Vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry

Abstract

The invention relates to a molecular beam mass spectrometry instrument, in particular to 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 photoelectric cathode and a differential electrode, the vacuum ultraviolet light source is arranged on the side wall of the ionization source cavity, and the ring electrode and the photoelectric cathode are sequentially arranged in the ionization source cavity in the light path direction of the vacuum ultraviolet light source. A sample injection conical opening and a differential electrode which are coaxial are arranged on the two side walls of the ionization source cavity in the direction perpendicular to the light path of the vacuum ultraviolet light source respectively, emitted light of the vacuum ultraviolet light source irradiates the photoelectric cathode to generate photoelectrons, the photoelectrons collide with a gas sample entering the ionization source cavity from the sample injection conical opening in the ring electrode, gas sample ions are obtained, and the gas sample ions are separated from the gas sample. And the gas sample ions are output through the differential electrode. According to the invention, a VUV light beam is utilized to irradiate a molecular beam vertically and crosswise from the side surface to achieve a threshold ionization effect; and meanwhile, the single-photon ionization effect is realized.

Description

Technical Field

[0001] The 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] The technologies commonly used to detect gas-phase active intermediates and free radicals mainly include molecular beam mass spectrometry and infrared spectroscopy, which are the main analytical tools for detection at present. Infrared spectroscopy often faces the problem of spectral peak overlap when analyzing active species in complex components, and also has its limitations in quantitative analysis. In contrast, molecular beam mass spectrometry is a more sensitive detection and analysis technology. It combines molecular beam sampling technology and mass spectrometry technology, and can detect and analyze active species in high-pressure systems online, such as free radicals and stable active intermediates.

[0003] At present, there are two types of molecular beam mass spectrometry ionization sources commonly used in laboratories: threshold electron ionization source (EI) and photoionization source (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 initial stage 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 10 -3 It works under Pa pressure and produces a low electron number density, which limits the improvement of sensitivity. Photoionization sources can be divided into two types, synchrotron radiation source and vacuum ultraviolet light source (VUV). Synchrotron radiation source has relatively stable light source performance, produces quasi-continuous flow photons, and the photon energy can be adjusted. The disadvantage is that the synchrotron radiation source instrument structure 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 low and can only ionize substances below 10.8eV, but the resulting spectrum is 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 work 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 lacks threshold ionization ability, and 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 view of the above problems, the object 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 the present invention is:

[0006] The 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 arranged in sequence in the ionization source cavity along the optical path direction of the vacuum ultraviolet light source, coaxial injection cone ports and differential electrodes are respectively arranged on the two 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 port in the ring electrode, gas sample ions are obtained, and the gas sample ions are output through the differential electrode.

[0007] A magnet I is arranged in the ionization source cavity near the vacuum ultraviolet light source, and a magnet II is arranged 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 ranges from 0 to 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 to the vicinity of 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 which are perpendicular to each other. The two through holes are used for passage of vacuum ultraviolet light and sample airflow 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 photoelectrons is 6-35V.

[0015] The side wall of the ionization source cavity is provided with an exhaust port and a through hole, wherein the through hole is tightly connected to a vacuum gauge, the exhaust port is connected to a gas valve and a mechanical vacuum pump, the vacuum gauge is used to measure the air 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 aperture, which is connected to the electrostatic lens and the mass analyzer in sequence, and the gas sample ions are directly introduced into the mass analyzer through the interface aperture 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 invention are as follows: the vacuum ultraviolet light photoelectron ionization source device for molecular beam mass spectrometry provided by the present invention utilizes a VUV light beam to vertically cross-irradiate a molecular beam from the side, and the electron beam is perpendicular to the direction of motion of the product ions, so the electron beam energy is relatively single, and the effect of threshold ionization can be achieved; and when the electron acceleration voltage is turned off, the effect of single photon ionization can be achieved. In order to compensate for the number of photoelectrons, research has been conducted on the use of magnetic and electric field enhancement to increase the trajectory of electron motion and photoelectric emission cathode materials, so as to further improve the detection sensitivity of the cold electron ionization source. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0020] In the figure: 1-sample gas injection tube, 2-injection 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 This is a diagram showing the threshold ionization effect in an embodiment of the present invention (sample: 10 ppm SO2). DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] See also Figure 1As shown, the present invention provides a vacuum ultraviolet 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 placed 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 optical path direction of the vacuum ultraviolet light source 11, and a coaxial injection cone 2 and a differential electrode 8 are respectively arranged on the two side walls of the ionization source cavity 14 along the optical path direction perpendicular to the vacuum ultraviolet light source 11, and the injection cone 2 is connected to a sample gas injection tube 1; the emission light of the vacuum ultraviolet light source 11 irradiates the photocathode 5 to generate photoelectrons, and the photoelectrons collide with neutral plume particles of 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.

[0024] In the embodiment of the present invention, a magnet I12 is provided in the ionization source cavity 14 near the vacuum ultraviolet light source 11, and a magnet II4 is provided at intervals on the side of the photocathode 5 away from the vacuum ultraviolet light source 11. The magnetic fields generated by the magnet I12 and the magnet II4 improve the utilization rate of the photoelectrons.

[0025] Specifically, magnet Ⅰ12 and magnet Ⅱ4 are both flat plate structures, and a circular through hole is set in the middle of magnet Ⅰ12; magnet Ⅰ12 and magnet Ⅱ4 are both made of neodymium iron boron material, and the magnetic field strength range is 0-3000Guass, preferably 900Guass; the magnetic field strength, size and applied voltage of magnet Ⅰ12 and magnet Ⅱ4 are the same.

[0026] Furthermore, a grid 15 is provided on the side of the magnet Ⅰ12 away from the vacuum ultraviolet light source 11, and a negative voltage is applied to the grid 15; 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 cubic structure and is provided with two through holes perpendicular to each other, and the two through holes are used for passage of vacuum ultraviolet light and sample airflow respectively.

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

[0029] In the embodiment of the present invention, the distance between the photocathode 5 and the ring electrode 3 is 0.5-10mm, preferably 1mm; the distance between the injection cone 2 and the ring electrode 3 is 0.5-10mm, preferably 2mm; the diameter of the injection cone 2 is 0.05-5mm, preferably 0.1mm; the negative voltage V1 applied to the photocathode 5 is 0 to -20V, the positive voltage V2 applied to the ring electrode 3 is 5-15V, and the energy of the generated photoelectrons is 6-35V. When the vacuum ultraviolet lamp 11 irradiates the photocathode 5, photoelectrons are generated by the photoelectric effect and then accelerated to an energy of (V2-V1)eV. By changing the voltage V1 of the photocathode 5 and 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, and a synchrotron radiation light source. Preferably, the vacuum ultraviolet light source 11 is a low-pressure discharge lamp light source (deuterium lamp).

[0031] In the embodiment of the present invention, a gas extraction port and a through hole are provided on the side wall of the ionization source cavity 14, wherein the through hole is tightly connected to the vacuum gauge 13, the gas extraction port is connected to the gas valve 6 and the mechanical vacuum pump 7, the vacuum gauge 13 is used to measure the air pressure in the ionization source cavity 14 in real time, and the gas valve 6 can adjust the gas flow. The gas valve 6 is a vacuum valve with adjustable flow, 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 light photoelectron ionization source device for molecular beam mass spectrometry, and its working principle is: vacuum ultraviolet lamp 11 irradiates photocathode 5 to generate photoelectrons, and the photoelectrons collide and ionize with sample gas entering through sample gas injection tube 1 under the action of electric field and magnetic field. A differential interface small hole is arranged on differential electrode 8, and the differential interface small hole is connected to electrostatic lens 9, that is, ions obtained by ionization of gas sample are directly introduced into electrostatic lens 9 through differential interface small hole on differential electrode 8; electrostatic lens 9 is connected to mass analyzer 10, and gas sample ions are directly introduced into mass analyzer 10 through interface small hole on electrostatic lens 9.

[0035] Vacuum ultraviolet photoelectron ionization source performance test. Measure the sample 10ppm SO2(N2), Figure 2 The mass spectra are measured when the photocathode voltage is 0V and 10V. The gas pressure is 0.1Pa, and each spectrum is accumulated for 30s. It can be seen from the figure that when the photocathode voltage is 0V, no signal is detected. When the photocathode voltage is 10V, a signal can be detected, and the signal has 18 counts. By adjusting the voltage of the photocathode, the energy of the photoelectrons can be changed to achieve the threshold ionization ability.

[0036] The vacuum ultraviolet light photoelectron ionization source device for molecular beam mass spectrometry provided by the present invention utilizes a vacuum ultraviolet light beam to vertically cross-irradiate a molecular beam from the side, and the electron beam is perpendicular to the movement direction of the product ions, so the electron beam energy can be individually adjusted to achieve a threshold ionization effect; when the electron acceleration voltage is turned off, a single photon ionization effect can be achieved. In order to improve the ionization efficiency, the magnetic field is enhanced to increase the electron movement trajectory, further improving 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 protection scope 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 protection scope 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 arranged in sequence 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 arranged 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).

2. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 1, characterized in that: A magnet I (12) is arranged in the ionization source cavity (14) near the vacuum ultraviolet light source (11), and a magnet II (4) is arranged 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.

3. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 2, 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, and the magnetic field strength ranges from 0 to 3000 Guass. The magnetic field strength and applied voltage of the magnet I (12) and the magnet II (4) are the same.

4. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 2, characterized in that: A grid (15) is provided on the 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 to the vicinity of the grid (15), they are pushed back, further increasing the movement path of the photoelectrons.

5. 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 which are perpendicular to each other. The two through holes are used for passage of vacuum ultraviolet light and sample airflow respectively.

6. 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.

7. 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 -20V, the positive voltage applied to the ring electrode (3) is 5-15V, and the energy of the generated photoelectrons is 6-35V.

8. 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.

9. 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).

10. The vacuum ultraviolet photoelectron ionization source device for molecular beam mass spectrometry according to claim 9, 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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