JPH0572701B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a single focusing mass spectrometer that uses a fan-shaped uniform magnetic field. (Prior Art) Mass spectrometry is a method of separating ions of different masses under an electric field and/or a magnetic field to obtain the distribution of ion masses, and is used in chemical analysis and the like. A mass spectrometer consists of an ion source that generates ions, a mass separation system that separates ions with different masses by appropriately using a magnetic field and/or an electric field, and detects and records the separated ions. It consists of a detection recording system. As a mass separation system, for example, a single focusing type that focuses the direction and velocity (energy) of accelerated ions using a fan-shaped uniform magnetic field, or a single focusing type that focuses the direction and velocity (energy) of accelerated ions using a fan-shaped uniform magnetic field, or a fan-shaped electric field as well as a fan-shaped uniform magnetic field. There is a double convergence type. The single-focus mass spectrometer was first developed by Dempster, and then first-order ion optics was perfected by Herzog, which is now widely used. Most of the devices that have been put into practical use use a 60° or 90° sector-shaped uniform magnetic field in a right-angle input/output method. On the other hand, attempts to improve device performance through ion optical considerations were made by Kerwin.
Several attempts have been made, including wide-angle convergence, but none have been put to practical use. This is thought to be due to the following reasons. That is, the calculations that form the basis of these proposals deal only with ion trajectories within the orbital plane and ignore the effects of fringing fields. However, even if the magnetic field is uniform, the distribution of the magnetic field differs when the ion deviates from the orbital plane in the edge field, which causes the ion trajectory to shift, which causes aberrations. For this reason, when the device was actually built, the improvement in performance was not as much as expected, which is probably why it was never put into practical use. In contrast, a trajectory calculation method has recently been developed that calculates the trajectory of an ion passing through the edge field using a third-order approximation.
A calculation method that can determine the second- and third-order aberrations of any ion optical system has been established, and using this method, the aberrations of various single-focus mass spectrometers can be investigated from an ion optical perspective, and this aberration can be investigated. Relatedly, various items such as asymmetrical arrangement, curved boundaries, and oblique entrance/exit are being studied. Next, the results of the study will be explained. First, to explain the fan-shaped uniform magnetic field, the fourth
In the figure, 1 is a magnetic field, 2 is an ion source, 3 is a slit, 4 is an ion beam from the ion source 2, 5 is a collector slit, and 6 is an ion collector. In addition, φm is the deflection angle of the magnetic field 1, rm is the orbital radius of the ion beam 4, ε ₁ and ε ₂ are the incident and exit angles of the ion beam 4, respectively, l ₁ is the distance from the slit 3 to the magnetic field 1, and l ₂ is the distance from the magnetic field 1 to the collector slit 5, and R ₁ and R ₂ are the radius of curvature of the boundary surface of the magnetic field 1. In _addition , the deviation from the optical axis of ions that leave the ion source 2 and reach the collector 6, the so ^{- called} aberration Xf, is _{a quadratic} approximation: 〓〓β ² ... is given by (1). where,
is the angle of inclination of Note that since we are considering a single-focus mass spectrometer, the energy deviation of the ions δ
It is assumed that there is no. γ is the mass deviation ratio. A _x is the image magnification, and A〓 is the mass dispersion coefficient. Since the remaining coefficients give aberrations, it is desirable that they be as small as possible. Also, under normal conditions, α, β
is less than 0.01, and when considering a device with a resolution of several thousand or less, there is no need to worry about the third-order coefficient as long as it is about 100 or less. Therefore, the second-order aberration coefficient as shown in equation (1) is considered here. Therefore, as a means to reduce the aberration, l ₁
It is conceivable to make and l ₂ asymmetric. It is known that by making l ₁ and l ₂ asymmetric, the image magnification A _x can be made to an arbitrary value. The resolution R of the mass spectrometer is the width of the ion source slit.
Then, it is given by the following equation (2). R=A〓/2(sAx+△)...(2) Here, △ is the spread of the image due to aberration. In equation (2), if △ is ignored, it can be seen that a larger resolution can be obtained by making Ax smaller. However, when the aberration coefficients were calculated for different Ax values, it was found that the aberrations rapidly increased as Ax became smaller. Now, as a representative example, for the case where φm = 90° and the magnetic pole spacing is 0.05γm, Ax of each aberration coefficient is
Figure 5 shows the changes in From Figure 5, it can be seen that when Ax=0.5, A〓〓 is three times larger than when Ax=1, and A _y〓 and A〓〓 are about twice as large. From this, it can be determined that as the aberration Δ in equation (2) increases, the resolution worsens, and that it is not preferable to make l ₁ and l ₂ asymmetric. In addition, in the figure, the aberration coefficient is A〓
The reason for this is that when considering resolution, the ratio between the magnitude of mass dispersion and the magnitude of aberration is an issue. Next, the curved boundary of the magnetic field and aberration will be explained. For a fan-shaped magnetic field, if both boundary surfaces are curved with a radius of curvature R ₁ = R ₂ = rmcot ³ 1/2φm (3), quadratic convergence with respect to α can be obtained. The conditions at this time are that when φm = 90°, R ₁ =
R ₂ = rm, and when φm = 60°, R ₁ = R ₂ =
It is 0.192rm. At this time, the results of calculations of other aberration coefficients are shown in the following table. For comparison, data on straight boundaries are also shown.

[Table] From this table, A〓〓 is 0 for the curved boundary, but the other aberration coefficients all show larger values compared to the straight boundary. It can also be said that adding a lens is not effective in reducing aberrations. Furthermore, when changing the input and output angles (ε ₁ , ε ₂ ) and the deflection angle (φm) for the linear boundary magnetic field,
From the measurement results of how the four second-order aberration coefficients and other important ion optical parameters change, when ε ₁ = ε ₂ = ε, tanε = (0.68 to 0.69) tanφm/ 2 If the angles of incidence and exit (ε ₁ , ε ₂ ) and angle of deflection (φm) are set to satisfy ...(3), the mass dispersion will be large, the aberration coefficient will be small, and a single converging mass will be achieved. It is known that the shape of analyzers is also becoming smaller. By the way, in isotope analysis, mass spectrometry is often applied to elements with significantly different abundance ratios. Taking helium (He) as an example, ³ He/ ⁴ He = 10 ^-6 to 10 ^-8 . In such a case, the number of ³ He ions in the mass spectrometer is less than several dozen. This ³ He
When trying to detect ions with a normal single focusing mass spectrometer, the noise level increases due to scattering, reflection, straying or floating of ⁴ He ions, and ³ He
becomes extremely difficult to detect. Therefore, for the analysis of such isotopes, a single convergence mass spectrometer having a large mass dispersion and a large convergence coefficient as described above can be effectively used. However, if this single convergence mass spectrometer is downsized to, say, 1/3 of the size of a conventional single convergence mass spectrometer, scattering, reflection, straying, floating, etc.
⁴ The problem was that the spatial density of He ions increased by more than three times, making it impossible to obtain a satisfactory S/N ratio. (Object of the Invention) The present invention has been made in view of the above-mentioned problems in conventional mass spectrometers, and provides a mass spectrometer that detects elements with significantly different abundance ratios with low noise and high accuracy. is intended to provide. (Structure of the Invention) Therefore, the present invention is characterized in that it includes an ion exclusion means for eliminating ions that have deviated from the ion trajectory from the magnetic field analysis section to the detection section, which is made up of a fan-shaped uniform magnetic field. The ion removal means is preferably a metal pipe that is provided to surround the ion trajectory from the magnetic field analysis section to the imaging slit and is grounded, and a metal pipe that is arranged between the magnetic field analysis section and the imaging slit and that is grounded. These are a plurality of electrode plates with slits grounded to the ground, or a plurality of deflection electrodes arranged between the imaging slit and the detection section. (Effects of the Invention) According to the present invention, an ion exclusion means is provided between the magnetic field analysis section and the detection section to exclude ions that have deviated from the ion trajectory from the magnetic field analysis section to the detection section. Or, since it prevents ions from reaching the detection unit due to floating, etc., it is possible to perform isotope ratio analysis with extremely low noise and high accuracy even when performing mass spectrometry of elements with significantly different abundance ratios such as isotope analysis. I can do it. (Embodiments) Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. An ionization chamber 11 is provided in a vacuum container (not shown) which is evacuated to a high vacuum by a vacuum evacuation system. Particles (usually positive ions) ionized in this ionization chamber 11 are accelerated by an electrode 12,
Energy is obtained and extracted as a particle beam 14 from the exit slit 13. The particle beam 14 is then bent according to its mass and charge by the Lorentz force under a fan-shaped uniform magnetic field 15 which is a magnetic field analysis section. In FIG. 1, magnet pole piece surfaces that generate a sector-shaped uniform magnetic field 15 are provided on the upper and lower sides of the figure, and the direction of the magnetic field is downward in the plane of the paper. The intensity of the particle beam 14 that exits the fan-shaped uniform magnetic field 15 and passes through the metal pipe 16, the electrode plate 17 with a plurality of slits, the imaging slit 18, and the deflection electrode 19 with a plurality of slits is determined by the detection unit. is amplified by a secondary electron multiplier tube 20, and an amplifier 21
is detected as an electrical signal. By continuously changing the intensity of the sector-shaped uniform magnetic field 15, ion species can be discriminated in time series based on the output of the amplifier 21 with respect to the intensity of the sector-shaped uniform magnetic field 15. The metal pipe 16 is connected to the ion trajectory 2 of the particle beam 14 from the magnetic field analyzer 15 to the imaging slit 18.
It is arranged near the magnetic field analysis section 15 so as to surround the magnetic field analysis section 2 . The metal pipe 16 is grounded. On the other hand, the electrode plate 17 is arranged between the metal pipe 16 and the imaging slit 18, and the slits 17a are arranged along the ion trajectory 22. The electrode plate 17 is an electrode plate 17 adjacent to ions that have left the ion trajectory 22 and are scattered, reflected, strayed, or floating in the vacuum container.
In order to prevent the ions from entering the ions 17, it is preferable to bend the ends thereof in the direction in which the ions fly. The electrode plate 17 is grounded to earth. Further, the deflection electrode 19 is connected to the imaging slit 18 and 2.
While being arranged between the secondary electron multiplier tube 20,
Like the electrode plate 17, its slit 19a is arranged along the ion trajectory 22 from the imaging slit 18 to the secondary electron multiplier 20. One of the deflection electrodes 19 has a positive high voltage.
E ₁ is applied and the rest are grounded to earth. If the mass spectrometer is configured like this, the ions leaving the fan-shaped uniform magnetic field 15 pass through the metal pipe 16 and the slit 17a of the electrode plate 17, and then enter the imaging slit 18. Ions that deviate from the ion trajectory 22 do not enter the metal pipe 16, but exit from the metal pipe 16. Ions that have come out of the metal pipe 16 no longer enter the ion trajectory 22 inside the metal pipe 16. Furthermore, there is a possibility that a very small amount of ions that deviate from the ion trajectory 22 will be incident on the metal pipe 16, but these ions will be scattered within the metal pipe 16 and will enter the ion trajectory 22 from between the next electrode plates 17. The image forming slit 18 is scattered and rejected.
It is never incident on . Ions that have passed through the imaging slit 18 and have an energy lower than the theoretical ion energy of the ions to be detected are repelled by the deflection electrode 19 to which a positive high voltage _E1 is applied, and exit the ion trajectory 22. be excluded. In this way, the ions that have left the ion orbit 22 are sequentially removed from the ion orbit 22 by the metal pipe 16, the electrode plate 17, and the deflection electrode 19. Orbit 22
Ions having the theoretical ion energy that have flown through will be incident. Therefore, elements with significantly different abundance ratios can be detected with low noise. Incidentally, when ³ He was detected using the mass spectrometer shown in FIG. 1, the results shown in FIG. 2 were obtained. Furthermore, when ³ He was detected using the mass spectrometer shown in FIG. 1 without the metal pipe 16, electrode plate 17, and deflection electrode 19,
The results shown in FIG. 3 were obtained. As is clear from a comparison between Figures 2 and 3, the mass spectrometer shown in Figure 1 has a significantly lower noise level and detects ³ He with high accuracy.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of an embodiment of the mass spectrometer according to the present invention, Fig. 2 is a graph showing the results of detecting ³ He using the mass spectrometer having the configuration shown in Fig. 1, and Fig. 3 Figure 4 is a graph showing the results of detecting ³ He using a mass spectrometer without ion exclusion means; Figure 5 is
It is a graph showing the relationship between Ax and each aberration coefficient. 14...Ion orbit, 15...Fan-shaped uniform magnetic field,
16... Metal pipe, 17... Electrode plate (17a...
... slit, 18... imaging slit, 19... deflection electrode (19a... slit), 20... secondary electron multiplier, 22... ion trajectory.

Claims (1)

[Claims] 1. Ions that are incident on a magnetic field analysis section consisting of a fan-shaped uniform magnetic field have their masses discriminated according to the magnetic field strength, and then enter the detection section from the magnetic field detection section through an imaging slit and are detected. In a mass spectrometer,
A mass spectrometer characterized in that an ion exclusion means is provided between the detection section and the magnetic field analysis section to exclude ions that have deviated from the ion trajectory from the magnetic field analysis section to the detection section. 2. The mass spectrometer according to claim 1, wherein the ion exclusion means is a metal pipe that is provided to surround the ion trajectory from the magnetic field analysis section to the imaging slit and is grounded. Device. 3. The ion exclusion means is an electrode plate with a plurality of slits arranged between the magnetic field analyzer and the imaging slit and grounded, and the slits of these electrode plates are arranged along the ion trajectory. The mass spectrometer according to claim 1, characterized in that the mass spectrometer is arranged as follows. 4. The mass spectrometer according to claim 3, wherein the electrode plate has an end bent in a direction in which ions fly. 5 The ion exclusion means is a plurality of deflection electrodes arranged between the imaging slit and the detection section,
5. The mass spectrometer according to claim 1, wherein a positive voltage is applied to one of the deflection electrodes.