Ion source assembly with multiple elliptical filaments

Abstract

An electron impact ion source assembly for a mass spectrometer includes an anode extending along an axis and surrounding an ionization volume. At least two filaments, each configured to emit thermionic electrons, are positioned outside the ionization volume and proximate to the anode. Each of the at least two filaments includes an elliptical portion and a non-elliptical portion at either end of the elliptical portion. The non-elliptical portions are mounted in a fixed position relative to the anode to maintain a constant distance between the elliptical portion and the anode. The elliptical portions extend along a plane that intersects a plane perpendicular to the axis of the anode at a non-zero angle.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application is a non-provisional application of and claims the benefit of and priority to U.S. Provisional Patent Application No. 63 / 290,436, filed December 16, 2021, the entire contents of which are incorporated herein by reference.

[0002] [Technical field] The present disclosure relates to an ion source assembly including a plurality of filaments, each filament having an elliptical central extension. [Background technology]

[0003] Mass spectrometry is a common analytical technique used to measure the mass-to-charge ratio of ions in a sample to determine the chemical composition of the sample. In general, mass spectrometry involves ionizing a sample, separating the ions according to their mass-to-charge ratio, detecting the separated ions, and displaying the results as a spectrum showing the signal intensity of the detected ions as a function of their mass-to-charge ratio.

[0004] Ionization of samples, particularly gas samples, can be performed using electron impact ionization sources, also known as electron ionization (EI) ion sources. An EI ion source includes an electron source, which can be a filament that is heated to a temperature that emits electrons. The filament used can be a thin wire composed of a refractory metal that is uncoated or coated with a metal oxide. Heating of the filament can be performed resistively by passing an electric current through the filament. Thermal electrons emitted from the filament are accelerated through the wall or anode and enter the ionization volume. The movement of the electrons is guided by the electric field resulting from the potential difference maintained by a control unit between the filament, the anode and possibly other electrodes, and by the shape and positional relationship of each of the members. The anode defines at least one opening through which a certain percentage of the electrons can pass through the anode and enter the ionization volume. In general, it is desirable to maintain the temperature and the various potentials of the filament so that a constant electron emission current enters the ionization volume. Additional electrodes, such as electron repellers, can be included for steering the electrons. Within the ionization volume, at least a portion of the accelerated or energetic electrons collide with molecules of the gas sample in the ionization volume, the electrons having sufficient energy to ionize and / or fragment the gas molecules upon collision with them to produce ions.

[0005] These ions are then accelerated and directed towards the mass filter by another potential established using ion optical elements that are part of the ion source. Some ion sources also include an ion repeller located upstream of the ionization volume. The ion repeller is set to a specific potential to help control the trajectory of the ions created in the ionization volume. The ion repeller may be a flat or planar electrode or may be concave in the direction towards the mass filter. When the ions with different mass to charge ratios reach the mass filter, they are separated spatially or temporally. The ions are then detected by an ion detector and a mass spectrum is determined from the output of the ion detector.

[0006] The filament of an EI ion source has a finite useful life. To emit a sufficient number of electrons, the filament must operate at temperatures between 1500 and 2400 K. At these high temperatures, the filament wire (and coating, if present) will eventually evaporate, resulting in filament breakage. Filament breakage may also occur as a result of changes in the crystal structure of the filament wire that occur at high operating temperatures. Additionally, the electron emitting surface of the filament may be chemically altered by the gases in the system, which increases the work function of the electron emitting surface while decreasing the electron emission efficiency of the electron emitting surface. If there are no electrons or insufficient electrons available to ionize the gas sample due to a broken, deformed, or chemically "contaminated" filament, the mass spectrometer will no longer function satisfactorily. Processes that are monitored and / or controlled based on data generated by the mass spectrometer must therefore be stopped or otherwise "run blind" until there is an opportunity to replace the filament. Replacing a filament is a time-consuming and inconvenient process. Because the filament is often located inside a processing vacuum chamber, the processing vacuum chamber must be vented to perform this replacement. It is therefore desirable to reduce the frequency of filament replacement, and even more desirable to be able to schedule filament replacement in advance so that the filament can be replaced at the same time that the processing chamber is off-line for other maintenance activities.

[0007] One commonly used method to address this shortcoming of the EI ion source is to include in the ion source a second filament that is placed near the anode and can operate when the first filament fails. The two filaments are usually copies of each other, mirrored about a plane extending along the ion optical axis of the ion source. Arranging the filaments in this way is done to try to maintain consistent performance of the mass spectrometer by allowing ions to form in the same region of the ionization volume regardless of which filament is used. This ensures that ions are created where the electric field can guide the ions to be successfully injected into the mass filter, and where the electric field is high enough to overcome space charge effects on sensitivity. However, one of the disadvantages of this type of EI ion source is that, due to the limited space generally available near the anode, each of the two filaments is shorter than the filament that would be used if there was only one filament. The relationship between the electron emission current density leaving an electron emitting surface and the temperature and work function of that surface is expressed by the Richardson equation as a monotonically increasing function of temperature. Because the total electron emission current depends on the area of ​​the emitting surface, a shorter filament must be operated at a higher temperature to obtain the same total emission current. Thus, two short filaments operated in series will not last twice as long as one long filament. In fact, the combined operating life of two short filaments may not be as long as that of a single "normal" length filament. Furthermore, a shorter filament will necessarily lose more heat to the mounting arrangement than a longer filament. This heat loss is due to the lower thermal resistance offered by the short path along the wire from the central region of the filament to the attachment point compared to a single long filament. As a result, a higher temperature is required at the hottest part of the filament (near the center) to maintain the total electron emission at a required level, shortening the operating life of the filament.

[0008] Two important properties of an ion source are sensitivity (the number of ions that can be produced at an acceptable rate per unit pressure and injected into the mass filter) and linearity (the degree to which sensitivity is independent of pressure). These properties cannot be completely ignored in an attempt to extend filament life. For example, the emission current and / or operating pressure can be reduced in an attempt to extend filament life by reducing the temperature of the filament. However, the reduction in emission current or operating pressure comes at the expense of sensitivity and / or ion current.

[0009] These are just some of the drawbacks associated with ion sources currently used in mass spectrometers. Summary of the Invention

[0010] An embodiment of an ion source assembly for a mass spectrometer includes a cylindrical anode structure that at least partially defines an ionization volume. At least two filaments are disposed proximate to the anode and function as an electron source used to ionize an analyte sample by electron bombardment. The filaments are generally elliptical in shape at a central portion and tangent to the elliptical portion at straight end sections. Although two embodiments are described herein, the ion source assembly of the present invention is not limited to the two described embodiments. In one embodiment, the filaments may be mounted parallel to each other or as copies of each other rotated 180° about the central axis of the anode structure. The filaments are attached to a support member at a location on the straight end sections and positioned such that the central elliptical portion is a fixed distance from the cylindrical anode structure. Additionally, the filaments are mounted such that one end of the semi-major axis of the generally elliptical portion of each filament is located at a similar axial position of the cylindrical anode.

[0011] An embodiment of an electron impact ion source assembly for a mass spectrometer includes a cylindrical anode extending along an axis and surrounding an ionization volume, and first and second filaments, each of which emits thermoelectrons. The first and second filaments are disposed outside the ionization volume and proximate to the cylindrical anode. Each of the first and second filaments includes an elliptical central extension and non-elliptical extensions at opposite ends of the elliptical central extension. The non-elliptical extensions are mounted in a fixed position relative to the cylindrical anode to maintain a constant distance between the elliptical central extension and the cylindrical anode. Each elliptical central extension extends along a plane that intersects a plane perpendicular to the axis of the cylindrical anode at a non-zero angle.

[0012] Another embodiment of an electron impact ion source assembly for a mass spectrometer includes an anode extending along an axis and surrounding an ionization volume, and at least two filaments, each of which emits thermoelectrons. The at least two filaments are disposed outside the ionization volume and proximate to the anode. Each of the at least two filaments includes an elliptically-shaped portion and a non-elliptical portion at each end of the elliptical portion. The non-elliptical portion is mounted in a fixed position relative to the anode to maintain a constant distance between the elliptical portion and the anode. Each elliptical portion extends along a plane that intersects a plane perpendicular to the axis of the anode at a non-zero angle.

[0013] In one embodiment, each elliptical portion includes an apex, and the at least two filaments are positioned such that the apexes are at the same depth relative to the anode. In one embodiment, each non-zero angle is created by rotating the at least two filaments about an axis of rotation that passes through each of the apexes of the at least two filaments. In one embodiment, the at least two filaments extend along parallel planes. In one embodiment, the axis of rotation intersects with the axis of the anode at an angle of 90°. In another embodiment, the at least two filaments are mirror images of each other rotated at an angle about the axis of the anode. In a further embodiment, at least one of the at least two filaments is coated with a metal oxide. [Brief description of the drawings]

[0014] The present invention briefly summarized above can be more particularly described with reference to embodiments, some of which are illustrated in the accompanying drawings. It should be noted, however, that the accompanying drawings illustrate only typical embodiments of the present invention and are not intended to limit the scope of the present invention, which may allow other embodiments that are equally effective. Therefore, for a better understanding of the nature and objects of the present invention, reference can be made to the following detailed description in conjunction with the drawings.

[0015] [Figure 1] 1 shows a schematic cross-sectional view of one embodiment of an ion source assembly for a prior art mass spectrometer.

[0016] [Diagram 2] FIG. 1 shows a perspective view of one embodiment of an ion source assembly for a prior art mass spectrometer.

[0017] [Figure 3A] FIG. 1 shows a top perspective view of one embodiment of a dual filament ion source assembly for a prior art mass spectrometer.

[0018] [Figure 3B]FIG. 1 shows a side perspective view of one embodiment of a dual filament ion source for a prior art mass spectrometer.

[0019] [Figure 4] FIG. 3C shows a schematic cross-sectional view of the embodiment of FIG. 3B.

[0020] [Diagram 5] 1 shows a side perspective view of one embodiment of an ion source assembly for a mass spectrometer according to the present invention.

[0021] [Figure 6A] FIG. 2 illustrates a bottom view of an embodiment of multiple filaments disposed around an anode of an embodiment of an ion source assembly according to the present disclosure.

[0022] [Figure 6B] FIG. 6B shows a side perspective view of the embodiment of FIG. 6A.

[0023] [Figure 6C] 6A-6B show elevational views of the embodiment of FIG.

[0024] [Figure 6D] 6A-6C depict elevational views illustrating exemplary orientations of ends of multiple filaments relative to one another.

[0025] [Figure 7A] FIG. 6B illustrates the embodiment of FIG. 6A further including one embodiment of a mounting element for holding and attaching the filament in place relative to the anode.

[0026] [Figure 7B] 6B embodiment includes the mounting element embodiment of FIG. 7A.

[0027] [Figure 7C] 6C includes the mounting element embodiment of FIGS. 7A and 7B.

[0028] [Figure 7D] 6D embodiment includes the mounting element embodiment of FIGS. 7A-7C.

[0029] [Figure 8A] FIG. 13 illustrates a bottom view of another embodiment of multiple filaments disposed around an anode of an embodiment of an ion source assembly according to the present disclosure.

[0030] [Figure 8B] 8B shows a side perspective view of the embodiment of FIG. 8A.

[0031] [Figure 8C] FIG. 8C is a front view of the embodiment of FIGS. 8A and 8B.

[0032] [Figure 8D] 8A-8C illustrating another exemplary orientation of the ends of multiple filaments relative to one another. FIG.

[0033] [Figure 9A] 8B shows the embodiment of FIG. 8A further including another embodiment of a mounting element for holding and attaching the filament in place relative to the anode.

[0034] [Figure 9B] 8B embodiment includes the mounting element embodiment of FIG. 9A.

[0035] [Figure 9C] 8C includes the mounting element embodiment of FIGS. 9A and 9B.

[0036] [Figure 9D] 8D embodiment includes the mounting element embodiment of FIGS. 9A-9C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The following description relates to various embodiments of an ion source assembly with multiple elliptical filaments. It will be understood that the versions described herein are examples embodying certain inventive concepts detailed herein. To this end, other variations and modifications will be readily apparent to those skilled in the art. Furthermore, certain terminology is used throughout this description to provide an appropriate frame of reference with respect to the accompanying drawings. Terms such as "upper", "lower", "outer", "inner", "top", "bottom", "first", "second", etc. are not intended to limit these concepts unless specifically indicated. The terms "about" or "approximately" as used herein may refer to a range of 80% to 125% of the claimed or disclosed value. With respect to the drawings, their purpose is to depict the salient features of the ion source assembly with multiple elliptical filaments and are not provided to scale for clarity.

[0038] As shown in Figures 1 and 2, a portion of a mass spectrometer 100 is shown that includes an electron ionization (El) ion source assembly 101. The El ion source assembly (ion source assembly) 101 includes an electron source 102, an anode 105 that at least partially defines or surrounds an ionization volume 106, one or more electron repellers 107, and one or more ion repellers 112. As shown, the electron source is a filament 102, such as a thin wire, that is heated to a temperature (1500-2400K) at which the filament 102 thermionically emits electrons. The filament 102 is comprised of a high melting point metal wire 103 that is coated with a metal oxide 104. The filament 102 is connected to a current source to allow an electric current to be passed through the filament 102 to heat the filament 102 to a temperature at which electrons are emitted from the filament 102. As shown, the anode 105 is spaced apart from the filament 102 and generally disposed between the filament 102 and the ionization volume 106. As shown, the anode 105 may at least partially surround the ionization volume 106. Additional electrodes, such as one or more electron repellers 107, are provided and used to guide the electrons emitted from the filament 102. As shown, the electron repeller 107 is disposed radially outward from the filament 102 such that the filament 102 is disposed between the electron repeller 107 and the anode 105. The thermal electrons emitted from the filament 102 are guided by the electron repeller 107 and accelerated by a potential difference established between the filament 102 and the anode 105 of the ionization volume 106. The potentials present on the components of the ion source assembly 101 are established and maintained by a control unit (not shown) or control electronics (not shown). These potentials may also be adjusted via the control unit (not shown). The electron trajectories also depend on the shape of the anode and filament and their relative positions. Electrons emitted from the filament 102 pass through an aperture 105a defined on the anode 105 and into the ionization volume 106, where at least a portion of the electrons collide with molecules of the gas sample present in the ionization volume 106.The electrons have sufficient energy so that when they collide with gas molecules, they ionize and / or fragment the gas molecules to produce ions.

[0039] As shown in Figures 1 and 2, the prior art ion source assembly 101 further includes one or more optical elements 108 that define an ion outlet, which may be an aperture 108a or a grid. The one or more optical elements 108 establish a potential that acts to accelerate and guide ions generated in the ionization volume 106 into a mass filter 109. The mass filter 109 separates ions of various mass-to-charge ratios in space or time. The ions are then detected by an ion detector 110 and a mass spectrum is determined from the output of the ion detector 110. The ion detector 110 is in electrical communication with an interface 111 where the output of the ion detector 110 and / or the mass spectrum are displayed and / or recorded. In some embodiments of the ion source assembly 101, an ion repeller 112 is positioned upstream of the ionization volume 106 and is set to a specific potential to help control the trajectory of the generated ions.

[0040] 2, the filament 102 is a single filament extending in length from a first end to a second end, the first end of the filament 102 is connected to and supported by a first support member 211a, and the second end of the filament 102 is connected to and supported by a second support member 211b.

[0041] 3A-4 show one embodiment of a prior art dual filament ion source assembly 301. As shown, the single long filament 102 of the embodiment of FIG. 1 is replaced with two short filaments 302a, 302b. The first filament 302a extends a length from a first end connected to and supported by a first support member 311a to a second end. The second filament 302b extends a length from a first end connected to and supported by a second support member 311b. The second ends of both the first filament 302a and the second filament 302b are connected to and supported by a third or common support member 311c. The length of the first filament 302a and the length of the second filament 302b together may approximate the length of the single filament 102 of the embodiment of FIGS. 1 and 2. The first filament 302a and the second filament 302b are positioned with respect to a vertical plane V such that they are mirror images of each other. The vertical plane V extends along or is parallel to the optical axis A of the ion source assembly 301. This dual filament ion source has the advantage over a single filament version that the user can continue to operate the equipment after the first filament fails and know when to schedule maintenance, but does not have an increased overall lifespan for the reasons discussed above. The two filaments are roughly half the length of the single filament and each can withstand operation for significantly less time than half of the single filament.

[0042] As shown in the prior art ion source, the filaments are circular or form a circle together and are concentric with the cylindrical anode in order to generate a radial electric field between the filament and the anode that is of constant strength along the intermediate extension of the filament. Also, the intermediate extension of the filament 102, 302a, 302b is located at a specifically selected depth (D in FIG. 4) in the anode 105 or at a position along the optical axis A of the ion source assembly 301 relative to the anode 105. Most of the electrons emitted by the filament 102, 302a, 302b are emitted from the intermediate extension of the filament because this is the part of the filament where the temperature is highest. Positioning the electron source so that the intermediate extension of the filament is at or near the selected depth provides electron trajectories that ensure that ions are generated in the most favorable position for the performance of the ion source assembly in the ionization volume 106.

[0043] These prior art embodiments of the ion source assembly 101, 301 are provided to illustrate the general concept and operation of the ion source assembly. The ion source assembly 500 of the present invention will now be described with reference to FIGS. 5-9D. Some components of the ion source assembly 500 are similar or similar to components of the prior art ion source assemblies 101, 301 described above and therefore will not be described in detail or at all. The embodiments of the ion source assembly 500 described below are not limiting and other embodiments are contemplated and encompassed by the present disclosure. For example, the described embodiment of the ion source assembly 500 includes two electron sources, but other embodiments may include more than two electron sources.

[0044] As shown in Figures 5-7D, the ion source assembly 500 includes an upper section 502 and a lower section 501. The lower section 501 includes ion optical elements 518, which, when positioned downstream of the upper section 502, aid in the extraction of ions from the upper section 502 and the injection of ions into a mass filter (not shown), similar to prior art ion sources. The upper section 502 includes an ionization volume 510 (Figures 7A, 9A) surrounded by a cylindrical anode 503, around which a first filament 504 and a second filament 505 are disposed. The first filament 504 and the second filament 505 are supported in position relative to the anode 503 and electrically connected by mounting or support elements 506a-506c, screws 507, and insulators 511. The anode 503 must be at least partially transparent to the electrons emitted from the first filament 504 and the second filament 505. As shown, the anode 503 includes a cage structure that defines a number of openings 503a (FIG. 6B) for allowing electrons emitted from the first filament 504 and the second filament 505 to pass through the ionization volume 510. Surrounding the anode 503 and filaments 504, 505 is an electron repeller 508 that is generally concentric with the anode 503 and the first filament 504 and the second filament 505. All of the various components that make up the top section 502 of the ion source assembly 500 are held in relative position by a common attachment to a top plate 509.

[0045] 6A, each of the first filament 504 and the second filament 505 includes an elliptical central or mid-section or emission extension 540, 550 and generally straight end sections 542, 544, 552, 554 on either side of the mid-section 540, 550 and extending along the axis of the filament. The first filament 504 and the second filament 505 are positioned relative to the anode 503 such that their mid-sections 540, 550 are generally elliptical in shape and the end sections 542, 544, 552, 554 of each filament 504, 505 extend tangentially from the mid-section 540, 550. The combined elliptical structure of the first 504 and second 505 filaments allows the first 504 and second 505 filaments to be positioned such that the planes of the first 504 and second 505 filaments are angled with respect to the axis z of the anode (or optical axis A in FIG. 1 ) and maintain a constant distance between each filament 504, 505 and the anode 503 over the middle section 540, 550 of each filament 504, 505. The middle section 540, 550 of each filament 504, 505 is hotter than the rest of the filament during operation and is therefore where the majority of electrons are emitted. Furthermore, by positioning the filaments 504, 505 at an appropriately selected angle, selecting an elliptical shape for that angle, and including straight tangent sections for attaching the filaments to a post or other means, the filaments 504, 505 themselves can be significantly longer than the prior art dual filament designs and still have the same length of emitting middle section 540, 550 as the original single filament design. By positioning the filaments 504, 505 at an angle to the axis z of the anode, the mid-section 540, 550 of each filament 504, 505 can be positioned at the same depth D (FIG. 6C), resulting in consistent performance among multiple filaments 504, 505.

[0046] The angle α with respect to the xy plane perpendicular to the axis z of the anode, at which the plane Ep1 of the first filament 504 and the plane Ep2 of the second filament 505 are respectively situated, is ideally kept as small as possible while leaving sufficient clearance for mounting the filaments and avoiding electrical shorts between the filaments (except for the desired connection at a shared common connection location). The filaments are further situated to place the middle portion 540, 550 of each filament 504, 505 at the correct depth D (FIG. 6C) or position along the axis z of the anode, similar to the case of the single and double circular filaments of the prior art (FIG. 4D). By keeping the rotation angle small and keeping the center at the proper depth D within the anode, the electron trajectory starting from either the first filament 504 or the second filament 505 will most closely resemble that of a single circular filament embodiment, resulting in similar mass spectrometer performance. Furthermore, because the length of each filament 504, 505 approaches that of a single circular filament of the prior art, the lifetime of each filament 504, 505 will approach that of a single circular filament. As a result, the operating time of the mass spectrometer between required maintenance procedures can be substantially doubled compared to prior art single filament ion sources and more than doubled compared to prior art dual filament ion sources.

[0047] 6A-6D show various views of the arrangement of the first filament 504 and the second filament 505 of the ion source assembly 500. Specifically, FIGS. 6A-6D show a first example of the arrangement of the filaments 504, 505 arranged relative to the anode 503. (Some components of the upper portion 502 of the ion source assembly 500 are omitted in FIGS. 6A-9D for clarity.) As specifically shown in FIG. 6C, a true ellipse is defined by the angle α between the planes Ep1, Ep2 of the filaments and the xy plane perpendicular to the axis z of the anode. In this embodiment, the planes Ep1, Ep2 of the two filaments are inclined in opposite directions as shown in FIG. 6C, and each filament 504, 505 is rotated about its center such that one end of each filament is displaced in the +Z direction and the other end is displaced in the -Z direction, and rotated about an axis L that passes through the center of the filament and intersects both the filament and the Z axis at 90°. 6A, the radial distance from the filaments 504, 505 to the anode 503 is constant across the elliptical central sections 540, 550 of the filaments 504, 505. The ellipse of the filament is essentially the intersection of a cylinder that is the radius of the anode 503 plus the desired spacing between the filament and the anode, with a plane that makes an angle (π / 2-α) from the axis of the cylinder.

[0048] The filament arrangement shown in Figures 6A-7D is rotationally symmetric. In other words, the filaments 504, 505 are the same but rotated 180° about the z-axis relative to each other. This rotationally symmetric shape may potentially perform better than other embodiments because the angle of rotation of the filaments from the vertical plane xy is reduced, resulting in electron emission occurring at a narrower range of depths into the anode. Another embodiment may include additional filaments that are further copies of the filaments 504, 505, each rotated at an angle relative to each other and to the filaments 504, 505. The angles of rotation about the z-axis and from the xy plane may be selected to increase the ease and efficiency of construction of the ion source assembly 500. Figures 7A-7D correspond to the embodiment of the filament arrangement shown in Figures 6A-6D, but further including filament supports 506a-506c. As shown, there are three filament supports 506a-506c, where a first filament support 506a secures a first end of a first filament 504 and a third filament support 506c secures a first end of a second filament 505. A second filament 506b secures a second end of both the first filament 504 and the second filament 504. The filament supports 506a-506c are secured to a top plate 509 and may include a variety of shapes and structures for supporting and holding the first filament 504 and the second filament 505 relative to the anode 503.

[0049] 8A-8D and 9A-9D show another embodiment in which a first filament 504 rotates about an axis of rotation relative to a second filament 505. The first filament 504 and the second filament 505 in FIGS. 8 and 9 are tilted with respect to their respective centers such that both ends of the first filament 504 move in the +z direction and both ends of the second filament 505 move in the -z direction by an angle β. In other words, the axes of rotation of the filaments L' and L'' in the second embodiment are tangential to the filaments 504, 505 at their respective hot spots and lie in the xy plane.

[0050] In the two embodiments, the two filaments 504, 505 are shown tilted by equal and opposite angles α, β, although the angles need not be equal. However, it is often desirable to maintain physical symmetry to obtain consistent performance when switching filaments. Also note that the distance of the filament emission region (540, 550) of each filament 504, 505 from the anode 503 is constant, as is the depth D of each filament emission region into the anode 103. Furthermore, the location of the overall filament emission region remains largely unchanged compared to the single filament embodiment.

[0051] The above-described embodiment of the ion source assembly 500 has many advantages over the prior art. For example, by having the intermediate or emitting extensions 540, 550 of the filaments 504, 505 be elliptical, and by mounting the filaments 504, 505 to maintain a constant distance between the intermediate portions 540, 550 of the filaments 504, 505 and the anode 503, the electric field between the filaments 504, 505 and the anode 503 is radial (with respect to the anode) and has a constant strength along the emitting extensions 540, 550 of the filaments 504, 505 (as would be the case if a single circular filament were used). Furthermore, locating the vertices W (FIG. 9A) of the two filaments at the same position (same depth D) along the axis A of the anode means that the majority of ions are generated at a similar position in this direction within the ionization volume 510, regardless of which filament 504, 505 the ionizing electrons are generated from. Thus, the generated ions will experience similar extraction and focusing fields for injection into the mass filter regardless of which filament 504, 505 is used.

[0052] Finally, the filament length at which significant electron emission occurs in prior art ion source assemblies containing two approximately semicircular filaments located in a common plane is shorter than that used in the disclosed elliptical configuration when the same filament wire diameter is used at the same (optimum) filament-to-anode distance. These long filaments have a longer heat emission section because the thermal conductivity per unit length of the filament depends on the material and cross-sectional area. For a given total emission, these long filaments operate at a lower temperature than circular short filaments because the emission increases with increasing temperature (Richardson's equation) because the electron emission per unit area depends strongly and monotonically on temperature. These long filaments have improved lifetime because the filament lifetime is inversely proportional to the operating temperature and the temperature gradient (spatial and temporal).

[0053] Although the present invention has been particularly shown and described with reference to certain exemplary embodiments, those skilled in the art will understand that various changes in detail can be made therein without departing from the spirit and scope of the invention as supported by the written description and drawings. Moreover, when an exemplary embodiment is described with reference to a particular number of elements, it will be understood that the exemplary embodiment can be practiced utilizing fewer or more than the particular number of elements.

Claims

1. 1. An electron impact ion source assembly for a mass spectrometer, comprising: a cylindrical anode extending along an axis and surrounding an ionization volume; a first filament and a second filament, each configured to emit thermionic electrons, disposed outside the ionization volume and proximate to the cylindrical anode; each of the first filament and the second filament includes an elliptical central extension and non-elliptical extensions on either side of the elliptical central extension; the non-elliptical extension is configured to be mounted in a fixed position relative to the cylindrical anode to maintain a constant distance between the elliptical central extension and the cylindrical anode; An electron impact ion source assembly, wherein each elliptical central extension extends along a plane that intersects a plane perpendicular to the axis of the cylindrical anode at a non-zero angle.

2. 2. The electron impact ion source assembly of claim 1, wherein each elliptical central extension includes an apex, and the first and second filaments are positioned such that their respective apexes are at the same depth relative to the cylindrical anode.

3. 3. The electron impact ion source assembly of claim 2, wherein each non-zero angle is obtained by rotating the first and second filaments about an axis of rotation passing through the vertex of each of the first and second filaments.

4. 2. The electron impact ion source assembly of claim 1, wherein the first filament rotates about a first axis of rotation and the second filament rotates about a second axis of rotation, the first and second axes of rotation extending along a common plane perpendicular to the axis of the cylindrical anode, and the first and second filaments rotate equally and in opposite directions such that the first and second filaments extend along parallel planes.

5. 4. The electron impact ion source assembly of claim 3, wherein said axis of rotation intersects said axis of said cylindrical anode at an angle of 90 degrees.

6. 6. The electron impact ion source assembly of claim 5, wherein the second filament is identical to the first filament, and the second filament is rotated 180 degrees about the axis of the cylindrical anode relative to the first filament.

7. Further comprising a third filament, the third filament comprising: an elliptical central extension including an apex; and non-elliptical extensions at either end of said elliptical central extension. the elliptical central extension extends along a plane that intersects the axis of the cylindrical anode at a non-zero angle; 2. The electron impact ion source assembly of claim 1, wherein the third filament is positioned such that the elliptical central extension is at a fixed distance from the cylindrical anode, and the apex of the third filament is at approximately the same depth as the apexes of the first and second filaments.

8. 8. The electron impact ion source assembly of claim 7, wherein the non-zero angle is obtained by rotating the third filament about an axis of rotation that passes through the apex of the third filament and intersects the axis of the cylindrical anode at 90 degrees.

9. 9. The electron impact ion source assembly of claim 8, wherein the third filament is identical to the first and second filaments and rotates relative to the first and second filaments about the axis of the cylindrical anode.

10. The electron impact ion source assembly of claim 1 , wherein at least one of the first filament and the second filament is coated with a metal oxide.

11. 1. An electron impact ion source assembly for a mass spectrometer, comprising: an anode extending along an axis and surrounding the ionization volume; at least two filaments, each configured to emit thermionic electrons, disposed outside the ionization volume and proximate to the anode; each of the at least two filaments includes an elliptical portion and a non-elliptical portion at each end of the elliptical portion; the non-elliptical portion is configured to be mounted in a fixed position relative to the anode to maintain a constant distance between the elliptical portion and the anode; The elliptical portion extends along a plane that intersects a plane perpendicular to the axis of the anode at a non-zero angle.

12. 12. The electron impact ion source assembly of claim 11, wherein each elliptical portion includes an apex, and the at least two filaments are positioned such that their respective apexes are at the same depth relative to the anode.

13. 13. The electron impact ion source assembly of claim 12, wherein each non-zero angle is produced by rotating the at least two filaments about an axis of rotation passing through the vertex of each of the at least two filaments.

14. The electron impact ion source assembly of claim 11 , wherein the at least two filaments extend along parallel planes.

15. 14. The electron impact ion source assembly of claim 13, wherein the axis of rotation intersects the axis of the anode at an angle of 90 degrees.

16. 12. The electron impact ion source assembly of claim 11, wherein the at least two filaments are identical to each other and rotate at an angle about the axis of the anode relative to each other.

17. The electron impact ion source assembly of claim 11 , wherein at least one of the at least two filaments is coated with a metal oxide.

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