Method for mass spectrometric detection of ammonia gas based on VUV lamp

Abstract

The invention belongs to the technical field of mass spectrometric detection and analysis, and relates to a method for mass spectrometric detection of ammonia gas based on a VUV lamp. A time-of-flight mass spectrum is used as an analyzer, a photoinduced dibromomethane chemical ionization source is used as a mass spectrum ionization source, gas to be detected enters the mass spectrum ionization source in a negative pressure form through a capillary tube, and meanwhile, 2000ppmv dibromomethane reagent enters the mass spectrum ionization source in a negative pressure form through another capillary tube; the method comprises the following steps: performing addition reaction on dibromomethane reagent ions generated by ionizing dibromomethane gas by using a VUV lamp and ammonia molecules to form [CH2Br2.NH3] < + > characteristic peaks with the abundance ratio of m / z 189, m / z 191 and m / z 193 being 1: 2: 1; according to the method, the defect that the ammonia gas cannot be detected through VUV lamp single-photon ionization is overcome, the qualitative analysis capacity and the signal intensity of the ammonia gas are improved, and the online analysis capacity can meet the detection requirement of the ammonia gas in electrocatalytic reaction ammonia synthesis.

Description

Technical Field

[0001] The invention belongs to the technical field of mass spectrometry detection and analysis, and relates to a mass spectrometry rapid analysis method for ammonia detection in a process of synthesizing ammonia by catalytic reaction. Background Art

[0002] Ammonia is a colorless, irritating gas with high water solubility. Due to its low boiling point, it is widely used in food processing and refrigeration as an environmentally friendly refrigerant. It is also an important chemical in the manufacture of nitric acid, fertilizers, resins, explosives and other compounds. The widespread use of ammonia inevitably leads to a huge potential for occupational exposure. Workers may be harmed by exposure to ammonia because ammonia can irritate and burn the skin, mouth, throat, lungs and eyes, and even damage the lungs, reduce lung function at low concentrations, and cause death at high concentrations. Many countries have established occupational exposure limits for ammonia. The concentration-time weighted average allowed in China is 20 mg / m 3 The permissible concentration-short-term exposure limit is 30 mg / m 3 . At the same time, ammonia is a common component in human exhaled breath, produced through the metabolism of amino acids. The exhaled ammonia level of a healthy human body ranges from 50ppb to 1500ppb, and increases to 15000ppb in patients with chronic kidney disease. Exhaled ammonia levels have been shown to be closely related to a variety of diseases such as chronic kidney disease and Helicobacter pylori infection, and can therefore be used for clinical diagnosis and metabolic testing. In addition, ammonia is vital to the global economy as a raw material for fertilizers, industrial and household chemicals, and is considered to be a future fuel substitute and hydrogen removal molecule. Therefore, it is of great significance to explore the reaction mechanism in the catalytic synthesis of ammonia.

[0003] At present, the representative detection methods of ammonia include colorimetry, nuclear magnetic resonance, surface Raman, ion chromatography and electron bombardment-quadrupole mass spectrometry. As the gold standard for detecting ammonia, colorimetry usually uses indigo phenol dyes or Nessler reagents. Ammonia reacts with these reagents to produce color changes, and then the color changes are captured and detected by a spectrophotometer. As the gold standard, colorimetry is highly versatile in different scenarios, but it has the problems of long pre-treatment time and color development being interfered by other ions. As a common method for ammonia detection, ion chromatography has the characteristics of high sensitivity, but it also requires a long pre-treatment time like colorimetry, and its detection time is long, the instrument is expensive, and professional operators are required. At the same time, this method is difficult to distinguish other amine substances due to the limitation of its principle. In addition, there are reports on the use of Fourier transform nuclear magnetic resonance spectrometer to detect ammonia, but its complex instrument, large sample volume, long detection time and high detection cost limit its application in the laboratory. The above three methods can only realize offline detection, and cannot realize continuous monitoring of the catalytic synthesis of ammonia reaction process. In addition to the above three methods for detecting ammonia, surface Raman method and electron bombardment-quadrupole mass spectrometry can realize online detection of ammonia. The surface Raman method is based on a microscope system and a silver substrate on which the liquid sample is placed, which can achieve a detection speed of 1 to 2 seconds and can detect the reaction in situ, but its working time is limited by the oxidation of the silver substrate, making it difficult to continuously monitor the reaction process for a long time. Electron bombardment-quadrupole mass spectrometry has the characteristics of high sensitivity detection, but due to the high-energy electron bombardment ionization source, it is difficult to avoid the generation of OH+ during the detection process. Its mass number is the same as that of NH3, m / z 17, and it is difficult to distinguish the two with the resolution of quadrupole mass spectrometry.

[0004] The soft ionization mass spectrometry developed in recent years can directly analyze most compounds without sample pretreatment, and has the characteristics of rapidity, high efficiency, and sensitivity. Among them, the VUV light-induced dibromomethane chemical ionization source is one of them. It uses a VUV lamp to ionize dibromomethane gas molecules, and then the charge transfer, association reaction, and substitution reaction between dibromomethane molecular ions and other substances to be tested significantly improve the detection sensitivity of the test substance, thereby realizing the detection of trace substances. Due to its high sensitivity and easy operation, it has attracted widespread attention. Compared with other methods, the time-of-flight mass spectrometry based on the photoinduced dibromomethane chemical ionization source can obtain richer spectral information and is more conducive to the detection of complex matrices and reaction processes. Therefore, if a method for detecting ammonia processes in catalytic processes based on the photoinduced dibromomethane chemical ionization source can be developed, it will have important application significance. Summary of the invention

[0005] In view of the problem that there is currently a lack of a method for ammonia detection that can be fast, efficient, sensitive and simultaneously obtain rich spectral information, the present invention provides a method for detecting ammonia by mass spectrometry based on a VUV lamp, which makes up for the deficiency that single-photon ionization of ammonia by a VUV lamp cannot detect ammonia, improves the sensitivity of ammonia detection, has the characteristics of being fast, efficient, sensitive, and rich in information, and is suitable for qualitative detection and process monitoring of electrocatalytic synthesis of ammonia.

[0006] In order to achieve the above-mentioned invention object, the present invention adopts the following technical solutions:

[0007] The invention provides a method for detecting ammonia by mass spectrometry based on a VUV lamp. The method uses a time-of-flight mass spectrometer as an analyzer and a photoinduced dibromomethane chemical ionization source as a mass spectrometry ionization source. Dibromomethane reagent ions generated by ionizing dibromomethane gas with a VUV lamp react with ammonia molecules to form [CH2Br2·NH3] with an abundance ratio of 1:2:1 among m / z 189, m / z 191 and m / z 193. + Characteristic peak.

[0008] In the above technical solution, further, the concentration of dibromomethane is 2000 ppmv.

[0009] In the above technical solution, further, the voltage difference between the beginning and the end of the ionization source is 1V; and the gas pressure in the ionization source is 400Pa.

[0010] In the above technical solution, further, the method comprises the following steps:

[0011] (1) establishing a standard mass spectrum of ammonia gas to obtain characteristic peaks of ammonia gas standard;

[0012] (2) Detect the mass spectrum of the ammonia sample to be tested. If the peaks are the same as those of the mass spectrum of the ammonia standard, then the sample to be tested contains ammonia.

[0013] In the above technical scheme, further, the step (1) is: ammonia standard gas is introduced into a mass spectrometer injection capillary with a three-way connection, and the other end of the three-way connection is connected to the tail gas discharge; the injection flow rate of the CH2Br2 reagent and the clean air flow rate are controlled to obtain a CH2Br2 reagent gas with a concentration of 2000ppmv at the outlet of the gas mixing chamber, and introduced into another mass spectrometer injection capillary with a three-way connection, and the other end of the three-way connection is connected to the tail gas discharge; [CH2Br2·NH3] is formed under the action of a VUV lamp. + , enter the time-of-flight mass spectrometer, and obtain the standard mass spectrum of ammonia in the positive ion mode.

[0014] In the above technical solution, further, the injection flow rate of the CH2Br2 reagent and the clean air flow rate are 5.514 μL / min and 1 L / min respectively; the ammonia standard gas uses nitrogen as the balance gas.

[0015] In the above technical scheme, further, the step (2) is: purge the ammonia generated by the electrocatalytic device, carry out the ammonia with a carrier gas, pass it into a mass spectrometer injection capillary, and obtain a mass spectrum of the ammonia sample using the same method as the step (1).

[0016] In the above technical solution, further, the carrier gas is humid nitrogen, dry argon or dry nitrogen.

[0017] In the above technical solution, further, the ammonia gas is diluted with dry nitrogen before being introduced into the mass spectrometer injection capillary.

[0018] The photoinduced dibromomethane chemical ionization source of the present invention is a kind of soft ionization mode. The present invention uses dibromomethane gas with a concentration of 2000ppmv to enter the ionization source cavity, and reaches the optimal reaction concentration with ammonia under the action of a VUV lamp. The concentration of dibromomethane gas is controlled by a flow injection pump and a gas flow meter: the injection flow rate of pure CH2Br2 reagent and the clean air flow rate are set to 5.514μL / min and 1L / min respectively to the heating cavity by using a flow injection pump, and a CH2Br2 reagent gas with a concentration of 2000ppmv is obtained at the outlet of the gas mixing chamber. This method is a liquid phase compound dynamic gas distribution method (CN103163007A). The voltage difference between the beginning and end of the ionization source is reduced to 1V to reduce E / N, which is most conducive to the formation of [CH2Br2·NH3] + The gas pressure in the ionization source is set to 400Pa for [CH2Br2·NH3] + The optimal gas pressure is formed. Further increasing the gas pressure will lead to the appearance of fragment ions, which is not conducive to the detection of ammonia.

[0019] Since the concentration of electrocatalytically synthesized ammonia is relatively high, it will affect the duty cycle of dibromomethane inside the photoinduced dibromomethane chemical ionization source and thus reduce the accuracy of the method. The present invention uses a three-way pipe to introduce dry nitrogen to dilute the outlet gas, and then connects the capillary injection tube of the time-of-flight mass spectrometer through the three-way pipe, and the other end is used as tail gas exhaust to avoid damaging the vacuum system of the time-of-flight mass spectrometer. The electrocatalytically synthesized ammonia is directly ionized by the photoinduced dibromomethane chemical ionization source after being diluted with dry nitrogen. The ions are shaped by the radio frequency quadrupole and the electrostatic transmission area in a vacuum absorption manner, and finally enter the time-of-flight mass spectrometer. The analysis is performed in the positive ion mode, the sample signal intensity is greatly enhanced, and the clarity of the obtained spectrum is also greatly improved, which effectively avoids the influence of high-concentration ammonia on the accuracy of the detection method.

[0020] Compared with the prior art, the present invention has the following beneficial effects: the present invention can realize rapid, efficient, sensitive ammonia detection while acquiring rich spectral information, and is successfully applied to the detection of ammonia in electrocatalytic ammonia synthesis, and can track the generation process of ammonia during the electrocatalytic reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Mass spectrum of ammonia standard gas in photoinduced dibromomethane chemical ionization mode;

[0022] Figure 2 The relationship between the signal intensity of the standard spectrum peak of ammonia and the ionization source pressure in the photoinduced dibromomethane chemical ionization mode;

[0023] Figure 3 The relationship between the signal intensity of the ammonia standard spectrum peak and the pressure difference between the beginning and the end of the ionization source in the photoinduced dibromomethane chemical ionization mode;

[0024] Figure 4 Relationship between the signal intensity of the ammonia standard spectrum peak and the concentration of dibromomethane in the photoinduced dibromomethane chemical ionization mode;

[0025] Figure 5 Comparison of the signal intensity of ammonia standard gas and ammonia produced by electrocatalysis after dilution in the photoinduced dibromomethane chemical ionization mode;

[0026] Figure 6 Mass spectra of ammonia characteristic peaks under different carrier gas conditions in photoinduced dibromomethane chemical ionization mode. DETAILED DESCRIPTION

[0027] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0028] Example 1

[0029] This embodiment uses a photoionization time-of-flight mass spectrometer as an analyzer, a photoinduced dibromomethane chemical ionization source as a mass spectrometer ionization source, and optimizes the ionization source gas pressure and voltage conditions. The specific steps are:

[0030] ① Optimization of ionization source pressure

[0031] The ammonia concentration was fixed at 0.4 ppmv and the dibromomethane concentration was fixed at 2000 ppmv. The ionization source pressure was adjusted from 100 Pa to 400 Pa at intervals of 30 Pa by adjusting the valve on the ionization source side. The signal intensity of the characteristic peak of ammonia was observed and the obtained Figure 2 As a result, the optimal gas pressure was determined to be 400Pa.

[0032] ②Optimization of ionization source voltage

[0033] The ammonia concentration was fixed at 0.4ppmv and the dibromomethane concentration was fixed at 2000ppmv. The voltage difference between the start and end of the ionization source was adjusted from 21V to 1V in 5V intervals. The signal intensity of the characteristic peak of ammonia was observed and the result was Figure 3 As a result, it was determined that the optimal ionization source start-end voltage difference was 1V.

[0034] ③ Optimization of dibromomethane concentration

[0035] The ammonia concentration was fixed at 0.4ppmv, the ionization source pressure was 400Pa, and the voltage difference between the start and end of the ionization source was 1V. The concentration of dibromomethane gas was controlled by changing the injection speed of dibromomethane by the flow injection pump, and the signal intensity of the ammonia characteristic peak was observed. Figure 4 As a result, it was determined that the optimal methylene bromide concentration that could ensure strength and sufficiency was 2000 ppmv.

[0036] ④Dilution of ammonia produced by electrocatalytic reaction

[0037] Use high-purity nitrogen to dilute the ammonia produced by the electrocatalytic device outlet through a three-way connection, and compare the intensity of 0.4ppmv ammonia standard gas. If the intensity is similar, it is considered to be diluted to an appropriate concentration and tested. The results are as follows Figure 5 shown.

[0038] Example 2

[0039] This embodiment uses a photoionization time-of-flight mass spectrometer as an analyzer and a photoinduced dibromomethane chemical ionization source as a mass spectrometer ionization source. The specific steps are:

[0040] ① Establish a standard mass spectrum of ammonia

[0041] Adjust the photoionization time-of-flight mass spectrometry ionization source pressure to 400Pa, the ionization source start-end voltage difference to 1V, use ammonia standard gas (concentration 1ppmv, nitrogen as balance gas), use a flow controller to pass a mass spectrometry injection capillary with a three-way flow rate of 100mL / min, and the other end of the three-way is connected to the tail gas discharge. Use a flow injection pump to set the injection flow rate and clean air flow rate of the pure CH2Br2 reagent to the heating chamber to 5.514μL / min and 1L / min respectively, and obtain a CH2Br2 reagent gas with a concentration of 2000ppmv at the outlet of the gas mixing chamber. Pass another mass spectrometry injection capillary with a three-way, and the other end of the three-way is connected to the tail gas discharge. This method is a liquid phase compound dynamic gas distribution method (CN103163007A). Under the action of the VUV lamp, the CH2Br2 reagent gas is ionized into CH2Br2 + , and ammonia gas undergoes substitution reaction to form [CH2Br2·NH3] + The ions are shaped by the radio frequency quadrupole and the electrostatic transfer zone by vacuum absorption, and finally enter the time-of-flight mass spectrometer and are analyzed in the positive ion mode to obtain the standard mass spectrum of ammonia.

[0042] ②Sample testing

[0043] A. Qualitative detection of electrocatalytic ammonia synthesis

[0044] The reaction of electrocatalytic synthesis of ammonia N2+3H2O=3 / 2O2+2NH3 is carried out in a detachable secondary battery measuring device (EQ-STC) of an electrocatalytic device, and the electrocatalytic device is purged, and ammonia is carried out with moist nitrogen that can produce ammonia, dry argon that cannot produce ammonia, or dry nitrogen that cannot produce ammonia as a carrier gas. The ammonia at the outlet of the electrocatalytic device passes through a mass spectrometer injection capillary with a three-way connection, and the other end of the three-way connection is connected to the tail gas for discharge. A flow injection pump is used to set the injection flow rate of pure CH2Br2 reagent and the clean air flow rate to 5.514μL / min and 1L / min respectively to the heating chamber, and a CH2Br2 reagent gas with a concentration of 2000ppmv is obtained at the outlet of the gas mixing chamber, and another mass spectrometer injection capillary with a three-way connection is passed, and the other end of the three-way connection is connected to the tail gas for discharge. This method is a liquid phase compound dynamic gas distribution method (CN103163007A). Under the action of VUV lamp, CH2Br2 reagent gas is ionized into CH2Br2 + , reacts with electrocatalytically synthesized ammonia to form [CH2Br2·NH3] + The ions are vacuum-absorbed and shaped by the radio frequency quadrupole and electrostatic transmission zone, and finally enter the time-of-flight mass spectrometer, where they are analyzed in the positive ion mode to obtain the ammonia mass spectrum. Figure 6 As shown in the figure, when the carrier gas is humid nitrogen, peaks appear at m / z 189, m / z 191, and m / z 193, which are determined to be molecules [CH2Br2·NH3] + The peak with an abundance ratio of 1:2:1 is compared with Figure 1 By comparing the mass spectrum of ammonia standard gas, it can be determined that this is the characteristic peak of ammonia.

[0045] The above detection results indicate that the method of the present invention can be used for qualitative detection of ammonia in electrocatalytic synthesis of ammonia.

[0046] For any technician familiar with the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for detecting ammonia gas by mass spectrometry based on a VUV lamp, characterized in that: The method uses a time-of-flight mass spectrometer as an analyzer and a photoinduced dibromomethane chemical ionization source as a mass spectrometer ionization source. Dibromomethane reagent ions generated by ionizing dibromomethane gas with a VUV lamp react with ammonia molecules to form [CH2Br2·NH3] with an abundance ratio of 1:2:1 among m / z 189, m / z 191 and m / z 193. + Characteristic peak.

2. The method according to claim 1, characterized in that The concentration of dibromomethane is 2000 ppmv.

3. The method according to claim 1, characterized in that The voltage difference between the beginning and the end of the ionization source is 1V; the gas pressure in the ionization source is 400Pa.

4. The method according to claim 1, characterized in that: The method comprises the following steps: (1) establishing a standard mass spectrum of ammonia gas to obtain characteristic peaks of ammonia gas standard; (2) Detect the mass spectrum of the ammonia sample to be tested. If the peaks are the same as those of the mass spectrum of the ammonia standard, then the sample to be tested contains ammonia.

5. The method according to claim 4, characterized in that The step (1) comprises: introducing ammonia standard gas into a mass spectrometer sampling capillary with a three-way connection, the other end of the three-way connection being connected to exhaust gas; controlling the injection flow rate of CH2Br2 reagent and the flow rate of clean air, obtaining CH2Br2 reagent gas with a concentration of 2000ppmv at the outlet of the gas mixing chamber, introducing the gas into another mass spectrometer sampling capillary with a three-way connection, the other end of the three-way connection being connected to exhaust gas; forming [CH2Br2·NH3] under the action of a VUV lamp. + , enter the time-of-flight mass spectrometer, and obtain the standard mass spectrum of ammonia in the positive ion mode.

6. The method according to claim 5, characterized in that The injection flow rate of CH2Br2 reagent and the clean air flow rate are 5.514 μL / min and 1 L / min respectively; the ammonia standard gas uses nitrogen as the balance gas.

7. The method according to claim 4, characterized in that The step (2) is: purging the ammonia generated by the electrocatalytic device, carrying out the ammonia with a carrier gas, passing the ammonia into a mass spectrometer injection capillary, and obtaining a mass spectrum of the ammonia sample using the same method as the step (1).

8. The method according to claim 7, characterized in that The carrier gas is wet nitrogen, dry argon or dry nitrogen.

9. The method according to claim 8, characterized in that The ammonia gas is diluted with dry nitrogen gas to a concentration range of 0.4-10 ppmv before being introduced into the mass spectrometer injection capillary.

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