Ion detector and mass analyzer

Abstract

To provide an ion detector capable of improving ion detection efficiency for both positive ions and negative ions.SOLUTION: An ion detector 1 includes a conversion dynode 3 for emitting electrons E upon incidence of ions P thereto, an electron multiplier unit 4 having a first-stage dynode 42a to which electrons E are incident, a box electrode 5 having an ion passage port 51a through which the ions P pass and an electron passage port 51b through which the electrons E pass, a conductive shielding unit 6 that is arranged at the ion passage port 51a and has a gap 6a through which the ions P pass, and a potential applying unit 8 that applies a first potential to the conversion dynode 3, applies a second potential to the first-stage dynode 42a, applies a third potential to the box electrode 5, and applies a fourth potential to the shielding unit 6. The first potential and the third potential have a polarity opposite to the polarity of the ions P. The second potential is higher than the first potential. The third potential and the fourth potential are substantially the same potential.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an ion detector and a mass spectrometer. [Background technology]

[0002] A known ion detector for detecting positive ions includes a conversion dynode that emits electrons when ions are incident on it, an electron multiplier section that multiplies the electrons emitted from the conversion dynode, and a box electrode that houses the conversion dynode and has a first opening that passes ions traveling to the conversion dynode and a second opening that passes electrons traveling from the conversion dynode to the first stage dynode (see, for example, Patent Document 1). In such an ion detector, a box electrode may be used to suppress stray light, which could become noise, from entering the conversion dynode. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2011-086403 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned ion detector, in order to enable detection of negative ions as well as positive ions, it is conceivable to make the potential applied to the conversion dynode switchable. Specifically, it is conceivable to apply a positive potential to the conversion dynode, and cause positive ions released from the conversion dynode in response to incidence of negative ions to be incident on the first dynode. In that case, the conversion efficiency from negative ions to positive ions is low, and as a result, there is a risk that the detection efficiency of negative ions will decrease.

[0005] An object of the present invention is to provide an ion detector and a mass spectrometer that can improve the ion detection efficiency for both positive ions and negative ions. [Means for solving the problem]

[0006] The ion detector of the present invention is [1] "an ion detector comprising: a conversion dynode having a conversion region that emits electrons when ions are incident thereon; an electron multiplier unit having a first stage dynode onto which the electrons are incident; a box electrode that houses the conversion dynode and has a first opening through which the ions proceeding to the conversion region and a second opening through which the electrons proceeding from the conversion region to the first stage dynode pass; a conductive shielding unit that is disposed at the first opening and has a gap through which the ions proceeding to the conversion region pass; and a potential application unit that applies a first potential to the conversion dynode, a second potential to the first stage dynode, a third potential to the box electrode, and a fourth potential to the shielding unit, wherein the first potential and the third potential have polarities opposite to those of the ions, the second potential is higher than the first potential, and the third potential and the fourth potential are substantially the same potential."

[0007] In the ion detector described in [1] above, when detecting positive ions, a first potential, which is a negative potential, is applied to the conversion dynode, and a second potential higher than the first potential is applied to the first stage dynode of the electron multiplier. At this time, a third potential, which is a negative potential, is applied to the box electrode housing the conversion dynode, and a fourth potential, which is substantially the same potential as the third potential, is applied to the shielding portion disposed in the first opening of the box electrode. This allows positive ions to be incident on the conversion region of the conversion dynode through the gap in the shielding portion disposed in the first opening of the box electrode. Also, electrons emitted from the conversion region can be incident on the first stage dynode through the second opening of the box electrode. At this time, the box electrode suppresses mutual interference between the electric field that causes positive ions to be incident on the conversion region and the electric field that causes electrons to be incident on the first stage dynode, so that positive ions can be reliably incident on the conversion region and electrons can be reliably incident on the first stage dynode. Furthermore, since the shielding portion prevents an electric field that attracts electrons from being formed in the box electrode through the first opening, the electrons can be made to enter the first dynode reliably. Therefore, the ion detector described in [1] above can improve the detection efficiency of positive ions.

[0008] In the ion detector described in [1] above, when detecting negative ions, a first potential, which is a positive potential, is applied to the conversion dynode, and a second potential higher than the first potential is applied to the first stage dynode of the electron multiplier. At this time, a third potential, which is a positive potential, is applied to the box electrode housing the conversion dynode, and a fourth potential, which is substantially the same potential as the third potential, is applied to the shielding portion disposed in the first opening of the box electrode. This allows negative ions to be incident on the conversion region of the conversion dynode through the gap in the shielding portion disposed in the first opening of the box electrode. Also, electrons emitted from the conversion region can be incident on the first stage dynode through the second opening of the box electrode. At this time, the box electrode suppresses mutual interference between the electric field that causes negative ions to be incident on the conversion region and the electric field that causes electrons to be incident on the first stage dynode, so that negative ions can be reliably incident on the conversion region and electrons can be reliably incident on the first stage dynode. Furthermore, since the shielding portion prevents an electric field that pushes back electrons from being formed in the box electrode through the first opening, the electrons can be made to enter the first dynode reliably. Therefore, the ion detector described in [1] above can improve the detection efficiency of negative ions. As described above, the ion detector described in [1] above can improve the ion detection efficiency for both positive ions and negative ions.

[0009] The ion detector of the present invention may be [2] "the ion detector according to the above [1], in which the shielding portion has, as the gaps, a plurality of gaps arranged two-dimensionally along the first opening." According to the ion detector according to [2], when detecting positive ions, it is possible to reliably allow positive ions to pass through the first opening while reliably suppressing the formation of an electric field that attracts electrons emitted from the conversion region in the box electrode via the first opening. When detecting negative ions, it is possible to reliably allow negative ions to pass through the first opening while reliably suppressing the formation of an electric field that pushes back electrons emitted from the conversion region in the box electrode via the first opening. Therefore, it is possible to further improve the detection efficiency for both positive ions and negative ions.

[0010] The ion detector of the present invention may be [3] "the ion detector according to the above [1] or [2], in which the first opening is disposed on one side of a first center line passing through the center of the conversion region when viewed in a direction perpendicular to the first center line, the second opening is disposed on the other side of the first center line when viewed in the direction perpendicular to the first center line, and the first stage dynode faces the second opening." According to the ion detector according to [3], since the first stage dynode faces the second opening of the box electrode, an electric field for causing electrons to enter the first stage dynode can be easily and reliably formed, and electrons can be more reliably caused to enter the first stage dynode. In addition, since the first opening of the box electrode is disposed on one side of the first center line of the conversion region and the second opening of the box electrode is disposed on the other side of the first center line of the conversion region, it is possible to suppress the electric field for causing electrons to enter the first stage dynode from affecting the ions to be detected proceeding to the conversion region, and the ions to be detected can be more reliably caused to enter the conversion region.

[0011] The ion detector of the present invention may be [4] "the ion detector according to any one of the above [1] to [3], in which the first potential and the third potential are substantially the same potential." According to the ion detector according to [4], it is possible to simplify the structure of the potential application unit and facilitate switching of the polarities of the first potential and the third potential according to the polarity of the ions to be detected.

[0012] The ion detector of the present invention may be [5] "the ion detector according to any one of the above [1] to [4], wherein the area of ​​the gap is 50% or more of the area of ​​the first opening." According to the ion detector according to [5], the ions to be detected can be more reliably incident on the conversion region of the conversion dynode through the gap in the shielding part arranged in the first opening of the box electrode. Therefore, the ion detection efficiency can be further improved for both positive ions and negative ions.

[0013] The ion detector of the present invention may be [6] "the ion detector according to any one of the above [1] to [5], further comprising a partition having an aperture for passing the ions proceeding to the first opening." According to the ion detector according to [6], it is possible to prevent electrons or ions that may become noise from entering the conversion region. Therefore, it is possible to improve the detection accuracy of ions for both positive ions and negative ions.

[0014] The ion detector of the present invention may be the ion detector according to the above [6], further comprising a deflection electrode, the box electrode being disposed on one side of the second center line of the aperture when viewed from a direction perpendicular to the second center line, the deflection electrode being disposed on the other side of the second center line when viewed from the direction perpendicular to the second center line, and the potential applying unit applying a ground potential or a potential having the same polarity as that of the ions to the deflection electrode. According to the ion detector according to the above [7], an electric field that causes the ions to be detected to proceed to the first opening of the box electrode can be reliably formed, so that the ions to be detected can be more reliably incident on the conversion region. Therefore, the ion detection efficiency can be further improved for both positive ions and negative ions.

[0015] The ion detector of the present invention may be [8] "the ion detector according to the above [6] or [7], in which the first opening is inclined so as to move away from the aperture as it approaches the second center line when viewed from a direction perpendicular to the second center line of the aperture." According to the ion detector according to [8], the ions to be detected that have passed through the aperture can be reliably made incident on the first opening of the box electrode, so that the ions to be detected can be more reliably made incident on the conversion region. Therefore, the ion detection efficiency can be further improved for both positive ions and negative ions.

[0016] The mass spectrometer of the present invention is [9] "a mass spectrometer comprising an ionization unit, a mass separation unit, and an ion detector, wherein the ion detector comprises: a conversion dynode having a conversion region that emits electrons when ions are incident thereon; an electron multiplier unit having a first stage dynode onto which the electrons are incident; a box electrode that houses the conversion dynode and has a first opening through which the ions proceeding to the conversion region and a second opening through which the electrons proceeding from the conversion region to the first stage dynode pass; a conductive shielding unit that is disposed at the first opening and has a gap through which the ions proceeding to the conversion region pass; and a potential application unit that applies a first potential to the conversion dynode, applies a second potential to the first stage dynode, applies a third potential to the box electrode, and applies a fourth potential to the shielding unit, wherein the first potential and the third potential have polarities opposite to those of the ions, the second potential is higher than the first potential, and the third potential and the fourth potential are substantially the same potential."

[0017] In the mass spectrometer described in [9] above, the trajectory of the electrons emitted from the conversion dynode is optimized by applying appropriate potentials to the conversion dynode, the first dynode of the electron multiplier section, the box electrode, and the shielding section. This makes it possible to improve the detection efficiency of positive and negative ions even when the energy distribution and spatial distribution of ions transmitted through the mass separation section are widely spread.

[0018] The mass spectrometer of the present invention may be

[10] "the mass spectrometer according to the above [9], in which the mass separator is a quadrupole mass separator." According to the mass spectrometer according to

[10] , the mass separator can be specifically configured. Effect of the Invention

[0019] According to the present invention, it is possible to provide an ion detector and a mass spectrometer that can improve the ion detection efficiency for both positive ions and negative ions. [Brief description of the drawings]

[0020] [Figure 1] FIG. 2 is a configuration diagram of an ion detector according to an embodiment. [Diagram 2] FIG. 4 is a diagram showing an example of an electric field distribution formed in an ion detector. [Diagram 3] FIG. 4 is a diagram showing an example of an electric field distribution formed in an ion detector. [Figure 4] FIG. 4 is a diagram showing the detection efficiency of ions in an ion detector. [Diagram 5] FIG. 1 is a configuration diagram of a mass spectrometer according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and duplicated explanations will be omitted. [Ion detector according to one embodiment]

[0022] As shown in FIG. 1, the ion detector 1 includes a partition section 2, a conversion dynode 3, an electron multiplier section 4, a box electrode 5, a shield section 6, a deflection electrode 7, and a potential applying section 8. The ion detector 1 is housed inside a housing 201 (see FIG. 5) of a mass spectrometer 200, and is disposed in a vacuum space. The material of the partition section 2 is a metal such as stainless steel. An aperture 21a is formed in a wall section 21 of the partition section 2. The aperture 21a has a center line (second center line) A1 parallel to the X-axis direction. The shape of the aperture 21a when viewed from the X-axis direction is, for example, a circular shape. Hereinafter, one direction perpendicular to the X-axis direction is referred to as the Y-axis direction, and a direction perpendicular to both the X-axis direction and the Y-axis direction is referred to as the Z-axis direction.

[0023] In this embodiment, the ion detector 1 is used as a part of a mass spectrometer 200. In the mass spectrometer 200, a sample is ionized by electrospray ionization. The mass spectrometer 200 selects ions P to be detected from the generated ions by using a quadrupole. The ions P to be detected are introduced into the ion detector 1 through an aperture 21a. The ion detector 1 detects the incident ions P. In this embodiment, the ions P may be positive ions or negative ions. The potential of the housing 201 (see FIG. 5) is described as a ground potential.

[0024] The conversion dynode 3 has a surface including a conversion region 3a. The conversion region 3a emits electrons E when positive ions or negative ions are incident on the conversion region 3a. The conversion region 3a is a concave region on the opposite side to the space that emits the electrons E. The material of the conversion dynode 3 is a metal such as stainless steel. The conversion dynode 3 is supported by a support member (not shown) in the box electrode 5. The conversion dynode 3 is disposed on one side in the Y-axis direction with respect to the center line A1. The conversion region 3a faces one side in the X-axis direction (the aperture 21a side) on a plane α that includes the center line A1 and is perpendicular to the Z-axis direction. The conversion region 3a has a center line (first center line) A2 that is parallel to the X-axis direction.

[0025] A first potential is applied to the conversion dynode 3 by the potential applying unit 8. The first potential is a potential of a polarity opposite to that of the ion P. As an example, the first potential is −10 kV when the ion P is a positive ion. In this case, the ion P is incident on the conversion region 3a with an energy of, for example, 10 keV, so that the electron E is emitted from the conversion region 3a. As an example, the first potential is +5.5 kV when the ion P is a negative ion. In this case, the ion P is incident on the conversion region 3a with an energy of, for example, 5.5 keV, so that the electron E is emitted from the conversion region 3a. Generally, regardless of whether the ion P is a positive ion or a negative ion, the amount of electrons E emitted increases as the energy of the ion P incident on the conversion region 3a increases. In other words, the higher the absolute value of the first potential, the higher the conversion efficiency from the ion P to the electron E can be.

[0026] The electron multiplier section 4 is disposed on one side in the Y-axis direction with respect to the center line A2 (opposite side to the center line A1). In this embodiment, the electron multiplier section 4 has a metal wall section 41, a plurality of dynodes 42, and an anode 43. The electron multiplier section 4 is supported by a support member (not shown). An electron passing port 41a that passes electrons E is formed in the wall section 41. The electron passing port 41a opens in the Y-axis direction. The shape of the electron passing port 41a when viewed from the Y-axis direction is, for example, a quadrangle. The plurality of dynodes 42 and the anode 43 are arranged on a plane α. The material of each dynode 42 and the anode 43 is, for example, a metal such as stainless steel. Of the plurality of dynodes 42, the first stage dynode 42a faces the electron passing port 41a in the Y-axis direction. In the electron multiplier section 4, electrons E that have passed through the electron passing port 41a are incident on the surface of the first-stage dynode 42a, causing secondary electrons (not shown) to be emitted from the first-stage dynode 42a. The secondary electrons are multiplied by each subsequent dynode 42 and are incident on the anode 43. The anode 43 is connected to a signal processing circuit via a capacitor (not shown). The anode 43 is configured to output the secondary electrons as a pulse signal.

[0027] A second potential is applied to the first-stage dynode 42a by the potential application unit 8. The second potential is higher than the first potential of the conversion dynode 3. As an example, the second potential is +7 kV both when the ion P is a positive ion and when the ion P is a negative ion. A predetermined potential is applied to each dynode 42 other than the first-stage dynode 42a by the potential application unit 8 so that secondary electrons emitted from the first-stage dynode 42a proceed sequentially to each dynode 42 in the subsequent stage. As an example, a potential higher than +7 kV is applied to each dynode 42 other than the first-stage dynode 42a both when the ion P is a positive ion and when the ion P is a negative ion. A predetermined potential is applied to the anode 43 by the potential application unit 8. As an example, the predetermined potential is +9 kV both when the ion P is a positive ion and when the ion P is a negative ion. In the ion detector 1, a second potential is applied to the wall portion 41 of the electron multiplier unit 4 by the potential application unit 8. That is, in this embodiment, regardless of the polarity of the ions P, the electron multiplier section 4 is floated to a positive polarity.

[0028] The box electrode 5 accommodates the conversion dynode 3 so as to separate the space in which the conversion dynode 3 is disposed from the space in which the electron multiplier section 4 is disposed. The box electrode 5 is disposed on one side (the conversion dynode 3 side) in the Y-axis direction with respect to the center line A1, and is disposed between the center line A1 and the electron multiplier section 4 on the plane α. The box electrode 5 is shaped, for example, like a rectangular parallelepiped box. The box electrode 5 is made of a metal such as stainless steel. The box electrode 5 is supported by a support member (not shown). When viewed from the X-axis direction, the box electrode 5 does not overlap the aperture 21a, but overlaps the wall section 21. This makes it possible to suppress stray light that is generated in the mass spectrometer 200 and may become noise from entering the box electrode 5.

[0029] The wall portion 51 is formed with an ion passage port (first opening) 51a that passes ions P traveling to the conversion region 3a. When viewed from the Z-axis direction, the ion passage port 51a is formed on the plane α at a corner portion on the aperture 21a side of the box electrode 5. When viewed from the Z-axis direction (direction perpendicular to the center line A2), the ion passage port 51a is disposed on one side (center line A1 side) of the center line A2. When viewed from the Z-axis direction (direction perpendicular to the center line A1), the ion passage port 51a is inclined so as to move away from the aperture 21a as it approaches the center line A1. The shape of the ion passage port 51a when viewed from a direction perpendicular to the ion passage port 51a is, for example, a quadrangle.

[0030] An electron passing port (second opening) 51b is formed in the wall portion 51, which passes electrons E traveling from the conversion region 3a to the first dynode 42a of the electron multiplier section 4. When viewed from the Z-axis direction (direction perpendicular to the center line A2), the electron passing port 51b is disposed on the other side (the electron multiplier section 4 side) of the center line A2. The electron passing port 51b faces the electron passing port 41a on the plane α. The shape of the electron passing port 51b when viewed from the Y-axis direction is, for example, circular.

[0031] A third potential is applied to the box electrode 5 by the potential application unit 8. The third potential is a potential of a polarity opposite to that of the ions P. The third potential is substantially the same potential as the first potential. Here, substantially the same potential means that the third potential is 90% or more and 110% or less when the first potential is 100%. In this embodiment, the third potential is the same potential as the first potential. As an example, the third potential is −10 kV when the ions P are positive ions. As a result, the incident positive ions proceed toward the ion passage port 51a while gradually moving away from the center line A1 of the aperture 21a to one side in the Y-axis direction. As an example, the third potential is +5.5 kV when the ions P are negative ions. As a result, the incident negative ions proceed toward the ion passage port 51a while gradually moving away from the center line A1 of the aperture 21a to one side in the Y-axis direction.

[0032] The shielding portion 6 is disposed at the ion passage port 51a, and is electrically connected to the box electrode 5 by, for example, contacting the box electrode 5. When viewed from the Z-axis direction (perpendicular to the center line A1), the shielding portion 6 is inclined so as to move away from the aperture 21a as it approaches the center line A1, similar to the ion passage port 51a. The material of the shielding portion 6 is a conductive material, for example, a metal such as stainless steel. The shielding portion 6 has a plurality of gaps 6a that allow the ions P traveling to the conversion region 3a to pass through. The plurality of gaps 6a are two-dimensionally arranged along the ion passage port 51a. In this embodiment, the shielding portion 6 is a lattice-shaped member, and the plurality of gaps 6a are arranged in a matrix. The total area of ​​the plurality of gaps 6a is, for example, 50% or more and 97% or less of the area of ​​the ion passage port 51a.

[0033] A fourth potential is applied to the shielding unit 6 by the potential application unit 8 via the box electrode 5. The fourth potential is a potential of the opposite polarity to the polarity of the ions P. The fourth potential is substantially the same potential as the third potential. Here, substantially the same potential means that the fourth potential is 90% or more and 110% or less when the third potential is 100%. In this embodiment, the fourth potential is the same potential as the third potential. As an example, the fourth potential is −10 kV when the ions P are positive ions. As an example, the fourth potential is +5.5 kV when the ions P are negative ions.

[0034] When viewed from the Z-axis direction (perpendicular to the center line A1), the deflection electrode 7 is disposed on the other side (opposite to the box electrode 5) of the center line A1. The material of the deflection electrode 7 is a metal such as stainless steel. The deflection electrode 7 is supported by a support member (not shown) so as to be located on the plane α. When viewed from the Z-axis direction, a part of the deflection electrode 7 extends along the Y-axis direction. When viewed from the Z-axis direction, the other part of the deflection electrode 7 extends from the part toward the wall portion 21 along the X-axis direction. When viewed from the X-axis direction, the deflection electrode 7 does not overlap with the aperture 21a, but overlaps with the wall portion 21. When viewed from the Y-axis direction, at least a part of the deflection electrode 7 overlaps with the ion passage port 51a.

[0035] A ground potential or a potential having the same polarity as that of the ion P is applied to the deflection electrode 7 by the potential application unit 8. As an example, the potential is +500 V when the ion P is a positive ion. As an example, the potential is −500 V when the ion P is a negative ion.

[0036] The potential applying unit 8 applies a predetermined potential to each of the conversion dynode 3, the electron multiplier unit 4, the box electrode 5, and the deflection electrode 7. The potential applying unit 8 applies a predetermined potential to the shielding unit 6 via the box electrode 5. The potential applying unit 8 is electrically connected to each of the conversion dynode 3, the electron multiplier unit 4, the box electrode 5, and the deflection electrode 7 by wiring or the like. As described above, when the ion P is a positive ion, the potential applying unit 8 applies a first potential (e.g., −10 kV) to the conversion dynode 3, applies a second potential (e.g., +7 kV) higher than the first potential to the first stage dynode 42a of the electron multiplier unit 4, applies a third potential (e.g., −10 kV) that is the same potential as the first potential to the box electrode 5, applies a fourth potential (e.g., −10 kV) that is the same potential as the third potential to the shielding unit 6, and applies, for example, +500 V to the deflection electrode 7. When the ion P is a negative ion, the potential applying unit 8 applies a first potential (e.g., +5.5 kV) to the conversion dynode 3, applies a second potential (e.g., +7 kV) higher than the first potential to the first stage dynode 42a of the electron multiplier unit 4, applies a third potential (e.g., +5.5 kV) that is the same potential as the first potential to the box electrode 5, applies a fourth potential (e.g., +5.5 kV) that is the same potential as the third potential to the shielding unit 6, and applies, for example, -500 V to the deflection electrode 7. In other words, the potential applying unit 8 can switch the polarity of the first potential and the third potential and change the value of each potential according to the polarity of the ion P to be detected. As an example, the potential applying unit 8 is configured by a combination of multiple power sources or a power source and resistor division.

[0037] In this embodiment, the first stage dynode 42a faces the electron passing port 51b across the electron passing port 41a. When viewed from the Y-axis direction, the outer edge of the electron passing port 41a is located inside the outer edge of the electron passing port 51b. When viewed from the Y-axis direction, at least the conversion region 3a of the conversion dynode 3 is located inside the outer edge of the electron passing port 51b.

[0038] In the ion detector 1 configured as above, when detecting positive ions, a predetermined potential is applied by the potential application unit 8 to each of the conversion dynode 3, the electron multiplier unit 4, the box electrode 5, the shielding unit 6, and the deflection electrode 7. This creates an electric field that causes positive ions to be incident on the conversion region 3a and an electric field that causes electrons E to be incident on the first stage dynode 42a. In this state, positive ions are incident from the aperture 21a, and the positive ions pass through the ion passing port 51a of the box electrode 5. Then, the positive ions are incident on the conversion region 3a of the conversion dynode 3. The incidence of the positive ions causes electrons E to be emitted from the conversion region 3a. Because an electric field that causes electrons E to be incident on the first stage dynode 42a is created, the electrons E emitted from the conversion region 3a pass through the electron passing port 51b of the box electrode 5 and the electron passing port 41a of the electron multiplier unit 4, and are incident on the first stage dynode 42a. When electrons E are incident on the first dynode 42a, secondary electrons (not shown) are emitted from the first dynode 42a. The secondary electrons are multiplied by each subsequent dynode 42 and are incident on the anode 43. The anode 43 then outputs the secondary electrons as a pulse signal, causing the ion detector 1 to detect positive ions.

[0039] In the ion detector 1, when detecting negative ions, a predetermined potential is applied by the potential application unit 8 to each of the conversion dynode 3, the electron multiplier unit 4, the box electrode 5, the shielding unit 6, and the deflection electrode 7. This creates an electric field that causes negative ions to enter the conversion region 3a and an electric field that causes electrons E to enter the first stage dynode 42a. In this state, negative ions enter through the aperture 21a and pass through the ion passing port 51a of the box electrode 5. Then, the negative ions enter the conversion region 3a of the conversion dynode 3. The incidence of the negative ions causes electrons E to be emitted from the conversion region 3a. Because an electric field that causes electrons E to enter the first stage dynode 42a is created, the electrons E emitted from the conversion region 3a pass through the electron passing port 51b of the box electrode 5 and the electron passing port 41a of the electron multiplier unit 4, and enter the first stage dynode 42a. When electrons E are incident on the first dynode 42a, secondary electrons (not shown) are emitted from the first dynode 42a. The secondary electrons are multiplied by each subsequent dynode 42 and are incident on the anode 43. The anode 43 then outputs the secondary electrons as a pulse signal, causing the ion detector 1 to detect negative ions. That is, in the ion detector 1, because the electron multiplier section 4 is floated to positive polarity, the electrons E emitted from the conversion region 3a can be made incident on the first dynode 42a even when negative ions are being detected.

[0040] As described above, in the ion detector 1, when detecting positive ions, a first potential, which is a negative potential, is applied to the conversion dynode 3, and a second potential higher than the first potential is applied to the first stage dynode 42a of the electron multiplier section 4. At this time, a third potential, which is a negative potential, is applied to the box electrode 5 housing the conversion dynode 3, and a fourth potential, which is substantially the same potential as the third potential, is applied to the shielding section 6 arranged at the ion passing port 51a of the box electrode 5. This allows positive ions to be incident on the conversion region 3a of the conversion dynode 3 through the multiple gaps 6a of the shielding section 6 arranged at the ion passing port 51a of the box electrode 5. Also, electrons E emitted from the conversion region 3a can be incident on the first stage dynode 42a through the electron passing port 51b of the box electrode 5. At this time, the box electrode 5 prevents the electric field that causes positive ions to be incident on the conversion region 3a and the electric field that causes electrons E to be incident on the first dynode 42a from interfering with each other, so that the positive ions can be reliably incident on the conversion region 3a and the electrons E can be reliably incident on the first dynode 42a. Furthermore, the shielding portion 6 prevents an electric field that attracts the electrons E from being formed in the box electrode 5 via the ion passing port 51a, so that the electrons E can be reliably incident on the first dynode 42a. Therefore, the ion detector 1 can improve the detection efficiency of positive ions.

[0041] In the ion detector 1, when detecting negative ions, a first potential, which is a positive potential, is applied to the conversion dynode 3, and a second potential higher than the first potential is applied to the first stage dynode 42a of the electron multiplier section 4. At this time, a third potential, which is a positive potential, is applied to the box electrode 5 housing the conversion dynode 3, and a fourth potential, which is substantially the same potential as the third potential, is applied to the shielding section 6 arranged at the ion passing port 51a of the box electrode 5. This allows negative ions to be incident on the conversion region 3a of the conversion dynode 3 through the multiple gaps 6a of the shielding section 6 arranged at the ion passing port 51a of the box electrode 5. Also, electrons E emitted from the conversion region 3a can be incident on the first stage dynode 42a through the electron passing port 51b of the box electrode 5. At this time, the box electrode 5 prevents the electric field that causes negative ions to enter the conversion region 3a and the electric field that causes electrons E to enter the first dynode 42a from interfering with each other, so that the negative ions can be reliably injected into the conversion region 3a and the electrons E can be reliably injected into the first dynode 42a. Furthermore, the shielding portion 6 prevents an electric field that pushes back the electrons E from being formed in the box electrode 5 via the ion passing port 51a, so that the electrons E can be reliably injected into the first dynode 42a. Therefore, the ion detector 1 can improve the detection efficiency of negative ions. As described above, the ion detector 1 can improve the ion detection efficiency for both positive ions and negative ions.

[0042] In the ion detector 1, the shielding portion 6 has a plurality of gaps 6a arranged two-dimensionally along the ion passing port 51a. This makes it possible to reliably prevent an electric field that attracts electrons E emitted from the conversion region 3a from being formed in the box electrode 5 through the ion passing port 51a when detecting positive ions, while allowing the positive ions to pass through the ion passing port 51a. When detecting negative ions, it makes it possible to reliably prevent an electric field that pushes back electrons E emitted from the conversion region 3a from being formed in the box electrode 5 through the ion passing port 51a, while allowing the negative ions to pass through the ion passing port 51a. This makes it possible to further improve the ion detection efficiency for both positive and negative ions.

[0043] In the ion detector 1, when viewed from a direction perpendicular to the center line A2 that passes through the center of the conversion region 3a, the ion passing port 51a is located on one side of the center line A2, and when viewed from a direction perpendicular to the center line A2, the electron passing port 51b is located on the other side of the center line A2, and the first stage dynode 42a faces the electron passing port 51b. As a result, because the first stage dynode 42a faces the electron passing port 51b of the box electrode 5, an electric field that causes electrons E to be incident on the first stage dynode 42a can be easily and reliably formed, and the electrons E can be more reliably incident on the first stage dynode 42a. In addition, since the ion passage port 51a of the box electrode 5 is located on one side of the center line A2 of the conversion region 3a, and the electron passage port 51b of the box electrode 5 is located on the other side of the center line A2 of the conversion region 3a, it is possible to prevent the electric field that causes electrons E to be incident on the first dynode 42a from affecting the detection target ions P proceeding to the conversion region 3a, and it is possible to more reliably cause the detection target ions P to be incident on the conversion region.

[0044] In the ion detector 1, the first potential and the third potential are substantially the same potential, which makes it possible to simplify the structure of the potential application unit 8 and facilitate switching of the polarities of the first potential and the third potential according to the polarity of the ions P to be detected.

[0045] In the ion detector 1, the total area of ​​the multiple gaps 6a is 50% or more of the area of ​​the ion passing port 51a. This allows the ions P to be detected to be more reliably incident on the conversion region 3a of the conversion dynode 3 through the multiple gaps 6a of the shielding portion 6 arranged at the ion passing port 51a of the box electrode 5. This makes it possible to further improve the ion detection efficiency for both positive ions and negative ions.

[0046] The ion detector 1 includes a partition section 2 having an aperture 21a that allows the ions P traveling to the ion passage port 51a to pass therethrough. This makes it possible to prevent electrons or ions that may become noise from entering the conversion region 3a. This makes it possible to improve the detection accuracy of ions for both positive and negative ions.

[0047] The ion detector 1 includes a deflection electrode 7, the box electrode 5 being disposed on one side of the center line A1 of the aperture 21a when viewed from a direction perpendicular to the center line A1, and the deflection electrode 7 being disposed on the other side of the center line A1 when viewed from a direction perpendicular to the center line A1, and the potential applying unit 8 applies a ground potential or a potential having the same polarity as that of the ions P to the deflection electrode 7. This can reliably form an electric field through which the ions P to be detected proceed to the ion passage port 51a of the box electrode 5, and therefore the ions P to be detected can be more reliably incident on the conversion region 3a. This can further improve the ion detection efficiency for both positive and negative ions.

[0048] In the ion detector 1, the ion passing port 51a is inclined so as to move away from the aperture 21a as it approaches the center line A1 when viewed from a direction perpendicular to the center line A1. This ensures that the detection target ions P that have passed through the aperture 21a are incident on the ion passing port 51a of the box electrode 5, and therefore the detection target ions P can be more reliably incident on the conversion region 3a. This makes it possible to further improve the ion detection efficiency for both positive ions and negative ions.

[0049] In the ion detector 1, the total area of ​​the multiple gaps 6a is 97% or less of the area of ​​the ion passing port 51a. This makes it possible to more reliably prevent an electric field that attracts electrons E emitted from the conversion region 3a from being formed in the box electrode 5 via the ion passing port 51a when detecting positive ions. When detecting negative ions, it makes it possible to more reliably prevent an electric field that pushes back electrons E emitted from the conversion region 3a from being formed in the box electrode 5 via the ion passing port 51a. This makes it possible to further improve the ion detection efficiency for both positive ions and negative ions. [Example of electric field distribution]

[0050] With reference to (a) and (b) of FIG. 2, the advantages of the above-mentioned shielding portion 6 in the case where the ion P is a positive ion will be described. In (a) of FIG. 2, the electric field distribution formed in an ion detector 101, which is different from the above-mentioned ion detector 1, in the case where the ion P is a positive ion is shown by equipotential lines. The ion detector 101 differs from the ion detector 1 in that it does not include the shielding portion 6. In (b) of FIG. 2, the electric field distribution formed in the above-mentioned ion detector 1 in the case where the ion P is a positive ion is shown by equipotential lines. As described above, the first potential of the conversion dynode 3 is −10 kV, the second potential of the first stage dynode 42a of the electron multiplier portion 4 is +7 kV, the third potential of the box electrode 5 is −10 kV, the fourth potential of the shielding portion 6 is −10 kV, and the potential of the deflection electrode 7 is +500 V.

[0051] 2(a), in the ion detector 101, an electric field EF1 that causes electrons E to enter the first dynode 42a is formed, the electric field EF1 spreading from the electron passing port 51b into the box electrode 5. In addition, in the ion detector 101, an electric field EF2 that attracts electrons E is formed, the electric field EF2 spreading from the ion passing port 51a into the box electrode 5. As a result, the electrons E are influenced not only by the electric field EF1 but also by the electric field EF2, so that the electrons E are attracted to the ion passing port 51a. As a result, in the ion detector 101, the electrons E are less likely to enter the first dynode 42a.

[0052] 2(b), in the ion detector 1, an electric field EF3 that causes electrons E to be incident on the first dynode 42a is formed, the electric field EF3 spreading from the electron passing port 51b into the box electrode 5. In the ion detector 1, the shielding portion 6 is disposed at the ion passing port 51a, and therefore, unlike the ion detector 101, the formation of an electric field that attracts the electrons E is suppressed in the box electrode 5. As a result, compared to the ion detector 101, the electrons E emitted from the conversion region 3a can be more reliably incident on the first dynode 42a.

[0053] The incidence efficiency of electrons E emitted from the conversion region 3a onto the first dynode 42a was simulated for the ion detector 101 shown in Fig. 2(a) and the ion detector 1 shown in Fig. 2(b). The value obtained by dividing the mass of the ion P by the charge of the ion P (hereinafter also referred to as "m / z") was 50. The incidence efficiency was 76.1% for the ion detector 101, whereas the incidence efficiency was 99.8% for the ion detector 1. As a result of this simulation, it was found that when the ion P is a positive ion, the amount of electrons E incident onto the first dynode 42a increases by arranging the shielding portion 6 at the ion passage port 51a.

[0054] 3(a) and (b), the advantages of the above-mentioned shielding unit 6 in the case where the ion P is a negative ion will be described. In FIG. 3(a), the electric field distribution formed in the above-mentioned ion detector 101 in the case where the ion P is a negative ion is shown by equipotential lines. In FIG. 3(b), the electric field distribution formed in the above-mentioned ion detector 1 in the case where the ion P is a negative ion is shown by equipotential lines. As described above, the first potential of the conversion dynode 3 is +5.5 kV, the second potential of the first stage dynode 42a of the electron multiplier unit 4 is +7 kV, the third potential of the box electrode 5 is +5.5 kV, the fourth potential of the shielding unit 6 is +5.5 kV, and the potential of the deflection electrode 7 is -500 V.

[0055] 3(a), in the ion detector 101, an electric field EF4 that causes electrons E to enter the first dynode 42a is formed, the electric field EF4 spreading from the electron passing port 51b into the box electrode 5. In addition, in the ion detector 101, an electric field EF5 that pushes back electrons E is formed, the electric field EF5 spreading from the ion passing port 51a into the box electrode 5. As a result, the electrons E are affected not only by the electric field EF4 but also by the electric field EF5, so that the electrons E are pushed back to the side opposite the ion passing port 51a. As a result, in the ion detector 101, it is difficult for the electrons E to enter the first dynode 42a.

[0056] 3(b), in the ion detector 1, an electric field EF6 that causes electrons E to be incident on the first dynode 42a is formed, the electric field EF6 spreading from the electron passing port 51b into the box electrode 5. In the ion detector 1, the shielding portion 6 is disposed at the ion passing port 51a, and therefore, unlike the ion detector 101, the formation of an electric field that pushes back the electrons E is suppressed in the box electrode 5. As a result, compared to the ion detector 101, the electrons E emitted from the conversion region 3a can be more reliably incident on the first dynode 42a.

[0057] The incidence efficiency of electrons E emitted from the conversion region 3a onto the first dynode 42a was simulated for the ion detector 101 shown in Fig. 3(a) and the ion detector 1 shown in Fig. 3(b). The m / z value for ion P was 50. The incidence efficiency for the ion detector 101 was 69.1%, whereas the incidence efficiency for the ion detector 1 was 91.2%. As a result of this simulation, it was found that when the ion P is a negative ion, the amount of electrons E incident on the first dynode 42a increases by arranging the shielding portion 6 at the ion passing port 51a. [Ion detection example]

[0058] The ion intensity of the ions detected by the ion detector 1 will be described with reference to (a) and (b) of FIG. 4. (a) of FIG. 4 shows the results when positive ions are detected by the ion detector 1, and (b) of FIG. 4 shows the results when negative ions are detected by the ion detector 1. The vertical axis of each of (a) and (b) of FIG. 4 shows the value (intensity ratio) obtained by dividing the ion intensity detected by the ion detector 1 by the ion intensity detected by a conventional ion detector (hereinafter referred to as the "conventional example"). The horizontal axis of each of (a) and (b) of FIG. 4 shows the value (m / z) obtained by dividing the mass of the ion to be detected by the charge of the ion. The conventional example does not include the box electrode 5, the shielding portion 6, and the deflection electrode 7, and the potential applied to each member in the conventional example is different from that of the ion detector 1.

[0059] The potentials of the components of the ion detector 1 in FIG. 4(a) will be described. In the ion detector 1, the first potential of the conversion dynode 3 is −10 kV, the second potential of the first stage dynode 42a of the electron multiplier 4 is +7 kV, the potential of the anode 43 is +9 kV, the third potential of the box electrode 5 is −10 kV, the fourth potential of the shielding unit 6 is −10 kV, and the potential of the deflection electrode 7 is +500 V. The potentials of the components of the conventional example in FIG. 4(a) will be described. In the conventional example, the potential of the conversion dynode is −10 kV, the potential of the first stage dynode of the electron multiplier is −2 kV, and the potential of the anode is ground potential. As shown in FIG. 4(a), when detecting positive ions, the detection efficiency of the ion detector 1 was comparable to that of the conventional example.

[0060] The potentials of the components of the ion detector 1 in FIG. 4(b) will be described. In the ion detector 1, the first potential of the conversion dynode 3 is +5.5 kV, the second potential of the first stage dynode 42a of the electron multiplier section 4 is +7 kV, the potential of the anode 43 is +9 kV, the third potential of the box electrode 5 is +5.5 kV, the fourth potential of the shielding section 6 is +5.5 kV, and the potential of the deflection electrode 7 is −500 V. The potentials of the components of the conventional example in FIG. 4(b) will be described. In the conventional example, the potential of the conversion dynode is +10 kV, the potential of the first stage dynode of the electron multiplier section is −2 kV, and the potential of the anode is ground potential. As shown in FIG. 4(b), when detecting negative ions, the detection efficiency of the ion detector 1 was superior to that of the conventional example in the region where the horizontal axis value was 200 or less. [Mass spectrometer according to an embodiment]

[0061] A specific configuration of the above-mentioned mass spectrometer 200 will be described with reference to FIG. 5. The mass spectrometer 200 includes an ion detector 1, a housing 201, an ionization section 202, a first mass separation section (mass separation section) 203, a collision section 204, a second mass separation section (mass separation section) 205, and a pulse count detection section 206. In the mass spectrometer 200, the first mass separation section 203, the collision section 204, and the second mass separation section are arranged in series in this order. The ion detector 1 includes a detection unit 10 including a partition section 2, a conversion dynode 3, an electron multiplier section 4, a box electrode 5, a shielding section 6, and a deflection electrode 7, and a potential application section 8. The housing 201 accommodates the detection unit 10, the ionization section 202, the first mass separation section 203, the collision section 204, and the second mass separation section 205. The potential application section 8 and the pulse count detection section 206 are arranged outside the housing 201.

[0062] The ionization unit 202 generates ions from a sample introduced into the ionization unit 202. As an example, a liquid sample is introduced into the ionization unit 202 from a liquid chromatograph. In the ionization unit 202, the sample is ionized by, for example, electrospray ionization.

[0063] The first mass separation section 203 selectively transmits ions having a predetermined m / z value from among the ions generated by the ionization section 202. In this embodiment, the first mass separation section 203 is a quadrupole mass separation section equipped with four electrodes 203a. Each electrode 203a is a rod-shaped electrode and is arranged around a predetermined central axis. A predetermined potential is applied to each electrode 203a by a potential application section (not shown), and an electric field whose phase changes at high speed is formed. Ions having a predetermined m / z value oscillate stably when passing through the electric field, and can therefore transmit through the first mass separation section 203.

[0064] The collision section 204 dissociates the ions (precursor ions) that have passed through the first mass separation section 203 to generate product ions. The collision section 204 has a plurality of electrodes 204a and a collision chamber 204b. The number of the plurality of electrodes 204a is, for example, eight. The electrodes 204a are arranged around a predetermined central axis in the collision chamber 204b. A collision gas for colliding with the precursor ions is introduced into the collision chamber 204b. As an example, the collision gas is argon gas. The precursor ions that have entered the collision chamber 204b collide with the collision gas, so that the precursor ions are dissociated and product ions are generated. A predetermined potential is applied to each electrode 204a by a potential application section (not shown), and a high-frequency electric field is formed. The product ions are accelerated by the electric field, so that they smoothly proceed toward the outside of the collision chamber 204b.

[0065] The second mass separation section 205 selectively transmits ions having a predetermined m / z value from the ions (product ions) generated by the collision section 204. In this embodiment, the second mass separation section 205 is a quadrupole mass separation section equipped with four electrodes 205a. Each electrode 205a is a rod-shaped electrode and is arranged around a predetermined central axis. A predetermined potential is applied to each electrode 205a by a potential application section (not shown), and an electric field whose phase changes at high speed is formed. Ions having a predetermined m / z value oscillate stably when passing through the electric field, and can therefore transmit through the second mass separation section 205.

[0066] The ions transmitted through the second mass separation section 205 enter the ion detector 1 as ions P to be detected. The pulse count detection section 206 counts the pulse signals output from the anode 43 described above. The mass spectrometer 200 is a quadrupole mass spectrometer equipped with a first mass separation section 203 and a second mass separation section 205, which are quadrupole mass separation sections. In the mass spectrometer 200, the signal-to-noise ratio (S / N) can be improved by the two-stage mass separation by the first mass separation section 203 and the second mass separation section 205. Therefore, the mass spectrometer 200 is suitable for detecting trace components contained in a sample.

[0067] According to the configuration of mass spectrometer 200 as described above, in the ion detector 1, the trajectory of electrons E emitted from the conversion dynode 3 is optimized by applying appropriate potentials to the conversion dynode 3, the first stage dynode 42a of the electron multiplier section 4, the box electrode 5, and the shielding section 6. This makes it possible to improve the detection efficiency of ions for both positive and negative ions, even when the energy distribution and spatial distribution of ions transmitted through the first mass separation section 203 or the second mass separation section 205 are widely spread.

[0068] In the mass spectrometer 200, the first mass separator 203 is a quadrupole type mass separator, and the second mass separator 205 is a quadrupole type mass separator. This allows the first mass separator 203 and the second mass separator 205 to be specifically configured. [Variations]

[0069] The present invention is not limited to the above-described embodiment. In the ion detector 1, one potential application unit 8 applies a predetermined potential to each of the conversion dynode 3, the electron multiplier unit 4, the box electrode 5, the shielding unit 6, and the deflection electrode 7, but each of the multiple potential application units 8 may apply a predetermined potential to at least one of the conversion dynode 3, the electron multiplier unit 4, the box electrode 5, the shielding unit 6, and the deflection electrode 7.

[0070] In the ion detector 1, the shielding portion 6 is electrically connected to the box electrode 5, but the shielding portion 6 does not have to be electrically connected to the box electrode 5. As an example, the shielding portion 6 may be supported by a support member (not shown). In this case, the potential applying portion 8 may be electrically connected to the shielding portion 6 by wiring or the like, so that a fourth potential may be applied to the shielding portion 6.

[0071] In the ion detector 1, the shielding portion 6 is a lattice-shaped member, but may be a honeycomb-shaped member. The multiple gaps 6a of the shielding portion 6 are arranged in a matrix, but may be arranged two-dimensionally in another form. Furthermore, the multiple gaps 6a may be arranged one-dimensionally. In that case, each gap 6a may be formed in a slit shape. It is sufficient that the shielding portion 6 has at least one gap 6a.

[0072] In the ion detector 1, the electron passing port 51b is located on the other side of the center line A2 when viewed from the Z-axis direction, and the first stage dynode 42a faces the electron passing port 51b, but the conversion region 3a, the electron passing port 41a, the first stage dynode 42a and the electron passing port 51b may be aligned on the plane α.

[0073] In the ion detector 1, the third potential is substantially the same as the first potential, but the third potential may not be substantially the same as the first potential. As an example, when the ion P is a positive ion, the first potential may be −10 kV and the third potential may be −11.5 kV. As an example, when the ion P is a negative ion, the first potential may be +5.5 kV and the third potential may be +6.1 kV.

[0074] In the ion detector 1, the total area of ​​the plurality of gaps 6a is 50% or more of the area of ​​the ion passing opening 51a, but the area of ​​the gaps 6a may be less than 50% of the area of ​​the ion passing opening 51a.

[0075] The ion detector 1 may be used as a part of a mass spectrometer that uses TOF (Time of Flight), or may be used as a part of a mass spectrometer that uses plasma as an ion source.

[0076] Although the mass spectrometer 200 includes the first mass separation section 203 and the second mass separation section 205, it is sufficient that the mass spectrometer 200 includes at least one of the first mass separation section 203 and the second mass separation section 205. The mass spectrometer 200 does not need to include the collision section 204. [Explanation of symbols]

[0077] DESCRIPTION OF THE REFERENCE NUMERALS 1...ion detector, 2...partition section, 3...conversion dynode, 3a...conversion region, 4...electron multiplier section, 5...box electrode, 6...shielding section, 6a...gap, 7...deflection electrode, 8...potential application section, 21a...aperture, 42a...first stage dynode, 51a...ion passage port (first opening), 51b...electron passage port (second opening), 200...mass analyzer, 202...ionization section, 203...first mass separation section (mass separation section), 205...second mass separation section (mass separation section), A1...center line (second center line), A2...center line (first center line), E...electrons, P...ions.

Claims

1. a conversion dynode having a conversion region that emits electrons when ions are incident thereon; an electron multiplier section having a first stage dynode onto which the electrons are incident; a box electrode containing the conversion dynode and having a first opening for passing the ions traveling to the conversion region and a second opening for passing the electrons traveling from the conversion region to the first stage dynode; a conductive shielding portion disposed at the first opening and having a gap for passing the ions traveling to the conversion region; a potential applying unit that applies a first potential to the conversion dynode, a second potential to the first stage dynode, a third potential to the box electrode, and a fourth potential to the shielding unit, the first potential and the third potential have polarities opposite to those of the ions; the second potential is higher than the first potential, The third potential and the fourth potential are substantially the same potential.

2. The ion detector according to claim 1 , wherein the shielding portion has, as the gap, a plurality of gaps that are two-dimensionally arranged along the first opening.

3. the first opening is disposed on one side of a first center line passing through a center of the conversion area when viewed in a direction perpendicular to the first center line, the second opening is disposed on the other side of the first center line when viewed from the direction perpendicular to the first center line, 3. The ion detector according to claim 1, wherein the first stage dynode faces the second opening.

4. 3. The ion detector according to claim 1, wherein the first potential and the third potential are substantially the same potential.

5. The ion detector according to claim 1 , wherein an area of ​​the gap is 50% or more of an area of ​​the first opening.

6. The ion detector according to claim 1 , further comprising a partition having an aperture for passing the ions traveling to the first opening.

7. Further comprising a deflection electrode, the box electrode is disposed on one side of a second center line of the aperture when viewed in a direction perpendicular to the second center line, the deflection electrode is disposed on the other side of the second center line when viewed from the direction perpendicular to the second center line, The ion detector according to claim 6 , wherein the potential applying unit applies a ground potential or a potential having the same polarity as that of the ions to the deflection electrode.

8. The ion detector according to claim 6 , wherein the first opening is inclined so as to move away from the aperture as it approaches the second center line when viewed in a direction perpendicular to the second center line of the aperture.

9. An ionization unit; A mass separator; an ion detector; The ion detector comprises: a conversion dynode having a conversion region that emits electrons when ions are incident thereon; an electron multiplier section having a first stage dynode onto which the electrons are incident; a box electrode containing the conversion dynode and having a first opening for passing the ions traveling to the conversion region and a second opening for passing the electrons traveling from the conversion region to the first stage dynode; a conductive shielding portion disposed at the first opening and having a gap for passing the ions traveling to the conversion region; a potential applying unit that applies a first potential to the conversion dynode, a second potential to the first stage dynode, a third potential to the box electrode, and a fourth potential to the shielding unit, the first potential and the third potential have polarities opposite to those of the ions; the second potential is higher than the first potential, The third potential and the fourth potential are substantially the same potential.

10. The mass separation unit is a quadrupole mass separation unit. The mass spectrometer of claim 9.

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