Patent Application
20090032702
This invention relates in general to ion sources and, more particularly, to an ion source having an electron source configured to selectively provide electrons.
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.
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.
In the accompanying drawings:
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
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
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 SiO2. 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
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.
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
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
In the example of
The third slot 120 (shown by hidden lines in
The filaments 46 and 48 are both hairpin filaments that, as shown in
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.
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 (
In the disclosed embodiment, the filament 48 is illustrated in
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.
