US3217160A

US3217160A - Ion source for mass spectrometers with means to separately mount electrodes of different potential

Info

Publication number: US3217160A
Application number: US220410A
Authority: US
Inventors: Craig Robert Derek; Willdig Edward
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1961-09-04
Filing date: 1962-08-30
Publication date: 1965-11-09
Anticipated expiration: 1982-11-09
Also published as: GB961334A

Description

Nov. 9, 1965 CRAIG ETAL 3,217,160

ION SOURCE FOR MASS SPECTROMETERS WITH MEANS T0 SEPARATELY MOUNT ELECTRODES OF DIFFERENT POTENTIAL Filed Aug. 30, 1962 INVENTORS:

Roberz Derek Crew/ '9 Edward W/l/d/lg ATTORNEV United States l atent C 3,217,160 ION SOURCE FOR MASS SPECTRQMETERS WITH MEANS TO SEPARATELY MQUN'I ELEC- TRODES OF DIFFERENT POTENTIAL Robert Derek Craig, Bowdon, and Edward Willdig, Stretford, England, assignors to Associated Electrical lndustries Limited, a British company Filed Aug. 30, 1962, Ser. No. 220,410 Claims priority, application Great Britain, Sept. 4, 1961, 31,739/ 61 8 Claims. (Cl. 25041.9)

The present invention relates to ion sources for mass spectrometers of the type in which a sample to be analyzed is first ionized. The sample may be a gas, liquid or a solid. The ions produced in the ion source are subsequently accelerated from the ion source into the main body of the mass spectrometer.

In order to eliminate background impurities, it is desirable to bake the ion source at the same time as evacuating it. Also in order to clean the components of the source it is desirable to be able to dismantle the source. When the source is reassembled it is essential that the various components are correctly spaced from each other.

The object of the present invention is to provide an improved ion source for a mass spectrometer.

According to the present invention an ion source for a mass spectrometer comprises a chamber adapted to be evacuated, a removable cover plate closing said chamber, means for producing a beam of ions from a sample Within said chamber, a plurality of components within said chamber adapted to be maintained respectively at high voltages relative to ground and a plurality of other components within said chamber adapted to be maintained respectively at low voltages relative to ground, together with means for supporting all said high voltage components from said cover plate and means for supporting all said low voltage components from the main body of said chamber.

According to a preferred embodiment of the invention in which the ion source comprises an electron beam adapted to bombard molecules of the sample material so as to produce ions, a magnet assembly for collimating said electron beam is mounted within said chamber.

In order that the invention may be more readily understood reference will now be made to the accompanying drawings in which:

FIG. 1 is a side view of an ion source embodying the invention sectioned on an axial plane,

FIG. 2 is an end view of the ion source sectioned on the planes IIII in FIG. 1, and

FIG. 3 is a plan view of the ionization chamber of the ion source.

With reference to FIGS. 1 and 2 the ion source comprises a chamber 1 having a main body 2 and a cover plate 3 located by an annular boss 53 and by long dowels 54, and sealed with a metal seal 55. The chamber is adapted to be evacuated through a duct 4 leading to a suitable evacuating device. The end wall 5 of the chamber is formed with a narrow channel 6 which is covered with a device 7 having a slit 8 which may be of variable size. The channel 6 connects the chamber to a duct 9 which can be closed by a valve mechanism 11. This valve mechanism preferably includes a ball 12 adapted to engage on a seating 13 extending obliquely across the duct 9.

The slit device 7, which may be the variable type described in co-pending patent application No. 220,411, filed of even date herewith, now Patent No. 3,187,179, and is supported from the end wall 5 and is covered by a plate 14 with a central slit 15 supported also from the end wall. The main body of the chamber, the end wall and the com- Patented Nov. 9, 1965 ponents supported therefrom are adapted to be maintained at a low potential such as ground.

The cover plate 3 includes a plurality of electrically insulating bushings 21, of ceramic or glass, for example, each supporting a high voltage lead 22 adapted to be connected to a suitable high voltage source. The ends of the leads 22 are protected by a cover plate 56. Each bushing 21 extends through an aperture 23 in the cover plate 3 and is mounted on a metal sleeve 24 projecting inwardly of the chamber from the edge of the aperture 23. An annular space 25 is therefore formed between the outer portion of the bushing and the sides of the aperture 23 and the sleeve 24. The surfaces of each bushing 21 and each sleeve 24 within the chamber 1 are preferably smooth.

An ionization chamber assembly is supported on two pillars 28 of electrically insulating material from the cover plate 3. The ionization chamber is illustrated more clearly in FIG. 3 and comprises a box 29 containing an electron gun assembly. The electron gun assembly comprises a filament 31, and an electron accelerating electrode 32. The ionization chamber also includes an ion accelerating electrode 34 and an electron trap electrode 30 behind a suitable apertured plate 33. The box has side walls 35, end walls 36, a base 37 on which the ion accelerating electrode is supported and an apertured lid which is not illustrated in FIG. 3. The electrons emitted from the filament 31 are accelerated through the gap in the electrode 32 towards the trap 30. The electron beam is collimated by two permanent magnets 38, supported within the inner surface of the body 2 and located substantially co-axially with the electron beam.

The box 29 is mounted on heat insulators 41 secured to a plate 42 and means may be provided for heating the box or for maintaining its temperature substantially constant. The plate 42 is mounted on supports 43, each of which comprises a central conductor surrounded with an electrically insulating sleeve. The plate 42 is insulated from the conductors in the supports 43. The supports 43 also support two

half plates

44, 45 which extend across the lid of the box 29 and are separated by a small gap 46. The

half plates

44, 45 may be maintained at different potentials by means of the conductors in the supports 43.

The supports 43 and the leads 47 to the box 29 are mounted on a plate 48 supported on the pillars 28. All the conductors for the ionization chamber assembly are at relatively high potentials of the order of 10 kv., and are connected respectively to conductors 22 by leads 57, only some of which are illustrated in the figure for clarity.

The two permanent magnets 38 are supported on a circular yoke 49 attached to the body 2. An aperture 51 is formed in the wall of the chamber 1 substantially in the same transverse plane as the box 29 and this aperture is used to introduce samples into the ionization region through which the electron beam passes. The samples are either in gaseous form in which case they pass along a tube in a well known manner, or in a solid form in which case they may be inserted in a sample insertion device, for example as described in co-pending patent application No. 220,409, filed of even date herewith, now Patent No. 3,158,740 and heated so that they are evaporated directly into the region of the electron beam. A plurality of cartridge type heaters 52 are located in apertures in the wall of the chamber and are used for baking the whole ion source.

In operation of the ion source, the sample is introduced into the chamber, either along a tube if the sample is a gas or on an insertion device if the sample is a solid. In the latter case the chamber is then reevacuated and the sample is vaporized. A current is passed through the filament 31 which is thereby heated and electrons are emitted. The electrons are accelerated by the electron gun assembly and collimated by the magnetic field induced by the magnets 38 and on striking the sample molecules produce ions. The ions so produced are accelerated by the electric field induced by the potential on the electrode 34 and pass out of the box 2a through the aperture in the lid and through the gap 46, the slit 15 in plate 14 and the adjustable slit 8 in the device 7. When the valve 11 is open the ions pass into the body of the mass spectrometer where they are analyzed in a well known manner.

After the analysis of the sample has taken place it may be necessary to dismantle the ion source so as to clean away any deposits on the component parts. In particular it may be necessary to clean all the deposits off the surfaces of the high voltage bushings 21. Since these surfaces are smooth and do not include any re-entrant surfaces, this is a simplified job. It will be appreciated that the bushings and the sleeves 24 are so designed that the only re-entrant surface is that in the annular space 25, and this re-entrant surface lies externally of the chamber, and therefore deposits do not form thereon. Therefore with such an arrangement there is less likelihood of deposits being left on a re-entrant surface and eventually causing breakdown of the high voltage bushing.

All the high voltage components are supported from the cover plate 3 and are spaced an apreciable distance from the cover plate. With this arrangement it is possible to use electrically insulating members of an appreciable length to support the high voltage components, and to use heat insulators so that the electrically insulating surfaces operate at low temperatures and the electrically insulating characteristics are improved. The

half plates

44, 45 are maintained at a high potential and are supported from the cover plate 3 whereas the plate 14 is maintained at a low potential and is supported from the end Wall 5. The gap between the half plates and the plate 14 is normally small for example about 1 millimeter, and this construction avoids the insulation problems which would arise if the high voltage and low voltage plates were supported from the same member.

Since the slit device 7 requires to be positioned accurately relative to the analyzing portions of the main spectrometer to which the ion source is to be connected, it is advantageous to mount the slit device on the end wall and not on the cover plate 3. Also since it may be an advantage to make a removably gas tight seal between the slit device 7 and the end wall 5 so that the slit itself provides the only connection for pumping between the chamber 1 and the channel 6, it is preferable to mount the slit device on the end wall 5.

Further, if the slit device 7 includes control means for varying the size of the slit, it is preferable to mount the device on the end wall 5 instead of on the cover plate 3 so that the control means need not be dismantled when the cover plate is removed for cleaning purposes.

It may also be necessary to clean deposits off the components of the ionization chamber assembly while it may not be necessary to clean the low voltage components.

This construction enables the ionization chamber assembly to be removed for cleaning without moving the low voltage components.

Since the seals between the bushings 21 and the sleeve 24 are mounted within the chamber and are uniformly heated when the chamber is baked, there are no cold spots on each seal which might cause adverse condensation effects. Also the supports for the high voltage ionization chamber assembly are long and therefore little heat is transmitted from the ionization chamber assembly to the cover plate. Therefore the main supports 28 do not become hot and there is less chance of voltage breakdown over these supports.

It will be appreciated that the constructional details of the ion source described are not limiting and could be varied without affecting the invention.

The arrangement of the components of the ion source as described above provide several advantages in addition to those already mentioned. The cover plate 3 is located accurately relative to the body 2 and the end wall 5 by means of the annular boss 53 and as the cover plate is withdrawn the long dowels 54 ensure that the high voltage components move axially and do not knock against other components, in particular a sample insertion device, and that the bushings 21 are not damaged by being knocked against the body 2. In particular, the high voltage components including the ionization chamber can be removed in order to replace a filament, for example, without interfering with a sample insertion device in its operational position extending within the ion source. This is of major importance when the sample insertion device includes a glass tube supplying a gas sample together with electrical heating wires, thermocouples and electrical insulation.

What we claim is:

1. An ion source for a mass spectrometer comprising a chamber adapted to be evacuated and including a main body and a removable cover plate closing said chamber, means for locating-said cover plate accurately relative to the main body of said chamber, means for producing a beam of ions from a sample within said chamber, a plurality of components within said chamber adapted to be maintained respectively at high voltages relative to ground and a plurality of other components within said chamber adapted to be maintained respectively at low voltages relative to ground, together with means for supporting all said high voltage components from said cover plate and means for supporting all said low voltage components from the main body of said chamber.

2. An ion source for a mass spectrometer comprising a chamber adapted to be evacuated and including a main body and a removable cover plate closing said chamber, means for locating said cover plate accurately relative to the main body of said chamber, means for producing an electron beam adapted to bombard molecules of a sample within said chamber so as to produce ions, a magnet as- ;sembly located within said chamber, supported from said main body and adapted to collimate said electron beam, a plurality of components within said chamber adapted 'to be maintained respectively at high voltages relative to ground and a plurality of other components within said chamber adapted to be maintained respectively at low voltages relative to ground, together with means for supporting all said high voltage components from said cover plate and means for supporting all said low voltage components from the main body of said chamber.

3. An ion source for a mass spectrometer comprising a chamber adapted to be evacuated and including a main body and a removable cover plate closing said chamber, means for locating said cover plate accurately relative to the main body of said chamber, means for producing a beam of ions from a sample within said chamber, a plurality of components Within said chamber adapted to be maintained respectively at high voltages relative to ground and a plurality of other components within said chamber adapted to be'maintained respectively at low woltages relative to ground, insulating supporting members for supporting all said high voltage components from said cover plate, heat insulators forming part of at least some of said supporting members so as to reduce the conduction of heat to said supporting members, and means for supporting all said low voltage components from the main body of said chamber.

4. An ion source for a mass spectrometer comprising a chamber adapted to be evacuated and including a main body and a removable cover plate closing said chamber, long dowels locating said cover plate relative to the main body of said chamber and adapted to control the direction of movement of said cover plate as it is being withdrawn from the main body of said chamber in order to open said chamber, means for producing a beam of ions from a sample within said chamber, a plurality of components within said chamber adapted to be maintained respectively at high voltages relative to ground and a plul rality of other components within said chamber adapted to be maintained respectively at low voltages relative to ground, together with means for supporting all said high voltage components from said cover plate and means for supporting all said low voltage components from the main body of said chamber.

5. The structure of claim 4 wherein the dowels are longer than the thickness of the cover plate and are mounted on the main body and extend through the entire thickness of the cover plate to form a long longitudinal guide for the cover plate in shifting the cover plate relative to the body.

6. An ion source for a mass spectrometer comprising a chamber adapted to be evacuated and including a main body and a removable cover plate adapted to close said main body, means for locating said cover plate accurately relative to said main body, a plurality of components within said chamber adapted to be maintained respectively at high voltages relative to ground and including means for producing a beam of ions from a sample within said chamber, a plurality of other components within said chamber adapted to be maintained respectively at low voltages relative to ground and including means for controlling the width of said ion beam emitted from said chamber, means for supporting all said high voltage components from said cover plate, and means for supporting all said low voltage components from said main body.

7. An ion source for a mass spectrometer comprising a chamber adapted to be evacuated and including a main body and a removable cover plate adapted to close said main body, means for locating said cover plate accurately relative to said main body, a plurality of components within said chamber adapted to be maintained respectively at high voltages relative to ground and including means for producing an electron beam adapted to bombard molecules of a sample within said chamber so as to produce ions and means for forming said ions into a beam, a plurality of other components within said chamher adapted to be maintained respectively at low voltages relative to ground and including means for controlling the width of the ion beam emitted from said chamber and a magnet producing a magnetic field for collimating said electron beam, means for supporting all said high voltage components from said cover plate, and means for supporting all said low voltage components from said main body.

8. An ion source for a mass spectrometer comprising a chamber adapted to be evacuated and including a main body and a removable cover plate adapted to close said main body, means for locating said cover plate accurate ly relative to said main body, a plurality of components within said chamber adapted to be maintained respectively at high voltages relative to ground and including means for producing a beam of ions from a sample within said chamebr, a plurality of other components Within said chamber adapted to be maintained respectively at low voltages relative to ground and including means for controlling the width of the ion beam emitted from said chamber, electrically insulating supporting members for supporting all said high voltage components from said cover plate, heat insulators forming part of at least some of said supporting members so as to reduce the conduction of heat along said supporting members from said ion producing means, and means for supporting all said low voltage components from said main body.

References Cited by the Examiner UNITED STATES PATENTS Rickard, from the Review of Scientific Instruments, vol. 28, No. 11, November 1957, pages 967 and 968,

RALPH NILSON, Primary Examiner.

Claims

1. AN ION SOURCE FOR A MASS SPECTROMETER COMPRISING A CHAMBER ADAPTED TO BE EVACUATED AND INCLUDING A MAIN BODY AND A REMOVABLE COVER PLATE CLOSING SAID CHAMBER, MENAS FOR LOCATING SAID COVER PLATE ACCURATELY RELATIVE TO THE MAIN BODY OF SAID CHAMBER, MEANS FOR PRODUCING A BEAM OF IONS FROM A SAMPLE WITHIN SAID CHAMBER, A PLURALITY OF COMPONENTS WITHIN SAID CHAMBER ADAPTED TO BE MAINTAINED RESPECTIVELY AT HIGH VOLTAGES RELATIVE TO GROUND AND A PLURALITY OF OTHER COMPONENTS WITHIN SAID CHAMBER ADAPTED TO BE MAINTAINED RESPECTIVELY AT LOW VOLTAGES RELATIVE TO GROUND, TOGETHER WITH MEANS FOR SUPPORTING ALL SAID HIGH VOLTAGE COMPONENTS FROM SAID COVER PLATE AND MEANS FOR SUPPORTING ALL SAID LOW VOLTAGE COMPONENTS FROM THE MAIN BODY OF SAID CHAMBER.