Mass analysis device

Abstract

The mass spectrometer of the present invention suppresses charging in an ion source and reduces the burden of maintenance work on the device. One embodiment of the mass spectrometer of the present invention is a mass spectrometer having an ion source for ionizing components contained in a sample gas, wherein the ion source (3) comprises: an ionization chamber (30) having an ion ejection port (301) and forming a space substantially separated from the outside; a reflection electrode (31) disposed in the ionization chamber and used to form a push-out electric field for pushing ions generated in the ionization chamber to the outside through the ion ejection port; and a voltage generating unit (7) selectively applying a first voltage for forming the push-out electric field and a second voltage of positive polarity having an absolute value greater than the first voltage for forming a charge elimination electric field to the reflection electrode.

Description

Technical Field

[0001] The present invention relates to a mass spectrometer, and more particularly to a mass spectrometer using an ion source implemented by electron ionization (EI), chemical ionization (CI), or negative chemical ionization (NCI). Background Art

[0002] In a gas chromatograph-mass spectrometer (GC-MS), a mass spectrometer primarily utilizes ionization methods such as EI, CI, or NCI to ionize compounds in a sample gas. In this mass spectrometer, compounds in the sample gas introduced into an ionization chamber are ionized using an appropriate ionization method such as the one described above. The generated ions are then transported via an ion transmission optical system to a mass separation unit, such as a quadrupole mass filter, where they are separated and detected based on their mass-to-charge ratio (strictly speaking, "m / z" in italics, but generally referred to as "mass-to-charge ratio" in this specification).

[0003] Figure 6 This is a schematic diagram of the configuration of a general EI ion source disclosed in Patent Document 1, etc. For convenience of explanation, three mutually orthogonal axes, X, Y, and Z, are defined in space.

[0004] The ion source 3 includes a box-shaped ionization chamber 30 made of conductive components. A repeller electrode 31 is disposed within the ionization chamber 30. An electron inlet 302 is formed on the upper wall of the ionization chamber 30, and an electron outlet 303 is formed on the lower wall. A filament 32 is disposed outside the electron inlet 302, and a trap electrode (actually, the filament) 33 is disposed outside the electron outlet 303. Furthermore, a pair of magnets 34 and 35 are disposed outside the filament 32 and the trap electrode 33 to sandwich the filament 32 and the trap electrode 33. An ion ejection port 301 is formed on the front wall of the ionization chamber 30 (the wall opposite to the wall where the repeller electrode 31 is disposed), and an extraction electrode 36 is disposed outside the ion ejection port. A sample gas inlet tube 304 is connected to a side wall of the ionization chamber 30. The ionization chamber 30 is grounded, with a potential of 0V.

[0005] During analysis, a current is supplied to the filament 32 from a power supply (not shown), causing the filament 32 to generate heat and generate thermal electrons. A DC voltage having a predetermined potential difference is applied to the filament 32 and the trap electrode 33, respectively. Due to this potential difference, the thermal electrons generated in the filament 32 are accelerated and move to the trap electrode 33. As a result, a thermal electron flow is formed in the ionization chamber 30 as a whole, which moves in the negative direction of the Y axis (see Figure 6 (dashed arrows in the figure). Sample components (compounds) in the sample gas supplied to the ionization chamber 30 via the sample gas inlet pipe 304 come into contact with the thermal electrons and are ionized. Magnets 34 and 35 form a magnetic field with magnetic flux lines oriented along the Y-axis, and this magnetic field suppresses the spread of the thermal electron flow in the X- and Z-axis directions.

[0006] A DC voltage having the same polarity as that of the ions originating from the sample and having an absolute value slightly higher than the potential of the ionization chamber 30 (here, 0V) is applied to the reflection electrode 31. As a result, in the ionization chamber 30, an electric field is formed between the reflection electrode 31 and the ion ejection port 301 to push the ions from the reflection electrode 31 side toward the ion ejection port 301 side. By the action of this push-out electric field, the ions generated near the center of the ionization chamber 30 move toward the ion ejection port 301. On the other hand, a DC voltage having an opposite polarity to that of the ions is applied to the extraction electrode 36, and the extraction electric field thus formed enters the interior of the ionization chamber 30 through the ion ejection port 301. By the action of both the above-mentioned push-out electric field and this extraction electric field, the ions are extracted from the ionization chamber 30 along the X-axis direction through the ion ejection port 301 (refer to Figure 6 Thick arrows in the figure) for quality analysis.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2016-157523 Summary of the Invention

[0010] Technical problem to be solved by the invention

[0011] In the above-mentioned ion source 3, various substances such as compound molecules contained in the sample gas or ions derived therefrom adhere to the inner wall surface of the ionization chamber 30, forming a contamination layer. If the degree of contamination becomes serious, the surface of the contamination layer will be charged due to the adhesion of electrons. If the electric field in the ionization chamber 30 is disturbed by this charging, the ions generated in the ionization chamber 30 will become easily attracted to the inner wall of the ionization chamber 30, causing collisions and disappearing. As a result, the amount of ions transmitted to the subsequent mass separator may be reduced, the analytical sensitivity may be reduced, and correct analysis may be hindered. Therefore, generally speaking, when it is suspected that the inner wall of the ionization chamber is contaminated due to charging problems, the user will stop the device and remove the ionization chamber to perform maintenance operations such as cleaning it.

[0012] Since the above maintenance work is performed every time charging is presumed to have occurred, it is a huge burden for the user. In addition, if such maintenance work is performed, not only does it take time to disassemble and clean the device, but it also takes time to start the device to restore the vacuum chamber to a vacuum state. For this reason, there is also the problem of being unable to perform measurement work for a certain period of time, resulting in a decrease in the operating rate of the device. In addition, if charging becomes serious during the analysis, there is also the problem of not being able to obtain reliable analysis results, making the analysis itself futile.

[0013] The present invention has been made to solve the above-mentioned technical problems, and an object of the present invention is to provide a mass spectrometer capable of eliminating or reducing charge-up generated in an ion source without stopping the device.

[0014] Solutions for solving the above technical problems

[0015] One aspect of the mass spectrometer of the present invention, which has been developed to solve the above-mentioned technical problems, is a mass spectrometer including an ion source for ionizing components included in a sample gas, the ion source including:

[0016] an ionization chamber having an ion ejection port and forming a space separated from the outside;

[0017] A reflection electrode is provided in the ionization chamber and is used to form an electric field to push ions generated in the ionization chamber outward through the ion ejection port;

[0018] The voltage generating unit selectively applies a first voltage for forming the push-out electric field and a second voltage of positive polarity having an absolute value greater than the first voltage for forming a charge-eliminating electric field to the reflective electrode.

[0019] The ion source is an ion source that performs ionization using thermal electrons, and specifically, can be an ion source implemented by the EI method, the CI method, or the NCI method.

[0020] Effects of the Invention

[0021] In the mass spectrometer of the present invention, when a highly positive voltage (the aforementioned second voltage) is applied to the reflected electrons, a strong electric field is formed between the reflecting electrode and the inner wall of the ionization chamber, attracting the electrons. This causes the charged electrons to peel off from the surface of the contamination layer formed on the inner wall of the ionization chamber, where they come into contact with the reflecting electrode and are extinguished.

[0022] In this way, the mass spectrometer according to the above-mentioned scheme of the present invention can eliminate or reduce the charge generated in the ionization chamber without stopping the device and without releasing the vacuum state of the device. As a result, when charge is generated in the ion source, the frequency of the troublesome maintenance operations such as stopping the device, disassembling the device and cleaning the ionization chamber, which are previously performed, can be reduced, and the burden on the user can be reduced. In addition, the state in which the device cannot be used to perform such maintenance operations can be reduced, and the operating rate of the device can be improved. In addition, since charge can be eliminated or reduced in the middle of the analysis, it is possible to avoid adverse conditions caused by charge in the middle of the analysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a configuration diagram of the main parts of a mass spectrometer as one embodiment of the present invention.

[0024] Figure 2 is a schematic diagram of the state of electrons inside the ion source in the mass spectrometer of this embodiment. Figure 2 (A) is a schematic diagram of a case where a voltage of +1 V is applied to the reflective electrode. Figure 2 (B) is a schematic diagram when a voltage of +100 V is applied to the reflective electrode.

[0025] Figure 3 1 is a diagram showing an example of a voltage waveform applied to a reflection electrode in the mass spectrometer according to the present embodiment.

[0026] Figure 4 1 and 2 are diagrams showing simulation results of electron orbits inside the ionization chamber (in the absence of a magnetic field for converging hot electrons).

[0027] Figure 5 It is a diagram showing the simulation results of electron orbits inside the ionization chamber (when a magnetic field for converging hot electrons is present).

[0028] Figure 6 This is a schematic diagram of the general structure of an EI ion source. DETAILED DESCRIPTION

[0029] A mass spectrometer as one embodiment of the present invention will be described with reference to the drawings.

[0030] Figure 1 FIG1 is a diagram showing the configuration of the main parts of the mass spectrometer of this embodiment. This mass spectrometer is a single quadrupole mass spectrometer.

[0031] like Figure 1 As shown, the mass spectrometer of this embodiment includes an EI ion source 3, an ion transmission optical system 4, a quadrupole mass filter 5 as a mass separator, and an ion detector 6 along an ion optical axis C inside a chamber 1 evacuated by a vacuum pump (not shown).

[0032] EI ion source 3 with Figure 6 The ion sources shown have the same structure. Specifically, the EI ion source 3 includes an ionization chamber 30, which is generally rectangular and made of a conductive material such as metal; a reflective electrode 31 disposed within the ionization chamber 30; a filament 32 disposed outside an electron inlet 302 formed on the upper wall of the ionization chamber 30; a trap electrode 33 disposed outside an electron outlet 303 formed on the lower wall of the ionization chamber 30; a pair of magnets 34 and 35 disposed so as to sandwich the filament 32 and trap electrode 33; and an extraction electrode 36 disposed outside the ion ejection port 301. Furthermore, a sample gas inlet pipe 304 is connected to the side wall of the ionization chamber 30.

[0033] The ionization chamber 30 is grounded and its potential is 0 V. Under the control of the control unit 8, the reflection voltage generating unit 7 applies a predetermined voltage to the reflection electrode 31. Figure 1 Although not described in the text, this mass spectrometer includes a voltage generator that applies a predetermined voltage to the filament 32, trap electrode 33, extraction electrode 36, and the like included in the EI ion source 3. Furthermore, this device also includes a voltage generator that applies a predetermined voltage to the subsequent ion transmission optical system 4, quadrupole mass filter 5, and the like.

[0034] Next, refer to Figure 2 and Figure 3 The operation of mass analysis performed in the mass spectrometer of this embodiment will be described. Figure 2 Schematic diagram of the state of electrons inside the ionization chamber 30 . Figure 3 3 is a diagram showing an example of a voltage waveform applied to the reflective electrode 31 .

[0035] For example, a sample gas containing a plurality of compounds separated over time in a chromatographic column of a gas chromatograph (not shown) is introduced into the ionization chamber 30 through the sample gas inlet tube 304. An electric current is supplied to the filament 32, thereby heating the filament 32 and generating thermal electrons. Due to the voltages applied to the filament 32 and the trap electrode 33, respectively, a potential difference is formed between them, and the thermal electrons are given energy due to the potential difference. As a result, the thermal electrons are emitted from the filament 32 and travel toward the trap electrode 33. That is, a thermal electron flow is formed from the filament 32 toward the trap electrode 33. The thermal electrons fly in a manner rotating around the magnetic flux lines in the magnetic field formed by the magnets 34 and 35. As a result, the diffusion of the thermal electron flow along the X-axis direction and the Z-axis direction is suppressed.

[0036] Under the control of the control unit 8, the reflection voltage generating unit 7 applies the following voltage to the reflection electrode 31: Figure 3 The pulse voltage shown has a low-level voltage value of +1V and a high-level voltage value of +100V. The low-level voltage value is the same as the voltage applied to the reflector electrode 31 in conventional mass spectrometers to form a repulsion electric field within the ionization chamber 30. The pulse interval of the pulse voltage applied to the reflector electrode 31 is at least 10 times greater than the pulse width. For example, the interval between adjacent pulses is 1 μsec, and the pulse width is 0.01 to 0.1 μsec.

[0037] In the ionization chamber 30, the compound molecules in the sample gas come into contact with the thermal electrons and are ionized. Figure 2 As shown in (A), when the voltage applied to the reflection electrode 31 is +1V, the push-out electric field formed in the ionization chamber 30 due to the potential difference between the reflection electrode 31 and the inner wall of the ionization chamber 30 (in this case, 1V) has the effect of pushing the ions (positive ions) generated in the above manner along the approximately X-axis direction, that is, toward the ion ejection port 301. On the other hand, a DC voltage with a polarity opposite to that of the ions is applied to the extraction electrode 36, and the extraction electric field generated thereby reaches the interior of the ionization chamber 30 through the ion ejection port 301. This extraction electric field has the effect of attracting ions. Due to the effects of both the push-out electric field and the extraction electric field, the ions generated by contact with the thermal electrons in the ionization chamber 30 are extracted to the outside through the ion ejection port 301 and are then introduced into the ion transmission optical system 4.

[0038] In the ion transmission optical system 4, ions are temporarily converged near the ion optical axis C and then sent to the quadrupole mass filter 5. A predetermined voltage, which is a direct current voltage plus a high-frequency voltage (RF voltage), is applied to the four rod electrodes constituting the quadrupole mass filter 5. This voltage selectively allows only ions with a specific mass-to-charge ratio corresponding to the voltage to pass through the quadrupole mass filter 5. The ion detector 6 generates and outputs a detection signal corresponding to the amount of ions that have arrived. Therefore, for example, by controlling the applied voltage so that the mass-to-charge ratio of ions passing through the quadrupole mass filter 5 varies within a predetermined range, it is possible to acquire mass spectrum data showing the ion intensities within the predetermined mass-to-charge ratio range.

[0039] Compound molecules contained in the sample gas and a portion of the generated ions collide with the inner wall of the ionization chamber 30 and adhere to the inner wall, forming a contamination layer 30A. Figure 2 Figure (A) depicts only the contamination layer 30A formed on the front wall of the ionization chamber 30, where the ion ejection port 301 is formed. However, the contamination layer forms on the entire inner wall of the ionization chamber 30, albeit to varying degrees. Because this contamination layer 30A is insulating, if some electrons, such as thermal electrons, adhere to the surface of the contamination layer 30A, they generate charge. This charge is a major factor in disrupting the electric field formed within the ionization chamber 30. However, in this device, applying a voltage of +100V to the reflective electrode 31 eliminates the charge as described below.

[0040] like Figure 2 As shown in (B), when the voltage applied to reflective electrode 31 increases to +100V, the potential difference between reflective electrode 31 and the inner wall of ionization chamber 30 increases significantly compared to the previous potential difference of 1V. Consequently, electrons adhering to contamination layer 30A formed on the inner wall of ionization chamber 30 are stripped away by the strong positive electric field (charge-eliminating electric field) and travel toward reflective electrode 31. These electrons then contact reflective electrode 31 and disappear. This allows charge to be eliminated, or even if it cannot be completely eliminated, it can be reduced.

[0041] Because the strong charge-eliminating electric field also affects hot electrons, the trajectory of some of the hot electrons entering the ionization chamber 30 through the electron inlet 302 is bent and absorbed by the reflective electrode 31. However, the acceleration energy imparted to the hot electrons is high, so not all of the hot electrons disappear. Instead, at least some of the hot electrons contribute to the ionization of sample components within the ionization chamber 30. Therefore, while the +100V voltage is applied to the reflective electrode 31, although the ionization efficiency decreases, ionization itself continues. Furthermore, while the charge-eliminating electric field also affects the behavior of generated ions, the movement of ions, which are much more massive than electrons, is slower than that of electrons. Therefore, by shortening the time during which the +100V voltage is applied to the reflective electrode 31, the ejection of ions from the ionization chamber 30 is virtually unaffected.

[0042] In the above example, a +100V voltage is applied to the reflective electrode 31 to eliminate charging. However, a voltage of tens to hundreds of volts can also be applied to remove electrons attached to the contamination layer 30A. Furthermore, as mentioned above, to minimize the impact on ionization and ion behavior, the pulse width of the high-voltage pulse is desirably as short as possible. However, if this duration is too short, electrons removed from the contamination layer 30A will not reach the reflective electrode 31, and the charging effect will not be fully enhanced. The pulse width depends on the pulse voltage value; the higher the voltage, the shorter the pulse width. Furthermore, the appropriate pulse width also depends on the distance between the reflective electrode 31 and the inner wall of the ionization chamber 30. Therefore, the appropriate pulse width and voltage value can be determined through experiments or simulations. Similarly, the pulse interval is set to 1μs in the above example, but it can be appropriately varied to minimize its impact on ionization and ion behavior.

[0043] Figure 4 as well as Figure 5 3 is a diagram showing simulation results of electron orbits inside the ionization chamber 30 . Figure 4 This is the case where there is no magnetic field for converging hot electrons generated by the magnets 34 and 35. Figure 5 This is the case where there is a magnetic field for converging thermal electrons generated by the magnets 34 and 35 .

[0044] like Figure 4 As shown in FIG. 1 , in the absence of a hot electron converging magnetic field, the electrons peeled off from the contamination layer on the inner surface of the front wall of the ionization chamber 30 travel almost straight toward the reflective electrode 31. Figure 5 As shown, in the presence of a hot electron converging magnetic field generated by magnets 34 and 35, not only the hot electron flow but also electrons stripped from the contamination layer on the inner surface of the front wall of ionization chamber 30 are affected by the Lorentz force generated by the magnetic field. Consequently, the electrons travel toward reflective electrode 31 while performing a complex motion, avoiding and circumventing areas of strong magnetic fields. In either case, electrons stripped from contamination layer 30A eventually reach reflective electrode 31, where they contact and disappear.

[0045] In the mass spectrometer of this embodiment, as described above, a positive high voltage is periodically applied to the reflector electrode 31 during analysis to eliminate charging. This prevents charging in the ion source 3 during analysis, which could reduce the amount of ions available for analysis. Consequently, high analytical sensitivity can be ensured. Furthermore, even if a certain contamination layer forms, charging is less likely to occur, reducing the frequency of maintenance work required to remove the contamination layer.

[0046] In the mass spectrometer of the above-mentioned embodiment, in addition to periodically performing the charge elimination operation during analysis, charge elimination can also be performed by applying an appropriate voltage of +100 V or a voltage in the range of tens to hundreds of V to the reflective electrode 31 for a specified time, as needed, for example, at an appropriate time point corresponding to the operation performed by the user.

[0047] In addition, the above description is an example of the case where positive ions are generated in the ion source 3, but it is obvious that in the case of generating negative ions, it is sufficient to apply a negative voltage of, for example, -1V to the reflecting electrode 31 during the period of forming the repulsion electric field, and to apply a high voltage of positive polarity of, for example, +100V to the reflecting electrode 31 during the period of forming the charge elimination electric field.

[0048] In addition, the ion source of the mass spectrometer in the above embodiment is an EI ion source, but the present invention can also be applied to a mass spectrometer having an ion source that ionizes components derived from a sample in an ionization chamber and ejects the generated ions from the ionization chamber using a reflection electrode, specifically a mass spectrometer having a CI ion source or an NCI ion source.

[0049] The above-described embodiment is merely an example of the present invention, and it goes without saying that appropriate modifications, changes, or additions to the present invention within the scope of the gist of the present invention are also encompassed by the scope of the present patent claims.

[0050] For example, the configuration other than the ion source is not limited to that described in the above embodiment and can be modified as appropriate. Therefore, the mass spectrometer of the present invention is not limited to application to a single quadrupole mass spectrometer but can also be applied to various mass spectrometers, including time-of-flight mass spectrometers, ion trap mass spectrometers, tandem mass spectrometers, and ion mobility-mass spectrometers.

[0051] [Various options]

[0052] It is self-evident to those skilled in the art that the above-mentioned exemplary embodiments are specific examples of the following aspects.

[0053] (Item 1) One aspect of the mass spectrometer of the present invention is a mass spectrometer including an ion source for ionizing a component included in a sample gas, the ion source including:

[0054] an ionization chamber having an ion ejection port and an interior space substantially separated from the exterior;

[0055] A reflection electrode is provided in the ionization chamber and is used to form an electric field to push the ions generated in the ionization chamber outward through the ion ejection port;

[0056] The voltage generating unit selectively applies a first voltage for forming the push-out electric field and a second voltage of positive polarity having an absolute value greater than the first voltage for forming a charge-eliminating electric field to the reflective electrode.

[0057] According to the mass spectrometer described in Item 1, it is possible to eliminate or reduce the charge generated in the ionization chamber without stopping the device or releasing the vacuum state of the device. As a result, when charge is generated in the ion source, the frequency of the troublesome maintenance work previously performed can be reduced, thereby reducing the burden on the user. In addition, it is possible to reduce the number of times the device becomes unusable due to such maintenance work, thereby improving the operating rate of the device. In addition, since charge can be eliminated or reduced in the middle of the analysis, it is possible to avoid adverse conditions caused by charge occurring in the middle of the analysis.

[0058] (Item 2) In the mass spectrometer described in Item 1, the voltage generating unit may apply a pulsed voltage to the reflective electrode, in which the first voltage is at a low level and the second voltage is at a high level.

[0059] In the mass spectrometer described in item 2, ionization can be performed in the ionization chamber mainly when the pulse voltage is at a low level, and charge elimination can be performed when the pulse voltage is at a high level. This can prevent the ion source from being charged during analysis.

[0060] (Item 3) In addition, in the mass spectrometer described in Item 2, it can be set as: based on the time when electrons charged in the inner wall of the ionization chamber are attracted by the charge cancellation electric field and reach the reflecting electrode, the width of the pulse of the second voltage is determined.

[0061] The mass spectrometer described in Item 3 can reliably eliminate charge while the pulse voltage is at a high level. Furthermore, it is unnecessary to extend the duration of the charge elimination operation beyond necessity, and the influence on ion generation in the ion source can be minimized.

[0062] Description of Reference Numerals

[0063] 1 chamber

[0064] 3 Ion source

[0065] 30 Ionization Chamber

[0066] 301 Ion ejector

[0067] 302 electron inlet

[0068] 303 electronic exhaust port

[0069] 304 Sample gas inlet tube

[0070] 30A pollution layer

[0071] 31 reflective electrode

[0072] 32 filament

[0073] 33 well electrode

[0074] 34, 35 Magnet

[0075] 36 Lead electrode

[0076] 4 Ion transport optical system

[0077] 5 Quadrupole mass filter

[0078] 6 Ion detector

[0079] 7 Reflected voltage generation unit

[0080] 8 Control Unit

[0081] C ion optical axis.

Claims

1. A mass spectrometer comprising an ion source for ionizing components contained in a sample gas, wherein: The ion source comprises: an ionization chamber having an ion ejection port and an interior space substantially separated from the exterior; A reflection electrode is provided in the ionization chamber and is used to form an electric field to push the ions generated in the ionization chamber outward through the ion ejection port; The voltage generating unit selectively applies to the reflecting electrode a first voltage for forming the pushing-out electric field and a second voltage having a positive polarity greater than the first voltage and an absolute value thereof for forming a charge-eliminating electric field for removing electrons from a contamination layer formed on an inner wall of the ionization chamber facing the reflecting electrode.

2. The mass spectrometer according to claim 1, wherein The voltage generating section applies a pulse voltage to the reflective electrode, in which the first voltage is at a low level and the second voltage is at a high level.

3. The mass spectrometer according to claim 2, wherein The width of the pulse having the second voltage value is determined by the time it takes for the charged electrons to be attracted by the charge canceling electric field on the inner wall of the ionization chamber and reach the reflective electrode.

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