Quadrupole electrode and manufacture thereof

Abstract: An improved quadrupole electrode used in a mass spectrometer, etc., which comprises two pairs of electrodes (1, 2, 3, 4), the sections of whose mutually-facing surfaces are formed in the shape of a hyperbola or circle, and whose electrode bars are made of ceramic, and further whose surfaces are covered with layers (5) made of conductive metal. And its manufacturing method characterized in that such four electrodes are assembled at predetermined intervals. Since the electrode bars are mainly made of ceramic, the bars can be formed precisely, and the quadrupole electrode can be easily assembled without troublesome adjustment. Thus a quadrupole electrode can be manufactured with a good reproducibility at a low price.

Description: TECHNICAL FIELD The present invention relates to a quadrupole electrode for use in the sensor part of a mass spectrometer or the like. BACKGROUND ART A quadrupole electrode used in a mass spectrometer of the like comprises four electrodes 11, 12, 13 and 14 formed in such a manner that opposed surfaces are hyperbolic in their cross section as shown in FIG. 4, or four electrodes 11', 12' 13' and 14' formed so as to have a circular cross section as shown in FIG. 5 are disposed in a positional relationship adjusted so that the electrodes are located at predetermined intervals. When ions are fed into the center of the quadrupole electrode in the direction indicated by an arrow, it becomes possible to take out ions having a particular mass to charge ratio with a high accuracy from the opposite side of the quadrupole electrode. In such a conventional quadrupole electrode, the distance between the electrode rods should be kept so accurately that a very highly accurate work is required in assembling the quadrupole electrode and five days or more are necessary for the assembly and adjustment of the quadrupole electrode. Further, a change in the distance between the electrodes caused during the analysis should be minimized. For example, Japanese Patent Laid-Open No. 30056/1983 describes the use of an electrode produced by subjecting a metallic material to extrusion or drawing into a V-shaped electrode for the purpose of reducing the weight of the electrode and, at the same time, improving the dimensional accuracy. Further, Japanese Patent Laid-Open No. 87743/1984 and Japanese Utility Model Laid-Open No. 64562/1985 describe the shape of electrode rods which are easy to assemble into a quadrupole electrode. Further, other various designs have been proposed in the art, f.i. in US-A-4 158 771 wherein ceramic electrodes are braced by outer rings. In the conventional quadrupole electrode, in order to bring the accuracy of the distance between the constituent electrodes to a predetermined value, it is a common practice to use a method which comprises manually assembling a quadrupole electrode, introducing a monitor gas for confirming the accuracy and repeating a check on the accuracy to correct the distance between the electrodes. Therefore, the object of the present invention is to provide quadrupole electrodes which can be disposed with a high dimensional accuracy without any such troublesome work and the predetermined accuracy of the distance between the electrodes can be kept high during the use thereof. The present invention provides a quadrupole electrode according to claim 1 to achieve this object. The present invention also provides a process for producing a quadrupole electrode as set forth in claim 3. Thus, the present invention has been made with a view to facilitating the formation of a quadrupole electrode with a high accuracy and a good reproducibility. In the present invention, a high accuracy within ±5 µm can be attained in the distance between the electrodes and a change in the distance between the electrodes during the use thereof in the analysis can be minimized by using an insulating ceramic having a low coefficient of thermal expansion and subjected to high-accuracy working as the material of the electrode and, after coating the surface of the electrode with a conductive metal, assembling four electrodes, and incorporating the resultant quadrupole electrode in a mass spectrometer. In order to improve the accuracy of assembling a quadrupole electrode and, at the same time, to shorten the time necessary for the adjustment of the accuracy, it is necessary to assemble at once the electrodes into a quadrupole electrode through reference planes finished with a predetermined accuracy. When a metal is used as the material of the electrode, however, there occurs a problem that the insulation between the electrodes cannot be maintained. This problem can be solved through the use of an insulating ceramic. Since ceramic has a low coefficient of thermal expansion and a light weight, it is advantageous in that the dimensional stability against a change in the temperature can be maintained and improved and the handleability is good. An Si 3 N 4 ceramic having a coefficient of thermal expansion of 4 (x10 -6 /°C) or less suffices for this purpose. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of one embodiment of the present invention. FIG. 2 is a graph showing the results of measurements of scattering of the peak waveforms in a mass spectra given by a mass spectrometer. FIG. 3 is an explanatory view of an embodiment wherein the electrode of the present invention is incorporated in a mass spectrometer. FIG. 4 is an explanatory perspective view of one construction of the conventional quadrupole electrode. FIG. 5 is an explanatory perspective view of another construction of the conventional quadrupole electrode. BEST MODE FOR CARRYING OUT THE INVENTION The invention will now be described in more detail with reference to FIG. 1. Numerals 1, 2, 3 and 4 designate four electrodes previously subjected to high-accuracy working, and the body of each electrode rod is made of an Si 3 N 4 ceramic as it has an insulating property and a low coefficient of thermal expansion. The present inventors have made intensive studies through the use of various ceramics and, as a result, have found that an Si 3 N 4 ceramic having a coefficient of thermal expansion of 4(x10 -6 /°C) or less suffices for this purpose. This is because the distance between the electrodes of the quadrupole electrode of a mass spectrometer where a high resolution is required is as large as at least 20 mm and, in this case, a change in the distance between the electrodes with the elapse of time is believed to affect the accuracy of analysis. The use of an Si 3 N 4 ceramic electrode having a low coefficient of thermal expansion enables the distance between the electrodes to be kept with an accuracy as high as ±5 µm, that is, the analytical accuracy to be sufficiently maintained, even when use is made of a quadrupole electrode having a large distance between the electrodes. Numeral 5 designates a conductive metal layer formed for coating the surface of the ceramic therewith for the purpose of allowing the ceramic to function as an electrode. The formation of the metal layer enables the insulating ceramic to function as the electrode. The metal layer may comprise any conductive metal, and it is also possible to use a single phase composed of Mo, W, Au, Pt, Ti, Cu, Ag, Ni or the like or an alloy or a composite phase composed of these materials. The thickness is preferably 1 mm or less. When the thickness exceeds 1 mm, there is a possibility that peeling occurs unfavorably. The coating may be conducted through the formation of a thin film according to a vapor deposition process or coating according to the wet paste method. If necessary, the metallized layer may be machined to maintain the accuracy. An electrode terminal can be formed by passing a conductive lead wire through a hole 7 of each of the electrode rods 1, 2, 3 and 4 for conduction to a conductive metal layer formed on the hyperbolic surface of the ceramic electrode rod. The lead wire is fixed with a nut 8. Thus, four ceramic electrodes are formed independently of each other. These electrodes can be assembled with a high accuracy by fixing reference planes 1', 2', 3' and 4' of the electrodes to each other by lapping and jointing the electrodes to each other directly or through a jig 6 such as a chip. The jointing is conducted through the use of an active metal layer for a ceramic, fine particles of a ceramic, or the like. Thus, it has become possible to facilitate assembling of four ceramic electrodes each made of a ceramic coated with a conductive metal into a quadrupole electrode with a high accuracy. In the drawing, numeral 9 designates a lead wire. Example 1 An electrode body having a distance between the opposed electrodes of 8.6 mm and a length of 200 mm was made of an Si 3 N 4 ceramic material having a coefficient of thermal expansion of 3.2 x 10 -6 /°C as a ceramic material, and the hyperbolic face thereof was machined with a high accuracy. Thereafter, an active metal (Ti-Cu-Ag) was deposited thereon in a thickness of 5 µm, and Ni was further deposited thereon in a thickness of 1 µm to form electrodes. These electrodes were assembled into a quadrupole electrode as shown in FIG. 1. As shown in FIG. 3, an ion source 16 for forming ions was mounted on one end of the quadrupole electrode 15, while a secondary electron multiplier 17 for detecting ions was mounted on the other end thereof. This assembly was incorporated as a quadrupole mass spectrometer in an ultrahigh vacuum apparatus where it was baked at 300°C. Thereafter, He, N 2 , Ar, Kr and Xe gases were flowed, and this procedure was repeated several times to measure a scattering in the peak waveform of a mass spectrum. As a result, the peak waveform of the quadrupole mass spectrometer, in which a conventional metal electrode (Mo electrode) was used, was in the split parabolic form as shown in FIG. 2(b). Also, the scattering of the peak height was large. This scattering of the peak waveform is believed to be attributable to the scattering of the dimensional accuracy. On the contrary, the peak waveform of the quadrupole mass spectrometer, in which the Si 3 N 4 ceramic quadrupole electrode was used, was in the parabolic form as shown in FIG. 2(a), and scarcely any scattering of the peak height was observed. Thus, the use of the Si 3 N 4 ceramic quadrupole electrode has made it possible to simplify the assembling and adjustment of the electrode and maintain a high analytical accuracy. Example 2 Si 3 N 4 ceramic electrode rods for forming a quadrupole electrode having a distance between the electrode rods of 8.6 mm and a length of 200 mm was machined into a predetermined shape having a predetermined dimension, which was then subjected to finish working so that the section became hyperbolic. The hyperbolic part was coated with Ti, Cu, Ag and Ni each in a thickness of 1 µm by ion plating to form a conductive film having a thickness of 4 µm in total. A Kovar rod of 1.6  was inserted into a hole previously formed in each electrode and then the electrodes were joined and fixed by means of an active metal solder. The four Si 3 N 4 ceramic electrodes were fixed one to another with the reference planes thereof abutting against each other and soldered to each other with an active metal solder via Si 3 N 4 chips, 5 x 5 in area and 10 mm long, in a jointing furnace under the conditions of 800°C and 10 min. The time taken for the assembling was 10 hr, and the accuracy of the distance between the electrodes in the assembling was within ±5 µm, which enabled the assembling time to be remarkably reduced. The quadrupole electrode thus assembled was incorporated in a vacuum apparatus, where baking was repeated ten times at 300°C. Then, the scattering of the peak waveform in a mass spectrum was measured. It was found that the waveform was parabolic as shown in FIG. 2 (a) and no scattering of the peak height was observed. On the contrary, the peak waveform given by the conventional metal (Mo) quadrupole electrode was in the split parabolic form as shown in FIG. 2 (b) and the scattering of the peak height was significant. INDUSTRIAL APPLICABILITY In the present invention, since each electrode rod is mainly made of a ceramic which is easily shaped with a high dimensional accuracy, the adjustment of the positional relationship between the electrodes during assembling can be made without much effort, which enables a quadrupole electrode having a high performance to be provided with a good reproducibility. Further, since a ceramic is used as the main material, it is possible to provide a quadrupole electrode having a light weight at a low cost as opposed to a quadrupole electrode wherein Mo or stainless steel is used as the main material.

Claims: A quadrupole electrode comprising two pairs of opposed electrodes, characterized in that each of the four electrodes (1, 2, 3 or 4) is made of an electrode rod, which is an Si 3 N 4 ceramic having a coefficient of thermal expansion of 4x10 -6 /°C or less, and the opposed inner face of each electrode is coated with a coating layer (5) of a conductive metal and provided with reference planes (1', 2', 3' or 4') at both ends thereof to directly joint the reference planes of adjoining electrodes, the reference planes having jig insertion parts at the end thereof, the electrodes being previously fixed with a predetermined dimensional accuracy by jointing directly the adjoining reference planes and inserting jigs (6) made of the Si 3 N 4 ceramic into the jig insertion parts. A quadrupole electrode according to claim 1, wherein the section of the opposed inner face of each electrode is hyperbolic or circular. A process for producing a quadrupole electrode comprising: abutting reference planes of four electrodes (1, 2, 3 or 4) which are made of an Si 3 N 4 ceramic having a coefficient of thermal expansion of 4x10 -6 /°C or less, have an inner surface coated with a coating layer (5) of a conductive metal and provided with reference planes (1', 2', 3' or 4') at both ends thereof to directly joint the reference planes of adjoining electrodes and jig insertion parts at the ends of adjoining reference planes in such a manner that two pairs of the electrodes are arranged opposite to each other; inserting jigs (6) made of the Si 3 N 4 ceramic into the jig insertion parts; and fixing the electrodes with a predetermined distance between the opposed electrodes at a predetermined dimensional accuracy. A process for producing a quadrupole electrode according to claim 3, wherein the section of the opposed inner face of each electrode is formed in hyperbolic or circular shape.

Inventors: ABE T, HIROKI S, MIYAKE M, MURAKAMI Y, TAKANO Y, YAMAKAWA A, ABE, T., HIROKI, S., MIYAKE, M., MURAKAMI, Y., TAKANO, Y., YAMAKAWA, A., ABE, T., JAPAN ATOMIC ENERGY RESEARCH INST., HIROKI, S., JAPAN ATOMIC ENERGY RESEARCH INST., MIYAKE, M., ITAMI WORKS OF SUMITOMO EL. IND. LTD., MURAKAMI, Y., JAPAN ATOMIC ENERGY RESEARCH INST., TAKANO, Y., ITAMI WORKS OF SUMITOMO EL. IND.LTD., YAMAKAWA, A., ITAMI WORKS OF SUMITOMO EL. IND.LTD
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1992
Main classification: H01J49
IPC codes: H01J49