Method for determining nitrogen isotope of nitrogen in gas sample

Abstract

The present invention discloses a nitrogen isotope determination method for nitrogen in a gas sample, and relates to the technical field of isotope analysis, the method comprises: instrument performance pretreatment: before sample determination, determining that an instrument is in a stable working state by checking the vacuum degree, detecting the interference peak, tuning an ion source, and testing the stability and linearity of reference gas; according to the method, a glass sample bottle is adopted to collect a gas sample, and before collection, a bidirectional inert gas replacement-vacuumizing coupling sampling process is adopted to treat the sample bottle. Residual air in the sample bottle and air attached to the bottle wall are effectively removed, and the sample pollution risk is remarkably reduced; the CO2 adsorption trap adopts a gradient filling structure and is matched with a constant-temperature heating sleeve, so that the contact time of CO2 and an adsorption material is prolonged, meanwhile, the influence of adsorption heat release on the adsorption efficiency is avoided, and the CO2 removal rate is improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of isotope analysis, and particularly relates to a nitrogen isotope determination method for nitrogen in a gas sample. BACKGROUND

[0002] A stable isotope ratio mass spectrometer is a high-precision analysis instrument and plays an important role in scientific research. The stable isotope ratio mass spectrometer can accurately measure the relative abundance of different isotopes in a sample and provides key data support for research in many fields. When connected with an elemental analyzer (EA), the stable isotope ratio mass spectrometer can measure the contents of C, H, N, S and O elements and related isotope ratios. When connected with a trace gas concentration system, the stable isotope ratio mass spectrometer can concentrate and purify CO2, CH4 and N2O in air and measure the isotope ratios. This combined technology has been widely applied in the determination of microbial metabolic rates and the tracing of element metabolic pathways.

[0003] The stable isotope ratio mass spectrometer in the prior art has high precision and multiple functions, but cannot directly measure the nitrogen isotope ratio of nitrogen in a gas sample. This defect limits the application range of the instrument in the research of nitrogen elements. In addition, during sample collection, the residual air in the glass sample bottle and the air adhered to the bottle wall can easily interfere with the sample and affect the accuracy of the measurement. Especially when the nitrogen content in the sample is low, the interference is more significant, which may cause the measured result to deviate from the true value. Furthermore, the CO2 adsorption trap usually uses a single particle size of adsorption material, and has the problems of short contact time of CO2 and the adsorption material, low adsorption efficiency, heat release during the CO2 adsorption process, and further influence on the reliability of the measured result. In the nitrogen-oxygen separation process, the traditional technology has the problems of long oxygen evacuation time and insufficient nitrogen adsorption stability. The long oxygen evacuation time prolongs the entire measurement period and reduces the measurement efficiency. The insufficient nitrogen adsorption stability easily leads to nitrogen leakage, which affects the separation effect and the subsequent measurement accuracy. These defects jointly limit the research efficiency and accuracy of the migration and transformation process of nitrogen elements among components in an ecological system. SUMMARY

[0004] The purpose of the present application is to make up for the deficiencies of the prior art, and provide a method for determining the nitrogen isotope of nitrogen in a gas sample, which is based on the combination of a trace gas concentration system and a stable isotope ratio mass spectrometer, and realizes efficient and accurate determination of the nitrogen isotope of nitrogen in a gas sample by optimizing the sample collection process, improving the structure of the CO2 adsorption trap and the nitrogen-oxygen separation technology, using a two-way inert gas displacement-vacuum coupling sampling process to effectively remove the residual air in the sample bottle and the air attached to the bottle wall, thereby reducing the risk of sample contamination, using a gradient filling structure for the CO2 adsorption trap and cooperating with a constant temperature heating jacket to improve the CO2 removal rate, and using a gradient temperature-pressure swing adsorption coupling technology to shorten the oxygen evacuation time and improve the nitrogen adsorption stability.

[0005] The present application provides the following technical solution to solve the above technical problems: a method for determining the nitrogen isotope of nitrogen in a gas sample, which comprises the following specific steps: Instrument performance pretreatment: Before sample determination, check the vacuum degree, detect the interference peak, tune the ion source, test the reference gas stability and linearity to determine that the instrument is in a stable working state; Sample collection and injection: glass sample bottles are used to collect gas samples, and a two-way inert gas displacement-vacuum coupling sampling process is used to treat the sample bottles before sampling, and the sample bottles are sealed after sampling; helium is used as the carrier gas, and the instrument automatic program controls the pneumatic twelve-way valve to load the sample into the valve and evacuate the front-end gas, so that the rear-end sample remains in the quantitative ring, and the volume of each injection is consistent; Multi-step impurity removal of the sample: the sample in the quantitative ring is loaded into liquid nitrogen cold hydrazine by another route of helium, and the water vapor, hydrocarbons and volatile organic compounds in the sample are removed by low temperature condensation, and CO2 and residual water are removed by a CO2 adsorption trap with a gradient filling structure and a magnesium perchlorate adsorption trap, respectively; Nitrogen-oxygen separation: the impurity-removed sample is introduced into a molecular sieve chromatographic column with a specific pore size, and a gradient temperature-pressure swing adsorption coupling technology is used to quickly evacuate the oxygen and stably retain the nitrogen; Mass spectrometric detection and analysis: the nitrogen is sent to the mass spectrometer by back-flushing the chromatographic column, and the signal intensity is recorded after ionization and separation, and the 15 N and 14 N isotope ratio of nitrogen in the gas sample is calculated by combining the reference gas.

[0006] Further, in the instrument performance pretreatment step, the system vacuum degree of the stable isotope ratio mass spectrometer is checked to meet the ionization requirements, high-purity helium is introduced to detect the interference peak signal in the system blank, the ion source is automatically tuned by the instrument program to ensure the success of the peak centering, the nitrogen isotope reference gas with the same injection amount is measured multiple times to calculate the isotope ratio standard deviation to complete the reference gas stability test, and the reference gas with gradient injection amount is configured to measure the isotope ratio and calculate the linear coefficient to complete the reference gas linearity test.

[0007] Further, in the instrument performance pretreatment step, high-purity helium is introduced to detect the interference peak signal in the system blank, and the blank detection needs to ensure that the water peak 18 signal, the nitrogen peak 28 signal, the argon peak 40 signal, and the carbon dioxide peak 44 signal meet the determination requirements, that is, meet the project acceptance indicators that the water peak 18 signal < 1.5E-09, the nitrogen peak 28 signal < 3.0E-10, the argon peak 40 signal < 3.0E-11, and the carbon dioxide peak 44 signal < 1.5E-10, and the reference gas stability test standard deviation is less than a set value, and the reference gas linearity test linear coefficient is less than a set value.

[0008] Further, in the sample collection and injection step, the gas sample includes an air sample, an environmental monitoring gas sample, or a spiked air sample, the sample bottle is sealed and stored at 20-25°C, and the storage time is not more than 72h.

[0009] Further, in the sample collection and injection step, before collection, the sample bottle is treated by a two-way inert gas replacement-vacuum coupling sampling process, specifically, before collection, the glass sample bottle is first vacuumed to a vacuum degree ≤ 1 × 10 -3 mbar, then high-purity helium is introduced to reach the normal pressure, and the operation is repeated for 3 times, then the helium is introduced in the reverse direction, flows into the gas outlet and flows out of the gas inlet, and the air adhered to the bottle wall is replaced.

[0010] Further, in the sample multi-step impurity removal step, the liquid nitrogen cold hydrazine is maintained at a low temperature of -196°C, the water vapor, hydrocarbons and volatile organic compounds (VOCs) in the sample are condensed into liquid or solid state by low temperature, and are left in the cold hydrazine, so as to realize preliminary impurity removal and avoid the interference of such impurities on the subsequent adsorption and separation steps.

[0011] Further, in the sample multi-step impurity removal step, the gradient filling structure of the CO2 adsorption trap is that the aminomodified molecular sieve with a coarse particle size of 2-3mm, a medium particle size of 1-2mm and a fine particle size of 0.5-1mm is sequentially filled from the gas inlet end to the gas outlet end, and the adsorption trap is wrapped with a constant temperature heating jacket, and the heating jacket controls the temperature to be 30-40°C.

[0012] Further, in the nitrogen-oxygen separation step, the pore size model of the molecular sieve chromatographic column is 5A, and the column temperature control and carrier gas pressure regulation are precisely controlled through instrument automation program.

[0013] Further, in the nitrogen-oxygen separation step, the specific parameters of the gradient temperature-varying pressure swing adsorption coupling technology are: the column temperature is increased from 25 DEG C to 40 DEG C at a rate of 2 DEG C / min, and the carrier gas pressure is decreased from 0.1 MPa to 0.08 MPa; after the oxygen is exhausted, the column temperature is decreased to 20 DEG C, and the carrier gas pressure is restored to 0.1 MPa, so as to improve the adsorption stability of nitrogen in the chromatographic column, prevent nitrogen leakage, and ensure that nitrogen is left in the chromatographic column.

[0014] Further, in the mass spectrometric detection and analysis step, the helium carrier gas flow direction is switched to blow back the chromatographic column, and the remaining nitrogen is sent to the stable isotope ratio mass spectrometer ion source. The ion source ionizes the nitrogen molecules, separates them through the mass analyzer, and records the ion signal intensity through the detector. Combined with the reference gas isotope ratio, the 15 N and 14 N isotope ratio in the sample is calculated.

[0015] Compared with the prior art, the nitrogen isotope determination method for nitrogen in a gas sample has the following beneficial effects: I. The present application adopts a bidirectional inert gas displacement-vacuum coupling sampling process, which effectively removes the residual air in the sample bottle and the air attached to the bottle wall through multiple vacuum-pumping and helium-filling and reverse helium displacement, significantly reducing the risk of sample contamination. The CO2 adsorption trap adopts a gradient filling structure and cooperates with a constant temperature heating jacket, which prolongs the contact time of CO2 and the adsorption material, and avoids the influence of adsorption heat release on the adsorption efficiency, thereby improving the CO2 removal rate.

[0016] II. The present application uses gradient temperature-varying pressure swing adsorption coupling technology to synergistically regulate temperature and pressure, enhances the desorption difference between oxygen and molecular sieve, shortens the oxygen exhaust time, and improves the adsorption stability of nitrogen in the nitrogen retention stage through low temperature and high pressure conditions, thereby avoiding nitrogen leakage. The whole method steps are closely connected, and the synergistic effect of the optimization measures in each link significantly improves the efficiency and accuracy of the nitrogen isotope determination of nitrogen, and provides reliable technical support for the research of nitrogen element.

[0017] Other advantages, objects, and features of the present application will be set forth in part in the following specification, and in part will become apparent to those skilled in the art from a study of the following, or can be learned from the practice of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to make the technical solutions in the embodiments of the present application or the prior art clearer, the accompanying drawings needed in the embodiments or prior art description will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0019] Figure 1 A flow chart of a nitrogen isotope determination method for nitrogen in a gas sample; Figure 2 A schematic diagram of a background scanning spectrum; Figure 3 A schematic diagram of a blank verification result. DETAILED DESCRIPTION

[0020] In order to further illustrate the technical means and effects adopted by the present application to achieve the predetermined application purpose, the specific embodiments, structures, features and effects according to the present application will be described in detail below with reference to the accompanying drawings and preferred embodiments.

[0021] The present application provides a nitrogen isotope determination method for nitrogen in a gas sample. The method is based on the combination of a trace gas concentration system (iso flow GHG) and a stable isotope ratio mass spectrometer (isoprime precisION). By optimizing the sample collection process, improving the structure of the CO2 adsorption trap and the nitrogen-oxygen separation technology, efficient and accurate determination of the nitrogen isotope of nitrogen in the gas sample is achieved. In terms of sample collection, the present application uses a bidirectional inert gas displacement-vacuum coupling sampling process to effectively remove the residual air in the sample bottle and the air attached to the bottle wall, significantly reducing the risk of sample contamination. In terms of CO2 adsorption, the present application designs a CO2 adsorption trap with a gradient filling structure and cooperates with a constant temperature heating jacket to prolong the contact time of CO2 and the adsorption material, while avoiding the influence of adsorption heat release on the adsorption efficiency and improving the CO2 removal rate. In terms of nitrogen-oxygen separation, the present application uses a gradient temperature-pressure swing adsorption coupling technology to enhance the desorption difference of oxygen and molecular sieve through the coordinated regulation of temperature and pressure, shorten the oxygen evacuation time, and improve the nitrogen adsorption stability under low temperature and high pressure conditions during the nitrogen retention stage to avoid nitrogen leakage. The synergistic effect of these optimization measures significantly improves the efficiency and accuracy of nitrogen isotope determination of nitrogen, providing reliable technical support for nitrogen element related research.

[0022] As shown in Figure 1 the following is a specific embodiment of a nitrogen isotope determination method for nitrogen in a gas sample proposed by the present application: Instrument performance pretreatment: start the mass spectrometer vacuum system, and wait until the ion source vacuum degree reaches 5×10 -6After the pressure is below 1 mbar, 99.999% high-purity helium is introduced to test the system blank, and the background scanning spectrum is analyzed, as shown in Figure 2 The water peak 18 signal is 1.2E-10, the nitrogen peak 28 signal is 0.8E-10, the argon peak 40 signal is 0.5E-11, and the carbon dioxide peak 44 signal is 0.9E-10, which meets the project acceptance indicators of water peak 18 signal <1.5E-09, nitrogen peak 28 signal <3.0E-10, argon peak 40 signal <3.0E-11, and carbon dioxide peak 44 signal <1.5E-10, without obvious interference. The instrument automatically tunes the program, adjusts the acceleration voltage, focusing voltage and other parameters, completes the 28N2 + , 29N2 + , 30N2 + peak centering, and the tuning result shows that the resolution is 144.31 and the peak center voltage deviation is +0.20V, which meets the measurement requirements. The nitrogen isotope reference gas is selected to carry out stability test, and the isotope ratio (29 / 28) is measured for 8 times in succession, with a standard deviation of 0.02‰. Ten gradient sample amounts of reference gas are configured to carry out linear test, with a linear coefficient of 0.03‰ / nA, which all meet the requirements of the project on instrument performance.

[0023] Sample collection and sampling: 12 mL glass sample bottles are used as collection containers. First, vacuumize to 1×10 - 3 mbar, introduce high-purity helium to normal pressure, repeat 3 times to exclude residual air in the bottle; then introduce helium from the gas outlet of the sample bottle and flow out from the gas inlet, replace the air attached to the bottle wall for 5 min, and then collect outdoor air samples. Immediately after sampling, seal with silica gel plug and store at 20-25℃ environment, the storage time is not more than 72h. At the same time, two types of samples are prepared: one type is low, medium and high concentration gradient air samples (theoretical δ 15 N is -1.2‰, 20.2‰ and 62.3‰ respectively), and the other type is 6 bag standard air samples (theoretical δ 15 N is about 23.3‰), which are used for method precision and accuracy verification. Start the trace gas concentration system (isoflow GHG) automatic program, set the helium carrier gas flow rate to 15 mL / min, and load the gas in the sealed sample bottle into the pneumatic twelve-way valve. Through valve switching control, the gas in the front end that may be contaminated is emptied, and only the sample in the rear end is left in the 50 μL quantitative ring. The instrument program controls the sampling volume to ensure the consistency of parallel samples.

[0024] Sample multi-step impurity removal: The sample in the quantitative ring was loaded into liquid nitrogen cold trap at a flow rate of 15 mL / min, and the water vapor, hydrocarbons and volatile organic compounds (VOCs) in the sample were condensed into liquid or solid state by low temperature, and remained in the cold trap to achieve preliminary impurity removal, avoiding the interference of such impurities on the subsequent adsorption and separation steps. The sample treated by the cold trap was sequentially introduced into the CO2 adsorption trap and the magnesium perchlorate adsorption trap. The CO2 adsorption trap was filled with specific adsorption material, which could efficiently fix CO2 in the sample. The magnesium perchlorate adsorption trap was filled with anhydrous magnesium perchlorate particles, which were used to adsorb the residual water in the sample. Finally, a mixed gas mainly composed of nitrogen and oxygen was obtained, which ensured the subsequent separation effect.

[0025] Nitrogen-oxygen separation: A 5Å pore size molecular sieve chromatographic column was selected, and a gradient control strategy was used to separate nitrogen and oxygen. First, the column temperature was increased from 25°C to 40°C at a rate of 2°C / min, and the helium carrier gas pressure was reduced from 0.1 MPa to 0.08 MPa. The temperature and pressure were cooperated to enhance the desorption difference of oxygen and molecular sieve, and accelerate the oxygen passing through the chromatographic column. Through real-time monitoring by the instrument, the oxygen was completely discharged from the chromatographic column within 3-5 min. Then the column temperature was reduced to 20°C and the carrier gas pressure was restored to 0.1 MPa, which improved the adsorption stability of nitrogen in the chromatographic column, prevented nitrogen leakage, and ensured that nitrogen was left in the chromatographic column.

[0026] Mass spectrometric detection and result analysis: Switch the helium flow direction to blow the molecular sieve chromatographic column at a flow rate of 15 mL / min, and send the remaining nitrogen to the stable isotope ratio mass spectrometer ion source. After confirming that the ion source tuning state is normal, start the detection. The ion source ionizes the nitrogen molecules into 28N2 + , 29N2 + , 30N2 + , which are separated by the mass analyzer, and the detector records the ion signal intensity. The single sample analysis time is controlled within 14 min, which meets the acceptance index of less than 15 min.

[0027] Result verification: Blank verification: The clean sample bottle filled with high-purity helium was analyzed. The blank peak height was 0.28 nA, which was much lower than the actual sample peak height (5.73-29.32 nA), and there was no blank pollution, as shown in Figure 3 . Precision verification: The outdoor air sample and two groups of standard addition samples were determined for 6 times in succession. The δ 15 N average value of the outdoor air sample was -1.22‰ (standard deviation 0.03‰), and the δ 15 N average values of the two groups of standard addition samples were 20.21‰ and 62.27‰ (standard deviation ≤0.05‰), respectively, as shown in the following table.

[0028] Accuracy verification: 6 bags of standard sample δ 15 The average value of N determination is 23.34‰ (standard deviation 0.03‰), and the parallel determination result sent to a third-party laboratory is 29.78‰, with a relative deviation of 12%, and the accuracy meets the requirements of scientific research analysis, as shown in the following table.

[0029]

[0030] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Although the present application has been disclosed as above with a preferred embodiment, it is not intended to limit the present application. Any person skilled in the art can make some changes or modifications to the above disclosed technical content without departing from the scope of the technical solution of the present application, and any equivalent embodiments with equivalent changes are equivalent. Any modification, equivalent change and modification of the above embodiments made in accordance with the technical essence of the present application are still within the scope of the technical solution of the present application.

Claims

1. A method for determining nitrogen isotopes in a gaseous sample, characterized in that, The method includes the following specific steps: Instrument performance pretreatment: Before sample measurement, the instrument is ensured to be in a stable working state by checking the vacuum level, detecting interference peaks, tuning the ion source, and testing the stability and linearity of the reference gas. Sample collection and injection: Gas samples are collected using glass sample vials. Before collection, the sample vials are treated with a two-way inert gas replacement-vacuum coupling sampling process. After sampling, the sample vials are sealed. Helium is used as the carrier gas. The instrument's automated program controls a pneumatic 12-way valve to load the sample into the valve and purge the gas at the front end, so that the sample at the back end remains in the quantitative loop, and the volume of each injection is kept consistent. Multi-step sample removal: The sample in the quantitative loop is loaded into liquid nitrogen and hydrazine through another helium gas stream. The water vapor, hydrocarbons and volatile organic compounds in the sample are removed by condensation at low temperature. CO2 and residual moisture are removed by CO2 adsorption traps and magnesium perchlorate adsorption traps with gradient-filled structures, respectively. Nitrogen-Oxygen Separation: The purified sample is passed through a molecular sieve column with a specific pore size. Gradient temperature rise-pressure swing adsorption coupling technology is used to rapidly vent oxygen and stably retain nitrogen. Mass spectrometry detection and analysis: Nitrogen gas is sent to the mass spectrometer via a backflushing column. After ionization and separation, the signal intensity is recorded, and the nitrogen concentration is calculated using a reference gas. 15 N and 14 N isotope ratio.

2. The method for determining nitrogen isotopes in a gas sample according to claim 1, characterized in that, In the instrument performance preprocessing step, the system vacuum degree of the stable isotope ratio mass spectrometer is checked to ensure that it meets the ionization requirements. High-purity helium gas is introduced to detect interference peak signals in the system blank. The ion source is automatically tuned by the instrument's built-in program to ensure successful peak alignment. The same amount of nitrogen isotope reference gas is measured multiple times and the standard deviation of the isotope ratio is calculated to complete the reference gas stability test. The reference gas with gradient injection volume is configured to measure the isotope ratio and calculate the linear coefficient to complete the reference gas linearity test.

3. The method for determining nitrogen isotopes in a gas sample according to claim 2, characterized in that, In the instrument performance preprocessing step, interference peak signals in the blank detection system are introduced into high-purity helium. The blank detection must ensure that the water peak 18 signal, nitrogen peak 28 signal, argon peak 40 signal, and carbon dioxide peak 44 signal meet the measurement requirements, that is, meet the project acceptance criteria of water peak 18 signal < 1.5E-09, nitrogen peak 28 signal < 3.0E-10, argon peak 40 signal < 3.0E-11, and carbon dioxide peak 44 signal < 1.5E-10. In addition, the standard deviation of the reference gas stability test is less than the set value, and the linear coefficient of the reference gas linearity test is less than the set value.

4. The method for determining nitrogen isotopes in a gas sample according to claim 1, characterized in that, In the sample collection and injection steps, the gas samples include air samples, environmental monitoring gas samples, or spiked air samples. After the sample bottles are sealed, they are stored at 20-25℃ for no more than 72 hours.

5. The method for determining nitrogen isotopes in a gas sample according to claim 1, characterized in that, In the sample collection and injection steps, the sample vials are pretreated using a bidirectional inert gas replacement-vacuum coupling sampling process. Specifically, the glass sample vials are evacuated to a vacuum level ≤1×10⁻⁶ before collection. -3 mbar, then introduce inert helium to atmospheric pressure, repeat this operation 3 times, then introduce helium in reverse, flowing in from the outlet of the sample bottle and out from the inlet, replacing the air adhering to the bottle wall.

6. The method for determining nitrogen isotopes in a gas sample according to claim 1, characterized in that, During the multi-step impurity removal process of the sample, liquid nitrogen and cold hydrazine are maintained at a low temperature of -196°C.

7. The method for determining nitrogen isotopes in a gas sample according to claim 1, characterized in that, In the multi-step impurity removal process of the sample, the gradient filling structure of the CO2 adsorption trap is as follows: from the gas inlet end to the gas outlet end, it is filled with amino-modified molecular sieves with coarse particle size of 2-3 mm, medium particle size of 1-2 mm, and fine particle size of 0.5-1 mm in sequence, and the outer layer of the adsorption trap is wrapped with a constant temperature heating jacket, and the temperature of the heating jacket is controlled at 30-40℃.

8. The method for determining nitrogen isotopes in a gas sample according to claim 1, characterized in that, In the nitrogen-oxygen separation step, the molecular sieve column has a pore size of 5 Å, and the column temperature and carrier gas pressure are precisely controlled by the instrument's automated program.

9. The method for determining nitrogen isotopes in a gas sample according to claim 1, characterized in that, In the nitrogen-oxygen separation step, the specific parameters of the gradient temperature-pressure swing adsorption coupling technology are as follows: the column temperature is increased from 25℃ to 40℃ at a rate of 2℃ / min, while the carrier gas pressure is decreased from 0.1MPa to 0.08MPa; after the oxygen is purged, the column temperature is reduced to 20℃ and the carrier gas pressure is restored to 0.1MPa.

10. The method for determining nitrogen isotopes in a gas sample according to claim 1, characterized in that, In the mass spectrometry detection and analysis step, the helium carrier gas flow direction is switched to the backflushing column, and the remaining nitrogen gas is sent to the ion source of the stable isotope ratio mass spectrometer. The ion source ionizes the nitrogen molecules, and after separation by the mass analyzer, the ion signal intensity is recorded by the detector. Combined with the reference gas isotope ratio value, the nitrogen content in the sample is calculated. 15 N and 14 N isotope ratio.

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