Carbon isotope content testing method for low-concentration dissolved organic carbon water sample and cold trap sample introduction front end

Abstract

The invention discloses a method for testing the content of carbon isotopes in a low-concentration dissolved organic carbon water sample and a cold trap sample introduction front end, and belongs to the technical field of testing of the content of organic carbon in a water body, and the method comprises the following steps: (1) carrying out sample collection on a low-concentration water body, and carrying out ultrafiltration concentration on the collected water body sample to increase the DOC concentration in the concentrated water body, a concentrated water body sample is obtained; (2) absorbing the concentrated water body sample by using kaolin, and repeatedly coating the dried kaolin sample for multiple times to obtain a plurality of small kaolin samples with the same weight; (3) calculating the number of times of combustion sample introduction according to the CO2 concentration of the sample in the pre-test, collecting the sample gas for multiple times by using a cold trap sample introduction front end, and testing the carbon isotope of the sample; and (4) calibrating the test data of the carbon isotope, determining the delta 13CDOC in the water body, and completing the test. The method can solve the problems of high test cost, poor test accuracy and low test efficiency when the delta 13CDOC in the low-concentration water body is measured in the prior art.

Description

Technical Field

[0001] This invention belongs to the field of water organic carbon content testing technology, specifically relating to a method for testing the carbon isotope content of water samples with low concentration of dissolved organic carbon and a cold trap sample introduction front end. Background Technology

[0002] Lakes, rivers, and other water bodies are crucial components of the global carbon cycle, primarily serving to store carbon generated in land and water bodies in sediments and to continuously release greenhouse gases such as CO2 into the atmosphere. Dissolved organic carbon (DOC) is a significant component of the aquatic carbon pool, and its stable carbon isotope ratio (δ¹⁸O) is crucial for its stability. 13 C) It has important scientific research value in the study of low-concentration water bodies.

[0003] Organic carbon in natural water bodies (such as lakes, ponds, and reservoirs) mainly originates from terrestrial vegetation, aquatic biological activity, and human activities. By comparing the δ¹⁸O₂ values ​​of different water bodies and different time periods... 13 C DOC Changes can effectively track the rate, extent, and transformation pathways of organic carbon degradation, and thereby assess the self-purification capacity and carbon cycle rate of water bodies. δ 13 C DOC It is also an important tool for tracing pollution sources. For example, organic carbon produced by fossil combustion has a significantly lighter δ-carbon content. 13 The C characteristic can be used to detect and identify man-made pollution such as industrial wastewater and oil spills. This is achieved by analyzing the δ-ray concentration in natural water bodies. 13 C DOC Long-term monitoring can help identify hidden sources of pollution, providing precise direction for environmental remediation. (Water body δ) 13 C DOC The determination of δ can help researchers calibrate carbon cycle models. By analyzing δ... 13 C DOC It can indicate the interconversion between various forms of carbon in water bodies, enabling a deeper understanding of the processes and mechanisms of carbon sinks, helping to estimate carbon fluxes from different sources and their contribution to the carbon cycle, thereby improving our understanding of regional differences in the global carbon cycle and providing support for addressing climate change.

[0004] Currently commonly used δ 13 C DOC The detection methods include wet oxidation using a GasBench II gas isotope ratio mass spectrometer and TOC-CRDS using a total organic carbon analyzer and cavity ring-down spectrometer. Wet oxidation requires a high concentration of oxidant in the collected water sample; however, post-oxidation combustion may produce incomplete oxidation, leading to delta-carbon emissions. 13 C DOCThe results tend to favor the lighter values. Furthermore, the water sample pretreatment process is cumbersome, requiring a series of steps including filtration, oxidation, acidification, purging, reaction, and centrifugation. The testing and conversion process is time-consuming and labor-intensive, and can easily lead to the decomposition of DOC in natural water bodies, resulting in lower concentrations of δ-carboxylic acid (DOC). 13 C DOC The accuracy of the measurements is relatively low. Finally, the high cost of multi-purpose gas sample preparation devices makes them difficult for most research institutions to prepare, increasing testing costs. The TOC-CRDS technology requires a Picaro G2201 low-range extension front-end to allow the instrument to measure CO2 concentrations between 200-380 ppm. Furthermore, this method allows direct injection of freshwater samples with DOC concentrations ≥2.5 mg / L, but for lower concentrations, evaporation and solid-phase extraction are necessary for concentration. Therefore, the DOC content of the water must be measured before the experiment, and due to the limited instrument volume, the total sample volume must be ≤30 ml. Secondly, phosphoric acid is used to acidify the water sample; however, incomplete acidification may occur for water with high impurity levels. Further increasing the reagent concentration may clog the instrument lines and damage the instrument. Finally, according to the measurement results, the testing time for each sample is approximately 45 minutes, which is relatively long.

[0005] Therefore, it is necessary to construct a low-cost, low-concentration δ-ray distillation method for water bodies. 13 C DOC The measurement techniques and methods are of great significance. Summary of the Invention

[0006] The purpose of this invention is to provide a method for testing the carbon isotope content of water samples with low concentrations of dissolved organic carbon, and a cold trap sample introduction front end, thereby solving the problem of existing techniques for determining the carbon isotope content of low concentrations of dissolved organic carbon in water. 13 C DOC The problems of high testing costs, poor testing accuracy, and low testing efficiency.

[0007] A method for determining the carbon isotope content in a low-concentration dissolved organic carbon aqueous sample, comprising the following steps: (1) Collect samples of low-concentration water bodies, and concentrate the collected water samples by ultrafiltration to increase the DOC concentration in the concentrated water body to obtain concentrated water samples. (2) Use kaolin to absorb the concentrated water sample after step (1), and repeatedly package the dried kaolin sample to obtain multiple small kaolin samples of the same weight for subsequent isotope determination. (3) The packaged single kaolin sample is sent into the elemental analyzer for high-temperature combustion. The gas after combustion is sent into the carbon isotope analyzer to pre-test the CO2 concentration of the single kaolin sample and record the maximum CO2 concentration. It is determined whether the CO2 concentration is within the optimal detection range of the carbon isotope analyzer. If the maximum CO2 concentration is within the optimal detection range of the carbon isotope analyzer, proceed directly to step (4). If the maximum CO2 concentration is greater than the upper limit of the optimal detection range of the carbon isotope analyzer, return to step (2) and reduce the weight of the packaged sample. If the maximum CO2 concentration is less than the lower limit of the optimal detection range of the carbon isotope analyzer, calculate the number of times the kaolin sample is burned and injected. Based on the calculated number of times the kaolin sample is injected, the sample is burned multiple times into the elemental analyzer and the sample gas is collected using the cold trap injection front end until the CO2 concentration of the sample reaches the optimal detection range of the carbon isotope analyzer. Then the gas is sent into the carbon isotope analyzer for carbon isotope testing of the sample. (4) The carbon isotope test data obtained in step (3) are calibrated to determine the δ-carbon content in the water. 13 C DOC Complete the test.

[0008] Furthermore, step (1) specifically includes: (1.1) Sample collection: Low-concentration water samples were collected and filtered using an all-glass vacuum filtration device. 4-5 L of water samples were filtered using a 0.7 μm sterile filter membrane and stored in a 4 ℃ refrigerator. (1.2) Acidification treatment: The filtered water sample was acidified with 2 mol / L hydrochloric acid to remove DIC from the water, and then heated at 60 °C for more than 2 h. (1.3) Water concentration: A peristaltic pump was used to concentrate the acidified water sample by ultrafiltration. The sample was then concentrated to 10 ml using a tangential flow PES ultrafiltration membrane.

[0009] Furthermore, step (2) specifically includes: (2.1) Sample adsorption: Kaolin was used to absorb the concentrated water sample, and the fully absorbed kaolin was placed in an oven for low-temperature drying at 60 ℃ for 4 h. (2.2) Sample packaging: The dried kaolin samples were repeatedly packaged using a 1 / 1,000,000 balance and an ultra-clean tin bag, with each kaolin sample weighing 10 mg.

[0010] Furthermore, step (4) specifically involves: (4.1) Correction of raw data: Determining element δ using standard substances 13 The measured value of C is used as a benchmark to calibrate the δ of water samples. 13 C DOC The standard substances are GBW04494, GBW04495, and GBW04497. For every 20 kaolin samples, a standard sample of 9 standard substances is required for calibration. Each of the three standard substances is repeated 3 times. (4.2) Data Calculation The average value of the three sets of standard substances is calculated using the AVERAGE function, and the standard deviation is calculated using the STDEV function. The corresponding slope, error, and correlation coefficient are calculated using the SLOPE, INTERCEPT, and CORREL functions, ensuring that the correlation coefficient is greater than 0.999. The measured carbon isotope data are calibrated using the formula "calibration value = slope * measured value + error".

[0011] A cold trap sample inlet for testing the carbon isotope content of a low-concentration dissolved organic carbon water sample includes a liquid nitrogen condensation device and a drying tube connected to the exhaust port of an elemental analyzer. The drying tube is connected to a first branch tube, with a three-way valve (first) installed in the middle of the first branch tube. A three-way valve (third) is installed at the end of the first branch tube furthest from the drying tube. The first and third ports of the three-way valve (first) are respectively connected to the first branch tube. The second port of the three-way valve (first) is connected to a second branch tube, and the third port of the three-way valve (third) is connected to the first branch tube. The second port of the three-way valve three is connected to the inlet end of the liquid nitrogen cooling gas collection pipe of the liquid nitrogen condensation device, and the first port of the three-way valve three is connected to a compressed nitrogen source; a third branch pipe is also provided, one end of which is connected to the inlet of the carbon isotope analyzer, and the other end of which is connected to the outlet end of the liquid nitrogen cooling gas collection pipe of the liquid nitrogen condensation device. A three-way valve two is provided in the middle of the third branch pipe, the first port of the three-way valve two is connected to the second branch pipe, and the third port and the second port of the three-way valve two are connected to the third branch pipe.

[0012] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. In this invention, by performing ultrafiltration concentration on a large number of water samples and employing multiple injections combined with gas collection at the cold trap injection front end, the concentration is brought to the standard required for conventional carbon isotope analyzers. This effectively solves the problem that conventional carbon isotope analyzers cannot measure δ-carbon dioxide (DOC) content in natural water samples due to their low DOC content. 13 The problem of CDOC. At the same time, the functions of conventional carbon isotope analyzers have been expanded, enabling them to determine carbon isotopes in water samples in addition to measuring carbon isotopes in gaseous and solid samples.

[0013] 2. This invention simplifies the existing methods for handling δ in water bodies. 13 C DOC The proposed method simplifies the water sample pretreatment process significantly, requiring only filtration and acidification. Furthermore, the use of an elemental analyzer for high-temperature oxidation ensures complete oxidation, greatly improving data accuracy. Simultaneously, the high temperature and small sample size reduce the combustion time, significantly shortening the measurement time for a single sample. Without requiring a low-range extension front-end device, multiple injections combined with cold trap gas collection at the front end increase the CO2 concentration of the sample gas, meeting the carbon isotope measurement requirements of conventional carbon isotope analyzers and reducing sample testing costs. Attached Figure Description

[0014] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort, wherein: Figure 1 This is a schematic diagram of the ultrafiltration concentration of water samples and the preparation of kaolin samples in this invention; Figure 2 δ in this invention 13 C DOC Schematic diagram of the measurement process; Figure 3 The image shows the test results of the pre-test sample of this invention. Figure 4 The figure shows the test results of the multi-injection gas collection instrument of this invention; Figure 5 For the sample δ of this invention 13 C DOC Diagram of the calibration process. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0016] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0017] Refer to the instruction manual. Figure 1-5 , A method for determining the carbon isotope content in a low-concentration dissolved organic carbon aqueous sample, comprising the following steps: (1) Samples of low-concentration water were collected and filtered using an all-glass vacuum filtration device. 4-5 L of water sample (depending on the cleanliness of the water sample) was filtered using a 0.7 μm sterile filter membrane (Haicheng Shijie, China). In this test, the filtered water sample was 5 L. The filtered water sample was stored in a 4℃ refrigerator. A 2 mol / L hydrochloric acid solution was prepared to acidify the filtered water sample to remove DIC. Approximately 75 ml of 2 mol / L hydrochloric acid was added to the 5 L water sample to completely eliminate DIC, and the sample was heated at 60℃ for at least 2 hours. If the water sample contained a large amount of impurities, the amount of acid used needed to eliminate DIC. A peristaltic pump (BT102S, Riefer, China) was used to concentrate the acidified water samples via ultrafiltration (16-gauge peristaltic pump tubing, 350 rpm). A tangential flow PES ultrafiltration membrane (Sartorius, Germany) was then used to concentrate the water sample to 10 ml. Through ultrafiltration, the water sample volume was reduced from the initial 5 L to 10 ml. This reduction to 1 / 500 of the original volume theoretically increases the sample concentration by 500 times. The water samples before and after concentration were analyzed using a total organic carbon (DOC) and total nitrogen (TN) analyzer. Before concentration, the DOC content in the water sample was 2.134 mg / L, while after concentration, the concentration was 1015.65 mg / L (this concentration exceeded the instrument's detection limit; the result was obtained after re-dilution).

[0018] (2) Kaolin was used to absorb the concentrated water sample after step (1), and the completely absorbed kaolin was placed in an oven for low-temperature drying (60℃, 4h). The dried kaolin sample was repeatedly packaged using a 1 / Million balance (Sartorius, Germany) and an ultra-clean tin bag. The weight of each kaolin sample was 10 mg for subsequent isotope determination. A total of 10 samples were packaged for this sample analysis.

[0019] (3) By pre-testing the CO2 concentration of a single kaolin sample, the number of combustion injections is calculated, and the sample gas is collected using multiple cold trap injection front ends to test the carbon isotopes of the sample.

[0020] Specifically, the packaged kaolin samples (10 mg / sample) were placed in an elemental analyzer (ECS4024, Italy) for high-temperature combustion (980°C). The combustion time for a single sample was approximately 6 minutes. The gas produced after combustion passed through a drying tube and then entered the cold trap sample inlet until the sample concentration reached the optimal detection range of the Picarro G2201-I high-precision carbon isotope analyzer (CO2 gas measurement range: 2000-4000 ppm).

[0021] The cold trap sample inlet includes a liquid nitrogen condensation device and a drying tube connected to the exhaust port of the elemental analyzer. The drying tube is connected to a first branch pipe. A three-way valve (VoV1) is installed in the middle of the first branch pipe, and a three-way valve (VoV3) is installed at the end of the first branch pipe furthest from the drying tube. The first port (farthest from the drying tube) and the third port (closest to the drying tube) of the three-way valve (VoV1) are respectively connected to the first branch pipe. The second port of the three-way valve (VoV1) is connected to a second branch pipe, and the third port of the three-way valve (VoV3) is connected to the first branch pipe. The second port of the three-way valve (VoV3) is connected to the liquid nitrogen cooling system of the liquid nitrogen condensation device. The inlet end of the gas collecting pipe is connected to a compressed nitrogen source at the first port of the three-way valve three. A third branch pipe is also provided, one end of which is connected to the inlet of the carbon isotope analyzer, and the other end of which is connected to the outlet end of the liquid nitrogen cooling gas collecting pipe of the liquid nitrogen condensation device. A three-way valve two is provided in the middle of the third branch pipe, the first port of which is connected to the second branch pipe, and the third port (the side away from the outlet end of the liquid nitrogen cooling gas collecting pipe) and the second port (the side closer to the outlet end of the liquid nitrogen cooling gas collecting pipe) of the three-way valve two are connected to the third branch pipe.

[0022] A single kaolin sample was subjected to a combustion pre-test. Three-way valve one was turned towards the second port, closing the first port; three-way valve two was turned towards the first port, closing the second port, thus forming gas path A. After combustion of the single sample, the gas directly entered the Picaro G2201-I high-precision carbon isotope analyzer. The maximum CO2 concentration was recorded to determine if the gas concentration was within the optimal testing range. The gas path is as follows: Figure 2 The red arrow indicates (gas path A). If the maximum CO2 concentration is between 2000-4000 ppm, proceed directly to step (4); if the CO2 concentration is less than 2000 ppm, proceed to operation B; if the CO2 concentration is greater than 4000 ppm, return to step (2) and reduce the weight of the packaged sample.

[0023] In this preliminary test, the maximum CO2 concentration in the sample gas was 703.257 ppm (e.g., Figure 3 (As shown).

[0024] Multiple injections were performed to collect the gas. Since the CO2 concentration was less than 2000 ppm, the number of injections required was calculated based on the pre-test results of a single kaolin sample combustion. The maximum CO2 concentration in the sample gas was 703.257 ppm. In this test, the kaolin sample was injected 5 times to bring the CO2 concentration to approximately 3500 ppm, thus bringing the concentration within the 2000-4000 ppm range. The liquid nitrogen condenser was placed in liquid nitrogen, and the temperature was allowed to drop to -80℃ before injection. The three-way valve was switched from one port to the first port and closed the second port; the three-way valve was switched from the third port to the second port and closed the first port; the three-way valve was switched from the second port to the second port and closed the first port, forming gas path B. The combusted gas was dried through a drying tube and then entered the liquid nitrogen condenser, where it became solid (due to the small gas volume, the pipeline would not become blocked). The gas path is as follows... Figure 2 The blue arrow indicates (Gas path B).

[0025] Multiple sample mixtures were analyzed. After five injections, the three-way valve was turned to the first port, and the compressed nitrogen source was opened. The liquid nitrogen was removed, and the temperature was allowed to rise. Once the temperature reached approximately -50°C, CO2 gas began to enter the Picaro G2201-I high-precision carbon isotope analyzer under the influence of nitrogen. The maximum CO2 gas concentration was recorded as 3519.568 ppm (e.g., ...). Figure 4 (As shown).

[0026] (4) The carbon isotope test data obtained in step (3) are calibrated to determine the δ-carbon content in the water. 13 C DOC Complete the test.

[0027] ① Original data correction Determining element δ using standard substances 13 The measured value of C is used as a benchmark to calibrate the δ of the sample. 13 C DOC The standard reference materials are: GBW04494, GBW04495, and GBW04497. For every 20 kaolin samples, standard samples using 9 of the standard reference materials are required for calibration, with each of the three standard reference materials being repeated 3 times. The δ values ​​obtained in this test 13 C DOC The original value was -13.534.

[0028] ② Data Calculation The average value of the three sets of standard substances is calculated using the "AVERAGE function," and the standard deviation is calculated using the "STDEV function." The corresponding slope, error, and correlation coefficient are calculated using the "SLOPE function," "INTERCEPT function," and "CORREL function," ensuring that the correlation coefficient is greater than 0.999 (under normal instrument use, the correlation coefficient will remain above 0.99; generally, a correlation coefficient greater than 0.999 is considered acceptable for experimental determination). The measured carbon isotope data are calibrated using the formula "calibration value = slope * measured value + error."

[0029] The δ values ​​obtained in this test 13 C DOC The correction value is -3.893 (e.g.) Figure 5 (As shown).

[0030] The above description constitutes an embodiment of the present invention. The foregoing descriptions are preferred embodiments of the present invention. Unless there is a clear contradiction or a prerequisite for a particular preferred embodiment, the preferred embodiments can be arbitrarily combined and used. The embodiments and specific parameters described are merely for clearly illustrating the verification process of the invention and are not intended to limit the scope of patent protection of the present invention. The scope of patent protection of the present invention is still determined by its claims. Similarly, any equivalent structural changes made based on the description and drawings of the present invention should also be included within the scope of protection of the present invention.

Claims

1. A method for testing the carbon isotope content of a low concentration dissolved organic carbon sample, characterized by, The method comprises the following steps: (1) collecting samples from low-concentration water bodies, concentrating the collected water samples by ultrafiltration to increase the DOC concentration in the concentrated water, and obtaining concentrated water samples; (2) using kaolin to absorb the concentrated water samples treated in step (1), repeatedly packaging the dried kaolin samples multiple times to obtain multiple kaolin samples of the same weight for subsequent isotope determination; (3) sending the packaged single kaolin sample into an elemental analyzer for high-temperature combustion, sending the combustion gas into a carbon isotope analyzer, pre-testing the CO2 concentration of the single kaolin sample and recording the maximum CO2 concentration, judging whether the CO2 concentration is within the optimal detection range of the carbon isotope analyzer, if the maximum CO2 concentration is within the optimal detection range of the carbon isotope analyzer, directly entering step (4); if the maximum CO2 concentration is greater than the upper limit of the optimal detection range of the carbon isotope analyzer, returning to step (2) and reducing the packaging weight; if the maximum CO2 concentration is less than the lower limit of the optimal detection range of the carbon isotope analyzer, calculating the combustion sample injection times of the kaolin sample, according to the calculated combustion sample injection times of the kaolin sample, repeatedly injecting and combusting the sample into the elemental analyzer and collecting the sample gas at the front end of the cold trap before injection, until the sample CO2 concentration reaches the optimal detection range of the carbon isotope analyzer, and then sending the gas into the carbon isotope analyzer for testing the carbon isotope of the sample; (4) calibrating the test data of carbon isotope obtained in step (3) to determine the δ 13 C DOC in the water body, and completing the test.

2. A method for testing the carbon isotope content of a low concentration dissolved organic carbon sample according to claim 1, characterized in that, In step (1), the following steps are specifically provided: (1.1) sample collection: Collecting samples from low-concentration water bodies, filtering the collected samples using a full-glass vacuum filtration device, filtering 4-5 L of water sample using a sterile filter membrane with a pore size of 0.7 μm, and storing the filtered water sample in a 4℃ refrigerator; (1.2) acidification treatment: Configuring hydrochloric acid with a specification of 2 mol / L to acidify the filtered water sample, removing DIC in the water body, and heating at 60℃ for more than 2 h; (1.3) water concentration: Ultrafiltrating and concentrating the acidified water sample using a peristaltic pump, ultrafiltering the sample using a tangential flow PES ultrafiltration membrane, concentrating the sample to 10 ml, and obtaining the concentrated water sample.

3. A method for testing the carbon isotope content of a low concentration dissolved organic carbon sample according to claim 1, characterized in that, Step (2) is specifically as follows: (2.1) sample adsorption: Using kaolin to absorb the concentrated water sample, and placing the completely absorbed kaolin in an oven for low-temperature drying, with a drying temperature of 60℃ and a drying time of 4 h; (2.2) sample packaging: Using a millionth balance and a super-clean tin capsule to repeatedly package the dried kaolin sample multiple times, and the weight of the kaolin sample is 10 mg per sample.

4. A method for testing the carbon isotope content of a low concentration dissolved organic carbon sample according to claim 1, characterized in that, Step (4) is specifically as follows: (4.1) original data correction: The measured value of the standard substance element δ 13 The measured value of C is used as a reference to calibrate the δ 13 C DOC The standard substance is GBW04494, GBW04495, and GBW04497. Each 20 kaolin samples are calibrated using 9 standard samples of the standard substance, and each of the three standard substances is repeated 3 times. (4.2) data calculation Calculating the average value of the three standard substances by the AVERAGE function, calculating the standard deviation by the STDEV function, calculating the corresponding slope, error and correlation coefficient by the SLOPE function, INTERCEPT function and CORREL function, ensuring that the correlation coefficient is greater than 0.999, and calibrating the measured carbon isotope data by "calibration value = slope * measured value + error".

5. A cold-trap inlet front end for implementing the method of testing the carbon isotope content of a low-concentration dissolved organic carbon sample according to any one of claims 1-4, characterized in that, The device comprises a liquid nitrogen condensing device, a drying tube communicated with an exhaust port of an elemental analyzer, a first branch pipe communicated with the drying tube, a three-way valve one installed in the middle of the first branch pipe, a three-way valve three installed at an end of the first branch pipe away from the drying tube, a second branch pipe communicated with a second interface of the three-way valve one, a third interface of the three-way valve three communicated with the first branch pipe, a first interface of the three-way valve three communicated with a liquid nitrogen cooling gas collecting pipe of the liquid nitrogen condensing device, and a compressed nitrogen source communicated with a second interface of the three-way valve three. The device further comprises a third branch pipe, one end of the third branch pipe communicated with an air inlet of a carbon isotope analyzer, the other end of the third branch pipe communicated with an air outlet of the liquid nitrogen cooling gas collecting pipe of the liquid nitrogen condensing device, and a three-way valve two installed in the middle of the third branch pipe, a first interface of the three-way valve two communicated with the second branch pipe, and a third interface and a second interface of the three-way valve two communicated with the third branch pipe.

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