Method for measuring carbon isotope of low-concentration soluble organic carbon in water body

Abstract

The invention discloses a method for determining carbon isotopes of low-concentration soluble organic carbon in a water body, and belongs to the technical field of organic carbon detection. The determination method comprises the following steps: combining a TOC analyzer and a stable isotope mass spectrometer, and converting DOC in a water body into CO2 gas by adopting the TOC analyzer; cO2 gas generated by the TOC analyzer is enriched for the first time through the cold trap T1; the CO2 gas frozen by the cold trap T1 is enriched for the second time through a cold trap T2; introducing the CO2 subjected to secondary enrichment into a stable isotope mass spectrometer, and determining the carbon isotope composition of the CO2. Aiming at a low-concentration DOC water body, the carbon isotope composition of DOC in the water body is determined on the basis of combination of a wet oxidation technology and stable isotope mass spectrometry detection, a wet oxidation method is used for increasing the volume of a water sample to generate more CO2 for stable isotope mass spectrometry determination, so that more accurate carbon isotope composition is obtained, and the method is efficient, time-saving, labor-saving and accurate.

Description

Technical Field

[0001] The present invention belongs to the technical field of organic carbon detection, and in particular relates to a method for determining carbon isotopes of low-concentration dissolved organic carbon in water. Background Art

[0002] Dissolved organic carbon (DOC) is a crucial component of the global carbon cycle. The ocean's DOC carbon pool holds 662 Pg·C, comparable to the global atmospheric CO2 carbon pool (approximately 870 Pg·C at a CO2 concentration of 419 ppm). Global riverine DOC flux to the ocean is 0.24 Pg·C / year, playing a crucial role in maintaining marine primary productivity and supporting marine biodiversity. Therefore, understanding the sources and fluxes of DOC in rivers and the ocean is crucial for understanding the global carbon cycle and developing proactive responses to global climate change.

[0003] Dissolved organic carbon carbon isotopes (δ 13 C-DOC) plays a key role in solving the above problems, because DOC from different sources often has different δ 13 C-DOC composition, such as DOC produced by plant litter decomposition and soil leaching, has a δ 13 The C-DOC value inherits the carbon isotope characteristics of terrestrial vegetation (δ 13 C-DOC=-27‰±5‰); algae photosynthesis increased, water δ 13 The C-DOC value is close to -25‰; the microbial metabolism consumes DOC, resulting in a decrease in DOC concentration, and the remaining water δ 13 The C-DOC value gradually becomes positive. These mechanisms can be 13 Quantification of C-DOC value.

[0004] Realize water body δ 13 The key steps in C-DOC measurement are mainly divided into two parts: the first is to convert DOC into CO2 gas, and the second is to select an appropriate method to introduce the generated CO2 into a stable isotope mass spectrometer for measurement. The two are organically combined and neither can be missing. It should be noted that dissolved carbon in water bodies comes in the form of dissolved inorganic carbon (DIC) and dissolved organic carbon. Therefore, DIC needs to be removed before converting DOC to CO2. The common method for DIC removal is to add phosphoric acid, which reacts with DIC to produce CO2, and then use a purge method to remove the dissolved CO2.

[0005] Currently, methods for converting DOC to CO2 include wet oxidation and high-temperature combustion, each with its own advantages and disadvantages. The wet oxidation method involves heating water at around 100°C in the presence of an oxidant, such as sodium persulfate, to produce CO2. Therefore, specialized equipment is not required, and CO2 can be produced manually offline. For example, the oxidant and water sample are placed in a headspace vial, placed in an oil bath, sand bath, or water bath, and heated at 100°C for 2 hours. This method has no strict requirements for water volume; even approximately 40 mL of water can be oxidized to produce sufficient CO2, making it suitable for water samples with a wide range of DOC concentrations. The high-temperature combustion method involves injecting water into a reactor and then burning it at around 650°C with oxygen to produce CO2. While this method offers higher oxidation efficiency, it imposes strict requirements on water volume. The actual volume of water sampled for combustion in the reactor is typically in the microliter range. Furthermore, high salinity in samples can reduce the oxidation efficiency of the oxidant. This method limits the water sample volume, making it difficult to generate sufficient CO2 for carbon isotope determination using a stable isotope mass spectrometer for low-concentration DOC samples.

[0006] In summary, while the wet oxidation method has no strict requirements for water sample volume, its oxidation efficiency may not be as high as the high-temperature combustion method. The high-temperature combustion method has high oxidation efficiency but has strict requirements for water sample volume. Currently, complete sets of laboratory TOC analyzers based on both principles are available for sale, such as the Aurora 1030 (wet oxidation method, 10ppb-30,000ppm) and the Shimadzu TOC-L series (high-temperature combustion method, 4ppb-30,000ppm). Both have a wide detection range for DOC concentrations in water and low detection limits.

[0007] In the prior art, there are two types of stable isotope mass spectrometers for carbon isotope determination by introducing CO2 gas into a stable isotope mass spectrometer, namely, an isotope ratio stable isotope mass spectrometer and a laser spectrometer. Representative products of the isotope ratio stable isotope mass spectrometer include the Thermo Delta and MAT253plus series, and the Elementar Isoprime series, and representative products of the laser spectrometer include the Picarro G2201-i series. Although the principles are different, the above instruments can all obtain accurate carbon isotope composition of CO2 gas. The main difference between the two is that the isotope ratio stable isotope mass spectrometer requires pure CO2 gas (0.5-2mL), while the laser spectrometer can measure CO2 mixed gas (continuous injection for 5min, requiring at least 50mL of mixed gas). Summary of the Invention

[0008] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a method for determining the carbon isotope composition of low-concentration dissolved organic carbon in water bodies, which is based on wet oxidation technology combined with stable isotope mass spectrometry detection to determine the carbon isotope composition of low-concentration DOC in water bodies. For low-concentration DOC water samples, the wet oxidation method is used to increase the volume of the water sample to produce more CO2, thereby meeting the amount of CO2 required for the stable isotope mass spectrometer measurement and obtaining a more accurate carbon isotope composition.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] The present invention provides a method for determining carbon isotopes of low-concentration dissolved organic carbon in water, comprising the following steps:

[0011] (1) Use TOC analyzer to convert DOC in water into CO2 gas;

[0012] (2) The CO2 gas generated by the TOC analyzer in step (1) is first enriched by the cold trap T1;

[0013] (3) The CO2 gas frozen in the cold trap T1 in step (2) is enriched for the second time by the cold trap T2;

[0014] (4) The CO2 secondary enriched in step (3) is introduced into a stable isotope mass spectrometer to determine its carbon isotope composition.

[0015] Preferably, in step (1), the water DOC is converted into CO2 gas using a complete set of TOC analyzers, model Aurora 1030W.

[0016] Preferably, in step (1), a phosphoric acid solution with a mass concentration of 10% reacts with DIC in water, and the generated CO2 gas is purged by nitrogen. DOC in water reacts with sodium persulfate at 100°C, and the generated CO2 gas is carried by nitrogen. After passing through a water trap, reduced copper and a desiccant, the CO2 gas enters a non-dispersive infrared detector to detect its CO2 concentration. The water body is a low-concentration DOC water body, and the minimum detected DOC concentration is 10 ppb.

[0017] Preferably, in step (2), the cold trap T1 is controlled to be in the load mode, and the nitrogen carries the CO2 gas, enters the six-way valve, and then enters the cold trap T1 placed in liquid nitrogen, the CO2 gas is completely frozen, and the nitrogen is exhausted; after the generated CO2 gas is released, it is all captured and enriched by the cold trap T1, and the six-way valve is connected to another helium gas to purge the cold trap T2. At this time, the cold trap T2 is in the load mode and is not placed in liquid nitrogen, and the helium from the cold trap T1 is exhausted.

[0018] Preferably, in step (3), in the injection mode, the cold trap T1 is raised and separated from the liquid nitrogen, and the CO2 immediately turns into gas. The helium gas begins to purge the CO2 in the cold trap T1 and enters the cold trap T2; when the cold trap T1 is ready to be raised and the mode is changed from Load to Inject, the cold trap T2 is lowered into the liquid nitrogen in advance, and the mode is still in Load mode. The CO2 gas in the cold trap T1 is carried by the helium gas into the cold trap T2, achieving secondary enrichment, and the remaining nitrogen in the cold trap T1 is completely purged.

[0019] Preferably, in step (4), the cold trap T2 is switched to the Inject mode, and the helium gas is used to purge all the CO2 gas in the cold trap T2 into the stable isotope mass spectrometer. The helium gas carries the CO2, which is separated by GC and dehydrated by Nafion, and then directly introduced into the stable isotope mass spectrometer to determine its carbon isotope value.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The present invention targets low-concentration DOC water bodies and determines the carbon isotope composition of low-concentration dissolved organic carbon in the water body based on wet oxidation technology combined with stable isotope mass spectrometry detection. The high-temperature combustion method is difficult to increase the water sample volume to meet the CO2 amount required by the stable isotope mass spectrometer. The wet oxidation method is used to increase the water sample volume to produce more CO2 for stable isotope mass spectrometry measurement, thereby obtaining a more accurate carbon isotope composition.

[0022] 2. A complete set of TOC analyzers is preferred, and the traditional manual offline CO2 production method and method is abandoned, which has the problems of air leakage and unclean DIC purge, thus affecting the determination of DOC carbon isotope composition. The CO2 produced by the TOC analyzer is collected and purified, and then introduced into the existing complete set of isotope stable isotope mass spectrometer to determine the carbon isotope composition, which is efficient, time-saving, labor-saving and accurate.

[0023] 3. By building a CO2 collection and purification system, the existing TOC analyzer is connected with the stable isotope mass spectrometer to achieve accurate determination of carbon isotopes in low-concentration DOC samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the carbon isotope determination process of low-concentration DOC samples in the examples.

[0025] Figure 2 Schematic diagram of the working process of the cold trap T1 and the six-way valve in the embodiment.

[0026] Figure 3 Schematic diagram of the working process of the cold trap T2 and the eight-way valve in the embodiment.

[0027] Figure 4It is the measurement result of potassium hydrogen phthalate standard solution in the embodiment. DETAILED DESCRIPTION

[0028] In order to more fully understand and demonstrate the technical solutions, objectives, and advantages of the present invention, the following is a further detailed and complete description of the technical effects produced by the present invention in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present invention, and not all of them. It should be pointed out that for those skilled in the art, other embodiments obtained without departing from the concept of the present invention are all within the scope of protection of the present invention.

[0029] like Figure 1 As shown, a method for determining carbon isotopes of low-concentration dissolved organic carbon in water is exemplarily proposed, comprising the following steps:

[0030] (1) Water DOC is converted into CO2 gas

[0031] The method of converting water DOC into CO2 gas uses a complete set of TOC analyzers, model Aurora1030W, such as Figure 1 As shown in part (I), a phosphoric acid solution (10%) reacts with aqueous DIC, producing CO2 that is purged with nitrogen. Then, through the reaction of aqueous DOC with sodium persulfate (at approximately 100°C), the generated CO2 is carried by the nitrogen and passes through a water trap, reduced copper, and a desiccant before entering a non-dispersive infrared detector (NDIR) for CO2 concentration measurement.

[0032] (2) First enrichment of CO2 gas with the help of cold trap T1

[0033] The CO2 gas produced by the TOC analyzer is first enriched with the help of cold trap T1. Figure 1 As shown in section (II) in the figure. Cold trap T1 is controlled to be in Load mode. Nitrogen carrying CO2 enters the six-way valve and then enters cold trap T1, which is placed in liquid nitrogen. The CO2 gas is completely frozen and the nitrogen is evacuated. The CO2 gas generated by the TOC analyzer is completely released (approximately 2 minutes) and is completely captured by cold trap T1, which enriches it. The six-way valve is connected to another gas line (helium), preparing to purge the first enriched CO2 gas to cold trap T2. At this time, cold trap T2 is also in Load mode, but is not placed in liquid nitrogen, and the helium gas from cold trap T1 is also evacuated.

[0034] (3) Second enrichment of CO2 gas with the help of cold trap T2

[0035] The CO2 gas frozen in the cold trap T1 is enriched for the second time, such as Figure 1As shown in Section (III). In Inject mode, cold trap T1 rises and detaches from the liquid nitrogen, and the CO2 immediately turns into gas. Helium (He) begins to purge the CO2 from cold trap T1 and into cold trap T2. When cold trap T1 is about to be raised and the mode is changed from Load to Inject, cold trap T2 must be lowered into the liquid nitrogen in advance. The mode remains in Load mode. The CO2 gas in cold trap T1 is carried by the helium into cold trap T2 for secondary enrichment. At this time, the remaining nitrogen in cold trap T1 is completely purged.

[0036] (4) DOC carbon isotope determination

[0037] The secondary enriched CO2 can be introduced into a stable isotope mass spectrometer to determine its carbon isotope composition, such as Figure 1 (IV) Switching cold trap T2 to Inject mode allows helium to purge all CO2 from cold trap T2 into the stable isotope mass spectrometer. Using the Gasbench II multi-purpose gas extraction device, the helium carries the CO2, which, after GC separation and Nafion dehydration, is directly introduced into the stable isotope mass spectrometer to determine its carbon isotope composition.

[0038] Through the above method, the carbon isotope of DOC can be stably measured.

[0039] The working process of the six-way valve in the cold trap T1 is as follows Figure 2 As shown, the six-way valve can be divided into Load mode and Inject mode when working. The CO2 generated by the TOC analyzer enters position 1 of the six-way valve together with N2 (carrier gas). Positions 2 and 5 of the six-way valve are connected by a 1 / 16-inch stainless steel to form a quantitative loop with a volume of about 2 mL (which can be customized to meet the freezing requirements of the liquid nitrogen cup).

[0040] In load mode, the CO2 and N2 mixture enters from position 1, enters the quantitative loop from position 2, then enters position 5, and is finally exhausted from position 6. At this time, the liquid nitrogen cup is inserted to freeze all the CO2 passing through the quantitative loop, achieving enrichment, while some N2 still exists in the quantitative loop. At the same time, helium is provided by a cylinder, entering from position 3, exiting from position 4, and entering port 1 of the eight-way valve. This ensures that the eight-way valve in cold trap T2 is always purged with helium, minimizing the impact of air.

[0041] In Inject mode, the CO2 and N2 produced by the TOC analyzer no longer enter the quantitative loop. Instead, they enter from position 1 and are directly exhausted from position 6. At the same time, helium enters from position 3 and enters the quantitative loop through position 2. When the liquid nitrogen cup is removed (cold trap T1 is removed from the liquid nitrogen cup), the helium can carry the CO2 and some N2 forward, passing through position 5 and entering position 4, thereby smoothly entering port 1 of the eight-way valve. It can then undergo secondary purification through the eight-way valve and ultimately be introduced into the stable isotope mass spectrometer.

[0042] The working process of the eight-way valve in the cold trap T2 is as follows Figure 3 As shown, the purpose of the eight-way valve is to achieve secondary purification of CO2 and completely purge the N2 introduced by the six-way valve (in some embodiments, another six-way valve can also be used to achieve this purpose). The purpose of using the eight-way valve is to allow the generated CO2 gas to be introduced into the stable isotope mass spectrometer for carbon isotope determination using the Gasbench peripheral device of the stable isotope mass spectrometer. The standard configuration of this device is a pneumatic eight-way valve. The original quantitative loop volume is only 100μL. The present invention replaces it with a cold trap T2 that can be enriched with liquid nitrogen.

[0043] In load mode, the CO2 and N2 mixture from the six-port valve, along with the helium, enters position 1 of the eight-port valve, passes through position 2 to the quantitative loop, then enters the eight-port valve from position 5, and is finally exhausted from position 6. This allows the complete secondary enrichment of the CO2 from the six-port valve, while the N2 is purged by the helium, completely eliminating its impact on the stable isotope mass spectrometer. Simultaneously, another helium stream enters the eight-port valve from position 3, enters the chromatographic column through position 4, and ultimately enters the stable isotope mass spectrometer, preemptively purging the air from the line.

[0044] In Inject mode, liquid nitrogen is removed, and another helium gas enters from position 3, enters the quantitative loop from position 2, and the enriched CO2 gas enters the eight-way valve through position 5. Then, it enters the chromatographic column and water removal membrane from position 4, and finally enters the stable isotope mass spectrometer to achieve accurate carbon isotope determination. At this time, the gas from the eight-way valve enters from position 1 and is exhausted from position 8, which does not affect the normal operation of the eight-way valve.

[0045] Example 1

[0046] (1) Preparation of DOC carbon isotope standard solution

[0047] Common high-grade pure chemical reagents for preparing DOC standard solution include potassium hydrogen phthalate, which has a solubility of 25g / 100mL in pure water at 25℃. The carbon isotopes of the chemical reagents used to prepare DOC solution are determined by elemental analyzer + stable isotope mass spectrometry. The standard material uses the international standard material IAEA-600 (δ 13C=-27.771‰). The calibrated laboratory DOC standard carbon isotope composition is potassium hydrogen phthalate (δ 13 C = -29.1‰).

[0048] The above-mentioned high-grade pure chemical reagents were dissolved in ultrapure water to prepare 1000 mg / L DOC stock solution, and then diluted to DOC gradient solutions of 0.25 mg·C / L, 0.5 mg·C / L, 1.0 mg·C / L, 2.0 mg·C / L, 3.0 mg·C / L, 4.0 mg·C / L, and 5.0 mg·C / L.

[0049] (2) Measured results

[0050] The measured effect is as follows Figure 4 As shown, the initial operating conditions of the TOC analyzer are as follows:

[0051] The concentration of phosphoric acid solution is 10% (v / v);

[0052] The concentration of sodium persulfate solution is 10% (m / v);

[0053] Injection volume: 1 mL;

[0054] DOC reaction time: 4 minutes and 5 seconds;

[0055] DOC reaction temperature: 100°C;

[0056] The residence time of cold trap No. 1 in liquid nitrogen is 2 minutes;

[0057] The No. 2 cold trap stays in liquid nitrogen for about 2 minutes.

[0058] result

[0059] (1) Different DOC concentrations showed an excellent linear relationship with the CO2 signal value generated by the stable isotope mass spectrometer, indicating that all DOC in the solution was oxidized to CO2 gas and all of it was introduced into the stable isotope mass spectrometer for detection ( Figure 4 a).

[0060] (2) The background CO2 signal value generated by the reaction of pure water with phosphoric acid + sodium persulfate is stable at around 1500 mV, and the standard deviation of its carbon isotope value is stable within 0.3‰.

[0061] (3) A good CO2 background concentration is conducive to better deriving the carbon isotope composition of the sample to be tested. The isotope value of the gas to be tested is obtained according to the two-source mixing model. The model is shown in formula (1):

[0062] δ 13 C sample =(δ 13 Cmeasure ×M measure -δ 13 C blank ×M blank ) / (M measure -M blank ) (1)

[0063] According to the above formula, in the accurate determination of δ 13 C blank In the case of 13 C measure , the isotope value of the sample to be tested can be accurately calculated.

[0064] In summary, the present invention builds a CO2 collection and purification system, connects the existing TOC analyzer with the stable isotope mass spectrometer, and realizes the accurate determination of carbon isotopes of low-concentration DOC samples, which has application value in China.

[0065] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for determining carbon isotopes of low-concentration dissolved organic carbon in water, characterized in that: The TOC analyzer is coupled with a stable isotope mass spectrometer, which includes the following steps: (1) Use TOC analyzer to convert DOC in water into CO2 gas; (2) The CO2 gas generated by the TOC analyzer in step (1) is first enriched by the cold trap T1; (3) The CO2 gas frozen in the cold trap T1 in step (2) is enriched for the second time by the cold trap T2; (4) The CO2 secondary enriched in step (3) is introduced into a stable isotope mass spectrometer to determine its carbon isotope composition.

2. The method for determining carbon isotopes of low-concentration dissolved organic carbon in water according to claim 1, characterized in that: In step (1), the water DOC is converted into CO2 gas using a complete set of TOC analyzers, model Aurora 1030W.

3. The method for determining carbon isotopes of low-concentration dissolved organic carbon in water according to claim 1, characterized in that: In step (1), a phosphoric acid solution with a mass concentration of 10% reacts with DIC in water, and the generated CO2 gas is purged by nitrogen. DOC in water reacts with sodium persulfate at 100°C, and the generated CO2 gas is carried by nitrogen. After passing through a water trap, reduced copper, and a desiccant, it enters a non-dispersive infrared detector to detect its CO2 concentration. The water body is a low-concentration DOC water body, and the minimum detected DOC concentration is 10 ppb.

4. The method for determining carbon isotopes of low-concentration dissolved organic carbon in water according to claim 1, characterized in that: In step (2), the cold trap T1 is controlled to be in the Load mode, and the nitrogen carries the CO2 gas, enters the six-way valve, and then enters the cold trap T1 placed in liquid nitrogen. The CO2 gas is completely frozen and the nitrogen is exhausted. After the generated CO2 gas is released, it is all captured and enriched by the cold trap T1. The six-way valve is connected to another helium gas to directly purge the cold trap T2. At this time, the cold trap T2 is in the Load mode and is not placed in liquid nitrogen. The helium from the cold trap T1 is exhausted.

5. The method for determining carbon isotopes of low-concentration dissolved organic carbon in water according to claim 1, characterized in that: In step (3), in the Inject mode, the cold trap T1 is raised and separated from the liquid nitrogen, and the CO2 immediately turns into gas. The helium gas begins to purge the CO2 in the cold trap T1 and enters the cold trap T2. When the cold trap T1 is ready to be raised and the mode is changed from Load mode to Inject mode, the cold trap T2 is lowered into the liquid nitrogen in advance, and it is still in Load mode. The CO2 gas in the cold trap T1 is carried by the helium gas into the cold trap T2, achieving secondary enrichment, and the remaining nitrogen in the cold trap T1 is completely purged.

6. The method for determining carbon isotopes of low-concentration dissolved organic carbon in water according to claim 1, characterized in that: In step (4), the cold trap T2 is switched to the Inject mode, and the helium gas is used to purge all the CO2 gas in the cold trap T2 into the stable isotope mass spectrometer. The helium gas carries the CO2, which is separated by GC and dehydrated by Nafion, and then directly introduced into the stable isotope mass spectrometer to determine its carbon isotope value.

7. The method for determining carbon isotopes of low-concentration dissolved organic carbon in water according to claim 1, characterized in that: The six-way valve in the cold trap T1 is divided into load mode and inject mode when working. The CO2 generated by the TOC analyzer enters the six-way valve position 1 together with the N2. The six-way valve positions 2 and 5 are connected by a 1 / 16-inch stainless steel to form a quantitative loop with a volume of 2 mL. When the six-way valve is in load mode, the CO2 and N2 mixture enters from position 1, enters the quantitative loop from position 2, then enters position 5, and is finally exhausted from position 6. At this time, a liquid nitrogen cup is inserted to freeze all the CO2 passing through the quantitative loop to achieve enrichment; at the same time, helium is provided by a cylinder, enters from position 3, exits from position 4, and enters the cold trap T2, which is constantly purged with helium to minimize the impact of air; When the six-way valve is in Inject mode, the CO2 and N2 generated by the TOC analyzer do not enter the quantitative loop. Instead, they enter through position 1 and are directly exhausted through position 6. Meanwhile, helium enters through position 3 and enters the quantitative loop through position 2. When the liquid nitrogen protection of cold trap T1 is removed, the helium carries the CO2 and some N2 forward, passing through position 5, entering position 4, and then entering cold trap T2.

8. The method for determining carbon isotopes of low-concentration dissolved organic carbon in water according to claim 1, characterized in that: The eight-way valve in the cold trap T2 is divided into load mode and inject mode when working, which is used to achieve secondary purification of CO2 and completely purge part of the N2 brought by the six-way valve; When the eight-way valve is in the load mode, the CO2 and N2 mixed gas from the six-way valve enters position 1 of the eight-way valve together with the helium, enters the quantitative loop through position 2, then enters the eight-way valve from position 5, and is finally exhausted from position 6. All CO2 is secondary enriched, and the N2 is purged by the helium. At the same time, another line of helium enters the eight-way valve from position 3, enters the chromatographic column through position 4, and finally enters the stable isotope mass spectrometer to purge the air in the pipeline. When the eight-way valve is in the Inject mode, the liquid nitrogen is removed, and another line of helium enters from position 3, enters the quantitative loop from position 2, and the enriched CO2 gas enters the eight-way valve through position 5, and then enters the chromatographic column and water removal membrane from position 4, and finally enters the stable isotope mass spectrometer to achieve accurate determination of carbon isotopes. At this time, the gas in the eight-way valve enters from position 1 and is discharged from position 8.

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