Patent Publication Number: US-7902529-B2

Title: Method and apparatus for selectively providing electrons in an ion source

Description:
TECHNICAL FIELD 
     This invention relates in general to ion sources and, more particularly, to an ion source having an electron source configured to selectively provide electrons. 
     BACKGROUND 
     Existing mass spectrometers have an ion source that produces ions of a sample material. These ions are then processed by a mass analyzer which includes a mass detector. Some existing ion sources produce ions using a technique known as electron ionization (EI). Particles of a sample material that are referred to as analytes are supplied in a gas phase to an ion volume having a relatively low pressure, and a stream of electrons is also supplied to the ion volume. The electrons directly strike the sample analytes, and the resulting energy exchange is sufficient to cause ionization, producing ions characteristic of the sample material. These ions are then supplied to the mass analyzer. 
     A different type of ion source produces ions using a technique known as chemical ionization (CI). The analytes of the sample material are supplied in a gas phase to an ion volume, and a reagent gas such as methane is also supplied to the ion volume. Further, a stream of electrons is supplied to the ion volume. The ion volume is configured so that the inflow of the reagent gas maintains a relatively high pressure within the ion volume, thereby ensuring a density for the reagent gas that increases the probability of collisions between the incoming electrons and the molecules of the reagent gas. When electrons collide with the molecules of the reagent gas, the collisions produce ions of the reagent gas. The ions of the reagent gas then react with the analytes of the sample gas, in order to form further ions that are characteristic of the sample material. These further ions are then supplied to the mass analyzer. 
     In both EI and CI, an electron source is configured to selectively provide the stream of electrons to the ion volume. The electron source includes a filament that is energized to emit electrons for the stream. It is advantageous to provide a second filament. When one of the filaments burns out, an operator can continue running samples with the other filament. As such, the mass spectrometer is not rendered completely inoperative by a burned-out filament, and can continue operating with minimum disruption. 
     In one approach, two separate filaments are provided with one filament on each side of the ion volume. While this approach has been generally adequate for its intended purposes, it has not been entirely satisfactory in all respects. As one example, this approach increases the cost associated with manufacturing the mass spectrometer. Moreover, the design of this approach is more complex due to the electrical and mechanical connections that are required on both sides of the ion volume. In another approach, two separate filaments are supported on common structure that is positioned on one side of the ion volume. The two filaments are spaced from each other in a direction transverse to a direction of electron travel to the ion volume. 
     SUMMARY 
     One of the broader forms of the invention involves an apparatus that includes an electron source for selectively providing a first stream of electrons that travels in a direction along an imaginary line to a location that is remote from the electron source, and for selectively providing a second stream of electrons that travels in the direction along the line to the location, the electron source including: a first electron emitter for selectively emitting electrons for the first stream; and a second electron emitter for selectively emitting electrons for the second stream. 
     Another of the broader forms of the invention involves a method for operating an apparatus having an electron source that includes first and second electron emitters that can each selectively emit electrons, the method including: selectively producing a first stream of electrons that travels from the first electron emitter in a direction along an imaginary line to a location remote from the electron source; and selectively producing a second stream of electrons that travels from the second electron emitter in the direction along the line to the location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a block diagram of a mass spectrometer that embodies aspects of the present invention. 
         FIG. 2  is a diagrammatic perspective view of a filament assembly that is a component of the mass spectrometer of  FIG. 1 . 
         FIG. 3  is an exploded diagrammatic view of the filament assembly of  FIG. 2 . 
         FIG. 4  is a diagrammatic fragmentary perspective view showing a portion of the filament assembly of  FIG. 2  in an enlarged scale. 
         FIG. 5  is a schematic view of the circuitry of a filament supply that is a component of the mass spectrometer of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of a mass spectrometer (MS)  10  that embodies aspects of the present invention. The mass spectrometer  10  includes an ion source  12 , a mass analyzer  14 , a gas chromatograph  16 , a source  18  of a reagent gas, a vacuum source  20 , and a control system  22 . The disclosed mass spectrometer  10  is configured for chemical ionization (CI), but could alternatively be configured for electron ionization (EI). 
     The mass analyzer  14  is a type of device that is known in the art, and in fact could be any of a number of commercially-available devices. The mass analyzer  14  may include a not-illustrated device to separate ions based on their mass-to-charge ratios, examples of which include but are not limited to a quadrupole filter, a linear ion trap, a rectilinear ion trap, a three-dimensional ion trap, a cylindrical ion trap, a Fourier transform ion cyclotron resonance filter, an electrostatic ion trap, a Fourier transform electrostatic filter, a time-of-flight filter, a quadrupole time-of-flight filter, a hybrid analyzer, or a magnetic sector. Further, the mass analyzer  14  may include a not-illustrated detector that can detect ions. Since the mass analyzer  14  in  FIG. 1  is a known type of device, it is not described here in further detail. 
     The gas chromatograph  16  is also a known type of device, and could be any of a number of commercially-available devices. The gas chromatograph  16  serves as a source of particles of a sample material that are referred to as analytes. In particular, the gas chromatograph  16  outputs analytes that are atoms or molecules of the sample material in a gas phase. The sample analytes delivered by the gas chromatograph  16  travel to the ion source  12  through a gas chromatograph (GC) column  26  of a known type. For example, the GC column  26  may be a fused silica capillary tube of a type well known in the art. Alternatively, instead of the gas chromatograph  16  and GC column  26 , the sample analytes may optionally be generated by a liquid chromatograph (LC) and delivered by an LC column. 
     The reagent gas source  18  is also a known type of device, and produces a flow of a reagent gas such as methane. The vacuum source  20  is a known type of system, and is operatively coupled to both the ion source  12  and the mass analyzer  14 , in order to maintain a vacuum in interior regions during normal operation. 
     The control system  22  includes circuitry of a known type, and is operatively coupled to various other components of the mass spectrometer  10 . In the disclosed embodiment, the control system  22  includes a digital signal processor (DSP) that is indicated diagrammatically at  28 . The DSP  28  executes a software program that determines how the system  22  controls other components of the mass spectrometer  10 . The DSP  28  could alternatively be a microcontroller, or some other form of digital processor. As another alternative, the DSP  28  could be replaced with a state machine or a hardwired circuit. The control system  22  includes an output  23  that controls the gas chromatograph  16  and an output  24  that controls the reagent gas source  18 . The control system  22  further includes a line  25  that communicates with the mass analyzer  14  for transmitting and receiving data. In addition, the control system  22  includes other outputs that control various other components of the mass spectrometer  10 , in a manner discussed later. It is to be understood that line  25  and the other lines to and from the controller may be provided by either a wired or a wireless transmission, or both. 
     The ion source  12  has therein an electrically conductive housing  34  with a chamber serving as an ion volume  36 . The housing  34  has two openings  38  and  40  that provide communication between the ion volume  36  and the exterior of the housing. The opening  38  serves as an electron opening or an electron inlet port, and the opening  40  serves as an ion opening or an ion outlet port in a manner discussed herein. A gas supply conduit  30  extends from the reagent gas source  18  to the housing  34 , and an electrically-operated valve  32  is provided along the conduit to control gas flow through the conduit. The valve  32  is controlled by an output  33  of the control system  22 . The conduit  30  opens into the ion volume  36  through a gas inlet port  42 . The end of the GC column  26  remote from the gas chromatograph  16  has an end portion that projects a short distance into the ion volume  36  through an opening in the housing  34 . 
     The ion source  12  includes near the housing  34  an electron source  44 . The electron source  44  includes a filament assembly  45  having two electron emitters, which may be of the thermionic emitter-type and take the form of filaments  46  and  48  having generally hairpin configurations that are positioned in relative overlying relationship to each other along an imaginary line  49  that extends through the electron inlet port  38  and into the ion volume  36 . As shown in  FIG. 1 , (and more clearly in  FIGS. 3 and 4  described below), the filaments may be disposed transverse to each other and have a hairpin configuration defining emission sections generally centered on the imaginary line  49 . Alternatively, the filaments  46  and  48  may optionally include ribbon filaments, coil filaments, or combinations thereof. When energized, each filament  46  and  48  can emit a stream of electrons that propagates along the imaginary line  49  through the electron inlet port  38  to a target location  50 , which may be a point or region within the ion volume  36 . The electron source  44  includes a filament supply  52 . The filament supply  52  can selectively energize either of the filaments  46  and  48 . The filament supply  52  is controlled by an output  53  of the control system  22 , so that the control system can selectively turn each of the filaments  46  and  48  on and off, in a manner discussed later. When energized, the filaments  46  and  48  are negatively biased with respect to the ion volume  36 . The filament supply includes an output  55  coupled to the ion volume  36 . The difference in potential between the ion volume  36  and the filaments  46  and  48  establishes the energy of the electrons as they travel to the ion volume. The filament supply  52  also includes an output  54  coupled to the control system  22  that indicates to the control system when either of the filaments  46  and  48  is burned out. 
     By way of further contradistinction with regard to past devices that include two filaments, the plural filament configurations of the embodiment of the present invention have less build up of insulative layers of neutral molecules. Insulative layers tend to build up less on an inactive filament of the embodiment of the present invention for at least two reasons. Firstly, the filaments are positioned close to each other. Therefore, heat from the active filament is transferred to the inactive filament such that condensation on the inactive filament is reduced. Secondly, the electrical potential on the inactive filament is substantially the same as or close to the electrical potential on the active filament. The small or non-existent electrical potential difference between the two filaments reduces the energy with which particles, such as electrons, impact the neutral molecules that have been adsorbed or condensed on the inactive filament. Less energetic impacts from the particles on the neutral molecules, which may include carbon or silicon for example, will tend away from decomposition of these neutral molecules into less volatile subcomponents such as carbon or SiO 2 . Hence, reducing the potential difference between the filaments greatly reduces the likelihood that the neutral molecules will be impacted by high energy particles and remain on the inactive filament. Since fewer neutrals condense on the inactive filament because of heating of the inactive filament from the proximate active filament, there will be fewer neutrals on the inactive filament to be impacted in the first place. For these reasons, the insulative layer of material that would otherwise build up on the inactive filament is greatly reduced or eliminated. 
     The electron source  44  further may include an electron gate  56  of a known type. For example, an electron gate  56  may include one or more lens(es) that can be operated in one or more of a focusing or gating mode. The electron gate  56  may be provided between the filaments  46  and  48  and the electron inlet port  38 , or may be omitted all together. When included, the electron gate  56  is controlled by an output  57  of the control system  22 . The control system  22  can thus selectively and independently “open” and “close” the electron gate  56 . When the electron gate  56  is open, the stream of electrons flowing along line  49  propagates through the gate and into the ion volume  36 . On the other hand, when the electron gate  56  is closed, it interrupts the stream of electrons, so that the stream of electrons is inhibited from traveling to and entering the ion volume  36 . 
     The ion source  12  includes a set of magnets  58  of a known type. The magnets  58  generate a magnetic field that is aligned parallel with the imaginary line  49  to help keep the stream of electrons collimated. The ion source  12  further includes a set of lens elements  59  of a known type. The lens elements  59  are disposed between the ion volume  36  and the mass analyzer  14 . The lens elements  59  are controlled by one or more outputs  60  of the control system  22 . 
     The ion volume  36  is used for chemical ionization (CI). The general principles of CI are known in the art, and are therefore described only briefly here, and not in detail. During operation, the valve  32  remains open to allow a continuous flow of the reagent gas to pass through the conduit  30  and into the ion volume  36 . As shown diagrammatically in  FIG. 1 , the ion volume  36  has only a few very small openings including openings  38  and  40 . Thus, due to these relatively small openings  38  and  40  and also the flow of reagent gas into the interior of the ion volume  36 , the ion volume  36  is maintained at a relatively high pressure. 
     The gas chromatograph  16  contains a sample material, and produces analytes of the sample material such as atoms or molecules thereof, which are supplied through the GC column  26  in a gas phase to the ion volume  36 . When the electron gate  56  is open and allows a stream of electrons to flow along line  49  to enter the ion volume  36 , the electrons collide primarily with molecules of the high pressure reagent gas to form ions of the reagent gas. The relatively high pressure within the ion volume  36  ensures a density of the reagent gas that promotes such collisions in order to produce ions of the reagent gas. The ions of the reagent gas then react with the analytes of the sample gas in order to form ions characteristic of the individual analytes. Gas flowing out of the ion volume  36  through the ion outlet port  40  carries with it these ions. 
     The control system  22  applies an electrical potential to the ion volume  36  through the control line  37 , and also applies at least one electrical potential to the lens elements  59 . The potential between the ion volume  36  and lens elements  59  extracts and focuses the ions of sample material generated within the volume  36 . In particular, the ions travel along a path  61  from the ion volume  36 , through the outlet  40 , and through the lens elements  59  to the mass analyzer  14 . The path  61  of ions travel is approximately perpendicular to the stream of electrons flowing along the line  49 . Even though the description above relates to a mass spectrometer operating by CI, the mass spectrometer  10  may alternatively be configured to operate by electron ionization (EI). In the case of EI, no reagent gas from source  18  is supplied to the ion volume  36 , openings  38  and  40  may be made larger, and ions characteristic of the sample material are formed directly from interactions of the sample material with the electrons. 
       FIG. 2  is a diagrammatic perspective view of the filament assembly  45  of  FIG. 1 , and  FIG. 3  is an exploded diagrammatic view of the filament assembly. In  FIGS. 2 and 3 , the filament assembly  45  includes a base  62 , a housing  64 , an optional electron lens  68 , and two filaments  46  and  48 . All or part of the housing or some other electrical element in a region of the filaments  46  and  48  makes up a first portion of the ion source that can be electrically biased relative to the ion volume  36  in order to urge electrons toward the ion volume. The ion volume  36  may be in the form of an enclosure. All or part of the enclosure forming the ion volume, or some other electrical element proximate the target location  50  makes up a second portion of the ion source  12  that can be electrically biased relative to the filaments  46  and  48  in order to urge electrons toward the ion volume  36 . It is to be understood that the electron lens  68  may provide all or part of the electron gate  56  described herein, and may be termed a third portion that can be electrically biased relative to both the filaments  46 ,  48  and the ion volume  36 . 
     The base  62  includes an insulating body  63  that is formed of a ceramic material. In the disclosed embodiment, the body  63  is made of a ceramic material that is available commercially under the trade name MYCALEX from Crystex Composites LLC of Clifton, N.J. However, the body  63  could alternatively be made of any other suitable material that is an electrical insulator. The body  63  includes a mid portion  72 , a bottom flange  74 , and a top portion  76 . The mid portion  72  has an approximately cylindrical shape with a flat surface  78  on one side. The bottom flange  74  projects radially outwardly and has an arcuate shape that surrounds the curved side of the mid portion  72 . The top portion  76  has a cylindrical shape with a smaller diameter than the mid portion  72 . The top portion  76  is positioned on the mid portion  72  such that the center axes of the mid portion and top portion are aligned. 
     The base  62  further includes three filament terminal posts  82 - 84  partially disposed within the body  63  and secured therein. The terminal posts  82 - 84  are L-shaped and made of stainless steel. Two of the terminal posts  82  and  83  have portions  86  and  87 , respectively, extending horizontally outwardly through the flat surface  78  of the mid portion  72 , and have portions  90  and  91 , respectively, extending vertically upwardly through the top surface of a top portion  76 . The third terminal post  84  has a portion  88  extending horizontally outwardly through the curved side of the mid portion  72 , and a portion  92  extending vertically upwardly through the top surface of the top portion  76 . Alternatively, the portion  88  of the third terminal post  84  may optionally extend horizontally outwardly through the flat surface  78  of the mid portion, with the portion  88  substantially parallel to the portions  86  and  87  of terminal posts  82  and  83 , respectively. 
     The base  72  further includes a support post  94  disposed partially within the base. The support post  94  is made of stainless steel. The support post  94  is T-shaped with a portion  95  extending horizontally outwardly through the flat surface  78  of the mid portion  72  and two portions  96  and  97  extending horizontally outwardly through the curved side of the mid portion  72  at spaced locations. 
     The housing  64  includes a shield portion  65  that has a generally cylindrical shape. A shallow cylindrical recess  98  is formed by inner surfaces of the shield portion, as shown for illustrative purposes by a cut away portion in  FIG. 3 . The cylindrical recess  98  opens downwardly from an upper surface of the inner surfaces of the cylindrical recess  98 . The shield portion  65  is made of stainless steel. The top of the shield portion  65  includes a vertical square opening  100  and a vertical oval opening  102  that each extend therethrough and communicate with the recess  98 . The shield portion  65  also has an upward projection  104  disposed at an outer edge of the opening  100 . The shield portion  65  is provided over and receives the top portion  76  of the base  62  within the recess  98 . The opening  102  is sized to easily receive the portions  91  and  92  of the terminal posts  83  and  84  so that the edge of the opening  102  does not contact the portions  91  and  92 . The opening  100  is sized to snugly receive the portion  90  of the terminal post  82 , and the projection  104  is welded to the portion  90 . 
     The housing  64  further includes a cover portion  66  that has a generally cylindrical shape with a cylindrical recess  109  formed by inner surfaces of the housing  64 , as shown for illustrative purposes by a cut away portion in  FIG. 3 . The cylindrical recess  109  opens downwardly as shown in  FIG. 3 . The cover portion  66  is made of stainless steel. The cover portion  66  includes a plurality of vertical slots  111  and  112  that extend upwardly from a bottom edge of the cover portion  66  at circumferentially spaced locations. In  FIG. 3 , only two slots are visible. However, any number of equally spaced slots may be provided, for example. The top of the cover portion  66  includes a vertical opening  114  that extends therethrough and communicates with the recess  109 . The imaginary line  49  extends centrally through the opening  114 . The cover portion  66  is provided over and covers the shield portion  65 , with the shield portion received snugly within the lower end of the recess  109 . The cover portion  66  may be welded or otherwise fixed to the shield portion  65 . 
     In the example of  FIGS. 2 and 3 , the electron lens  68  has a partially cylindrical shape with a partially cylindrical recess  115  that opens downwardly. The electron lens  68  is made of stainless steel. The electron lens  68  has an opening  117  on one side. The electron lens  68  includes a radially outwardly projecting bottom flange  116  with three vertical slots  118 ,  119 , and  120  that extend upwardly from a bottom edge of the electron lens  68  at circumferentially spaced locations. The electron lens  68  may be placed over the base  62  with the flange  116  of the lens  68  being supported by the flange  74  of the base  62 . The respective portions  86  and  87  of the terminal posts  82  and  83 , and the portion  95  of the support post  94  that extend through the flat surface  78  of the mid portion  72  also extend through the rectangular opening of the lens  68 . Two of the slots  118  and  119  are sized to snugly receive the portions  96  and  97  of the support post  94  that project outwardly from the mid portion  72  of the base  62 . The electron lens  68  may be welded or otherwise fixed to the portions  96  and  97  of the support post  94 . 
     The third slot  120  (shown by hidden lines in  FIG. 3 ) is sized to easily receive the portion  88  of the terminal post  84  that projects outwardly from the curved side of the mid portion  72  so that an outer edge of the slot  120  does not contact the portion  88 . The non-contacting fit avoids electrical contact between the terminal post  84  and the electron lens  68 . The top of the electron lens  68  includes an opening  122  that is coaxially aligned with the opening  114  of the cover portion  66 . Accordingly, the imaginary line  49  also extends centrally through the opening  122  of the electron lens  68 . The electron lens  68  is positively biased with respect to the ion volume  36  ( FIG. 1 ). In this regard, it is to be understood that all or part of the electron lens  68  may be considered an electrically conductive element that is in a region proximate to the filaments  46 ,  48 . Alternatively, other elements having insulative or semiconductive properties may be capable of being biased and/or acting as an electron lens or gate for urging ions toward the ion volume  36 . 
     The filaments  46  and  48  are both hairpin filaments that, as shown in  FIG. 2 , extend upwardly through the opening  114  of the cover portion  66 . The filament  46  extends vertically upwardly through the opening  114  by a greater distance than the filament  48 . It is to be understood that greater heat is retained in the filaments when they are disposed to a greater degree within the volume of the cover portion  66 . However, the emission section of the filaments  46  and  48  will be shorter when the filaments  46  and  48  extend to a lesser degree through the opening  114  because of the geometry of the opening and a position of the portions of the filaments that are emitting electrons during operation. Also, when the filaments are deeper within the cover portion  66  users cause the filaments to get hotter in order to compensate for fewer electrons that will pass through the opening  114  of the cover. This can result in quicker burn out of filaments. Thus, there are a number of parameters and factors that can be adjusted with associated trade-offs. Nevertheless, the basic principles applied to the disclosed embodiment may be generally extended to other embodiments without departing from the spirit and scope of the invention. 
       FIG. 4  is a diagrammatic fragmentary perspective view showing part of the filament assembly  45  of  FIGS. 2 and 3  in an enlarged scale. Each filament  46  and  48  includes a small diameter refractory metal wire with a hairpin configuration. The diameter of the filaments may be approximately 0.004 in. (inch) although filaments having other diameters may be utilized. The filaments  46  and  48  have curved elongate emission sections  126  and  128 , respectively, at their tips. The imaginary line  49  is normal to and extends through central axes of the emission sections  126  and  128 , the emission sections  126  and  128  being vertically separated by a small distance  129 , as measured from center to center of the filaments  46 ,  48 . The distance  129  may be any value in a range from approximately 0.008 in. to approximately 0.2 in. from center to center of the filaments  46  and  48 . As may be appreciated with 0.004 in. diameter filaments an actual spacing between outer diameters of the filaments will be approximately 0.004 in. at a lower end of this range. Examples of distances  129  within this range that may be applied include any value in a range from 0.010 to 0.015 in. from center to center of the filaments  46  and  48 . When the filaments or their materials are somewhat flexible, the practical limit is that at which the filaments will not physically engage each other or otherwise act as a single filament. For example, a distance between the outer diameters of the filaments may be 0.002 in. in some cases. 
     The filament  46  has arms  130  and  132  extending downwardly at a diverging angle to each other from opposite sides of the emission section  126 . The filament  46  has legs  134  and  136  horizontally extending from the lower end of each arm  130  and  132 , respectively. The filament  48  has arms  138  and  140  extending downwardly at a diverging angle to each other from opposite sides of the emission section  128 . The filament  48  has legs  142  and  144  horizontally extending from the lower end of each arm  138  and  140 , respectively. The leg  144  of filament  48  is L-shaped. 
     The filaments  46  and  48  are welded to the terminal posts  82 - 84 . More particularly, the legs  136  and  144  of the filaments  46  and  48  are both welded to the portion  90  of the terminal post  82 . The second leg  134  of filament  46  is welded to the portion  91  of the terminal post  83 . The second leg  142  of filament  48  is welded to the portion  92  of the terminal post  84 . Accordingly, a current flowing through terminal posts  82  and  83  will energize filament  46  and a current flowing through terminal posts  82  and  84  will energize filament  48 , in a manner discussed later. A first imaginary plane (not illustrated) includes the line  49  and a centerline of the filament  46 , and a second imaginary plane (not illustrated) includes the line  49  and a centerline of the filament  48 . These two imaginary planes are arranged at an angle with respect to each other. In the disclosed embodiment this angle is about 60°. However, the angle could alternatively be 90°. Further alternatively, the angle between these planes could be any of a variety of other angles including angles in a range from approximately 0° to approximately 90°. 
     Although the filaments  46  and  48  are shown as having a hairpin configuration and are operated as thermionic emitters commonly referred to as hot wire filaments, it is to be understood that other types of electron emitters could be substituted for the filaments  46  and/or  48  without departing from the spirit and scope of the invention. For example, electron emitters may be provided by field emitters, which may include electron discharge needles. Field emitters that include a plate with whiskers could be implemented if a very narrow tip is provided. The filaments  46  and  48  shown and described in the embodiment of this disclosure may be made of rhenium. Alternatively, the filaments  46  and  48  may optionally include tungsten, thoriated tungsten, thoriated tungsten rhenium, thoriated iridium, yttria coated rhenium, or any other suitable material. In addition, even though the filaments  46  and  48  are disclosed as hairpin filaments, it is understood that other types of filaments may optionally be used, such as ribbon filaments or coil filaments. As such, the filaments  46  and  48  may include combinations of different filament types, sizes/thicknesses, and/or different materials. For example, one filament may be a ribbon filament made of tungsten and the other filament may be a coil filament made of rhenium. A variety of combinations of filament types and materials are within the scope of this disclosure. The combinations of filament types and materials can be optimized for particular applications such as one or more of CI and EI, for example. Furthermore, it is to be understood that the term electron emitter as used herein may refer to more elements than the filaments or portions of the filaments that emit electrons. For example, the term electron emitter may refer to any number of elements that work together to emit electrons including one or more of a filament, any power source, a control for operating the power to the filaments, and any structural or operational elements supporting the filaments and their function. 
     Incorporation of different filament types may also require a different housing or other structure for supporting the filaments. For example, a platform like the shield described herein would be better adapted for supporting ribbon filaments. The relative positioning of the filaments may be described in terms of the spacing of the filaments or center lines of the filaments from the target location for the electrons. For example, the first filament may be spaced from the location by a first distance and the second filament may be spaced from the location by a second distance. The second distance may be greater than the first distance. On the other hand, with some filament types and geometries it is possible to make the distances from each of the filaments to the target location equal, such as by intertwining coil filaments for example. 
     Whereas the filaments  46 ,  48  are shown and described as hairpin type filaments positioned with a spacing between the emission sections, it is to be understood that the filaments  46 ,  48  could be replaced by coil filaments. Geometries of coil filaments may include two crossed coils, two coaxial coils of different diameters one inside the other, or two intertwined coils. As may be appreciated, by using different coil diameters, placing the coils in a coaxial relation, and/or intertwining the coils the emission sections may be located relative to each other as desired. In particular, the coils may be intertwined such that a distance from the emission section of each is at the same distance from the target location for the electrons. This can be achieved while keeping both filaments and their emission sections aligned with the electron entrance hole of the ionization volume. 
       FIG. 5  is a schematic diagram of the circuitry of the filament supply  52  of  FIG. 1 . The filament supply  52  is electrically coupled at  53  to the control system  22  for receiving control signals from the control system. The filament supply  52  includes a filament drive  150  that provides a current for energizing one of the filaments  46  and  48 . The filament drive  150  is controlled by a control signal  151 , and has a positive terminal  152  and negative terminal  154 . The filament supply  52  includes a switch  156  that has six contacts  158 - 163  arranged in a double-pole double-throw configuration. Two contacts  160  and  163  are electrically coupled to the positive terminal  152  of the filament drive  150 , and two contacts  161  and  162  are electrically coupled to the negative terminal  154  of the filament drive  150 . The contact  158  is electrically coupled to filament terminal post  84  and the contact  159  is electrically coupled to filament terminal post  83 . The third filament terminal post  82  is electrically coupled to the negative terminal  154  of the filament drive  150 . 
     The switch  156  can be selectively switched between two states by a select circuit  168  that is controlled by a control signal  169  from the control system  22 . The select circuit  168  may include a not-illustrated solenoid having a plunger coupled to the two movable switch contacts. The select circuit  168  can selectively energize either filament  46  or  48 . The switch  156  is shown facilitating a current flow through filament  48  in one state. More particularly, the switch  156  is shown with contacts  158  and  160  closed and contacts  159  and  162  closed. As such, the filament drive  150  is operatively providing a current flow through terminal posts  82  and  84  and thus, filament  48  is energized by the filament drive. The inactive filament  46  is not energized or may be de-energized by connecting it to or holding it at the potential of the negative terminal  154  of the filament drive  150 . 
     In the other state (not shown), the switch  156  operates with contacts  158  and  161  closed and contacts  159  and  163  closed. As such, the filament drive  150  is operatively providing a current flow through terminal posts  82  and  83  and thus, filament  46  is energized by the filament drive. The inactive filament  48  is not energized or may be de-energized by connecting it to or holding it at the potential of the negative terminal  154  of the filament drive  150 . Energizing the filaments or electron emitters is for the purpose of producing the stream of electrons and may include causing a current flow through one of the electron emitters while preventing or inhibiting a current flow through the other of the electron emitters. 
     The negative terminal  154  of the filament drive  150  is electrically coupled to an electron energy circuit  172  that is controlled by a control signal  173  from the control system  22 . The filaments  46  and  48  have their respective legs  136  and  144  electrically coupled to the negative terminal  154 . The filaments  46  and  48  are electrically biased at a negative potential with respect to the potential of ion volume  36  so that emitted electrons are encouraged to travel to the ion volume. The difference in potential between the ion volume  36  and the filaments  46  and  48  establishes the energy of the electrons as they travel to the ion volume. The electron energy circuit  172  is electrically coupled to the ion volume  36  at output  55 . In the disclosed embodiment, the ion volume  36  is grounded and filaments  46  and  48  are electrically biased at −70 V which generates electrons with an energy of 70 eV. As such, the filament drive  150  is electrically biased with the negative terminal  154  at the potential of −70 V for the desired electron energy. Alternatively, the bias potential may range from about 0 V to about −300 V. In addition, the housing  64 , including the shield portion  65  and the cover portion  66 , is also electrically coupled to the negative terminal  154  via terminal post  82  and is negatively biased with respect to the ion volume  36  at the same bias potential. 
     In this regard, it is to be understood that all or part of the housing  64  may be considered an electrical element that is in a region proximate to the filaments  46 ,  48 , and thus form at least part of a first portion as described above. The housing  64  or any part of it may be electrically insulated from the filaments and may be electrically biased relative to one or more of the filaments and the enclosure forming the ion volume  36 . All or part of the enclosure forming the ion volume  36  may be considered to be a second portion that is proximate to the target location  50 . At least one of the filaments may be electrically biased relative to one or more of the housing  64  and the enclosure forming the ion volume  36 . Thus, at least one of the filaments may be electrically biased relative to at least one of the first and second portions. Alternatively, other elements having electrically conductive, insulative, or semiconductive properties may be capable of being biased and may be alternatively or additionally substituted for all or part of the housing  64  and the enclosure forming the ion volume  36 . These elements form portions of the ion source that are capable of being biased to urge the electrons from the filaments  46 ,  48  toward the ion volume. 
     The filament supply  52  further includes a detect circuit  174  of a known type for detecting whether the filament drive  150  is supplying a current through the currently selected filament. In other words, if there is a filament burnout (i.e. open circuit) with one of the filaments  46  and  48 , the detect circuit  174  relays this information to the control system  22  at output  54 . As such, the control system  22  can actuate the switch  156  to energize the non-burned out filament  46  or  48 , and continue operation of the mass spectrometer  10 . Also, an operator is notified of the filament burnout condition. The current scan may need to be restarted, but the mass spectrometer  10  is not rendered completely inoperative by a burned-out filament, and can continue operating with a minimum of disruption. 
     The filament supply  52  further includes an electron lens bias circuit  175  that is controlled by a control signal  176  from the control system  22 . The lens bias circuit  175  provides a bias potential to the electron lens  68  of the filament assembly  45  via the support post  94 . The electron lens  68  is positively biased with respect to the ion volume  36 . In the disclosed embodiment, the electron lens  68  is electrically biased at a potential of +15 V with respect to the ion volume  36 . Alternatively, the bias potential may optionally range from 0 V to about +150 V. The electron lens is regularly kept at or above the potential of the ion volume. Since the filament is more negative than the ion volume, the electron lens is usually never at a potential between the ion volume and filament. However, depending on the specific geometries and desired energy for electrons entering the ion volume, the electron lens  68  may be placed at the same potential as the ion volume or at a potential between that of the ion volume and the filaments  46 ,  48 . Even though the bias potentials of the filaments  46  and  48 , housing  64 , and electron lens  68  have been disclosed with respect to a grounded ion volume  36 , the ion volume may alternatively float at a desired potential and the bias potentials of the filaments, housing, and electron lens may be set relative to the potential of the ion volume. 
     As previously disclosed, in a filament burnout condition the mass spectrometer  10  can continue to operate with the non-burned out filament. The filament burnout may occur during a sample run. The operator does not have to delay the sample run and shut down the mass spectrometer  10  to repair and/or replace the burned out filament. Since the two filaments will typically not have completely identical characteristics, it will usually be necessary to scrap any scan that was in progress and restart that scan, but this can be quickly and efficiently accomplished during the sample run that is already in progress. The operator can thus restart the interrupted scan with the non-burned out filament without delay. As such, the operator can wait to repair and/or replace the filament during a down-time or a scheduled maintenance of the mass spectrometer  10 . In the situation where one or both filaments have burned out, the modular configuration of the filament assembly  45  ( FIGS. 2 and 3 ) allows for easy removal and replacement. The filament assembly  45  readily connects to the mass spectrometer  10  via the filament terminal posts  82 - 84  and electron lens support post  94 , the mass spectrometer having not-illustrated electrical connectors that cooperate with the posts  82 - 84  and  94 . The filament assembly  45  may be repaired with new filaments, or replaced with a new filament assembly. 
     In the disclosed embodiment, the filament  48  is illustrated in  FIG. 5  as the active filament and filament  46  as the inactive filament. The filament  46  partially extends through the opening  114  of the cover portion  64  ( FIG. 3 ) by a greater distance than filament  48 . As such, filament  46  is closer to the ion volume  36  ( FIG. 1 ) than filament  48 . It is contemplated that filament  46  will typically be energized first, because there is no direct obstruction between it and the ion volume. Then, after the filament  46  is burned out, the filament  48  will be energized. When the filament  46  burns out, the central emission section may be missing, and in that case there would be no direct obstruction between the filament  48  and the ion volume. But even when filament  48  is active and filament  46  is still entirely present, it has been observed that a filament drive current required to produce a specified electron emission current from filament  48  is not significantly different from that for a single filament configuration. In addition, the number and stability of characteristic ions of the sample material produced is substantially the same for CI and EI modes of operation when the instrument is operated with both the dual filament configuration and the single filament configuration. The number and stability of the characteristic ions of the sample material produced is a measure or indication of the emitted electrons which reach the ion volume such that it can be seen that two filaments in accordance with the embodiment of the present invention has no significant adverse affect on the results obtained from an instrument. 
     While not illustrated, a biased electron collector/reflector may be placed generally on line  49  on an opposite side of the enclosure forming the ion volume from the opening  38 . An opening in the enclosure may be provided to enable passage of electrons from the ion volume for at least one of collection and reflection in the biased ion collector/reflector. A feedback/control line may connect the collector/reflector to the control system  22  for sending and/or receiving signals to aid in regulating the emission currents/voltages. 
     Although one selected embodiment has been illustrated and described in detail, it will be understood that it is exemplary, and that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims. For example, it is to be understood that more than two filaments can be implemented in place of the two filament configuration shown and described above without departing from the spirit and scope of the invention. That is, three, four, five, or any number of filaments could be placed adjacent to each other to provide redundancy when an active filament burns out. The plurality of filaments may be aligned axially with emission sections aligned on the line of travel of the electrons into the ion volume.