to near dryness, a step of adding 6 mL of concentrated nitric acid to repeat the acid removal process once is also included to completely remove hydrofluoric acid.

[0010] Furthermore, in step S2, for other metal elements besides lithium that are susceptible to interference from polyatomic ions, the collision cell mode (He mode) of ICP-MS is used for analysis.

[0011] Furthermore, the cation exchange chromatography separation step in step S3 uses AG50W-X8 type cation exchange resin with a mesh size of 200-400 mesh, which is packed into a PFA chromatographic column with a resin bed height of 20 cm. Before packing, the resin is also washed by alternating soaking with 30% nitric acid, 6 mol / L hydrochloric acid and ultrapure water, repeated 10 times to activate the resin.

[0012] Further, the determination of the optimal lithium collection range by elution curve in step S3 includes: before sample loading, the matrix of the test solution is replaced, evaporated to dryness and then reconstituted with 0.5 mol / L hydrochloric acid, after sample loading, elution is performed with 0.5 mol / L hydrochloric acid, the effluent is collected in segments, the concentration of lithium and interfering ions sodium and aluminum in each segment of the effluent is measured, and the volume of the discarded early elution solution and the volume of the lithium-containing fraction to be collected are determined according to the lithium peak position.

[0013] Furthermore, after the lithium isotope separation and purification described in step S3, the recovery rate and residual interfering ions need to be evaluated: if the overall lithium recovery rate is less than 95%, or the sodium / lithium ratio in the sample after column chromatography is greater than 1, or the aluminum concentration is higher than the lithium concentration, the sample needs to be purified by column chromatography again until the requirements are met.

[0014] Furthermore, before performing lithium isotope separation and purification on the test solution in step S3, it is necessary to ensure that the total amount of lithium in the test solution is not less than 100 ng. If it is less than this value, lithium is enriched by heating and concentration. When determining the lithium isotope ratio using the standard-sample cross method in step S4, the lithium concentration in the test sample solution is adjusted to be consistent with that of the standard solution to ensure that the instrument signal intensity is matched.

[0015] Furthermore, it also includes step S5: evaluating the precision and accuracy of the whole process analytical method by analyzing standard substances, calculating spiked recoveries and relative standard deviations (RSD); wherein the spiked recoveries are between 93.07% and 107.58%, and the relative standard deviations of the lithium determination results are between 1.4% and 5.0%.

[0016] Furthermore, in step S4, the LSVEC standard was measured multiple times using MC-ICP-MS combined with the SSB method. The measured lithium isotope ratio ranged from -0.38‰ to 0.13‰, and the relative standard deviation (RSD) ranged from 0.04% to 0.44%.

[0017] Compared with the prior art, the present invention achieves the following beneficial technical effects: This invention establishes a systematic analytical method covering multiple types of environmental media, overcoming the shortcomings of existing technologies that lack a complete process technical system. Through targeted pretreatment optimization, innovative content determination strategies, and rigorous purification quality control, it significantly improves the accuracy of trace lithium analysis and the precision of lithium isotope determination in complex matrix samples, providing reliable technical support for environmental source apportionment and pollution tracing research. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 The figure shows the results of lithium content determination and spike recovery experiments on different samples in the embodiments of the present invention; where (a) is the lithium content and spike recovery rate of different samples; and (b) is the recovery rate of each element of the standard substance.

[0020] Figure 2 This is a rinsing curve obtained during the lithium isotope separation and purification process in an embodiment of the present invention.

[0021] Figure 3 This is a flowchart of a high-precision analysis method for lithium and its isotopes in complex environmental media according to the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. 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.

[0023] This invention provides a high-precision analysis method for lithium and its isotopes covering a variety of complex environmental media. For example... Figure 3 As shown, the method consists of four steps, and each step will be explained in detail below with specific operations.

[0024] S1: Different pretreatment processes are used to prepare the test solution for different types of environmental samples. First, step S1 involves preparing the test solution using differentiated pretreatment procedures for different types of environmental samples. This is the foundation of the entire method, and its purpose is to select the optimal digestion or treatment method based on the physicochemical properties of the sample matrix, transferring the target element lithium and other metal elements into the solution in a soluble form, while minimizing or removing organic matter and some matrix interference, thus laying the foundation for subsequent accurate quantification and high-precision isotope analysis.

[0025] S2: The lithium and other metal elements in the test solution obtained in step S1 were analyzed by inductively coupled plasma mass spectrometry (ICP-MS); the lithium element was analyzed in No gas mode and rhodium (Rh) was used as an online internal standard element for calibration. Secondly, step S2 involves analyzing the lithium and other metal element content of the test solution obtained in step S1 using inductively coupled plasma mass spectrometry (ICP-MS). In this step, the invention makes a crucial optimization: the conventional analysis mode of ICP-MS, namely No gas mode, is used for lithium analysis. This is because lithium has a low mass number (6 and 7), eliminating mass spectrometric interference from isotopes or polyatomic ions. Furthermore, No gas mode offers the highest ion transport efficiency, achieving the lowest detection limit, which is essential for trace analysis of low-lithium concentration samples such as those from water and biological samples. Simultaneously, the invention selects rhodium (Rh) as an online internal standard element. Rh has extremely low or undetectable background levels in all the environmental media involved in this invention. Introduced online via a three-way valve, it is mixed with the sample solution and then fed into the ICP-MS. This effectively corrects for physical effects caused by complex matrices (such as viscosity differences and surface tension variations) and signal drift caused by prolonged instrument operation, thereby ensuring the accuracy of the content determination.

[0026] S3: The test solution obtained in step S1 is subjected to lithium isotope separation and purification, including cation exchange chromatography separation step, and the optimal lithium collection range is determined by the elution curve to obtain the purified lithium sample. Next, step S3 involves lithium isotope separation and purification of the test solution obtained in step S1. The accuracy of isotope ratio determination is highly dependent on the purity and recovery rate of lithium in the test solution. Therefore, this step aims to efficiently separate lithium from the matrix interfering elements (especially sodium, aluminum, calcium, magnesium, etc.) present in large quantities in the sample. This invention employs cation exchange chromatography separation technology and determines the optimal lithium collection range through a key sub-step—elution curve. This means that for each batch of newly packaged resin, a elution experiment needs to be performed beforehand with a standard solution or representative sample, collecting the effluent in segments, determining the concentrations of lithium and major interfering ions, and plotting the elution curve as shown in the diagram. Figure 2 The elution curves shown allow for precise determination of the volume of the initial elution solution to be discarded and the volume of the lithium-containing fraction to be collected, ensuring complete lithium recovery and optimal separation from interfering ions.

[0027] S4: The lithium isotope ratio of the lithium sample purified in step S3 was determined with high precision using a multi-receiver inductively coupled plasma mass spectrometer (MC-ICP-MS) combined with the standard-sample cross-method. Finally, step S4 involves using a multi-receiver inductively coupled plasma mass spectrometer (MC-ICP-MS) combined with a standard-sample cross-validation method to perform high-precision determination of the lithium isotope ratio in the purified lithium sample from step S3. MC-ICP-MS is the preferred instrument for high-precision isotope ratio determination. To correct for the instrument's own mass fractionation effect, this invention employs a standard-sample cross-validation method. This method involves measuring a lithium isotope standard solution (e.g., IRMM-016) before and after measuring a sample, and using the average of the two standard measurements as a baseline to calculate the lithium isotope ratio (δ) of the intermediate sample. 7 Li) value. By using this alternating measurement method, the influence of instrument fluctuations over time on the measurement results can be eliminated to the greatest extent, thereby achieving high-precision isotope ratio analysis.

[0028] In one embodiment of this application, the present invention employs different pretreatment methods for different types of environmental samples to achieve optimal digestion and sample recovery. The pretreatment procedures for four sample types are detailed below with specific operational examples.

[0029] For water samples (including river water, sewage, and tap water), the pretreatment is relatively simple, primarily aimed at removing suspended particulate matter. Immediately after sample collection, the sample is filtered through a 0.45 μm membrane, and the filtrate is collected. Then, an equal volume of 5% dilute nitric acid is added to the filtrate, acidifying it to a matrix of 2.5% HNO3. Acidification stabilizes the metal ions in the solution, preventing hydrolysis or adsorption by the container walls. The acidified sample is then stored at 4°C until analysis. For water samples with extremely low lithium content that require subsequent isotope analysis, concentration is necessary before purification, as detailed later.

[0030] For soil and sediment samples (such as sediment and soil), these samples have complex matrices and contain a large amount of silicate minerals. Therefore, a mixed acid system containing hydrofluoric acid is necessary to completely disrupt the silicon lattice and release lithium completely. This invention uses an anti-aqua regia-hydrofluoric acid mixed acid system composed of nitric acid, hydrochloric acid, and hydrofluoric acid in a volume ratio of 3:1:1. The specific operation is as follows: The collected sediment or soil sample is dried to constant weight using a vacuum freeze dryer, ground using a pulverizer or agate mortar, and passed through a 200-mesh sieve. 0.1 g (accurate to 0.0001 g) of the pretreated sample is accurately weighed into a Teflon digestion tube, and 4 mL of the above mixed acid system is carefully added. The digestion tube is placed in a super microwave digester and the preset digestion program is run. The specific parameters of this program will be detailed later. After digestion, the temperature inside the vessel is allowed to drop to room temperature. The digestion tube is then removed and placed in a 170°C acid removal apparatus. The lid is opened and the acid is removed until nearly dry (approximately 0.1-0.2 mL remains). The purpose of this step is to remove a large amount of acid, preparing for subsequent volume adjustment and instrumentation. Finally, bring the volume to 10 mL with 2.5% HNO3, shake well, and it is ready for analysis.

[0031] For plant samples (such as shrub branches and leaves), the main matrix is ​​organic matter, and the goal of digestion is to completely oxidize the organic matter. This invention uses concentrated nitric acid for digestion. The specific operation is as follows: The collected plant sample is freeze-dried under vacuum and then pulverized. 0.1 g of the sample is accurately weighed into a Teflon digestion tube, and 3 mL of concentrated nitric acid is added. The digestion tube is placed in a super microwave digester and the preset digestion program is run; the specific parameters of this program will be detailed later. After digestion, the digestion tube is removed and subjected to acid removal at 170℃ until nearly dry. Finally, 2.5% HNO3 is used to bring the volume to 10 mL for analysis.

[0032] For hair samples, in addition to containing organic matter such as keratin, the surface may also be contaminated with exogenous pollutants such as grease and dust. Therefore, a strict cleaning step is added to the pretreatment process before digestion. The specific operation is as follows: the hair is cut into small sections of about 1 cm, and ultrasonically cleaned sequentially with acetone, ultrapure water, and acetone, 30 min each time, repeated 3 times, to thoroughly remove surface grease and exogenous contaminants. After cleaning, the hair is vacuum freeze-dried, then cut into small pieces with clean scissors and pulverized with a grinder. 0.1 g of the pretreated hair sample is accurately weighed into a digestion tube, and 3 mL of concentrated nitric acid and 2 drops of 30% H2O2 are added. The addition of H2O2 helps to more thoroughly oxidize organic matter and improve digestion efficiency. The digestion tube is placed in a super microwave digester and the preset digestion program is run. The specific parameters of this program will be detailed later. The subsequent acid removal and volume adjustment steps are the same as for plant samples, and the final volume is adjusted to 10 mL for analysis.

[0033] In the embodiments of this application, the present invention optimizes the parameters of programmed temperature rise for different types of samples to ensure complete digestion and safe and controllable operation.

[0034] For soil and sediment samples, the digestion procedure is as follows: heat from room temperature to 150°C over 10 minutes and maintain at 150°C for 10 minutes; then heat from 150°C to 220°C over 5 minutes and maintain at 220°C for 20 minutes.

[0035] For plant samples, the digestion procedure is as follows: heat to 150°C over 10 minutes and hold for 5 minutes, then heat to 220°C over 10 minutes and hold for 20 minutes.

[0036] For hair samples, the digestion procedure was as follows: the temperature was increased to 160°C over 10 minutes and held for 30 minutes, then increased to 180°C over 10 minutes and held for 30 minutes.

[0037] These procedures, through stepwise heating and holding, ensure that the sample and acid react fully under high temperature and pressure, while avoiding the risk of a sudden pressure rise due to excessively rapid heating.

[0038] For soil and sediment samples, the use of hydrofluoric acid in pretreatment can cause severe corrosion to the ICP-MS injection system (especially the quartz rectangular tube and sampling cone). Therefore, this invention adds a step of "adding 6 mL of concentrated nitric acid to repeat the acid removal process once" after the initial acid removal to near dryness. The purpose of this step is to use the high-boiling-point nitric acid to completely remove the low-boiling-point hydrofluoric acid, thereby protecting the instrument and ensuring the stability of long-term analysis. After the repeated acid removal, the volume is adjusted to 2.5% HNO3.

[0039] In the embodiments of this application, in actual environmental sample analysis, in addition to focusing on lithium, it is often necessary to simultaneously determine multiple other metallic elements (such as vanadium (V), chromium (Cr), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), etc.) for comprehensive pollution assessment. These elements are easily affected by polyatomic ions (such as ArCl) in ICP-MS analysis. + CaO + Interference from elements such as helium (He) can occur. Therefore, in step S2 of this invention, when analyzing these susceptible metal elements, the system switches to the collision cell mode (KED mode, using helium as the collision gas) of ICP-MS. Helium molecules collide with polyatomic ions, reducing their kinetic energy and preventing them from entering the mass analyzer through energy discrimination. The atomic ions of the target element are less affected, thereby achieving the purpose of eliminating interference and accurate quantification.

[0040] The resin used in the lithium isotope separation and purification step of this invention is AG50W-X8 type strong acid cation exchange resin with a mesh size of 200-400 mesh. This resin has the characteristics of high exchange capacity, uniform particle size, and good separation efficiency. The resin is packed into a PFA (perfluoroalkoxy) chromatographic column. PFA has excellent chemical inertness, can withstand strong acids, and has a low background, so it will not introduce contamination. Care must be taken during packing to avoid generating air bubbles, and the resin bed should be allowed to settle naturally until it stabilizes at a height of 20 cm.

[0041] Before initial use or reuse after a long period of disuse, the resin must be activated to remove any adsorbed impurities. The activation steps of this invention are as follows: the resin is alternately immersed in 30% nitric acid, 6 mol / L hydrochloric acid, and ultrapure water, with each immersion lasting a specific time, repeated 10 times. This alternating immersion in strong acids effectively removes impurities remaining from the resin synthesis process and metal ions adsorbed during use, restoring the resin's exchange capacity. The activated resin is then finally immersed in ultrapure water for later use.

[0042] In one embodiment of this application, the core steps of determining the optimal lithium collection range through elution curves to ensure efficient lithium purification are disclosed.

[0043] To ensure that the sample solution and eluent matrix are consistent and to avoid affecting the separation effect due to different acid media, matrix replacement is required before column chromatography. The specific procedure is as follows: evaporate the sample solution to be separated and purified to dryness at 170℃, and then add an appropriate amount of 0.5 mol / L hydrochloric acid to redissolve it, so that the sample to be tested is in the same acid system as the eluent.

[0044] After matrix replacement, the elution curve is plotted. First, equilibrate the packed resin column with 20 mL of 0.5 mol / L hydrochloric acid. After equilibration, carefully add 0.5 mL of the matrix-replaced sample solution (or the standard solution for testing) to the column. Once the sample solution has completely penetrated the resin bed, wash the tube walls with a small amount of 0.5 mol / L hydrochloric acid. Then, begin elution with 0.5 mol / L hydrochloric acid, continuously collecting 1 mL aliquots in centrifuge tubes, for a total of 35 or more aliquots, until lithium is completely eluted. Use ICP-MS to determine the concentration of lithium and major interfering ions (such as sodium (Na) and aluminum (Al) in each aliquot. Plot the elution curve with elution volume (mL) on the x-axis and elemental concentration (μg / L or cps) on the y-axis. Figure 2 As shown in the figure, the peak position and width of lithium, as well as its separation from interfering peaks such as sodium and aluminum, can be clearly seen through this curve. Based on this, the volume of the initial eluent (i.e., all eluent from the beginning to the appearance of the lithium peak) that needs to be discarded during actual sample separation and the volume of the lithium-containing fraction that needs to be collected (i.e., all eluent corresponding to the appearance and end of the lithium peak) can be accurately determined.

[0045] In the actual column separation process, the waste liquid volume and sample intake volume determined according to the elution curve were accurately collected. The specific operation is as follows: After activating the resin, 0.5 mL of sample after matrix replacement was loaded onto the column. After the sample had completely entered the resin bed, 0.5 mL and 1 mL of 0.5 mol / L hydrochloric acid were added sequentially for elution until complete entry. Then, according to the determined waste liquid volume, the corresponding volume of 0.5 mol / L hydrochloric acid was added for elution and discarded. After the waste liquid was discarded, 1 mL of 0.5 mol / L hydrochloric acid was added, and this 1 mL eluent was collected in a centrifuge tube and labeled "check1". Subsequently, according to the determined sample intake volume, the corresponding volume of 0.5 mol / L hydrochloric acid was added, and this lithium-containing fraction was collected in a PFA container. After the sample collection was completed, another 1 mL of 0.5 mol / L hydrochloric acid was added, and this 1 mL eluent was collected in a centrifuge tube and labeled "check2". Check1 and check2 were used for subsequent recovery evaluation to monitor whether lithium premature leakage or incomplete elution occurred during the separation process.

[0046] After completing the column separation and collecting the lithium-containing fraction according to the above steps, two core assessments are required to ensure the purification effect.

[0047] First, recovery rate assessment. The collected lithium-containing fraction is evaporated to dryness, reconstituted, and a small amount is diluted. The lithium concentration is determined by ICP-MS and compared with the lithium concentration of the sample taken before column chromatography to calculate the overall recovery rate. This invention requires an overall lithium recovery rate of no less than 95%. If the recovery rate is less than 95%, the samples collected during column chromatography (check1 and check2) must be analyzed to check for lithium content. If lithium is detected in check1, it indicates insufficient waste liquid volume or improper flow rate control, leading to lithium leakage before expected collection. If lithium is detected in check2, it indicates insufficient sample volume, resulting in some lithium not being collected. In either case, the detection of lithium in check1 or check2 indicates a failure of the separation process, and the sample must be purified again using column chromatography.

[0048] Second, assessment of residual interfering ions. Similarly, the concentrations of sodium and aluminum in the post-column sample are determined using ICP-MS. This invention requires that the sodium / lithium ratio and aluminum / lithium ratio in the post-column sample not exceed 1. Sodium and aluminum are the most abundant matrix elements in environmental samples. If they enter MC-ICP-MS along with lithium, they will cause severe matrix effects, leading to distortion of isotope ratio measurements. If the sodium / lithium ratio or aluminum / lithium ratio is greater than 1, it indicates that a single column pass has not effectively removed interference. In this case, the sample solution needs to be evaporated to dryness again, and the entire column pass procedure (starting from reconstitution and loading) needs to be repeated until the above requirements are met. For samples with extremely high initial sodium / lithium ratios, a second column pass usually achieves an acceptable level.

[0049] Furthermore, this invention specifies two key prerequisites for analysis.

[0050] First, the total lithium content requirement before column loading. Sufficient signal intensity is required for isotope ratio determination. This invention stipulates that before cation exchange chromatography separation of the sample solution, the total lithium content in the solution must be no less than 100 ng, and a maximum of 200 ng is recommended. For water samples with extremely low lithium content (such as clean river water and tap water), due to their simple matrix, digestion is not required. They can be directly concentrated by heating after filtration through a 0.45 μm filter membrane as described in step S1. Specifically, an appropriate amount of water sample is placed in a PFA crucible and evaporated at 180°C on a hot plate. During this process, a small amount of concentrated nitric acid and hydrogen peroxide can be added to remove any possible trace organic matter until the lithium concentration in the solution reaches a total lithium content of no less than 100 ng per 0.5 mL (i.e., the total volume of the sample loaded for subsequent cation exchange chromatography separation). The concentrated sample can then be used for subsequent matrix replacement and column separation.

[0051] Second, concentration matching requirements before instrumentation. When using the standard-sample cross-reference method for MC-ICP-MS determination, the purified sample solution must first undergo matrix replacement: the sample is evaporated to dryness, then reconstituted with 2.5% nitric acid (the same as the standard solution), ensuring the sample and standard are in the same acidic medium. Subsequently, the lithium concentration of the purified sample is determined by ICP-MS, and based on the measurement results, it is diluted with 2.5% nitric acid to a concentration matching that of the IRMM-016 standard solution (typically 50-100 ng / mL) to ensure instrument signal intensity matching. This is because the signal intensity of ICP-MS is directly proportional to the concentration; if the concentration difference is too large, a concentration-dependent mass fractionation effect will be introduced, affecting the accuracy of the measurement results. Therefore, before formal instrumentation, the lithium concentration of the purified sample is first determined by ICP-MS, and then, based on the measurement results, it is diluted with 2.5% nitric acid to a concentration matching that of the IRMM-016 standard solution (typically 50-100 ng / mL).

[0052] This invention verifies the reliability of the method through systematic evaluation metrics. During method development, to comprehensively evaluate the precision and accuracy of the entire analytical process, three parallel samples were set up for independent analysis of each type of environmental sample. The relative standard deviation (RSD) was calculated to characterize the method's precision. The results show that the RSD for lithium in multi-media samples is 1.4%–5.0%, indicating good precision.

[0053] To assess the accuracy of the method, spiked recovery experiments and analyses using certified reference materials were performed. The results of the spiked recovery experiments showed that, as... Figure 1 As shown in (a), the overall recoveries of the spiked samples ranged from 93.07% to 107.58%, with soil samples at 93.07%, sediment samples at 107.58%, plant samples at 106.30%, and hair samples at 105.21%. Further full-process validation was conducted using soil reference material (GBW07404) and shrub branch and leaf reference material (GBW07603), with the results shown below. Figure 1 As shown in (b), the measured values ​​of each element are close to the standard values, indicating that this method has excellent accuracy for different matrix samples.

[0054] This invention achieves an extremely high level of precision in lithium isotope determination. To evaluate the precision of the isotope determination, pure standard solutions and actual samples were repeatedly measured over a long period. IRMM-016 standard solution was used as the standard sample, and the LSVEC standard was repeatedly tested using the standard-sample cross-validation method (SSB method) (its δ... 7 The standard value for Li is 0‰. The results, as shown in Table 1, are as follows: δ 7The measured Li values ​​ranged from -0.38‰ to 0.13‰, which is highly consistent with the LSVEC recommended values, indicating extremely high accuracy. Furthermore, the relative standard deviation (RSD) of multiple measurements was between 0.04% and 0.44%, demonstrating the instrument's high precision.

[0055] Table 1. Determination results of standard reference materials used for precision evaluation

[0056] In summary, the multi-environmental medium lithium content and lithium isotope analysis method established in this invention has the characteristics of high sensitivity, good precision and strong matrix tolerance, and can provide a reliable methodological basis for subsequent regional lithium pollution characteristic analysis and environmental risk assessment.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, 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 do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for high-precision analysis of lithium and its isotopes in complex environmental media, characterized by, Includes the following steps: S1: Different pretreatment processes are used to prepare the test solution for different types of environmental samples. S2: The lithium and other metal elements in the test solution obtained in step S1 were analyzed by inductively coupled plasma mass spectrometry (ICP-MS); the lithium element was analyzed in No gas mode and rhodium (Rh) was used as an online internal standard element for calibration. S3: The test solution obtained in step S1 is subjected to lithium isotope separation and purification, including cation exchange chromatography separation step, and the optimal lithium collection range is determined by the elution curve to obtain the purified lithium sample. S4: The lithium isotope ratio of the lithium sample purified in step S3 was determined with high precision using a multi-receiver inductively coupled plasma mass spectrometer (MC-ICP-MS) combined with the standard-sample cross-method.

2. The analytical method according to claim 1, characterized in that, The environmental samples to be tested in step S1 include water samples, soil and sediment samples, and biological samples; the differentiated pretreatment process specifically includes: For water samples, after filtration through a 0.45 μm filter membrane, the solution was acidified with nitric acid to a 2.5% HNO3 matrix. For soil and sediment samples, after drying and grinding, a mixed acid system of anti-aqua regia-hydrofluoric acid, which is composed of nitric acid, hydrochloric acid and hydrofluoric acid in a volume ratio of 3:1:1, is used for digestion in a super microwave digester. The acid is removed at 170℃ until it is nearly dry, and then the volume is adjusted with 2.5% HNO3. For plant samples, after drying and pulverizing, they were digested in a super microwave digester with concentrated nitric acid, and the acid was removed at 170℃ until nearly dry, and then the volume was adjusted with 2.5% HNO3. For hair samples, after alternating ultrasonic cleaning, drying, and pulverization with acetone-water, a mixture of concentrated nitric acid and hydrogen peroxide was used for digestion in a super microwave digester. The acid was removed at 170°C until nearly dry, and then the volume was adjusted with 2.5% HNO3.

3. The analytical method according to claim 2, characterized in that, The digestion program of the super microwave digester is a programmed temperature control. For soil and sediment samples, the digestion program includes: heating to 150°C for 10 min and holding for 10 min, then heating to 220°C for 5 min and holding for 20 min. For plant samples, the digestion program includes: heating to 150°C for 10 min and holding for 5 min, then heating to 220°C for 10 min and holding for 20 min. For hair samples, the digestion program includes: heating to 160°C for 10 min and holding for 30 min, then heating to 180°C for 10 min and holding for 30 min. Furthermore, for soil and sediment samples, after removing acid to near dryness, a step of adding 6 mL of concentrated nitric acid to repeat the acid removal process once is included to completely remove hydrofluoric acid.

4. The analytical method according to claim 1, characterized in that, In step S2, for metal elements other than lithium that are susceptible to interference from polyatomic ions, the collision cell mode (He mode) of ICP-MS is used for analysis.

5. The analytical method according to claim 1, characterized in that, The cation exchange chromatography separation step in step S3 uses AG50W-X8 type cation exchange resin with a mesh size of 200-400 mesh, which is packed into a PFA chromatographic column with a resin bed height of 20 cm. Before packing, the resin is also activated by alternately soaking and washing it with 30% nitric acid, 6 mol / L hydrochloric acid and ultrapure water 10 times.

6. The analytical method according to claim 1, characterized in that, The determination of the optimal lithium collection range by elution curve in step S3 includes: before sample loading, the matrix of the test solution is replaced, evaporated to dryness and then reconstituted with 0.5 mol / L hydrochloric acid. After sample loading, the solution is eluted with 0.5 mol / L hydrochloric acid, and the effluent is collected in segments. The concentrations of lithium and interfering ions sodium and aluminum in each segment of the effluent are measured. The volume of the discarded early elution solution and the volume of the lithium-containing fraction to be collected are determined based on the lithium peak position.

7. The analytical method according to claim 1 or 6, characterized in that, After the lithium isotope separation and purification described in step S3, the recovery rate and residual interfering ions need to be evaluated: if the overall lithium recovery rate is less than 95%, or the sodium / lithium ratio in the sample after column chromatography is greater than 1, or the aluminum concentration is higher than the lithium concentration, the sample needs to be purified by column chromatography again until the requirements are met.

8. The analytical method according to claim 1, characterized in that, Before performing lithium isotope separation and purification on the test solution in step S3, it is necessary to ensure that the total amount of lithium in the test solution is not less than 100 ng. If it is less than this value, lithium is enriched by heating and concentration. When determining the lithium isotope ratio using the standard-sample cross method in step S4, the lithium concentration in the test sample solution is adjusted to be consistent with that of the standard solution to ensure that the instrument signal intensity is matched.

9. The analytical method according to claim 1, characterized in that, The method also includes step S5: evaluating the precision and accuracy of the whole process analytical method by analyzing standard substances, calculating spiked recoveries and relative standard deviations (RSD); wherein the spiked recoveries are between 93.07% and 107.58%, and the relative standard deviations of the lithium determination results are between 1.4% and 5.0%.

10. The analytical method according to claim 1, characterized in that, In step S4, the LSVEC standard was measured multiple times using MC-ICP-MS combined with the SSB method. The measured lithium isotope ratio ranged from -0.38‰ to 0.13‰, and the relative standard deviation (RSD) ranged from 0.04% to 0.44%.

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