JPS61237358A - Mass spectrometer - Google Patents

Abstract

PURPOSE:To improve the accuracy with low noise in mass spectrograph of such element as having remarkably high existing ratio by providing reflection preventing means in Farady cap thereby preventing reflection and dispersion of incident ions at the Farady cap. CONSTITUTION:In order to prevent the incident ions from departing to the outside of Farady cap 21 through reflection or dispersion, a reflection preventing board 25 and an ion reflection preventing mesh 26 are provided respectively at the slit board side 23 and the cap 22 side. Said boards 25 are planted at three sides of square slit board 23 while surrounding the slit board 23 in reverse direction from the advancing direction of the incident ions. While said mesh 26 is arranged in the cap 22 in three layers approximately in perpendicular against the ion orbit 17.

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. As a mass separation system, even 1! , the direction and velocity of the accelerated ion (
There is a single convergence type that converges energy (energy) using a fan-shaped uniform magnetic field, and a double convergence type that converges the fan-shaped uniform magnetic field using a fan-shaped electric field.

A single convergence mass spectrometer is a Dempster
), and then the -order approximation of ion optics was perfected 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 the wide-angle convergence system (Erwin), 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 ionic orbits in the orbital plane, and only the fringing
This is because the influence of g fieldcl) is ignored.

However, even if the magnetic field is uniform, if the edge group deviates from the orbital plane, the distribution force C of the magnetic field will be different, and the ion trajectory will therefore shift, causing 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 an edge group 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. 6, 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 and exit angles of the ion beam 4, respectively, and 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 leaving the ion source 2 and reaching the collector 6, the so-called aberration Xr, 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 α 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. γ 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. It is known that by making 11 and 12 asymmetrical, the image magnification AX can be made arbitrarily large. The resolution R of the mass spectrometer is given by the following equation (2), where S is the width of the slit of the ion source.

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.

Now, as a typical example, FIG. 7 shows changes in each aberration coefficient with respect to Ax in the case of φm=90', magnetic pole gap O, and OSγl.

It can be seen from FIG. 7 that when Ax=0.5, Aαα is three times larger than when Ax=1, and A, β, and Aββ are about twice as large. From this, when the aberration Δ in equation (2) becomes extraordinary, the resolution worsens, and 11 and 1
It can be determined that it is not preferable to make 2 asymmetrical. Note that in the figure, the value obtained by dividing the aberration coefficient by Aγ is shown, but this is because when considering resolution, the ratio between the magnitude of mass dispersion and the magnitude of aberration becomes a problem.

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
=rmcot' 1/2φm - (3) By applying the bending, quadratic convergence with respect to α is obtained. The conditions at this time are R4 = R2 = rm when φm = 90°, and R1 = R2 = 0.192 outside of φm = 60''.
It is 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.

From this table, it can be seen that Aαα is O for the curved boundary, but the Ike's aberration coefficients are all larger than those for the straight boundary. Therefore, adding a curve to the magnetic field boundary also reduces aberration. It can be said that it is not effective in reducing the size.

Furthermore, for the magnetic field at the straight boundary, the angle of input and output (ε1
．． ε2) and deflection angle (φm), four 2
These are the measurement results that measure how the order aberration coefficients and other important ion optical parameters change.
2” ε, tan t=<0.68~0.
69) janφm/2 (3) If the input and output angles (ε1, ε2) and deflection angles (φll1) are set to satisfy the following, mass dispersion is large,
It is known that the aberration coefficient can be reduced and the shape of a single convergence mass spectrometer can also be made 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, 3He/'He=10-6 to IQ-8.

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.

In this case, it is convenient to receive 'He, which has a large abundance ratio, in a Faraday cup and amplify 3He in a secondary electron multiplier so that they can be measured simultaneously. Such a mass spectrometer is shown in FIG.

As shown in FIG. 8, an ionization chamber 11 is provided in a vacuum container (not shown) that is evacuated to a high vacuum by an evacuation system. For example, when helium (He) is introduced into this ionization chamber 11, ionized 'He and '
He is accelerated by the electrode 12, obtains energy, and is extracted from the output slot 13 as a particle beam 14. The particle beam 14 is then separated into 3He ions and 'He ions according to their mass and charge by the Lorentz force under a fan-shaped uniform magnetic field 15 which is a magnetic field analysis section. The 3He ions and 'He ions exiting the fan-shaped uniform magnetic field 15 take different ion trajectories 16 and 17, respectively. In the middle of the ion trajectory 16, there is an imaging slit 18. A deflection electrode 19 and a secondary electron multiplier 20 are arranged, and a Faraday cup 21 is arranged in the middle of another ion trajectory 17. As shown in FIG. 9, this Faraday cup 21 is a stainless steel cup 2 having a rectangular parallelepiped shape.
2, and a slit plate 2 arranged in front of the opening 22a.
The 'He ions taking the ion trajectory 17 pass through the slit 7) 23a of the slit plate 23, collide with the cup 22, become neutral atoms, and are detected as an ion current. On the other hand, 'He ions taking an ion trajectory 16 are transmitted through an amplifier 24 connected to the output of a secondary electron multiplier 20.
is detected as an electrical signal.

By the way, in the mass spectrometer having the above configuration, the ionization chamber 11 has a voltage ranging from 2X10-'Torr to 5xi
When helium is introduced at o-6 Torr, the ion current detected by the Faraday cup 21 becomes approximately 20 pA, so it can be detected by the Faraday cup 21 as described above, but the 'He ions incident on the Faraday cup 21 y) It is reflected and scattered repeatedly, and from the orbit 16 of the 3He ion, it passes through the imaging slit 18 and enters the secondary electron multiplier tube 2.
If it is incident on 0, it becomes noise. At this time, the 3He ion is about 2X10-"A, and the number of ions is about 1
About 3 microscopic secondary electron multipliers 77 to 20
When the 'He ions reflected by the Radhe cup 21 are incident, they cause a large amount of noise, making it completely impossible to measure 'He.

(Object of the Invention) The present invention has been made to solve the above-mentioned problems, and it is possible to prevent reflection and scattering of incident ions in the Faraday cup, improve the S/N ratio, and perform stable measurements. The aim is to provide a mass spectrometer with

(Structure of the Invention) Therefore, the present invention provides a mass spectrometer in which the ion species to be detected is incident on a Faraday cup and an ion detection means, which are respectively arranged in the middle of the ion trajectory, in which the Faraday cup prevents ion reflection. It is characterized by having the means.

The ion anti-reflection means is preferably an anti-reflection plate erected to surround a slit of a slit plate disposed at the front of a metal cup constituting the Faraday cup.

Preferably, the ion antireflection means is a mesh arranged so as to be perpendicular to the trajectory of ions entering the interior of the metal cup constituting the Faraday cup.

(Effects of the Invention) According to the present invention, since the Faraday cup is provided with an anti-reflection means to prevent reflection and scattering of ions incident on the Faraday cup, abundance ratios differ significantly as in isotope analysis. Even in the case of mass spectrometry of elements, isotopes can be measured simultaneously with low noise and with high precision using ion detection means such as a Faraday cup and a secondary electron multiplier.

(Example) Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

A mass spectrometer according to the present invention is shown in FIG.

In the mass spectrometer shown in FIG. 1, the mass spectrometer shown in FIG. As means, an antireflection plate 25 is provided on the slit plate 23 side, and a mesh 26 for preventing ion reflection is provided on the cup 22 side.

As shown in FIG. 2, the anti-reflection plate 25 is erected on three sides of the rectangular slit plate 23 so as to surround the slit plate 23 in a direction opposite to the traveling direction of the incident ions. In addition, the mesh 26 for preventing ion reflection is
As shown in FIG. 3, three layers are arranged inside the cup 22 at right angles to the ion trajectory 17.

In this way, the ions emitted from the Faraday cup 20 will be reflected or scattered, and the Faraday cup 2
1. They go outside and become stray ions or floating ions, which become 2.
The number of electrons incident on the secondary electron multiplier 20 is significantly reduced, resulting in a high S
/N ratio makes it possible to simultaneously measure isotopes using the 77 rad cup 21 and the secondary electron multiplier 20.

By the way, with the mass spectrometer shown in Figure 1, 'He and 'H
When detecting e, as shown in Fig. 4,
For 'He, results shown by curves 1 and 2 were obtained, respectively.

On the other hand, when similar analysis was performed using the mass spectrometer shown in FIG. 8, as shown in FIG.
Analysis results indicated by □ were obtained.

As is clear from comparing Figures 4 and 5, the mass spectrometer shown in Figure 1 has a much lower noise level.
It can be seen that 3He is detected with high accuracy. In comparison, in Fig. 5, "He" is completely buried in the noise of reflected "He" and cannot be detected at all.

In the above embodiment, even if either the antireflection plate 25 or the mesh 26 is provided as the ion antireflection means, mass spectrometry of elements with significantly different abundance ratios can still be performed with a high S/ It can be measured by N ratio.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the configuration of an embodiment of a mass spectrometer according to the present invention, FIG. 2 is a perspective view of a 77-rad cup of the mass spectrometer shown in FIG. 1, and FIG. 3 is a perspective view of a 77-rad cup of the mass spectrometer shown in FIG. Figures 4 and 5 are graphs showing the helium analysis results of Figure 1 and a conventional mass spectrometer, respectively, and Figure 6 shows the deflection angle of ions in a fan-shaped uniform magnetic field. Explanatory diagram showing the relationship between and the human exit angle, No. 7
The figure is a graph showing the relationship between Ax and each aberration coefficient, FIG. 8 is an explanatory diagram of a conventional mass spectrometer, and FIG. 9 is a perspective view of the 77 Radhe cup of the mass spectrometer shown in FIG. 15... Fan-shaped uniform magnetic field, 16.17... Ion orbit, 20... Secondary electron multiplier, 21... Faraday cup, 22... Cup, 23
...Slit plate, 25...Anti-reflection plate, 26...
Metush. Patent applicant: Murata Manufacturing Co., Ltd. Agent: Patent attorney: Aoyama Bohaha, 2 persons No. 3 M
! 8g Figure 4 Figure 5 Figure 9 @ 2M Figure 6

Claims (3)

[Claims] (1) Ions incident on a magnetic field analyzer consisting of a fan-shaped uniform magnetic field have their masses discriminated according to the magnetic field strength, and are then emitted from the magnetic field detector and placed in the middle of the ion trajectory of the ion species to be detected. 1. A mass spectrometer in which ions are incident on a Faraday cup and an ion detection means, wherein the Faraday cup is equipped with an ion reflection prevention means. (2) A patent characterized in that the ion anti-reflection means is an anti-reflection plate erected to surround the slit of a slit plate arranged in the front part of a metal cup constituting a Faraday cup. A mass spectrometer according to claim 1. (3) A patent characterized in that the ion reflection prevention means is a mesh arranged so as to be substantially perpendicular to the trajectory of ions entering the interior of the metal cup constituting the Faraday cup. A mass spectrometer according to claim 1.