JPS588107B2 - quadrupole lens - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a quadrupole lens used in a mass spectrometer, etc.
The present invention provides a quadrupole lens that is easy to manufacture and can form an electric field extremely close to a hyperbolic electric field.

As is well known, quadrupole lenses are widely used as mass filters.

In order to make the electric field formed by this quadrupole lens an ideal quadrupole electric field, it is necessary to use four electrodes with hyperbolic cross-sections, but manufacturing is extremely difficult due to processing accuracy etc. There is a drawback.

For this reason, conventionally, four round bar electrodes C having a circular cross section as shown in FIG. 1a are used side by side.

The electric field generated using an electrode with a circular cross section is shown in Figure 1b.
It is generally expanded into a power class having terms of second order, sixth order, tenth order, fourteenth order, etc. of the distance r from the lens center.

The second-order term is a hyperbolic electric field, which is an ideal quadrupole electric field, and the sixth-order and subsequent terms represent deviations from the hyperbolic electric field.

When using electrodes with this circular cross section, the distance from the center of the quadrupole lens to each electrode as shown in Figure 1a is R.
0, and if the radius of the electrode is R, then if R/R0 is selected to be about 1,147, the 6th order term can also be set to 0, so R
The radius and positional relationship of the electrodes are set so that /R6 becomes this value, but higher-order deviation terms cannot be made zero, so in conventional quadrupole lenses, the distance from the center of the lens Aberrations increase as the distance increases, making it difficult to improve both sensitivity and resolution.

The present invention was made in view of these points, and an object of the present invention is to provide a quadrupole lens that is easy to manufacture and can form an electric field extremely close to a hyperbolic number electric field, which is an ideal quadrupole electric field. A total of 12 needle-like electrodes are arranged at specific positions, and a specific potential is applied to these electrodes.Examples of the present invention will be described below in detail with reference to the drawings.

FIG. 2 is a diagram for explaining the electrode positional relationship of the quadrupole lens according to the present invention, and the origin 0 of the orthogonal coordinates is set to coincide with the central axis of the lens.

Needle wire electrodes 1, 2, 3, and 4 are arranged at equal intervals on a circumference of a radius ra centered on the origin 0, and on a circumference of a radius rb centered on the origin 0. is a needle wire electrode 5,
6, 7, 9, 9, 10, 11, and 12 are arranged.

electrodes 5, 12, electrodes 6, 7, electrodes 8, 9 and electrode 10,
11 are arranged at positions separated by an angle φ from the electrodes 1, 2, 3, and 4, respectively.

Also, the potential given to electrodes 1 and 3 is −A, and the potential given to electrodes 2 and 4 is
The potential given to +A, and the electrodes 5, 8, 9, 12
It is generated when the potential applied to the electrodes is +B and the potential applied to the electrodes 6, 7, 10, and 11 is -B.

The state of the potential distribution is expressed by the following potential function V(r, θ) using new coordinates (r, θ).

However, λ=ra/rb, ρa=r/ra.

In equation (1), the first term (second-order term with respect to r) gives a hyperbolic electric field, which is an ideal quadrupole electric field, and the terms after the second term are the sixth-order term from the dipole electric field, 10th order, 1
This represents a fourth-order shift.

In the above equation, B/A, λ, φ are independent variables, and co
Since there is no value of φ that makes the terms snφ (n-2, 6, 10, 14, . . .) 0, a solution is found in which only the 6th and 10th order terms are made zero at the same time.

In this case, since there are three independent variables, the solution is not just one, but an example of a typical solution with a practical value is as follows.

(Solution 1) If λ=l φ=22.5° is as follows.

V(r,θ)=4A(ρ2acos2θ+0.143p
14acos14θ+・・・・・・・・・）・・・・・・
(2) (Solution 2) When A=B, φ=21.83° and λ=0.956, and in this case, V(r, θ) is as follows.

Of these two solutions, the one given by equation (3) is 1
It can be seen that the fourth order term is small.

In reality, the quadrupole lens is placed in a vacuum container at ground potential.

Therefore, in order to take into account the influence of this vacuum container, we will consider an arrangement in which the electrode shown in FIG. 2 is placed in a metal cylinder with reference to FIG. 3.

In Figure 3, S represents the inner surface of the cylinder,
Components other than electrodes 2, 6, and 7 are omitted.

The electric field applied in such a case is derived below by considering the mirror image of each electrode with respect to the metallic cylinder.

The mirror image of each electrode arranged on the circumference of a circle with radius ra is formed on the circumference of radius ra*a2/ra, and the mirror image of each electrode arranged on the circumference of radius rb is formed with radius rb*=
It is formed on the circumference of a2/rb.

Further, a potential having the same absolute value and the opposite sign as the potential applied to the electrode is applied to the mirror image of the electrode.

When the potential function in this case is expanded by a power, it becomes as follows.

Similarly, since there are three independent variables, A/B, λ, and φ, it is possible to obtain a solution that simultaneously makes the 6th and 10th order terms zero, that is, simultaneously satisfies the following two equations.

The solutions when B/A is 1, 2, 1, 4, 1, 6, 1, 8 are shown in the following table.

Further, in the case of λ=1, that is, in the case of ra=rb, the fifth equation and the fifth equation (6) become as shown below.

A+2Bcos6φ=0 ・・・・・・・・・・・・
...(5')A+2Bcos10φ=0 ...
・・・・・・・・・・・・(6') Therefore, in this case, the solution is { φ=22.5° B=A√2・・・・・・・・・( 7), which is exactly the same as (solution 2) when there is no mirror image.

An example of an electric field diagram corresponding to such a solution plotted using an electronic computer is introduced below.

FIG. 4 corresponds to the solution shown by equation (7), and FIG. 5 corresponds to the solution when A=B, and μa=0.
This is the case of 8.

Also, the dotted lines in the figure indicate equipotential lines in the case of an ideal hyperbolic electric field.

As is clear from this figure, the area within a radius of 0.5a can be regarded as an ideal hyperbolic electric field.

Next, an example in which a quadrupole lens constructed using 12 needle electrodes is used as a mass spectrometer is shown in Figure 6 and its A-B.
This will be explained in detail using FIG. 7 for showing the cross section and electrical connection relationship.

In FIG. 6, reference numerals 13, 14, 15, 16, and 17 are needle-like electrodes made of materials such as tungsten and molybdenum.
On both sides of each of electrodes 3 and 14, wire-shaped electrodes 18, 19 and 20, 21 are arranged, and electrodes 15, 16, 1
Needle wire electrodes 22, 23, and 24 are arranged opposite to 7.

Inner circle 2 where electrodes 13, 14, 16, 23 are arranged
The radius ra of electrode 15 is set to 4.38 mm.
, 17, 18, 19, 20, 21, 22, 24 are arranged, the radius rb of the outer circle 26 is set to 4.65 mm.

Also, φ is set at 20°.

Moreover, -(
The potential represented by U+Vcosωt) is
6, 17 and 22, 23, 24 have +(U+Vcosω
A potential represented by t) is given.

Returning to FIG. 6, each of the electrodes is held by doughnut-shaped holding plates 27 and 28 made of an insulator.

The holding plates 27 and 28 are fixed to a cylinder 29 made of metal.

An ion source 30 is arranged in front of the cylinder 29, and ions generated from the ion source 30 are introduced into a quadrupole field formed in the cylinder 29 through a focus slit 31 and a ground slit 32. Ru.

The ions emitted from the quadrupole field are introduced into an ion multiplier 33, and the output signal of the ion multiplier 33 is supplied to a recorder (not shown).

As mentioned above, ions generated from the ion source chamber 30 pass through the focus slit 31 and the ground slit 32.
It is introduced into the cylinder 29 through the.

As described above, a low-aberration quadrupole lens is formed within the cylinder 29 and has an electric field extremely close to the ideal hyperbolic electric field that can eliminate not only the 6th-order term but also the 10th-order term. Ions introduced into the lens are separated with extremely high resolution according to their mass-to-charge ratio.

In addition, a wide range up to the vicinity of the needle wire electrode is an ideal hyperbolic electric field, and the range of the electric field in which ions can be energized is wide, thus enabling highly sensitive detection.

Furthermore, in the case of a quadrupole lens using conventional round bar electrodes, extremely high manufacturing precision is required, but in the quadrupole lens based on the present invention, the dimensional precision of the needle-like electrodes does not need to be so high. It is easy and inexpensive to manufacture.

[Brief explanation of the drawing]

FIG. 1a is a diagram for schematically showing a conventional quadrupole lens, FIG. 1b is a diagram for explaining the electric field formed by the conventional quadrupole lens, and FIG. FIG. 3 is an explanatory diagram for calculating the electric field created by the polar lens, and FIG. 3 is an explanatory diagram for calculating the electric field formed when the mirror image of each electrode by the cylinder is also considered. FIG. 5 illustrates the field formed by a quadrupole lens according to the invention, and FIGS.
The figure is a diagram for illustrating an example in which a quadrupole lens based on the present invention is incorporated into a mass spectrometer. C... Round bar electrode, 1 to 24... Needle wire electrode, 25... Inner circle, 26... Outer circle, 27, 28... ...Retaining plate, 29...
Cylinder, 30...Ion source, 31...
・Focus slit, 32...Earth slit, 33...Ion multiplier.

Claims (1)

[Scope of Claims] 1. A cylinder with an inner diameter a formed of a conductor, four needle-like electrodes arranged at equal intervals on a circumference with a radius ra from the center of the cylinder, and a circumference with a radius rb. eight needle-like electrodes arranged above and placed at an angle φ on both sides of each of the four needle-like electrodes, and the potential difference between the cylinder and the four needle-like electrodes is A, When the potential difference between the cylinder and the eight needle wire electrodes is B, A{1-(ra/a)12}+2B(ra/rb)6
{1-(rb/n)12}×cos6φ=0 and A{1-(ra/a)20}+2B(ra/rb)10
A quadrupole lens characterized in that a, ra, rb, φ, A, and B are selected to satisfy {1-(ra/a)20}×cos10φ=0. 2. The relationship ra=rb holds true between ra and rb, and φ-2. 2. The quadrupole lens according to claim 1, wherein a is arbitrarily selected from 5° and A/B√2.