Ion source assembly with multiple elliptical filaments

Abstract

An electron impact ion source assembly for use in a mass spectrometer includes an anode extending along an axis and surrounding an ionization volume. The at least two filaments are each configured to thermionically emit electrons and are positioned outside the ionization volume and proximate the anode. The at least two filaments each include an elliptically shaped portion and non-elliptically shaped portions on both ends of the elliptically shaped portion. The non-elliptically shaped portion is configured to be mounted in a fixed position relative to the anode to maintain a constant distance between the elliptically shaped portion and the anode. The elliptically shaped portion extends along a plane that intersects a plane perpendicular to an axis of the anode at a non-zero angle.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This is a non-provisional application of and claims the benefit of and priority to U.S. Provisional Patent Application No. 63 / 290,436, filed on December 16, 2021. The entire contents of said application are hereby incorporated by reference. Technical Field

[0003] The present disclosure relates to an ion source assembly having a plurality of filaments, wherein each filament has an elliptically shaped center length. Background Art

[0004] Mass spectrometry is a common analytical technique for measuring the mass-to-charge ratio of ions in a sample in order to determine the chemical composition of the sample. Typically, mass spectrometry requires ionizing the 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 the mass-to-charge ratio.

[0005] An electron impact ionization source (also known as an electron ionization (EI) ion source) can be used to ionize a sample (particularly a gaseous sample). The EI ion source includes an electron source, which can be a filament heated to a temperature that emits electrons. The filament used can be a thin wire composed of a refractory metal, which is uncoated or coated with a metal oxide. The heating of the filament can be performed resistively by passing an electric current through the filament. Thermionically emitted electrons from the filament are accelerated through the wall or anode and enter the ionization volume. The movement of electrons is guided by an electric field and the geometric shape and positional relationship between these parts, and the electric field is generated by the potential difference maintained between the filament and the anode and possibly other electrodes by means of a control unit. The anode defines at least one opening that enables a certain percentage of electrons to pass through the anode and enter the ionization volume. It is generally desirable to maintain the filament temperature and various potentials so that a constant electron emission current enters the ionization volume. Additional electrodes (such as electron repellers) can be included to manipulate electrons. Inside the ionization volume, at least some of the accelerated or high-energy electrons will collide with molecules of the gas sample in the ionization volume. The electrons have sufficient energy that upon collision with the gas molecules, they will ionize and / or fragment the gas molecules to produce ions.

[0006] These ions are then accelerated and manipulated into a mass filter by means of other potentials established using ion optical elements (which are part of the ion source). Some ion sources also include an ion repeller positioned upstream of the ionization volume. The ion repeller can be set to a specific potential to help control the trajectory of the ions generated in the ionization volume. The ion repeller can be a flat or flat electrode, or it can be concave in the direction toward the mass filter. When ions with various mass-to-charge ratios arrive at the mass filter, they are separated in space or time. These ions are then detected by an ion detector, and a mass spectrum is determined based on the output of the ion detector.

[0007] The filament of the EI ion source has a limited service life. In order to emit a sufficient number of electrons, the filament must be operated at a temperature between 1500-2400K. At these high temperatures, the filament wire (and the coating, if present) eventually evaporates, resulting in filament failure. Filament failure may also occur due to changes in the crystal structure of the filament wire that occur at high operating temperatures. In addition, the electron emission surface of the filament may be chemically changed by the gas in the system, which increases the work function of the electron emission surface while reducing the electron emission efficiency of the electron emission surface. When there are no electrons or not enough electrons available to ionize the gas sample due to filament breakage, deformation or chemical "poisoning", the mass spectrometer no longer operates satisfactorily. Therefore, the process monitored and / or controlled based on the data generated by the mass spectrometer will have to be stopped, otherwise it will "run blindly" until there is an opportunity to replace the filament. Replacing the filament is a time-consuming and inconvenient process because the filament is usually located inside the process vacuum chamber, so that the process vacuum chamber must be vented to implement this replacement. Therefore, it is desirable to reduce the frequency of filament replacement, and even more desirable to be able to schedule when proactive filament replacement occurs so the filament can be replaced while the process chamber is offline for other maintenance activities.

[0008] A common method to solve this shortcoming of the EI ion source is to include a second filament in the ion source, which is positioned near the anode and can be put into operation when the first filament fails. The two filaments are usually copies of each other and are mirrored about the plane extending along the ion optical axis of the ion source. Positioning the filament in this way is to try to maintain the consistent performance of the mass spectrometer by making it possible to form ions in the same area of ​​the ionization volume regardless of which filament is used. This ensures that ions are generated at the following positions: the position where the electric field can manipulate the ions so that the ions are successfully injected into the mass filter, and the position where the electric field is high enough to overcome the effect of space charge on sensitivity. However, a disadvantage of this type of EI ion source is that the available space near the anode is usually limited, so each of the two filaments is shorter than the filament used when there is only a single filament. The relationship between the electron emission current density leaving the electron emission surface and the temperature and work function of the surface is described by the Richardson equation as a monotonically increasing function of temperature. Since the total electron emission current depends on the area of ​​the emission surface, in order to obtain the same total emission current, the shorter filament must be operated at a higher temperature. Thus, two shorter filaments running sequentially do not last twice as long as one long filament. In fact, the combined operating life of the two short filaments may not even be as good as the operating life of a single filament of "normal" length. Furthermore, a shorter filament necessarily loses more heat in its mounting arrangement than a longer filament. This heat loss is due to the lower thermal resistance provided by the shorter path along the wire from the central region of the filament to the mounting point compared to a longer single filament. Therefore, in order to keep the total electron emission at the desired level, a higher temperature is required at the hottest part of the filament (near the center), which shortens the operating life of the filament.

[0009] Two key properties of an ion source are sensitivity (the number of ions produced and injected into the mass filter at an acceptable rate per unit pressure) and linearity (the degree to which the sensitivity is independent of pressure). These properties cannot be completely ignored when trying to extend filament life. For example, the emission current and / or operating pressure can be reduced in order to reduce the filament temperature and, therefore, extend filament life. However, reducing the emission current or operating pressure is done at the expense of sensitivity and / or ion current.

[0010] These are just some of the disadvantages associated with the ion sources currently used in mass spectrometers. Summary of the invention

[0011] An embodiment of an ion source assembly for use in a mass spectrometer includes a cylindrical anode structure that at least partially defines an ionization volume. At least two filaments are located near the anode and act as electron sources for ionizing the sample being analyzed by electron bombardment. The shape of the filament in its central portion is approximately elliptical, with a straight end section tangent to the elliptical portion. Two embodiments are described herein, however, the ion source assembly of the present invention is not limited to the two embodiments described. In one embodiment, the filaments can be installed so as to be parallel to each other, or alternatively, they can be installed as copies of each other, rotated 180° around the central axis of the anode structure. The filaments are mounted to a support member at multiple positions on the straight end sections, and are positioned so that the central elliptical portion is a fixed distance from the cylindrical anode structure. The filaments are also installed so that one end of the semi-major axis of the approximately elliptical portion of each of the filaments is positioned at a similar position in the direction of the axis of the anode cylinder.

[0012] An embodiment of an electron bombardment ion source assembly for use in a mass spectrometer includes: a cylindrical anode extending along an axis and surrounding an ionization volume; and a first filament and a second filament each emitting electrons thermionically. The first filament and the second filament are positioned outside the ionization volume and proximate to the cylindrical anode. The first filament and the second filament each include an elliptically shaped central length and non-elliptically shaped lengths at both ends of the elliptically shaped central length. The non-elliptically shaped lengths are structured to be mounted in a fixed position relative to the cylindrical anode to maintain a constant distance between the elliptically shaped central length and the cylindrical anode. Each elliptically shaped central length extends along a plane that intersects a plane perpendicular to the axis of the cylindrical anode at a non-zero angle.

[0013] Another embodiment of an electron bombardment ion source assembly for use in a mass spectrometer includes: an anode extending along an axis and surrounding an ionization volume; and at least two filaments that each emit electrons thermionically. The at least two filaments are positioned outside the ionization volume and close to the anode. The at least two filaments each include an elliptically shaped portion and non-elliptically shaped portions on both ends of the elliptically shaped portion. The non-elliptically shaped portion is structured to be mounted in a fixed position relative to the anode to maintain a constant distance between the elliptically shaped portion and the anode. Each elliptically shaped portion extends along a plane that intersects a plane perpendicular to the axis of the anode at a non-zero angle.

[0014] In an embodiment, each elliptically shaped portion includes a vertex, and the at least two filaments are each positioned so that each vertex is at the same depth relative to the anode. In an embodiment, each non-zero angle is caused by the rotation of the at least two filaments about a rotation axis extending through the vertex of each of the at least two filaments. In an embodiment, the at least two filaments extend along parallel planes. In an embodiment, the rotation axis intersects the axis of the anode at an angle of 90°. In another embodiment, the at least two filaments are images of each other, rotated through a certain angle around 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

[0015] The invention briefly outlined above may be described in more detail by reference to embodiments, some of which are illustrated in the accompanying drawings. However, it is worth noting that the accompanying drawings illustrate only common embodiments of the invention and are therefore not to be considered as limiting the scope of the invention, as the invention may admit of other embodiments with the same effect. Therefore, to further understand the nature and purpose of the invention, reference may be made to the following specific embodiments understood in conjunction with the accompanying drawings, in which:

[0016] Figure 1 schematically illustrates a cross-sectional view of an embodiment of a prior art ion source assembly for use in a mass spectrometer;

[0017] Figure 2 illustrates a perspective view of an embodiment of a prior art ion source assembly for use in a mass spectrometer;

[0018] Figure 3A A top perspective view of an embodiment of a prior art dual filament ion source assembly for use in a mass spectrometer is illustrated;

[0019] Figure 3B A side perspective view of an embodiment of a prior art dual filament ion source for use in a mass spectrometer is shown;

[0020] Figure 4 Schematically illustrated Figure 3B A cross-sectional view of an embodiment of the present invention;

[0021] Figure 5 illustrates a side perspective view of an embodiment of an ion source assembly according to the present disclosure for use in a mass spectrometer;

[0022] Fig. 6A illustrates a bottom plan view of an embodiment of a plurality of filaments positioned around an anode of an embodiment of an ion source assembly according to the present disclosure;

[0023] Figure 6B Diagram Fig. 6A A perspective side view of an embodiment of;

[0024] Figure 6C Diagram Fig. 6A - a front view of an embodiment of B;

[0025] Fig.6D Diagram Fig. 6A - A front view of an embodiment of C, which specifically shows an exemplary orientation of the ends of multiple filaments relative to each other;

[0026] Fig. 7A Diagram Fig. 6A An embodiment of the invention further comprising an embodiment of a mounting element for holding and mounting the filament in position relative to the anode;

[0027] Figure 7B Diagram Figure 6B An embodiment comprising Fig. 7A Embodiments of mounting elements;

[0028] Figure 7C Diagram Figure 6C An embodiment comprising Fig. 7A - an embodiment of the mounting element of B;

[0029] Fig.7D Diagram Fig.6D An embodiment comprising Fig. 7A - Embodiments of mounting elements of C;

[0030] Fig. 8A A bottom plan view illustrating another embodiment of a plurality of filaments positioned about an anode of an embodiment of an ion source assembly according to the present disclosure;

[0031] Figure 8B Diagram Fig. 8A A perspective side view of an embodiment of;

[0032] Figure 8C Diagram Fig. 8A - a front plan view of an embodiment of B;

[0033] Fig.8D Diagram Fig. 8A - a side plan view of an embodiment of -C, which specifically illustrates another exemplary orientation of the ends of the plurality of filaments relative to each other;

[0034] Fig.9A Diagram Fig. 8A an embodiment of the invention further comprising another embodiment of a mounting element for holding and mounting the filament in position relative to the anode;

[0035] Fig. 9B Diagram Figure 8B An embodiment comprising Fig.9A Embodiments of mounting elements;

[0036] Fig. 9C Graphics Figure 8C An embodiment comprising Fig.9A - an embodiment of the mounting element of B; and

[0037] Fig.9D Graphics Fig.8D An embodiment comprising Fig.9A - Example of a mounting element of C. DETAILED DESCRIPTION

[0038] The following discussion relates to various embodiments of an ion source assembly having multiple elliptical filaments. It will be understood that the versions described herein are examples that embody certain inventive concepts as detailed herein. To this end, other variations and modifications will be apparent to those skilled in the art. In addition, certain terms are used throughout this discussion to provide a suitable frame of reference with respect to the accompanying drawings. Unless otherwise expressly stated, these terms (such as "upper," "lower," "outward," "inward," "top," "bottom," "first," "second," etc.) are not intended to limit these concepts. The terms "about" or "approximately" as used herein may refer to a range of 80%–125% of a claimed or disclosed value. With respect to the accompanying drawings, their purpose is to depict the salient features of an ion source assembly having multiple elliptical filaments and are not set to a particular scale.

[0039] refer to Figure 1 and Figure 2, a portion of a mass spectrometer 100 is shown, which includes an electron ionization (EI) ion source assembly 101. The EI ion source assembly (ion source assembly) 101 includes an electron source 102, an anode 105 that at least partially surrounds or defines 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 thermally emits electrons. The filament 102 can be composed of a refractory metal wire 103, which can be coated with a metal oxide 104. The filament 102 can be connected to a current source so that current can pass through the filament 102 so as 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 is generally positioned between the filament 102 and the ionization volume 106. As shown, the anode 105 can at least partially surround the ionization volume 106. Additional electrodes (such as one or more electron repellers 107) can be included and used to manipulate the electrons emitted from the filament 102. As shown, the electron repeller 107 is positioned radially outward from the filament 102 so that the filament 102 is positioned between the electron repeller 107 and the anode 105. Thermionically emitted electrons from the filament 102 can be manipulated by the electron repeller 107 and accelerated by the 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 can also be adjusted via a control unit (not shown). The trajectory of the electrons also depends on the geometry of the anode and the filament and their relative positions. Electrons emitted from the filament 102 pass through an opening 105a defined in the anode 105 and enter the ionization volume 106, where at least some of the electrons will collide with molecules of the gas sample present in the ionization volume 106. The electrons have sufficient energy that upon collision with gas molecules they will ionize and / or fragment the gas molecules to produce ions.

[0040] like Figure 1 and Figure 2As shown in the prior art ion source assembly 101, the prior art ion source assembly 101 also includes one or more optical elements 108, which define an ion exit port, which can be an orifice 108a or a grid as shown in the figure. The one or more optical elements 108 establish an electrical potential, which is used to accelerate and manipulate the ions generated in the ionization volume 106 to enter the mass filter 109. The mass filter 109 separates ions with various mass-to-charge ratios in space or time. Then, the ion detector 110 detects these ions, and the mass spectrum is determined based on the output of the ion detector 110. The ion detector 110 can be in electrical communication with an interface 111 on which the output and / or mass spectrum of the ion detector 110 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 electrical potential to help control the trajectory of the generated ions.

[0041] refer to Figure 2 The filament 102 is a single filament extending a certain length from the first end to the second end. The first end of the filament 102 is connected to and supported by the first support member 211a, and the second end of the filament 102 is connected to and supported by the second support member 211b.

[0042] Figure 3A Figure 4 shows an embodiment of a prior art dual filament ion source assembly 301. As can be seen from the figure, the two shorter filaments 302a and 302b have been replaced. Figure 1 The single long filament 102 of the embodiment of the present invention. The first filament 302a extends a certain length from the first end connected to and supported by the first support member 311a to the second end. The second filament 302b extends a certain length from the first end connected to and supported by the second support member 311b to the second end. The second ends of the first filament 302a and the second filament 302b are both connected to and supported by the third or common support member 311c. The length of the first filament 302a and the length of the second filament 302b added together can be close to Figure 1 and Figure 2 The length of a single filament 102 of an embodiment of the present invention. The first filament 302a and the second filament 302b are positioned relative to a vertical plane V extending along or parallel to the optical axis A of the ion source assembly 301 so that they are mirror images of each other. While this dual filament ion source does have some advantages over the single filament version in that the user can continue to operate the instrument after the failure of the first filament and know to schedule maintenance, it does not have an extended overall life for the reasons described above. The length of the two filaments is approximately half the length of a single filament, and the operating life of each filament is significantly less than half the time of the operating life of a single filament.

[0043] As shown in prior art ion sources, the filament is circular or co-formed with a circular shape and is concentric with a cylindrical anode so as to generate a radial electric field between the filament and the anode that has a constant magnitude along the mid-length of the filament. In addition, the mid-length of the filament 102, 302a, 302b is located at a specifically selected depth ( 102 ) in the anode 105 relative to the anode 105 along the optical axis A of the ion source assembly 301. Figure 4 The majority of electrons emitted by the filament 102, 302a, 302b are emitted from the middle length of the filament because this is where the highest filament temperature exists. Positioning the electron source so that the middle length of the filament is at or near the selected depth provides electron trajectories that ensure ions are produced in the most favorable locations within the ionization volume 106 with respect to ion source component performance.

[0044] These prior art embodiments of ion source assemblies 101, 301 have been provided to explain the general concepts and operation of ion source assemblies. Figure 5 –9D describes the present invention ion source assembly 500 (ion source assembly). Some components of the ion source assembly 500 are similar or identical to components of the prior art ion source assemblies 101, 301 described previously and will not be explained in detail, or at all. The embodiments of the ion source assembly 500 described below are not limiting, as 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, however, other embodiments may include more than two electron sources.

[0045] refer to Figure 5 –7D, ion source assembly 500 includes an upper section 502 and a lower section 501. Lower section 501 includes ion optical element 518, which, when placed downstream of upper section 502, helps extract ions from upper section 502 and injects these ions into a mass filter (not shown), similar to the ion source of the prior art. Upper section 502 includes ionization volume 510 (enclosed in cylindrical anode 503) Fig. 7A , Fig.9A ), a first filament 504 and a second filament 505 are arranged around a cylindrical anode 503. The first filament 504 and the second filament 505 are supported in position relative to the anode 503 and are electrically connected by mounting elements or support elements 506a-c and screws 507 and an insulator 511. The anode 503 must be at least partially transparent to electrons leaving the first filament 504 and the second filament 505. As shown in the figure, the anode 503 includes a plurality of openings 503a ( Figure 6B) of the cage structure, the opening 503a enables the electrons emitted by the first filament 504 and the second filament 505 to enter the ionization volume 510. The electron repeller 508 surrounds the anode 503 and the filaments 504, 505, and is approximately concentric with the anode 503 and the first filament 504 and the second filament 505. The various components comprising the upper section 502 of the ion source assembly 500 are held in relative position by being mounted together to the upper plate 509.

[0046] like Fig. 6A As specifically shown in FIG. 1 , the first filament 504 and the second filament 505 each include a central elliptical section or middle section or emission length 540, 550 and approximately straight end sections 542, 544, 552, 554 on either side of the middle section 540, 550 and extending along the filament axis. The first filament 504 and the second filament 505 are positioned relative to the anode 503 such that the middle section 540, 550 thereof forms an approximately elliptical shape, wherein the end sections 542, 544, 552, 554 of the filaments 504, 505 each extend tangentially from the middle section 540, 550. The combined elliptical configuration of the first filament 504 and the second filament 505 enables them to be mounted such that the planes of the first filament 504 and the second filament 505 are relative to the anode axis z (or Figure 1 The filaments 504, 505 are angled to the optical axis A in FIG. 1 , and a constant distance between each filament 504, 505 and the anode 503 is still maintained at the middle section 540, 550 of each filament 504, 505. The middle section 540, 550 of each filament 504, 505 is where the vast majority of electrons are emitted because, in operation, it is hotter than other parts of the filament. Furthermore, by mounting the filaments 504, 505 at a carefully selected angle, selecting an elliptical shape that matches the angle, and including a straight tangential section by which the filament is mounted to a support post or other device, the filaments 504, 505 themselves can be much longer than prior art dual filament designs, and even have an emitting central section 540, 550 that is as long as the original single filament design. By positioning the filaments 504, 505 at an angle relative to the anode axis z, the middle section 540, 550 of each filament 504, 505 can be positioned at the same depth D ( Figure 6C ) at a location so as to provide consistent performance between the multiple filaments 504, 505.

[0047] The angle α of the plane Ep1 and the plane Ep2 of the first filament 504 and the second filament 505, respectively, mounted relative to the xy plane perpendicular to the anode axis z is ideally kept as small as possible while still leaving enough clearance to mount the filaments and avoid electrical shorting between the filaments (except for the desired connection where they share a common connection). The filaments are also mounted so that the middle portion 540, 550 of each filament 504, 505 is placed at the correct depth D along the anode axis z ( Figure 6C ) or position, as is done in the case of single circular filament and double circular filament of the prior art ( Figure 4 By keeping the rotation angle small and centered at an appropriate depth D in the anode, the electron trajectories starting from the first filament 504 or the second filament 505 will be very similar to those of the circular single filament embodiment, and similar mass spectrometer performance will result. In addition, because the length of each filament 504, 505 is close to the length of a single circular filament of the prior art, the life of each filament 504, 505 will be close to the life of a single circular filament. Therefore, the operating time of the mass spectrometer between required maintenance actions can be effectively doubled compared to the single filament ion source of the prior art, and more than doubled compared to the dual filament ion source of the prior art.

[0048] Fig. 6A -D shows multiple views of the arrangement of the first filament 504 and the second filament 505 of the ion source assembly 500. In particular, Fig. 6A -D shows a first example of an arrangement of filaments 504, 505 positioned relative to anode 503. Fig. 6A In FIG. 9D , some components of the upper portion 502 of the ion source assembly 500 are omitted for clarity. Figure 6C The angle α between the planes Ep1 and Ep2 of the filaments and the xy plane perpendicular to the anode axis z defines a precise ellipse. In this embodiment, the planes Ep1 and Ep2 of the two filaments are inclined in opposite directions (e.g. Figure 6C ), and each filament 504, 505 rotates around its center by a rotation about an axis L that passes through the center of the filament and intersects the two filaments and the z-axis at 90°, so that one end of each filament is displaced in the +z direction and the other end is displaced in the -z direction. Fig. 6A As can be seen in the figure, the radial distance from the filaments 504, 505 to the anode 503 is constant across the central elliptical sections 540, 550 of the filaments 504, 505. The elliptical shape of the filaments is essentially the intersection of a cylinder with a radius equal to the radius of the anode 503 plus the desired filament-to-anode spacing, and the plane is at an angle to the cylinder axis.

[0049] Fig. 6AThe filament arrangement shown in FIG. 7D is rotationally symmetric. In other words, the filaments 504, 505 are identical, but are rotated 180° relative to each other about the z-axis. In some cases, this rotationally symmetric geometry can improve performance over other embodiments because it provides for a smaller rotation angle of the filaments from the vertical plane xy, and therefore electron emission occurs over a smaller depth range in the anode. In another embodiment, additional filaments can be included, which would be additional copies of the filaments 504, 505, and each rotated a certain angle relative to each other and the filaments 504, 505. The rotation angles about z and out of the xy plane can be selected to increase the ease of construction and efficiency of the ion source assembly 500. Fig. 7A –D corresponds to Fig. 6A -D, but also includes filament supports 506a-c. As shown, there are three (3) filament supports 506a-c, such that 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 both the first filament 504 and the second filament 504. The filament supports 506a-c are anchored to an upper plate 509 and may include a variety of shapes and configurations to support and hold the first filament 504 and the second filament 505 relative to the anode 503.

[0050] Fig. 8A –D and Fig.9A -D shows another embodiment, in which the first filament 504 rotates about the rotation axis relative to the second filament 505. The first filament 504 and the second filament 505 of Figures 8 and 9 are tilted about their respective centers so that both ends of the first filament 504 move in the +z direction, while both ends of the second filament 505 move in the -z direction by a certain angle β. In other words, the filament rotation axes L' and L" of the second embodiment are tangent to the filaments 504, 505 at their respective hot spots and are located in the xy plane.

[0051] Although both embodiments show two filaments 504, 505 tilted equally and at relative angles α, β, the angles are not necessarily equal. However, it is generally desirable to maintain physical symmetry to help with consistent performance when switching between filaments. It should also be noted 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. In addition, the position of the entire filament emission region varies very little compared to the single filament embodiment.

[0052] The foregoing embodiments of the ion source assembly 500 have many advantages over the prior art. For example, the elliptical shape of the middle portion or emission length 540, 550 of the filaments 504, 505 and the mounting of the filaments 504, 505 to maintain a constant distance from the middle portion 540, 550 of the filaments 504, 505 to the anode 503 cause the electric field between the filaments 504, 505 and the anode 503 to be radial (relative to the anode) and have a constant magnitude along the emission length 540, 550 of the filaments 504, 505 (similar to what is seen when using a single circular filament). In addition, the apex W ( Fig.9A ) are positioned at the same position (same depth D) along the anode axis A, meaning that most ions will be generated at similar positions along this direction inside the ionization volume 510, regardless of which filament 504, 505 the ionizing electrons originate from. Therefore, regardless of which filament 504, 505 is used, the generated ions will experience similar extraction and focusing fields for injection into the mass filter.

[0053] Finally, the filament lengths for which significant electron emission occurs for prior art ion source assemblies comprising two nearly semicircular filaments located in a common plane are less than the filament lengths used in the disclosed elliptical configuration when the same filament wire diameter is used with the same (optimal) filament-to-anode distance. Since the thermal conductivity per unit length of the filament depends on the material and the cross-sectional area, these longer filaments will have a proportionally longer thermal emission segment. Since the electron emission per unit area depends strongly and monotonically on temperature, emission increases with increasing temperature (Richardson equation), these longer filaments will operate at lower temperatures than their circular shorter counterparts for a given total emission. Since the filament lifetime is inversely proportional to the operating temperature and the temperature gradient (spatial and temporal), these longer filaments have an improved lifetime.

[0054] Although the present invention has been specifically shown and described with reference to certain exemplary embodiments, it will be appreciated by those skilled in the art that various changes in detail may be implemented therein without departing from the spirit and scope of the present invention as may be supported by the written description and the accompanying drawings. In addition, where exemplary embodiments are described with reference to a certain number of elements, it will be appreciated that these exemplary embodiments may be practiced with or less than or more than the certain number of elements.

Claims

1. An electron bombardment ion source assembly for use in a mass spectrometer, comprising: a cylindrical anode extending along an axis and surrounding an ionization volume; as well as a first filament and a second filament, the first filament and the second filament each being configured to thermionically emit electrons, wherein the first filament and the second filament are positioned outside the ionization volume and proximate to the cylindrical anode; wherein the first filament and the second filament each include an elliptically shaped central length and non-elliptically shaped lengths at both ends of the elliptically shaped central length, wherein the non-elliptical length is configured to be mounted in a fixed position relative to the cylindrical anode to maintain a constant distance between the elliptically shaped center length and the cylindrical anode, and Therein, each elliptically shaped central length extends along a plane that intersects a plane perpendicular to the axis of the cylindrical anode at a non-zero angle.

2. The electron bombardment ion source assembly according to claim 1, wherein: Each elliptically shaped central length includes an apex, wherein the first filament and the second filament are positioned such that each apex is at a same depth relative to the cylindrical anode.

3. The electron bombardment ion source assembly according to claim 2, wherein: Each non-zero angle is a result of rotation of the first filament and the second filament about an axis of rotation extending through a vertex of each of the first filament and the second filament.

4. The electron bombardment ion source assembly according to claim 1, wherein: The rotation of the first filament is about a first rotation axis and the second filament is about a second rotation axis, wherein the first rotation axis and the second rotation axis extend along a common plane perpendicular to the axis of the cylindrical anode, and wherein equal but relative rotation of the first filament and the second filament causes the first filament and the second filament to extend along parallel planes.

5. The electron bombardment ion source assembly according to claim 3, wherein: The rotation axis intersects the axis of the cylindrical anode at an angle of 90°.

6. The electron bombardment ion source assembly according to claim 5, wherein: The second filament is identical to the first filament, and wherein the second filament is rotated 180° relative to the first filament around the axis of the cylindrical anode.

7. The electron bombardment ion source assembly according to claim 1, further comprising a third filament, wherein the third filament comprises: the center length of the elliptically shaped portion including the vertex, and non-elliptical lengths at both ends of the elliptical shaped central length, wherein The elliptically shaped central length extends along a plane that intersects the axis of the cylindrical anode at a non-zero angle wherein the third filament is positioned such that the elliptically shaped center length is a constant distance from the cylindrical anode, and wherein an apex of the third filament is at approximately the same depth as an apex of each of the first filament and the second filament.

8. The electron bombardment ion source assembly according to claim 7, wherein: The non-zero angle results from a rotation of 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°.

9. The electron bombardment ion source assembly according to 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 bombardment ion source assembly according to claim 1, wherein: At least one of the first filament and the second filament is coated with a metal oxide.

11. An electron bombardment ion source assembly for use in a mass spectrometer, comprising: an anode extending along the axis and surrounding the ionization volume; as well as at least two filaments, the at least two filaments each configured to thermionically emit electrons, wherein the at least two filaments are positioned outside the ionization volume and proximate to the anode; wherein each of the at least two filaments comprises an elliptically shaped portion and non-elliptically shaped portions at both ends of the elliptically shaped portion, wherein the non-elliptically shaped portion is configured to be mounted in a fixed position relative to the anode to maintain a constant distance between the elliptically shaped portion and the anode, and Therein, the elliptically shaped portion extends along a plane that intersects a plane perpendicular to the axis of the anode at a non-zero angle.

12. The electron bombardment ion source assembly according to claim 11, wherein: Each elliptically shaped portion includes an apex, wherein the at least two filaments are each positioned such that each apex is at a same depth relative to the anode.

13. The electron bombardment ion source assembly according to claim 12, wherein: Each non-zero angle results from a rotation of the at least two filaments about an axis of rotation extending through a vertex of each of the at least two filaments.

14. The electron bombardment ion source assembly according to claim 11, wherein: The at least two filaments extend along parallel planes.

15. The electron bombardment ion source assembly according to claim 13, wherein: The rotation axis intersects the axis of the anode at an angle of 90°.

16. The electron bombardment ion source assembly according to claim 11, wherein: The at least two filaments are identical to each other and are rotated at a certain angle relative to each other around the axis of the anode.

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

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