Method for continuously testing three stable carbon isotopes in soil

Abstract

The invention provides a method for continuously testing three stable carbon isotopes in soil, and belongs to the technical field of soil detection. The pretreatment process of stable carbon isotope testing of three carbon forms (carbonate inorganic carbon, soluble organic carbon and non-soluble organic carbon) of soil is simplified and integrated, then the same stable isotope ratio mass spectrometer is externally connected with a multipurpose online gas preparation and introduction device and an element analyzer, and the external connection sequence is controlled; and efficient and continuous testing of three carbon isotopes is realized. Compared with a traditional method in which single carbon forms are respectively treated and independently detected, the method has the advantages that the time cost of sample pretreatment and instrument analysis is reduced, error accumulation caused by sample transfer and multiple pretreatment is avoided, and the detection result is more accurate.

Description

Technical Field

[0001] This invention belongs to the field of soil testing technology, specifically relating to a method for continuously testing three stable carbon isotopes in soil. Background Technology

[0002] Soil ecosystems play a crucial role in the carbon cycle of terrestrial ecosystems. Dynamic changes in the soil carbon cycle not only affect soil fertility and ecosystem stability but are also closely linked to global climate change. Soil carbon exists primarily in three forms: carbonate inorganic carbon, dissolved organic carbon, and insoluble organic carbon. These forms occupy different proportions in the soil carbon pool and undergo diverse transformation processes. Accurate analysis of the stable carbon isotope composition of these three forms is of great significance for in-depth research into the sources, transformations, and migration patterns of soil carbon.

[0003] However, traditional analytical methods typically require cumbersome pretreatment and independent testing for each single form of carbon, resulting in low efficiency and the introduction of errors. Summary of the Invention

[0004] The purpose of this invention is to provide a method for continuously testing three stable carbon isotopes in soil. The method provided by this invention has high efficiency and accuracy.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for continuously testing three stable carbon isotopes in soil, comprising the following steps: (1) Mix the soil sample with phosphoric acid and acidify it to obtain the carbonate inorganic carbon sample to be tested; (2) Mix the soil sample with water, shake it in the dark and then separate the solid and liquid to obtain the extract; mix the extract with the oxidant solution, introduce helium gas, and then carry out the oxidation reaction and let it stand to obtain the dissolved organic carbon sample to be tested; (3) When the inorganic carbon content in the soil sample is ≥3wt%, the soil sample is mixed with hydrochloric acid, and then shaken, allowed to stand and separated into solid and liquid in sequence to obtain a solid. The solid is washed with water and dried to obtain the non-dissolved organic carbon sample to be tested. When the inorganic carbon content in the soil sample is <3wt%, the soil sample is moistened with water, placed in a concentrated hydrochloric acid atmosphere, and then dried to obtain the non-dissolved organic carbon sample to be tested. (4) The stable carbon isotope ratios in the carbonate inorganic carbon sample to be tested obtained in step (1) and the dissolved organic carbon sample to be tested obtained in step (2) are tested sequentially using a multi-purpose online gas preparation and introduction device-stable isotope ratio mass spectrometer. Then, the stable carbon isotope ratios in the non-dissolved organic carbon sample to be tested obtained in step (3) are tested using an elemental analyzer-stable isotope ratio mass spectrometer to obtain the three stable carbon isotope ratios in the soil.

[0006] Preferably, the mass concentration of phosphoric acid in step (1) is 98-99%; and the acidification time is 48 hours.

[0007] Preferably, in step (2), the mass ratio of the soil sample to the volume of water is 2g:(38~42)mL; the temperature of the light-shielded vibration is 20~30℃, and the time of light-shielded vibration is 22~25h.

[0008] Preferably, the oxidant solution in step (2) includes potassium persulfate, phosphoric acid and water; the concentration of potassium persulfate in the oxidant solution is 38~42 g / L; and the volume ratio of the extract to the oxidant solution is (3~5):1.

[0009] Preferably, the oxidation reaction temperature in step (2) is 95~100℃ and the oxidation reaction time is 0.5~1.5h.

[0010] Preferably, the concentration of hydrochloric acid in step (3) is 0.4~0.6 mol / L; the mass ratio of the soil sample to the volume of hydrochloric acid is 1 g: (28~32) mL.

[0011] Preferably, in step (3), when the mass of the soil sample is ≤5mg, the amount of water used is 20μL; when 5mg < the mass of the soil sample <10mg, the amount of water used is 40μL.

[0012] Preferably, the time spent in the concentrated hydrochloric acid atmosphere in step (3) is 22-25 hours.

[0013] Preferably, the conditions of the multi-purpose online gas preparation and introduction device in step (4) of the stable isotope ratio mass spectrometer include: column temperature 48~52℃, carrier gas flow rate 95~105mL / min; the conditions of the stable isotope ratio mass spectrometer include: ionization method is electron bombardment, ion source high voltage 9~10kV.

[0014] Preferably, the conditions of the elemental analyzer in step (4) of the elemental analyzer-stable isotope ratio mass spectrometer include: reaction tube temperature 950~1000℃, column temperature 58~62℃, carrier gas flow rate 170~190mL / min, reference gas flow rate 48~52mL / min, oxygen flow rate 240~260mL / min, and oxygen passage time 4~6s.

[0015] This invention provides a method for continuously testing three stable carbon isotopes in soil, comprising the following steps: (1) mixing a soil sample with phosphoric acid and acidifying it to obtain a carbonate inorganic carbon sample to be tested; (2) mixing the soil sample with water, shaking it in the dark, and then separating the solid and liquid to obtain an extract; mixing the extract with an oxidant solution, introducing helium gas, and then performing an oxidation reaction followed by standing to obtain a soluble organic carbon sample to be tested; (3) when the inorganic carbon content in the soil sample is ≥3wt%, mixing the soil sample with hydrochloric acid, and then performing shaking, standing, and solid-liquid separation sequentially to obtain a solid; washing the solid with water and then drying it to obtain a non-soluble organic carbon sample to be tested. Organic carbon sample; when the inorganic carbon content in the soil sample is <3wt%, the soil sample is moistened with water and then placed in a concentrated hydrochloric acid atmosphere and then dried to obtain the non-dissolved organic carbon sample to be tested; (4) the stable carbon isotope ratio in the carbonate inorganic carbon sample to be tested obtained in step (1) and the dissolved organic carbon sample to be tested obtained in step (2) are tested sequentially using a multi-purpose online gas preparation and introduction device-stable isotope ratio mass spectrometer, and then the stable carbon isotope ratio in the non-dissolved organic carbon sample to be tested obtained in step (3) is tested using an elemental analyzer-stable isotope ratio mass spectrometer to obtain the three stable carbon isotope ratios in the soil. This invention simplifies and integrates the pretreatment process for stable carbon isotope testing of three carbon forms in soil (carbonate inorganic carbon, dissolved organic carbon, and insoluble organic carbon). Then, it uses a single stable isotope ratio mass spectrometer connected to a multi-purpose online gas preparation and introduction device and an elemental analyzer, and controls the order of external connections to achieve efficient and continuous testing of the three carbon isotopes. Compared with the traditional method of processing and detecting each carbon form separately, this reduces the time cost of sample pretreatment and instrument analysis, avoids the accumulation of errors caused by sample transfer and multiple pretreatments, and makes the detection results more accurate. Detailed Implementation

[0016] This invention provides a method for continuously testing three stable carbon isotopes in soil, comprising the following steps: (1) Mix the soil sample with phosphoric acid and acidify it to obtain the carbonate inorganic carbon sample to be tested; (2) Mix the soil sample with water, shake it in the dark and then separate the solid and liquid to obtain the extract; mix the extract with the oxidant solution, introduce helium gas, and then carry out the oxidation reaction and let it stand to obtain the dissolved organic carbon sample to be tested; (3) When the inorganic carbon content in the soil sample is ≥3wt%, the soil sample is mixed with hydrochloric acid, and then shaken, allowed to stand and separated into solid and liquid in sequence to obtain a solid. The solid is washed with water and dried to obtain the non-dissolved organic carbon sample to be tested. When the inorganic carbon content in the soil sample is <3wt%, the soil sample is moistened with water, placed in a concentrated hydrochloric acid atmosphere, and then dried to obtain the non-dissolved organic carbon sample to be tested. (4) The stable carbon isotope ratios in the carbonate inorganic carbon sample to be tested obtained in step (1) and the dissolved organic carbon sample to be tested obtained in step (2) are tested sequentially using a multi-purpose online gas preparation and introduction device-stable isotope ratio mass spectrometer. Then, the stable carbon isotope ratios in the non-dissolved organic carbon sample to be tested obtained in step (3) are tested using an elemental analyzer-stable isotope ratio mass spectrometer to obtain the three stable carbon isotope ratios in the soil.

[0017] Unless otherwise specified, the present invention does not impose any special restrictions on the source of the raw materials, and commercially available products well known to those skilled in the art can be used.

[0018] This invention involves mixing soil samples with phosphoric acid and acidifying them to obtain a carbonate inorganic carbon sample to be tested.

[0019] In this invention, the soil sample is preferably dried, ground and sieved in sequence before use.

[0020] The present invention does not impose any special limitations on the drying operation; drying to constant weight can be achieved using technical solutions well known to those skilled in the art.

[0021] The present invention does not impose any special limitations on the grinding operation; any technical solution known to those skilled in the art can be used.

[0022] The present invention does not impose any special limitations on the sieving operation; any technical solution known to those skilled in the art can be used to ensure that the mesh size of the soil sample is within the required range.

[0023] In this invention, the mesh size of the soil sample is preferably ≥100 mesh.

[0024] In one embodiment, the present invention places a soil sample in a headspace vial, then fills it with helium gas, and then adds phosphoric acid for acidification.

[0025] In one embodiment, the carbonate content in the soil sample may be 200 μg.

[0026] In one embodiment, the purity of the helium gas is 99.999%; the time for filling the helium gas is 8 minutes; and the flow rate of the helium gas is 100 mL / min.

[0027] In this invention, the mass concentration of phosphoric acid is preferably 98-99%, more preferably 99%.

[0028] The present invention does not have a special limitation on the amount of phosphoric acid used, as long as it is sufficient to fully submerge the soil sample.

[0029] In this invention, the acidification time is preferably 48 hours, and the acidification is preferably carried out at room temperature. This invention controls the acidification time and temperature to achieve carbon dioxide equilibrium.

[0030] This invention controls the amount of each raw material, reaction time, and other parameters within the above-mentioned range, which enables phosphoric acid to react fully with the carbonate in the soil sample, thereby further improving the accuracy of detection.

[0031] This invention involves mixing a soil sample with water, shaking it in the dark, and then separating the solid and liquid components to obtain an extract. The extract is then mixed with an oxidant solution, helium gas is introduced, and an oxidation reaction is carried out. After standing, the sample is obtained as a dissolved organic carbon sample to be tested.

[0032] In this invention, the water is preferably ultrapure water.

[0033] In this invention, the preferred mass ratio of the soil sample to the volume of water is 2g:(38~42)mL, and more preferably 2g:40mL.

[0034] In this invention, the temperature of the light-shielded oscillation is preferably 20~30℃, more preferably 25℃; the time of the light-shielded oscillation is preferably 22~25h, more preferably 24h.

[0035] The present invention does not impose any special limitations on the solid-liquid separation operation; the extract can be obtained by using technical solutions well known to those skilled in the art.

[0036] In one implementation method, the solid-liquid separation is vacuum filtration.

[0037] In one implementation method, the present invention uses tweezers to place the PES filter membrane into the sand core of the sand core filter, adds 100mL of ultrapure water for filtration, and then adds the system after light-proof shaking for vacuum filtration. After vacuum filtration, the PES filter membrane is discarded, the sand core is rinsed with tap water, and then vacuum filtered three times with ultrapure water (300mL each time). The sand core is then dried at 60℃. The same set of sand cores is used to treat the same soil sample.

[0038] In this invention, the oxidant solution preferably includes potassium persulfate, phosphoric acid and water; the concentration of potassium persulfate in the oxidant solution is preferably 38~42 g / L, more preferably 40 g / L.

[0039] As one embodiment, the oxidant solution is prepared by adding 2g of potassium persulfate to ultrapure water, adding 100μL of 85% phosphoric acid, and making up to 50mL to obtain the oxidant solution.

[0040] In this invention, the volume ratio of the extract to the oxidant solution is preferably (3~5):1, more preferably 4:1.

[0041] In this invention, the mixing time of the extract and the oxidant solution is preferably 0.5~1.5 h, more preferably 1 h; the mixing is preferably carried out at room temperature and under static conditions. In this invention, an oxidant solution is added and allowed to stand to remove inorganic carbon.

[0042] In one embodiment, the mixing of the extract and the oxidant solution is carried out in a headspace vial; after mixing, the headspace vial is sealed with a cap with a rubber septum.

[0043] In this invention, the helium introduction time is preferably 4-6 minutes, more preferably 5 minutes; the helium flow rate is preferably 90-110 mL / min, more preferably 100 mL / min. This invention uses helium to remove carbon dioxide generated from dissolved inorganic carbon and to replace headspace gas. After the helium introduction is complete, the inlet needle is removed first.

[0044] In this invention, the temperature of the oxidation reaction is preferably 95~100℃, more preferably 98℃; the time of the oxidation reaction is preferably 0.5~1.5h, more preferably 1h.

[0045] In this invention, the settling time is preferably 22-25 hours, more preferably 24 hours; the settling temperature is preferably 20-30°C, more preferably 25°C. The settling process in this invention allows the carbon dioxide in the headspace and liquid phase to reach equilibrium.

[0046] This invention controls parameters such as the amount of each raw material, reaction temperature, and time within the above-mentioned range, which can further improve the accuracy of detection.

[0047] As one implementation method, when the inorganic carbon content in the soil sample is ≥3wt%, the present invention mixes the soil sample with hydrochloric acid, and then performs oscillation, settling and solid-liquid separation in sequence to obtain a solid. The solid is then washed with water and dried to obtain the non-dissolved organic carbon sample to be tested.

[0048] In this invention, the concentration of the hydrochloric acid is preferably 0.4~0.6 mol / L, more preferably 0.5 mol / L.

[0049] In this invention, the preferred mass ratio of the soil sample to the volume of hydrochloric acid is 1 g: (28~32) mL, and more preferably 1 g: 30 mL.

[0050] In this invention, the mixing time of the soil sample and hydrochloric acid is preferably 2 to 4 hours, more preferably 3 hours.

[0051] In this invention, the oscillation time is preferably 10-12 hours.

[0052] In this invention, the settling time is preferably 2-4 hours, more preferably 3 hours. During the settling process, intermittent shaking is preferably performed to disperse the air. This invention does not impose specific limitations on the interval and number of intermittent shaking cycles; they can be selected according to actual needs to ensure sufficient air dispersion.

[0053] The present invention does not impose any particular limitation on the solid-liquid separation operation; any technical solution well known to those skilled in the art can be used to obtain the solid. As one embodiment, the solid-liquid separation is centrifugation; the centrifugation speed is 3000 rpm; and the centrifugation time is 10 minutes.

[0054] In one embodiment, the present invention mixes the solid after solid-liquid separation with ultrapure water, shakes for 15 minutes for washing, discards the supernatant, and repeats this process until the pH value of the supernatant is the same as that of the ultrapure water.

[0055] In one implementation method, the mass ratio of the soil sample to the volume of ultrapure water is 1 g: 30 mL.

[0056] This invention does not specify a particular number of repetitions, as long as the pH value of the supernatant is the same as that of ultrapure water. This invention includes water washing to ensure no hydrochloric acid residue remains.

[0057] In this invention, the drying temperature is preferably 45~55℃, more preferably 50℃. This invention does not have a specific limitation on the drying time; drying to a constant weight is sufficient.

[0058] As another implementation method, when the inorganic carbon content in the soil sample is <3wt%, the present invention wets the soil sample with water, places it in a concentrated hydrochloric acid atmosphere, and then dries it to obtain the non-dissolved organic carbon sample to be tested.

[0059] In this invention, the water is preferably ultrapure water.

[0060] In this invention, when the mass of the soil sample is ≤5mg, the amount of water used is preferably 20μL; when 5mg < the mass of the soil sample < 10mg, the amount of water used is preferably 40μL.

[0061] In one embodiment, the present invention places a soil sample in a silver cup, which is then placed in a 96-well tray. Water is added to the silver cup, and the 96-well tray is placed in an evaporating dish containing concentrated hydrochloric acid at the bottom. The present invention moistens the soil sample with water and then uses the vapors from the concentrated hydrochloric acid to remove inorganic carbon from the soil sample.

[0062] In this invention, the concentration of the concentrated hydrochloric acid is preferably 11.5~12 mol / L, more preferably 12 mol / L.

[0063] In one embodiment, the outer diameter of the evaporating dish can be 10-30 cm; the amount of concentrated hydrochloric acid used can be 100-300 mL.

[0064] In this invention, the time for immersion in concentrated hydrochloric acid atmosphere is preferably 22-25 hours, more preferably 24 hours.

[0065] As one implementation method, the drying can be carried out in a fume hood.

[0066] This invention controls the amount of each raw material, reaction temperature, and time within the above-mentioned range, which can further improve the accuracy of detection.

[0067] In the three pretreatment processes of soil samples (obtaining the sample of inorganic carbonate carbon to be tested, the sample of dissolved organic carbon to be tested, and the sample of non-dissolved organic carbon to be tested), this invention adjusts the time configuration of the three pretreatment processes according to the processing procedures and required processing times of each soil sample in the three pretreatment processes, so as to achieve efficient and continuous pretreatment of soil samples and provide uniform and stable sample input for continuous testing.

[0068] After obtaining the carbonate inorganic carbon sample, the dissolved organic carbon sample, and the non-dissolved organic carbon sample to be tested, this invention uses a multi-purpose online gas preparation and introduction device-stable isotope ratio mass spectrometer to sequentially test the stable carbon isotope ratios in the carbonate inorganic carbon sample and the dissolved organic carbon sample, and then uses an elemental analyzer-stable isotope ratio mass spectrometer to test the stable carbon isotope ratios in the non-dissolved organic carbon sample to obtain the three stable carbon isotope ratios in the soil.

[0069] In this invention, the preferred conditions for the multi-purpose online gas preparation and introduction device in the stable isotope ratio mass spectrometer include: column temperature 48~52℃, carrier gas flow rate 95~105mL / min, and helium as the carrier gas; more preferably: column temperature 50℃, carrier gas flow rate 100mL / min, helium as the carrier gas, and helium purity 99.999%.

[0070] In this invention, the conditions of the stable isotope ratio mass spectrometer in the multi-purpose online gas preparation and introduction device-stable isotope ratio mass spectrometer preferably include: ionization method is electron bombardment, and ion source high voltage is 9~10kV; more preferably: ion source high voltage is 9.5kV.

[0071] In this invention, the preferred conditions for the elemental analyzer in the elemental analyzer-stable isotope ratio mass spectrometer include: reaction tube temperature 950~1000℃, column temperature 58~62℃, carrier gas flow rate 170~190mL / min, carrier gas is helium, reference gas flow rate 48~52mL / min, reference gas is carbon dioxide, oxygen flow rate 240~260mL / min, and oxygen flow time 4~6s; more preferably: reaction tube temperature 980℃, column temperature 60℃, carrier gas flow rate 180mL / min, carrier gas is helium with a purity of 99.999%, reference gas flow rate 50mL / min, reference gas is carbon dioxide with a purity of 99.999% (calibrated to VPDB), oxygen flow rate 250mL / min, oxygen purity of 99.999%, and oxygen flow time 5s.

[0072] Based on the characteristics and requirements of testing three forms of carbon isotopes, this invention uses the same stable isotope ratio mass spectrometer and rationally arranges the debugging sequence and time of external equipment (multi-purpose online gas preparation and introduction device and elemental analyzer) to achieve continuous testing of the three carbon isotopes.

[0073] This invention first connects a multi-purpose online gas preparation and introduction device to a stable isotope ratio mass spectrometer, and then sequentially tests the stable carbon isotope ratios of inorganic carbon and dissolved organic carbon in soil carbonates. Before the sample is processed, the various parameters and consumables of the multi-purpose online gas preparation and introduction device-stable isotope ratio mass spectrometer system are checked, including the pressure, flow rate and purity of gases such as He and CO2, to ensure a stable gas supply.

[0074] Secondly, during the stable carbon isotope testing of soil carbonate inorganic carbon and dissolved organic carbon, the dried soil non-dissolved organic carbon samples were pre-treated by packaging: an appropriate amount of the treated sample was weighed using a 1 / 1,000,000 balance and placed in a tin cup.

[0075] Secondly, one day before completing the stable carbon isotope testing of soil carbonate inorganic carbon and dissolved organic carbon, prepare the gas and hardware for the elemental analyzer-stable isotope ratio mass spectrometer system in advance. Regarding gas preparation, check the pressure and flow rate of the carrier gas (He), combustion gas (O2), and reference gas (CO2). Regarding hardware preparation, check the consumption of magnesium perchlorate in the water trap. Simultaneously, check the usage record of the combustion furnace reaction tubes, confirming the type of reaction tube, the number of tin cups tested, and the approximate range of sample injection volumes to determine whether ash removal or replacement of the reaction tubes is necessary. After ash removal, the reaction tubes must be heated at 980℃ for at least 4 hours before sample processing. Newly replaced reaction tubes should be heated at 980℃ for at least 12 hours before sample processing can begin.

[0076] Finally, the elemental analyzer was connected to a stable isotope ratio mass spectrometer for stable carbon isotope testing of soil non-dissolved organic carbon. After the elemental analyzer entered the sample preparation state, the packaged sample was placed into the autosampler, and appropriate parameters were set before starting the test. During the test, all instrument parameters were monitored to ensure the normal operation of components such as the combustion reduction furnace, and to avoid incomplete combustion of the sample or blockage of the gas flow path affecting the test results. At least three empty tin cups were tested before each batch of samples, followed by at least three standard samples. Two additional standard samples were tested between every ten samples during the testing period, and two more standard samples were tested at the end of the sequence to ensure testing accuracy and stability.

[0077] This invention simplifies and integrates pretreatment methods for testing stable carbon isotopes of three carbon forms in soil. By identifying commonalities in operation and optimizing the time configuration of the three pretreatment methods, it achieves efficient pretreatment of soil samples, providing uniform and stable sample input for continuous testing. Optimizing processing conditions yields the most efficient continuous processing method. Finally, by rationally planning the debugging and operation time of the external equipment for the stable isotope ratio mass spectrometer, efficient continuous testing of the three carbon isotopes is achieved. This invention enables continuous testing of stable carbon isotopes of three forms in soil, improving testing efficiency and accuracy. Compared to traditional methods that process and detect each carbon form separately, it reduces the time cost of sample pretreatment and instrument analysis (the pretreatment-test cycle for the same soil sample can be compressed to within 4 days, while traditional three independent methods require approximately 7 to 10 days), avoids error accumulation due to sample transfer and multiple pretreatments, and helps provide more reliable data support for a deeper understanding of the carbon cycle processes and carbon emission characteristics of soil ecosystems.

[0078] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0079] Raw materials and consumables required for the example: Soil samples: collected in the field, dried and ground through a 100-mesh sieve.

[0080] Acids: 85% phosphoric acid, 99% phosphoric acid, 0.5 mol / L hydrochloric acid, 12 mol / L hydrochloric acid.

[0081] Reagent: Potassium persulfate.

[0082] Ultrapure water: resistivity ≥18.2MΩ / cm.

[0083] High-purity gases: Helium 99.999%, Carbon dioxide 99.999%, Oxygen 99.999%.

[0084] Consumables: 0.22~0.45μm PES filter membrane, 12mL borosilicate glass bottle, tin cup (5×8mm), 1mL syringe (with needle), 150mL conical flask, sintered glass filter, 40mL reagent bottle, 100mL reagent bottle, 12mL headspace vial (with butyl rubber septum).

[0085] Oxidizing agent solution: Add 2.0 g of potassium persulfate to ultrapure water, add 100 μL of 85% phosphoric acid, and make up to 50 mL to obtain an oxidizing agent solution with a potassium persulfate concentration of 40 g / L.

[0086] Example 1 A method for continuously testing three stable carbon isotopes in soil is as follows: (1) Place the soil sample (containing 200 μg of carbonate) in a 12 mL headspace bottle, purge with helium for 8 min (flow rate 100 mL / min), add 99% phosphoric acid (enough to cover the soil sample), let stand for 48 h to acidify, so that the carbon dioxide reaches equilibrium, and obtain the carbonate inorganic carbon sample to be tested. (2) Weigh 2.0g of soil sample into a 150mL conical flask, add 40mL of ultrapure water, shake at 25℃ in the dark for 24h, filter to obtain extract; add 4mL of extract and 1mL of oxidant solution to a 12mL headspace vial, let stand for 1h to remove inorganic carbon, then seal with a cap with rubber septum, and purge with helium gas for 5min (flow rate 100mL / min) to remove carbon dioxide generated from dissolved inorganic carbon and replace headspace gas. After purging, first remove the gas inlet needle, then react in a 98℃ oven for 1h, and let stand in a 25℃ environment for 24h to allow carbon dioxide in the headspace and liquid phase to reach equilibrium, and obtain the dissolved organic carbon sample to be tested; (3) When the inorganic carbon content of the soil sample is ≥3wt%, place 1g of soil sample in a centrifuge tube, add 30mL of 0.5mol / L hydrochloric acid, open the lid and let stand for 3h, cover the tube, use a shaker to rotate up and down for 11h, let stand for 3h (shaking intermittently to release gas), then centrifuge at 3000rpm for 10min, discard the supernatant, then add 30mL of ultrapure water and shake for 15min, repeat this process until the pH value of the supernatant is the same as that of the ultrapure water, then discard the supernatant, dry at 50℃ to constant weight, and obtain the non-dissolved organic carbon sample to be tested; When the inorganic carbon content of the soil sample is <3wt%, 5mg of soil sample is placed in a silver cup and then in a 96-well tray. 20μL of ultrapure water is added to the silver cup. The tray is then placed in an evaporating dish (OD=18cm) containing 150mL of concentrated hydrochloric acid (12mol / L). The sample is left to stand for 24h and then dried in a fume hood for 3h to obtain the non-dissolved organic carbon sample to be tested. (4) First, connect the multipurpose gas online preparation and introduction device to the stable isotope ratio mass spectrometer, and then determine the stable carbon isotope ratio of the carbonate inorganic carbon sample and the dissolved organic carbon sample to be tested in sequence. Then, connect the elemental analyzer to determine the stable carbon isotope ratio of the non-dissolved organic carbon sample to be tested, and obtain the three stable carbon isotope ratios in the soil. The conditions of the multipurpose gas online preparation and introduction device are: column temperature 50℃, carrier gas flow rate 100mL / min, carrier gas is helium, and the purity of helium is 99.999%. The conditions for the stable isotope ratio mass spectrometer were as follows: ionization method was electron impact, and the ion source high voltage was 9.5 kV; the conditions for the elemental analyzer were as follows: reaction tube temperature 980℃, column temperature 60℃, carrier gas flow rate 180 mL / min, carrier gas was helium with a purity of 99.999%, reference gas flow rate 50 mL / min, reference gas was carbon dioxide with a purity of 99.999% (calibrated to VPDB), oxygen flow rate 250 mL / min, oxygen purity of 99.999%, and oxygen flow time 5 s.

[0087] Ten soil samples were tested using the method in Example 1. The total dry weight of the samples consumed for pretreatment and testing was less than 10g. Each sample was tested twice in parallel. The results are shown in Table 1.

[0088] Table 1 shows the results of the ratios of three stable carbon isotopes in 10 soil samples tested using the method in Example 1.

[0089] The three standard samples were tested using the method in Example 1, and the results are shown in Table 2.

[0090] Table 2 shows the results of the ratios of three stable carbon isotopes in the standard samples tested using the method of Example 1.

[0091] As shown in Table 1, the peak height (44 m / z) of CO2 generated during the testing of the samples was above 0.5V. The accuracy of the test results met the expected technical specifications, namely: the accuracy of inorganic carbon stable carbon isotope testing reached ±0.2‰ within one standard deviation; the accuracy of soluble organic carbon stable carbon isotope testing reached ±0.5‰ within one standard deviation; and the accuracy of insoluble organic carbon stable carbon isotope testing reached ±0.2‰ within one standard deviation. As shown in Table 2, the results of the ratios of the three stable carbon isotopes in the standard samples tested using the method of this invention were basically consistent with the calibration values, proving that the method of this invention has high detection accuracy.

[0092] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for continuously testing three stable carbon isotopes in soil, characterized in that, Includes the following steps: (1) Mix the soil sample with phosphoric acid and acidify it to obtain the carbonate inorganic carbon sample to be tested; (2) Mix the soil sample with water, shake it in the dark and then separate the solid and liquid to obtain the extract; mix the extract with the oxidant solution, introduce helium gas, and then carry out the oxidation reaction and let it stand to obtain the dissolved organic carbon sample to be tested; (3) When the inorganic carbon content in the soil sample is ≥3wt%, the soil sample is mixed with hydrochloric acid, and then shaken, allowed to stand and separated into solid and liquid in sequence to obtain a solid. The solid is washed with water and dried to obtain the non-dissolved organic carbon sample to be tested. When the inorganic carbon content in the soil sample is <3wt%, the soil sample is moistened with water, placed in a concentrated hydrochloric acid atmosphere, and then dried to obtain the non-dissolved organic carbon sample to be tested. (4) The stable carbon isotope ratios in the carbonate inorganic carbon sample to be tested obtained in step (1) and the dissolved organic carbon sample to be tested obtained in step (2) are tested sequentially using a multi-purpose online gas preparation and introduction device-stable isotope ratio mass spectrometer. Then, the stable carbon isotope ratios in the non-dissolved organic carbon sample to be tested obtained in step (3) are tested using an elemental analyzer-stable isotope ratio mass spectrometer to obtain the three stable carbon isotope ratios in the soil.

2. The method according to claim 1, characterized in that, In step (1), the mass concentration of phosphoric acid is 98-99%; the acidification time is 48 hours.

3. The method according to claim 1, characterized in that, In step (2), the mass ratio of soil sample to water volume is 2g:(38~42)mL; the temperature of the light-shielded vibration is 20~30℃, and the time of light-shielded vibration is 22~25h.

4. The method according to claim 1, characterized in that, The oxidant solution in step (2) includes potassium persulfate, phosphoric acid and water; the concentration of potassium persulfate in the oxidant solution is 38~42 g / L; the volume ratio of the extract to the oxidant solution is (3~5):

1.

5. The method according to claim 1, characterized in that, The oxidation reaction in step (2) is carried out at a temperature of 95~100℃ and for a time of 0.5~1.5h.

6. The method according to claim 1, characterized in that, In step (3), the concentration of hydrochloric acid is 0.4~0.6 mol / L; the mass ratio of the soil sample to the volume of hydrochloric acid is 1 g: (28~32) mL.

7. The method according to claim 1, characterized in that, In step (3), when the mass of the soil sample is ≤5mg, the amount of water used is 20μL; when 5mg < the mass of the soil sample <10mg, the amount of water used is 40μL.

8. The method according to claim 1, characterized in that, The time spent in the concentrated hydrochloric acid atmosphere in step (3) is 22~25h.

9. The method according to claim 1, characterized in that, The conditions for the multi-purpose online gas preparation and introduction device in step (4) of the stable isotope ratio mass spectrometer include: column temperature 48~52℃, carrier gas flow rate 95~105mL / min; the conditions for the stable isotope ratio mass spectrometer include: ionization method is electron bombardment, ion source high voltage 9~10kV.

10. The method according to claim 1, characterized in that, The conditions of the elemental analyzer in step (4) of the elemental analyzer-stable isotope ratio mass spectrometer include: reaction tube temperature 950~1000℃, column temperature 58~62℃, carrier gas flow rate 170~190mL / min, reference gas flow rate 48~52mL / min, oxygen flow rate 240~260mL / min, and oxygen passage time 4~6s.

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