Method for detecting ethylene in atmosphere by mass spectrometry 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 ethylene in atmosphere based on a VUV lamp. A flight time 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 PEEK capillary tube, and meanwhile, 2000ppmv dibromomethane reagent enters the mass spectrum ionization source in a negative pressure form through another PEEK capillary tube; dibromomethane reagent ions generated by ionizing dibromomethane gas through a VUV lamp and ethylene molecules are subjected to a substitution reaction to form [CH2Br.C2H4] < + > characteristic peaks with the abundance ratio of m / z 121 to m / z 123 being 1: 1. According to the method, the defect that ethylene cannot be detected through VUV lamp single-photon ionization is overcome, the qualitative and quantitative analysis capacity and the signal intensity of ethylene are improved, and the sensitivity can meet the detection requirement of trace ethylene in the atmosphere.

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 ethylene in the atmosphere. Background Art

[0002] Ethylene, the simplest olefin, has four hydrogen atoms bound to a pair of double-bonded carbon atoms. It has a smaller kinetic diameter and a similar specific gravity to air (1.178 kg·m at 15°C). -3 and 1.225kg·m -3 ). Therefore, it can diffuse freely in the air, with an average concentration of about 2 parts per billion (ppbv). Despite its small size and negligible concentration in nature, it plays an important role in the fields of environmental and health sciences. From an environmental perspective, the average concentration of ethylene in the atmosphere is around 2 ppbv. The concentration of ethylene in urban atmosphere is relatively high, mainly produced by automobile exhaust and industrial emissions. It is one of the main precursors for the rapid formation of ozone in the atmosphere of urban areas.

[0003] At present, the representative detection methods of ethylene include gas chromatography, electrochemical sensor method, photoacoustic spectroscopy and proton transfer mass spectrometry. Gas chromatography is accurate and sensitive. However, due to factors such as complex pre-treatment and long detection time, it is not suitable for rapid detection of ethylene in the atmosphere. Electrochemical sensing has the characteristics of low cost. However, the sensor material is sensitive to external factors such as temperature, humidity, and interfering gases, resulting in a limited detection life. Photoacoustic spectroscopy has the characteristics of fast detection speed and high sensitivity. However, its detection principle of fixed laser frequency detection leads to a single detection target for this method, and less available information in the detection results. Proton transfer mass spectrometry is similar to photoacoustic spectroscopy, with high sensitivity but a single detection target, and it takes a long time to detect when it reaches a lower sensitivity.

[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 qualitative and quantitative method based on the photoinduced dibromomethane chemical ionization source can be developed to detect ethylene in the atmosphere, it will have important application significance. Summary of the invention

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

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

[0007] The present invention provides a method for detecting ethylene 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. The dibromomethane reagent ions generated by the VUV lamp ionizing dibromomethane gas undergo a substitution reaction with ethylene molecules to form [CH2Br·C2H4] with an abundance ratio of 1:1 between m / z 121 and m / z 123. + 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 ethylene, obtaining characteristic peaks of ethylene standards, and establishing an ethylene standard curve;

[0012] (2) detecting the mass spectrum of the sample to be tested and comparing it with the mass spectrum of the ethylene standard;

[0013] (3) Substitute the peak intensity of the characteristic peak of the sample to be tested into the ethylene standard curve to obtain the ethylene concentration of the sample to be tested.

[0014] In the above technical scheme, further, the step (1) is: ethylene standard gas is introduced into a mass spectrometer sampling 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 sampling capillary with a three-way connection, and the other end of the three-way connection is connected to the tail gas discharge; [CH2Br·C2H4] is formed under the action of a VUV lamp. + , enter the time-of-flight mass spectrometer, and obtain the ethylene standard mass spectrum in the positive ion mode; dilute the ethylene standard gas to obtain ethylene standard gas of different concentrations, and obtain the corresponding signal intensity, and fit to obtain the ethylene standard curve.

[0015] 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; and the ethylene standard gas is diluted with nitrogen.

[0016] In the above technical solution, further, step (2) is: collecting outdoor air, passing it into a mass spectrometer injection capillary, and obtaining a mass spectrum of the air sample using the same method as step (1).

[0017] In the above technical solution, further, the method for collecting outdoor air is: using a diaphragm pump in conjunction with a Tedlar air bag to collect; before the outdoor air is sampled, it is first connected to a Nafion drying tube through a tetrafluoro tube for dehumidification, and the outlet of the Nafion drying tube is connected to a sampling capillary.

[0018] The present invention also provides application of the above method in tracking and monitoring ethylene concentration in the atmosphere.

[0019] The photoinduced dibromomethane chemical ionization source of the present invention is a kind of soft ionization mode. In order to achieve the best reaction concentration with ethylene under the action of VUV lamp, the present invention uses dibromomethane gas with a concentration of 2000ppmv to enter the ionization source cavity. 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 the flow injection pump, and the CH2Br2 reagent gas with a concentration of 2000ppmv is obtained at the outlet of the gas mixing chamber. The voltage difference between the beginning and the end of the ionization source is reduced to 1V to reduce E / N, which is most conducive to the formation of [CH2Br·C2H4] + The pressure in the ionization source was set to 400 Pa to form [CH2Br·C2H4] + Further increasing the pressure will lead to the appearance of fragment ions, which is not conducive to the detection of ethylene in the sample.

[0020] Since the humidity of outdoor air is relatively high, it will affect the duty cycle of dibromomethane inside the photoinduced dibromomethane chemical ionization source and thus reduce the sensitivity of the method. The present invention uses a Nafion drying tube to dehumidify the atmospheric sample: a Tedlar air bag is used to collect the atmosphere, and the outlet of the air bag filled with outdoor air is connected to the Nafion drying tube for dehumidification through a tetrafluoro tube, and the outlet of the Nafion drying tube is directly connected to an injection capillary of a mass spectrometer. After being dehumidified by the Nafion drying tube, the sample directly enters the photoinduced dibromomethane chemical ionization source for ionization, and 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, and are analyzed in the positive ion mode, the sample signal intensity is greatly enhanced, and the clarity of the obtained spectrum is also greatly improved, effectively avoiding the interference of outdoor air humidity on ethylene in the outdoor air.

[0021] Compared with the prior art, the present invention has the following beneficial effects: the method of the present invention can realize fast, efficient, sensitive and simultaneously obtain rich spectral information of ethylene detection, has been successfully applied to the detection of ethylene in outdoor air, and can track the changes in ethylene concentration in outdoor air within a day. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Mass spectrum of ethylene standard gas in photoinduced dibromomethane chemical ionization mode;

[0023] Figure 2 The relationship between the signal intensity of the ethylene standard spectrum peak and the ionization source pressure in the photoinduced dibromomethane chemical ionization mode;

[0024] Figure 3 The relationship between the signal intensity of the ethylene standard spectrum peak and the voltage difference between the start and end of the ionization source in the photoinduced dibromomethane chemical ionization mode;

[0025] Figure 4 The relationship between the signal intensity of the ethylene standard spectrum peak and the concentration of dibromomethane gas in the photoinduced dibromomethane chemical ionization mode;

[0026] Figure 5 Relationship between ethylene standard gas concentration and signal intensity in photoinduced dibromomethane chemical ionization mode;

[0027] Figure 6 Mass spectrum of outdoor air in photoinduced dibromomethane chemical ionization mode;

[0028] Figure 7 Mass spectrum of outdoor air near major traffic roads under photoinduced dibromomethane chemical ionization mode;

[0029] Figure 8 Tracking chart of ethylene concentration changes in outdoor air along major traffic roads over a day under the photoinduced dibromomethane chemical ionization mode. DETAILED DESCRIPTION

[0030] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0031] Example 1

[0032] 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 ionization source pressure, the voltage difference between the start and end of the ionization source, and the dibromomethane concentration are optimized for the detection of ethylene. The specific steps are:

[0033] ① Optimization of ionization source pressure

[0034] The ethylene concentration was fixed at 1 ppmv and the dibromomethane concentration was 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 ethylene was observed. Figure 2 As a result, the optimal gas pressure was determined to be 400Pa.

[0035] ②Optimization of ionization source voltage

[0036] The ethylene concentration was fixed at 0.1ppmv and the dibromomethane concentration was 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 ethylene was observed and the result was obtained. Figure 3 As a result, it was determined that the optimal ionization source start-end voltage difference was 1V.

[0037] ③ Optimization of dibromomethane concentration

[0038] The ethylene concentration was fixed at 1ppmv, 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. The signal intensity of the ethylene characteristic peak was observed and the result was Figure 4 As a result, it was determined that the optimal methylene bromide concentration that could ensure strength and sufficiency was 2000 ppmv.

[0039] Example 2

[0040] 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:

[0041] ① Establish the standard mass spectrum and standard curve of ethylene

[0042] Ethylene standard gas (concentration 1ppmv, nitrogen as balance gas) was used, and a flow controller was used to pass a mass spectrometer injection capillary with a three-way valve at a flow rate of 100mL / min, and the other end of the three-way valve was connected to the tail gas discharge. A flow injection pump was 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 was obtained at the outlet of the heating chamber, and another mass spectrometer injection capillary with a three-way valve was passed, and the other end of the three-way valve was 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 ethylene undergoes substitution reaction to form [CH2Br·C2H4] +The ions are vacuum-absorbed through the radio frequency quadrupole and electrostatic transmission zone in the time-of-flight mass spectrometer and finally enter the time-of-flight mass spectrometer for analysis in the positive ion mode to obtain the ethylene standard mass spectrum. The ethylene standard gas is diluted by another flow controller (nitrogen is used as the dilution gas) to obtain the signal intensity corresponding to the ethylene standard gas of different concentrations. The ethylene standard curve is obtained by linear simulation using Origin software ( Figure 5 );The voltage difference between the beginning and the end of the ionization source is reduced to 1V to reduce E / N, which is most conducive to the formation of [CH2Br·C2H4] + The gas pressure in the ionization source was set to 400Pa.

[0043] ②Sample testing

[0044] A diaphragm pump is used in conjunction with a Tedlar air bag to collect outdoor air. The outlet of the air bag filled with outdoor air is connected to a Nafion drying tube for dehumidification through a PTFE tube, and the outlet of the Nafion drying tube is directly connected to an injection capillary of a mass spectrometer. 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, into the heating chamber. A CH2Br2 reagent gas with a concentration of 2000ppmv is obtained at the outlet of the gas mixing chamber, and is passed into another mass spectrometer injection capillary with a three-way connection. The other end of the three-way connection is connected to the exhaust 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 undergoes substitution reaction with ethylene in the air to form [CH2Br·C2H4] + 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. They are analyzed in the positive ion mode to obtain the air mass spectrum, such as Figure 6 As shown in Figure 2, a peak appears at m / z 121. Figure 1 By comparing the mass spectrum of ethylene standard gas, it can be determined that this is the characteristic peak of ethylene. Substituting the peak intensity into Figure 5 The corresponding ethylene concentration can be obtained from the relationship diagram between ethylene standard gas concentration and signal intensity.

[0045] Example 3

[0046] The equipment model adopted in the present embodiment and the method for establishing the standard mass spectrum of ethylene are the same as in Example 1, and the detected air source is the air collected by the side of the main traffic road. Using the Tedlar air bag that has been used to measure the background of the air bag, a diaphragm pump is used to sample the side of the main traffic road outdoors at intervals of 1 hour from 9 am, and the samples are collected until 21 pm. The air bag outlet equipped with the air by the side of the main traffic road is connected to the Nafion drying tube for dehumidification through a tetrafluoro tube, and a sampling capillary of a mass spectrum is directly connected at the outlet of the Nafion drying tube. Using a flow injection pump to the heating chamber, pure CH2Br2 and the injection flow rate of the reagent and the clean air flow rate are set to 5.514 μL / min and 1L / min respectively, and the CH2Br2 reagent gas with a concentration of 2000ppmv is obtained at the outlet of the gas mixing chamber, and another mass spectrometry sampling capillary with a three-way is passed, and the other end of the three-way is connected to the tail gas and discharged, and the method is a kind of liquid phase compound dynamic gas distribution method (CN103163007A). Under the action of VUV lamp, CH2Br2 reagent gas is ionized into CH2Br2 + , and undergoes substitution reaction with ethylene in the air to form [CH2Br·C2H4] + 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. They are analyzed in the positive ion mode to obtain the air mass spectrum, such as Figure 7 As shown in the figure, strong peaks appear at m / z 121 and 123. Substituting the peak intensity into Figure 4 The corresponding ethylene concentration can be obtained from the relationship between ethylene standard gas concentration and signal intensity. The results are statistically shown as a line graph. Figure 8 .

[0047] The above detection results indicate that the method of the present invention can be used for the qualitative and quantitative detection of ethylene in the atmosphere.

[0048] 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 ethylene 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. The dibromomethane reagent ions generated by ionizing dibromomethane gas with a VUV lamp undergo a substitution reaction with ethylene molecules to form [CH2Br·C2H4] with an abundance ratio of 1:1 between m / z 121 and m / z 123. + 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 ethylene, obtaining characteristic peaks of ethylene standards, and establishing an ethylene standard curve; (2) detecting the mass spectrum of the sample to be tested and comparing it with the mass spectrum of the ethylene standard; (3) Substitute the peak intensity of the characteristic peak of the sample to be tested into the ethylene standard curve to obtain the ethylene concentration of the sample to be tested.

5. The method according to claim 4, characterized in that The step (1) comprises: introducing ethylene 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 [CH2Br·C2H4] under the action of a VUV lamp. + , enter the time-of-flight mass spectrometer, and obtain the ethylene standard mass spectrum in the positive ion mode; dilute the ethylene standard gas to obtain ethylene standard gas of different concentrations, and obtain the corresponding signal intensity, and fit to obtain the ethylene standard curve.

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 ethylene standard gas uses nitrogen as the balance gas.

7. The method according to claim 4, characterized in that The step (2) is: collecting outdoor air, passing it into a mass spectrometer injection capillary, and obtaining a mass spectrum of the air sample using the same method as the step (1).

8. The method according to claim 7, characterized in that The method for collecting outdoor air is: using a diaphragm pump in combination with a Tedlar air bag to collect the outdoor air; before the outdoor air is sampled, it is first connected to a Nafion drying tube through a tetrafluoro tube for dehumidification, and the outlet of the Nafion drying tube is connected to a sampling capillary.

9. Application of the method according to any one of claims 1 to 8 in tracking and monitoring the concentration of ethylene in the atmosphere.

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