Ion source assembly with multiple ionization volumes for use in a mass spectrometer - Patent Application 20070122997

Abstract

An ion source assembly for use in a mass spectrometer includes a first anode defining a first ionization volume and a first electron source positioned proximate to the first anode and configured to generate electrons that pass through the first anode and enter the first ionization volume. The ion source assembly further includes a second anode defining a second ionization volume and a second electron source positioned proximate to the second anode and configured to generate electrons that pass through the second anode and enter the second ionization volume. At least one optical element is positioned proximate to the first ionization volume and defines an aperture. The first and second anodes and the first and second ionization volumes are positioned along an ion optical axis of the mass spectrometer, and the first anode is positioned between the second anode and the aperture.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority under the relevant portions of 35 U.S.C. § 119 to U.S. patent application Ser. No. 17 / 357,348, filed June 24, 2021, and U.S. patent application Ser. No. 63 / 117,708, filed November 24, 2020. Each of the above applications is incorporated by reference in its entirety.

[0002] The present disclosure relates to an ion source assembly comprising multiple ionization volumes, each ionization volume including a separate electron source. [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 its chemical composition. Generally, mass spectrometry requires ionization of the sample, separation of the ions according to their mass-to-charge ratio, detection of the separated ions, and display of the results as a spectrum showing the signal intensity of the detected ions as a function of their mass-to-charge ratio. Ionization of samples, particularly gaseous samples, can be performed using electron impact ionization sources, also known as electron ionization (EI) sources. EI ion sources contain a source of electrons, which can be a filament heated to a temperature at which it emits electrons. The filament used can be a thin wire composed of a high-melting-point metal, either uncoated or coated with a metal oxide. Heating of the filament can be performed resistively by passing an electric current through the filament. Electrons are thermionically emitted from the filament and accelerated through a wall or anode into the ionization volume. Electron movement is induced by a potential difference maintained between the filament and the anode by a control unit. The anode defines at least one opening through which a certain percentage of electrons can pass through the anode and enter the ionization volume. It is generally desirable to maintain the temperature and variable potential of the filament so that a constant electron emission current enters the ionization volume. Additional electrodes, such as an electron repeller, can be included to guide the electrons. Inside the ionization volume, at least a portion of the accelerated or energetic electrons collide with molecules of the gas sample in the ionization volume. Upon colliding with the gas molecules, the electrons have sufficient energy to ionize and / or fragment the gas molecules to produce ions.

[0004] These ions are then accelerated and guided into the mass filter by other potentials established on 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 may be set to a specific potential to help control the trajectories of the ions generated in the ionization volume. The ion repeller may be a flat or planar electrode, or may be concave in the direction toward the mass filter. As 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.

[0005] The filament in 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) eventually evaporates, resulting in filament breakage. Filament breakage can also occur as a result of changes in the crystalline structure of the filament wire that occur at high operating temperatures. Additionally, the electron-emitting surface of the filament can be chemically altered by gases in the system, increasing the work function of the electron-emitting surface while decreasing the electron-emitting efficiency of the surface. If a broken, deformed, or chemically "contaminated" filament results in no electrons or insufficient electrons being available to ionize the gas sample, the mass spectrometer will no longer function satisfactorily. Therefore, any processes monitored and / or controlled based on data generated by the mass spectrometer must be stopped or otherwise "run blind" until an opportunity to replace the filament exists. Filament replacement is a time-consuming and inconvenient process. Because the filament is often located inside the process vacuum chamber, the process vacuum chamber must be vented to perform this replacement. Therefore, it is desirable to reduce the frequency of filament replacements, and even more desirable to be able to schedule filament replacements in advance so that the filament can be replaced at the same time the process chamber is offline for other maintenance activities.

[0006] One commonly adopted method to address this shortcoming of EI ion sources is to include a second filament located near the anode, capable of operating if the first filament fails. The two filaments are typically mirrored copies of each other, mirrored about a plane extending along the ion optical axis of the ion source. Arranging the filaments in this manner is done to maintain consistent mass spectrometer performance by ensuring that ions are formed 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 them for successful injection into the mass filter, and where the electric field is high enough to overcome the effects of space charge on sensitivity. However, one drawback of this type of EI ion source is that, due to the generally limited space available near the anode, each of the two filaments is shorter than the filament used when only one filament is used. 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 total electron emission current depends on the area of ​​the emitting surface, a shorter filament must be operated at a higher temperature to achieve the same total emission current. Therefore, two short filaments operated in series will not have twice the lifespan of one long filament. In fact, the combined operating life of two short filaments may not be as long as that of a single filament of "normal" length. Furthermore, a shorter filament necessarily loses more heat to the mounting structure than a longer filament. This heat loss is due to the lower thermal resistance offered by the shorter path along the wire from the central region of the filament to the attachment point compared to a long single filament. As a result, a higher temperature is required at the hottest part of the filament (near the center) to maintain the required level of total electron emission, shortening the filament's operating life.

[0007] Two important properties of an ion source are sensitivity (the number of ions that can be generated 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 lifetime. For example, the emission current and / or operating pressure can be reduced in an attempt to extend filament lifetime by lowering the filament temperature. However, reducing the emission current or operating pressure comes at the expense of sensitivity and / or ion current.

[0008] These are just some of the drawbacks associated with ion sources currently used in mass spectrometers. Summary of the Invention [Means for solving the problem]

[0009] An embodiment of an ion source assembly for use in a mass spectrometer includes a first anode defining a first ionization volume and a first electron source positioned proximate to the first anode and configured to generate electrons that pass through the first anode and enter the first ionization volume. The embodiment of the ion source assembly further includes a second anode defining a second ionization volume and a second electron source positioned proximate to the second anode and configured to generate electrons that pass through the second anode and enter the second ionization volume. At least one optical element is positioned proximate to the first ionization volume and defines an ion outlet from the ion source. The first and second anodes and the first and second ionization volumes are positioned along an ion optical axis of the mass spectrometer, and the first anode is positioned between the second anode and the ion outlet.

[0010] In one embodiment, the first electron source includes a first filament and the second electron source includes a second filament, the first filament and the second filament configured to be heated to emit electrons. In one embodiment, the first filament and the second filament are configured with approximately the same dimensions. In one embodiment, the first filament and the second filament are configured with different dimensions. In one embodiment, the first filament and the second filament are configured with the same material. In one embodiment, the first filament is configured with a different material than the second filament. In one embodiment, one of the first filament and the second filament is configured with a tungsten alloy. In one embodiment, one of the first filament and the second filament is configured with iridium coated with an oxide. In one embodiment, the ion source assembly further includes a conductive end cap disposed between the first ionization volume and the second ionization volume, the conductive end cap capable of passing ions from the second ionization volume to the first ionization volume. In one embodiment, the conductive end cap is planar. In one embodiment, the conductive end cap comprises a concave shape in a direction towards the first ionization volume.

[0011] An embodiment of a method for operating an ion source in a mass spectrometer includes directing a current to a first electron source, heating the first electron source to emit electrons, directing the electrons emitted from the first electron source through a first anode to a first ionization volume, and using the electrons to generate ions in the first ionization volume. The method further includes diverting a current from the first electron source to a second electron source, heating the second electron source to emit electrons, directing the electrons emitted from the second electron source through a second anode to a second ionization volume, and using the electrons to generate ions in the second ionization volume. The ions are directed from the second ionization volume to an ion outlet using a potential applied to the first anode. The first anode, the second anode, the first ionization volume, and the second ionization volume are arranged along the ion optical axis of the mass spectrometer, and the first anode is positioned between the second anode and the ion outlet.

[0012] In one embodiment, the method of operating an ion source in a mass spectrometer further includes setting the potential of the first electron source equal to the potential of the first anode when the second electron source emits electrons and ions are generated in the second ionization volume. In another embodiment, the step of diverting current from the first electron source to the second electron source occurs after a failure of the first electron source.

[0013] An embodiment of a method for operating an ion source in a mass spectrometer to analyze a gas sample includes providing an ion source assembly including a first anode defining a first ionization volume and a first electron source constructed from a first material and configured to emit electrons into the first ionization volume through the first anode. The ion source assembly further includes a second anode defining a second ionization volume and a second electron source constructed from a second material and configured to emit electrons into the second ionization volume through the second anode. The second material is different from the first material and has different chemical properties than the first material. The ion source assembly further includes at least one ion optical element defining an ion outlet. One of the first electron source and the second electron source is operated for analysis of the gas sample based on the resistance of the first material and the second material to the gas sample. The first anode, the second anode, the first ionization volume, and the second ionization volume are arranged along the ion optical axis of the mass spectrometer, and the first anode is positioned between the second anode and the ion outlet.

[0014] A more particular description of the invention briefly summarized above can be had by reference to the 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 invention and should not therefore be considered as limiting its scope, since the invention may admit of other equally effective embodiments. Accordingly, for a better understanding of the nature and objects of the invention, reference may be made to the following detailed description read in conjunction with the drawings. [Brief explanation of the drawings]

[0015] [Figure 1] 1 shows a schematic cross-sectional view of one embodiment of a prior art ion source in a mass spectrometer. [Figure 2] 1 shows a perspective view of one embodiment of a prior art ion source for use in a mass spectrometer. [Figure 3A] 1 shows a top perspective view of one embodiment of a prior art ion source for use in a mass spectrometer. [Figure 3B] 1 shows a side perspective view of one embodiment of a prior art ion source for use in a mass spectrometer. [Figure 4] 1 shows a schematic cross-sectional view of one embodiment of an ion source assembly according to the present invention in a mass spectrometer. [Figure 5] FIG. 1 shows a top perspective view of one embodiment of an ion source assembly for use in a mass spectrometer according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] The following description relates to various embodiments of an ion source assembly with multiple ionization volumes. It will be understood that the versions described herein are examples embodying the specific 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," "forward," "rearward," "internal," "external," "front," "rearward," "top," "bottom," "inside," "outside," "first," and "second" are not intended to limit these concepts unless specifically indicated. As used herein, the terms "about" or "approximately" can 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 ionization volumes and are clearly not provided to scale.

[0017] FIG. 1 shows a cross-sectional view of a portion of a prior art mass spectrometer 100 having an electron ionization (EI) ion source 101. The EI ion source 101 includes an electron source 102, an anode 105 that defines or at least partially 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 (1500-2400 K) at which the filament 102 emits electrons. The filament 102 may be composed of a refractory metal 103 that may be coated with a metal oxide 104. The filament 102 may be connected to a current source to allow an electric current to pass through the filament 102 to heat it to a temperature at which electrons are emitted from the filament 102. As shown, the anode 105 is spaced from the filament 102 and is generally disposed between the filament 102 and the ionization volume 106. Additional electrodes 107, such as one or more electron repellers 107, may be provided and configured to guide electrons emitted from the filament 102. As shown, the electron repeller 107 is located radially outward from the filament 102, such that the filament 102 is located between the electron repeller 107 and the anode 105. Electrons thermionically emitted from the filament 102 may be 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 in the ion source components 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 electrons pass through an opening 105a (FIG. 2) defined on the anode 105 and enter 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. Upon colliding with gas molecules, the electrons have sufficient energy to ionize and / or fragment the gas molecules to produce ions.

[0018] As shown, the ion source 101 further comprises one or more optical elements 108 defining an ion outlet, which may be an aperture 108a or a grid, configured to establish a potential that acts to accelerate ions generated in the ionization volume 106 and direct them into a mass filter 109. The mass filter 109 spatially or temporally separates ions of various mass-to-charge ratios. The ions are then detected by an ion detector 110, and a mass spectrum is determined from the output of the ion detector 110. In one embodiment, the ion detector 110 is in electrical communication with an interface 111 on which the output of the ion detector 110 and / or the mass spectrum may be displayed. In some embodiments of the ion source 101, an ion repeller 112 is positioned upstream of the ionization volume 106 and is set to a specific potential to assist in controlling the trajectories of the generated ions.

[0019] Referring now to FIG. 2, a perspective view of an embodiment of the ion source 101 of FIG. 1 is shown. As shown, a single filament 102 extends a 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. FIGS. 3A-B show an embodiment of a prior art dual filament ion source 301. As can be seen, the single long filament 102 of the embodiment of FIG. 2 has been replaced with two short filaments 302a, 302b. The first filament 302a extends a length from a first end connected to and supported by the first support member 311a to a second end. The second filament 302b extends a length from a first end connected to and supported by the second support member 311b to a second end. The second ends of both the first filament 302a and the second filament 302b are connected to 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 202 of the embodiment of Figure 2. The first filament 302a and the second filament 302b are disposed with respect to a vertical plane V such that they are mirror images of each other. The vertical plane V extends along the optical axis of the ion source.

[0020] 4 and 5 illustrate an embodiment of a multiple ionization volume ion source assembly 401 ("ion source assembly") of the present invention for a mass spectrometer 400. The multiple ionization volume ion source assembly 401 generally comprises at least two ionization volumes. As shown, the ion source assembly 401 comprises a first electron source, such as a first filament 402, and a first anode 405 that defines or at least partially surrounds a first ionization volume 406. The ion source assembly 401 further comprises a second electron source, such as a second filament 412, disposed proximate to a second anode 415. The second filament 412 may be approximately the same length as the first filament 402. A plurality of support members 511a-d (FIG. 5) are configured to connect to and support the first filament 402 and the second filament 412. Each of the two filaments 402, 412 may be supported by two or more of the support members 511a-d. The second anode 415 at least partially surrounds the second ionization volume 416. As shown in FIG. 5 , the first anode 405 and the second anode 415 may define one or more openings 405a, 415a that traverse the first anode 405 and the second anode 415, respectively. The second anode 415 and the second ionization volume 416 are disposed upstream relative to the first anode 405 and the first ionization volume 406 along the ion optical axis Z of the ion source assembly 401.

[0021] Similar to prior art filaments, the first filament 402 and the second filament 412 of the ion source assembly 401 may be composed of a high-melting-point metal that may be coated with a metal oxide. The first filament 402 and the second filament 412 may be identical to one another or may be of different lengths and may be composed of different materials. Each of the first filament 402 and the second filament 412 may be connected to a current source so that an electric current can be passed through the first filament 402 and the second filament 412 to heat them to a temperature at which electrons are emitted from each of the first filament 402 and the second filament 412. As shown, a first anode 405 and a second anode 415 are spaced apart from the first filament 402 and the second filament 412. In this configuration, the first anode 405 is typically positioned between the first filament 402 and the first ionization volume 406, while the second anode 415 is typically positioned between the second filament 412 and the second ionization volume 416.

[0022] 4, the ion source assembly further includes one or more electron repellers 407 that act to guide electrons emitted from the first filament 402 and the second filament 412. As shown in FIG. 5, the electron repeller 407 is positioned radially outward from the first filament 402 and the second filament 412 such that the first filament 402 and the second filament 412 are positioned between the electron repeller 407 and the first anode 405 and the second anode 415. Thus, electrons thermionically emitted from the first filament 402 and / or the second filament 412 can be guided by the electron repeller 407 and accelerated by a potential difference established between the first filament 402 and the second filament 412 and the first anode 405 and the second anode 415. The ion source assembly 401 may further include a control unit / control electronics 450 in electrical communication with the ion source assembly 401 and configured to establish and maintain electrical potentials present on the components of the ion source assembly 401. These electrical potentials may also be adjusted via the control unit 450. Electrons emitted by the first filament 402 and the second filament 412 pass through openings 405 a, 415 a defined on the first anode 405 and the second anode 415, respectively, and enter the first ionization volume 406 and / or the second ionization volume 416. At least a portion of the electrons in the first ionization volume 406 and / or the second ionization volume 416 collide with molecules of the gas sample present in the ionization volumes 406, 416. Upon colliding with the gas molecules, the electrons have sufficient energy to ionize and / or fragment the gas molecules to produce ions.

[0023] The ions pass through one or more optical elements 408, each defining a passageway or orifice 408a, and into a mass filter 409, which spatially or temporally separates ions of various mass-to-charge ratios. The ions are then detected by an ion detector 410, and a mass spectrum is determined from the output of the ion detector 410. In one embodiment, the ion detector 410 is in electrical communication with an interface on which the output of the ion detector 410 and / or the mass spectrum may be displayed. In one embodiment, the interface may be part of the control unit 450. The ion source assembly 401 uses two long filaments, both positioned along the optical axis (or vertical plane V), which allows for good ion current linearity with pressure while also providing good sensitivity, thereby improving ion source lifetime and allowing for the use of different filament materials.

[0024] For example, when the second filament 402 is operating, the potential on the first anode 405 can be adjusted via the control unit 450 so that the first anode 405 acts as an extraction and focusing optic to transport ions generated in the second ionization volume 416 to the optics 408 and thereby into the mass filter 409. By using the first anode 405 as the extraction and focusing optic, the sensitivity and linearity of the ion source assembly 401 to pressure can be adjusted when the second filament 412 is in use. This level of adjustment would not be possible if the second filament 412 were located further from the mass filter 409 along the optical axis (vertical plane V) and deeper in the single, longer anode.

[0025] As mentioned above, in one embodiment, the second filament 412 may be a duplicate of the first filament 402, with a length approximately equal to the length of the second filament 412, doubling the time between filament replacement functions. In other embodiments, the second filament 412 may be a different length and / or be constructed from a different material than the first filament 402. In certain embodiments, the second filament 412 may be shorter than the first filament 402 to conserve space within the mass spectrometer. In this embodiment, the shorter second filament 412 may act as a "mini-spare" to allow continued use of the instrument until the next scheduled maintenance time, rather than necessarily doubling the operating time between failures.

[0026] In another embodiment, the second filament 412 may be constructed from a different material than the first filament 402, allowing the ion source assembly 401 to be used in a mass spectrometer to continuously analyze or monitor different gas samples, provided that the material of each filament is selected to optimally match the chemistry of each sample. For example, one filament may be constructed from a tungsten alloy, while the other filament may be constructed from oxide-coated iridium. This embodiment of the ion source assembly 401 may be particularly useful in portable GC / MS systems where the operator can switch from sampling the hydrogen carrier flow from the GC to the mass spectrometer to directly sampling room air through an appropriate vacuum interface (e.g., an orifice or membrane). In this case, a filament constructed from a tungsten alloy is used for the GC flow because hydrogen destroys the yttria coating present on the iridium filament. An yttria-coated iridium filament is used for air samples because oxygen in air quickly destroys hot tungsten filaments.

[0027] 4-5 , the first anode 405 of the ion source assembly 401 can include a section or first end cap 425 electrically connected to the first anode 405 and acting to separate the first ionization volume 406 from the second ionization volume 416. In another embodiment, the first end cap 425 can be a separate electrode similar to the ion repeller 112 of FIG. 1 . The first end cap 425 can be constructed from a conductive material configured to allow ions to pass through it. In one embodiment, the first end cap 425 is constructed at least in part from a conductive grid or mesh. The inclusion of the first end cap 425 between the first ionization volume 406 and the second ionization volume 416 has been shown to improve the performance of the ion source assembly 401, particularly when operating the second filament 412. However, the first end cap 425 can accumulate non-conductive surface contaminants over time, depending on the chemistry within the process chamber. This buildup of non-conductive film on the first end cap 425 can affect the performance of the ion source assembly 401, but such buildup may also affect the performance of any embodiment of any ion source.

[0028] The second anode 415 may also include a second end cap 426 that acts to better define the electrical potential inside the second ionization volume 416. The second end cap 426 may be similar to the first end cap 425. As such, the second end cap 426 may also be composed of a conductive material, but the conductive material need not allow ions to pass through the second end cap 426. In one embodiment, the conductive material of the second end cap 426 defines one or more openings that allow the free flow of gas into and out of the ionization volume 416. The second end cap 426 may be electrically connected to the second anode 415 and configured to operate at the same electrical potential as the second anode 415. In another embodiment, the second end cap 426 may be a separate electrode, similar to the previously described ion repeller 112. The second end cap 426 may have many different geometric shapes, including a flat or planar shape, or a shape that is generally concave toward the mass filter 409.

[0029] A further embodiment of the ion source assembly, not shown, comprises a third ionization volume, a third anode, and a third electron source located upstream of the second ionization volume 416. In this embodiment, one or more of the components may be the same as or similar to the components previously described with respect to the ion source assembly 401.

[0030] In a specific example, the ion source assembly 401 may be mounted on a residual gas analyzer (RGA) upstream of the mass filter 409 and ion detector 410. In this example, the mass filter 409 is a quadrupole mass filter, the ion detector 410 is a Faraday cup ion detector, and the RGA is used to monitor gases within a vacuum process chamber. The first filament 402 and second filament 412 of the ion source assembly 401 are two yttria-coated iridium filaments, each positioned adjacent to a respective anode. The two anodes are formed from 304 stainless steel mesh. Stainless steel ion optics 408, such as a focusing lens and exit aperture plate, are used to define the ion beam and inject it into the quadrupole mass filter 409. Initially, current from the control electronics is used to resistively self-heat the first filament 402 to a temperature such that 2 mA of emission current in the form of electrons emerges from the surface of the first filament 402. The first filament 402 is biased so that the center of the filament is at 110 V. The first anode 405 is biased by the control electronics to 212 V, the focusing lens is at 185 V, and the exit aperture plate 408 and electron repeller 407 are at 0 V. All of these voltages are relative to the wall of the vacuum chamber being monitored. Electrons emitted from the first filament 402 are accelerated by the 102 V difference between the first anode 405 and the first filament 402. They pass through the mesh of the first anode 405 into the first ionization volume 406. The electrons collide with molecules of the gas being monitored, causing them to be struck and released, fragmenting the gas molecules and producing ions that are analyzed by the quadrupole mass filter 409. While the first filament 402 is in use, the second anode 415 can be held at 215 V or some other voltage, and the second filament 412 can be held at 0 V. The quadrupole is operated in the normal way so that the central axis (pole zero) is at 202V.

[0031] Operation can continue in this manner until the first filament 402 fails. At that time, filament current is sent to the second filament 412, whose center is now biased to 110 V. The second anode 415 is set to 212 V, so that 2 mA of 102 eV electrons now enter the second ionization volume 416. The first anode 405 is now set to 206 V and acts to extract ions from the second ionization volume 416 and direct them toward the focusing lens and exit aperture plate 408, and then toward the quadrupole mass filter 409. When operating the second filament 412 as an electron source, the first filament 402 may be set to the same voltage as the first anode 405 (206 V in this example), so that if the first filament 402 breaks and touches the anode 405, there will be no current flow between the first anode 405 and the first filament 402. Operation of the RGA can then continue at these new potential settings as before the filament failure.

[0032] Although the present invention has been described primarily as a means of obtaining a similar ion current from the ion source using a second filament as that obtained from using a first filament, and while extending the operating time between required ion source maintenance operations, this is not necessarily the only way in which the ion source assembly 401 can be operated. The sensitivity and linearity of the ion source depend on the electric field determined in the ion source by the potentials applied to the various electrodes by the control electronics of the control unit. Therefore, it is possible to customize electrode settings for each of the first filament 402 and the second filament 412. In other words, when the first filament 402 is operating, the applied electrode settings optimize the resulting ion beam for its particular characteristics. Similarly, when the second filament 412 is operating, the applied electrode settings optimize the resulting ion beam for its particular characteristics. For example, electrode potentials can be selected to optimize linearity when using the first filament 402 and to optimize sensitivity when using the second filament 412. In an alternative example, the electrode potentials can be selected to direct ions of one energy from the first ionization volume 406 into the mass filter 409 and ions of another energy from the second ionization volume 416 into the mass filter 409. In another example, both filaments 402, 412 can be operated simultaneously, which will result in a wider range of ion energies entering the mass filter 409.

[0033] While 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, which may be supported by the written description and drawings. Furthermore, when exemplary embodiments are described with reference to a particular number of elements, it will be understood that the exemplary embodiments can be practiced utilizing fewer or more than the particular number of elements.

Claims

1. 1. An ion source assembly for use in a mass spectrometer, the ion source assembly comprising: a first anode at least partially surrounding the first ionization volume; a first electron source disposed proximate to the first anode and configured to generate electrons that pass through the first anode and enter the first ionization volume; a second anode at least partially surrounding the second ionization volume; a second electron source disposed proximate to the second anode and configured to generate electrons that pass through the second anode and into the second ionization volume; at least one optical element proximate the first ionization volume and defining an ion exit from the ion source; Equipped with the first and second anodes and the first and second ionization volumes are arranged along an ion optical axis of the mass spectrometer, and the first anode is positioned between the second anode and the ion outlet; an ion source assembly, wherein a current is selectively directed to one of the first electron source and the second electron source;

2. 10. The ion source assembly of claim 1, wherein the first electron source comprises a first filament and the second electron source comprises a second filament, the first filament and the second filament configured to be heated to emit electrons.

3. The ion source assembly of claim 2 , wherein the first filament and the second filament are constructed of approximately the same dimensions.

4. The ion source assembly of claim 2 , wherein the first filament and the second filament are constructed of different dimensions.

5. The ion source assembly of claim 2 , wherein the first filament and the second filament are constructed from the same material.

6. The ion source assembly of claim 2 , wherein the first filament is constructed of a different material than the second filament.

7. The ion source assembly of claim 6 , wherein one of the first filament and the second filament is constructed from a tungsten alloy.

8. The ion source assembly of claim 7 , wherein one of the first filament and the second filament is composed of iridium coated with an oxide.

9. 10. The ion source assembly of claim 1, further comprising a conductive end cap disposed between the first ionization volume and the second ionization volume, the conductive end cap allowing ions to pass from the second ionization volume to the first ionization volume.

10. The ion source assembly of claim 9 , wherein the conductive end cap is planar.

11. The ion source assembly of claim 9 , wherein the conductive end cap comprises a concave shape in a direction toward the first ionization volume.

12. 1. A method of operating an ion source in a mass spectrometer, said method comprising: directing a current to a first electron source; heating the first electron source to emit electrons; directing the electrons emitted from the first electron source into a first ionization volume through a first anode at least partially surrounding the first ionization volume; using the electrons to generate ions in the first ionization volume; diverting the current from the first electron source to a second electron source; heating the second electron source to emit electrons; directing the electrons emitted from the second electron source into a second ionization volume through a second anode at least partially surrounding the second ionization volume; using the electrons to generate ions in the second ionization volume; directing ions from the second ionization volume to an ion outlet using a potential applied to the first anode; Including, the first anode, the second anode, the first ionization volume, and the second ionization volume are arranged along an ion optical axis of the mass spectrometer, and the first anode is positioned between the second anode and the ion outlet.

13. 13. The method of claim 12, further comprising setting a potential of the first electron source equal to a potential of the first anode when the second electron source emits electrons and ions are generated in the second ionization volume.

14. The method of claim 12 , wherein the step of diverting the current from the first electron source to the second electron source occurs after a failure of the first electron source.

15. 1. A method of operating an ion source in a mass spectrometer for analyzing a gas sample, the method comprising: Providing an ion source assembly, the ion source assembly comprising: a first anode at least partially surrounding the first ionization volume; a first electron source constructed from a first material and configured to emit electrons through the first anode into the first ionization volume; a second anode at least partially surrounding the second ionization volume; a second electron source constructed from a second material and configured to emit electrons through the second anode into the second ionization volume, the second material being different from the first material and having different chemical properties than the first material; at least one ion optical element defining an ion outlet; and operating one of the first electron source and the second electron source for analysis of the gas sample based on resistance of the first material and the second material to the gas sample; Including, the first anode, the second anode, the first ionization volume, and the second ionization volume are arranged along an ion optical axis of the mass spectrometer, and the first anode is positioned between the second anode and the ion outlet.

16. The ion source assembly of claim 1 , wherein at least one of the first electron source and the second electron source at least partially surrounds at least one of the first anode and the second anode.

17. The method of claim 12 , wherein at least one of the first electron source and the second electron source at least partially surrounds at least one of the first anode and the second anode.

18. The method of claim 15 , wherein at least one of the first electron source and the second electron source at least partially surrounds at least one of the first anode and the second anode.

19. An ion source assembly as described in claim 1, wherein when the second electron source is operating, the first anode is adjusted to act as an extraction and focusing optical element configured to transport ions generated within the second ionization volume.

20. The method of claim 15, further comprising a step of adjusting the potential of the first anode so that when the second electron source is operating, the first anode acts as an extraction and focusing optical element configured to transport ions generated within the second ionization volume.

Images