US3421035A

US3421035A - Tubular ion source for high efficiency ion generation

Info

Publication number: US3421035A
Application number: US568175A
Authority: US
Inventors: Wilson M Brubaker
Original assignee: Bell and Howell Co
Current assignee: Bell and Howell Co
Priority date: 1966-07-27
Filing date: 1966-07-27
Publication date: 1969-01-07
Anticipated expiration: 1986-01-07

Description

Jan. 7, 1969 w. M. BRUBAKER 3,421,035

TUBULAR ION SOURCE FOR HIGH EFFICIENY ION GENERATION Filed July-27, 1966 SheefI v/ of 2v www Jan. 7, 1969 W. M. BRUBAKER 3,421,035

TL'BULAR ION SOURCE FOR HIGH EFFICIENCY ION GENERATION Fued'July 27, 196e sheet 2 of 2 ff /f i5 I NVE NTOR. Mam/4% inw/? BY www@ United States Patent O 3,421,035 TUBULAR ION SOURCE FOR HIGH EFFICIENCY ION GENERATION Wilson M. Brubaker, Arcadia, Calif., assignor to Bell & Howell Company, Chicago, Ill., a corporation of Illinois Filed July 27, 1966, Ser. No. 568,175 U.S. Cl. 313-63 Int. Cl. H05h 5/60 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to quadrupole mass filters and in particular to ion sources for use with such filters.

In conventional mass spectrometers, it is customary to provide an ion source for the spectrometer which is characterized by a relatively small and confined space into which molecules have been admitted. Electrons are directed in a beam through this space to ionize the molecules. These ions are then accelerated by means of suitable electrodes in the source toward the analyzer portion of the instrument for separation into discrete beams of ions of the dierent masses present in the sample being analyzed. Because the analyzer section of the mass spectrometer frequently cannot tolerate wide variations in ion energy, the formation of ions in a small volume is desirable and preferred in order to keep this energy variation quite small.

The limitation with respect to tolerable energy variations of ions in the source imposes a limitation on the intensity of the ionizing electron beam. In addition, a beam of too high an intensity produces space charge edects which have a deleterious effect on the operation of the source. In view of the preceding limitations and in order to maximize the effectiveness of the electron beam While simultaneously minimizing the influence of space charge, sources of conventional mass analyzers are made of quite small dimensions and the intensity of the electron beam is closely controlled.

In contrast to conventional spectrometers, sources used with non-magnetic mass filters, such as quadrupole filters, are not subject to such stringent requirements due to the ability of non-magnetic analyzers `to accept ions having a wide energy spread. This unique tolerance of such an analyzer means that it is possible to provide an ion source of relatively large dimension. `Up to now, however, it has been customary even in sources designed for quadrupole mass filters to provide one in which the ionizing electron beam is directed Ialong the longitudinal axis ofthe source. This design has several disadvantages. The first disadvantage is that electrons injected in the ionizing region along the axis are brought to rest and returned to the end of the source which is remote from the ion exit aperture. In the region of reversal, which is normally adjacent the ion exit aperture, the electrons possess a very low energy and hence are normally incapable of forming ions with consequent loss of efficiency and space charge build-up at the most sensitive portion of the source. In addition, because the electrons reverse their axial component of velocity in this area, the densityY of the electrons is very high causing a ydistinct aggravation in the space charge problem.

The present invention provides a new ion source for use with non-magnetic mass filters. In this source, electrons are directed transversely of the longitudinal axis of the source rather than along or parallel to the axis. Because of the size and design of the source, several further improvements are obtained which have the effect of optimizing the efficient and sensitivity of the source. By 'directing electrons in this transverse `direction in a relatively large source, the following -advantages yare obtained. Ionization occurs over a large volume. Space charge effects are minimized because the electrons in the ionizing portion of the volume all have high ionizing energies. This is because the electrons now experience a reversal of their direction of motion adjacent the walls of the source at a point removed -a substantial distance from the axis. In addition it is in the area along the axis that the electrons possess their highest energies and since the majority of sample molecules 4are concentrated in this region the efiiciency of ionization of the sample is optimized.

Where a cylindrical source is used and a suitable p0- tential imposed on the area enclosing the source, the electrons are directed back toward the axis of the source as they aproach the walls end, being focused on the axis, they thereby produce the maximum concentration of ionizing electrons along the axis. This concentration of electrons along the axis means that a substantial negative space charge is present along the axis. The existence of such a negative potential along the yaxis has the beneficial effect of causing the ions to be attracted to this region thereby substantially increasing the concentration of ions along the axis and at the exit aperture. Finally, the sensitivity of such a source is high because of the large `ionizing electron currents which are available.

The present invention provides an ion source comprising an enclosure having a longitudinal axis. Means are provided for introducing molecules into the enclosed area and means are also provided for generating and directing a sheet of electrons in a direction transverse to the longitudinal axis of the source to ionize the molecules. Additional means located within the source are provided to establish a potential gradient along the longitudinal axis to accelerate the ions in a predetermined direction and to establish an electronrepelling potential in the enclosure walls. An aperture is provided at one end of the source for permitting the ions to pass from the source.

In a preferred embodiment, a further refinement is provided with the source of this invention. This refinement is based on the configuration of a geometrical figure such as a sphere which has a central focus point. If two concentric spheres are provided one interiorly of the other, the interior sphere having a transparency for ions and a potential difference is established between the two spheres, ions enclosed within the outer space are accelerated toward the center of the spheres. By adapting this concept to the ion source of this invention, further improvements in the concentration of ions at the exit aperture can be achieved. As a practical matter, it is not necessary to provide a spherical source since a more conventional source simulating the electric field conditions existent in a segment of a sphere can be provided. For example, by establishing proper potentials on several electrodes in 'a tubular or cylindrical source, a field can be created which simulates that occurring within a sphere and accomplishes the same results. This and other refinements of the present invention will be more readily understood by reference to the following figures in which:

FIG. 1 is an illustration of one embodiment of the source of this invention,

FIG. 2 is an alternate embodiment of a part of the source shown in FIG. l,

FIG. 3 is a diagrammatic illustration of two spheres illustrating the focus of charged particles at the Center thereof,

FIG. 4 is an adaptation of the concept depicted in FIG. 3 to amore conventional ion source, and

FIG. 5 is an embodiment of an ion source incorporating the improvements of this invention.

Referring now to FIG. 1, there is shown therein an exploded, diagrammatic view of Ia quadrupole mass filter. The filter, as shown in FIG. l, consists essentially of two sections, an ion source 1f), represented by an elongated tube 20 and associated components, and an analyzer 12 represented by four rods 14 parallelly arranged Iabout a central axis. An electrode 16 having an aperture 18 1s located at the exit end of the source 10. The purpose of the aperture in elecerode 16 is to concentrate ions emerging from the source.

In the embodiment shown in FIG. l, source consists of an elongated cylinder 20, having a helical winding 22 located interiorly thereof 'and disposed coaxially therewith. A first Source of potential 24 is connected at the ends of helix 22 to establish a potential gradient within the source 10 such that postive ions are accelerated along the longitudinal axis of the source toward the aperture 18 in exit electrode 16. A second source of potential 34 is connected to cylinder 20 to impose a negative potential on the cylinder enclosing the source volume.

Three

elongated filaments

26, 2S and 30 are located interiorly of cylinder and are disposed generally parallel to the longitudinal axis of the cylinder 20. These filaments are connected to sources of potential capable of supplying suicient electrical power to cause electrons to be thermionically emitted from them. The negative potential on cylinder 20 repels these electrons causing them to be directed as a sheet or a cloud toward the axis of the cylinder in a direction essentially transverse t0 this axis. Sample molecules to be analyzed are admitted to the source through aperture 32 at one end of the cylinder 20. As the molecules drift in the space enclosed by cylinder 20 they are ionized by the electrons emitted from the three

filaments

26, 28 and 30. Once ionized, the molecules are accelerated along the length of the cylinder toward aperture 18 at the exit end of the cylinder under the influence of the potential gradient produced by helix 22. The ions then pass from the source into the analyzer section 12 of the filter. Here the ions are separated according to their mass-to-charge ratio by means, in la quadrupole mass filter, of the combined efects of the AC and DC fields created within the space enclosed by the rods.

The present invention provides several advantages over prior art sources used with filters of this type. In the prior art it is typical for the electron gun to direct electrons along the longitudinal axis of the source in order to obtain ionization. After such electrons have traversed the length of the source, they undergo a reversal of direction at the end of the source corresponding to aperture 18 and are accelerated back toward the entrance end 32 of the source. The reversal of electrons adjacent the exit from the source creates a region wherein the speed of travel and energy of the electrons is relatively low with the result that the electrons normally do not possess sufficient energy to ionize the molecules in this area. Furthermore, the relatively slow speed of travel of the electrons causes the occurrence of a substantial space charge in the area adjacent the exit aperture from the source causing a diffusion of ions in this region and interference with the emission of ions from the source into the analyzer.

With a source constructed according to the present invention, such problems are diminished or eliminated. Electrons are now emitted by a filament such that they are directed generally transversely of the axis of the source and experience a reversal of the direction of motion only adjacent the cylinder Walls of the source. This means that the reversal of electrons occurs at a point removed a substantial distance from the exit from the source. Since the major concentration of molecules occurs along the axis and a relatively small number are found adjacent the walls of the cylinder, the low energy of the electrons adjacent the cylinder walls has little effect on the overall operation of the source. By virtue of the transverse direction of travel of the electrons, the electrons have highest energy as they traverse the region surrounding the axis.

By the use of a negatively charged, cylindrical source, electrons are caused to pass through the axis of the source repeatedly as they oscillate back and forth between the walls of the source and the greatest concentration of the electrons is experienced along the axis of the source. In contrast with the deleterious effect of a concentration of electrons adjacent the exit aperture in prior art sources, the existence of a negative space charge surrounding the area of the axis has a beneficial rather than a deleterious effect in that now the negative space charge has the effect of urging the positive ions toward the axis thereby achieving a greater concentration of ions emitted from the source.

The concentration of high energy electrons along the axis of the source means ionization occurs under optimum conditions because the ionizing electron beam possesses approximately its maximum energy in this region. The efficiency of ionization is further enhanced when the source is used with instruments such as quadrupole filters because ionizing currents of a high intensity can now be employed due to the mass filters inherent capability of accepting ions having a wide energy spread.

As shown in FIG. l,

filaments

26, 28 and 30 extend substantially throughout the length of the source. Such an embodiment illustrates one of several ways in which the filament or filaments may be disposed within the source in order to produce electrons which cross the axis of the source in an approximately transverse direction. The physical length and shape of the filament has no particular significance and equally satisfactory results can be obtained by providing shorter filaments located preferably toward the exit end of the source. As will be described in more detail below, still another way in which the ionizing electron beam can be provided is by means of one or more circular filaments located interiorly of the source.

In FIG. l helix 22 is provided to establish a potential gradient urging ions toward the exit end of the source. The helix is constructed of a high resistance wire and a source of potential 24 is connected to the ends of the helix such that a potential gradient decreasing toward the exit end of the source is established.

In FIG. 2 is another embodiment of a means by which ions can be accelerated longitudinally through the source toward the rods of the filter. As shown therein, a series of loops 36 disposed about a common axis and spaced a predetermined distance from adjacent loops are connected by means of leads 38 to a potential divider 40 which in turn is connected to a source of potential 47 for establishing the same potential gradient as in the embodiment of FIG. l. In still another embodiment, a second cylinder is located concentrically with and interiorly of a first cylinder. The second cylinder is preferably constructed of a mesh material which is transparent to electrons permitting unimpeded movement `within the source while at the sarne time providing means ffor establishing the desired potential gradient by connecting the ends of the second cylinder to a source of potential of suitable polarity.

Another embodiment of an ion source suitable for use with a mass filter is shown in FIG. 3. As shown therein a section is taken through a pair of

spheres

44 and 46 concentrically disposed with respect to one another. By grounding interior sphere 46 and applying a higher potential to exterior sphere 44, a potential gradient directed toward the center 48 of the spheres is created. Assuming the effect of space charge is negligible, charged particles introduced into the space between

spheres

44 and 46 are accelerated along lines 50 toward the center 48 of the sphere. If sphere 46 is made of a material which is transparent to charged particles, the ions are accelerated toward and concentrated at the center.

The source 43, shown in FIG. 3, is illustrative of an ideal source in which all charged particles are accelerated toward and collected at a single point providing greatest concentration of ions. It is also possi-ble to accomplish the same result by taking portions, such as a solid segment `52 of two spheres and applying proper potentials to each of the two segments. Ions located within the space enclosed by the segments are accelerated toward the apex of the segments. The field can also be created by a series of electrodes having potentials corresponding to the potential distribution between two spheres.

The source 56 of FIG. 4 illustrates still another way in which, by imposing proper potentials on a series of electrodes, an electric field 54 simulating that created with concentric spheres can be created. Such a result is accomplished by providing a plurality of

field forming electrodes

58, 60, 62, 64, 66 and 68 and imposing a predetermined potential on each of the field forming electrodes. A circular filament 70 is added to the source 56 for generating electrons transversely of the axis 72 of the source. An aperture 74 is provided for admitting molecules into the source and a collimating electrode 76 is provided exteriorly of the source for directing ions focussed at aperture 78 in electrode 76 into an analyzer such as a quadrupole mass filter. A filament shield 80 surrounds

electrodes

58, 60, 62, 64 and 66 and filament 70. The purpose of the shield is to prevent the loss of electrons to adjacent surfaces and to provide a means for establishing a potential which will reilect electrons after their initial passage through the analyzing region in order to produce multiple traversals of the source by the electrons. The source of FIG. 4 combines the advantages of the source illustrated in FIG. 1 with the advantages of the ideal spherical source illustrated in FIG. l with the advantages of the ideal spherical source of FIG. 3 to obtain greater focussing and concentration of ions at the outlet from the source.

In FIG. 5` is shown a further refinement of an ion source utilizing the features of the source shown in FIG. 4. The source 82 comprises a housing 84 defining an ion chamber 86. Attached to housing 84 is a cover 88 having a sample inlet 90 for admitting sample molecules into chamber 86. Extending through one wall of housing 84 are a series of

electrodes

92, 94, 96, 98, 100 and 102. These electrodes are mounted by means of bolts 104 and are separated from one another by means of insulated spacers 106 located between adjacent electrodes. A filament 108 is located within chamber 86 and is provided with a pair of connectors 110 which pass through the wall of chamber 86 to the exterior for connection to a source of potential. A plate 114 having an outlet slit 112 is secured to the housing by means of bolt and nut arrangement 104 and a second bolt and nut assembly 116. Plate 114 is constructed such that slit 112 is located on the axis of the source and at the point of focus of the lfield created therein.

Electrodes

92, 94 and 98, respectively, are connected to sources of potential (not shown) for producing a potential gradient directed toward aperture 112 for accelerating ions toward the aperture. By providing a predetermined potential on each of these electrodes, the potential gradient of concentric spheres is simulated and the ions are focussed toward a center point located in aperture 112. Electrode 100 is provided with a curvature to more nearly simulate a spherical surface. The combined effect of the electrodes is to focus the ions at aperture 112. Electrodes -96 and 102 are rfocussing electrodes which are connected to adjustable sources of potential for obtaining optimum focussing at the exit aperture of the source. An additional function of these focussing electrodes is to counteract the effect of space charge at the aperture. Ions concentrated at the aperture are then emitted into the analyzer section of the filter (not shown) for separation according to their massto-charge ratios.

What is claimed is:

1. An ion source comprising:

an enclosure having a longitudinal axis;

means for introducing molecules into the enclosure;

means located within the enclosure for generating and directing a cloud of electrons in a direction generally transverse to the longitudinal axis of the enclosure to ionize sail molecules;

means for establishing an electron repelling potential on the enclosure walls;

elect-rode means located within the enclosure connectable to a source of electric potential for establishing a potential gradient within the enclosure to accelerate the ions in a predetermined direction along the longitudinal axis; and

an exit aperture for permitting the ions to pass from the enclosure.

2. A source according to claim 1 wherein the enclosure is a cylinder of revolution about the longitudinal axis.

3. A source according to claim 2 wherein lthe means for establishing the potential gradient is a helical coil located interiorly of the cylinder.

4. A source according to claim 2 wherein the means for establishing the potential gradient is a plurality of rings connected at spaced intervals along a potential divider.

5. A source according to claim 2 wherein the means for establishing the potential gradient is a second cylinder located interiorly of the cylinder of revolution.

6. An ion source comprising:

a tubular enclosure having a longitudinal axis and an outlet end;

means for introducing molecules into the enclosure;

lament means located within the enclosure -for generating an ionizing sheet of electrons in a direction transverse to the longitudinal axis;

means for connecting a source of electricpotential to the enclosure for electrostatically repelling electrons adjacent lthe walls of the enclosure; and

a plurality of field forming electrodes located within the enclosure for establishing a potential gradient along the longitudinal axis to accelerate ions toward the outlet end.

7. A source according to claim 6 wherein the iilament means is a circular coil disposed aboutthe longitudinal axis of the enclosed space and connectable externally to a source of electric power.

8. A source according to claim 6 including means located within the enclosure for electrostatically focusing ions at the outlet.

9. A source according to claim 8 wherein a predetermined electric potential is applied to vthe Ifield forming electrodes to simulate the potential gradient of two concentric spheres having their center located at the outlet.

References Cited UNITED STATES PATENTS 2,714,679 8/1955 Van de Graaff et al. 313-230 X 2,975,278 3/1961 Brubaker et al. 2604l.9 3,286,187 1l/1966 Gabor 313-230 X 3,297,894 l/1967 Hall et al. 313--63 JAMES W. LAWRENCE, Primary Examiner. R. L. JUDD, Assistant Examiner.

U. S. Cl. X.R. 3l3--230; Z50-41.9