Ion screening method and system in chemical ionization mass spectrometry

Abstract

The invention belongs to the technical field of ion detection, and particularly relates to a method and a system for screening ions in chemical ionization mass spectrometry. The screening method comprises the steps that an ion screening rod is arranged on an ion transmission path of an ion transmission area of the multistage differential chemical ionization mass spectrometer, main radio frequency voltage is applied to the ion screening rod, and full-time broadband control waveforms are coupled; the main radio frequency voltage is used for realizing collision cooling and radial focusing of ions; the full-time broadband control waveform is a broadband excitation waveform of a time domain and a frequency domain, the frequency of the full-time broadband control waveform is the same as the resonant frequency of reagent ions, and the full-time broadband control waveform is used for removing excessive reagent ions. Due to the fact that flying ions are restrained by a radio frequency electric field, the ions with different mass-to-charge ratios have specific resonant frequencies, when a full-time broadband waveform with the same resonant frequency as the reagent ions is externally applied, the reagent ions can be ejected out and annihilated on the ion screening rod, and therefore real-time screening of the reagent ions is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of ion detection, and in particular relates to an ion screening method and system in chemical ionization mass spectrometry. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention, and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] In order to ensure high sensitivity, chemical ionization mass spectrometry needs to generate excess reagent ions in the ionization zone and reaction zone. After entering the detector, the reagent ions collide with the surface of the microchannel plate to generate a current signal. However, if the microchannel plate is in a saturated or oversaturated state for a long time, its electron emission ability will decrease, and the gain multiple will decay rapidly with the increase of usage time; that is, the high-intensity signal of the reagent ion flow will cause the life of the microchannel plate detector to be greatly reduced; on the other hand, the ultra-high intensity of the reagent ions raises the baseline of the detection spectrum, which is not conducive to the monitoring of the weak signal of trace peroxyl radicals, which leads to poor quantitative stability of the instrument. Therefore, removing unnecessary excess reagent ions before the ion mass analyzer is an important improvement direction for improving the performance of chemical ionization mass spectrometry instruments.

[0004] At present, the commercial quadrupole time-of-flight mass spectrometer has the function of screening ions. It applies a time-varying radio frequency voltage to the saturated reagent ions in the flight process for scanning, quenches the saturated reagent ions in the quadrupole area, and prevents them from entering the mass analyzer to remove unnecessary excess saturated ions. Moreover, in the quadrupole or ion trap transmission system using voltage scanning, in the ion selection mode, the ion scans of different mass-to-charge ratios correspond to a certain voltage, and only one ion is mass analyzed at each time point. To obtain the full spectrum analysis of the target ion, multiple scans are required, which greatly reduces the analytical throughput of the instrument. Mo Ting et al. of Shanghai University invented a device for ion screening of time-of-flight mass spectrometer in the technical document (Chinese patent application number "CN201310728867.4") and Peng Zhen et al. invented a device for ion screening of time-of-flight mass spectrometer in the technical document (Chinese patent application number "CN201711368233.7"). In its technical solution, a pulse power supply adjusts pulse parameters to output reflected pulses to the reflector plate in the reflection area of ​​the time-of-flight mass analyzer. The pulse power supply adjusts the number of reflected pulses, so that the time-of-flight mass analyzer can achieve quantitative control of ion selection / screening through the reflector plate. The reflector plate can screen out the ions to be screened through a quantitative number of screening schemes, so that the ions to be retained are reflected to the detection area for detection, which greatly improves the service life of the detector in the detection area. The time-of-flight mass analyzer achieves quantitative selective screening of ions to be selected based on reflected pulses with changed pulse numbers. However, the ion screening system is located in the mass analyzer, and the pulse voltage applied to the reflector will also affect the movement of other ions and reduce the mass spectrometer resolution. The best ion screening mode is before the ions enter the mass analyzer. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention provides a method and system for ion screening in chemical ionization mass spectrometry, which screens different ions based on the frequency scanning principle, utilizes a full-time broadband control waveform in the ion transmission region to resonantly quench the reagent ions, and realizes real-time screening of supersaturated reagent ions during continuous sampling.

[0006] In order to achieve the above object, the technical solution of the present invention is:

[0007] In a first aspect, a method for ion screening in chemical ionization mass spectrometry comprises the following process:

[0008] An ion screening rod is arranged in the ion transmission path of the ion transmission region of the multi-stage differential chemical ionization mass spectrometer, a main radio frequency voltage is applied to the ion screening rod and a full-time broadband control waveform is coupled;

[0009] The main radio frequency voltage is used to achieve collision cooling and radial focusing of ions;

[0010] The full-time broadband control waveform is a broadband excitation waveform in the time domain and the frequency domain. The frequency of the full-time broadband control waveform is the same as the resonance frequency of the reagent ions and is used to remove excess reagent ions.

[0011] Optionally, the full-time broadband control waveform corresponds to the frequency coupled to the main radio frequency and is a broadband sine wave.

[0012] Optionally, the full-time broadband control waveform is modulated by a quadratic phase function.

[0013] Optionally, a DC voltage is applied to the ion screening rod.

[0014] Optionally, the full-time broadband control waveform is determined by two parts: a waveform function shown in formula (1) and a phase modulation function shown in formula (2):

[0015]

[0016]

[0017] In formula (1), w(t) is the waveform function; A m is the amplitude of the sine wave; f m is the frequency; t is the time; is the initial phase; n is any positive integer;

[0018] In formula (2), m(t) is the phase modulation function; F(ω) is the amplitude of the broadband waveform; P(n) is the data point related to the coefficient of the quadratic phase function; ω is the angular frequency; and N is the total number of data points.

[0019] After generating a full-time broadband waveform with multiple frequency components at each time point through equation (1), the quadratic phase function (P(n) = an 2 The (bn+c) modulation eliminates the drastic amplitude change of the broadband waveform, and the full-time broadband waveform after modulation can be expressed by formula (2).

[0020] In a second aspect, a system for ion screening in chemical ionization mass spectrometry comprises:

[0021] An ion screening rod is arranged in the ion transmission path of the ion transmission zone of the multi-stage differential chemical ionization mass spectrometer, and a digital radio frequency driving device is connected to the ion screening rod to execute the above-mentioned ion screening method in the chemical ionization mass spectrometer.

[0022] Optionally, the digital RF drive device can apply only the main RF voltage to the ion screening rod without coupling the full-time broadband control waveform.

[0023] Optionally, the ion screening rod includes at least one group of ion screening rod groups, each group of ion screening rod groups includes a first screening rod and a second screening rod, and the full-time broadband control waveforms applied to the first screening rod and the second screening rod have opposite phases.

[0024] Optionally, the first screening rod and the second screening rod are one of a quadrupole, a hexapole, an octopole or a dodecapole, and the electrode shape is one of a round rod electrode, a hyperbolic electrode or a rectangular electrode.

[0025] Optionally, the ion screening system in the chemical ionization mass spectrometer also includes a front end cover and a rear end cover, the ion screening rod is arranged between the front end cover and the rear end cover, the front end cover is an electrode plate with a first center hole, and the rear end cover is an electrode plate with a second center hole, the first center hole is connected to the ionization source, and the second center hole is connected to the mass analyzer.

[0026] Optionally, the front cover and the rear cover enclose a cavity, the ion screening rod is located in the cavity, and a set vacuum degree is maintained in the cavity.

[0027] The beneficial effects of the present invention are:

[0028] The present invention provides an ion screening method and removal method in a chemical ionization mass spectrometer. Since the flying ions are constrained by a radio frequency electric field, the ions of different mass-to-charge ratios have their specific resonant frequencies when passing through an ion screening rod system based on a quadrupole field. Therefore, when a full-time broadband waveform with the same resonance frequency as the reagent ion is applied from the outside, the resonance amplitude of the reagent ions can be increased to make them eject or collide and quench on the ion screening rod, thereby realizing real-time screening of the reagent ions. This mode can efficiently remove high-intensity reagent ions while retaining target ions. Through the design of ion manipulation by an alternating electric field in a digital radio frequency driven ion screening rod (such as a quadrupole system), the excess reagent ions remaining after the ion molecule reaction in the chemical ionization reaction zone are excited and quenched, avoiding their entry into the mass analyzer to ineffectively bombard a microchannel plate detector, causing ineffective loss of the electronic coating on the microchannel plate, and extending the service life of the microchannel plate detector; reducing the baseline lift and electrical noise interference caused by the reagent ions. The ion screening rod can simultaneously apply the main radio frequency voltage and the full-time broadband control waveform to realize ion screening during continuous transmission and improve the analysis throughput. When only the main RF voltage is applied and the full-time broadband control waveform is not coupled, the system only plays the role of ion transmission. All ions generated in the ionization zone and reaction zone can be transmitted to the mass analyzer of the mass spectrometer, and the total ion current intensity of the ionization zone and reaction zone can also be evaluated. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0030] Figure 1 It is a schematic diagram of the structure of the ion screening system in the chemical ionization mass spectrometry in Example 1.

[0031] Figure 2 The quadrupole in Example 1 is Figure 1 Cross-section view at the AA position.

[0032] Figure 3 Schematic diagram of the mode switching mode in Example 1, A) is a schematic diagram of the multi-reagent ion real-time screening mass spectrometry mode, and B) is a schematic diagram of the full ion transmission mode.

[0033] Among them, 1. front end cover; 11. first center hole; 2. ion screening rod; 21. first screening rod; 22. second screening rod; 3. rear end cover; 31. second center hole; 4. digital RF driving device; 5. ionization source. DETAILED DESCRIPTION

[0034] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0035] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0036] A method for ion screening in chemical ionization mass spectrometry includes the following process:

[0037] An ion screening rod is arranged on the side of the ion flight path of the ion transmission region of the multi-stage differential chemical ionization mass spectrometer, and a main radio frequency voltage is applied to the ion screening rod and a full-time broadband control waveform is coupled;

[0038] The main radio frequency voltage is used to achieve collision cooling and radial focusing of ions;

[0039] The full-time broadband control waveform is a broadband excitation waveform in the time domain and the frequency domain, and its frequency is the same as the resonance frequency of the reagent ions, and is used to remove excess saturated reagent ions.

[0040] In the above process, the reagent ions are affected by the voltage applied to the lateral ion screening rod when flying in the ion transmission area. Under the action of the main radio frequency voltage, they collide with the sample ions to be detected, cool and achieve radial focusing, and active peroxide free radical intermediates are generated in the process; because ions of different mass-to-charge ratios have specific resonant frequencies, the reagent ions of specific charge-to-mass ratios are removed under the influence of the full-time broadband control waveform, while the target ions of active peroxide free radical intermediates are retained, thereby reducing the baseline rise and electrical noise interference caused by the reagent ions, and improving the sensitivity to weak signals of trace peroxide free radicals.

[0041] Optionally, the frequency of the full-time broadband control waveform corresponds to the excitation frequency of ion removal and is different from the waveform of the main RF voltage.

[0042] Optionally, the frequency of the main RF voltage is between 1 and 3 MHz, and the peak voltage is between 0 and 800 V.

[0043] Optionally, the full-time broadband control waveform is modulated by a quadratic phase function to remove drastic amplitude changes of the broadband waveform.

[0044] Optionally, a DC voltage is applied to the ion screening rod to form an electric field gradient in the forward direction of the ion flight path, thereby driving the ions to move forward.

[0045] Optionally, the full-time broadband control waveform consists of two parts: waveform function formula (1) and phase modulation function formula (2):

[0046]

[0047]

[0048] In formula (1), w(t) is the waveform function; A m is the amplitude of the sine wave; f m is the frequency; t is the time, is the initial phase; n is any positive integer;

[0049] In formula (2), m(t) is the phase modulation function; F(ω) is the amplitude of the broadband waveform; P(n) is the data point related to the coefficient of the quadratic phase function; ω is the angular frequency; and N is the total number of data points.

[0050] After generating a full-time broadband waveform with multiple frequency components at each time point through equation (1), the quadratic phase function (P(n) = an 2 The (bn+c) modulation eliminates the drastic amplitude change of the broadband waveform, and the full-time broadband waveform after modulation can be expressed by formula (2).

[0051] From the above formula, it can be seen that in the full-time broadband control waveform, full-time refers to the entire time domain, so t in formula (1) includes each time point, thereby ensuring real-time screening of ions during continuous sampling; broadband means that there are multiple waveforms of different frequencies at each time point, thereby achieving simultaneous screening and removal of multiple reagent ions with different charge-to-mass ratios in the ion transmission stage.

[0052] The frequency of the full-time broadband waveform in formula (2) is between 10kHz and 200kHz, and the amplitude of the waveform is between 0 and 100V.

[0053] An ion screening system in a chemical ionization mass spectrometer, comprising:

[0054] An ion screening rod is arranged in the ion transmission path of the ion transmission zone of the multi-stage differential chemical ionization mass spectrometer, and a digital radio frequency driving device is connected to the ion screening rod to execute the above-mentioned ion screening method in the chemical ionization mass spectrometer.

[0055] Optionally, the digital RF drive device can apply only the main RF voltage to the ion screening rod without coupling the full-time broadband control waveform, thereby enabling the system to switch between two modes, namely: (1) multi-reagent ion real-time screening mass spectrometry mode; (2) full ion transmission mode.

[0056] Among them, (1) in the multi-reagent ion real-time screening mass spectrometry mode, a full-time broadband control waveform is coupled to the main RF voltage of the digital RF-driven ion screening rod to efficiently remove high-intensity reagent ions while retaining target ions, effectively improving the service life of the microchannel plate detector and reducing the baseline rise and electrical noise interference caused by reagent ions. (2) In the full ion transmission mode, there is only the main RF voltage on the digital RF-driven ion screening rod in the transmission zone, and the full-time broadband control waveform is not coupled. The ion screening rod only plays the role of ion collision cooling, radial focusing and transmission. All ions generated in the ionization zone and reaction zone can be transmitted to the mass spectrometer to facilitate the evaluation of the total ion current intensity in the ionization zone and reaction zone.

[0057] Optionally, the ion screening rod includes at least one group of ion screening rod groups, each group of ion screening rod groups includes a first screening rod and a second screening rod, and the full-time broadband control waveforms applied to the first screening rod and the second screening rod have opposite phases, which are used to make reagent ions with a specific charge-to-mass ratio pop out from the radial focusing path and annihilate onto the screening rod, thereby realizing real-time screening of reagent ions.

[0058] Optionally, the first screening rod and the second screening rod are one of a round rod electrode, a hyperbolic electrode or a rectangular electrode; when the first screening rod and the second screening rod are round rod electrodes and two groups of ion screening rods are provided in the system, the ion screening rod is a quadrupole, the rod diameter is 4 to 20 mm, and the rod diameter (d 杆) to the pole inscribed circle diameter (d0) is between 1.120 and 1.160.

[0059] Optionally, the ion screening system in the chemical ionization mass spectrometry also includes a front end cover and a rear end cover, the ion screening rod is arranged between the front end cover and the rear end cover, the front end cover is an electrode plate having a first center hole with an aperture of 0.5 to 3 mm, and the rear end cover is an electrode plate having a second center hole with an aperture of 1 to 5 mm, the first center hole is connected to the ionization source, and the second center hole is connected to the mass analyzer; since the ion screening method in the chemical ionization mass spectrometry can be achieved only by applying a main RF voltage to the ion screening rod and coupling a full-time broadband control waveform, but it needs to be combined with a mass spectrometer to realize the mass spectrometry function, the front end cover and the rear end cover can connect the system in series in the mass spectrometer, and the line connecting the first center hole and the second center hole is the ion flight path, and the ion screening rod is arranged on the side of the line connecting the first center hole and the second center hole.

[0060] Optionally, the front cover and the rear cover form a cavity, the ion screening rod is located in the cavity, a set vacuum degree is maintained in the cavity, and the working gas pressure is between 0.1 and 10 Pa, which is a commonly used technical solution for mass spectrometry instruments.

[0061] Example 1

[0062] A system for screening ions in chemical ionization mass spectrometry, such as Figure 1 As shown, it includes a front end cover 1 and a rear end cover 3, an ion screening rod 2 is arranged between the front end cover 1 and the rear end cover 3, and a digital radio frequency driving device 4 is connected to the ion screening rod 2, which is used to execute the ion screening method in the chemical ionization mass spectrometry; the front end cover 1 is an electrode plate with a first central hole 11, and the rear end cover 3 is an electrode plate with a second central hole 31, the first central hole 11 is connected to the ionization source 5, and the second central hole 31 is connected to the mass analyzer; the line connecting the first central hole 11 and the second central hole 31 is the ion flight path, and the ion screening rod 2 is arranged on the side of the line connecting the first central hole 11 and the second central hole 31.

[0063] The ion screening rod 2 is a quadrupole, and the cross-sectional view of the quadrupole at the AA position is as follows: Figure 2 As shown, it includes two ion screening rod groups, each of which includes a first screening rod 21 and a second screening rod 22. The phases of the full-time broadband control waveforms applied to the first screening rod 21 and the second screening rod 22 are opposite, and the first screening rod 21 and the second screening rod 22 are adjacent to each other. The first screening rod 21 and the second screening rod 22 have the same shape and size, and the pole diameter (d 杆 ) is 12mm, the pole diameter (d 杆 ) to the pole inscribed circle diameter (d0) is 1.148.

[0064] The working gas pressure in the cavity surrounded by the front cover 1 and the rear cover 3 and including the ion screening rod 2 is 0.1 Pa.

[0065] A DC voltage V is applied to the front cover 1. DC1 , a DC voltage V is applied to the rear end cover 3 DC3 , a DC voltage V is applied to the ion screening rod 2 DC2 , so that the front cover 1, the ion screening rod 2 and the rear cover 3 form an electric field gradient of >0.5V / cm, driving the ions to move along the line connecting the first center hole 11 and the second center hole 31.

[0066] The digital RF driving device 4 applies a main RF voltage to the ion screening rod 2 and couples a full-time broadband control waveform (RF); the main RF voltage frequency is between 1 and 3 MHz, and the peak-to-peak value of the voltage is between 0 and 800 V, which is used to achieve collision cooling and radial focusing of ions; the full-time broadband control waveform is a broadband excitation waveform in the time domain and frequency domain, and its frequency is the same as the resonance frequency of the reagent ions, which is used to remove excess saturated reagent ions; the full-time broadband control waveform is obtained according to equations (1) and (2):

[0067]

[0068]

[0069] In formula (1), A m is 4.65V, f m 18.2~40.5kHz, When the frequency ω of the full-time broadband waveform in formula (2) is between 10kHz and 200kHz and the waveform amplitude F(ω) is between 0 and 100V, it can convert H3O + and O2 + The reagent ion excitation quenching prevents the microchannel plate from being in a saturated or supersaturated state for a long time, thereby extending the life of the microchannel plate detector. On the other hand, it avoids the problem of baseline elevation in the detection spectrum, enhances the monitoring effect of weak signals of trace peroxide radicals (for example, alkanes, acyl peroxide radicals, such as CH3O2, CH2C(O)O2), and improves the quantitative stability of the instrument.

[0070] Since the digital RF drive device 4 can apply only the main RF voltage to the ion screening rod 2 without coupling the full-time broadband control waveform, the system can switch between (1) multi-reagent ion real-time screening mass spectrometry mode and (2) full ion transmission mode: Figure 3As shown in (A), (1) in the multi-reagent ion real-time screening mass spectrometry mode, the full-time broadband control waveform in this embodiment is coupled based on the main RF voltage of the digital RF driven ion screening rod, and a DC voltage V is applied to the front cover DC1 , a DC voltage V is applied to the rear end cover DC3 The ion screening rod is applied with a DC voltage V DC2 , which can efficiently remove high-intensity reagent ions while retaining target ions, effectively prolong the service life of the microchannel plate detector and reduce the baseline rise and electrical noise interference caused by reagent ions; Figure 3 As shown in (B), in the full ion transmission mode, the digital RF-driven ion screening rod in the transmission zone only has the main RF voltage, and is not coupled with the full-time broadband control waveform. A DC voltage V is applied to the front cover. DC1 , a DC voltage V is applied to the rear end cover DC3 The ion screening rod is applied with a DC voltage V DC2 The ion screening rod only plays the role of ion collision cooling, radial focusing and transmission. All ions generated in the ionization zone and reaction zone can be transmitted to the mass spectrometer to facilitate the evaluation of the total ion current intensity in the ionization zone and reaction zone.

[0071] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for ion screening in chemical ionization mass spectrometry, characterized in that: The process includes: An ion screening rod is arranged in the ion transmission path of the ion transmission region of the multi-stage differential chemical ionization mass spectrometer, a main radio frequency voltage is applied to the ion screening rod and a full-time broadband control waveform is coupled; The main radio frequency voltage is used to achieve collision cooling and radial focusing of ions; The full-time broadband control waveform is a broadband excitation waveform in the time domain and the frequency domain. The frequency of the full-time broadband control waveform is the same as the resonance frequency of the reagent ions and is used to remove excess reagent ions.

2. The ion screening method in chemical ionization mass spectrometry according to claim 1, characterized in that: The full-time broadband control waveform corresponds to the frequency coupled to the main radio frequency and is a broadband sine wave.

3. The ion screening method in chemical ionization mass spectrometry according to claim 1, characterized in that: The full-time broadband control waveform is modulated by a quadratic phase function.

4. The ion screening method in chemical ionization mass spectrometry according to claim 1, characterized in that: The ion screening rod is floated with a direct current voltage.

5. The ion screening method in chemical ionization mass spectrometry according to claim 1, characterized in that: The full-time broadband control waveform is determined by two parts: the waveform function shown in formula (1) and the phase modulation function shown in formula (2): In formula (1), w(t) is the waveform function; A m is the amplitude of the sine wave; f m is the frequency; t is the time, is the initial phase, n is any positive integer; In formula (2), m(t) is the phase modulation function; F(ω) is the amplitude of the broadband waveform; P(n) is the data point related to the coefficient of the quadratic phase function; ω is the angular frequency; and N is the total number of data points.

6. An ion screening system in chemical ionization mass spectrometry, characterized in that: An ion screening rod is arranged in the ion transmission path of the ion transmission zone of the multi-stage differential chemical ionization mass spectrometer, and a digital radio frequency driving device is connected to the ion screening rod for executing the ion screening method in the chemical ionization mass spectrometer as described in any one of claims 1-5.

7. The ion screening system in chemical ionization mass spectrometry according to claim 6, characterized in that: The digital RF drive device can apply only the main RF voltage to the ion screening rod without coupling the full-time broadband control waveform.

8. The ion screening system in chemical ionization mass spectrometry according to claim 6, characterized in that: The ion screening rods include at least one group of ion screening rods, each group of ion screening rods includes a first screening rod and a second screening rod, and the full-time broadband control waveforms applied to the first screening rod and the second screening rod have opposite phases.

9. The ion screening system in chemical ionization mass spectrometry according to claim 6, characterized in that: The first screening rod and the second screening rod are one of a quadrupole, a hexapole, an octopole or a dodecapole, and the electrode shape is one of a round rod electrode, a hyperbolic electrode or a rectangular electrode.

10. The ion screening system in chemical ionization mass spectrometry according to claim 6, characterized in that: The ion screening system in the chemical ionization mass spectrometer further comprises a front cover and a rear cover, the ion screening rod is arranged between the front cover and the rear cover, the front cover is an electrode plate having a first central hole, the rear cover is an electrode plate having a second central hole, the first central hole is connected to the ionization source, and the second central hole is connected to the mass analyzer; Optionally, the front end cover and the rear end cover enclose a cavity, the ion screening rod is located in the cavity, and a set vacuum degree is maintained in the cavity.

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