An off-axis curved photoionization source

Abstract

The present application relates to mass spectrometry instrument technical field, specifically, it is a kind of off-axis curved photoionization source, including ionization source cavity, sample tube, push-pull electrode, vacuum ultraviolet lamp, introduction electrode rod group, off-axis curved electrode rod group, lead-out electrode rod group and vacuum pump, the axial center line of the light source emission end of vacuum ultraviolet lamp and the axial center line of ion output port are on the same plane and present fixed angle of inclusion.The present application can make the sample ion produced by ionization discharge from ion output port after successfully passing through the curved path formed by introduction electrode rod group, off-axis curved electrode rod group, lead-out electrode rod group, effectively reduce the interference of photon and neutral molecule to subsequent analysis;On the other hand, by using the electrode rod group covered by insulating material, chemical ionization and electron impact ionization and other ionization pathways caused by photoelectron generation can also be avoided, so as to ensure the single and high efficiency of single-photon ionization, improve mass spectrometry qualitative and quantitative ability.

Description

Technical Field

[0001] This invention relates to the field of mass spectrometry analysis instrument technology, specifically to an off-axis bending photoionization source. Background Technology

[0002] Photoionization technology based on commercially available vacuum ultraviolet lamps is an attractive soft ionization technology. Photoionization itself has advantages such as excellent ionization efficiency, high molecular ion yield and easy spectrum interpretation. Furthermore, using a simple and compact vacuum ultraviolet lamp as the light source further reduces the cost of use. Therefore, it is widely used in environmental analysis, clinical diagnosis and industrial process monitoring.

[0003] Different ionization sources can be developed using photoelectrons generated by vacuum ultraviolet light in the reagent region. However, photoelectrons, through the influence of an electric field, can induce other ionization pathways besides photoionization, such as electron bombardment ionization and chemical ionization. The existence of multiple ionization modes narrows the linear dynamic range of sample measurement. Furthermore, common cylindrical ionization sources cannot effectively remove interference from electrons, photons, and other neutral particles, resulting in a high signal-to-noise ratio in mass spectrometry, which is detrimental to qualitative and quantitative analysis. Summary of the Invention

[0004] To address the above problems, the present invention aims to provide an off-axis bending photoionization source.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] An off-axis bending photoionization source includes an ionization source cavity, a sample inlet tube, a repulsion electrode, a vacuum ultraviolet lamp, an inlet electrode rod assembly, an off-axis bending electrode rod assembly, an outlet electrode rod assembly, and a vacuum pump.

[0007] The ionization source cavity is provided with a vacuum pump port and an ion output port, and the vacuum pump port is connected to the vacuum pump.

[0008] The vacuum ultraviolet lamp, repulsion electrode, lead-in electrode rod group, off-axis bending electrode rod group, and lead-out electrode rod group are respectively installed inside the ionization source cavity in sequence.

[0009] The introduced electrode rod group includes at least four or more even-numbered introduced electrode rods; the off-axis bending electrode rod group includes at least four or more even-numbered off-axis bending electrode rods; and the led-out electrode rod group includes at least four or more even-numbered led-out electrode rods. The number of introduced electrode rods, the number of off-axis bending electrode rods, and the number of led-out electrode rods are all equal. The axial centerlines of each introduced electrode rod are parallel to each other and evenly distributed on the circumference of the same circle A, which lies on a plane perpendicular to the axial centerlines of each introduced electrode rod. The arcuate axes of each off-axis bending electrode rod are evenly distributed on the circumference of the same circle B, which lies on a plane perpendicular to the arcuate axes of each off-axis bending electrode rod. The axial centerlines of each led-out electrode rod are parallel to each other and evenly distributed on the circumference of the same circle C. Circle C is located on a plane perpendicular to the axial center lines of each of the lead-out electrode rods. The radii of circles A, B, and C are equal. One end face of each lead-in electrode rod is located near the repulsion electrode and is spaced from the side of the repulsion electrode away from the vacuum ultraviolet lamp. The other end face of each lead-in electrode rod is located away from the repulsion electrode. One end face of each off-axis bent electrode rod is opposite to and spaced from the other end face of a corresponding lead-in electrode rod. The other end face of each off-axis bent electrode rod is opposite to and spaced from the one end face of a corresponding lead-out electrode rod. One end face of each lead-out electrode rod is located away from the ion output port, and the other end face of each lead-out electrode rod is located near the ion output port and is spaced from the ion output port.

[0010] One end of the sample inlet tube is located outside the ionization source cavity and is used to allow sample molecules to enter the ionization source cavity through the sample inlet tube. The other end of the sample inlet tube passes into the ionization source cavity and extends to the space between the repulsion electrode and the introduction electrode rod group.

[0011] The light emitted from the light source of the vacuum ultraviolet lamp passes through the repulsion electrode. The axial centerline of the light source of the vacuum ultraviolet lamp and the axial centerline of the ion output port are located on the same plane and form a fixed angle. A DC voltage is applied to the repulsion electrode, the lead-in electrode rod group, the off-axis bent electrode rod group, and the lead-out electrode rod group, and the DC voltage decreases sequentially. A radio frequency voltage is also applied to the lead-in electrode rod group, the off-axis bent electrode rod group, and the lead-out electrode rod group.

[0012] A valve is installed on the vacuum extraction port.

[0013] The vacuum extraction port is located at the bottom of the ionization source cavity, and the ion output port is located on the side of the ionization source cavity. The axial center line of the vacuum extraction port and the axial center line of the light source emitting end of the vacuum ultraviolet lamp are both perpendicular to the horizontal plane, and the axial center line of the ion output port is parallel to the horizontal plane.

[0014] The axial centerline of the injection tube is perpendicular to the axial centerline of the light source emitting end of the vacuum ultraviolet lamp.

[0015] A mass analyzer is provided on the outside of the ionization source cavity, and the sample input end of the mass analyzer is connected to the ion output port.

[0016] The axial centerline of the light source emitting end of the vacuum ultraviolet lamp, the centerline of the circular through hole of the repulsion electrode, and the center of circle A are all collinear, and the axial centerline of the ion output port is collinear with the center of circle C.

[0017] The repulsion electrode is an annular plate structure with a circular through hole in the middle.

[0018] One end face of each of the introduced electrode rods is located in the same plane A, and the other end face of each of the introduced electrode rods is located in the same plane B. One end face of each of the off-axis bent electrode rods is located in the same plane C, and the other end face of each of the off-axis bent electrode rods is located in the same plane D. One end face of each of the led-out electrode rods is located in the same plane E, and the other end face of each of the led-out electrode rods is located in the same plane F. Planes A, B, and C are all parallel to the repulsion electrode, and planes D, E, and F are parallel to each other. Planes C and D form a fixed angle, and this fixed angle is the same as the fixed angle between the axial centerline of the vacuum ultraviolet lamp light source and the axial centerline of the ion output port.

[0019] Different DC voltages are sequentially applied to the repulsive electrode, the lead electrode rod group, the off-axis bent electrode rod group, and the lead electrode rod group in descending order of voltage to form an ion transport electric field.

[0020] The applied radio frequency peak-to-peak amplitudes are equal and the phases are opposite for every two adjacent introduced electrode rods in the introduced electrode rod group; the applied radio frequency peak-to-peak amplitudes are equal and the phases are opposite for every two adjacent off-axis bent electrode rods in the off-axis bent electrode rod group; the applied radio frequency peak-to-peak amplitudes are equal and the phases are opposite for every two adjacent led-out electrode rods in the led-out electrode rod group; the surfaces of all introduced electrode rods, off-axis bent electrode rods and led-out electrode rods are covered with an insulating material layer of the same thickness.

[0021] The advantages and positive effects of this invention are as follows:

[0022] This invention, through the coordinated arrangement of an electrode rod assembly, an off-axis bent electrode rod assembly, and an extraction electrode rod assembly, ensures that the sample ions generated by ionization smoothly pass through the bent path formed by the introduced electrode rod assembly, the off-axis bent electrode rod assembly, and the extraction electrode rod assembly before being discharged from the ion output port. This effectively reduces the interference of photons and neutral molecules on subsequent analysis. On the other hand, by using an electrode rod assembly with a surface covered by insulating material, other ionization pathways such as chemical ionization and electron bombardment ionization caused by photoelectron generation can also be avoided, thus ensuring the singularity and efficiency of single-photon ionization and improving the qualitative and quantitative capabilities of mass spectrometry. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the present invention;

[0024] Figure 2 for Figure 1 A schematic diagram of the cross-sectional structure at point A.

[0025] In the figure: 1 is the ionization source cavity, 2 is the sample inlet tube, 3 is the repulsion electrode, 4 is the vacuum ultraviolet lamp, 5 is the inlet electrode rod assembly, 6 is the off-axis bent electrode rod assembly, 7 is the outlet electrode rod assembly, 8 is the needle valve, and 9 is the vacuum pump. Detailed Implementation

[0026] The following is in conjunction with the appendix Figure 1-2 The present invention will be described in further detail below.

[0027] An off-axis bending photoionization source, such as Figure 1-2 As shown, this embodiment includes an ionization source cavity 1, a sample inlet tube 2, a repulsion electrode 3, a vacuum ultraviolet lamp 4, an inlet electrode rod assembly 5, an off-axis bending electrode rod assembly 6, an outlet electrode rod assembly 7, and a vacuum pump 9.

[0028] The ionization source chamber 1 is equipped with a vacuum extraction port and an ion output port, with the vacuum extraction port connected to a vacuum pump 9. The vacuum pump 9 extracts gas from the ionization source chamber 1 through the vacuum extraction port, creating a negative pressure within the chamber; the gas pressure inside the ionization source chamber 1 is typically maintained at 10-1000 Pa. The vacuum pump 9 is a commercially available product, controlled by an external controller.

[0029] The vacuum ultraviolet lamp 4, the repulsion electrode 3, the lead electrode rod group 5, the off-axis bending electrode rod group 6, and the lead electrode rod group 7 are sequentially installed inside the ionization source cavity 1. The installation structure of the vacuum ultraviolet lamp 4, the repulsion electrode 3, the lead electrode rod group 5, the off-axis bending electrode rod group 6, and the lead electrode rod group 7 adopts existing technology.

[0030] The electrode rod group 5 includes at least four or more even-numbered electrode rods, the off-axis bending electrode rod group 6 includes at least four or more even-numbered off-axis bending electrode rods, and the electrode rod group 7 includes at least four or more even-numbered electrode rods. In this embodiment, the number of electrode rods, the number of off-axis bending electrode rods, and the number of electrode rods are all equal, which is four. Alternatively, a six-pole group or an eight-pole group with the same corresponding structure can be used instead.

[0031] The axial centerlines of each introduced electrode rod are parallel to each other and evenly distributed on the circumference of the same circle A, which lies on a plane perpendicular to the axial centerlines of each introduced electrode rod. The arc-shaped axes of each off-axis bent electrode rod are evenly distributed on the circumference of the same circle B, which lies on a plane perpendicular to the arc-shaped axes of each off-axis bent electrode rod. The axial centerlines of each led-out electrode rod are parallel to each other and evenly distributed on the circumference of the same circle C, which lies on a plane perpendicular to the axial centerlines of each led-out electrode rod. The radii of circles A, B, and C are equal, all being R. One end face of each introduced electrode rod is located near the repulsion electrode 3 and is spaced apart from the side of the repulsion electrode 3 away from the vacuum ultraviolet lamp 4. The other end face of each introduced electrode rod is located away from the repulsion electrode 3. One end face of each off-axis bent electrode rod is opposite to and coaxially placed with a gap from the other end face of the corresponding introduced electrode rod. The other end face of each off-axis bent electrode rod is opposite to and coaxially placed with a gap between it and the end face of a corresponding lead-out electrode rod. One end face of each lead-out electrode rod is located on the side away from the ion output port, and the other end face of each lead-out electrode rod is located on the side closer to the ion output port, with a gap between them.

[0032] One end of the injection tube 2 is located outside the ionization source chamber 1 and is used to allow sample molecules to enter the ionization source chamber 1 through the injection tube 2. The other end of the injection tube 2 is inserted into the ionization source chamber 1 and extends to the space between the repulsion electrode 3 and the introduction electrode rod assembly 5. One end of the injection tube 2 can be placed directly in an environment containing sample molecules, or it can be connected to the source of sample molecules.

[0033] The light emitted from the light source of the vacuum ultraviolet lamp 4 passes through the repulsion electrode 3. The axial centerline of the light source of the vacuum ultraviolet lamp 4 and the axial centerline of the ion output port are on the same plane and form a fixed angle. In this embodiment, the fixed angle is 90°, but the fixed angle can be set arbitrarily according to the usage requirements. A DC voltage is applied to the repulsion electrode 3, the lead-in electrode rod group 5, the off-axis bent electrode rod group 6, and the lead-out electrode rod group 7, and the voltage decreases sequentially. A radio frequency voltage is also applied to the lead-in electrode rod group 5, the off-axis bent electrode rod group 6, and the lead-out electrode rod group 7.

[0034] Specifically, in this embodiment, the vacuum extraction port is located at the bottom of the ionization source cavity 1, and the ion output port is located on the side of the ionization source cavity 1. The axial centerline of the vacuum extraction port and the axial centerline of the light source emitting end of the vacuum ultraviolet lamp 4 are both perpendicular to the horizontal plane, while the axial centerline of the ion output port is parallel to the horizontal plane. The axial centerline of the sample inlet tube 2 is perpendicular to the axial centerline of the light source emitting end of the vacuum ultraviolet lamp 4. The axial centerline of the light source emitting end of the vacuum ultraviolet lamp 4, the centerline of the circular through-hole of the repulsion electrode 3, and the center of circle A are all collinear. The axial centerline of the ion output port is collinear with the center of circle C. This design is convenient to use and ensures accurate detection. In this embodiment, the vacuum ultraviolet lamp 4 can be a commercially available low-pressure inert gas discharge lamp, such as a krypton (Kr) discharge lamp, a deuterium (D2) discharge lamp, or a xenon (Xe) discharge lamp. In this embodiment, the injection tube 2 can be a capillary tube made of stainless steel, quartz or polyether ether ketone (PEEK) with an inner diameter of 50-500μm and a length of 10-60cm.

[0035] Specifically, in this embodiment, a valve 8 is provided on the vacuum extraction port to control the opening and closing of the vacuum extraction port. The valve 8 is a commercially available needle valve, which is convenient to open and close.

[0036] Specifically, in this embodiment, a mass analyzer is provided on the outside of the ionization source cavity 1, and the sample input end of the mass analyzer is connected to the ion output port. The mass analyzer receives ions discharged from the ion output port for subsequent detection, and the configuration of the mass analyzer is in accordance with existing technology. In this embodiment, the mass analyzer can be a commercially available time-of-flight mass analyzer, quadrupole mass analyzer, ion trap mass analyzer, sector magnetic field mass analyzer, or ion cyclotron resonance mass analyzer.

[0037] Specifically, in this embodiment, the repulsion electrode 3 is an annular plate structure with a circular through hole in the center. The thickness of the repulsion electrode 3 is 1-8 mm, and the diameter of the circular through hole is 2-6 mm. The material of the repulsion electrode 3 can be a conductive metal (such as stainless steel) or a plate with a conductive metal layer plated on its surface.

[0038] In this embodiment, all lead-in electrode rods, off-axis bending electrode rods, and lead-out electrode rods are circular in cross-section, with a uniform cross-sectional radius of r, which can range from 2 to 20 mm. After determining the selected value of r, the common radius R of circles A, B, and C is determined according to the ratio r / (Rr) = 1.13. In this embodiment, all lead-in electrode rods, off-axis bending electrode rods, and lead-out electrode rods are made of conductive metal (such as stainless steel) and are covered with an insulating material layer of the same thickness (such as polytetrafluoroethylene or polyetheretherketone), with an insulating material layer thickness of 0.5-2 mm. All lead-in electrode rods and lead-out electrode rods can be of equal length for easy replacement and maintenance. In this embodiment, the off-axis bending electrode rod group 6 is divided into two parallel off-axis bending electrode rods with a larger radius of curvature and two parallel off-axis bending electrode rods with a smaller radius of curvature. Each off-axis bending electrode rod is quarter-circular, i.e., a 90° bending rod. Each off-axis bent electrode rod with a larger radius of curvature is located in the same plane as a corresponding off-axis bent electrode rod with a smaller radius of curvature, and the centers of their circular axes are at the same location. Depending on the application requirements, each off-axis bent electrode rod can also be an acute-angled, right-angled, or obtuse-angled bent rod.

[0039] In this embodiment, one end face of each introduced electrode rod is located in the same plane A, and the other end face of each introduced electrode rod is located in the same plane B. One end face of each off-axis bent electrode rod is located in the same plane C, and the other end face of each off-axis bent electrode rod is located in the same plane D. One end face of each led-out electrode rod is located in the same plane E, and the other end face of each led-out electrode rod is located in the same plane F. Planes A, B, and C are all parallel to the repulsion electrode 3, and planes D, E, and F are parallel to each other, with plane C perpendicular to plane D.

[0040] Specifically, in this embodiment, different DC voltages (range 10-100V) are sequentially applied to the repulsion electrode 3, the lead-in electrode rod group 5, the off-axis bent electrode rod group 6, and the lead-out electrode rod group 7 in descending order of voltage to form an ion transport electric field. In this embodiment, the applied radio frequency voltage frequency to the repulsion electrode 3, the lead-in electrode rod group 5, the off-axis bent electrode rod group 6, and the lead-out electrode rod group 7 is 0.5-5MHz, and the radio frequency peak-to-peak value is 10-500V. The peak-to-peak value amplitudes of the applied radio frequency voltages on every two adjacent lead-in electrode rods in the lead-in electrode rod group 5 are equal, but the phases are opposite. Similarly, the peak-to-peak value amplitudes of the applied radio frequency voltages on every two adjacent off-axis bent electrode rods in the off-axis bent electrode rod group 6 are equal, but the phases are opposite. The peak-to-peak value frequencies and the applied radio frequency voltages on the lead-in electrode rod group 5, the off-axis bent electrode rod group 6, and the lead-out electrode rod group 7 can be the same or different. The voltage settings at each location and the energizing methods for the repulsion electrode 3, the lead-in electrode rod group 5, the off-axis bending electrode rod group 6, and the lead-out electrode rod group 7 all adopt existing technologies.

[0041] Working principle:

[0042] During operation, the sample to be tested enters the ionization source chamber 1 through the sample inlet tube 2. The gas in the ionization source chamber 1 is extracted from the vacuum pump 9 through the vacuum extraction port, creating a negative pressure in the ionization source chamber 1, which allows the sample molecules to enter smoothly and efficiently. Through the coordinated arrangement of the introduced electrode rod group 5, the off-axis bent electrode rod group 6, and the extracted electrode rod group 7, the sample ions generated by ionization can bend and move along the path formed by the introduced electrode rod group 5, the off-axis bent electrode rod group 6, and the extracted electrode rod group 7 before being discharged from the ion output port, effectively reducing the interference of photons and neutral molecules on subsequent analysis. On the other hand, by using the electrode rod group whose surface is covered with insulating material, it is also possible to avoid other ionization pathways such as chemical ionization and electron bombardment ionization caused by photoelectron generation, thereby ensuring the single-photon ionization is singular and efficient, and improving the qualitative and quantitative capabilities of mass spectrometry.

Claims

1. An off-axis bending photoionization source, characterized in that: It includes an ionization source chamber (1), a sample inlet tube (2), a repulsion electrode (3), a vacuum ultraviolet lamp (4), an inlet electrode rod assembly (5), an off-axis bending electrode rod assembly (6), an outlet electrode rod assembly (7), and a vacuum pump (9). The ionization source cavity (1) is provided with a vacuum pump port and an ion output port, and the vacuum pump port is connected to the vacuum pump (9). The vacuum ultraviolet lamp (4), the repulsion electrode (3), the lead electrode rod group (5), the off-axis bending electrode rod group (6), and the lead electrode rod group (7) are respectively installed inside the ionization source cavity (1); The introduced electrode rod group (5) includes four or more even-numbered introduced electrode rods, the off-axis bending electrode rod group (6) includes four or more even-numbered off-axis bending electrode rods, and the led-out electrode rod group (7) includes four or more even-numbered led-out electrode rods. The number of introduced electrode rods, the number of off-axis bending electrode rods, and the number of led-out electrode rods are all equal. The axial center lines of each introduced electrode rod are parallel to each other and evenly distributed on the circumference of the same circle A. Circle A is located on a plane perpendicular to the axial center lines of each introduced electrode rod. The arc-shaped axes of each off-axis bending electrode rod are evenly distributed on the circumference of the same circle B. Circle B is located on a plane perpendicular to the arc-shaped axes of each off-axis bending electrode rod. The axial center lines of each led-out electrode rod are parallel to each other and evenly distributed on the circumference of the same circle C. On a plane perpendicular to the axial centerline of each of the lead-out electrode rods, the radii of circles A, B, and C are equal. One end face of each lead-in electrode rod is located near the repulsion electrode (3) and is spaced from the side of the repulsion electrode (3) away from the vacuum ultraviolet lamp (4). The other end face of each lead-in electrode rod is located away from the repulsion electrode (3). One end face of each off-axis bent electrode rod is opposite to the other end face of a corresponding lead-in electrode rod and is spaced apart. The other end face of each off-axis bent electrode rod is opposite to the one end face of a corresponding lead-out electrode rod and is spaced apart. One end face of each lead-out electrode rod is located away from the ion output port. The other end face of each lead-out electrode rod is located near the ion output port and is spaced apart from the ion output port. One end of the sample inlet tube (2) is located outside the ionization source cavity (1) and is used to allow sample molecules to enter the ionization source cavity (1) through the sample inlet tube (2). The other end of the sample inlet tube (2) is inserted into the ionization source cavity (1) and extends to the space between the repulsion electrode (3) and the introduction electrode rod group (5). The light emitted from the light source of the vacuum ultraviolet lamp (4) passes through the repulsion electrode (3). The axial center line of the light source of the vacuum ultraviolet lamp (4) and the axial center line of the ion output port are located on the same plane and form a fixed angle. DC voltage is applied to the repulsion electrode (3), the lead electrode rod group (5), the off-axis bent electrode rod group (6) and the lead electrode rod group (7) in sequence. Radio frequency voltage is also applied to the lead electrode rod group (5), the off-axis bent electrode rod group (6) and the lead electrode rod group (7).

2. The off-axis bending photoionization source according to claim 1, characterized in that: A valve (8) is provided on the vacuum extraction port.

3. The off-axis bending photoionization source according to claim 1, characterized in that: The vacuum extraction port is located at the bottom of the ionization source cavity (1), and the ion output port is located on the side of the ionization source cavity (1). The axial center line of the vacuum extraction port and the axial center line of the light source emitting end of the vacuum ultraviolet lamp (4) are both perpendicular to the horizontal plane, and the axial center line of the ion output port is parallel to the horizontal plane.

4. The off-axis bending photoionization source according to claim 1, characterized in that: The axial centerline of the sample inlet tube (2) is perpendicular to the axial centerline of the light source emitting end of the vacuum ultraviolet lamp (4).

5. An off-axis bending photoionization source according to claim 1, characterized in that: A mass analyzer is provided on the outside of the ionization source cavity (1), and the sample input end of the mass analyzer is connected to the ion output port.

6. The off-axis bending photoionization source according to claim 1, characterized in that: The axial centerline of the light source emitting end of the vacuum ultraviolet lamp (4), the centerline of the circular through hole of the repulsion electrode (3) and the center of circle A are all collinear, and the axial centerline of the ion output port is collinear with the center of circle C.

7. An off-axis bending photoionization source according to claim 1, characterized in that: The repulsion electrode (3) is an annular plate structure with a circular through hole in the middle.

8. An off-axis bending photoionization source according to claim 7, characterized in that: One end face of each of the introduced electrode rods is located in the same plane A, the other end face of each of the introduced electrode rods is located in the same plane B, one end face of each of the off-axis bent electrode rods is located in the same plane C, the other end face of each of the off-axis bent electrode rods is located in the same plane D, one end face of each of the led-out electrode rods is located in the same plane E, and the other end face of each of the led-out electrode rods is located in the same plane F. Planes A, B, and C are all parallel to the repulsion electrode (3), and planes D, E, and F are parallel to each other. Planes C and D form a fixed angle, and this fixed angle is the same as the fixed angle between the axial center line of the light source emitting end of the vacuum ultraviolet lamp (4) and the axial center line of the ion output port.

9. An off-axis bending photoionization source according to claim 1, characterized in that: Different DC voltages are sequentially applied to the repulsion electrode (3), the lead electrode rod group (5), the off-axis bending electrode rod group (6), and the lead electrode rod group (7) in order from high to low voltage to form an ion transport electric field.

10. An off-axis bending photoionization source according to claim 1, characterized in that: The peak-to-peak amplitude of the radio frequency applied to each pair of adjacent inlet electrode rods in the inlet electrode rod group (5) is equal and the phase is opposite; the peak-to-peak amplitude of the radio frequency applied to each pair of adjacent off-axis bent electrode rods in the off-axis bent electrode rod group (6) is equal and the phase is opposite; the peak-to-peak amplitude of the radio frequency applied to each pair of adjacent outlet electrode rods in the outlet electrode rod group (7) is equal and the phase is opposite; the surfaces of all inlet electrode rods, off-axis bent electrode rods and outlet electrode rods are covered with an insulating material layer of the same thickness.

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