Method for combined determination of trace elements in black carbonaceous mudstone

Abstract

The application discloses a method for combined determination of trace elements in black carbonaceous mudstone, and belongs to the technical field of quantitative analysis. The method comprises the following steps: taking a black carbonaceous mudstone sample in a platinum crucible, and burning the sample in a muffle furnace at 800 DEG C to 900 DEG C for 25 min to 35 min; after the burning, the sample is moved into a microwave digestion tank, and after being wetted, acid is added, and the sample is cold-digested for 8 h to 12 h and then is subjected to microwave digestion; the product is added with perchloric acid and heated until white smoke is generated; hydrochloric acid is added to dissolve salts; after cooling, the sample is diluted to a constant volume to obtain a test solution; and inductively coupled plasma emission spectrometry is used to determine vanadium and nickel elements. The method can make organic carbon fully escape, avoid abnormal pressure of the digestion tank, and expose wrapped elements; cold digestion and microwave digestion can avoid spattering or incomplete digestion; the method can remove hydrofluoric acid to prevent corrosion and precipitation interference; the method is accurate in constant volume, prevents fluorine ion corrosion, can simultaneously and accurately measure vanadium and nickel, and has high sensitivity and accuracy.

Description

Technical Field

[0001] This invention belongs to the field of quantitative analysis technology, specifically relating to a method for the combined determination of trace elements in black carbonaceous mudstone. Background Technology

[0002] Carbonaceous mudstone, a special sedimentary rock in coal-bearing strata, possesses both fuel utilization value and metal resource extraction potential. The detection of its vanadium, nickel, and other element content is of significant guiding importance for comprehensive resource utilization, geological exploration, and related process optimization. With the development of modern analytical techniques, inductively coupled plasma atomic emission spectrometry (ICP-AES) has become one of the mainstream techniques for metal element detection due to its advantages such as simultaneous multi-element determination, high sensitivity, and fast analysis speed. However, the effectiveness of sample pretreatment directly determines the accuracy and reliability of the final detection results. Existing pretreatment and detection methods for complex matrix samples are difficult to adapt to the characteristics of carbonaceous mudstone. For example, CN 103335881A discloses a sample dissolution method for aluminum-based grinding wheels. After removing organic components by high-temperature calcination and treating inorganic components by microwave digestion, elemental detection is performed using ICP-AES. Although this method simplifies the sample dissolution process for aluminum-based grinding wheels, the matrix composition of aluminum-based grinding wheels differs significantly from that of carbonaceous mudstone. Carbonaceous mudstone contains high levels of organic carbon and complex components such as clay minerals and quartz. Directly applying this method cannot achieve complete removal of carbon and complete digestion of the sample, which can easily lead to significant deviations in subsequent detection results. Traditional methods for detecting metallic elements in carbonaceous mudstone have many shortcomings. For example, the pretreatment process using sodium / calcium salt additives for oxidation roasting is cumbersome, time-consuming, and prone to introducing impurities. Conventional direct digestion methods are prone to generating a large number of bubbles during digestion due to the presence of organic carbon, resulting in sample loss. At the same time, matrix interference can also affect the accuracy of ICP-AES determination, failing to meet the need for rapid and accurate detection of vanadium and nickel in carbonaceous mudstone. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this specification provides one or more embodiments of a combined method for determining trace elements in black carbonaceous mudstone, comprising the following steps: S1, weighing a black carbonaceous mudstone sample and placing it in a platinum dish, then calcining it in a muffle furnace at 800℃~900℃ for 25min~35min to obtain a calcined sample; S2, transferring the calcined sample into a microwave digestion vessel, wetting it with deionized water, then adding hydrochloric acid, nitric acid, hydrofluoric acid, and hydrogen peroxide for cold digestion for 8h~12h, followed by microwave digestion according to a set program to obtain a microwave digestion product; S3, adding perchloric acid to the microwave digestion product, heating it at 160℃~180℃ until white fumes cease, then adding hydrochloric acid to dissolve the salts to obtain a salt solution; S4, cooling the salt solution and transferring it to a plastic volumetric flask for volume adjustment to obtain the test solution; S5, using inductively coupled plasma atomic emission spectrometry to determine the vanadium and nickel elements in the test solution.

[0004] In some embodiments, in step S1, the calcination temperature of the muffle furnace is 850°C and the calcination time is 30 min; the mass of the black carbonaceous mudstone sample is 0.25 g to 0.35 g.

[0005] In some embodiments, in step S2, the platinum dish is first rinsed multiple times with 8 mL to 10 mL of hydrochloric acid, and all the rinsing solution is transferred into a microwave digestion vessel; the amount of nitric acid added is 0.8 mL to 1.2 mL, the amount of hydrofluoric acid added is 2.8 mL to 3.2 mL, and the amount of hydrogen peroxide added is 0.8 mL to 1.2 mL.

[0006] In some embodiments, the microwave digestion procedure in S2 includes the following steps: S21, heating at a power of 1100W~1300W for 8min~12min to 110℃~130℃, and holding for 8min~12min; S22, heating at a power of 1100W~1300W for 4min~6min to 150℃~170℃, and holding for 4min~6min; S23, heating at a power of 1100W~1300W for 4min~6min to 180℃~200℃, and holding for 23min~27min.

[0007] In some embodiments, in step S3, the amount of perchloric acid used is 1.8 mL to 2.2 mL; the hydrochloric acid used to dissolve the salts is a hydrochloric acid solution with a volume ratio of 1:1, and the amount used is 9 mL to 11 mL.

[0008] In some embodiments, in step S4, the volume of the plastic volumetric flask is 90 mL to 110 mL; the solvent used for volume adjustment is deionized water with a resistivity greater than 18.0 MΩ·cm.

[0009] In some embodiments, the operating parameters of the inductively coupled plasma atomic emission spectrometer in step S5 are as follows: sample cleaning time 25s~35s, sample cleaning speed 1.7mL / min~2.0mL / min, sample analysis speed 1.4mL / min~1.6mL / min, integration time 4s~6s, power 1100W~1200W, cooling gas flow rate 13L / min~15L / min, carrier gas pressure 24psi~26psi, and auxiliary gas flow rate 0.8L / min~1.2L / min.

[0010] In some embodiments, before step S5, the method further includes: transferring vanadium single-element standard stock solution and nickel single-element standard stock solution into volumetric flasks, adding hydrochloric acid solution with a volume ratio of 1:1, and making up to volume to obtain a vanadium and nickel mixed standard working solution with a concentration of 90 μg / mL to 110 μg / mL; transferring different volumes of the vanadium and nickel mixed standard working solution into multiple volumetric flasks, adding hydrochloric acid solution with a volume ratio of 1:1, and making up to volume to obtain a series of standard solutions with concentration gradients of 0 μg / mL, 0.4 μg / mL to 0.6 μg / mL, 0.9 μg / mL to 1.1 μg / mL, 2.9 μg / mL to 3.1 μg / mL, 4.9 μg / mL to 5.1 μg / mL, and 9.9 μg / mL to 10.1 μg / mL.

[0011] In some embodiments, in step S5, the analytical spectral line used to determine vanadium is 311.071 nm, and the analytical spectral line used to determine nickel is 341.476 nm.

[0012] Beneficial effects 1. Place the black carbonaceous mudstone sample in a muffle furnace at 800℃~900℃ and burn for 25min~35min to allow the organic carbon to burn and escape completely, avoid abnormal pressure in the digestion vessel, and expose the encapsulated vanadium and nickel elements, thus solving the problem of high carbon content hindering digestion and interfering with the determination. 2. After calcination, the sample is first subjected to cold digestion for 8 to 12 hours and then microwave digestion according to the set program. This can avoid the splashing or incompleteness of direct microwave digestion, quickly and thoroughly decompose the insoluble matrix, release vanadium and nickel elements, and solve the problem of incomplete element release caused by the indigestion of clay minerals. 3. Add perchloric acid to the microwave digestion product and heat at 160℃~180℃ until white fumes are exhausted. Then add hydrochloric acid to dissolve the salts. This can remove residual hydrofluoric acid to avoid corrosion of the equipment and precipitation interference. At the same time, it converts the metal salts into soluble chlorides, avoids element loss, and solves the problems of hydrofluoric acid hazards and incomplete salt dissolution. 4. Cooling the salt solution and then making up the volume with a plastic volumetric flask ensures accurate volume, avoids fluoride ion corrosion of the vessel, and solves the problem of volume adjustment error caused by temperature changes and fluoride ion corrosion. 5. The inductively coupled plasma atomic emission spectrometer is used to determine vanadium and nickel in the test solution. It can simultaneously determine the two elements, with high sensitivity and accuracy, solving the problems of cumbersome operation and difficulty in simultaneous accurate determination of traditional methods. 6. A three-stage gradient heating microwave digestion program is adopted (first stage: heating at 1100W~1300W for 8min~12min to 110℃~130℃ and holding for 8min~12min; second stage: heating at the same power for 4min~6min to 150℃~170℃ and holding for 4min~6min; third stage: heating at the same power for 4min~6min to 180℃~200℃ and holding for 23min~27min), which can avoid boiling, abnormal pressure and inclusion formation, completely destroy the dense aluminosilicate structure, and solve the problems of sample loss, safety risks, insufficient release of target elements and energy waste. Detailed Implementation

[0013] This specification provides one or more embodiments of a method for the combined determination of trace elements in black carbonaceous mudstone, comprising the following steps: weighing a black carbonaceous mudstone sample and placing it in a platinum dish, calcining it in a muffle furnace at 800℃~900℃ for 25min~35min to obtain a calcined sample; transferring the calcined sample into a microwave digestion vessel, wetting it with deionized water, adding hydrochloric acid, nitric acid, hydrofluoric acid and hydrogen peroxide for cold digestion for 8h~12h, and then performing microwave digestion according to a set program to obtain a microwave digestion product; adding perchloric acid to the microwave digestion product, heating it at 160℃~180℃ until white fumes cease, then adding hydrochloric acid to dissolve the salts to obtain a salt solution; cooling the salt solution and transferring it to a plastic volumetric flask for volume adjustment to obtain the test solution; and using inductively coupled plasma atomic emission spectrometry to determine the vanadium and nickel elements in the test solution.

[0014] Black carbonaceous mudstone is rich in organic carbon and clay minerals. Organic carbon can interfere with subsequent acid digestion and instrumental measurements. The sample is first weighed and placed in a platinum dish, and then heated in a muffle furnace at 800℃~900℃ for 25min~35min. This temperature range and duration allow the organic carbon in the sample to be fully combusted and converted into carbon dioxide, thus avoiding the generation of a large amount of gas during the reaction of carbon and acid during subsequent digestion, which could cause abnormal pressure in the digestion vessel. It also destroys the organic structure of the sample, allowing the vanadium and nickel elements encased in the organic matter to be fully exposed, creating conditions for subsequent acid digestion. This solves the core problem of high carbon content hindering sample digestion and interfering with elemental measurements.

[0015] The ignited sample was transferred to a microwave digestion vessel. Wetting with deionized water first helps disperse the sample and prevents clumping, which could lead to uneven digestion. Hydrochloric acid, nitric acid, hydrofluoric acid, and hydrogen peroxide were then added for 8–12 hours of cold digestion. Hydrochloric acid initially dissolves metal oxides in the sample; nitric acid, as a strong oxidant, oxidizes residual organic components and low-valence metals; hydrofluoric acid specifically disrupts the silicon-oxygen bond structure of clay minerals such as kaolinite and hydromica in black carbonaceous mudstone, converting silicon to silicon tetrafluoride to dissolve insoluble aluminosilicate minerals; and hydrogen peroxide further enhances the digestion process. Oxidation effect: The cold digestion process allows the acid to slowly penetrate into the sample, pre-decompose some of the easily soluble components, and avoid the splashing or incomplete digestion caused by violent local reactions during direct microwave digestion. Then, microwave digestion is carried out according to the set program. By utilizing the thermal and non-thermal effects of microwaves, the acid can quickly and thoroughly decompose the sparingly soluble matrix in the sample under high temperature and high pressure, releasing vanadium and nickel elements from the mineral lattice and converting them into soluble ionic forms. This solves the problem that sparingly soluble matrices such as clay minerals are difficult to completely digest by conventional acid methods, resulting in incomplete release of vanadium and nickel elements.

[0016] After adding perchloric acid to the microwave digestion product, heating at 160℃~180℃ until white fumes cease to appear. The strong oxidizing property of perchloric acid can further digest residual trace organic matter. At the same time, perchloric acid can effectively drive away residual hydrofluoric acid in the digestion system, preventing hydrofluoric acid from corroding subsequent equipment and preventing the fluorosilicic acid formed by hydrofluoric acid with silicon from precipitating and interfering with the determination in subsequent steps. After the white fumes cease to appear, hydrochloric acid is added to dissolve the salts, which can convert the metal salts formed after digestion into soluble chlorides, ensuring that vanadium and nickel elements exist in a stable ionic form and avoiding the formation of insoluble salts that would cause element loss. This solves the problems of residual hydrofluoric acid in the digestion product and element loss caused by incomplete dissolution of salts after digestion.

[0017] After cooling the salt solution, it is transferred to a plastic volumetric flask and brought to volume. Transferring it while hot can cause volume errors due to thermal expansion and contraction of the solution, while transferring it after cooling ensures the accuracy of the volume. Using a plastic volumetric flask avoids the corrosion of glass volumetric flasks by residual trace amounts of fluoride ions, further ensuring the stability of the volume of the test liquid after volume adjustment, and making the concentration of vanadium and nickel elements in the test liquid uniform. This solves the problem of volume adjustment errors caused by temperature changes and fluoride ion corrosion, laying the foundation for subsequent accurate determination.

[0018] Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to determine vanadium and nickel in the test solution. This instrument uses argon plasma as the excitation source to excite vanadium and nickel atoms in the test solution to a high-energy state. When the excited atoms return to the ground state, they emit spectra with characteristic wavelengths. Quantitative analysis of vanadium and nickel can be achieved by detecting the intensity of the characteristic spectra. This method utilizes the high excitation efficiency of plasma to simultaneously determine two elements, and has high sensitivity and accuracy. It solves the problems of traditional methods being cumbersome to operate and difficult to accurately determine vanadium and nickel simultaneously, ultimately achieving efficient and accurate determination of vanadium and nickel in black carbonaceous mudstone.

[0019] In some embodiments, in S1, the calcination temperature of the muffle furnace is 850°C and the calcination time is 30 min; the mass of the black carbonaceous mudstone sample weighed is 0.25 g to 0.35 g, accurate to 0.0001 g.

[0020] Weighing 0.25g to 0.35g of black carbonaceous mudstone sample, accurate to 0.0001g, is the optimal range determined by considering the vanadium and nickel content levels in the black carbonaceous mudstone and the requirements of subsequent measurements. If the sample amount is too small, the absolute content of vanadium and nickel in the sample will be low, and subsequent instrument measurements are prone to large errors due to insufficient signal intensity, making it difficult to meet the accuracy requirements of microanalysis. If the sample amount is too large, the total amount of organic carbon in the sample will increase, which will not only increase the difficulty of completely removing organic carbon during the ignition stage, but also consume more acid reagents during the subsequent digestion process, and may even lead to incomplete digestion due to excessive matrix. At the same time, the element concentration in the test solution after volume adjustment may exceed the linear response range of the instrument, affecting the quantitative accuracy. Accurately weighing the sample to 0.0001g can ensure the maximum accuracy of sample weighing, reduce the weighing error between parallel samples, lay the foundation for the consistency of subsequent steps and the reliability of the final measurement results, and solve the problems of large measurement errors, incomplete digestion, or reagent waste caused by improper sample amount.

[0021] Choosing 850℃ as the calcination temperature for the muffle furnace is a suitable temperature determined based on the structural characteristics of organic carbon in black carbonaceous mudstone. Black carbonaceous mudstone has a high organic carbon content and complex structure; some recalcitrant organic components require sufficient temperature to be completely oxidized into carbon dioxide and released. If the temperature is below 850℃, unburned organic carbon will remain in the sample. This residual carbon will not only react with acid during subsequent digestion to produce a large amount of gas, causing abnormal pressure in the digestion vessel, but will also trap vanadium and nickel, hindering their release during subsequent digestion. If the temperature is too high, it will exacerbate the high-temperature wear of the platinum dish, shortening its lifespan, and may also cause excessive sintering of aluminosilicate minerals in the sample, forming a dense matrix that is more difficult to digest with acid, which is detrimental to the complete release of vanadium and nickel. The calcination temperature of 850℃ ensures the complete removal of organic carbon while avoiding problems caused by excessively high or low temperatures, further enhancing the effectiveness of the calcination step in removing organic carbon interference and exposing vanadium and nickel.

[0022] Setting the burning time to 30 minutes ensures that the organic carbon inside the sample is fully combusted. Black carbonaceous mudstone samples may have localized agglomeration or internal organic carbon inclusions. Heat transfer and oxygen contact require a certain amount of time. If the burning time is insufficient, although the organic carbon on the sample surface can be burned, the organic carbon inside cannot fully react with oxygen and will remain. If the burning time is too long, it will unnecessarily prolong the experimental cycle, increase energy consumption, and the prolonged exposure of the platinum dish to the high temperature environment will also accelerate its aging and wear. The 30-minute duration allows heat to be fully transferred to the interior of the sample, enabling all organic carbon to come into contact with oxygen and burn completely. This balances the burning effect with experimental efficiency and the wear and tear on the equipment, solving the problems of organic carbon residue due to insufficient burning time and low efficiency due to excessive burning time.

[0023] In some embodiments, in step S2, the platinum dish is first rinsed multiple times with 8 mL to 10 mL of hydrochloric acid, and all the rinsing solution is transferred into a microwave digestion vessel; the amount of nitric acid added is 0.8 mL to 1.2 mL, the amount of hydrofluoric acid added is 2.8 mL to 3.2 mL, and the amount of hydrogen peroxide added is 0.8 mL to 1.2 mL.

[0024] First, rinse the platinum dish multiple times with 8-10 mL of hydrochloric acid, transferring all the rinsing solution into the microwave digestion vessel. This is because after the sample is burned and transferred into the microwave digestion vessel, a small amount of sample components containing vanadium and nickel will remain on the inner wall of the platinum dish. If the volume of hydrochloric acid used for rinsing is too small, it will not be able to fully dissolve and transfer these residual components, leading to the loss of target elements in the sample and ultimately resulting in lower measurement results. If the volume of hydrochloric acid is too large, it will introduce too much chloride ion matrix, which will not only consume more oxidizing reagents in the subsequent digestion process, but may also cause the matrix concentration of the test solution to be too high after volume adjustment, interfering with the signal response of the inductively coupled plasma atomic emission spectrometer. Using 8-10 mL of hydrochloric acid for multiple rinsings ensures that all residual sample components on the platinum dish are completely dissolved and transferred to the microwave digestion vessel, avoiding the loss of target elements, while controlling the total amount of matrix introduced, so as not to cause additional interference to the subsequent digestion and measurement steps. This solves the problems of inaccurate measurement results caused by sample residues, as well as reagent waste and matrix interference.

[0025] The amount of nitric acid added is 0.8 mL to 1.2 mL. As a strong oxidizing agent, nitric acid can oxidize any remaining small amounts of reducing organic components and low-valence metal compounds in the sample after ignition, providing a basis for thorough microwave digestion. If the amount of nitric acid is insufficient, these reducing substances cannot be completely oxidized. The remaining reducing components will not only undergo side reactions with other acid reagents during microwave digestion, but may also encapsulate some vanadium and nickel elements, hindering their complete digestion. If the amount of nitric acid is excessive, a large amount of nitrogen oxides will be introduced, requiring a longer time to remove them in the subsequent acid removal step, unnecessarily prolonging the experimental cycle. At the same time, excessive nitrate ions may also interfere with the instrument signal during the measurement stage. The amount of nitric acid, 0.8 mL to 1.2 mL, is just right to meet the requirements for oxidizing residual reducing substances, avoiding both insufficient oxidation affecting the digestion effect and excessive reagent causing problems such as prolonged experimental cycle and signal interference.

[0026] The amount of hydrofluoric acid used is 2.8 mL to 3.2 mL. Black carbonaceous mudstone contains a large amount of clay minerals and quartz in the form of aluminosilicates. These components have a dense structure that encapsulates vanadium and nickel, making them difficult to dissolve with ordinary acid reagents. Hydrofluoric acid reacts with silicon to form volatile silicon tetrafluoride, disrupting the dense structure of the aluminosilicates and exposing the encapsulated vanadium and nickel, facilitating subsequent dissolution by acid reagents. If the amount of hydrofluoric acid is insufficient, the aluminosilicate structure cannot be completely destroyed, and some vanadium and nickel will remain encapsulated, unable to enter the test solution, leading to lower test results. If the amount of hydrofluoric acid is excessive, the excess hydrofluoric acid will corrode the inner wall of the microwave digestion vessel, shortening its lifespan. Furthermore, when adding perchloric acid to neutralize the excess hydrofluoric acid, more perchloric acid is needed, and residual fluoride ions may remain, forming insoluble compounds with metal ions in the test solution, interfering with the test results. A dosage of 2.8 mL to 3.2 mL of hydrofluoric acid can fully disrupt the dense structure of aluminosilicates, releasing all the encapsulated vanadium and nickel elements, while avoiding the problems of glassware corrosion, reagent waste, and measurement interference caused by excessive hydrofluoric acid.

[0027] The amount of hydrogen peroxide used is 0.8 mL to 1.2 mL. Under the high temperature and high pressure environment of microwave digestion, hydrogen peroxide decomposes to produce reactive oxygen free radicals, which have extremely strong oxidizing power. This can assist nitric acid in further oxidizing stubborn organic residues and bound metal components that are difficult to oxidize by nitric acid alone, thus improving the thoroughness of microwave digestion. If the amount of hydrogen peroxide is insufficient, it cannot provide enough reactive oxygen, and stubborn components cannot be completely oxidized and digested, resulting in some vanadium and nickel elements not being released. If the amount of hydrogen peroxide is too large, the large amount of oxygen produced by decomposition will cause the pressure inside the microwave digestion vessel to rise rapidly, exceeding the safe pressure range of the digestion vessel, posing a safety hazard. At the same time, excessive hydrogen peroxide will also require a longer time to completely decompose in the subsequent acid removal step, prolonging the experimental cycle. The amount of hydrogen peroxide used, 0.8 mL to 1.2 mL, can effectively enhance the oxidative digestion effect, ensuring that all difficult-to-digest components are completely decomposed, while maintaining the pressure inside the digestion vessel within a safe range, preventing safety risks due to abnormal pressure, and solving the problems of incomplete digestion of stubborn components and excessive pressure.

[0028] In some embodiments, the microwave digestion procedure in S2 includes the following steps: heating at a power of 1100W~1300W for 8min~12min to 110℃~130℃, and holding for 8min~12min; heating at a power of 1100W~1300W for 4min~6min to 150℃~170℃, and holding for 4min~6min; heating at a power of 1100W~1300W for 4min~6min to 180℃~200℃, and holding for 23min~27min.

[0029] The first stage of microwave digestion involves heating at 1100W~1300W for 8-12 minutes to 110℃~130℃, and holding for 8-12 minutes. After cold digestion, the sample still contains some readily soluble metal salts and residual mild reducing components. If the heating power is too high or the heating time is too short, the temperature inside the digestion vessel will rise sharply, causing the acid reagent and moisture in the sample to rapidly vaporize and trigger violent boiling. This will not only cause some sample to splash onto the inner wall of the digestion vessel and fail to participate in subsequent reactions, resulting in the loss of target elements, but will also cause the pressure inside the vessel to rise sharply and instantaneously, exceeding the safety threshold and posing a safety hazard. If the heating temperature is too low or the holding time is too short, the acid reagent cannot fully react with the readily soluble components, and some mild reducing components will not be initially oxidized. These components will then undergo violent side reactions with strong oxidizing reagents in the subsequent high-temperature digestion stage, interfering with the release of vanadium and nickel. Choosing the above parameters for the first stage of digestion can avoid boiling over and abnormal pressure through a stable heating rate and a suitable low temperature environment, ensuring that the sample and reagents are always in a stable reaction system. It can also allow the acid reagent to fully dissolve the easily soluble components and preliminarily oxidize the residual reducing substances, laying the foundation for subsequent deep digestion. This solves the problems of sample loss and safety risks caused by boiling over, as well as side reactions caused by insufficient pre-digestion.

[0030] The second stage of microwave digestion involves heating at 1100W~1300W for 4~6 minutes to 150℃~170℃, and holding for 4~6 minutes. After the first stage of pre-digestion, the remaining moderately insoluble components in the sample begin to be exposed. If the heating rate is too fast or the temperature is too high in this stage, it will cause large fluctuations in the pressure inside the digestion vessel, which may damage the seal of the digestion vessel. At the same time, local overheating of the sample will cause some unreacted components to form molten inclusions, which will encapsulate vanadium and nickel elements and hinder subsequent digestion. If the heating time is too long or the holding time is too long, it will unnecessarily consume the acid reagent in the digestion system, resulting in insufficient acid during subsequent deep digestion. Using the above parameters for the second-stage digestion allows the reaction temperature to be gradually increased at a moderate heating rate, enabling the acid reagent to slowly penetrate into the structural gaps of the moderately insoluble components and gradually destroy their aggregate structure. At the same time, the stable pressure environment avoids sealing failure and the formation of inclusions. The moderate holding time ensures that the moderately insoluble components are fully decomposed without excessive consumption of acid reagents, thus solving the problems of equipment damage caused by pressure fluctuations, the inability to release target elements due to inclusions, and the waste of acid reagents.

[0031] The third stage of microwave digestion involves heating the sample at 1100W~1300W for 4-6 minutes to 180℃~200℃, and holding the temperature for 23-27 minutes. At this point, only the most densely structured aluminosilicate minerals and a small amount of tightly bound vanadium and nickel elements remain in the sample. These components require a high-temperature environment to allow hydrofluoric acid to fully react with them, destroy the dense structure, and release the target elements. Insufficient heating power or excessive heating time will result in a slow temperature rise inside the digestion vessel, low reaction efficiency, and a prolonged overall experimental cycle. If the temperature is below 180℃, the reaction rate between hydrofluoric acid and aluminosilicates will decrease significantly, and even with extended holding time, the dense structure cannot be completely destroyed, leaving some vanadium and nickel elements encapsulated, leading to lower measurement results. If the temperature is above 200℃, it will accelerate the corrosion of the inner wall of the microwave digestion vessel, shortening its service life. At the same time, excessively high temperatures will cause reagents such as nitric acid and hydrogen peroxide to volatilize excessively, failing to fully exert their oxidation effect. If the holding time is less than 23 minutes, the destruction reaction of the dense structure cannot be completed, and the target elements will not be fully released. If the holding time is longer than 27 minutes, it will cause unnecessary energy consumption, and the excessively long high-temperature environment will increase the risk of damage to the digestion vessel. Choosing the above parameters for the third-stage digestion allows for the rapid attainment of the high-temperature environment required for the reaction through appropriate power and heating rate, ensuring the reactivity of hydrofluoric acid and oxidizing reagents. It also allows for sufficient holding time to completely destroy the dense structure, enabling the complete release of all encapsulated vanadium and nickel elements. Simultaneously, it controls the temperature within a safe range, avoiding excessive corrosion of the digestion vessel and excessive volatilization of reagents. This solves the problems of low measurement results, excessive wear and tear of the digestion vessel, and energy waste caused by incomplete digestion of the dense structure.

[0032] In some embodiments, in step S3, the amount of perchloric acid used is 1.8 mL to 2.2 mL; the hydrochloric acid used to dissolve the salts is a hydrochloric acid solution with a volume ratio of 1:1, and the amount used is 9 mL to 11 mL.

[0033] The microwave digestion product contains unreacted hydrofluoric acid and fluorides. These fluorides can combine with silicon in the system to form insoluble fluorosilicon compounds, and may also form stable complexes with vanadium and nickel, hindering the signal response of vanadium and nickel in subsequent measurements. Simultaneously, the residual hydrofluoric acid can corrode components such as the nebulization system of the inductively coupled plasma atomic emission spectrometer used in subsequent measurements, shortening the instrument's lifespan. In this case, 1.8 mL to 2.2 mL of perchloric acid is added to the microwave digestion product, and the mixture is heated at 160°C to 180°C until white fumes are exhausted. If the amount of perchloric acid is less than 1.8 mL, its oxidizing and fluoride-removing abilities are insufficient to completely remove the fluoride from the system, and the residual fluoride will cause the aforementioned complexation interference and instrument corrosion problems. If the amount of perchloric acid is greater than 2.2 mL, a small amount of excess perchloric acid will remain after heating to exhaustion. Its strong oxidizing properties will cause side reactions with some salts during subsequent hydrochloric acid dissolution, leading to changes in the valence state of vanadium and nickel, interfering with subsequent measurement results, and causing unnecessary waste of reagents. Choosing a perchloric acid dosage of 1.8 mL to 2.2 mL can completely remove fluoride from the system through the reaction of perchloric acid with hydrofluoric acid and the high-temperature fuming process, avoiding a series of problems caused by fluoride residue. It can also control the amount of reagent used, avoid side reactions and reagent waste caused by excessive perchloric acid, and ensure the stability of subsequent salt dissolution and element determination.

[0034] A 1:1 volume ratio hydrochloric acid solution ensures sufficient dissolving power to completely dissolve all salts, allowing vanadium and nickel to exist entirely as free ions. It also controls the system's acidity, avoiding spectral interference and reduced nebulization efficiency caused by excessive chloride ions, thus laying the foundation for accurate subsequent measurements. A dosage of 9-11 mL of hydrochloric acid provides sufficient reaction reagent for complete salt dissolution, ensuring vanadium and nickel enter the test solution without loss. It also avoids the problems of abnormal acidity, instrument corrosion, and reagent waste caused by excessive hydrochloric acid, ensuring the acidity of the test solution is within the suitable range for measurement.

[0035] In some embodiments, in step S4, the volume of the plastic volumetric flask is 90 mL to 110 mL; the solvent used for volume adjustment is deionized water with a resistivity greater than 18.0 MΩ·cm.

[0036] In some embodiments, after the preceding salt dissolution step, a very small amount of fluoride ions may still remain in the system, along with salts of target element ions such as vanadium and nickel, as well as matrix elements such as aluminum and iron. If a glass volumetric flask is used for volume adjustment, the silicon element in the glass will react with the residual fluoride ions to form insoluble fluorosilicone compounds. This not only corrodes the glass volumetric flask, leading to a decrease in its volumetric accuracy and compromising the accuracy of the volume adjustment operation, but also causes some vanadium and nickel ions to be lost due to the adsorption effect of the fluorosilicone compounds, causing subsequent measurement results to deviate from the true values. Plastic volumetric flasks, on the other hand, have excellent resistance to fluoride corrosion, completely preventing fluoride ions from reacting with the container components, while maintaining long-term volume stability and ensuring the reliability of the volume adjustment operation.

[0037] In some embodiments, the solvent used for volume adjustment is deionized water with a resistivity greater than 18.0 MΩ·cm. If the resistivity of the deionized water is lower than this value, it indicates that it contains a large number of impurity ions, such as common cations like sodium, potassium, and calcium, as well as anions like chloride and sulfate ions. These impurity ions, after entering the test solution, will generate their own characteristic emission signals during subsequent inductively coupled plasma atomic emission spectrometry (ICP-AES) measurements, creating strong background interference that masks the characteristic spectral lines of vanadium and nickel, leading to falsely high measurement results. Simultaneously, some impurity ions may also complex with vanadium and nickel ions, altering the form of the target ions, affecting their ionization efficiency in the plasma, interfering with the stability of the signal intensity, and further reducing the accuracy of the measurement results. Deionized water with a resistivity greater than 18.0 MΩ·cm, however, keeps the impurity ion content at an extremely low level, almost completely eliminating the introduction of additional interference signals and preventing any reaction with vanadium and nickel ions. This maximizes the purity and stability of the test solution, providing a clean medium environment for the ICP-AES spectrometer to accurately capture the characteristic signals of vanadium and nickel, ensuring the authenticity and reliability of the measurement results from the source.

[0038] In some embodiments, the operating parameters of the inductively coupled plasma atomic emission spectrometer in step S5 are as follows: sample cleaning time 25s~35s, sample cleaning speed 1.7mL / min~2.0mL / min, sample analysis speed 1.4mL / min~1.6mL / min, integration time 4s~6s, power 1100W~1200W, cooling gas flow rate 13L / min~15L / min, carrier gas pressure 24psi~26psi, and auxiliary gas flow rate 0.8L / min~1.2L / min.

[0039] The sample cleaning time was set to 25-35 seconds, and the cleaning speed to 1.7-2.0 mL / min. If the cleaning time is too short or the speed too fast, the residual analyte in the tubing cannot be completely replaced, leading to cross-contamination and causing the measurement results to deviate from the true value. If the cleaning time is too long or the speed too slow, the analyte will be excessively consumed. When the background concentration of the target element in the sample is low, the measurement may not be completed due to insufficient sample volume, which will also prolong the experimental cycle and reduce detection efficiency. This time and speed range ensures sufficient replacement of residual solution in the tubing, avoiding cross-contamination, while minimizing analyte loss while maintaining effective cleaning and high experimental efficiency.

[0040] The sample analysis speed is set to 1.4 mL / min to 1.6 mL / min. If the analysis speed is too fast, the residence time of the analyte in the plasma is too short, and vanadium and nickel cannot be fully atomized and ionized, resulting in insufficient characteristic emission signal intensity and making it difficult for the instrument to accurately capture the effective signal. If the analysis speed is too slow, the amount of analyte entering the plasma per unit time is too small, which will also weaken the signal intensity, prolong the single measurement time, and reduce the detection efficiency. This speed range ensures that the analyte obtains an appropriate residence time in the plasma, allowing vanadium and nickel to be fully atomized and ionized, while maintaining a reasonable detection rhythm to ensure the stability of the signal intensity and the detection efficiency.

[0041] The integration time is set to 4-6 seconds. If the integration time is too short, the instrument will acquire insufficient characteristic signal data, which cannot effectively counteract random noise interference, resulting in poor signal repeatability and decreased precision of the measurement results. If the integration time is too long, it will increase the duration of a single measurement, reduce detection efficiency, and long-term signal acquisition may introduce additional errors due to instrument drift. This integration time range can acquire sufficient signal data to effectively suppress noise interference and improve the precision of the results, while avoiding the efficiency reduction and instrument drift errors caused by excessive integration time, thus ensuring the accuracy of the measurement results.

[0042] The power is set to 1100W~1200W. If the power is too low, the plasma energy is insufficient to provide enough energy for the atomization and ionization of vanadium and nickel, resulting in incomplete atomization and ionization, weak characteristic emission signal intensity, and decreased instrument detection sensitivity. If the power is too high, the plasma temperature will be too high, which will excite more matrix elements to generate characteristic emission signals, forming background interference. At the same time, excessively high power will accelerate the wear and tear of instrument components, shorten the instrument's lifespan, and increase maintenance costs. This power range can provide suitable energy for the plasma, ensuring sufficient atomization and ionization of vanadium and nickel, while avoiding over-excitation of matrix elements to prevent interference, thus balancing the relationship between detection sensitivity, anti-interference capability, and instrument wear and tear.

[0043] The cooling gas flow rate is set to 13L / min~15L / min. If the cooling gas flow rate is too low, it will not be able to effectively remove the large amount of heat generated by the plasma, leading to excessively high torch temperature, accelerated torch aging and damage, and may even affect plasma stability. If the cooling gas flow rate is too high, it will excessively dilute the plasma, reducing its temperature and energy density, affecting the atomization and ionization efficiency of vanadium and nickel, and causing a decrease in signal strength. This flow rate range can effectively remove the heat generated by the plasma, maintain the normal operating temperature of the torch, and ensure the energy density and stability of the plasma, providing a stable environment for the atomization and ionization of vanadium and nickel.

[0044] The carrier gas pressure is set to 24 psi to 26 psi. If the carrier gas pressure is too low, the amount of analyte lifted will be insufficient, resulting in too little vanadium and nickel entering the plasma, weak characteristic emission signals, and the instrument will be unable to accurately determine low-content target elements. If the carrier gas pressure is too high, the amount of analyte lifted will be too large, leading to excessive plasma load, disrupting plasma stability, and potentially even extinguishing the plasma. Furthermore, excessive analyte entering the plasma will increase the probability of matrix interference. This pressure range provides suitable lifting power for the analyte, ensuring a sufficient amount enters the plasma while maintaining plasma stability and preventing exacerbation of matrix interference.

[0045] The auxiliary gas flow rate is set to 0.8 L / min to 1.2 L / min. If the flow rate is too low, a stable channel cannot be formed at the center of the plasma, affecting the plasma morphology and stability, leading to unstable atomization and ionization processes of vanadium and nickel, and large fluctuations in signal intensity. If the flow rate is too high, it will compress the plasma volume, reduce the plasma energy density, affect the atomization and ionization efficiency of vanadium and nickel, and increase argon consumption, thus increasing experimental costs. This flow rate range allows for the formation of a stable channel at the center of the plasma, maintaining its morphology and stability, ensuring stable atomization and ionization processes of vanadium and nickel, while avoiding excessive argon consumption and controlling experimental costs.

[0046] In some embodiments, the method further includes the following steps before S5: Vanadium and nickel single-element standard stock solutions were transferred to volumetric flasks, respectively. Hydrochloric acid solution with a volume ratio of 1:1 was added and the solution was brought to volume to obtain a mixed standard working solution of vanadium and nickel with a concentration of 90 μg / mL to 110 μg / mL. Different volumes of the vanadium and nickel mixed standard working solution were transferred to multiple volumetric flasks, and hydrochloric acid solution with a volume ratio of 1:1 was added and the volume was adjusted to obtain a series of standard solutions with concentration gradients of 0 μg / mL, 0.4 μg / mL~0.6 μg / mL, 0.9 μg / mL~1.1 μg / mL, 2.9 μg / mL~3.1 μg / mL, 4.9 μg / mL~5.1 μg / mL, and 9.9 μg / mL~10.1 μg / mL.

[0047] First, transfer vanadium and nickel single-element standard stock solutions to volumetric flasks, add hydrochloric acid solution at a 1:1 volume ratio, and dilute to volume to obtain a mixed vanadium and nickel standard working solution with a concentration of 90 μg / mL to 110 μg / mL. Since single-element standard stock solutions are typically highly concentrated, directly using them to prepare low-concentration standard solutions requires extremely small volumes. Even minor deviations in pipetting can lead to significant errors in the standard solution concentration, thus affecting the accuracy of subsequent calibration curves. Preparing a mixed standard working solution within this concentration range allows for better control of the transfer volume, effectively reducing the impact of pipetting errors on the solution concentration. Simultaneously, diluting to volume with hydrochloric acid solution at a 1:1 volume ratio ensures that the acid medium of the mixed standard working solution is consistent with that of the subsequent analyte, avoiding matrix effects introduced by medium differences. This ensures that the analyte and standard solutions are in similar atomization and ionization environments in the inductively coupled plasma atomic emission spectrometer, reducing measurement errors.

[0048] Different volumes of the aforementioned vanadium and nickel mixed standard working solution were transferred to multiple volumetric flasks, and diluted to volume with 1:1 hydrochloric acid solution to obtain a series of standard solutions with concentration gradients of 0 μg / mL, 0.4 μg / mL~0.6 μg / mL, 0.9 μg / mL~1.1 μg / mL, 2.9 μg / mL~3.1 μg / mL, 4.9 μg / mL~5.1 μg / mL, and 9.9 μg / mL~10.1 μg / mL. A 0 μg / mL solution was set as a blank to eliminate background interference introduced by reagents and experimental equipment during the measurement process, further improving the accuracy of the results. The gradient settings covering low, medium, and high concentrations cover the possible content range of vanadium and nickel in black carbonaceous mudstone, ensuring a good linear relationship in the calibration curve. This allows the instrument to accurately fit the correspondence between concentration and emission intensity, thus enabling precise determination of vanadium and nickel at different concentration levels in the test solution. If the concentration gradient is set too sparsely, it may not be able to fully reflect the linear relationship between concentration and signal, resulting in insufficient fitting accuracy of the calibration curve. If the concentration gradient exceeds the content range of the target element in the sample by too much, it will cause unnecessary waste of standard solution. At the same time, the nonlinear characteristics that may appear in the high concentration region will also affect the accuracy of the overall calibration curve. This gradient range ensures the accuracy of linear fitting while avoiding excessive consumption of solution.

[0049] In some embodiments, in step S5, the analytical spectral line used to determine vanadium is 311.071 nm, and the analytical spectral line used to determine nickel is 341.476 nm.

[0050] In some embodiments, when using inductively coupled plasma atomic emission spectrometry (ICP-AES) to determine vanadium and nickel in the test solution, the selection of analytical spectral lines requires comprehensive consideration of factors such as interference from coexisting elements, signal intensity, and background interference. After pretreatment, black carbonaceous mudstone still contains residual matrix components such as iron, aluminum, calcium, and magnesium in the test solution. The characteristic spectral lines of these components may overlap with some spectral lines of vanadium and nickel, or produce background absorption effects, thereby interfering with the effective signal detection of the target elements and leading to deviations in the measurement results. Therefore, it is necessary to screen for exclusive spectral lines with low interference levels and stable signal responses.

[0051] In some embodiments, during the operations of S1 to S5, a blank test is performed simultaneously. The operation steps of the blank test are completely consistent with those of S1 to S5, except that no black carbonaceous mudstone sample is added.

[0052] In the process of determining vanadium and nickel in black carbonaceous mudstone, various reagents such as hydrochloric acid, nitric acid, hydrofluoric acid, and perchloric acid are used in each step from sample ignition and microwave digestion to volume determination. At the same time, various vessels such as platinum dishes, microwave digestion vessels, and plastic volumetric flasks are used. Even if the reagents are purified and the vessels are cleaned with acid, trace amounts of vanadium and nickel may still remain. In addition, impurities in the environment may also enter the experimental system during the operation. These foreign vanadium and nickel elements will superimpose on the target element signals of the sample itself, causing the final measurement results to deviate from the true content of elements in the sample and fail to accurately reflect the actual level of vanadium and nickel in black carbonaceous mudstone.

[0053] A blank test was conducted simultaneously with the sample determination. The blank test followed the entire procedure of the sample determination, except that the black carbonaceous mudstone sample was not added. This operation allowed the blank test system to undergo the same reagent addition, glassware contact, and environmental exposure process as the sample determination. This enriched and captured all interference signals of vanadium and nickel elements caused by all external factors such as reagent residues, glassware contamination, and environmental introduction during the sample determination process, and obtained a blank response value that included all external interferences.

[0054] By subtracting the blank response value obtained from the blank test from the vanadium and nickel response values ​​obtained from the sample measurement, the interference of vanadium and nickel elements introduced by various factors other than the sample itself during the experiment can be accurately eliminated, thus completely eliminating the influence of these external interferences on the measurement results. This allows the final measurement data to truly reflect the actual content of vanadium and nickel elements in the black carbonaceous mudstone sample, effectively solving the technical problem of inaccurate measurement results caused by external interference during the experiment, and significantly improving the accuracy and reliability of the entire measurement method.

[0055] The method of the present invention will be described in detail below with reference to embodiments, comparative examples and experimental data.

[0056] Example 1 First, prepare a series of mixed vanadium and nickel standard working solutions. The specific steps are as follows: S01, transfer 10.00 mL of a 1000 μg / mL vanadium single-element standard stock solution and 10.00 mL of a 1000 μg / mL nickel single-element standard stock solution to 100 mL volumetric flasks, add 10 mL of hydrochloric acid solution (1:1 volume ratio), and then dilute to the mark with deionized water (resistivity greater than 18.0 MΩ·cm). Shake well to obtain a 100 μg / mL mixed vanadium and nickel standard working solution. Liquid; SO2, transfer 0 mL, 0.5 mL, 1 mL, 3 mL, 5 mL, and 10 mL of the above vanadium and nickel mixed standard working solution into six 100 mL volumetric flasks respectively. Add 10 mL of hydrochloric acid solution with a volume ratio of 1:1 to each volumetric flask, and then dilute to the mark with deionized water with a resistivity greater than 18.0 MΩ·cm. Shake well to obtain a series of standard solutions with concentration gradients of 0 μg / mL, 0.5 μg / mL, 1 μg / mL, 3 μg / mL, 5 μg / mL, and 10 μg / mL.

[0057] Simultaneously, a blank test was conducted. The operation procedure for the blank test was completely consistent with that of subsequent S1 to S5, except that no black carbonaceous mudstone sample was added.

[0058] S1. Weigh 0.3g (accurate to 0.0001g) of black carbonaceous mudstone sample and place it in a platinum dish. Put the platinum dish into a muffle furnace and ignite it at 850℃ for 30min to obtain the ignited sample.

[0059] S2. Carefully transfer the ignited sample into a microwave digestion vessel. After wetting with deionized water, rinse the platinum dish multiple times with 9 mL of hydrochloric acid. Transfer all the rinsing solution into the microwave digestion vessel. Then, add 1 mL of nitric acid, 3 mL of hydrofluoric acid, and 1 mL of hydrogen peroxide to the microwave digestion vessel and perform cold digestion for 10 hours. Afterward, tighten the microwave digestion vessel and perform microwave digestion according to the set program. The specific program is as follows: S21. Heat to 120°C at 1200W for 10 minutes and hold for 10 minutes; S22. Heat to 160°C at 1200W for 5 minutes and hold for 5 minutes; S23. Heat to 190°C at 1200W for 5 minutes and hold for 25 minutes. After digestion, the microwave digestion product is obtained.

[0060] S3. Transfer the microwave digestion product to the heating container of the graphite digester, add 2 mL of perchloric acid, heat at 170°C until white fumes are exhausted, then add 10 mL of hydrochloric acid solution with a volume ratio of 1:1, place on the graphite digester and heat to dissolve the salts, and obtain the salt solution.

[0061] S4. After cooling the salt solution to room temperature, transfer it to a 100mL plastic volumetric flask, dilute to the mark with deionized water with a resistivity greater than 18.0MΩ·cm, shake well, and obtain the test solution.

[0062] S5. Set the operating parameters of the inductively coupled plasma atomic emission spectrometer: sample cleaning time 30s, sample cleaning speed 1.85mL / min, sample analysis speed 1.48mL / min, integration time 5s, power 1150W, cooling gas flow rate 14L / min, carrier gas pressure 25psi, and auxiliary gas flow rate 1.0L / min. Establish a calibration curve using a series of prepared standard solutions, and then use the instrument to determine vanadium and nickel in the test solution. The analytical spectral line used for determining vanadium is 311.071nm, and the analytical spectral line used for determining nickel is 341.476nm.

[0063] Example 2 First, prepare a series of mixed vanadium and nickel standard working solutions. The specific steps are as follows: S01, transfer 10.00 mL of a 1000 μg / mL vanadium single-element standard stock solution and 10.00 mL of a 1000 μg / mL nickel single-element standard stock solution to 100 mL volumetric flasks, add 10 mL of hydrochloric acid solution (1:1 volume ratio), and then dilute to the mark with deionized water (resistivity greater than 18.0 MΩ·cm). Shake well to obtain a 100 μg / mL mixed vanadium and nickel standard working solution. Liquid; SO2, transfer 0 mL, 0.5 mL, 1 mL, 3 mL, 5 mL, and 10 mL of the above vanadium and nickel mixed standard working solution into six 100 mL volumetric flasks respectively. Add 10 mL of hydrochloric acid solution with a volume ratio of 1:1 to each volumetric flask, and then dilute to the mark with deionized water with a resistivity greater than 18.0 MΩ·cm. Shake well to obtain a series of standard solutions with concentration gradients of 0 μg / mL, 0.5 μg / mL, 1 μg / mL, 3 μg / mL, 5 μg / mL, and 10 μg / mL.

[0064] Simultaneously, a blank test was conducted. The operation procedure for the blank test was completely consistent with that of subsequent S1 to S5, except that no black carbonaceous mudstone sample was added.

[0065] S1. Weigh 0.35g (accurate to 0.0001g) of black carbonaceous mudstone sample and place it in a platinum dish. Put the platinum dish into a muffle furnace and ignite it at 850℃ for 30min to obtain the ignited sample.

[0066] S2. Carefully transfer the ignited sample into a microwave digestion vessel. After wetting with deionized water, rinse the platinum dish multiple times with 9 mL of hydrochloric acid. Transfer all the rinsing solution into the microwave digestion vessel. Then, add 1 mL of nitric acid, 3 mL of hydrofluoric acid, and 1 mL of hydrogen peroxide to the microwave digestion vessel and perform cold digestion for 10 hours. Afterward, tighten the microwave digestion vessel and perform microwave digestion according to the set program. The specific program is as follows: S21. Heat to 120°C at 1200W for 10 minutes and hold for 10 minutes; S22. Heat to 160°C at 1200W for 5 minutes and hold for 5 minutes; S23. Heat to 190°C at 1200W for 5 minutes and hold for 25 minutes. After digestion, the microwave digestion product is obtained.

[0067] S3. Transfer the microwave digestion product to the heating container of the graphite digester, add 2 mL of perchloric acid, heat at 170°C until white fumes are exhausted, then add 10 mL of hydrochloric acid solution with a volume ratio of 1:1, place on the graphite digester and heat to dissolve the salts, and obtain the salt solution.

[0068] S4. After cooling the salt solution to room temperature, transfer it to a 100mL plastic volumetric flask, dilute to the mark with deionized water with a resistivity greater than 18.0MΩ·cm, shake well, and obtain the test solution.

[0069] S5. Set the operating parameters of the inductively coupled plasma atomic emission spectrometer: sample cleaning time 30s, sample cleaning speed 1.85mL / min, sample analysis speed 1.48mL / min, integration time 5s, power 1150W, cooling gas flow rate 14L / min, carrier gas pressure 25psi, and auxiliary gas flow rate 1.0L / min. Establish a calibration curve using a series of prepared standard solutions, and then use the instrument to determine vanadium and nickel in the test solution. The analytical spectral line used for determining vanadium is 311.071nm, and the analytical spectral line used for determining nickel is 341.476nm.

[0070] Example 3 First, prepare a series of mixed vanadium and nickel standard working solutions. The specific steps are as follows: S01, transfer 10.00 mL of a vanadium single-element standard stock solution with a concentration of 1000 μg / mL and 10.00 mL of a nickel single-element standard stock solution with a concentration of 1000 μg / mL to 100 mL volumetric flasks, add 10 mL of hydrochloric acid solution with a volume ratio of 1:1, and then dilute to the mark with deionized water with a resistivity greater than 18.0 MΩ·cm. Shake well to obtain a vanadium and nickel mixed standard working solution with a concentration of 100 μg / mL. Solution; SO2, transfer 0 mL, 1 mL, 2 mL, 6 mL, 10 mL, and 20 mL of the above vanadium and nickel mixed standard working solution into six 100 mL volumetric flasks respectively. Add 10 mL of hydrochloric acid solution with a volume ratio of 1:1 to each volumetric flask, and then dilute to the mark with deionized water with a resistivity greater than 18.0 MΩ·cm. Shake well to obtain a series of standard solutions with concentration gradients of 0 μg / mL, 1 μg / mL, 2 μg / mL, 6 μg / mL, 10 μg / mL, and 20 μg / mL.

[0071] Simultaneously, a blank test was conducted. The operation procedure for the blank test was completely consistent with that of subsequent S1 to S5, except that no black carbonaceous mudstone sample was added.

[0072] S1. Weigh 0.5g (accurate to 0.0001g) of black carbonaceous mudstone sample and place it in a platinum dish. Put the platinum dish into a muffle furnace and ignite it at 900℃ for 60min to obtain the ignited sample.

[0073] S2. Carefully transfer the ignited sample into a microwave digestion vessel. After wetting with deionized water, rinse the platinum dish several times with 10 mL of hydrochloric acid. Transfer all the rinsing solution into the microwave digestion vessel. Then, add 2 mL of nitric acid, 5 mL of hydrofluoric acid, and 2 mL of hydrogen peroxide to the microwave digestion vessel and perform cold digestion for 12 hours. Afterward, tighten the microwave digestion vessel and perform microwave digestion according to the set program. The specific program is as follows: S21. Heat to 140°C at 1500W for 15 minutes and hold for 15 minutes; S22. Heat to 180°C at 1500W for 10 minutes and hold for 10 minutes; S23. Heat to 210°C at 1500W for 10 minutes and hold for 30 minutes. After digestion, the microwave digestion product is obtained.

[0074] S3. Transfer the microwave digestion product to the heating container of the graphite digester, add 3 mL of perchloric acid, heat at 200°C until white fumes are exhausted, then add 10 mL of hydrochloric acid solution with a volume ratio of 1:1, place on the graphite digester and heat to dissolve the salts, and obtain the salt solution.

[0075] S4. After cooling the salt solution to room temperature, transfer it to a 100mL plastic volumetric flask, dilute to the mark with deionized water with a resistivity greater than 18.0MΩ·cm, shake well, and obtain the test solution.

[0076] S5. Set the operating parameters of the inductively coupled plasma atomic emission spectrometer: sample cleaning time 60s, sample cleaning speed 2.0mL / min, sample analysis speed 1.6mL / min, integration time 10s, power 1300W, cooling gas flow rate 15L / min, carrier gas pressure 30psi, and auxiliary gas flow rate 1.5L / min. Establish a calibration curve using a series of prepared standard solutions, and then use the instrument to determine vanadium and nickel in the test solution. The analytical spectral line used for determining vanadium is 311.071nm, and the analytical spectral line used for determining nickel is 341.476nm.

[0077] Comparative Example 1 This comparative example provides a method for the joint determination of vanadium and nickel in black carbonaceous mudstone. The difference from Example 1 is that the ignition temperature of the muffle furnace in S1 is 700°C, while the other steps are the same as in Example 1.

[0078] Comparative Example 2 This comparative example provides a method for the joint determination of vanadium and nickel in black carbonaceous mudstone. The difference from Example 1 is that the calcination time of the muffle furnace in S1 is 20 min, and the other steps are the same as in Example 1.

[0079] Comparative Example 3 This comparative example provides a method for the joint determination of vanadium and nickel in black carbonaceous mudstone. The difference from Example 1 is that the burning step in S1 is omitted, and the weighed black carbonaceous mudstone sample is directly transferred into a microwave digestion vessel for subsequent operations. The remaining steps are the same as in Example 1.

[0080] Comparative Example 4 This comparative example provides a method for the joint determination of vanadium and nickel in black carbonaceous mudstone. The difference from Example 1 is that the cold digestion time in S2 is 6 hours, while the other steps are the same as in Example 1.

[0081] Comparative Example 5 This comparative example provides a method for the joint determination of vanadium and nickel in black carbonaceous mudstone. The difference from Example 1 is that step S23 of the microwave digestion procedure in S2 is to heat the sample to 170°C at a power of 1200W for 5 minutes and hold for 25 minutes. The remaining steps are the same as in Example 1.

[0082] Comparative Example 6 This comparative example provides a method for the joint determination of vanadium and nickel in black carbonaceous mudstone. The difference from Example 1 is that the amount of perchloric acid used in S3 is 1 mL, while the other steps are the same as in Example 1.

[0083] Comparative Example 7 This comparative example provides a method for the joint determination of vanadium and nickel in black carbonaceous mudstone. The difference from Example 1 is that the analytical spectral line used for determining vanadium in S5 is 309.311 nm, and the analytical spectral line used for determining nickel is 305.082 nm. The remaining steps are the same as in Example 1.

[0084] The products corresponding to the above embodiments and comparative examples were tested using the following methods: Appearance and color of the sample after ignition: The sample to be tested is placed in a muffle furnace and ignition is carried out according to the preset ignition temperature and time. After that, the sample is taken out and placed in a desiccator to cool to room temperature. The appearance and color of the sample are observed and compared by visual inspection combined with a standard color comparison card.

[0085] Clarity of the test solution: Take an appropriate amount of the test solution and transfer it into a clean, dry stoppered colorimetric tube. Place the colorimetric tube in front of a black background and observe the test solution horizontally to see if there is any turbidity, precipitation or flocculent suspension. If necessary, it can be compared with the same volume of deionized water.

[0086] Seal integrity of digestion vessel: After assembling the digestion vessel according to the standard assembly procedure, inject an appropriate amount of deionized water into the vessel, place it in a pressure testing device, apply a preset pressure and maintain it for a certain period of time, remove the digestion vessel, check whether there is water leakage on the outer surface of the vessel, and observe whether the water level inside the vessel drops abnormally.

[0087] White smoke residue during the acid removal stage: Place the container containing the digested sample solution on the acid removal device, set the preset acid removal temperature, and heat to remove the acid. Observe the release state of white smoke in the container throughout the process. When there are no obvious liquid droplets left on the inner wall of the container, and the white smoke changes from continuous large-scale emission to intermittent weak emission until it completely dissipates, record the white smoke residue at this stage.

[0088] Vanadium recovery rate: Take a black carbonaceous mudstone sample with known vanadium content, prepare a blank test solution according to steps S1-S4 of this method, and determine and record the vanadium content according to step S5. Take another black carbonaceous mudstone sample of the same batch with the same mass, add a quantitative amount of vanadium single-element standard solution in step S2, and perform the remaining operations as in steps S1-S5 to prepare a spiked test solution and determine and record the vanadium content. Calculate the vanadium recovery rate according to the formula: Recovery rate = (Spiked content - Blank content) / Spiked amount × 100%.

[0089] RSD of vanadium element determination: Take the same black carbonaceous mudstone sample and perform six parallel determinations following the complete steps S1 to S5 of this method. Accurately record the vanadium content determination results for each determination and calculate the relative standard deviation (RSD) of vanadium content based on the parallel determination results.

[0090] Nickel recovery rate: Take a black carbonaceous mudstone sample with known nickel content, prepare a blank test solution according to steps S1-S4 of this method, and determine and record the measured nickel content according to step S5. Take another black carbonaceous mudstone sample of the same batch with the same mass, add a quantitative amount of nickel single-element standard solution in step S2, and perform the remaining operations as in steps S1-S5 to prepare a spiked test solution and determine and record the measured nickel content. Calculate the nickel recovery rate according to the formula: Recovery rate = (Spiked content - Blank content) / Spiked amount × 100%.

[0091] RSD of nickel element determination: Take the same black carbonaceous mudstone sample and perform six parallel determinations following the complete steps S1 to S5 of this method. Accurately record the determination results of nickel element each time, and calculate the relative standard deviation (RSD) of nickel element content based on the parallel determination results.

[0092] Table 1 Performance Test Results ; Results analysis: In Example 1, the sample was uniformly grayish-white after ignition, indicating that the ignition parameters of 850℃ and 30min could completely oxidize and remove the organic carbon in the black carbonaceous mudstone, leaving no carbonaceous components. The test solution was clear and transparent without any turbidity, precipitation, or suspended matter, indicating that the microwave digestion program of cold digestion for 10 hours combined with three-stage gradient heating could achieve complete digestion of the sample, and that the perchloric acid thoroughly removed the acid and the hydrochloric acid fully dissolved the salts, with no insoluble mineral particles or fluoride precipitates produced. The digestion vessel showed no water stains or leakage, and the water level did not drop abnormally, proving that the pressure inside the digestion vessel was within the safe threshold range under the experimental conditions, and the sealing integrity was good. The white smoke completely dissipated during the acid removal stage, indicating that the perchloric acid could completely remove the hydrofluoric acid in the system under the heating condition of 170℃, leaving no fluorine residue. Under these conditions, the vanadium recovery rate reached 98.5% with a determination RSD of 1.2%, and the nickel recovery rate reached 99% with a determination RSD of 1%. The high recovery rates and extremely low relative standard deviations demonstrate the effectiveness of the 0.3g sample weight, optimal dosage of each acid reagent, standard operating parameters of the inductively coupled plasma atomic emission spectrometer, and the selection of specific analytical spectral lines at 311.071nm for vanadium and 341.476nm for nickel used in this embodiment. This forms a synergistic and compatible determination system, ensuring the complete release of vanadium and nickel from the sample matrix without loss, while effectively avoiding matrix interference through precise instrument detection. This results in accurate and precise determinations that meet the detection requirements. This optimal parameter combination is the foundation for the joint determination of vanadium and nickel in black carbonaceous mudstone, providing a reliable benchmark for subsequent fine-tuning of sample volume and instrument parameters.

[0093] In Example 2, the sample was grayish-white after ignition, the test solution was clear and transparent, the digestion vessel had no leakage and the water level did not drop abnormally, the white smoke completely dissipated during the acid removal stage, the vanadium recovery rate was 98.2% and the RSD was 1.3%, the nickel recovery rate was 98.8% and the RSD was 1.1%. Although the sample weight was slightly higher than in Example 1, the sample processing was complete, and the accuracy and precision of the vanadium and nickel determination also met the detection requirements.

[0094] In Example 3, the sample was grayish-white after ignition, the test solution was clear and transparent, the digestion vessel had no leakage and the water level did not drop abnormally, the white smoke completely dissipated during the acid removal stage, the vanadium recovery rate was 99.2% and the RSD was 0.8%, the nickel recovery rate was 99.5% and the RSD was 0.7%. After adjusting the sample weight, ignition conditions, digestion parameters and the working parameters of the inductively coupled plasma atomic emission spectrometer, the sample processing was more thorough, and the accuracy and precision of the vanadium and nickel determination were better than those in Examples 1 and 2.

[0095] In Comparative Example 1, the sample turned grayish-black after ignition, indicating that the organic carbon in the black carbonaceous mudstone was not completely removed. The test solution was turbid, and there was water leakage in the digestion vessel with a slight drop in water level. Although the white smoke completely dissipated during the acid removal stage, the recovery rate of vanadium was only 82.3% with a determination RSD of 5.6%, and the recovery rate of nickel was only 81.8% with a determination RSD of 5.8%. It can be seen that a low ignition temperature will lead to residual organic carbon, which can easily cause the digestion vessel to fail to seal and interfere with the signal detection of the inductively coupled plasma atomic emission spectrometer, thus significantly reducing the accuracy and precision of the determination results of vanadium and nickel.

[0096] In Comparative Example 2, the sample turned grayish-brown after ignition, indicating that the carbonaceous components in the black carbonaceous mudstone were not fully ignited and decomposed. The test solution was slightly turbid, the digestion vessel showed no leakage and the water level did not drop abnormally. The white smoke completely dissipated during the acid removal stage. The recovery rate of vanadium was 88.5% and the RSD was 4.2%. The recovery rate of nickel was 87.9% and the RSD was 4.5%. This indicates that insufficient ignition time will lead to incomplete digestion of the sample, resulting in lower vanadium and nickel determination results and poor repeatability.

[0097] In Comparative Example 3, no ignition step was performed. The test solution was severely turbid with flocculent suspended matter. The digestion vessel showed obvious water leakage and a significant drop in water level. Black soot residue remained during the acid removal stage. The vanadium recovery rate was only 65.2% with an RSD of 12.3%, and the nickel recovery rate was only 64.5% with an RSD of 13.1%. This indicates that the absence of high-temperature ignition to remove organic carbon causes a large amount of carbonaceous components in the sample to react violently with acid during microwave digestion, leading to overpressure leakage in the digestion vessel. Furthermore, the sample cannot be completely digested, severely interfering with the determination. This results in extremely poor accuracy and insufficient precision for the determination of vanadium and nickel.

[0098] In Comparative Example 4, the sample was grayish-white after ignition, the test solution was slightly turbid and contained a small amount of undissolved particles, the digestion vessel showed no leakage and the water level did not drop abnormally, the white smoke completely dissipated during the acid removal stage, the vanadium recovery rate was 90.2% and the RSD was 3.8%, the nickel recovery rate was 89.7% and the RSD was 4.0%. The short cold digestion time led to insufficient pre-reaction between the sample and the acid solution, and some sparingly soluble mineral components were not initially infiltrated and decomposed. The subsequent microwave digestion could not completely digest the sample, resulting in lower vanadium and nickel determination results, and the determination results of different parallel samples had large deviations.

[0099] In Comparative Example 5, the sample was grayish-white after ignition, the test solution was slightly turbid and contained undissolved mineral particles, the digestion vessel showed no leakage and the water level did not drop abnormally, the white smoke completely dissipated during the acid removal stage, the vanadium recovery rate was 85.6% and the RSD was 4.8%, the nickel recovery rate was 84.9% and the RSD was 5.1%, the maximum temperature of microwave digestion was too low, and it could not completely decompose the sparingly soluble silicates, oxides and other components in the sample, resulting in significantly lower vanadium and nickel determination results, which could not truly reflect the actual vanadium and nickel content in the sample.

[0100] In Comparative Example 6, the sample was grayish-white after ignition, and a small amount of precipitate was formed in the test solution. There was a faint white smoke residue during the acid removal stage. The digestion vessel showed no leakage and the water level did not drop abnormally. The recovery rate of vanadium was 92.1% with a determination RSD of 6.2%, and the recovery rate of nickel was 91.5% with a determination RSD of 6.5%. Insufficient perchloric acid was used, which could not effectively remove hydrofluoric acid from the microwave digestion products. The residual hydrofluoric acid could not only affect the complete dissolution of salts, but also corrode the precision parts of the instrument, resulting in large fluctuations and poor precision in the determination results of vanadium and nickel.

[0101] In Comparative Example 7, the sample was grayish-white after ignition, the test solution was clear and transparent, the digestion vessel had no leakage and the water level did not drop abnormally, and the white smoke completely dissipated during the acid removal stage. However, the recovery rate of vanadium was 88.0% and the RSD was 5.0%, while the recovery rate of nickel was 87.2% and the RSD was 5.3%. The selected vanadium and nickel analytical spectral lines had large interference and high background noise, resulting in poor signal stability of the inductively coupled plasma atomic emission spectrometer, which reduced the accuracy and precision of the vanadium and nickel determination results.

[0102] In summary, the example demonstrates that by rationally setting the sample ignition conditions, digestion parameters, acid removal operation, and instrument operating parameters, the black carbonaceous mudstone sample is adequately processed, the digestion vessel is well-sealed, and acid removal is thorough, resulting in high recovery rates and good precision for vanadium and nickel element determination. In contrast, the comparative example suffers from inadequate parameter settings in any step, such as ignition temperature, ignition time, cold digestion time, microwave digestion temperature, perchloric acid dosage, or selection of analytical spectral lines. This leads to incomplete sample processing, digestion vessel seal failure, incomplete acid removal, or interference with the measurement signal, ultimately resulting in vanadium and nickel element determinations that fail to meet the detection requirements in terms of accuracy and precision. This illustrates that parameter settings in each step of this method are crucial for ensuring the reliability of the joint determination of vanadium and nickel elements in black carbonaceous mudstone.

Claims

1. A method for combined determination of trace elements in black carbonaceous mudstones, characterized by Includes the following steps: S1. Weigh a black carbonaceous mudstone sample and place it in a platinum dish. Then, calcine it in a muffle furnace at 800℃~900℃ for 25min~35min to obtain the calcined sample. S2. The calcined sample is transferred into a microwave digestion vessel, moistened with deionized water, and then hydrochloric acid, nitric acid, hydrofluoric acid and hydrogen peroxide are added for cold digestion for 8h~12h. After that, microwave digestion is carried out according to the set program to obtain microwave digestion products. S3. After adding the microwave digestion product to perchloric acid, heat it at 160℃~180℃ until white fumes are exhausted, then add hydrochloric acid to dissolve the salts to obtain a salt solution. S4. After cooling the salt solution, transfer it to a plastic volumetric flask and make up to volume to obtain the test solution; S5. The vanadium and nickel elements in the test solution are determined using an inductively coupled plasma atomic emission spectrometer.

2. The method for combined determination of trace elements in black carbonaceous mudstone according to claim 1, characterized in that: In S1, the calcination temperature of the muffle furnace is 850℃ and the calcination time is 30min; the mass of the black carbonaceous mudstone sample is 0.25g~0.35g.

3. The method for joint determination of trace elements in black carbonaceous mudstone according to claim 1 or 2, characterized in that: In step S2, the platinum dish is first rinsed multiple times with 8 mL to 10 mL of hydrochloric acid, and the entire rinsing solution is transferred into a microwave digestion vessel. The amount of nitric acid added is 0.8 mL to 1.2 mL, the amount of hydrofluoric acid is 2.8 mL to 3.2 mL, and the amount of hydrogen peroxide is 0.8 mL to 1.2 mL.

4. The method for joint determination of trace elements in black carbonaceous mudstone according to any one of claims 1 to 3, characterized in that: The microwave digestion procedure in S2 includes the following steps: heating at a power of 1100W~1300W for 8min~12min to 110℃~130℃, and holding for 8min~12min; heating at a power of 1100W~1300W for 4min~6min to 150℃~170℃, and holding for 4min~6min; heating at a power of 1100W~1300W for 4min~6min to 180℃~200℃, and holding for 23min~27min.

5. The method for joint determination of trace elements in black carbonaceous mudstone according to any one of claims 1 to 4, characterized in that: In S3, the amount of perchloric acid used is 1.8 mL to 2.2 mL; the hydrochloric acid used to dissolve the salts is a hydrochloric acid solution with a volume ratio of 1:1, and the amount used is 9 mL to 11 mL.

6. The method for joint determination of trace elements in black carbonaceous mudstone according to any one of claims 1 to 5, characterized in that: In step S4, the volume of the plastic volumetric flask is 90 mL to 110 mL; the solvent used for volume adjustment is deionized water with a resistivity greater than 18.0 MΩ·cm.

7. The method for joint determination of trace elements in black carbonaceous mudstone according to any one of claims 1 to 6, characterized in that: In S5, the operating parameters of the inductively coupled plasma atomic emission spectrometer are as follows: sample cleaning time 25s~35s, sample cleaning speed 1.7mL / min~2.0mL / min, sample analysis speed 1.4mL / min~1.6mL / min, integration time 4s~6s, power 1100W~1200W, cooling gas flow rate 13L / min~15L / min, carrier gas pressure 24psi~26psi, and auxiliary gas flow rate 0.8L / min~1.2L / min.

8. The method for joint determination of trace elements in black carbonaceous mudstone according to any one of claims 1 to 7, characterized in that: Prior to S5, the following is also included: Vanadium and nickel single-element standard stock solutions were transferred to volumetric flasks, and hydrochloric acid solution with a volume ratio of 1:1 was added and the solution was brought to volume to obtain vanadium and nickel mixed standard working solutions with concentrations ranging from 90 μg / mL to 110 μg / mL. Different volumes of the vanadium and nickel mixed standard working solutions were transferred to multiple volumetric flasks, and hydrochloric acid solution with a volume ratio of 1:1 was added and the solution was brought to volume to obtain a series of standard solutions with concentration gradients of 0 μg / mL, 0.4 μg / mL to 0.6 μg / mL, 0.9 μg / mL to 1.1 μg / mL, 2.9 μg / mL to 3.1 μg / mL, 4.9 μg / mL to 5.1 μg / mL, and 9.9 μg / mL to 10.1 μg / mL.

9. The method for joint determination of trace elements in black carbonaceous mudstone according to any one of claims 1 to 8, characterized in that: In step S5, the analytical spectral line used to determine vanadium is 311.071 nm, and the analytical spectral line used to determine nickel is 341.476 nm.