WO2008103733A2 - Gas ionizer - Google Patents

Gas ionizer Download PDF

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Publication number
WO2008103733A2
WO2008103733A2 PCT/US2008/054425 US2008054425W WO2008103733A2 WO 2008103733 A2 WO2008103733 A2 WO 2008103733A2 US 2008054425 W US2008054425 W US 2008054425W WO 2008103733 A2 WO2008103733 A2 WO 2008103733A2
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Prior art keywords
conductor
gas
structures
method
nano
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PCT/US2008/054425
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French (fr)
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WO2008103733A3 (en
WO2008103733A9 (en
Inventor
Erkinjon G. Nazarov
Richard Lee Fink
Evgeny Krylov
Kenneth A. Markoski
Raanan A. Miller
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Applied Nanotech, Inc.
Sionex Corporation
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Priority to US90248707P priority Critical
Priority to US60/902,487 priority
Application filed by Applied Nanotech, Inc., Sionex Corporation filed Critical Applied Nanotech, Inc.
Publication of WO2008103733A2 publication Critical patent/WO2008103733A2/en
Publication of WO2008103733A9 publication Critical patent/WO2008103733A9/en
Publication of WO2008103733A3 publication Critical patent/WO2008103733A3/en

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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01JELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J27/00Ion beam tubes
    • H01J27/02Ion sources; Ion guns
    • H01J27/26Ion sources; Ion guns using surface ionisation, e.g. field effect ion sources, thermionic ion sources
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01JELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J49/00Particle spectrometers or separator tubes
    • H01J49/02Details
    • H01J49/10Ion sources; Ion guns
    • H01J49/14Ion sources; Ion guns using particle bombardment, e.g. ionisation chambers
    • H01J49/147Ion sources; Ion guns using particle bombardment, e.g. ionisation chambers with electrons, e.g. electron impact ionisation, electron attachment
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01JELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2201/00Electrodes common to discharge tubes
    • H01J2201/30Cold cathodes
    • H01J2201/304Field emission cathodes
    • H01J2201/30403Field emission cathodes characterised by the emitter shape
    • H01J2201/30434Nanotubes

Abstract

Field emission based ionization sources are provided, with the emitter (305) being a carbon nanotube field emitter. Such emitters can replace Ni-63 beta emitters. Ionization of a gas (308) that is flowed through the gap (310) between the emitter plates (301, 302) is performed by electron capture of the flow of electrons by the molecules in the gas (308).

Description

Gas Ionizer

This application for patent claims priority to U.S. Provisional Patent Application Serial No. 60/902,487, which is hereby incorporated by reference herein.

BACKGROUND

Radioactive Ni-63 is a beta emitter ionization source, which emits electrons. Generally, the source comes in the form of an electroplated foil. The Ni-63 source from an analytical standpoint is a very attractive source as it provides good ion chemistry and consumes no power. A commercial desire to eliminate radioactive Ni-63 ionization sources from analytical or chemical detection instruments stems from the enormous overhead required to test, regulate and account for all of these sources because of their radioactive nature. This applies as well to government applications, where for example, the U.S. Army alone has tens of thousands of ion mobility spectrometers in the field, each utilizing a radioactive Ni-63 source. The spectrometers must be wipe tested and checked for radioactive leaks at least once per year. Finding these hand-held devices in the field and sending them to facilities that perform these tests adds up to huge sums of money expended by both military and industrial users. Furthermore, during the period of the wipe test, the instrument is not available for use. Anon- radioactive ionization source that does not have to be wipe tested would result in a significant cost savings for both military and industrial users.

Recently, government agencies from the U.S. and other foreign countries have recognized the problem of orphaned radioactive sources worldwide. Such sources pose a security risk in the form of potential material for a "dirty bomb" or other illicit applications. Dirty bombs are not nuclear weapons; they are devices designed to disperse radioactive materials. Despite their relative low power, they can still cause casualties and contaminate the area surrounding the explosion with radioactive material. The largest concern is not from the immediate explosion, but from the hazards resulting from the radiation coming from the radioactive material. Since human senses cannot immediately detect low to moderate radiation levels, one would not know if a nearby explosion was a dirty bomb until authorities announced they had detected it with special sensor equipment.

Detonation of a dirty bomb would first cause fear and have a significant psychological impact on the population. Next, there would be health risks from radiation sickness and increased cancer rates to those exposed to the radiation directly, or through inhalation or ingestion. Finally there is the economic impact. After the contaminated area is identified and evacuated, the public would also face a significant cleanup challenge, depending on the area of the fallout. The methods for successful decontamination are known, however, this cleanup would be time-consuming and costly. The contaminated area would essentially be unusable until the cleanup is complete, adding to the economic impact. In short, a dirty bomb is not a weapon of mass destruction, but rather a weapon of mass disruption.

In addition to use of these materials in a dirty bomb, another concern is that lost or orphaned radiation sources could be inadvertently mixed with other recyclable material. This could result in a general dispersion of the material that would be difficult to follow or detect since there would be no single trigger event that would get the public's attention (such as a bomb blast).

Nuclear sources of radiation that are of concern include:

Cobolt-60 - Gamma emitter: Used to irradiate food to kill pathogens and cancer treatment.

Cesium- 137 - Beta and Gamma emitter: Used in medical and scientific equipment.

Americium-241 - Alpha emitter: Used in smoke detectors and moisture content gauges.

Tritium - Weak Beta emitter: Used for emergency exit signs that glow in the dark. Iridium-192 - Beta and Gamma emitter: Used for detecting flaws in concrete and welding.

Nickel-63 - Beta emitter: Used for gas ionization sources for chemical analysis.

SUMMARY

The details of one or more embodiments of the invention are set forth in the description below and the accompanying drawings. Other features will be apparent from the description, the drawings and the claims.

In one aspect, ionizing a gas includes flowing a gas between first and second conductors, in which the first conductor includes a first coating containing nano-structures, and applying a voltage potential between the first and second conductors such that molecules in the gas form ions. The electrons that result from the ionization of the molecules are emitted from or captured by the nano-structures.

In some implementations, ionizing the gas also include analyzing the ions formed between the first and second conductors. Analyzing the ions may include applying differential mobility spectrometry. In some cases, ionizing the gas also includes filtering out a first type of ions formed between first and second conductors. The filtering may include applying a filtering potential across ions exiting the first and second conductors.

In certain implementations, ionizing the gas includes correcting a trajectory of non- filtered ions. Correcting a trajectory of non- filtered ions may include applying an acceleration potential across the non-filtered ions.

In some examples, ionizing the gas includes reacting the ions formed between the first and second conductors with an analyte gas to form analyte ions.

In some cases, the voltage is a RF voltage.

In another aspect, a device for ionizing a gas includes a first conductor having a first coating, in which the first coating includes field emission nano-structures, a second conductor spaced apart from the first conductor, and a voltage source coupled to the first conductor and the second conductor to bias the field emission nano-structures.

In another aspect, an apparatus for ionizing gas includes an ionization source having electron field emission nano-structures, a voltage source coupled to the ionization source to bias the field emission nano-structures and an ion analysis device coupled to the ionization source.

In some implementations, the voltage potential is at a level to cause electron capture of the flow of electrons by the molecules in the gas.

In some cases, the voltage potential is at a level to cause removal of electrons from the molecules in the gas by electron impact ionization.

In certain implementations, the voltage potential is at a level to cause electron disassociation of electrons from the molecules in the gas.

In some examples, the second conductor includes a second coating containing nano- structures. The nano-structures may include carbon nanotubes. Each of the first conductor and second conductor may comprise a glass substrate.

In some implementations, the first conductor includes a plurality of spaced apart regions on a first dielectric substrate where an intermittent pattern of conductive films are deposited alternating between films including the first coating and films that do not include the first coating. The second conductor may also include a plurality of spaced apart regions on a second dielectric substrate where an intermittent pattern of conductive films are deposited alternating between films including a second coating containing nano-structures and films that do not include a second coating containing nano-structures. The first and second conductors may be positioned in a spaced apart relation to each other so that each region on each conductor having a conductive film including nano-structures emits electrons to a region on the opposing conductor having a conductive film that does not include nano-structures.

In certain cases, the first conductor includes a grid structure for the gas to flow through. Each grid may include a wire grid. In some cases, the second conductor includes a grid structure for the gas to flow through.

In some implementations, the gas includes a carrier gas and a sample gas different from the carrier gas.

In some examples, a pressure level between the first and second conductor is substantially equal to atmospheric pressure.

In some cases, the analyte gas is a vapor.

In certain implementations, the electrons flowing between the molecules and the first conductor are emitted from or captured by the nano-structures through quantum mechanical tunneling. The device may include a first filter electrode on the first conductor and a second filter electrode on the second conductor, in which the first filter electrode on the first conductor is spaced apart from and opposes the second filter electrode on the second conductor. The filter electrodes may be downstream of the ionization source.

In some examples, the device includes accelerator electrodes downstream of the ionization source.

In some cases, the device can include a first detection electrode on the first conductor and a second detection electrode on the second conductor, in which the first detection electrode on the first conductor is spaced apart from and opposes the second detection electrode on the second conductor. In certain implementations, a gas channel is coupled to an outlet of the ionization source.

In some examples, a gas source is coupled to an inlet of the ionization source. In certain cases, the ion analysis device is a differential mobility spectrometer, a high field asymmetric waveform ion mobility spectrometer (FAIMS), a time-of-flight ion mobility spectrometer, a mass spectrometer, or a macro ion mobility spectrometer.

Embodiments of the present invention may replace existing operating nuclear sources of radiation with alternative sources of radiation or alternative approaches that would eliminate the need for nuclear sources. This would have several advantages: 1) less material would be available for dirty bombs or other illicit activities; 2) fewer sources would be lost and the number of accidental exposure cases would be reduced; 3) administrative expenses would be reduced since both the user and the government would not have to keep track of the source; and 4) with fewer sources, more attention and resources can be placed in other hazard areas. Embodiments of the present invention replace any radioactive ion sources, including

Ni-63 beta emitters, with a field emission (FE) based ionization source. In one embodiment, the source is a carbon nanotube (CNT) field emitter. This FE -based ionization source may be useful for a whole class of devices that currently use radioactive materials for gas ionizers.

The CNT gas ionizer may be a nearly one-for-one replacement for the Ni-63, requiring only additional driver electronics to operate the source. The CNT ionizer operates over a wide temperature range (a range of -50 0C to +100 0C and greater). The source may be designed to fit almost any ion density need by increasing the source size and shape needed for analytical instruments needing such a source. Only minor additional weight and power are required to operate the CNT gas ionizer (basic instrument remains unchanged; electronics for driving the CNT-based source are very small compared to what is typically already available on the analytical tools such as ion mobility spectrometers that require gas ionization sources). An advantage of the CNT gas ionizer is that it can be switched on and off as needed. An additional advantage is that ion current, ion composition and ion or plasma chemistry can be controlled by the drive signals to the CNT gas ionizer, including the shape of the voltage pulse supplied to the CNT gas ionizer. Radioactive materials cannot be switched off and the energy of the emitted radiation is constant and thus cannot be modified easily.

There are many potential uses for the CNT-based ionizer. For example, tools that typically use radioactive Ni-63 sources for gas ionization include mass spectrometers, electron capture detectors, and ion mobility spectrometers, including differential ion mobility spectrometers such as those produced by Sionex. The CNT ionizer may be easily adapted for these tools to be used in place of the radioactive Ni-63 source. In another example, compact accelerator neutron generators typically use a penning ion source to accelerate deuterium into a metal hydride target that is loaded with deuterium or tritium. The penning ion source can be replaced with a carbon nanotube based ionizer source. This reduces the size of the non-radioactive neutron generator and allows for higher performance.

In some cases, radioactive ionizers (which utilize the radioactive isotope Am-241) are used in smoke detectors. Although the Am-241 source emits alpha particles, it still functions as an ionizer. The CNT-based ionizer may be used to replace these radioactive ionizers.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a comparison of a band gap under no electric field and under an applied electric field;

FIG. 2 A illustrates functionality of a high aspect ratio conductor concentrating an applied electric field;

FIG. 2B illustrates a diode-type field emission display structure using carbon nanotube emitters;

FIG. 3 illustrates an embodiment of the present invention;

FIG. 4 illustrates field modeling showing that a high density of CNT fibers shields an electric field from neighboring fibers;

FIG. 5 illustrates another embodiment of the present invention; FIG. 6 illustrates another embodiment of the present invention;

FIGS. 7A-7B illustrate another embodiment of the present invention; FIG. 8 illustrates another embodiment of the present invention; FIG. 9 is a photograph of another embodiment of the present invention; FIG. 10 illustrates an ionization source coupled to an analysis device; FIGS. 1 IA-11C show graphs depicting positive ion spectra obtained from a mass spectrometer;

FIGS. 12A-12C show graphs depicting positive ion spectra obtained from a mass spectrometer;

FIGS. 13A-13B show graphs depicting spectra obtained from a differential mobility spectrometer;

FIG. 14 illustrates another embodiment of the present invention; FIG. 15A illustrates an ionization source coupled to a gas channel; FIG. 15B illustrates an ionization source coupled to a gas channel; and FIG. 16 illustrates a system for filtering ions.

DETAILED DESCRIPTION

A mechanism for producing ionized gas molecules that does not require a radioactive source includes UV lamps. Typically, such sources are limited to ionization potentials less than 11.8 eV, as this is the bandgap of LiF windows used in the lamps. In addition, UV lamps are bulky, contain mercury and are hazardous to the eye. Furthermore, UV lamps have a limited life-time (a maximum of 1000 hours), are expensive, and need re-calibration after cleaning the windows. Another approach is to create a corona discharge in the gas. This is done by placing a high voltage across the gas between two electrodes, high enough to create an electrical breakdown of the gas between the electrodes. A corona discharge creates many ions, but it also creates undesirable ion species such as NOx " molecules that have high electron affinity, i.e., they hold on to the negative charge and do not give it up to other gas molecules. Another issue is that a corona discharge will also crack larger molecules that are often the point of interest for many of the applications for gas ionizers.

An alternative ionization mechanism that does not require radioactive sources, UV lamps or high voltages is electron field emission. Electron field emission entails the emission of electrons from a material, such as a conductor, under the influence of a strong electric field. In electron field emission, the potential barrier of the material is lowered by an applied electric field so that electrons can tunnel from the Fermi level of the material into vacuum or a gas environment (R. Gomer, Field Emission and Field Ionization in Condensed Phase, Accounts of Chemical Research, 5(2):41, 1972).

There are two scenarios in which electron field emission may be used for gas ionization. In a first case, a field emission structure is biased negatively, such that electrons are emitted from the field emitter structure into a gas environment. After emission, the electrons accelerate under the effect of the electric field and collide with the gas molecules. If the electrons have appropriate kinetic energy, they will be captured by the gas molecules upon collision and form negative ions. Alternatively, if the electron has sufficient kinetic energy, the impact frees one or more electrons from a gas molecule to form a positive ion. In a second case, the field emission structure is biased positively such that electrons are pulled from the gas molecules into the field emission structures, thus producing positive ions.

FIG. 1 illustrates band diagrams for a material under the influence of an electric field equal to 0 and 107 V/cm. The application of the electric field considerably reduces the potential barrier so that electrons can quantum mechanically tunnel through the barrier and escape the material. Alternatively, in the case of a positively biased structure, electrons quantum mechanically tunnel from gas molecules into the structure. In conductors, the barrier height is equivalent to the material's work function. An estimate of the efficiency of electrons emitted from the material surface follows the Fowler-Nordheim equation: Je = //S ~ Λexp[-0.68Φ3/2 /£]

where Je is current density of electrons emitted from surface S, I is the current, Φ is the material work function and E is electric field strength. The parameter A is a constant. As an example, an electric field of 107 V/cm applied to a material having a work function in the range of 4.5 to 5.5 eV could produce electron current densities up to 5 mA/cm2. According to the Fowler-Nordheim equation, an electric field on the order of several megavolts/cm (~ several 100 V/μm) is sufficient to produce electron emission from materials. Hence, it is the magnitude of the field strength that is important and not necessarily a high voltage. One way to practically achieve fields with such large magnitudes is to use conducting or semiconducting structures that have very high aspect ratios {e.g., structures that are tall and thin) and place them in an electric field. Because the high aspect ratios will concentrate the electric fields at the ends or tips of the structure, electron field emission can be achieved with applied electric fields as low as 1-10 V/μm since the electric field at the tips of these high aspect features can be as high as 100-1000 V/ μm. FIGS. 2A-2B illustrate this concept. As shown in FIG. 2A, the electric field F0 between two electrodes 12, 14 is determined by the voltage applied across the electrodes and the distance 16 between them. However, if a field emission structure 10 is positioned between the two electrodes 12, 14, the field F at the tip of the structure is concentrated. Depending on the ratio of the structure height h to radius r, the concentration of the field may be large enough to induce electron emission from the structure 10. The size restrictions necessary for achieving electron field emission may be achieved using micro and nano-structures. An example of a nano-structure that is suitable for electron field emission is a carbon nanotube. Certain single-walled and multi-walled carbon nanotubes have diameters in the range of 2-20 nanometers.

An example that illustrates using nanotubes as electron field emission structures is shown in FIG. 2B. The apparatus depicted in FIG. 2B includes a diode-type field emission display structure 18 using carbon nanotube emitters 20. The carbon nanotubes 20 are formed on a cathode 22 of the display and emit electrons when a potential 26 is applied across the cathode 22 and anode 24. The anode includes a phosphor coating 28. Upon exposure to the electrons ejected from the nanotubes 20, the phosphor coating 28 emits light.

An example of using field emission structures to produce ionized gas molecules is illustrated in FIG. 3. An ionization device 300 includes two conductor plates 301, 302. The plates are separated by a gap 310 having a height d that ranges from 50 - 10,000 microns and a width w (extending into the page). The conductor plates may comprise a conductor material or a conductor-coated insulator such as, for example, metal-coated glass or ceramic panels. In the example shown in FIG. 3, each of the conductor plates includes an insulator 303 and a conductive coating 304. The conductive coating 304 may be formed using fabrication techniques such as electron-beam deposition, sputtering or chemical vapor deposition. Other techniques for forming the coating 304 may be used as well.

One of the conductor plates 301 is also coated with micro or nano-structures that serve as electron field emission structures 305. The surface of the other conductor plate 302 may be relatively smooth. The field emission structures 305 include, but are not limited to, single- walled or multi-walled carbon nanotubes (including double wall), nanowires and microtips. The nanowires and microtips may be formed of a conducting material, such as metal, or semiconducting material, such as silicon. The field emission structures 305 may be formed using chemical vapor deposition or printed using inks or pastes. The aspect ratio for the micro and nano structures ranges from 10 - 10,000 (typically 100 - 1000). The field emission structures may be vertically aligned, as shown in FIG. 3. A potential source 306, such as a battery or power supply, is applied across the conductor plates 301, 302. Gas flow 308 provides the gas molecules to be ionized by the field emission structures 305. The flow of gas may occur at atmospheric pressure or very close to atmospheric pressure, although the ionization source may be operated under sub-atmospheric conditions as well. In some cases, both conductors 301, 302 are coated with field emission structures 305. Ionization device 300 is operated with the field emission structures 305 biased negatively by the power source 306. As explained above, the negative bias induces electrons to quantum mechanically tunnel from the field emission structures 305 into the gas environment located between the conductor plates 301 , 302. The extracted electrons accelerate due to the applied electric field that exists across the plates. The applied electric field may alternatively be provided by an AC field, a DC field or simultaneous application of both AC and DC fields. When an AC field is used, electron emission may occur for only a portion of the time that the field is applied. As a result, the electrons gain kinetic energy. In some cases, the electrons will collide with the gas molecules flowing through the device 300. When low voltages are used, the electrons do not experience strong acceleration and thus enable a "soft" plasma to form in the gap between the conductor plates such that ion chemistry is avoided. Accordingly, there is no danger of a corona discharge occurring or of cracking molecules that are of interest for gas ionizers. If the kinetic energy of the electrons is smaller than the ionization potential of the gas molecules, the electrons may be captured by the molecules (thus forming negative ions). For example, in the case of oxygen molecules (which have an electron affinity equal to 0.5 eV) passing through the device 300, the electrons may be captured to form negative oxygen ions. Alternatively, the electrons may pass through the gap 310 to the conductor plate 302.

If the applied voltage is increased, the electrons may gain enough kinetic energy such that, upon collision with the gas molecules, positive ions and secondary electrons are formed. This is known as electron impact ionization. However, at the interface close to the field emission structures 305, most electrons will not have gained enough kinetic energy for impact ionization, such that electron capture is the main process by which ionization occurs. Further from the field emission structure 305, the electron may have sufficient energy to create positive ions through electron impact ionization. Accordingly, it is possible to form both positive and negative ions similar to the process that takes place with radioactive 63Ni ionization sources. By controlling the voltage applied across the conductor plates and/or the gap height, it is possible to accelerate the electrons to a moderate level where a soft plasma forms but avalanche processes do not occur. The ions are formed at atmospheric pressure levels inside the gap 310, but the device may be configured for lower and higher pressures for other applications, ranging from sub-millitorr to a few atmospheres of pressure. There are several issues to consider when using micro and nano-structures for electron field emission. For example, given that the emission structures 305 will potentially operate in air or other gaseous environments, the tips of the emitters are susceptible to gas adsorption and the formation of physical and chemical bonds with the gas molecules. Accordingly, subsequent changes in work function and aspect ratio of the structures 305 are possible. Such physical and chemical changes may lead to degradation in the electron emission properties of the structures 305. In general, however, carbon nanotubes may inhibit these effects given that the carbon nanotube structures are relatively inert compared to most metals (i.e., oxide layers will not form on carbon nanotube surfaces). Additionally, inert gases including, for example, argon or helium, may be used to reduce such physical and chemical changes. Other gases, such as nitrogen, may be used as well.

In some cases, when a very high voltage is applied between electrodes, ions bombard the field emission structures 305 causing erosion damage. This erosion damage is mainly due to water molecules or oxygen ions that attach to the carbon nanotube material and convert it to carbon monoxide or carbon dioxide through chemical reaction, thus leading to a reduction in emitter lifetime. This is particularly true in high vacuum environments in which the ions have high kinetic energy upon impact with the field emission structures. However, if the ionization source 300 is operated at atmospheric pressure, the ions will experience high collision rates with other gas molecules prior to coming into contact with the field emission structures 305. Accordingly, ion erosion effects can be reduced. In addition, inert gas environments may also be used to reduce erosion of the field emission structures 305 due to chemical reaction.

When using multi-walled carbon nanotubes as the field emission structure, the density of nanotubes on the conductor plate may be controlled. In some cases, high densities of nanotubes reduce the overall effectiveness of the field emission structure, whether in air or in vacuum. FIG. 4 shows comparison plots of simulated electric field lines 403 over carbon nanotubes 401, in which the number of nanotubes 401 is 10, 50 or 250. As the number of nanotubes 401 increase on a surface, the nanotubes electrostatically shield the electric field 403 of adjacent structures, thus reducing the effective aspect ratio. To achieve good electron emission at low applied electric fields, in which a measurable amount of ions is generated, the carbon nanotubes may be printed on the conductor plate surface using an ink or paste having a controlled fiber density.

Referring again to FIG. 3, the field emission structures 305 may include high-purity carbon nanotube fibers that are anchored in an adhesive bonding material such as glass frit materials or inorganic polymer materials such as PPSQ. Accordingly, the nanotubes will not easily dislodge from the conductor plate surface as can happen with nanotubes that are formed by chemical vapor deposition. The carbon nanotube fibers are anchored to prevent arcing and short circuits. It is known how to anchor the CNT fibers to prevent arcing and short circuits and how to activate them after printing and firing to get the best performance. In addition, the nanotubes 305 may be arranged vertically to enhance the number of electrons emitted into the gap 310.

Table 1

Figure imgf000014_0001

Table 1 compares carbon nanotube (CNT) based emitters with Ni-63 beta emitters, plasma generators and UV lamps as ionization sources. In general, a CNT based field emission ionization source will have properties that are superior to the radioactive Ni-63 source and other alternative sources. Namely, 1) the intensity of the Ni-63 source, and thus the ion current generated by the source, cannot be changed by the user. In contrast, the ion current from a carbon nanotube based ionizer may be easily modulated by controlling the electron extraction voltage applied across the conductor plates as well as the duty factor. 2) The carbon nanotube based ionizer may be switched off. 3) There is no regulatory oversight required for field emission ionization sources that include carbon nanotubes. 4) The field emission devices are low power. 5) The field emission device may be easily retrofitted into tools currently using Ni- 63 sources. Finally, the cost of ownership may be much less compared to the Ni-63 sources and very competitive with other approaches. The electron field emission ionizer has another advantage. By varying the energy of the electrons emitted from the field emission structures (by changing the electrical bias on the electrode having the carbon nanotubes 305), the mixture of ions that are created may be partially controlled. This opens up the ionization tool to many other applications, such as APCI mass spectrometers, electron capture detectors, and ion mobility spectrometers, compact neutron generators, and smoke detectors.

An alternative design layout for the ionization source is shown in FIG. 5. The ionization source 500 of FIG. 5 is similar to the source 300 shown in FIG. 3, except that each conductor plate 501, 502 includes multiple separate conductive electrodes 503. As a result, the overall capacitance and power consumption of the device is reduced. As shown in FIG. 5, voltage is applied to the electrodes 503 in an alternating manner such that each electrode on a conductor plate has a polarity that is opposite to the polarity of an adjacent electrode on the same conductor plate. The electrodes 501, 502 may be fabricated using standard metal deposition, photolithography and etching or lift-off techniques as known in the art. The spacing between each electrode on a plate may be varied. Following formation of the conductive coatings 503, electron field emission structures 505, such as carbon nanotubes, are formed on the surface of the coatings 503. In some cases, the field emission structures 505 are formed only on electrodes 503 that have the same polarity. In other cases, the field emission structures 505 may be formed on either type of electrode. In another implementation, as illustrated in FIG. 6, an ionization source 600 includes two grids 601, 602 arranged parallel to each other. The grids 601, 602 may be formed of metal or an insulator that is covered with a conductive coating. A gas flow 608 is provided that passes through openings 612 in the grids 601, 602. The surface of one or both of the grids is coated with electron field emission structures 605. The size of openings 612 and the distance between the grids may be varied. The size of each of the grid openings 612 may be about the same as the height of the gap 610 that extends between grid 601 and grid 602. The ionization source design shown in FIG. 6 enables a higher gas flow rate due to the increased number of openings. As the gas flows through the openings 612, the gas molecules become ionized by the electrons emitted by the field emission structures 605. Furthermore, the flow of ions is not significantly affected by the field that extends between the grids.

FIGS. 7 A and 7B show additional implementations of an ionization source 700. FIG. 7A illustrates a cross-section of a single wire grid 701 on which electron field emission structures 705 are formed. As with the grids 601, 602 in the implementation of FIG. 6, the wire grid 701 may be formed of metal wires or insulators that are covered with a conductive coating. The wires 702 of the grid are separated by openings 712. The surface of the wire grid 701 is coated with field emission structures (not shown) such as carbon nanotubes. A gas flow 708 is provided that passes through the openings 712 in the wire grid 701. A voltage is applied to the wire grid 701 in an alternating manner such that each wire in the grid 701 has an opposite polarity from an adjacent wire. As the gas flows between the grid openings 712, the gas molecules become ionized by the electrons emitted from the field emission structures. Once formed, the ions do not experience significant drift due to the applied electric field between wires 702. However, because of the smaller area in which ionization occurs, fewer ions may be formed. The gas flow does not need to travel perpendicular to the openings but also may enter the openings 712 at oblique angles as shown in FIG. 7B.

The ionization source 300 shown in FIG. 3 requires fewer fabrication steps and is a simpler construction design than the other sources shown in FIGS. 5-7. Accordingly, the cost of fabricating the source 300 is potentially cheaper than the other devices. However, given the long path length that gas molecules follow when passing through the gap 310, any ions that are generated in the gap 310 are susceptible to significant drift or acceleration due to the applied electric field. In contrast, gas molecules passing through the ionization sources 600 and 700 do not experience significant drift due to an electric field extending between the openings in the grids.

An example of a mechanism to counteract ion drift is shown in FIG. 8. The design of the ionization source is similar to the device illustrated in FIG. 3 and includes two conductor plates 801, 802 having length L, a voltage source 806, a gap 810 of height h between the conductor plates, and electron field emission structures 805 formed on one of the conductor plates. A gas 808 flows through the gap between the plates 801, 802. Upon ionization of the gas molecules, the gas flow pushes the ions towards the exhaust end 814 of the ionization source 800. Close to the field emission structures 805, it is mainly negative ions 816 that are formed due to the electron capture mechanism as the electrons do not have enough kinetic energy to induce ionization by impact collision. Further into the center of the gap, however, electrons have gained enough energy to form positive ions 818 and secondary electrons by the impact collision mechanism. The velocity of an ion due to the electric field depends on the direction and magnitude of the electric field as well as the mobility of the ion. The time needed for an ion to pass across the gap 810 of height h can be determined using the equation ti = h2/KV where K is the ion coefficient of mobility and V is the potential applied across the conductor plates 801, 802.

For example, if the ions have a coefficient of mobility of 2-3 Cm2V 1S 1, a DC voltage equal to 30 V is applied across the conductor plates 801, 802, and the gap height is equal to 0.1 cm, the time necessary for the ion to travel across the height of the gap is about th = 100 μs. During the same period, the ions are moving transverse to the gap in the direction of gas flow. However, the velocity of ions in that transverse direction is equal to the velocity of the gas molecules. Accordingly, the time the molecules (and consequently ions) take to pass through the gap can be calculated as the volume of the gap 810 divided by flow consumption, i.e., t = hwL/F where F is the gas flow rate and w is the gap width. Using a gap height equal to 0.1 cm, gap width equal to 0.2 cm, a conductor plate length equal to 0.5 cm, and flow consumption of 8.3 cm /sec, the time an ion takes to pass through the gap in the direction of gas flow is approximately tι= 1.2 ms, which is significantly longer than the 100 μs the ion takes to travel across the gap and transverse to the gas flow. Therefore, the small number of ions that exit the gap are likely generated near the exhaust end 814 of the ionization source 800. In contrast, the ions generated at the entrance to the gap are likely to be neutralized by a conductor plate well before they reach the exhaust end 814.

To compensate for the ion drift, an RF voltage is applied across the conductor plates 801, 802. The RF potential has the harmonic waveform V(t) = A sin(Ωt) where A is a constant and Ω is the frequency. Due to the harmonic nature of the RF voltage, it is possible to increase the ion lifetime in the gap 810 and maintain an average ion displacement such that the ions do not collide with either of the conductor plates 801, 802. For ions traveling in the gap center in the example above, it is preferable that the frequency Ω may be greater than or equal to 0.1 MHz.

FIG. 9 shows an example of an ionization source 900 in which carbon nanotubes (not shown) are used as the electron field emission structure. The ionization source of FIG. 9 may be used in conjunction with ionization mass spectrometers, ion mobility spectrometers and differential mobility spectrometers. The device 900 includes a gas outlet 902 at a near end and a gas inlet (not shown) at a far end. The gas outlet has a cross-section of 2 mm by 0.5 mm and accommodates a gas flow rate of 0.5 liters per minute. The electron field emission ionization device may be operated in three modes. In a first positive mode, the carbon nanotube electron emission structures are biased positively such that electrons are pulled from gas molecules thus creating positive ions. In a second negative mode, the carbon nanotubes are biased negatively with a potential that is not sufficient to induce impact ionization. Instead, only negative ions obtained by the electron capture mechanism are produced. In a third, combined mode, the nanotubes are biased negatively with a voltage that is sufficient to allow both electron capture and impact ionization. Accordingly, the third mode produces both positive and negative ions.

Experiments have demonstrated that the device works effectively under atmospheric pressure conditions when exposed to both air and nitrogen. FIG. 10 shows a carbon nanotube based ionization source 1000 coupled to an analysis device 1002 for analyzing the ions generated by the ionization source 1000. The analysis device 1002 may include, for example, a mass spectrometer, an ion mobility spectrometer, a time-of- flight ion mobility spectrometer, a high field asymmetric waveform ion mobility spectrometer (FAIMS), a macro ion mobility spectrometer or a differential mobility spectrometer.

A carrier gas CG flows into the ionization device 1000 and is ionized by field emission structures formed within the ionization device. In some cases, the carrier gas CG includes a sample gas S containing gas molecules to be analyzed. Depending on the mode of operation of the ionization source (i.e., positive, negative, or combined) and the applied electric field, positive and negative ions may be formed within the gap region 1010 of the ionization source 1000. The ionized gas molecules then flow out of the ionization source 1000 and downstream into the analysis device 1002, where analysis and/or detection occurs. As shown in FIG. 10, the ionization source 1000 is separate from, but in communication with, the analysis device 1002 through gas channel 1005. Alternatively, the ionization source may be integrated into the analysis device to form an integrated system in a single device.

FIGS. 1 IA-11C show graphs illustrating positive ion spectra of air, air plus 0.1 part per million methylsalicylate (MS), and air plus 1 part per million MS after ionization by the ionization source 1000 in positive mode, in which the source 300 included carbon nanotubes as the electron field emission structures. MS is a stimulant of the chemical warfare agent sarin. The spectra of FIGS. 1 IA-11C were detected by an analysis device that includes an atmospheric pressure mass spectrometer, which was connected to the exhaust of a differential mobility spectrometer. All ions were measured without separation (RF = 0 V and compensation voltage = 0 V). Comparing the three frames, it can be seen that when only background air (which includes, among others, molecules of nitrogen, oxygen, water vapor, and carbon dioxide) is provided to the device 1000, positive water ions ((H2O)2H+, atomic mass = 37 Daltons) are generated (see FIG. 1 IA). However, when 0.1 parts per million of MS is incorporated into the background air, positive methylsalicylate ions (protonated ions MH+ with atomic mass = 153 Daltons) are produced as well (see FIG. 1 IB). Upon increasing the concentration of MS even further to 1 part per million, it is seen that the background positive water ions are absent (see FIG. HC). This suggests that upon reaction with the protonated monomer MS ions, the positive water ions are removed. FIGS. 12A-12C show mass spectrometer spectra of background air, air plus 1 part per million MS, and air plus 5 part per million MS when operating the ionization source 300 in negative mode. The electron field emission structures of the source 300 included carbon nanotubes. When flowing only background air under negative mode operation, negative oxygen ion species (O2 ", atomic mass = 32 Daltons) are produced. In addition, several types of nitrogen oxide ions (NO2 ", atomic mass = 46 Daltons; NO3", atomic mass = 62 Daltons; NOx) and solvated complexes (H[NOs]2 ", atomic mass = 125 Daltons) are also produced (see FIG. 12A) (see E.G. Nazarov, R. A. Miller, G. A. Eiceman, J. A. Stone, Miniature Differential Mobility Spectrometry Using Atmospheric Pressure Photoionization, Anal. Chem., 2006, 78:4553-4563). Upon introducing MS at a concentration of 1 part per million, the MS gas molecules can react with the negative oxygen ions to form MS ions (MO2 ", atomic mass = 184 Daltons; M-H, atomic mass = 151 Daltons; see FIG. 12B). Increasing the concentration further to 5 parts per million increases the formation of MO2 " and M-H ions (see FIG. 12C). All of these ion species have been formed using radioactive 63-Ni ion sources (see E.G. Nazarov, S.L. Coy, E.V. Krylov, R.A. Miller, G.A. Eiceman, Pressure Effects in Differential Mobility Spectrometry, Anal. Chem., 2006, 78:7697-7706)

Other devices may be used to analyze the ions generated from the ionization source 1000, as well. For example, FIG. 13A shows positive and negative spectra obtained from a differential mobility spectrometer coupled to the ionization source 1000 that includes carbon nanotubes as the electron field emission structures. The spectra include positive (data line 1300) and negative (data line 1302) ions generated from a carrier gas that includes only air ionized by the device 300. FIG. 13B shows comparison spectra in which the ionization source utilizes radioactive 63-Ni instead of carbon nanotube field emission structures. The spectra shown in FIG. 13B also includes positive (data line 1300) and negative (1302) ions.

A comparison of the positive ion spectra shows that similar positive ion species are generated by both the carbon nanotube source and the radioactive 63-Ni source. In addition, a comparison of the negative ion spectra shows that both ionization sources produce negative oxygen ion species (oxygen ions detected at Vc = -9 V for carbon nanotube source and at Vc = -11 V for 63-Ni source), which enable the ionization of MS molecules, if introduced as a sample gas. In contrast, however, when operating in the negative mode, the carbon nanotube ion source produces additional ion species, such as NO2 " and N(V ions (Nitrous oxide ions detected at Vc = -4.5 V and 0 V, see FIG. 13A) that are not produced by the radioactive 63-Ni source.

FIG. 14 shows an example of a system 1400 that includes a carbon nanotube electron field emission source 1402 integrated with a differential mobility spectrum analyzer 1404. The field emission source 1402 is located upstream for plasma ionization. Ions are generated for chemical analysis of a sample S in a carrier gas CG.

More particularly, the system 1400 of FIG. 14 includes an ionization source 1402, an ion filter 1404 in the filter region 1406 defined between filter electrodes 1408 and 1410, and a detector 1412 in a detection region 1414 between detector electrodes 1416 and 1418. Carbon nanotube electron field emission structures 1401 are formed on one or both electrodes 1403, 1405 of the ionization source. Asymmetric field and compensation bias signals or voltages are applied to the filter electrodes 1408 and 1410 by a drive circuit 1420 within a control unit 1422. The detector electrodes 1416 and 1418 are also under the direction of the drive circuit 1420 and the control unit 1422.

In operation, the carrier gas CG, is ionized in the plasma region 1401 forming ions ++,- - and the sample S is ionized creating both positive and negative ions, M+ and M". Based on differential mobility spectrometry ion filtering techniques, only certain ion species pass through the filter region 1406, while others are filtered out (i.e., they are neutralized by contact with the filter electrodes 1408 and 1410). Those that pass through are detected at the detector electrodes 1416, 1418. As depicted in FIG. 14, the electrodes 1403, 1408 and 1416 are coplanar and the electrodes 1405, 1410 and 1418 are coplanar, being formed on the substrates 1424 and 1426, respectively. Differential mobility spectrometry configurations are described in greater detail in U.S. Patent Nos. 6,495,823, 6,512,224 and 7,279,680, the entire contents of which are incorporated herein by reference.

In some cases, the ions formed from the electron field emission source may be used to react with separate analyte gas/vapor molecules to form additional ion species. For example, FIG. 15A shows a system 1500 that includes a carbon nanotube electron field emission ionization source 1502 coupled to an adjacent gas channel 1504. A transport gas 1506 is introduced at the entrance of the ionization source 1502. The transport gas 1506 then is ionized by the carbon nanotube field emission structures 1508. The ionized transport gas can include positive or negative ion species or a combination of both positive and negative ion species. Subsequently, the ionized transport gas merges with an analyte gas/vapor 1512 flowing through the gas channel 1502. Upon merging with the analyte gas/vapor 1512, a secondary ionization reaction takes place due to the collision of the ionized transport gas with molecules in the analyte gas/vapor stream 1512. Following the analyte molecule ionization due to charge exchange, analyte ion species 1514 exit an exhaust region 1516 of the gas channel 1504 due to downstream pressure generated by the analyte flow.

FIG. 15B shows an alternative arrangement of the system shown in FIG. 15A. In the arrangement of FIG. 15B, the flow of ionized analyte gas/vapor molecules is due to downstream pressure generated by the transport gas. A T-junction is established at the exhaust of the ionization source 1502 where analyte vapors/gas are ionized by ions exiting carbon nanotube field emission device 1502.

In some implementations, undesirable ions (e.g., NOx species) and/or neutral molecules may be filtered out of the exhaust flow so that only certain ions react with the analyte flow. For example, FIG. 16 shows a system 1600 for separating ions generated by an electron field emission device 1602 that includes carbon nanotube field emission structures 1604. The system 1600 includes a pair of filter electrodes 1606 arranged on either side of the outlet of the ionization device 1602. The polarity and magnitude of an electric potential applied across the filter electrodes 1606 may be selected to deflect undesirable positive or negative ions exiting the ionization device 1602 prior to entering the gas channel 1610. However, the ions that are desired to interact with the analyte flow 1612 can also have their trajectory affected by the filter electrodes 1606. Accordingly, an additional pair of accelerator electrodes 1614 are located at the entrance to the gas channel 1610. Applying an electric potential across the accelerator electrodes 1612 may serve to correct the flow trajectory of the desired ions such that they enter the gas channel 1610 and react with the analyte molecules.

Claims

WHAT IS CLAIMED IS:
1. A method for ionizing a gas comprising: flowing a gas between first and second conductors, the first conductor further comprising a first coating containing nano-structures; and applying a voltage potential between the first and second conductors causing molecules in the gas to form ions, wherein electrons flowing between the molecules and the first conductor are emitted from or captured by the nano-structures.
2. The method of claim 1, wherein the voltage potential is at a level to thereby cause an electron capture of the flow of electrons by the molecules in the gas.
3. The method of claim 1, wherein the voltage potential is at a level to thereby cause a removal of electrons from the molecules in the gas by electron impact ionization.
4. The method of claim 1 , wherein the voltage potential is at a level to thereby cause electron disassociation of electrons from the molecules in the gas.
5. The method of claim 1, wherein the second conductor comprises a second coating containing nano-structures .
6. The method of claim 1 , wherein the nano-structures further comprise carbon nanotubes.
7. The method of claim 1 , wherein the first coating containing nano-structures is deposited on a glass substrate.
8. The method of claim 1 , wherein the first conductor comprises a plurality of spaced apart regions on a first dielectric substrate where an intermittent pattern of conductive films are deposited alternating between films including the first coating and films that do not include the first coating, wherein the second conductor also comprises a plurality of spaced apart regions on a second dielectric substrate where an intermittent pattern of conductive films are deposited alternating between films including a second coating containing nano-structures and films that do not include a second coating containing nano-structures.
9. The method of claim 8, wherein the first and second conductors are positioned in a spaced apart relation to each other so that each region on each conductor having a conductive film including nano-structures emits electrons to a region on the opposing conductor having a conductive film that does not include nano-structures.
10. The method of claim 1, wherein the first conductor comprises a grid structure for the gas to flow through.
11. The method of claim 10, wherein the second conductor comprises a grid structure for the gas to flow through.
12. The method of claim 11 , wherein each grid comprises a wire grid.
13. The method of claim 1, wherein the gas includes a carrier gas and a sample gas different from the carrier gas.
14. The method of claim 1, wherein a pressure level between the first and second conductor is substantially equal to atmospheric pressure.
15. The method of claim 1, further comprising analyzing the ions formed between the first and second conductors.
16. The method of claim 15, wherein analyzing the ions comprises applying differential mobility spectrometry.
17. The method of claim 15, further comprising filtering out a first type of ions formed between first and second conductors.
18. The method of claim 17, wherein filtering comprises applying a filtering potential across ions exiting the first and second conductors.
19. The method of claim 15, further comprising correcting a trajectory of non- filtered ions.
20. The method of claim 19, wherein correcting a trajectory of non- filtered ions comprises applying an acceleration potential across the non-filtered ions.
21. The method of claim 1 , further comprising reacting the ions formed between the first and second conductors with an analyte gas to form analyte ions.
22. The method of claim 21 , wherein the analyte gas is a vapor.
23. The method of claim 1, wherein the electrons flowing between the molecules and the first conductor are emitted from or captured by the nano-structures through quantum mechanical tunneling.
24. The method of claim 1 , wherein the voltage is a RF voltage.
25. A device for ionizing a gas, the device comprising: a first conductor comprising a first coating, wherein the first coating includes field emission nano-structures; a second conductor spaced apart from the first conductor; and a voltage source to bias the field emission nano-structures, wherein the voltage source is coupled to the first conductor and the second conductor.
26. The device of claim 25, wherein the second conductor comprises a second coating and wherein the second coating includes field emission nano-structures.
27. The device of claim 25, wherein the nano-structures comprise carbon nanotubes.
28. The device of claim 25, wherein the first conductor and second conductor each comprises a glass substrate.
29. The device of claim 25, wherein each of the first conductor and second conductor comprises: an intermittent pattern of conductive film on a plurality of spaced apart regions; and a conductive coating on each intermittent pattern, wherein the conductive coating includes an alternating pattern of nano-structures on every other spaced apart region.
30. The device of claim 29, wherein the first and second conductors are positioned in a spaced apart relation to each other so that each region on each conductor having a conductive film including nano-structures emits electrons to a region on the opposing conductor having a conductive film that does not include nano-structures.
31. The device of claim 25, wherein the first and second conductor are collectively in the shape of a grid and wherein the grid comprises openings for gas to flow through.
32. The device of claim 31, wherein each of the first and second conductor of the grid comprises a wire.
33. The device of claim 25, wherein the first conductor is in the shape of a grid and the second conductor is in the shape of a grid and wherein each of the first conductor and second conductor comprises openings for gas to flow through.
34. The device of claim 25, further comprising a first filter electrode on the first conductor and a second filter electrode on the second conductor, wherein the first filter electrode on the first conductor is spaced apart from and opposes the second filter electrode on the second conductor.
35. The device of claim 25, further comprising a first detection electrode on the first conductor and a second detection electrode on the second conductor, wherein the first detection electrode on the first conductor is spaced apart from and opposes the second detection electrode on the second conductor.
36. The device of claim 25, wherein the voltage source is a RF voltage source.
37. An apparatus for ionizing gas, the apparatus comprising: an ionization source, wherein the ionization source comprises electron field emission nano-structures; a voltage source coupled to the ionization source to bias the field emission nano- structures; and an ion analysis device coupled to the ionization source.
38. The apparatus of claim 37, further comprising filter electrodes downstream of the ionization source.
39. The apparatus of claim 37, further comprising accelerator electrodes downstream of the ionization source.
40. The apparatus of claim 37, further comprising a gas channel coupled to an outlet of the ionization source.
41. The apparatus of claim 37, further comprising a gas source coupled to an inlet of the ionization source.
42. The apparatus of claim 37, wherein the electron field emission nano-structures comprise carbon nanotubes.
43. The apparatus of claim 37, wherein the ion analysis device is a differential mobility spectrometer.
44. The apparatus of claim 37, wherein the ion analysis device is selected from the group consisting of a time-of- flight ion mobility spectrometer, a mass spectrometer, a high field asymmetric waveform ion mobility spectrometer and a macro ion mobility spectrometer.
45. The apparatus of claim 37, wherein the voltage source is a RF voltage source.
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