US3443088A

US3443088A - Ion source with side and end walls having independent potentials

Info

Publication number: US3443088A
Application number: US534857A
Authority: US
Inventors: Harmon W Brown
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1966-03-16
Filing date: 1966-03-16
Publication date: 1969-05-06
Anticipated expiration: 1986-05-06
Also published as: GB1130468A; FR1514718A

Description

May 6, 1969 w. BROWN 3,443,088

ION SOURCE WITH SIDE AND END WALLS HAVING INDEPENDENT POTENTIALS Filed March 16, 1966 Sheet l or 2 ID r r T r r r W; PUMP GAS i SOURCE AMPLIFIER 2 1 M I P INVENTOR.

HARMON W, BROWN BY ORNEY May 6, 1969 H. w. BROWN ION SOURCE WITH SLIDE AND END WALLS HAVING INDEPENDENT POTENTIALS Sheet Filed March 16, 1966 N A m 4 Q 6 Ta I 2 1J 3 u N 7 |l 4 1J y 2 00 E f "NT Wm H m mw E O L v nnl HlH/V MQMMM/AW HHM m mm V w w v m0 w 20 J 2 H \i v FEGB N2(MASS 2a) N(MASS I4) INVENTOR. W. BROWN RNEY United States Patent Office 3,443,088 Patented May 6, 1969 US. Cl. 250-41.9 8 Claims ABSTRACT OF THE DISCLOSURE A cycloidal mass spectrometer is disclosed. The mass spectrometer includes an improved ion source. The ion source includes a substantially hollow cylindrical chamber having a cylindrical side wall and a pair; of disc shaped end walls. An electron beam is passed through the center of the chamber along a path falling in a midplane parallel to the plane of the end walls. The electron beam serves to ionize gas flowing into the ionizing chamber through one of the end walls to produce an ion beam which exits through a beam defining slot in the other end wall. The cylindrical side wall is operated at a potential intermediate the potentials applied to thev end walls such that the ions are produced in a relatively strong accelerating field which rapidly accelerates the' ions out of the ionizing beam path. By causing the ionizing beam to cross the ionizing chamber parallel to an eqpipotential plane in the central region of the ionizing chamber, the ions are produced in a region of uniform accelerating electric field and by electrons of a well-defined potential. This facilitates obtaining high resolution in the output of the spectrometer and facilitates determination of ionizing and dissociation potentials. Means are also provided for varying the beam potential.

Heretofore ion sources used in mass spectrometer have had several problems. First, they have been characterized by relatively low sensitivity as of 02x10 amps/torr for high resolution spectrometers, i.e., ion beam exit slit widths yielding mass resolution greater than 1000. Typically this low sensitivity is caused by one or more factors. One factor lea-ding to low sensitivity is the production of ions in a region of low accelerating electric field, i.e., less than 100 v./cm. such that the ions, once produced, are not quickly removed from the source. A second characteristic of some of the prior ion sources has been their poor ionization and dissociation potential resolution. One factor which causes this poor potential resolution is the production of the ions by electrons that must cross equipotentia'ls in the ionizing region, whereby the electrons ionize or dissociate gaseous material at different potentials along the directon of their ionizing trajectories. The result is that ionizing and dissociation potentials of the source are not well defined. Furthermore, many prior art ions sources have been relatively open structures permitting leakage of gas therefrom without being ionized. As a result the mass spectrometer apparatus is unnecessarily contaminated by the wasted material and furthermore more sample material is required than would otherwise be used.

In the present invention an improved ion source is provided which ionizes and/or dissociates the gaseous material to be analyzed in a relatively gas tight chamber and within a region thereof of relatively intense uniform electric field with the ionizing and/or dissociating electrons entering the ion production region in the plane of the equipotentials. In this manner ions are efficiently produced at well defined ionizing or dissociation potentials and the ions, once produced, are quickly removed through the ion beam exit slit, whereby an order of magnitude increase in sensitivity is obtained for a given high mass resolution.

The principal object of the present invention is the provision of an improved ion source for use in cycloidal mass spectrometers.

One feature of the present invention is the provision of a substantially closed ionizing chamber in the ion source having a minimum of gas leakage therefrom, other than through the ion beam exit slit, whereby efficient ionization of the gas introduced thereto is obtained.

\Another feature of the present invention is the provision of an ion source wherein the gas is ionized, within an ionizing chamber, in a region of relatively intense uniform electric field having an intensity greater than v./cm., whereby ions produced are rapidly removed from the ionizing region of the source through the beam exit slit.

Another feature of the present invention is the same as any one or more of the preceding wherein the ions are produced in a region of electric and/or structural symmetry of the ionizing chamber to assure uniformity of the electric field in which the ion beam emanates.

Another feature of the present invention is thesarne as any one or more of the preceding wherein the ions are produced in a region of uniform electric field by an elec tron beam which passes through the ionizing region substantially parallel to and in the plane of the electric equipo-tentials within the ion beam source region of the ionizing chamber, whereby the ionizing and/or dissociation potentials of the ion source are well defined.

Another feature of the present invention is the same as the preceding wherein the ion source includes an ionizing chamber formed by three electrodes, a pair of spaced separate end wall electrodes separated by an intervening ring electrode portion operating at a potential intermediate the potentials applied to the end wall electrodes, and wherein the ionizing region is located centrally of the ring electrode.

Other features and advantages of the present invention will become apparent upon a persual of the following specificaion taken in connection with the accompanying drawings wherein:

FIG. 1 is schematic drawing of a cycloidal mass spectrometer system employing features of the present invention,

FIG. 2 is a circuit diagram of the network for applying operating potentials to the electric field ion analyzer electrode array of FIG. -1,

FIG. 3 is an enlarged sectional view, partly schematic, of the ion source structure of FIG. 1 taken along line 3-3 in the direction of the arrows,

FIG. 4 is a view of the structure of FIG. 3 taken along line 44 in the direction of the arrows, and

FIG. 5 is a plot of detected ion current for N and N+ versus electron volts of the ionizing electron beam for the ion source of FIG. 3.

Referring now to FIG. 1 there is shown a cycloidal mass spectrometer system. More particularly, an array of generally rectangular shaped ring electrodes 1 are ins-ulatively supported within a thin rectangular vacuum envelope 2, only partially shown, from a heavy rectangular flange, not shown, which closes off one end of the vacuum envelope.

The separate rings 1 of the electrode array are operated at slightly different electric potentials derived from a voltage source 3 via leads 4 connected at nodes 5 of a voltage divider network 60. The different potentials applied to the different rings 1 establishes a region of uniform electric field E in the hollow interior of the ring electrode array. The electric field E is directed parallel to the line of development of the ring electrode array.

The electrode array is immersed in a uniform region of magnetic field H directed at right angles to the direction of the electric field E. The field H is conveniently produced by an electromagnet 7 with the vacuum envelope 2 being disposed in the narrow gap defined between a pair of pole pieces 8 of the magnet 7.

The envelope 2 is evacuated in use via pump 10 to a suitably low pressure as of l0 torrs. Gas to be analyzed by the analyzer section, including the array of electrodes 1, is introduced from a source 9 into the analyzer section through the vacuum envelope 2 via an inlet tubing 11 as of stainless steel. The inlet tubing 11 feeds gas at a desired rate into an ion source 12. The ion source ionizes the gas and projects it through a slot into the crossed magnetic field H and electric field E of the analyzer.

Under the influence of the crossed electric and magnetic fields the ions are caused to execute cycloidal trajectories. However, only ions of a certain mass number, for a given intensity of E and H, will be focused at a detector slot 13 a certain focal distance from the source and at the same electric potential. An ion detector 14 is positioned behind the slot 13 to produce an output corresponding to the number of ions under analysis having the certain predetermined focused mass number, if any.

The output is fed to an amplifier 15 which amplifies the detected signal and feeds it to the Y axis of an X-Y recorder 16 wherein it is recorded as a function of a scan of the magnetic field intensity H produced by a scan generator 17. The output of the recorder 16 is a mass spectrum of the sample under analysis.

Referring now to FIGS. 3 and 4, the ion source -12 includes a metallic ionizing chamber 21 as of stainless steel which may be rhodium plated to reduce corrosion and contamination and within which gas to be analyzed is ionized and formed into a beam 22. The ionizing chamber 21 is segmented and separated by thin insulating 'sheets 23 as of 0.005" thick mica to provide three

separate electrodes

24, 25 and 26. The center electrode 25 includes a hollow cylindrical bore as of 0.250" in diameter and 0.116 in axial length defining the central portion of the ionizing chamber 21. The ends of the ionizing chamber 21 are closed off by

transverse walls

27 and 28 forming portions of

electrodes

24 and 26, respectively. End wall 27 is centrally apertured to form a gas inlet passageway 29 in gas communication with an insulating section of the gas inlet pipe 11 for introducing gas, to be analyzed, into the ion source 12. The opposite end wall 28 includes an ion beam exit slit 31 formed by a pair of slightly spaced apart knife edge plates 32 as of stainless steel sealed over a cylindrical 'bore 34 centrally located of the end wall 28. Bore 34 is, for example, 0.200" in diameter and the beam exit slit 31 is approximately 0.001 to 0.0004 in width as defined by the spacing between the plates 32. The elongated axis of the ion beam exit slit 31 is parallel to the direction of the magnetic field H which threads through the ion source 12 and ion analyzer rings 1. The gas inlet end wall 27 is counter bored at 35 to provide mechanical symmetry with the bore 34 in the ion beam exit wall 28.

A pair of cylindrical electron beam passageways 36, axially aligned with the direction of the magnetic field H, and as of, for example, 0.040" in diameter, pass through the inner wall of the center electrode 25. The passageways 36 define an electron beam path 37 therebetween coinciding with and lying within the transverse structural plane of symmetry of the ionizing chamber 21. A filamentary thermionic emitter 38 is exially aligned with the beam passageways 36 for projecting a beam of electrons across the ionizing chamber over the beam path 37. The emitter 38 is heated by a current drawn from a battery 39. The central electrode 25 serves as the anode for the emitter 38 and the anode potential for the emitter 38 is supplied from a variable voltage power supply 41 connected between the filament 38 and its anode 25. The electron beam 37 serves to ionize and/or to dissociate gas particles within the electron beam path 37 inside the ionizing chamber 21 and is collected by a metallic collector electrode 40 operating at anode potential and covering over the beam exit hole 36.

Electrode 24 serves as the repeller electrode for the ion source 12 and is supplied with its independent operating electrical potential as of 160-200 volts from a variable voltage source 42. Electrode 26 serves as the beam exit electrode and is preferably operated at ground potential.-

The intermediate electrode 25 serves to produce a region of uniform intense electric field E as of more than volts/cm. over the central ionizing region 43 of the beam path 37 defined by the shaded region of the drawing, The central electrode 25 is preferably operated at a potential midway between the operating potentials applied to

electrodes

24 and 26. The operating potential for the central electrode 25 is derived from a centertap 44 of a voltage dividing network 45 formed by

resistors

46 and 47 as of 10 K9 each connected across the voltage supply 42.

In operation, gas to be analyzed by the cycloidal mass spectrometer is introduced into the ion source 12 via gas inlet pipe 11, 11 and inlet passageway 29. The gas is ionized by the electron beam in the beam path 37. Under the influence of the uniform electric field E, produced by the system of

electrodes

24, 25 and 26, the ions within the central beam path region 43 are rapidly swept through the ion beam exit slit 31 to form a well defined ribbonshaped ion beam 22 emerging from the exit slit 31. The central ring shaped electrode 25, operated at a potential midway between the repeller and exit electrode potentials and placed in a position of structural symmetry, allows the ionizing region 43 to be placed in a position of optimum electric field uniformity. By making the inside diameter of ring 25 larger than the axial length, the intensity of uniform electric field is made relatively large as of greater than 150 volts/ cm. averaged over the ionizing region 43. Thus, ions produced are rapidly withdrawn through the exit slit 31. As a result, the ion source 12 yielded a sensitivity of 2 10- amps/torr with exit and detector slits of the aforementioned dimensions giving a detected mass resolution greater than 1000 between half amplitude points on the detected mass peak.

Passing the ionizing electron beam path through the chamber 21 in a plane of electrical symmetry with the electrons directed parallel to the equipotentials of the uniform electric field E yields substantially improved definition of the ionizing and dissociation potentials of the ion source 12. For example, nitrogen gas introduced into the ionizing chamber 21 may undergo either one of the following reactions:

The first reaction (1) results in only ionizing the nitrogen gas to produce N ions with mass number 28. While monitoring this mass number on the mass spectrometer and decreasing the ionizing electron beam anode voltage, a plot of ion current versus ionizing electron volts is obtained as shown in FIG. 5. The point where the mass 28 ion goes to zero represents the ionizing potential in electron volts for the nitrogen gas under analysis. This is of importance to chemists and it is desired that this point be well defined. The ion source of the present invention permits good resolution of ionizing potential.

The second reaction (2) represents dissociation of the nitrogen gas molecule and the potential at which this occurs is of interest to chemists and, therefore, should be well defined. This potential is measured in the same way as the ionizing potential, only mass 14 is monitored instead of mass 28. The ion source 12 provides a well defined value for this potential as well.

Lastly, the ion source should not be wasteful of gas to be analyzed as unnecessary leaks in the ionizing chamber produce wasting of the sample and contamination of the spectrometer. In the ion source 12 of the present invention, with a beam exit slit 31 of the aforementioned dimensions, the chamber 21 was free of unnecessary leaks to the extent that the total leak rate taken through the source 12 from the inlet 29 for all openings, including the beam exit slit, was less than 2 liters/second for nitrogen gas.

The ion source 12 has been described as it would be used to produce a positive ion beam. However, the source is equally useful for producing negative ion beams by merely reversing the terminals of the voltage supply 42. The negative ion beam would be analyzed by reversing the direction of the magnetic field H, and the direction of electric field E.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: s

1. An ion source apparatus for providing a beam of ions for a cycloidal mass spectrometer including, means defining an ionizing chamber having a gas input passageway leading thereto for introduction of gaseous material to be ionized and subsequently mass analyzed, said chamber means having an ion beam exit slit in gas communication therewith through which the ion beam emerges from the ion source for mass analysis by the mass spectrometer, said chamber means having a pair of electrically conductive spaced end walls separated by an intervening region bounded by a surrounding electrically conductive side wall portion, said ion beam exit slit being disposed in one of said end walls, means for insulating said end and side walls each from the other to hold independent operating potentials, said chamber means having a pair of axially aligned beam passageways passing through opposite wall portions thereof and defining a beam path therebetween for passage of an electron beam through said chamber means along the beam path for ionizing gas within said chamber means, said beam passageways being located in said intervening surrounding side wall substantially midway of the length of its intervening portion to direct the beam path across said chamber approximately parallel to equipotential planes in the central region of said chamber, whereby ions are produced in a region of uniform electric field and by electrons of equal potential.

2. The apparatus according to claim 1 including means for applying operating potentials to said end and intervening side walls, said potential applying means applying a potential to said side wall portion which is intermediate the operating potentials applied to said end walls.

3. The apparatus according to claim 1 wherein said beam passageways define a beam path therebetween which lies within a plane of structural symmetry inside of said chamber.

4. The apparatus according to claim 2 wherein said electron beam passageways define a beam path therebetween which lies within a plane of electrical symmetry inside of said chamber.

5. The apparatus according to claim 1 wherein said intervening side wall portion has a characteristic minimum inside transverse dimension which is within i25% of being twice the axial extent of said intervening side wall portion, whereby a uniform central region of relatively intense electric field is produced centrally of said chamber coextensive with a central portion of the electron beam path.

6. The apparatus according to claim 1 wherein said ionizing chamber is substantially gas tight except for said gas inlet, ion beam exit slit and electron beam passageways and wherein the degree of gas tightness of said ionizing chamber is defined by its leak rate taken from said gas inlet through all other leaks and passageways and is less than 2 liters/second for N gas.

7. The apparatus according to claim 2 wherein the applied operating potentials are of such a magnitude combined with the dimensions of said ionizing chamber to produce a central ionizing region within said chamber traversed by the electron beam wherein the applied potentials produce an average electric field intensity greater than volts/cm.

8. The apparatus according to claim 1 in combination wtih a cycloidal mass spectrometer for mass analyzing the ion beam exiting from said ion beam exit slot of said ionizing chamber means.

References Cited UNITED STATES PATENTS 2,975,277 3/1961 Von Ardenne 313231 2,977,470 3/ 1961 Robinson. 3,265,890 8/1966 Briggs 250419 RALPH G. NILSON, Primary Examiner. S. C. SHEAR, Assistant Examiner.