US3086111A

US3086111A - Mass spectrometer for gases at low pressures

Info

Publication number: US3086111A
Application number: US28141A
Authority: US
Inventors: Peper Jan
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1959-05-19
Filing date: 1960-05-10
Publication date: 1963-04-16
Anticipated expiration: 1980-04-16
Also published as: DE1169698B; NL104008C; GB935184A; CH384249A; NL239331A

Description

April 16, 1963 J. PEPER 3,086,111

MASS SPECTROMETER FOR GASES AT LOW PRESSUR EIS Filed May 10, 1960 2 Sheets-Sheet 1 lNVENTOR JAN PEPER BY W r AGENT April 6, 1963 J. PEPER 3,086,111

MASS SPECTROMETER FOR GASES AT LOW PRESSURES Filed May 10, 1960 2 Sheets-Sheet 2 United States Patent GASES AT This invention relates to mass spectrometers for gases at low pressures, in which an electron beam of small cross-sectional area is projected in the direction of a homogeneous static magnetic field, which beam produces ionization of the gas to be examined, and in which provision is also made of a high-frequency electric field at right angles to the magnetic field, the high-frequency field being homogenized by electrodes arranged parallel to the direction of the magnetic field.

Mass spectrometers of the above-mentioned known type, which may dispense with homogenization of the high-frequency field and are referred to as omegatrons, are capable of determining the composition of a gas at very low pressures, since only ions of a certain massto-charge ratio describe spirally increasing paths at a certain frequency. By varying the frequency of the high-frequency field, the mass spectrum is scanned and from the collected ion current as a function of the frequency the intensity of the masses is determined.

It has, however, been found that many types of omegatrons have a lower sensitivity and/ or resolving power than would be expected on theoretical grounds. One of the reasons therefor is that, owing to the various voltages set up at the electrodes enclosing the rectangular ionization chamber, fields are produced in this chamber due to which the resonant frequency for a certain kind of ions is not the same throughout the entire chamber.

It is an object of the present invention to provide a mass spectrometer in which the above-mentioned disadvantages are at least substantially obviated.

In a mass spectrometer for gases at low pressures, in which an electron beam of small cross-sectional area is produced in the direction of a homogeneous static magnetic field, which beam brings about ionization of the gas to be examined, while provision is also made of a high-frequency electric field at right angles to the magnetic field, this high-frequency field being homogenized by electrodes arranged parallel to the direction of the magnetic field, according to the invention the chamber in which the gas to be examined is ionized by the electron beam is bounded by electrodes the boundaries of which comprise the lines of intersection of planes with a surface of revolution of three sheets, which surface is obtained by revolution of two conjugate hyperbolae about the axis of one of them, which coincides with the axis of the electron beam, while the electrodes the boundary lines of which are determined by the two outer sheets of the surface of revolution are at a small positive voltage relative to the electrodes the boundary lines of which are determined by the central sheet of the surface of revolution. Preferably, the asymptotes to the hyperbolae from which the surface of revolution is derived make an angle with the axis of revolution such that the tangent is equal to /2.

The electrode configuration in accordance with the invention ensures that the radial field strength produced by the small voltage difference between the centre elec trodes and the outer electroeds is proportional to the distance from the axis. Hence, the resonant frequency is constant for all ions throughout the entire chamber.

The voltage difference between the inner and outer electrodes is necessary to prevent the resonant ions from 3,986,111 Patented Apr. 16, 19 63 drifting too far in the axial direction, so that they would no longer contribute to the desired ion current.

In order that the invention may readily be carried into effect, an embodiment will now be described in detail, by way of example, with reference to the accompanying drawings, vin which:

FIG. 1 shows the conjugate hyperbolae from which the surface of revolution is derived,

FIG. 2 shows the electrodes, and

FIG. 3 shows a circuit arrangement comprising an omegatron in accordance with the invention partly in cross-section.

' FIG. 1 shows co-ordinate axes z and r at right angles to one another, in which two branches I, II, resp., III, IV of two conjugate hyperbolae are drawn between two asymptotes A and A The co-ordinate z represents the axial co-ordinates in the electrode system and the co-ordinate r indicates the distance from the axis. The surface of revolution determining the boundary lines of the electrodes is obtained by revolving the branches of the hyperbolae about the z-axis. Thus, three sheets are obtained, namely a centre sheet the axial section of which is given by the branches I and II, (the surfaces of revolution of branches I and II coincide) and two outer sheets the axial sections of which are given by the branches III and IV. The angle on made by the asymptotes A and A with the z-axis is such that tan oa=\/2. It can be proved that by this form the condition of proportionality between the radial field strength and the distance from the axis is exactly satisfied.

FIG. 2 is a perspective view of the electrodes bounding the ionization chamber. Electrodes 1 are obtained by intersection of planes parallel to the z-axis with the outer sheet corresponding with the branch IV and the electrodes 3 are obtained by intersection of these planes with the sheet corresponding with the branch III. For the sake of clearness, only part of the electrodes 1 are shown partially broken away. Electrodes 2 are obtained by intersection of the sheet corresponding with the branches I and II with the planes extending parallel to the z-axis. The electrodes 1 and 3 associated with the same plane are connected to one another electrically, as is shown in the drawing. The electrodes 2 which are split in two are also connected to one another electrically.

In the mass spectrometer shown in FIG. 3, the electrodes .1, 2 and 3 are shown partly in elevation and the remaining electrodes in cross-section. The remaining electrodes comprise a thermionic cathode 4 arranged within a screen electrode 5. A control electrode 6 may be used for maintaining the electron current constant. Two screening electrodes 7 and 8 are provided, similarly to the

electrodes

5 and 6 and the centre electrodes 1 and 3, are provided with apertures for the passage of an electron beam. A collector for the electron beam is designated 9. The electrode 5 and the cathode 4 are at a negative potential of about v. with respect to the earthed electrodes 7 and 8, while the electrode 6 is at a positive potential of about 10 v. with respect to the electrode 7. The electrodes 1 and 3 on the one hand and the electrode 2 on the other are connected to potentiometers 10 and 11 respectively, which both comprise a number of resistors. A positive voltage -of from 0.3 to 0.4 v. with respect to earth is produced across the potentiometer it), while the potentiometer 11 is at earth potential. A high-frequency voltage having an effective value of about 1 v. across terminals 12 is applied through the two potentiometers 10 and 11 to the electrodes. For the sake of clarity, not all the connec-. tions to the electrodes are shown. The ions which under the influence of the fields describe paths of increasing radius, are collected by the electrodes 2 and this ion flow is measured by means of a sensitive direct-current amplifier 13 connected between the potentiometer 11 and earth. The magnetic field is indicated by arrows H.

What is claimed is:

1. A mass spectrometer for gases at low pressures comprising means to generate and project an electron beam of given cross-sectional area in a given direction, means to generate a homogeneous static magnetic field in a region of gas through which the electron beam passes and ionizes the gas to be examined, means to generate a high-frequency electric field which is at right angles to the magnetic field and is homogenized by electrodes positioned parallel to the direction of the magnetic field, said region being bounded by electrodes the boundaries of which comprise the lines of intersection of planes with a surface of revolution of thre sheets obtained by revolving two conjugate hyperbolae about the axis of one of them, which coincides with the axis of the electron equal to V5.

References Cited in the file of this patent UNITED STATES PATENTS 2,829,260 Donner et al. Apr. 1, 1958 FOREIGN PATENTS 773,689 Great Britain May 1, 1957

Claims

1. A MASS SPECTROMETER FOR GASES AT LOW PRESSURES COMPRISING MEANS TO GENERATE AND PROJECT AN ELECTRON BEAM OF GIVEN CROSS-SECTIONAL AREA IN A GIVEN DIRECTION, MEANS TO GENERATE A HOMOGENEOUS STATIC MAGNETIC FIELD IN A REGION OF GAS THROUGH WHICH THE ELECTRON BEAM PASSES AND IONIZES THE GAS TO BE EXAMINED, MEANS TO GENERATE A HIGH-FREQUENCY ELECTRIC FIELD WHICH IS AT RIGHT ANGLES TO THE MAGNETIC FIELD AND IS HOMOGENIZED BY ELECTRODES POSITIONED PARALLEL TO THE DIRECTION OF THE MAGNETIC FIELD, SAID REGION BEING BOUNDED BY ELECTRODES THE BOUNDARIES OF WHICH COMPRISE THE LINES OF INTERSECTION OF PLANES WITH A SURFACE OF REVOLUTION OF THREE SHEETS OBTAINED BY REVOLVING TWO CONJUGATE HYPERBOLAE ABOUT THE AXIS OF ONE OF THEM, WHICH COINCIDES WITH THE AXIS OF THE ELECTRON BEAM, THE ELECTRODES THE BOUNDARY LINES OF WHICH ARE DETERMINED BY THE TWO OUTER SHEETS OF THE SURFACE OF REVOLUTION BEING AT A LOW POSITIVE POTENTIAL WITH RESPECT TO THE ELECTRODES THE BOUNDARIES LINES OF WHICH ARE DETERMINED BY THE CENTRE SHEET OF THE SURFACE OF REVOLUTION.