US3418513A

US3418513A - Mass spectrometer ion source with cooling means

Info

Publication number: US3418513A
Application number: US406607A
Authority: US
Inventors: Elliott Richard Martin
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1963-10-31
Filing date: 1964-10-26
Publication date: 1968-12-24
Anticipated expiration: 1985-12-24
Also published as: GB1102462A; DE1498552A1

Description

MAss SPECTROMETEH 10N soURcE WITH COOLING MEANS Filed OCt. 26, 1954 R. M. ELLIOTT 5 Sheets-Sheet l Dec. 24, 1968 k UU 7///////////,M/ m

Defn 24, 196s R. M, ELLIQTT 3,418,513

MASS SPECTROMETER ION SOURCE WITH COOLING MEANS Fi1ed oci. 2e, 1964 s sheets-sheet e 7 F1' 5.5. Fi El l 55 5% r3@ 40 j52 3,5P n l 30 51j h t .f'f 52 I 54 55 56 L51 -f MASS SPECTROMETER ION SOURCE WITH COOLING MEANS Filed Oct. 26. 1964 R. M. ELLIOTT f5 Sheets-Sheet 5 United States Patent O 3,418,513 MASS SPECTROMETER ION SOURCE WITH COOLING MEANS Richard Martin Elliott, Altrincham, England, assignor t Associated Electrical Industries Limited, London, Eng land, a British company Filed Oct. 26, 1964, Ser. No. 406,607 Claims priority, application Great Britain, Oct. 31, 1963, 43,063/63 9 Claims. (Cl. S13- 230) This invention relates to ion sources in which a sample material introduced therein is ionised. Such ion sources are used in a mass spectrometer in which the ions are subsequently analysed in a well-known manner so that the constituents of the sample can be determined.

Ions can be produced from a sample material simply by heating the material, but it is preferable for the sample to be converted to its gaseous state and then bombarded with an electron beam in order to produce-the ions. In the latter arrangement it is essential that ions representative of the constituent materials of the sample should be produced in the ion source and to minimize generation of spurious ions. Therefore none of the molecules reachling the electron beam should have been decomposed nor should they have acquired a high thermal energy by reason of one or more previous collisions with a heated surface.

Ionisation Iof the molecules takes place in a chamber which may be a closed box except for certain small open ings or a more open cage defined by rods or bars, but which in any case is constructed of metal and is defined by a number tof metal surfaces. When an electron beam is employed the ionisation chamber normally operates at temperatures in the range 50-250 C. This operating temperature can be varied by means of one or more heaters surrounding or inserted into the ionisation chamber block, but even when these heaters are switched off the operating temperature of the ionisation chamber will be from 50'-l50 C. because the presence of the ion source filament which produces the electron beam, runs at about 2000 C. and is usually in good thermal contact with the ionisation chamber. The operating temperature 0f the ionisation chamber produces heated metal surfaces on which the molecules of the sample may impinge before -being bombarded by the electron beam, resulting in either of the two unwanted conditions of the molecules described above.

An object of the present invention is to provide an improved ion source which is constructed so that the tend cncy for these adverse conditions to be produced is reduced.

According to the present invention, a mass spectrometer ion source comprises an ionisation chamber for ionising molecules of a sample material introduced into the source and cooling means by which the temperature of the housing defining the chamber is reduced to a value less than that which would exist in the absence of said means.

The cooling means may include means whereby a cooling fluid is brought into heat transferring relationship with the walls defining the chamber. In one embodiment of the invention ducting is arranged in thermal contact with the walls of the ionisation chamber and cooling fiuid may be passed through the ducting. Alternatively, the ducting may extend into the chamber.

In a further embodiment of the invention the ionisation chamber is in heat transferring relationship with a heat sink, such as a body of copper, and ducting through which a cooling liuid is passed is in thermal contact with the heat sink. The cooling fluid in these embodiments of the invention may, for example, be liquid nitrogen or water.

In a still further embodiment of the invention a thermoelectric cooling device may be mounted on the wall of the ionisation chamber, or alternatively the cooling device can be in thermal contact with a heat sink which supports the ionisation chamber.

By lowering the temperature of the ionisation chamber to a required value the probability of a molecule being adversely affected -by collision with a heated surface is reduced and therefore the efiicacy of the ion source in producing representative ions from the sample molecules is increased.

The sample material is conveniently vaporised to produce the sample molecules and the vapour may for eX- ample be produced at one end of a tube containing the sample. The surfaces from which the sample material is evaporated are independent of the surfaces defining the ionisation chamber and the sample molecules may make a number 0f collisions with the walls of the ionisation chamber before being ionised or escaping through one of the openings in the chamber. If the temperature of the metal surfaces defining the ionisation chamber is higher than that of the surfaces from which the sample is evaporated, the molecules may undergo further collisions with heated surfaces resulting in the undesirable effects described above.

On the other hand if the temperature of the surfaces defining the ionisation chamber is lower than the evaporation temperature of the sample then there is a high probability that the sample molecules will condense on their first collisions with the cooler surfaces. If decomposition lof the molecules takes place during these collisions the decomposition products will probably remain condensed and cannot subsequently reach the ionising electron beam and result in unwanted ions being produced.

Two useful ranges of temperatures of the surfaces defining the ionisation chamber can therefore be defined.

(l) The range of temperatures above the temperature at which evaporation of the sample is taking place. This range will be useful for those substances which are moderately stable with respect to collisions with metal sur faces, but where decomposition and thermal energy are required to be kept to a minimum. Here the expected effects of the collisions with the moderately cool surfaces are not serious and the molecules can be allowed to reevaporate into the ionisation region; in fact it is desirable that they should since if they remained condensed they would be lost to the system and the effective sensitivity would be greatly reduced.

This range of temperatures, which might be 0-300 C., is the most generally useful.

(2) The range of temperatures below those at which evaporation of the sample is taking place. Decomposition products can be prevented from reaching the ionisation region, but some sample molecules are also prevented from doing so and the sensitivity of the instrument is therefore reduced. This range would be used for those very unstable materials where the expected effects of any collision are so serious (i.e. decomposition) that it is desirable that the molecules, decomposed or otherwise, should be prevented from re-evaporating even though this means a severe reduction in sensitivity. The result will be that only a small proportion of molecules will reach the ionising region, but those that do will have arrived by line-of-sight paths without suffering any collisions.

The range of temperatures involved might be 190 C. to C. The temperature controlling means is therefore made flexible in its control so that the temperature can be selected over the wide range covered by cases (l) and In order that the invention may be more readily understood it will now be described by way of example with reference to the accompanying drawings; in which:

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

FIG. 2 is a side view of part of an alternative arrangement to that shown in FIG. l;

FIG. 3 is a plan view of the ionisation chamber shown in FIGS. l and 2;

FIG. 4 is a side elevation partly sectioned of an ion source in accordance with a further embodiment of the invention;

FIG. 5 is a side view partly sectioned on an axial plane of part of an ion source in accordance with a still further embodiment of the invention; and

FIG. 6 is a side elevation of an ionisation chamber similar to that shown in FIG. 3.

Referring to FIG. l, the part of an ion source which is illustrated comprises an ionisation chamber 1 with means for producing an electron beam for bombarding molecules of a sample material introduced into the chamber and a guide 2 is provided for the insertion of a probe carrying the sample material into the correct position into the chamber. The chamber is defined by a metal walled housing and is supported on plates 3, 4 of good thermally conducting material which are attached to a cooling block 5 acting as a heat sink. Block 5 is supported on an electrically insulating frame 6 from an end plate 7 which together with a domed cover member 7 (part of which is shown) form the outer casing of the ion source which encloses the ionisation chamber 1. The ion source can be evacuated through an opening 8 in the domed cover member 7. Insulating bushings 8 extend through the end plate 7 and support conductors 9 for connection to the electrical components of the ion source.

The ionisation chamber may be of any convenient form and a chamber which is suitable for the purpose is illustrated in more detail in FIG. 3. This chamber, which is described by way of example, comprises a box containing an electron gun assembly which comprises a filament 31 and an electron accelerating electrode 32. The ionisation chamber also includes an ion accelerating electrode 34 displaced laterally from the electron beam path 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. The electrons emitted from the filament 31 are accelerated through the gap of the electrode 32 toward the trap 30 and the electron beam is collimated by two permanent magnets (not shown) which are mounted on the inner surface of the casing of the ion source and located substantially coaxially with the electron beam.

The temperature control apparatus employed in the embodiment of the invention illustrated in FIG. 1 comprises ducting in the form of two concentric metal tubes 11 and 12 which project through the end plates 7 so that their inner ends extend into the heat sink 5. The inner end of the outer tube 12 is closed by a metal plate 13 and ts Within a tube 14 of an electrically insulating, but thermally conducting, material such as alumina. The tube 14 provides electrical insulation between the tubes 11, 12, and the components of the ionisation chamber. Tube 14 lits closely within the socket in the heat sink block 5. The outer ends of tubes 11, 12 are connected respectively to

unions

15, 16 by means of which a suitable cooling uid, for example liquid nitrogen or Water is passed along the tubes. At the inner ends of the tubes there is a good thermally conducting path between the cooling fluid and the block 5 whereby the temperature in the block can be maintained at a required value.

A cap 17 is provided extending around the outer ends of the conductors 9 and formed with a central aperture 18 through which the tubes 11, 12 extend freely. A tubular cover 19 extends `around the tubes 11, 12 rfrom the aperture 18 within the cap 17 in order to prevent condensation from forming on the outer surfaces of bushings 8'.

By suitably controlling the ow of the cooling iiuid through tubes 11, 12 the temperature of the block 5 can be maintained at any required value, and since the ionisation chamber 1 is in heat transferring relationship with the block 5 through the supporting plates 3, 4 the temperature of the walls of the ionisation chamber can be controlled to a required Ivalue.

In the alternative embodiment of the invention illustrated in FIG. 2 the only difference is that the ducting extends into the ionisation chamber 1 and is in good thermal contact with the walls thereof. The block 5' supports the ionisation chamber, but does not act as a heat sink therefor, and the tubes 11, 12 pass through an opening centrally of the block 5' and are not in contact with the block.

With this embodiment of the invention it is necessary that the tube 12 should be made from an electrically insulating material in order to provide the necessary electrical insulation between the tubes and the other components of the ion source.

FIG. 4 shows how the present invention can be applied to an ion source in which the ionisation mechanism is purely thermal, and in such an ion source the reasons for wanting a low temperature in the ionisation region are to reduce the number of spurious background ions from residual vacuum vapours (mainly hydrocarbons), and to obtain less ions which are produced from reevaporation of sample material which had condensed on the walls of the ion source during a previous analysis.

An ion source in which the ionisation is produced thermally is shown at 20 and is supported on a metal sliding bar 21 which has a central portion 22 of reduced diameter and upon which the ion source is mounted. The walls of the ionisation chamber forming part of the ion source are in heat transferring relationship with the portion 22 of the bar, and in accordance with the invention the portion 22 of the bar is cooled during the operation of the ion source to reduce the temperature of the walls of the ionisation chamber to Ia value less than that which will exist in the absence of the cooling arrangement. A heat sink in the form of a cylindrical body 23 has an arcuate groove 24 formed therein which is of the same radius of curvature as the periphery of the portion 22 of the metal bar 21 so that the heat sink can be located in good heat transferring relationship with the portion of the bar which supports the ion source. The heat sink forms the base of a housing for containing the cooling fluid such as liquid nitrogen. The housing comprises a double walled portion 25 which is joined to the heat sink by flexible bellows 26. The cooling uid in the housing lowers the temperature of the `heat sink 23 which in turn cools the walls of the ionisation chamber of the source 20.

The source is located within an evacuated chamber 27, part of the walls of which are provided by the portions of the bar 21 on each side of the portion 22 of reduced diameter, and to enable the ion source to be recharged the -heat sink is withdrawn from engagement with the bar by means of a mechanism including a cam 28, an operating lever 29 and a rod 29' connecting the heat sink to the cam 28. The bar is then moved axially to withdraw the ion source from the chamber 27.

In the embodiments of the invention described in connection with FIGS. l to 4 a cooling fluid has been employed, but in the embodiment of the invention illustrated in FIG. 5 a thermo-electric cooling device 41 is mounted against the wall of t-he ionisation chamber 1. Electrical connections 42 from the terminals of the cooling device pass through the end plate 7 of the ion source by means of insulating bushings 8. By applying a suitable potential to the terminals of the thermo-electric device cooling of the walls of the ionisation chamber can be brought about.

Referring to FIG. 6, radiation shields 40 are interposed between the filament 31 of an ion source and the entrance to the ion chamber. The shields in the form of thin plates each have an opening therein and the openings are aligned with the filament 31 and an opening in the accelerating electrode 32 through which the electrons enter into the ion chamber. The plates prevent much of the heat emitted by the filament from falling upon the walls of the ionisation chamber. The plates are connected in good thermal contact with the heat sink I5 (shown in FIG. 1) so that heat absorbed by the plates is rapidly conducted away from the vicinity of the ionisation chamber.

What I claim is:

1. A mass spectrometer ion source including a metal housing defining a chamber,

entry means for admission of vaporized sample material to the chamber,

an electron beam 4generator in the chamber for ionizing molecules of vaporized sample material therein, said generator tending in use to cause heating of the housing to a temperature above the vaporization temperature of said sample material,

said housing also having Iat least one exit for ions generated therein, and

means for preventing thermal change in the molecules due to collision with a heated internal surface of the housing prior to ionization, said means comprising means for cooling the metal housing toa temperature suiciently low as compared with the vaporization temperature of said sample material to prevent such thermal change from occurring but insufficiently low to cause condensation of all of the molecules there- 2. A mass spectrometer ion source as claimed in claim 1 in which the means for cooling the housing includes a tube of electrically insulating thermally conductive material capable of conveying cooling Huid, which tube is in electrically insulating thermally conductive relation with the housing and projects into said chamber.

3. A mass spectrometer ion source as claimed in claim 1 in which said means for cooling the housing comprises a thermo-electric cooling device in heat transferring relationship with the housing, said device having terminals connectible to a voltage source to bring about cooling of the device.

4. A mass spectrometer ion source including a metal housing defining a chamber,

entry means for admission of vaporized sample material to the chamber,

an electron beam generator in the chamber for ionizing molecules of vaporized sample material therein, said generator tending in use to cause heating of the housing to a temperature above the vaporization temperature of said sample material,

said housing 'also having at least one exit for ions generated therein, and

means for preventing thermal change in the molecules due to collision with a heated internal surface of the housing prior to ionization, said means comprising a heat sink in electrically conductive heat transferring relationship with said housing :and means in electrically insulating heat transferring relationship with the heat sink for cooling the heat sink and in consequence the metal housing to a temperature sufficiently low as compared with the vaporization temperature of said sample material to prevent such thermal change from occurring but insufficiently low to cause condensation of all of the molecules there- 5. A mass spectrometer ion source las claimed in claim 4 in which said means for cooling the heat sink comprises ducting which is capable of conveying cooling iiuid.

6. A mass spectrometer ion source as claimed in claim 5 in which said ducting includes a metal tube and a body of electrically insulating thermally conductive material is disposed between the tube and the heat sink.

7. A mass spectrometer ion source as claimed in claim 6 in which said body is of alumina.

8. A mass spectrometer ion source as claimed in claim 4 in which said electron beam generator includes an incandescible filament, and at least one apertured metal plate is positioned between said filament and part of said housing and is in heat transferring relation with said heat sink.

9. A mass spectrometer ion source including a metal housing defining a chamber,

a heat source in the chamber for ionizing a sample of the material introduced into said chamber, said heat source tending in use to cause heating of the housing to a temperature above the vaporization temperature of said sample material,

said housing also having at least one exit Ifor ions generated therein, and

means for preventing thermal change in the molecules due to collision with a heated internal surface of the housing prior to ionization, said means comprising means for cooling the metal -housing to a temperature sufficiently low as compared with the vaporization temperature of said sample material to prevent such thermal change from occurring but insufficiently low to cause condensation of all of the molecules thereon, and

said cooling means being a metal body in the form of a rod in circular cross section, said housing being mounted in a recess in said rod in heat transferring relationship with the wall defining the recess, the heat sink being engageable with a portion of the curved surface of the rod and being provided with a curved co-engaging surface of the same radius of curvature as the portion of the rod with which it engages.

References Cited UNITED STATES PATENTS 2,677,770 5/1954 Smyth 313-63 2,826,701 3/1938 Columbe 313-36 X 2,829,271 4/1958 Boucher 313-36 X 2,932,753 4/1960 Arnott 313-109 3,142,752 7/ 1964 Hammer 313-63 3,217,162 11/1965 Wehner 313-63 3,226,542 12/ 1965 Craig 250-41 3,226,598 12/ 1965 Van Nimwegen 313-63 JAMES W. LAWRENCE, Primary Examiner.

R. L. JUDD, Assistant Examiner.

U.S. Cl. X.R.

Claims

1. A MASS SPECTROMETER ION SOURCE INCLUDING A METAL HOUSING DEFINING A CHAMBER, ENTRY MEANS FOR ADMISSION OF VAPORIZED SAMPLE MATERIAL TO THE CHAMBER, AN ELECTRON BEAM GENERATOR IS THE CHAMBER FOR IONIZING MOLECULES OF VAPORIZED SAMPLE MATERIAL THEREIN, SAID GENERATOR TENDING IN USE TO CAUSE HEATING OF THE HOUSING TO A TEMPERATURE ABOVE THE VAPORIZATION TEMPERATURE OF SAID SAMPLE MATERIAL, SAID HOUSING ALSO HAVING AT LEAST ONE EXIT FOR IONS GENERATED THEREIN, AND MEANS FOR PREVENTING THERMAL CHANGE IN THE MOLECULES DUE TO COLLISION WITH A HEATED INTERNAL SURFACE OF THE HOUSING PRIOR TO IONIZATION, SAID MEANS COMPRISING MEANS FOR COOLING THE METAL HOUSING TO A TEMPERATURE