VUV lamp-based method for mass spectrometric detection of straight-chain alkane

Abstract

The invention relates to a method for detecting straight-chain alkane by mass spectrometry based on a VUV lamp, which comprises the following steps: taking a time-of-flight mass spectrum as an analyzer, taking nitrogen dioxide as a reagent molecule of chemical ionization, and enabling to-be-detected gas to enter a mass spectrum ionization source in a negative pressure form through a PEEK capillary tube; meanwhile, nitrogen dioxide gas with the volume concentration of 1% enters a mass spectrum ionization source in a negative pressure form through another PEEK capillary tube, and nitrogen dioxide ions generated by ionizing the nitrogen dioxide gas through a VUV lamp and straight-chain alkane molecules are subjected to a hydrogen abstracting reaction to generate # imgabs0 # ions. According to the method, the defects that the sensitivity of VUV lamp single-photon ionization detection of the straight-chain alkane is not enough and fragmentation is difficult to avoid are overcome, and the qualitative analysis capability of the device on the straight-chain alkane and the signal intensity are improved.

Description

Technical Field

[0001] The present invention belongs to the field of mass spectrometry analysis methods, and particularly relates to a rapid mass spectrometry analysis method for detecting linear alkanes. A time-of-flight mass spectrometer is used as an analyzer, and nitrogen dioxide is used as a chemical ionization reagent molecule. The gas to be measured enters the mass spectrometry ionization source in a negative pressure form through a PEEK capillary, and at the same time, a nitrogen dioxide gas with a volume concentration of 1% enters the mass spectrometry ionization source in a negative pressure form through another PEEK capillary. The reagent ions of nitrogen dioxide generated by ionizing the nitrogen dioxide gas with a VUV lamp react with the linear alkane molecules to form [C n H 2n+2 -H] + characteristic peaks. Background Art

[0002] Linear alkanes are a class of hydrocarbons whose molecules are composed of directly connected carbon and hydrogen atoms. Such compounds are usually represented by the general formula C n H 2n+2 where n represents the number of carbon atoms in the alkane. Linear alkanes are one of the basic members of the hydrocarbon family, having a simple molecular structure in which carbon atoms form a straight-chain structure and hydrogen atoms fill the vacancies of carbon atoms. These linear alkanes are widely present in nature and are mainly the main components in petroleum and natural gas. They are also the main components of fuels and are widely used in energy production and the chemical industry. In addition, linear alkanes are also the starting materials for many chemical syntheses and are used to prepare various chemical products. The detection of linear alkanes is of great significance in multiple fields. In the environmental field, monitoring the concentration of linear alkanes can provide information on air quality and the degree of air pollution. In the petroleum and natural gas industries, detecting the content of alkanes helps to evaluate fuel quality and treat impurities in fuels. In chemical production, linear alkanes are key intermediates in many synthesis processes, so detecting their concentration is crucial for optimizing the production process.

[0003] Analytical techniques such as gas chromatography-mass spectrometry (GC-MS) are usually used to detect linear alkanes. These methods can provide highly accurate and sensitive results to meet the requirements for alkane analysis in different fields. Saturated hydrocarbons (alkanes) are difficult to analyze by mass spectrometry due to their unique chemical characteristics of having ionizable functional groups and low basicity. The ionization methods used for analyzing alkanes usually include electron ionization, photoionization, flame ionization detector, and field ionization / field desorption, combined with gas chromatography. These methods are time-consuming and complex for rapid and simple original analysis.

[0004] In recent years, the soft ionization mass spectrometry developed can directly analyze most compounds without sample pretreatment, featuring rapidity, high efficiency, sensitivity, etc. Among them, the photoionization chemical reagent molecular ionization source based on VUV lamp is one of them. It ionizes the chemical reagent gas molecules through the VUV lamp, and then significantly improves the detection sensitivity of the analyte through charge transfer, association reaction and substitution reaction between the chemical reagent molecular ions and other analytes, realizing the detection of trace substances. Due to its high sensitivity and easy operation, etc., it has received extensive attention. The photoionization chemical reagent molecular ionization source based on VUV lamp has been successfully applied to the detection of gas molecules such as hydrogen sulfide, methanethiol, glyoxal, ethylene, ammonia, etc., and can detect samples in trace amounts and complex matrices. And compared with other methods, the time-of-flight mass spectrometry based on the photoionization chemical reagent molecular ionization source can obtain richer spectroscopic information, which is more conducive to the detection of complex matrices and reaction processes. Therefore, it is of great significance to invent a method for detecting linear alkanes based on photoionization nitrogen dioxide chemical ionization source that can achieve rapid, high sensitivity and low fragmentation rate.

[0005] Aiming at the problem that there is a lack of a method for detecting linear alkanes that is rapid, efficient, sensitive and has a low fragmentation rate, a method for detecting linear alkanes by mass spectrometry based on a VUV lamp of the present invention improves the sensitivity of linear alkane detection, and has the characteristics of rapidity, high efficiency, sensitivity, low fragmentation rate, etc., and is suitable for the highly sensitive and qualitative detection of linear alkanes. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to realize a method for detecting linear alkanes that is rapid, efficient, sensitive and has a low fragmentation rate: a time-of-flight mass spectrometry is used as an analyzer, and a photoionization nitrogen dioxide chemical ionization source is used as a mass spectrometry ionization source. The gas to be measured enters the mass spectrometry ionization source in a negative pressure form through a PEEK capillary, and at the same time, 1% nitrogen dioxide gas by volume concentration enters the mass spectrometry ionization source in a negative pressure form through another PEEK capillary. The nitrogen dioxide ions generated by ionizing the nitrogen dioxide gas with a VUV lamp react with the linear alkane molecules to form [C n H 2n+2 -H] + characteristic peaks, and are compared with the standard mass spectrometry spectrum of the single-photon ionization mode of linear alkanes established in advance. By comparing the mass spectrometry spectra obtained under the two ionization modes, it can be found that the photoionization nitrogen dioxide chemical ionization source has the characteristics of rapidity, high efficiency, sensitivity and low fragmentation rate.

[0007] The technical solution of the present invention is to use a time-of-flight mass spectrometry as an analyzer and a photoionization nitrogen dioxide chemical ionization source as a mass spectrometry ionization source. The specific steps are as follows:

[0008] ① Establish the mass spectrometry spectrum of nitrogen dioxide standard quality

[0009] Using nitrogen dioxide standard gas (volume concentration 1%, nitrogen as the balance gas), it is introduced into the three-way joint at a flow rate of 100 mL / min by a flow controller. One path enters the first sampling capillary of the mass spectrometry ionization source, and the other path enters the tail gas purification device. Nitrogen is introduced into another three-way joint at a flow rate of 100 mL / min by a flow controller. One path enters the second sampling capillary of the mass spectrometry ionization source, and the other path enters the tail gas purification device. Under the action of the VUV lamp, the nitrogen dioxide reagent gas is ionized into NO 2 + ions. It is shaped by the radio frequency quadrupole and the electrostatic transmission region in a vacuum suction manner and finally enters the time-of-flight mass spectrometer for analysis in the positive ion mode, thus obtaining the mass spectrometry spectrum of the nitrogen dioxide standard quality.

[0010] ② Establish the single-photon ionization mode mass spectrometry spectrum of the straight-chain alkane standard

[0011] Using straight-chain alkane standard gas (concentration 1 ppmv, nitrogen as the balance gas), it is introduced into the three-way joint at a flow rate of 100 mL / min by a flow controller. One path enters the first sampling capillary of the mass spectrometry ionization source, and the other path enters the tail gas purification device. Nitrogen is introduced into another three-way joint at a flow rate of 100 mL / min by a flow controller. One path enters the second sampling capillary of the mass spectrometry ionization source, and the other path enters the tail gas purification device. Under the action of the VUV lamp, the straight-chain alkane is ionized. It is shaped by the radio frequency quadrupole and the electrostatic transmission region in a vacuum suction manner and finally enters the time-of-flight mass spectrometer for analysis in the positive ion mode, thus obtaining the mass spectrometry spectrum of the straight-chain alkane standard.

[0012] ③ Establish the mass spectrometry spectrum of the straight-chain alkane standard in the nitrogen dioxide chemical ionization mode

[0013] Using straight-chain alkane standard gas (concentration 1 ppmv, nitrogen as the balance gas), it is introduced into the mass spectrometry sampling capillary with a three-way joint at a flow rate of 100 mL / min by a flow controller. Nitrogen dioxide standard gas (volume concentration 1%, nitrogen as the balancer) is introduced into another mass spectrometry sampling capillary with a three-way joint of the ion source at a flow rate of 100 mL / min by a flow controller. Under the action of the VUV lamp, the straight-chain alkane is ionized. It is shaped by the radio frequency quadrupole and the electrostatic transmission region in a vacuum suction manner and finally enters the time-of-flight mass spectrometer for analysis in the positive ion mode, thus obtaining the mass spectrometry spectrum of the straight-chain alkane standard.

[0014] ④ Detection results

[0015] Compare the single-photon ionization mode mass spectrometry spectrum of the sample obtained in ② with the mass spectrometry spectrum of the nitrogen dioxide chemical ionization mode obtained in ③ one by one, and observe the signal intensity of the main peak of the spectrum and the fragmentation rate of the straight-chain alkane.

[0016] The working principle and characteristics of the instrument used in the present invention are that the photoinduced nitrogen dioxide chemical ionization source is a kind of soft ionization method. In the present invention, nitrogen dioxide gas with a volume concentration of 1% enters the ionization source cavity and undergoes a hydrogen abstraction reaction with linear alkanes under the action of a VUV lamp. The pressure difference between Skimmer1 and the radio frequency bias voltage is reduced to 0.5 V to reduce E / N, which is most beneficial for the formation of [CnH 2n+2 -H] + . The gas pressure inside the ionization source is set to 600 Pa, which is the optimal gas pressure for the formation of [C n H 2n+2 -H] + .

[0017] In the present invention, the gas to be measured enters the mass spectrometry ionization source in a negative pressure form through a PEEK capillary, and at the same time, nitrogen dioxide gas with a volume concentration of 1% enters the mass spectrometry ionization source in a negative pressure form through another PEEK capillary. The nitrogen dioxide ions generated by ionizing nitrogen dioxide gas using a VUV lamp undergo a hydrogen abstraction reaction with linear alkane molecules to generate [C n H 2n+2 -H] + ions. The method of the present invention makes up for the deficiencies of insufficient sensitivity in detecting linear alkanes by single-photon ionization of a VUV lamp and inevitably generating fragmentation, and improves the qualitative analysis ability and signal intensity of the device for linear alkanes.

[0018] The present invention can achieve rapid, efficient, sensitive and low-fragmentation detection of linear alkanes, becoming a powerful means for the detection of linear alkanes. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Mass spectrometry diagram of nitrogen dioxide standard gas under photoinduced nitrogen dioxide chemical ionization mode;

[0020] Figure 2 Relationship diagram between the peak signal intensity of n-octane standard gas and the ionization source gas pressure under photoinduced nitrogen dioxide chemical ionization mode;

[0021] Figure 3 Relationship diagram between the main peak ratio of n-octane standard gas and the ionization source gas pressure under photoinduced nitrogen dioxide chemical ionization mode;

[0022] Figure 4 Relationship diagram between the peak signal intensity of n-octane standard gas and the pressure difference between Skimmer1 and the quadrupole bias under photoinduced nitrogen dioxide chemical ionization mode;

[0023] Figure 5 Relationship diagram between the main peak ratio of n-octane standard gas and the pressure difference between Skimmer1 and the quadrupole bias under photoinduced nitrogen dioxide chemical ionization mode;

[0024] Figure 6Comparison of the spectral peaks of n-octane standard gas between the photo-induced nitrogen dioxide chemical ionization mode and the single-photon ionization mode;

[0025] Figure 7 Comparison of the spectral peaks of n-undecane standard gas between the photo-induced nitrogen dioxide chemical ionization mode and the single-photon ionization mode. Specific implementation manner

[0026] The photoionization time-of-flight mass spectrometry consists of a VUV lamp ionization source, an ion transmission system, and a time-of-flight detector. Among them, the ionization source injects samples through two injection capillaries, and its gas pressure can be adjusted by the valve connected between the vacuum pump and the ionization source. The ion transmission system includes a radio frequency quadrupole and an electrostatic transmission region, and its voltage can be adjusted by an electronic control system. The time-of-flight detector consists of a pulse system and an ion detection system. The ion source chamber is connected to the quadrupole radio frequency quadrupole region through Skimmer1; one end of the two injection capillaries is respectively connected to the ion source chamber, and the other end is connected to one interface of two three-way joints.

[0027] Example 1

[0028] In this example, the R5020-TOF MS time-of-flight mass spectrometry is used as the analyzer, and the photo-induced nitrogen dioxide chemical ionization source is used as the mass spectrometry ionization source. The specific steps are as follows:

[0029] ① Establish the mass spectrometry spectrum of nitrogen dioxide standard quality

[0030] Use nitrogen dioxide standard gas (volume concentration 1%, nitrogen as the balance gas), and use a flow controller to introduce it into the first interface of the three-way joint connected to the first injection capillary at a flow rate of 100 mL / min. One way enters the ion source chamber through the second interface of the three-way joint and then enters the first injection capillary of the mass spectrometry ionization source, and the other way enters the sealed container filled with molecular sieve with an air inlet and an air outlet through the third interface of the three-way joint (the third interface is connected to the air inlet).

[0031] Use a flow controller to introduce nitrogen into the first interface of another three-way joint connected to the second injection capillary at a flow rate of 100 mL / min. One way enters the ion source chamber through the second interface of the three-way joint and then enters the second injection capillary of the mass spectrometry ionization source, and the other way enters the sealed container filled with molecular sieve with an air inlet and an air outlet through the third interface of the three-way joint (the third interface is connected to the air inlet).

[0032] In the ion source chamber, under the action of the VUV lamp, the nitrogen dioxide reagent gas is ionized into NO 2 + ions, shaped by the radio frequency quadrupole and the electrostatic transmission region in a vacuum suction manner, and finally enter the time-of-flight detector for analysis in the positive ion mode, that is, the Figure 1Mass spectrum of nitrogen dioxide standard quality. Select 600 Pa as the ionization source chamber pressure and 0.5 V as the pressure difference between the skimmer1 voltage before the quadrupole and the quadrupole bias voltage in the ion transport system for the experiment.

[0033] ② Establish the mass spectrum of linear alkane standard in single photon ionization mode

[0034] Use a linear alkane standard gas (concentration 1 ppmv, nitrogen as the balance gas). Using a flow controller, introduce it into the first interface of the tee connected to the first sampling capillary at a flow rate of 100 mL / min. One path enters the ion source chamber through the second interface of the tee and then through the first sampling capillary of the mass spectrometry ionization source. The other path enters a sealed container filled with molecular sieve with an air inlet and an air outlet (the third interface is connected to the air inlet) through the third interface of the tee.

[0035] Using a flow controller, introduce nitrogen into the first interface of another tee connected to the second sampling capillary at a flow rate of 100 mL / min. One path enters the ion source chamber through the second interface of the tee and then through the second sampling capillary of the mass spectrometry ionization source. The other path enters a sealed container filled with molecular sieve with an air inlet and an air outlet (the third interface is connected to the air inlet) through the third interface of the tee.

[0036] In the ion source chamber, the linear alkane is ionized under the action of the VUV lamp, shaped by the radio frequency quadrupole and the electrostatic transmission region in a vacuum suction manner, and finally enters the time-of-flight detector for analysis in the positive ion mode, thus obtaining Figure 6 、 Figure 7 The upper part of the mass spectrum of linear alkane standard in single photon ionization mode. Select 600 Pa as the ionization source chamber pressure and 0.5 V as the pressure difference between the skimmer1 voltage before the quadrupole and the quadrupole bias voltage in the ion transport system for the experiment.

[0037] ③ Establish the mass spectrum of linear alkane standard in nitrogen dioxide chemical ionization mode

[0038] Use a linear alkane standard gas (concentration 1 ppmv, nitrogen as the balance gas). Using a flow controller, introduce it into the first interface of the tee connected to the first sampling capillary at a flow rate of 100 mL / min; One path enters the ion source chamber through the second interface of the tee and then through the first sampling capillary of the mass spectrometry ionization source. The other path enters a sealed container filled with molecular sieve with an air inlet and an air outlet (the third interface is connected to the air inlet) through the third interface of the tee.

[0039] Using a flow controller, nitrogen dioxide standard gas (volume concentration 1%, with nitrogen as the balancer) is introduced into the first interface of another three-way connected to the second sampling capillary at a flow rate of 100 mL / min. One path enters the second sampling capillary of the mass spectrometry ionization source through the second interface of the three-way and then enters the ion source chamber, and the other path enters a sealed container filled with molecular sieve with an air inlet and an air outlet through the third interface of the three-way (the third interface is connected to the air inlet).

[0040] In the ion source chamber, nitrogen dioxide is ionized under the action of a VUV lamp, and then undergoes a hydrogen abstraction reaction with long-chain alkanes to generate [C n H 2n+2 -H] + ions. They are shaped by a radio frequency quadrupole and an electrostatic transmission region in a vacuum suction manner and finally enter the time-of-flight detector for analysis in the positive ion mode.

[0041] By changing the air pressure in the ionization source through the ionization source side valve, the variation of the main peak intensity and fragment peak intensity of the n-octane sample with the ionization source air pressure (200, 300, 400, 500, 600 Pa) is investigated, and the results are as Figure 2 . The peak intensities are statistically analyzed to investigate the variation of the main peak ratio with the ionization source air pressure, and the results are as Figure 3 .

[0042] By changing the voltage difference between the skimmer1 voltage in front of the quadrupole and the quadrupole offset voltage in the ion transport system through the electronic control system, the variation of the main peak intensity and fragment peak intensity of the n-octane sample with the voltage difference between the two (0.5, 1.5, 2.5, 3.5, 4.5 V) is investigated, and the results are as Figure 4 . The peak intensities are statistically analyzed to investigate the variation of the main peak ratio with the voltage difference, and the results are as Figure 5 .

[0043] Considering the main peak intensity and main peak ratio of n-octane comprehensively, 600 Pa is selected as the air pressure in the ionization source chamber, and 0.5 V is selected as the voltage difference between the skimmer1 voltage in front of the quadrupole and the quadrupole offset voltage in the ion transport system for subsequent experiments.

[0044] The n-octane and n-undecane samples are detected using the nitrogen dioxide chemical ionization mode respectively to obtain mass spectra. After subtracting the nitrogen dioxide standard quality spectrum obtained in step ①, the lower half of the mass spectra of the straight-chain alkane standard products in the nitrogen dioxide chemical ionization mode is obtained. Figure 6 、 Figure 7

[0045] ④ Detection results

[0046] The single-photon ionization mode mass spectra of the samples obtained in step ② ( Figure 6 、 Figure 7 ​The upper half of the figure) and the mass spectrum of the nitrogen dioxide chemical ionization mode obtained in step ③ ( Figure 6 , Figure 7 The lower half of the figure) were compared one by one to observe the signal intensity of the main peak of the spectrum and the fragmentation rate of the straight-chain alkanes. The signal intensity of the main peak of the n-octane and n-undecane standards in the nitrogen dioxide chemical ionization mode was stronger than that in the single-photon ionization mode, and the degree of fragmentation was significantly reduced.

Claims

1. A method for mass spectrometric detection of straight-chain alkanes based on a VUV lamp, characterized in that: a time-of-flight mass spectrometer is used as an analyzer, and nitrogen dioxide is used as a chemical ionization reagent molecule; the photoionization time-of-flight mass spectrometer includes a VUV lamp ionization source, an ion transmission system, and a time-of-flight detector, wherein the ionization source is sampled through two sampling capillaries; the ion transmission system includes a radio frequency quadrupole and an electrostatic transmission region; the ion source chamber is connected to the radio frequency quadrupole region through Skimmer1; one end of each of the two sampling capillaries is connected to the ion source chamber, and the other end is connected to one interface of two three-way joints; a straight-chain alkane standard gas (concentration feasible range 1 ppbv - 10 ppmv, preferred range 10 ppbv - 1 ppmv, nitrogen as balance gas) is used, and is introduced into the first interface of the three-way joint connected to the first sampling capillary at a flow rate of 50 - 400 mL / min (flow rate feasible range 50 - 200 mL / min, preferred range 50 - 150 mL / min) by a flow controller; one path enters the mass spectrometry ionization source through the first sampling capillary and then enters the ion source chamber, and the other path enters the tail gas purification device through the third interface of the three-way joint; nitrogen dioxide standard gas (concentration feasible range 1% - 20%, preferred range 1% - 10%, nitrogen as balance gas) is introduced into the first interface of the other three-way joint connected to the second sampling capillary at a flow rate of 50 - 400 mL / min (flow rate feasible range 50 - 200 mL / min, preferred range 50 - 150 mL / min) by a flow controller, one path enters the mass spectrometry ionization source through the second sampling capillary and then enters the ion source chamber, and the other path enters the tail gas purification device through the third interface of the three-way joint; 200 - 700 Pa is selected as the air pressure of the ion source chamber (air pressure feasible range 550 - 600 Pa, preferred air pressure 600 Pa), and an experiment is carried out with a voltage difference of 0.3 - 10 V between the voltage of Skimmer1 before the quadrupole and the quadrupole bias voltage in the ion transmission system (voltage difference feasible range 0.3 V to 4.5 V, preferred voltage difference 0.4 - 0.6 V), and analysis is carried out in the positive ion mode.

2. The method for mass spectrometric detection of straight-chain alkanes based on a VUV lamp according to claim 1, characterized in that: the straight-chain alkane is a straight-chain alkane with carbon number C4 - C20, preferably in the range of C6 - C16, and more preferably in the range of C8 - C12.

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