JPS61237357A - Mass spectrometer - Google Patents

Abstract

PURPOSE:To improve the accuracy while reduce the noise, in mass spectrograph of such element as having remarkably high existing ratio by providing means for removing the ions deviated from the orbit of ion. CONSTITUTION:A metal pipe 16 is arranged near the magnetic field analyzing section 15 while surrounding the ion orbit 22 of particle beam 14 between said section 15 and an imaging slit 18 then grounded, while an electrode board 17 is arranged between said pipe 16 and slit 18 then grounded. A deflection electrode 19 is arranged between said slit 18 and secondary electron multiplier tube 20 where positive (plus) high voltage E1 is applied onto one of them and the remainder is grounded. The ions existed from an uniform fan-type filed 15 will pass through said pipe 16 and the slit 17a of the electrode board 17 and enter into said slit 18. The majority of the ions deviated from the ion orbit 22 will never enter into said pipe 16 and the minority entering into said pipe 16 are removed from next electrode board 17 and never enter into said slit 18.

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 determine 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 these ions into ions of different masses using a magnetic field and/or an electric field, and detects and records the separated ions. It consists of a detection recording system. Examples of mass separation systems include a single focusing type that focuses the direction and velocity (energy) of accelerated ions using a fan-shaped uniform magnetic field, and a single focusing type that focuses the direction and velocity (energy) of accelerated ions using a fan-shaped uniform magnetic field as well as a fan-shaped electric field. There is a double convergence type.

A single convergence mass spectrometer is a Dempster
), and then the -order approximation of ion optics was completed by Herzog and is now widely used. Many of the devices in practical use are 6
A fan-shaped uniform magnetic field of 0° or 90° is used in a right-angle entrance/exit method. On the other hand, attempts to improve the performance of devices using ion optical considerations are
Several attempts have been made, such as wide-angle convergence (eru+in), 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 only deal with ion trajectories within the orbital plane, and the edge fields (fringin
g field) is ignored. 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, and therefore the ion trajectory shifts, which causes aberrations. For this reason, when the device was actually built, the improvement in performance was not as noticeable 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, and a calculation method that can calculate the second- and third-order aberrations of any ion optical system has been developed. This has been established and used to examine the aberrations of various single-focus mass spectrometers from an ion optical perspective, and various items related to these aberrations, such as asymmetrical arrangement, curved boundaries, and radiation emission, have been investigated. ing.

Next, the results of the study will be explained.

First, to explain the fan-shaped uniform magnetic field, in Fig. 4, 1 is the magnetic field, 2 is the ion source, 3 is the slit, 4 is the ion beam from the ion source 2, 5 is the collector slit, 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 and ε2 are the incident angle and exit angle of the ion beam 4, respectively, 11 is the distance from the slit 3 to the magnetic field 1,
12 is the distance from the magnetic field 1 to the collector slit 5, R,
, R2 is 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: Xf=AXX+Aαα+Aγγ+Aααα2+A, ,Y
2+A, βYβ+Aβββ2 (1) Given by.

Here, X and Y are deviations from the optical axis in the orbital plane and in a direction perpendicular to the orbital plane when the ions enter the slit 3, and a and β are the angles of inclination of the ion beam 4.

Note that since we are considering a single focusing mass spectrometer, it is assumed that there is no deviation δ in the energy of ions. χ is the mass deviation ratio.

AX is the image magnification, and Aγ is the mass dispersion coefficient. Since the remaining coefficients give aberrations, it is desirable that they be as small as possible. Further, under normal conditions, α and β are 0.01 or less, and when considering an apparatus 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, first, as a means to reduce the aberration, 11 and 12
It is conceivable to make it asymmetric. l.

It is known that by making and 12 asymmetric, the image magnification AX can be set to an arbitrary value. Then, the resolution R of the mass spectrometer is given by S, the width of the slit of the ion source.
i) is given by the following equation (2).

Here, Δ is the spread of the image due to aberration.

In equation (2), it can be seen that if Δ is ignored, 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.

As a typical example, FIG. 5 shows the changes in each aberration coefficient with respect to Ax in the case where φm=90° and the magnetic pole spacing is 0.05γm.

It can be seen from FIG. 5 that when AX is 0.5, Aαα is three times larger, and A, β, and Aββ are about twice as large as when Ax=1. This means that if the aberration Δ in equation (2) becomes large, the resolution will worsen, and 11 and 12
It can be judged that it is not desirable to make it asymmetric. In addition, in the figure, the value obtained by dividing the aberration coefficient by Ax is shown, but
This is because when considering resolution, the ratio between the magnitude of mass dispersion and the magnitude of aberration becomes an issue.

Next, the curved boundary of the magnetic field and aberration will be explained.

For a fan-shaped magnetic field, the radius of curvature R, = R2 = on the image boundary surface
By applying the bend rmcot31/2φm−(3), we obtain two large bundles regarding α. The other conditions are R1"R2"rm when φm=90',
When φm=60', R1=R2=0.192 rm. 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.

In this table, Aαα is 0 for the curved boundary, but the other aberration coefficients all show larger values compared to the straight boundary, and therefore the magnetic field boundary surface is curved. It can be said that this is not effective in reducing aberrations.

Furthermore, for the magnetic field at the straight boundary, the angle of input and output (ε1
．． Here are the measurement results of how the four second-order aberration coefficients and other important ion optical parameters change when changing ε2) and deflection angle (φl11). ε1=
When ε2=ε, tan ε=, (0,68~0.
69) tanφIIl/2 (3) If the angle of incidence and exit (ε1.ε2) and deflection angle (φm) are set to satisfy the following, the mass dispersion is large and the aberration coefficient is It is known that the size of the single convergence mass spectrometer is becoming smaller, and the shape of the single convergence mass spectrometer 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, koHe/'He=10-6 to 1O-a. In such a case, the number of 3He ions in the mass spectrometer is less than several tens. If you try to detect this 'He ion with a normal single focusing mass spectrometer, 'H
The noise level increases due to scattering, reflection, straying, or floating of e ions, making it extremely difficult to detect 3He.

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, our single convergence mass spectrometer is, for example, 1/3 of the conventional single convergence mass spectrometer.
When the size of the He ion is reduced to a certain extent, the spatial density of 'He ions due to scattering, reflection, straying, or floating becomes more than three times higher, 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 eliminate ions that have deviated from the ion trajectory from the magnetic field analysis section to the detection section.

Since ions due to reflection, straying, floating, etc. are prevented from reaching the detection unit, isotope ratio analysis can be performed with extremely low noise even when mass spectrometry is performed on elements with significantly different abundance ratios such as isotope analysis. It can be done with high precision.

(Example) Hereinafter, the present invention will be described in detail 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. This ionization chamber 1
Particles (usually positive ions) ionized in the particle beam 1 are accelerated by the electrode 12, gain energy, and are extracted from the exit slit 13 as a particle beam 14. The particle beam 14 is then passed through a fan-shaped uniform magnetic field 15 which is a magnetic field analysis section.
is bent according to its mass and charge by the Lorentz force under . In FIG. 1, magnet pole piece surfaces that generate a fan-shaped uniform magnetic field 15 are provided on the upper and lower sides of FIG. 1, and the direction of the magnetic field is downward in the plane of the paper. Exiting the fan-shaped uniform magnetic field 15, the metal pipe 16. Electrode plate 17 with multiple slits. The intensity of the particle beam 14 having a diameter of the imaging slit 18 and the deflection electrode 19 with a plurality of slits is
Amplified by a secondary electron multiplier tube 20 which is a detection section,
1 is detected as an electrical signal. When the intensity of the sector-shaped uniform magnetic field 15 is continuously changed, the sector-shaped uniform magnetic field 15
The ion species can be discriminated in time series from the output of the amplifier 21 with respect to the intensity.

The metal pipe 16 is arranged near the magnetic field analysis section 15 so as to surround the ion trajectory 22 of the particle beam 14 from the magnetic field analysis section 15 to the imaging slit 18 . 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 slit 17
a is arranged along the ion trajectory 22 6
The electrode plate 17 has its edges oriented in the direction in which the ions fly in order to prevent ions that have strayed from the ion trajectory 22 and are scattered, reflected, strayed, or floating in the vacuum container from invading the adjacent electrode plates 17°17. It is preferable to bend it. The electrode plate 17 is grounded.

Further, the deflection electrode 19 is arranged between the imaging slit 18 and the secondary electron multiplier 20, and similarly to the electrode plate 17, the deflection electrode 19a is arranged between the imaging slit 18 and the secondary electron multiplier 20.
It is arranged along the ion trajectory 22 leading to the secondary electron multiplier 20. A positive (plus) high voltage E1 is applied to one of the deflection electrodes 19, and the remaining ones are grounded.

If the mass spectrometer has such a configuration, the ions leaving the fan-shaped uniform magnetic field 15 pass through the metal pipe 16, pass through the sulin 17a of the electrode plate 17, and then pass through the imaging slit 1.
However, the ions that deviate from the ion trajectory 22 do not enter the metal pipe 16 and 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.

In addition, there is a possibility that ions slightly deviated from the ion orbit 22 may be incident on the metal pipe 16, but these ions will be scattered within the metal pipe 16 and will be redirected from between the next electrode plates 17 to the ion orbit 22. The light is scattered outside and eliminated, and does not enter the imaging slit 18.

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 are eliminated from the ion trajectory 22. be done.

In this way, the ions that have left the ion orbit 22 are sequentially transferred to the metal pipe 16. Since the ions are removed from the ion trajectory 22 by the electrode plate 17 and the deflection electrode 19, the ions having the theoretical ion energy that have flown through the ion trajectory 22 are incident on the secondary electron multiplier 20. Therefore,
Elements with significantly different abundance ratios can be detected with low noise.

Incidentally, when 3He was detected using the mass spectrometer shown in FIG. 1, the results shown in FIG. 2 were obtained.

In addition, in the mass spectrometer shown in FIG. 1, the metal pipe 16,
When 3He was detected using a device without the electrode plate 17 and deflection electrode 19, the results shown in FIG. 3 were obtained.

As is clear from comparing Figures 2 and 3, the mass spectrometer shown in Figure 1 has a much lower noise level.
It can be seen that 'He is detected 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 3He using the mass spectrometer having the configuration shown in FIG. 1, and FIG. A graph showing the results of detecting He using a mass spectrometer without ion exclusion means. Fig. 4 is an explanatory diagram showing the relationship between the ion deflection angle and the human exit angle in a fan-shaped uniform magnetic field. Fig. 5 The figure 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 orbit. Patent Applicant Murata Manufacturing Co., Ltd. Agent Patent Attorney Aoyama 2 people Figure 1 Figure 4 Figure 2 HD+H. Figure 3 Figure 5 AX

Claims (1)

[Scope of 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. A mass spectrometer for mass detection, 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. Analysis equipment. (2) 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. Mass spectrometer. (3) The ion exclusion means is an electrode plate with a plurality of slits arranged between the magnetic field analysis section and the imaging slit and grounded, and these electrode plates have a plurality of slits arranged so that the ion trajectory is above the ion trajectory. The mass spectrometer according to claim 1, characterized in that the mass spectrometer is arranged along. (14) Claim 3, wherein the electrode plate has an end bent in a direction in which ions fly.
The mass spectrometer described in Section 1. (5) The ion exclusion means is a plurality of deflection electrodes arranged between the imaging slit and the detection section, and a positive voltage is applied to one of these deflection electrodes. A mass spectrometer according to any one of claims 1 to 4, characterized in that: