US3379874A - Ion source for a mass spectrometer with specific electrode structure to accelerate and focus ions - Google Patents

Description

prl 23, 1968 c. w. BAKER 3,379,874

vION SOURCE FOR A MASS SPECTROMETER WITH SPECIFIC ELECTRODE STRUCTURE To ACCELERATE AND FOCUS TONS Filed June 29, 1964 2 Sheets-Sheet l April 23, 1968 c. w. BAKER 3,379,374

ION SOURCE FOR A MASS SPECTROMETEH WITH SPECIFIC ELECTRODE STRUCTURE TO ACCELERATE AND FOCUS IONS Filed June 29, 1964 2 Sheets-Sheet 2 United States Patent O 3,379,874 ION SOURCE FOR A MASS SPECTROMETER WITH SPECIFIC ELECTRODE STRUCTURE TO AC- CELERATE AND FOCUS IONS Clarence William Baker, Monrovia, Calif., assigner to Bell & Howell Company, Chicago, lll., a corporation of Illinois Filed June 29, 1964, Ser. No. 378,910 4 Claims. (Cl. Z50-41.9)

This invention relates to mass analyzers and in particular to an improved ion source for use with such analyzers.

Generally speaking, mass analyzers, as for example, mass spectrometers, consist of three parts: an ion source, an analyzing sector and a collector section. Located within the ion source in many types of instruments and interposed in the direction of ion travel are three or more electrodes to which various potentials are applied in order to accelerate the ions before they -pass into the analyzing sector, These electrodes are normally thin metallic members in which apertures or slits have been cut in order that the ions may pass through them and be collimated into a thin beam or ribbon of ions before passing into the analyzing sector.

Since there is normally a difference of potential established between the first and second electrode, an electrode field exists in the space between the two electrodes, and this field has been found to be distorted adjacent the apertures in these two electrodes. It has further been found that such distortion has a significant lens effect on the direction of acceleration of the ions. As the ions pass through the aperture in the first electrode, they are deflected toward the optical or central axis of the analyzer. As they pass through the field adjacent the slit in the second electrode, they are deflected away from this central axis. The result is that the ion yield at the slit in the third electrode is substantially reduced over that available at the slit in the second electrode.

Other problems have also been encountered. For eX- ample, ions impinging on the sides of the slit in the second electrode are found to cause secondary emission of electrons which were attracted to and deposited on the back of the first electrode. This current flowing from the second to the first electrode tends to load down the accelerating voltage supply causing a varying accelerating voltage and consequent low ion yield.

One possible means of increasing the ion yield at the third electrode slit is to increase the width of the slit in the second electrode. In this way a greater number of ions pass through the slit in the second electrode. However, a wider angle of acceptance due to the wider slit also produces greater distortion of the adjacent electric field and increases ion scatter. Thus, the two effects offset each other.

The present invention increases the ion yield at the third electrode by providing an electrode system for collimating and propelling ions as a beam outwardly from an ion source. The system comprises a first electrode having an aperture therein, a second electrode having a grid or array of curved conductors which is aligned with the first electrode and displaced from it in the direction of ion propulsion. Located on the side of the second electrode opposite the first electrode is a third electrode aligned with the first two, this electrode also having an aperture therein for the passage of ions. Connected to the first and second electrodes are means for imposing a potential between the first and second electrode for propelling the ions.

In an alternate form of the invention, a plurality of grid-type electrodes are provided in the area bounded 3,379,874 Patented Apr. 23, 1968 by the first and third electrodes. Increasing the number of electrodes has the effect of forming a more predictable electric field and minimizing distortion due to surface charges or nearby conductors in the array. In still another alternate to the invention, a second electrode in the form of a mesh is located between the first and third electrodes.

These and other features of the invention will be more clearly understood by reference to the following figures where:

FIG. 1 is a schematic representation of a mass spectrometer provided with an ion source in accordance with the prior art,

FIG. 2 is a perspective view of an ion source provided with an electrode in accordance with one form of the invention,

FIG. 3 is a plan view of the ion source depicted in FIG. 2 showing the equipotential distribution produced by an electrode provided in accordance with the invention,

FIG. 4 is an ion source with a plurality of grids provided in the space between the first and third electrodes, and

FIG. 5 is a plan view of grid electrode showing the potential distribution adjacent a grid electrode provided with vertical conductors.

Referring to FIG. 1, a conventional mass spectrometer 5 is shown schematically and includes an iron source 6, an analyzer section 7 and a collector section .9. Within the iron source are located spark electrodes 10 and 11 connected at opposite ends to the secondary Winding of a Tesla transformer 13, The midpoint of the transformer winding is connected to the positive side of a high voltage supply 14. First, second and third electrodes 1'6, 17 and 1-8 are arranged serially, electrode 16 being connected through a suitable voltage divider 20 to the high voltage supply 14 and serving as the first electrode in a system for propelling ions from the spark source as a beam toward electrodes 17 and 18. Electrode 17 has an aperture 4 defining the geometrical width of the beam and electrode 18 has an aperture 19 acting as a limit on angular divergence of the beam passing therethrough, both electrodes being grounded as shown.

The analyzing sector 7 in this particular type of mass analyzer comprises an electric sector 21 constituting curved plates 22 and 23 arranged serially with respect to the electrodes, and a magnetic sector 28. The two plates 22 and 23 of the electric sector are connected to opposite sides of a deiiecting voltage supply 24, and, as illustrated, both plates are at a pre-assigned potential with respect to ground.

The magnetic sector is formed in the conventional manner by a pair of magnetic poles, one of which, pole piece 26, is shown in the drawing. In accordance with normal practice, a second identical pole piece (not shown) is disposed parallel to and spaced from the pole piece 26 to form a magnetic field therebetween transversely of the direction of ion travel into the magnetic field from the deiiector. The collector section 9 frequently comprises a photographic plate 8 immersed in the magnetic field for detecting mass dispersion therein.

The accelerating field configuration in conventional ion sources is shown in FIG. 1 by the dotted lines 15 located in the area between the first and second electrodes. It will be observed that the equipotential lines are parallel to the first and second electrodes in the middle area between the two electrodes. However, in the areas adjacent the apertures 3 and 4 in the two electrodes the field is deformed as indicated. Adjacent the first electrode the field has a bulge or dip toward the aperture 3. Near the second electrode the field has a similar deformation in the opposite direction. Arrows 25 and 27 normal to the i equipotential lines from which they extend indicate the direction of deflection of ions as they encounter the field distribution at those particular points. As will be discussed in more detail in connection with FIG. 3, the shape of the electric field has a direct bearing on the focusing or defocusing of the ions on the various apertures or slits.

The illustration in FIG. 1 is based on a` double focusing mass spectrometer, and in particular, one used for the analysis of solid materials. However, the invention is not limited to this particular type of mass analyzer or ion source. A field forming electrode system provided in accordance with this invention is equally well adaptable to other types of ion sources, such as for example, ones in which ions are created by electron bombardment, thermal emission, or other means such as are used in a wide range of analyzer types.

Referring now to FIGS. 2 and 3, there is illustrated in perspective and plan view a mass spectrometer ion source provided with a field forming grid in accordance with this invention. In addition, coordinate axes, X, Y and Z, are

shown to facilitate description of the relations between the various electrodes. As in FIG. 1, the ion source is provided with a Tesla transformer 29 which is connected to spark electrodes 3f) and 3l and to a high voltage supply 33. A first electrode 37, also referred to in the art as the first aperture, is provided with a circular aperture 32 and is located adjacent the spark electrodes. By means of connection 35 the first electrode is also connected to the high voltage supply 33. Displaced from the first electrode in the direction of the analyzing sector is a second electrode 34, which will be referred to as the field forming grid, which consists of an array or grid of curved conductors 36 disposed in the ion beam path. The particular configuration of the grid of electrode 34, as shown in FIGS. 2 and 3, is the preferred configuration according to this invention. This preferred configuration comprises a number of curved conductors located transversely of the major or long axis of the slit 38 in a third electrode 39, usually referred to as the object slit. As illustrated in FIG. 2 these conductors are arranged paralle! to each other and to the X-Y plane. FIG. 3 more graphically illustrates the curvature which is built into the conductors of the field forming grid 34. As shown, the curvature of the conductors is in one plane only, the horizontal or X-Y plane.

To produce a curved or shaped electric field, a potential difference between the first aperture and the combination of field forming grid and object slit is established,

This is accomplished by connecting the first aperture to the high voltage supply by means of the connection 35, while the grid and object slit are grounded. The curvature of the electric field represented by equipotential lines 4t), 42, 44, 46, 48 and 50 is caused by the aperture 32 and the shape of the grid 34. Examining the equipotential distribution of FIG. 3 in more detail, we find that in the region adjacent the aperture 32 the equipotential lines 4o and 42 have a bulge or dip in the direction of the spark electrodes. This deformation of the electric field is caused by the opening 32 in the electrode. As the space between the first and second electrodes is traversed, the effect of the curvature of the second electrode 34 is encountered. This effect is indicated by the shape of the equipotential lines 44, 46, 4S and S0.

The curvature `of the second electrode 34 is determined by its distance from the aperture 38. In order to obtain maximum ion yields at the object slit, the curvature of the field forming grid 34 in the embodiment shown in FIG. 3 is circular and is arranged such that its center of focus is located in the center of the slit 38, and the grid is positioned such that its axis of symmetry lies in the same plane as the Z axis and the major axis of the slit 38. In instances where the field forming grid is placed such that the axis of symmetry does not coincide with this plane, its shape must be adjusted in order that focusing is still obtained at the center of the slit 38 if maximum ion yield is still to be obtained.

The direction of this acceleration is indicated by arrows 52 and S4. The reason for this behavior is that charged particles placed in an electric field are accelerated in the direction of the highest potenial gradient, i.e., the direction of the shortest distance between adjacent equipotential lines. Arrows 52 and 54 represent th's shortest distance between adjacent equipotential lines 48 and Eil. Since the curvature of the electrode 34 determines the shape of the electric field in the region adjacent the electrode, the curvature of equipotential lines is nearly identical to that of the electrode and hence the direction of ion acceleration coincides with the radii of curvature of the grid. In this way the ions are focused on the center of the slit 3S.

As illustrated in FIG. 2 the second electrode 34 has a curvature in the X-Y plane only. Under these conditions focusing is also confined to the X-Y plane. However, in certain instances it may be useful to achieve focusing in two planes. This is accomplished by changing the shape and configuration of the second electrode 34 so that it is new in the form of a partial sphere. In this condition the grid has the capability of not only focusing in the X-Y plane as shown in FIG. 3, but also in the X-Z plane.

It should be noted that the preceding discussion is postulated on the use of a grid provided with conductors which are arranged transversely of the major axis of the object slit; in this case, horizontally. This is important for, as is shown in FIG. 5, if the wires 41 of a field forming grid 43 were located parallel to the major axis, i.e., vertically, a slight amount of defocusing would then be encountered in the X-Y plane which would detract from the major focusing effect due to the curvature of the grid itself. With conductors arranged transversely, the grid is also subject to the minor defocusing effect: however, the performance of the system is not impaired, since the defocusing is in the X-Z plane, and hence does not detract from the lens effect in the X-Y plane.

Still another alternative is to provide a mesh-type array as the second electrode and provide it with curvature in two planes as above. While yielding a major focusing effect on both the major and minor axis of slit 38, such a configuration is subject to the same slight disadvantage due to the deformation of the electric field adjacent the wires of the mesh electrode as indicated above. As shown in conjunction with FIG. 5 spaced apart conductors contribute minor defocusing effects opposing the major effect. A mesh is also subject to decreased electrical transparency since more ions traveling between the first and third electrodes would encounter the conductors of the second electrode. This reduces the ion yield in the analyzing sector.

Multiple grids, as shown in FIG. 4, are provided in the inter-electrode space between the first aperture 37 and object slit 39 where it is desired to form a more predictable field and to minimize field distortion due to surface charges or nearby conductors. These electrodes are designated 56 and 58. For focusing purposes the curvature of the extra electrodes 56 and 5S is also selected such that their center of focus is located at the center of the slit 38. In the embodiment shown in FIG. 4 electrodes 56 and 5S are circular segments as is electrode 34. In addition, the electrodes S6 and 53 are connected to the high voltage supply at 60 and 62 such that their potential is intermediate that of the first aperture 37 and the object slit 39. To achieve greatest ion yields, the electrodes in this configuration are arranged such that corresponding conductors in each electrode are located in the same plane. This means that corresponding conductors in adjacent electrodes are located in the electrical shadow of each other so that the effective electrical transparency of the combination is the same as if only one field forming grid were intcrpocd in this interclccfrode space.

Although the electrode system of this invention has been described in conjunction with an ion source for use with mass spectrometer and in particular a solids mass spectrometer using a spark source, such an illustration is intended to be used by way of example only. The electrode system of this invention is capable of being used with any type of ion source in the various mass spectrometers or analyzers which are common in the art.

I claim:

1. 1n a rnass analyzer the combination comprising:

an ionization chamber,

eans for producing ions within the chamber,

a first electrode with an aperture provided therein located adjacent the ion producing means,

a third electrode with an elongated vertical slit provided therein located on the side of the first electrode oppoiste the ion producing means, the slit being aligned with the aperture in the first electrode,

a second electrode mounted on the third electrode on the side thereof adjacent the first electrode, the second electrode comprising a framework of horizontal curved, parallel conductors, each of said conductors being arranged traveresly with respect to the vertical slit in the third electrode and in a convex relation with respect to the first electrode and a concave relation with respect to the third electrode, and

means for imposing a potential difference between the first and second electrodes whereby an electric field is created between the first and second electrodes, and ions passing through the aperture in the first electrode are accelerated by the difference of potential between the first and second electrodes and focused on the slit in the third electrode by means of the electric field.

2. In a mass spectrometer, the combination comprising:

an ionization chamber,

means for producing ions within the chamber,

a first electrode with a circular aperture provided therein located adjacent the ion producing means,

a first source of potential connected to the first electrode,

a third electrode with a vertical slit provided therein having a major and a minor axis defining one boundary of the ionization chamber, the slit being aligned with the aperture in the first electrode,

a second electrode mounted yon the third electrode on the side adjacent the first electrode, the second electrode including a framework of spaced, horizontal parallel conductors aligned with the minor axis of the slit in the third electrode, each of the conductors forming circular segments with centers coinciding with the major axis of the third electrode slit such that each `of said conductors in said framework is curved convexly in relation to the first electrode and concavely in relation to the third electrode, and a second source of potential connected to the second and third electrodes, the second source being lower in magnitude than the first whereby ions passing through the aperture in the first electrode are accelerated by the difference of potential between the first and second electrodes and are focused on the major axis of the slit in the third electrode by means of an electric field existing between the first and second electrodes due to the difference of potential between the two electrodes.

3. In a mass spectrometer, the combination comprising:

an ionization chamber,

a spark source of positive ions located within the chamber,

a radio frequency source connected to the spark source,

a first electrode with a circular aperture provided therein located adjacent the spark source and electrically connected to the spark source,

a first source of potential connected to the spark source and the rst electrode,

a third electrode located on the side of the first electrode opposite the spark source and aligned with the first electrode, the third electrode including an elongated vertical slit having a major and a minor axls,

a second electrode mounted on the third electrode on the side adjacent the first electrode and electrically connected to the third electrode, the second electrode including a framework of spaced, arcuate, parallel wires aligned with the minor axis of the elongated slit in the third electrode, the center of curvature of the wires coinciding with the major axis of the third electrode slit such that each of said conductors in said framework is curved convexly in relation to the first electrode and concavely in relation to the third electrode, and

a second source of potential connected to the second and third electrodes, the second source being lower in magnitude than the first, whereby the ions created by the spark source diffuse through the aperture in the first electrode, are accelerated by the difference of potential between the first and second electrodes and are focused on the major axis of the elongated slit in the third electrode by means of an electric field existing between the first and second electrodes due to the difference of potential established therebetween.

4. The combination according to claim 3 wherein at least one additional electrode is located in the space between the first and third electrodes, said electrode being congruent with the second electrode and comprising a framework of spaced, arcuate, parallel wires aligned with the minor axis of the slit in the third electrode and with the wires of the second electrode, the center of curvature of the wires of said additional electrode coinciding with the major axis of the third electrode slit such that each of said w-ires in said additional electrode is convex with respect to the first electrode and concave with respect to the third electrode.

References Cited UNiTED STATES PATENTS 3/1957 Lawrence z Z50-41.9 9/1958 Robinson Z50-41.9

Claims (1)

1. IN A MASS ANALYZER THE COMBINATION COMPRISING: AN IONIZATION CHAMBER, MEANS FOR PRODUCING IONS WITHIN THE CHAMBER, A FIRST ELECTRODE WITH AN APERTURE PROVIDED THEREIN LOCATED ADJACENT THE ION PRODUCING MEANS, A THIRD ELECTRODE WITH AN ELONGATED VERTICAL SLIT PROVIDED THEREIN LOCATED ON THE SIDE OF THE FIRST ELECTRODE OPPOSITE THE ION PRODUCING MEANS, THE SLIT BEING ALIGNED WITH THE APERTURE IN THE FIRST ELECTRODE, A SECOND ELECTRODE MOUNTED ON THE THIRD ELECTRODE ON THE SIDE THEREOF ADJACENT THE FIRST ELECTRODE, THE SECOND ELECTRODE COMPRISING A FRAMEWORK OF HORIZONTAL CURVED, PARALLEL CONDUCTORS, EACH OF SAID CONDUCTORS BEING ARRANGED TRAVERESLY WITH RESPECT TO THE VERTICAL SLIT IN THE THIRD ELECTRODE AND IN A CONVEX RELATION WITH RESPECT TO THE FIRST ELECTRODE AND A CONCAVE RELATION WITH RESPECT TO THE THIRD ELECTRODE, AND MEANS FOR IMPOSING A POTENTIAL DIFFERENCE BETWEEN THE FIRST AND SECOND ELECTRODES WHEREBY AN ELECTRIC FIELD IS CREATED BETWEEN THE FIRST AND SECOND ELECTRODES, AND IONS PASSING THROUGH THE APERTURE IN THE FIRST ELECTRODE ARE ACCELERATED BY THE DIFFERENCE OF POTENTIAL BETWEEN THE FIRST AND SECOND ELECTRODES AND FOCUSED ON THE SLIT IN THE THIRD ELECTRODE BY MEANS OF THE ELECTRIC FIELD.

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