A vacuum ultraviolet light photoelectron ionization source device for molecular beam mass spectrometry

Abstract

The present application relates to molecular beam mass spectrometry instrument, especially to a kind of vacuum ultraviolet light photoelectron ionization source device for molecular beam mass spectrometry.It includes vacuum ultraviolet light source, ionization source cavity, ring electrode, photocathode and differential electrode, wherein vacuum ultraviolet light source is arranged on the side wall of ionization source cavity, ring electrode and photocathode are sequentially arranged in ionization source cavity along the light path direction of vacuum ultraviolet light source, coaxial sampling cone and differential electrode are respectively arranged on the two side walls of ionization source cavity along the direction perpendicular to the light path of vacuum ultraviolet light source, the emitted light of vacuum ultraviolet light source irradiates photocathode to generate photoelectron, photoelectron collides with gas sample entering from sampling cone in ring electrode to obtain gas sample ion, and gas sample ion is output through differential electrode.The present application uses VUV light beam to irradiate molecular beam from side vertical intersection, to achieve threshold ionization effect;Single-photon ionization effect is realized simultaneously.

Description

Technical Field

[0001] The present invention relates to a molecular beam mass spectrometry instrument, in particular to a vacuum ultraviolet light photoelectron ionization source device for molecular beam mass spectrometry. Background Art

[0002] Molecular beam mass spectrometry and infrared spectroscopy are currently the primary analytical tools for detecting reactive intermediates and free radicals in the gas phase. However, infrared spectroscopy often faces peak overlap when analyzing reactive species in complex compositions and also has limitations in quantitative analysis. In contrast, molecular beam mass spectrometry is a more sensitive technique. Combining molecular beam sampling with mass spectrometry, it enables online detection and analysis of reactive species, such as free radicals and stable reactive intermediates, in high-pressure systems.

[0003] Currently, there are two types of ionization sources commonly used in molecular beam mass spectrometry laboratories: threshold electron ionization (EI) and photoionization (PI). Threshold electron ionization uses a hot filament to emit electrons. This ionization source has a simple structure, small size, and is easy to place. Therefore, it has been widely used in the early stages of the development of molecular beam mass spectrometry technology. However, due to the easy oxidation of the filament, the hot filament can only work at a temperature of 10 -3 It works under Pa gas pressure and produces a low electron number density, which limits the improvement of sensitivity. Photoionization sources can be divided into two types, synchrotron radiation sources and vacuum ultraviolet sources (VUV). Synchrotron radiation sources have relatively stable light source performance, produce quasi-continuous flow photons, and the photon energy can be adjusted. The disadvantage is that the structure of the synchrotron radiation source instrument is too complicated, the instrument volume is too large, and the price is expensive. Therefore, the equipment cannot be widely used. Low-pressure inert gas discharge lamps are small in size, low in power consumption, easy to install and use, can produce continuous flow, and the luminous flux can reach 10 12 photos / s, photoelectrons can be generated through the photoelectric effect, and photoelectrons can be fully utilized to ionize neutral particles. The photon energy they generate is relatively low and can only ionize substances below 10.8eV, but the resulting spectra are simple and easy to analyze. Photoionization, photoelectron ionization, and photochemical ionization sources have been developed using VUV lamps that continuously emit photons. The detection sensitivity has been effectively improved by increasing the gas pressure or assisting with reagent molecules. The "cold" electrons induced by light can operate under higher gas pressure conditions (0.1Pa), which is beneficial to improving the detection sensitivity of low-abundance intermediates. However, in the coaxial structure, the movement directions of electrons, ions, and gas molecules are coaxial, the electron energy is constantly changing, and there is a lack of threshold ionization ability. The energy of the ions also depends on the spatial position in the electric field, resulting in a loss of ion reception efficiency. This coaxial structure is difficult to use for molecular beam mass spectrometry. Summary of the Invention

[0004] In response to the above problems, the purpose of the present invention is to provide a vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry. The electron beam energy obtained by the device can be individually adjusted to achieve a threshold ionization effect; when the electron acceleration voltage is turned off, the single-photon ionization effect can be achieved.

[0005] In order to achieve the above purpose, the technical solution adopted by this utility model is:

[0006] The present invention provides a vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry, comprising a vacuum ultraviolet light source, an ionization source cavity, a ring electrode, a photocathode and a differential electrode, wherein the vacuum ultraviolet light source is arranged on the side wall of the ionization source cavity, the ring electrode and the photocathode are sequentially arranged in the ionization source cavity along the optical path direction of the vacuum ultraviolet light source, a coaxial injection cone and a differential electrode are respectively provided on both side walls of the ionization source cavity along the optical path direction perpendicular to the vacuum ultraviolet light source, the emission light of the vacuum ultraviolet light source irradiates the photocathode to generate photoelectrons, the photoelectrons collide with a gas sample entering through the injection cone in the ring electrode to obtain gas sample ions, and the gas sample ions are output through the differential electrode.

[0007] A magnet I is provided in the ionization source cavity near the vacuum ultraviolet light source, and a magnet II is provided at intervals on the side of the photocathode away from the vacuum ultraviolet light source. The magnetic fields generated by the magnets I and II improve the utilization rate of photoelectrons.

[0008] The magnet I and the magnet II are both flat plate structures, and a circular through hole is provided in the middle of the magnet I;

[0009] The magnet I and the magnet II are both made of neodymium iron boron, and the magnetic field strength range is 0-3000 Guass. The magnetic field strength and applied voltage of the magnet I and the magnet II are the same.

[0010] A grid is provided on the side of the magnet I away from the vacuum ultraviolet light source, and a negative voltage is applied to the grid; when the photoelectrons move near the grid, they are pushed back, further increasing the movement path of the photoelectrons.

[0011] The ring electrode is a cubic structure and is provided with two through holes perpendicular to each other. The two through holes are used for passage of vacuum ultraviolet light and sample air flow respectively.

[0012] The material of the photocathode is a metal electrode or a flat plate with a conductive metal layer plated on the surface.

[0013] The distance between the photocathode and the ring electrode is 0.5-10 mm; the distance between the injection cone and the ring electrode is 0.5-10 mm; the diameter of the injection cone is 0.05-5 mm;

[0014] The negative voltage applied to the photocathode is 0 to -20V, the positive voltage applied to the ring electrode is 5-15V, and the energy of the generated photoelectron is 6-35V.

[0015] The side wall of the ionization source cavity is provided with a gas exhaust port and a through hole, wherein the through hole is in sealing connection with a vacuum gauge, the gas exhaust port is connected with a gas valve and a mechanical vacuum pump, the vacuum gauge is used for measuring the gas pressure in the ionization source cavity in real time, and the gas valve can adjust the gas flow.

[0016] The differential electrode is provided with a differential interface small hole, the differential interface small hole is sequentially connected with an electrostatic lens and a mass analyzer, and the gas sample ions are directly introduced into the mass analyzer through the interface small hole on the electrostatic lens.

[0017] The mass analyzer is a time-of-flight mass analyzer, a quadrupole mass analyzer or an ion trap mass analyzer.

[0018] The advantages and beneficial effects of the present application are as follows: the vacuum ultraviolet light photoelectron ionization source device for molecular beam mass spectrometry provided by the present application utilizes a VUV light beam to irradiate a molecular beam from the side, the electron beam is perpendicular to the direction of motion of the product ions, the energy of the electron beam is relatively single, and the threshold ionization effect can be achieved; and when the electron acceleration voltage is turned off, the single-photon ionization effect can be achieved. In order to compensate for the number of photoelectrons, research is carried out from the aspects of utilizing a magnetic field and an electric field to enhance and increase the electron motion trajectory and photoemission cathode material, so as to further improve the detection sensitivity of the cold electron ionization source. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 It is a structural schematic diagram of the vacuum ultraviolet light photoelectron ionization source device for molecular beam mass spectrometry of the present application.

[0020] In the figure: 1-sample gas inlet tube, 2-inlet cone, 3-ring electrode, 4-magnet II, 5-photocathode, 6-gas valve, 7-mechanical vacuum pump, 8-differential electrode, 9-electrostatic lens, 10-mass analyzer, 11-vacuum ultraviolet light source, 12-magnet I, 13-vacuum gauge, 14-ionization source cavity, 15-grid;

[0021] Figure 2 It is a threshold ionization effect display diagram (sample: 10ppm SO2) in the embodiment of the present application. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is described in detail below in combination with the drawings and specific embodiments.

[0023] Reference Figure 1As shown, the present application provides a vacuum ultraviolet light photoelectron ionization source device for molecular beam mass spectrometry, comprising a vacuum ultraviolet light source 11, an ionization source cavity 14, a ring electrode 3, a photocathode 5 and a differential electrode 8, wherein the vacuum ultraviolet light source 11 is arranged on the side wall of the ionization source cavity 14, and the vacuum ultraviolet lamp 11 is arranged outside the ionization source cavity 14, the ring electrode 3 and the photocathode 5 are sequentially arranged in the ionization source cavity 14 along the light path direction of the vacuum ultraviolet light source 11, coaxial sampling cone ports 2 and differential electrodes 8 are respectively arranged on the two side walls of the ionization source cavity 14 along the direction perpendicular to the light path direction of the vacuum ultraviolet light source 11, and the sampling cone ports 2 are connected with the sample gas sampling tube 1; the emitted light of the vacuum ultraviolet light source 11 irradiates the photocathode 5 to generate photoelectrons, the photoelectrons collide with the neutral plume particles of the gas sample entering from the sampling cone port 2 in the ring electrode 3 to obtain gas sample ions, and the gas sample ions are output through the differential electrode 8.

[0024] In the embodiment of the present application, a magnet I 12 is arranged in the ionization source cavity 14 close to the vacuum ultraviolet light source 11, and a magnet II 4 is arranged away from the photocathode 5 and away from the vacuum ultraviolet light source 11, and the magnetic fields generated by the magnet I 12 and the magnet II 4 improve the utilization rate of photoelectrons.

[0025] Specifically, the magnet I 12 and the magnet II 4 are both flat plate structures, and the magnet I 12 is provided with a circular through hole in the middle; the magnet I 12 and the magnet II 4 are both made of neodymium iron boron material, and the magnetic field strength ranges from 0 to 3000 Guass, preferably 900 Guass; the magnetic field strength, size and applied voltage of the magnet I 12 and the magnet II 4 are all the same.

[0026] Further, the magnet I 12 is provided with a grid 15 on the side away from the vacuum ultraviolet light source 11, and the grid 15 is negatively charged; when the photoelectrons move to the vicinity of the grid 15, they are pushed back by the electric field generated by the grid 15, further increasing the movement path of the photoelectrons.

[0027] Preferably, the ring electrode 3 is a square structure and is provided with two through holes perpendicular to each other, and the two through holes are respectively used for the passage of vacuum ultraviolet light and sample gas flow.

[0028] Specifically, the material of the photocathode 5 is a metal electrode (such as cadmium, stainless steel, etc.) or a flat plate coated with a conductive metal layer (gold). The photocathode 5 material selects a photocathode with higher quantum efficiency and higher stability to increase the photoelectron density. The optical axis of the light emitted by the vacuum ultraviolet lamp 11 is coaxial with the magnet I 12, the grid 15, the ring electrode 3, the photocathode 5 and the magnet II 4 in the middle; the gas in the sample gas sampling tube 1 reaches the inside of the ionization source cavity 14 through the sampling cone port 2, and the outlet faces the center area of the circular through hole of the ring electrode 3.

[0029] In an embodiment of the present invention, the distance between the photocathode 5 and the ring electrode 3 is 0.5-10 mm, preferably 1 mm; the distance between the injection cone 2 and the ring electrode 3 is 0.5-10 mm, preferably 2 mm; the diameter of the injection cone 2 is 0.05-5 mm, preferably 0.1 mm; the negative voltage V1 applied to the photocathode 5 is 0 to -20 V, and the positive voltage V2 applied to the ring electrode 3 is 5-15 V, and the energy of the generated photoelectrons is 6-35 V. When the vacuum ultraviolet lamp 11 irradiates the photocathode 5, photoelectrons are generated through the photoelectric effect and then accelerated to an energy of (V2-V1) eV. By changing the voltage V1 of the photocathode 5 while maintaining the voltage V2 of the ring electrode 3, the electron energy can be changed without affecting the extraction of ions.

[0030] Specifically, the vacuum ultraviolet light source 11 is a low-pressure discharge lamp light source, a laser, or a synchrotron radiation light source. Preferably, the vacuum ultraviolet light source 11 is a low-pressure discharge lamp light source (deuterium lamp).

[0031] In an embodiment of the present invention, a gas extraction port and a through hole are formed on the side wall of the ionization source chamber 14. The through hole is sealed with a vacuum gauge 13, and the gas extraction port is connected to a gas valve 6 and a mechanical vacuum pump 7. The vacuum gauge 13 is used to measure the air pressure within the ionization source chamber 14 in real time. The gas valve 6 can adjust the gas flow. The gas valve 6 is a flow-adjustable vacuum valve, specifically a vacuum baffle valve, a vacuum needle valve, or a vacuum butterfly valve.

[0032] Furthermore, a differential interface hole is provided on the differential electrode 8, and the differential interface hole is connected to the electrostatic lens 9 and the mass analyzer 10 in sequence, that is, the ions obtained by ionization of the gas sample are directly introduced into the electrostatic lens 9 through the differential interface hole on the differential electrode 8, and then directly introduced into the mass analyzer 10 through the interface hole on the electrostatic lens 9.

[0033] Specifically, the mass analyzer 10 is a time-of-flight mass analyzer, a quadrupole mass analyzer, or an ion trap mass analyzer.

[0034] The present invention provides a vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry. Its operating principle is as follows: a vacuum ultraviolet lamp 11 irradiates a photocathode 5, generating photoelectrons. Under the action of electric and magnetic fields, the photoelectrons collide and ionize with sample gas entering through a sample gas inlet tube 1. A differential interface aperture is provided on a differential electrode 8, which is connected to an electrostatic lens 9. That is, ions obtained by ionizing the gas sample are directly introduced into the electrostatic lens 9 through the differential interface aperture on the differential electrode 8. The electrostatic lens 9 is connected to a mass analyzer 10, and the gas sample ions are directly introduced into the mass analyzer 10 through the interface aperture on the electrostatic lens 9.

[0035] Vacuum ultraviolet photoelectron ionization source performance test. Measure 10ppm SO2(N2) in the sample. Figure 2 The following are mass spectra obtained at photocathode voltages of 0 V and 10 V. The gas pressure was 0.1 Pa, and each spectrum was collected for 30 seconds. As can be seen from the figure, no signal is detected when the photocathode voltage is 0 V. However, a signal of 18 counts is detected when the photocathode voltage is 10 V. By adjusting the photocathode voltage, the energy of the photoelectrons is varied, achieving threshold ionization capability.

[0036] The present invention provides a vacuum ultraviolet (VUV) photoelectron ionization source for molecular beam mass spectrometry. This device utilizes a VUV beam to cross-irradiate the molecular beam from the side, perpendicularly. The electron beam is perpendicular to the direction of motion of the product ions, allowing the electron beam energy to be individually controlled to achieve threshold ionization. When the electron acceleration voltage is turned off, single-photon ionization can be achieved. To improve ionization efficiency, magnetic field enhancement is used to enlarge the electron trajectory, further enhancing the detection sensitivity of the cold electron ionization source.

[0037] The above description is only an embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modification, equivalent replacement, improvement, expansion, etc. made within the spirit and principle of the present invention are included in the scope of protection of the present invention.

Claims

1. A vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry, characterized in that: The invention comprises a vacuum ultraviolet light source (11), an ionization source cavity (14), a ring electrode (3), a photocathode (5) and a differential electrode (8), wherein the vacuum ultraviolet light source (11) is arranged on the side wall of the ionization source cavity (14), the ring electrode (3) and the photocathode (5) are sequentially arranged in the ionization source cavity (14) along the optical path direction of the vacuum ultraviolet light source (11), and a coaxial injection cone (2) and a differential electrode (8) are respectively provided on the two side walls of the ionization source cavity (14) along the optical path direction perpendicular to the vacuum ultraviolet light source (11), the emission light of the vacuum ultraviolet light source (11) irradiates the photocathode (5) to generate photoelectrons, and the photoelectrons collide with the gas sample entering through the injection cone (2) in the ring electrode (3) to obtain gas sample ions, and the gas sample ions are output through the differential electrode (8); A magnet I (12) is provided in the ionization source cavity (14) near the vacuum ultraviolet light source (11), and a magnet II (4) is provided at intervals on the side of the photocathode (5) away from the vacuum ultraviolet light source (11). The magnetic field generated by the magnet I (12) and the magnet II (4) improves the utilization rate of photoelectrons.

2. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 1, characterized in that: The magnet I (12) and the magnet II (4) are both flat plate structures, and a circular through hole is provided in the middle of the magnet I (12); The magnet I (12) and the magnet II (4) are both made of neodymium iron boron material, with a magnetic field strength range of 0-3000 Guass. The magnetic field strength and applied voltage of the magnet I (12) and the magnet II (4) are the same.

3. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 1, characterized in that: A grid (15) is provided on one side of the magnet I (12) away from the vacuum ultraviolet light source (11), and a negative voltage is applied to the grid (15); when the photoelectrons move near the grid (15), they are pushed back, further increasing the movement path of the photoelectrons.

4. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 1, characterized in that: The ring electrode (3) is a cubic structure and is provided with two through holes perpendicular to each other, the two through holes being used for passage of vacuum ultraviolet light and sample airflow respectively.

5. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 1, characterized in that: The material of the photocathode (5) is a metal electrode or a flat plate with a conductive metal layer plated on the surface.

6. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 1, characterized in that: The distance between the photocathode (5) and the ring electrode (3) is 0.5-10 mm; the distance between the injection cone (2) and the ring electrode (3) is 0.5-10 mm; the diameter of the injection cone (2) is 0.05-5 mm; The negative voltage applied to the photocathode (5) is 0 to -20 V, the positive voltage applied to the ring electrode (3) is 5-15 V, and the energy of the generated photoelectrons is 6-35 V.

7. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 1, characterized in that: The side wall of the ionization source cavity (14) is provided with an exhaust port and a through hole, wherein the through hole is tightly connected to a vacuum gauge (13), and the exhaust port is connected to a gas valve (6) and a mechanical vacuum pump (7). The vacuum gauge (13) is used to measure the gas pressure in the ionization source cavity (14) in real time, and the gas valve (6) can adjust the gas flow rate.

8. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 1, characterized in that: The differential electrode (8) is provided with a differential interface aperture, which is connected to the electrostatic lens (9) and the mass analyzer (10) in sequence, and the gas sample ions are directly introduced into the mass analyzer (10) through the interface aperture on the electrostatic lens (9).

9. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 8, characterized in that: The mass analyzer (10) is a time-of-flight mass analyzer, a quadrupole mass analyzer or an ion trap mass analyzer.

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