Electromagnetic energy may be employed to facilitate examination of the composition of an unknown gas via photochemistry applications such as soft ionization and photo-fragmentation. The vacuum ultraviolet (VUV) region of the electromagnetic spectrum is particularly useful in these applications because the energies of VUV photons (generally 6-124 eV) correspond to electronic excitation and ionization energies of most chemical species. Vacuum ultraviolet (VUV) light is generally defined as light having wavelengths in the range of 10-200 nanometers.
Most existing systems involve generating VUV light remotely from the area to be exposed, for example using a resonance lamp, frequency-multiplied laser, or synchrotron, and attempting to deliver this light to the area of interest, typically by passing the VUV light through a window. However, window materials and refractive optics in this wavelength range are scarce or non-existent, so it is often impractical to direct or concentrate VUV light. The windows that are employed typically absorb a large fraction of light in this wavelength spectrum, and reflective optics can become contaminated in a less-than perfectly clean environment. In addition, lasers and synchrotrons can be prohibitively expensive and can require large amounts of power and space.
So-called “windowless” photoionization devices (“ionization devices”) allow a greater portion of the light spectrum to be incident on a sample. However, in known windowless ionization devices, positive ions of the plasma (“plasma ions”) and electrons of the plasma (“plasma electrons”) can travel through the aperture through which the light of the plasma is desirably transmitted. The presence of the plasma ions in the ionization region can result in interfering peaks with analyte ions of the sample, and ultimately reduce the reliability of the detection of analyte ions of interest. Plasma electrons and ions can undesirably give rise to hard ionization of the analyte ions of the sample in an uncontrolled manner, either through electron impact ionization or ion-molecule charge transfer reactions.
What is needed, therefore, are better systems and methods of generating VUV light and delivering the VUV light to an area of interest.
In accordance with a representative embodiment, an ionization device comprises: a plasma source configured to generate a plasma. The plasma comprises light, plasma ions and plasma electrons. The plasma source comprises an aperture disposed such that at least part of the light passes through the aperture and is incident on a gas sample. The ionization device further comprises an ionization region; and a plasma deflection device comprising a plurality of electrodes configured to establish an electric field, wherein the electric field substantially prevents the plasma ions from entering the ionization region.
In accordance with another representative embodiment, method of exposing a sample gas to an excitation light is disclosed. The method comprises: generating a plasma comprising light, plasma ions and plasma electrons; passing at least a portion of the light from the plasma through an aperture to an ionization region; passing a gas sample through the ionization region; and generating an electric field to substantially prevent the plasma ions from entering the ionization region.
In accordance with another representative embodiment, an ionization device, comprises: a channel having an inlet end and an outlet end, the inlet end being configured to receive a gas sample; a plasma source configured to generate light, plasma ions and plasma electrons, the plasma source comprising an aperture disposed such that at least part of the light passes through the aperture and is incident on the gas sample released from the outlet end of the channel; and a plurality of electrodes configured to establish an electric field to guide plasma ions. The electric field substantially prevents the plasma ions from exiting through the aperture. The ionization device comprises a magnet configured to establish a magnetic field to guide plasma electrons. The magnetic field substantially prevents the plasma electrons of the plasma from exiting through the aperture and the electric field and the magnetic field are orthogonal.
The representative embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.
In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the representative embodiments. Such methods and apparatuses are clearly within the scope of the present teachings.
An effective strategy for irradiating gas samples for photochemistry applications is to produce a high density light in a geometry that is convenient for coupling to the flow of a sample gas. Described below are representative embodiments of an ionization device that allows for efficient coupling of photons of a desired wavelength (e.g., vacuum ultraviolet (VUV) light) to a flowing gas sample.
In a representative embodiment, a plasma is created in a structure and an aperture in the structure allows for windowless emission of photons (e.g., VUV photons) that are incident on sample ions in an ionization region. A plasma deflection device is provided between the aperture and the ionization region. The plasma deflection device comprises deflection electrodes, which generate a static electric field in the region between the aperture and the ionization region. The electric field deflects (through attraction or repulsion) positive ions of the plasma that traveled through the aperture and substantially prevents these ions from reaching the ionization region. In an embodiment, the plasma deflection device also comprises magnets, which generate a static magnetic field in the region between the aperture and the ionization region. The static magnetic field substantially prevents electrons from reaching the ionization region. The magnitude of the magnetic field is great enough to influence the motion of plasma electrons, but not great enough to influence the motion of plasma ions, which are comparatively massive.
The magnetic field may be oriented orthogonal to the electric field, or parallel to the electric field. As described more fully below, with the magnetic field oriented orthogonal to the electric field the plasma electrons that travel through the aperture drift in a direction that is orthogonal to both the electric field and the magnetic field in a so-called E X B (where “X” designates the cross product) drift. Orientation of the magnetic field is selected so that the plasma electrons do not drift into the ionization region. With the magnetic field oriented parallel (or anti-parallel) to the electric field, plasma electrons that travel through the aperture are subjected to the Lorentz force. Orientation of the magnetic field is selected so that the plasma electrons are deflected away from the ionization region.
In another representative embodiment, a plasma is created in a toroidal cavity constructed with an aperture oriented along an inner surface or wall thereof that allows for windowless emission of photons (e.g., VUV photons) directed radially inward to the flowing gaseous sample. In accordance with representative embodiments, static electric and magnetic fields create a plasma from a source gas. In accordance with representative embodiments, the electric and magnetic fields are orthogonal everywhere in the ionization device. This causes a drift of the plasma electrons in the direction of the cross-product of the electric field vector and the magnetic field vector (E X B). As a result of the EXB drift, movement of the plasma electrons is the superposition of a relatively fast circular motion around a point (commonly referred to as the guiding center) and a relatively slow drift of this point in circular motion according to the geometries of the ionization device. By contrast, due to their relatively large mass and the selection of a comparatively weak static magnetic field, the plasma ions are not significantly influenced by EXB drift but rather accelerate axially in the static electric field. As described more fully below, the orientation and magnitude of the static electric and magnetic fields according to the representative embodiments aids in preventing plasma ions and plasma electrons from being directed into the ionization region of the ionization device.
Light from the plasma source 201 is emitted through an aperture (not shown in
Plasma ions and plasma electrons may undesirably be emitted through the aperture at the end 206 of the plasma source 201. As noted above, it is undesirable for plasma ions and plasma electrons to enter the ionization region 202. In a representative embodiment, the plasma ions that are emitted at the end 206 are deflected by the static electric field 204 in a direction away from the ionization region (y-direction in the coordinate system of
Plasma ions and plasma electrons that are emitted from the aperture at the end 206 of the plasma source 201 and can form a quasi-neutral, plasma-like environment. Formation of such a quasi-neutral plasma-like environment in close proximity to the deflection electrodes of the plasma deflection device 203 can serve to screen the static electric field 204 and diminish its influence on the plasma ions. If the length over which plasma ions and plasma electrons effectively screen the electrostatic potential applied to the deflection electrodes of the plasma deflection device 203 is less than the distance between the deflection electrodes, the usefulness of the static electric field 204 in preventing plasma ions from reaching the ionization region 202 is undesirably diminished.
In a representative embodiment, static electric field 204 is provided in the plasma deflection device 203. Plasma ions are influenced by the static electric field 204 and are deflected away from the ionization region. For example, with the illustrative orientation of the static electric field 204 as depicted in
In the presently described embodiment, the static electric field 204 and the static magnetic field 205 are oriented parallel to one another. It is contemplated that the static electric field 204 and the static magnetic field 205 are oriented anti-parallel to one another.
Plasma ions are influenced by the static electric field 204 and are deflected away from the ionization region (again in the y-direction). Plasma electrons having a velocity component orthogonal to the static magnetic field 205 are subjected to a magnetic component (q v X B) of the Lorentz force (q(E+vXB)), where v is the velocity of the electron, q is the charge of the electron, E is the electric field and B is the magnetic field. The magnetic component beneficially retards the motion of the plasma electrons in the x-direction. Ultimately, a significant portion of the plasma electrons that are emitted from the end 206 of the plasma source 201 are deflected away from the ionization region by the plasma deflection device 203. As such, application of the static electric field 204 and the static magnetic field 205 in the plasma deflection device 203 as depicted in
The various components of the ionization device 300 that are usefully electrically conducting are made of a suitable electrically conductive material such as stainless steel. The various components of the ionization device 300 that are required to be electrically insulating are made of a suitable electrical insulator such as a high-temp plastic (e.g., Vespel®), or a suitable machinable ceramic material (e.g., Macor®, alumina or boron nitride). The magnets of the representative embodiments are illustratively rare-earth magnets, known to one of ordinary skill in the art.
The ionization device 300 comprises a first plasma source 303 and, optionally, a second plasma source 304. The first and second plasma sources 303, 304 are illustratively as described in U.S. Patent Application Publication 20110109226, incorporated by reference above. Notably, the second plasma source 304 provides redundant function to the first plasma source 303 and its function is not described in further detail.
The ionization device 300 comprises a deflection device 305 disposed adjacent to an aperture (not shown in
In certain embodiments, the electric field is orthogonal to the magnetic field. As such, plasma ions are influenced by the static electric field and are deflected away from the ionization region 307. The plasma electrons are subjected to E X B drift and are deflected in the z direction (i.e., out of the plane of the page) in the coordinate system depicted in
In other embodiments, the static electric field is parallel (or antiparallel) to the static magnetic field. Plasma ions are influenced by the static electric field and are deflected away from the ionization region 307. Plasma electrons having a velocity component orthogonal to the static magnetic field are subjected to a magnetic component of the Lorentz force and are deflected away from the ionization region.
The deflection device 305 optionally comprises a first magnet 312 and a second magnet 313. The first and second magnets 312, 313 are of opposite polarity and create a radial magnet field. The first and second magnets 312, 313 may comprise permanent magnets or electromagnets known to one of ordinary skill in the art. Like the first and second deflection electrodes 310, 311, the first and second magnets 312, 313 are disposed annularly around the axis of symmetry 301 so that each of the first and second magnets 312, 313 deflect plasma electrons from both the first plasma source 303 and the second plasma source 304.
A first aperture 314 is provided between the first plasma source 303 and the ionization region 307, and a second aperture 315 is disposed between the second plasma source 304 and the ionization region 307. In a representative embodiment, the first and second apertures 314, 315 are approximately 600 μm in width (z-direction in the depicted coordinate system) and approximately 250 μm in height (x-direction in the depicted coordinate system). The first and second deflection electrodes 310, 311 are separated (in the x-direction) by approximately 1.0 mm, and the ionization region 307 has a radius (in the y-z plane) of approximately 3.0 mm It is noted that the absolute dimensions of the components and their spacing is merely illustrative. However, the scale of the dimensions is controlled to ensure suitably sufficient field strengths needed to ensure deflection of ions and electrons away from the ionization region 307.
The first and second apertures 314, 315 provide windowless illumination of the sample gas by the light from the generated plasmas. Plasma ions and plasma electrons can traverse the first and second apertures 314, 315 and travel vertically (−y direction and y direction, respectively, in the coordinate system of
In certain embodiments the first and second magnets 312, 313 are configured to provide the static magnetic field that is orthogonal to the direction of the static electric field established between the first and second deflection electrodes 310, 311. As such, in the coordinate system depicted
In certain embodiments the first and second magnets 312, 313 are configured to provide the static magnetic field that is parallel (or antiparallel) to the direction of the static electric field established between the first and second deflection electrodes 310, 311. As such, in the coordinate system depicted
A plasma 406 is created in a cavity 407, which substantially encircles the channel 402. The cavity 407 is formed in a structure 409, which comprises an aperture 408 along an inner wall 409′ of the structure. As described more fully below, the aperture 408 along inner wall 409′ allows photons (e.g., VUV photons) created in the plasma 406 to be incident on the gas sample 405 at the outlet 404 of the channel 402 and to cause photoionization of the gas sample 405.
A plasma anode 410 is disposed at one end of the cavity 407 and a plasma cathode 411 is disposed at the opposing end of the cavity 407. An outer magnet 412 is provided in a recess 413 of the housing 401 and substantially encircles the cavity 407. An inner magnet 414 substantially encircles the channel 402 as depicted. Notably, the outer and inner magnets 412, 414 are of opposite polarity and create a radial magnet field. The outer and inner magnets 412,414 may comprise permanent magnets or electromagnets known to one of ordinary skill in the art. In a representative embodiment, the outer and inner magnets 412, 414 provide a field strength in the range of 2000 Gauss to approximately 10000 Gauss.
An optional plasma electron deflection electrode 415 is disposed near the outlet 404 of the channel 402. The plasma electron deflection electrode 415 substantially encircles the channel 402 near the outlet 404 as depicted in
Ion extraction optics 418 are provided adjacent to the plasma ion deflection electrode 416. An ionized gas sample 419 is provided at an exit 420 of the ionization device 400. In mass spectrometer 100, the exit 420 is connected to the mass analyzer 102. In a representative embodiment, suitable voltage differences are maintained between the ion extraction optics to ensure movement of the ions from ionization region 417 and the mass analyzer 102.
The ionization device 400 is disposed about an axis of symmetry 421, which defines an axial direction of the present teachings. As described below, an electrostatic voltage difference is established between the plasma anode 410 and the plasma cathode 411 in the axial direction. A magnetic field is established by the outer and inner magnets 412, 414 in an inward radial direction (i.e., orthogonal to the axial direction) as depicted by arrows 422 in
An inlet port (not shown in
The plasma anode 410 and the plasma cathode 411 are connected to an energy source (not shown). The energy source may be configured to provide energy to the source gas in the form of a DC voltage, a pulsed voltage, or an oscillating signal with some appropriate frequency such as RF or microwave to generate and maintain a plasma.
In representative embodiments, cavity 407 is illustratively toroidal. In operation, a source gas is supplied to an inlet port (not shown in
The magnetic field is oriented radially inward (i.e., perpendicular to the axis of symmetry 421, depicted by arrows 422). This orthogonal orientation of the static electric and magnetic fields creates an EXB-drift wherein movement of the electrons of the plasma 406 is the superposition of a relatively fast circular motion around the guiding center and a relatively slow drift of this point in the direction of EXB (i.e., rotationally about the axis of symmetry 421 as depicted by arrow 423). Stated somewhat differently, the motion of the plasma electrons of plasma 406 is azimuthal with a substantially constant velocity in arcs around the axis of symmetry 421. The magnetic field traps the electrons of the plasma 406 in an EXB drift orbit about the axis of symmetry 421. The plasma electrons ionize the source gas introduced into the cavity 407 and aid in sustaining the plasma 406. The plasma ions created by the plasma electrons are not significantly influenced by the comparatively weak magnetic field, and rather are accelerated by the axial electrostatic force between the plasma anode 410 and the plasma cathode 411, further sustaining the plasma 406.
As described more fully below, in addition to plasma creation, the plasma anode 410 and the plasma cathode 411 serve to confine plasma ions and plasma electrons to the cavity 407 and thus substantially prevent plasma ions and plasma electrons from traveling through the aperture 408 and into the ionization region 417. Similarly, in addition to plasma creation, the inner and outer magnets 414, 412 serve to confine electrons in the cavity and thus substantially prevent plasma electrons from traveling through aperture and into the ionization region. As such, the plasma anode 410 and the plasma cathode 411 in conjunction with the inner and outer magnets 414, 412 function as a deflection device in accordance with a representative embodiment.
The relative orientation of the orthogonal electric and magnetic fields of the ionization device 400 not only functions to create and sustain plasma 406, but also functions to substantially confine the electrons and ions of the plasma 406 within the cavity 407. Because there is no window over aperture 408, there is a potential for leakage of ions and electrons through the aperture 408, into the ionization region 417 and into the channel 402. Such ions and electrons could contaminate the sample gas/ions and result ultimately in inaccurate measurements by the mass spectrometer 100. Beneficially, and as described above, the ions of the plasma 406 are guided strongly by the electric field between the plasma anode 410 and the plasma cathode 411, and are substantially prevented from exiting the aperture 408. The electrons of the plasma 406 are confined in the EXB drift rotationally about the axis of symmetry, and are substantially prevented from exiting through the aperture 408 as well.
A gas sample 405 is provided at the inlet 403. A plasma 406 is created in cavity 407 of structure 409, which substantially encircles the channel 402. The structure 409 is illustratively an electrical insulator (e.g., high-temp plastic or suitable machinable ceramic material) that isolates the cavity 407 from electric fields generated to deflect plasma ions and plasma electrons, and from ion extraction optics 418 to ensure that the electric field in the cavity 407 is axial (i.e., parallel to the axis of symmetry 421). The aperture 408 along inner wall 409′ allows photons (e.g., VUV photons) created in the plasma 406 to be incident on the gas sample 405 at the outlet 404 of the channel 402 and to cause photoionization of the gas sample 405.
As depicted in
The magnetic field is oriented radially inward (i.e., perpendicular to the axis of symmetry 421, depicted by arrows 422). This orthogonal orientation of the static electric and magnetic field creates an EXB-drift wherein movement of the electrons of the plasma 406 is the superposition of a relatively fast circular motion around the guiding center and a relatively slow drift of this point in the direction of EXB (i.e., rotationally about the axis of symmetry 421 as depicted by arrow 423). Stated somewhat differently, the motion of the plasma electrons of plasma 406 is azimuthal with a substantially constant velocity in arcs around the axis of symmetry 421. The magnetic field traps the electrons of the plasma 406 in an EXB drift orbit about the axis of symmetry 421. The plasma electrons ionize the source gas introduced into the cavity 407 and aid in sustaining the plasma 406. The plasma ions created by the plasma electrons are not significantly influenced by the comparatively weak magnetic field, and rather are accelerated by the axial electrostatic force between the plasma anode 410 and the plasma cathode 411, further sustaining the plasma 406.
Optional plasma electron deflection electrode 415 is disposed near the outlet 404 of the channel 402. The plasma electron deflection electrode 415 substantially encircles the channel 402 near the outlet 404 as depicted in
While representative embodiments are disclosed herein, one of ordinary skill in the art appreciates that many variations that are in accordance with the present teachings are possible and remain within the scope of the appended claims. The invention therefore is not to be restricted except within the scope of the appended claims.
Number | Name | Date | Kind |
---|---|---|---|
4476392 | Young | Oct 1984 | A |
5481107 | Takada et al. | Jan 1996 | A |
7332715 | Russ, IV et al. | Feb 2008 | B2 |
7618806 | Reilly et al. | Nov 2009 | B2 |
20080296485 | Benter et al. | Dec 2008 | A1 |
20090121127 | Orlando et al. | May 2009 | A1 |
20100032559 | Lopez-Avila et al. | Feb 2010 | A1 |
20110109226 | Cooley et al. | May 2011 | A1 |
Entry |
---|
International Search Report mailed Jan. 17, 2013 for Application No. PCT/US2012/040407. |
Number | Date | Country | |
---|---|---|---|
20130001416 A1 | Jan 2013 | US |