Patent Application
20120248301
The present invention relates generally to ionization sources and systems. More particularly, the invention relates to a UV-LED ionization device that includes a UV-LED ionization source and process that produces photoemission electrons for indirect or direct ionization of analytes.
Attention in the scientific community has avoided exploring LEDs as a viable light source because of their low light intensity (i.e., photon flux) or low photon energy. UV LEDs in the region of 240 nm to 280 nm are now becoming commercially available, although their flux is still limited. Traditional photon-based ion sources have either very high photon energies (e.g., Vacuum UV or VUV lamps and lasers) resulting in direct photo-ionization via Single Photon Ionization (SPI) or via a high intensity or focused laser beam(s), resulting in Multi-Photon Ionization (MPI) or Resonance-Enhanced Multi-Photon Ionization (REMPI). Other UV-laser techniques including, e.g., UV Pulsed-Laser Fragmentation (UV-PLF) have been successfully used to measure photo-fragments of explosive residues using a focused UV laser (MPI). However this approach requires a focused laser beam to permit MPI, and resulting ions are not the parent or close-parent ion fragments, but NO2. With photons that have an energy of between 4 eV to 5 eV per, current UV LEDs lack the ability to directly photo-ionize organic or atmospheric components. In addition, the light from a UV LED has both insufficient energy, for SPI to occur (most organic molecules ionize at >8 eV) and a photon flux that is too low for MPI or REMPI to occur. Photon flux (φ) [(photons/sec)] measured at a distance from an LED is given by Equation [1]:
φ=P/(1.602×10−19 J*1240 nm/λ) [1]
Here, (P) is the measured power (Watts), and (A) is the wavelength (nm). For currently available UV-LEDS, LEDs with 280 nm and a power of 500 μW generates 7×1014 photons/sec, whereas a UV-LED at 240 nm and 22 μW generates 3×1013 photons/sec. Thus, currently available UV-LEDs are incapable of generating ions via SPI, MPI, or REMPI. Accordingly, new devices and approaches are needed that can take advantage of these low-energy, low-flux light sources to ionize selected organics for analysis.
The present invention includes a UV-LED ionization device that includes a UV-LED photoemission ionization source and process for indirect chemical ionization or direct electron capture ionization of analytes for ion analysis. The UV-LED ionization source does not include another electron-generating source, e.g., electron-generating filaments, beta-emitting sources, or other electron-generating sources. The UV-LED ionization source produces UV light of a selected wavelength that generates photoemission electrons from the conducting surface when energy of the UV light exceeds the work function upon contact with the conducting surface. The photoemission electrons provide ionization of an analyte.
The process for UV-LED photoemission ionization includes generating UV light of a preselected wavelength with the UV-LED source in the absence of another electron-generating source. UV light from the UV-LED source contacts a conducting surface that includes a preselected material located adjacent the UV-LED source to generate photoemission photoelectrons from the conducting surface when energy of the UV light exceeds the work function of the surface upon contact with the conducting surface. A preselected analyte is then ionized with photoemission electrons released from the conducting surface directly or indirectly upon contact with the analyte. The method can further include the step of determining the ionized analyte in an ion analyzer for identification.
In some embodiments, the analyte is located on the conducting surface. In other embodiments, the analyte is a vapor-phase analyte.
In another embodiment, the analyte is located on a non-conducting surface and ionized by photoemission electrons released from the conducting surface.
In one embodiment, the UV-LED source is located in the same ionization volume as the analyte.
In one embodiment, the UV-LED ionization source is positioned adjacent a conducting surface made of a preselected material.
In one embodiment, the UV-LED source is coupled to an ion analyzer located adjacent the conducting surface.
In a preferred embodiment, the UV light is non-coherent UV light. In a preferred embodiment, the UV-LED source provides UV light of a wavelength from about 200 nm to about 400 nm. The UV-LED source is characterized in that it produces a low quantity of ozone as a by-product of the ionization of the analyte. In some embodiments, the quantity of ozone is below about 100 parts per billion (ppb). In other embodiments, the quantity of ozone produced is a quantity of ozone below a monolayer of ozone coverage on the conducting surface.
In various embodiments, the conducting surface includes preselected materials including, but not limited to, e.g., metals, alloys, metalloids, and polymers.
In various embodiments, the conducting surface includes a metal. In various embodiments, the form of the metal conducting surface includes, e.g., a mesh, a foil, a coating, or a bulk material. In other embodiments, the conducting surface includes a metal alloy. In one embodiment, the conducting surface includes a metal alloy composed of stainless steel.
In another embodiment, the conducting surface includes a metalloid.
In some embodiments, the conducting surface includes a modified surface. In some embodiments, the modified surface includes a metal coating. In some embodiments, the modified surface includes an oxidized metal. In some embodiments, the oxidized metal is oxidized with air or oxygen gas. In some embodiments, the oxidized metal is oxidized with a reagent that leaves oxygen on the surface of the metal. The oxidizing reagent can include an acid, an acid gas, a base, or another like reagent. Exemplary acids include, e.g., nitric (HNO3), phosphoric (H3PO4), and sulfuric (H2SO4). An exemplary base is sodium hydroxide (NaOH). In some embodiments, the modified surface includes a polymer coating. In some embodiments, the modified surface includes a conductive polymer.
In various embodiments, analytes ionized in accordance with the invention include, but are not limited to, e.g., vapor-phase analytes, liquid-phase analytes, solid-phase analytes, vapor-phase analytes, residues, explosives, including analytes in these various forms. In some embodiments, the analyte is a liquid residue. In other embodiments, the analyte is a solid residue. In yet other embodiments, the analyte is a vapor or gas-phase analyte. In some embodiments, the analyte is ionized from off a conducting surface. In some embodiments, the analyte is located on or in an organic-containing matrix located on the conducting surface. In some embodiments, the analyte is an explosive. Examples include, but are not limited to, e.g., TNT, RDX, PETN, or mixtures of these explosives.
In some embodiments, ionization of the analyte occurs at a pressure at or above ambient pressure. In other embodiments, ionization of the analyte occurs at a reduced pressure. In one embodiment, the reduced pressure is a vacuum pressure. In some embodiments, ionization of the analyte occurs at a temperature greater than or equal to ambient temperature. In some embodiments, ionization of the analyte occurs at a temperature less than or equal to ambient temperature.
In some embodiments, the UV-LED is adapted for use in portable mass analyzers including, e.g., MS, IMS, and like instruments that generate ions at atmospheric pressure that provide both an increase in ion intensity and a reduction in size of the instruments.
In various embodiments, the UV-LED source is coupled to an ion analyzer positioned adjacent the conducting surface to provide analysis of ionized analytes, and the method further includes determining the ionized analyte in an ion analyzer for identification of the analyte.
In some embodiments, the ionizing of the analyte involves a proton abstraction (removal) reaction. In some embodiments, the ionizing of the analyte involves formation of a chemical adduct in the vapor phase. In some embodiments, the ionizing analyte is fragmented to form ion and neutral products. In some embodiments, the ionizing of the analyte involves a charge-transfer reaction with a reagent gas. In various embodiments, the reagent gas is selected from: SF6, NO2, OH, O2, O3, CH3OH, CH3CN, CH3COOH, HCl, NH3, CH14, CH2Cl2, CF2Cl2, C4H10, halogenated hydrocarbons, including combinations of these gases. In some embodiments, the ionizing of the analyte involves an electron capture reaction with an emitted photoemission electron. In some embodiments, the ionizing of the analyte involves an electron capture reaction with oxygen. In other embodiments, the ionizing involves an electron capture reaction with emitted photoemission electrons. In various embodiments, photoemission ionization via electron capture (EC) is used to ionize samples that provides for detection of various chemical species in ambient air.
In one embodiment, electron capture (EC) ionization is performed at ambient pressure to provide a softer ionization approach than is obtainable with conventional, low-pressure EC ionization, which yields simpler mass spectra and higher parent ion yields.
In some embodiments, photoelectrons are generated on a surface that is separate from the surface on which the sample is placed. In these embodiments, the sample can be a liquid residue or a solid residue. In some embodiments, the surface is a non-conductive surface. In some embodiments, the non-conductive surface includes a glass, a polymer, a ceramic, or a ceramic glass. In some embodiments, a non-conducting surface provides the mounting surface for the analyte, and a conducting surface provides the source of photoemission electrons. In some embodiments, the conducting surface is positioned separate from or distal to the non-conducting surface to provide the necessary electrons for ionization of the sample from off the non-conducting surface.
In some embodiments, photoelectrons are generated from the metal surface (e.g., oxidized aluminum or stainless steel) via impinging low-energy UV light (280 nm) generated by a single UV-LED operating at atmospheric pressure. The UV-LED source generates small photocurrents in the metals and has the potential to replace traditional Electron Impact (EI) sources. Low-energy photoelectrons result in both direct electron capture by the analyte, or via chemical ionization with O2−. Ion generation occurs without use of high electric fields such as corona discharge or ESI. As such, no disrupting electric fields are generated in the source region.
A more complete appreciation of the invention will be readily obtained by reference to the following description of the accompanying drawing in which like numerals in different figures represent the same structures or elements.
a plots signal intensity of reagent/background ions produced with one embodiment of the invention from various conducting surfaces in open air.
b plots total ion count (TIC) produced with another embodiment of the invention from various metal conducting surfaces.
a-4c present mass spectra showing background ions produced with one embodiment of the invention from different metal foil surfaces.
a-5b present mass spectra for two organic compounds ionized from a metal surface with one embodiment of the invention.
a is a mass spectrum for an exemplary explosive residue ionized from a metal surface, according to an embodiment of the invention.
b is a plot showing signal intensity for an exemplary explosive ionized from various surfaces, according to various embodiments of the invention.
A UV-LED photoemission ionization source and process are detailed that generate photoemission electrons that provide indirect ionization or direct electron capture ionization of analytes including, e.g., volatile organic vapors and surface residues via low-energy electron capture and subsequent ion-neutral reactions that find application in mass spectrometry or ion mobility spectrometry.
Photoemission electrons released from various conducting surfaces and modified conducting surfaces provide ionization of analytes either directly or indirectly, as detailed further herein.
As shown in TABLE 1, energy of the initial photoemission electron from a non-oxidized aluminum surface irradiated with 280 nm UV light is 0.18 eV. For a stainless steel surface, energy of the photoemission electron with 280 nm UV light is 0.03 eV. Metal oxide surfaces typically have a higher effective work function upon oxidation. However, oxides of zinc and aluminum yield lower effective work function values upon oxidation. Thin oxide layers on gold also lower the threshold for photoemission by up to 1 eV. Exposure of many metal surfaces to atmosphere may also lower the work function due to thin-layers of organic residues or chemisorptions from the atmosphere.
Efficiency of photoelectron generation is another factor for selection of metal conducting surfaces for photoemission applications. At 5 eV, for example, stainless steel yields 10−6 electrons per photon. At an equal light intensity, aluminum yields 10−5 electrons/photon, providing a 10× greater yield of photoemission electrons per photon compared with stainless steel. Based on photon flux data for the UV-LED source, the photoelectron yield for the aluminum surface corresponds to an expected electron current of 0.5 nW (3×109 electrons/sec) at 240 nm or 10 nW (7×1010 electrons/sec) at 280 nm. For the stainless steel surface, the photoelectron yield corresponds to an expected electron current of 0.05 nW (3×108 electrons/sec) at 240 nm or 1 nW (7×109 electrons/sec) at 280 nm.
a plots total ion count (signal intensity) generated from various metal surfaces positioned in open air (i.e., external to the ion source of
a-4c present mass spectra showing background ions obtained in air at ambient temperature using the UV-LED source as the UV light (280 nm) irradiated three different conducting (i.e., metal foil) surfaces including, e.g., a lead foil (
Although all metals produce small quantities of all ions listed in TABLE 2, three distinct ion-classes are typically produced: 1) metals that generate O2− reagent ions predominantly, 2) metals that yield NOx− reagent ions predominantly, and 3) metals that yield CO3− reagent ions predominantly. Stainless steel and Co:Ni:Cr alloys generate O2− reagent ions predominantly upon irradiation with low-energy UV light. Aluminum surfaces generate NOx− reagent ions predominantly upon irradiation with low-energy UV light. Lead and cadmium surfaces generate CO3− reagent ions predominantly upon irradiation with low-energy UV light. While ion generation described herein was demonstrated in atmospheric air resulting in chemical ionization typically involving O2− reagent ions, the ion generation process can also be achieved with other reagent (electrophillic) gases capable of receiving the photoemission electron from the conducting surface, including, but not limited to, e.g., SF6, CF4 NO2, OH, O2, O3, CH3OH, CH3CN, CH3COOH, HCl, NH3, CH4, CH2Cl2, CF2Cl2, C4H10, SO2F2, halogenated hydrocarbons, as well as combinations of these various gases. Thus, no limitations are intended. Other background ions include, but are not limited to, e.g., HCO2− (m/z 45); O2(H2O)− (m/z 50); CO3− (m/z 59.99); H2O(HCO2)− (m/z 63); O2(H2O2)− (m/z 68); O2(HCO2)− (m/z 77); [(C4H10O2)—H]− (m/z 89); [H2O(C4H10O2)—H]− (m/z 107); and O2(C4H10O2)− (m/z 121). Thus, no limitations are intended.
Analytes may be ionized directly as vapor-phase analytes, or be ionized directly from conducting surface into the headspace above conducting surface. Analytes may be of any form that is volatile including, but not limited to, e.g., liquids, residues, powders, particles, suspensions, vapors, bound-species, and combinations of these various analyte forms. In exemplary tests, several classes of organic vapors were analyzed and found to be effectively detected. Detected organic vapors include, but are not limited to, e.g., organic acids, heterocyclic aromatics, and halo-organic solvents. Electronegative molecules are preferred for vapor detection, but are not limited thereto. Ion types resulting from ionization are principally from two chief pathways: 1) reaction with an atmospheric reagent molecule or 2) via electron capture. For example, a dominant ionization pathway for organic vapors involves photoemission from an oxidized conducting surface (e.g., an oxidized metal surface), electron capture by oxygen, and then proton abstraction (removal) of the analyte. A primary benefit of UV-LED source is the ability to generate ions for detection of several classes of organic vapors without the need of high-energy potential fields or low pressure vacuum systems. In some embodiments, for example, an analyte may placed on conducting surface as a liquid prior to being ionized or may be ionized as a dry residue from conducting surface. Thus, no limitations are intended. Ionization then proceeds by exposing conducting surface to UV light emanating from UV-LED ion source. Ionized analytes are subsequently introduced as vapor-phase analytes into an ion analyzer (e.g., MS, IMS) positioned adjacent the conducting surface via air flow (i.e., suction) or ion steering.
a-5b present mass spectra for two organic acids, Trifluoroacetic acid (TFA) (100 mM), (Agilent, Santa Clara, Calif., USA) and Oleic acid (CAS #112-80-1) (Sigma-Aldrich, St. Louis, Mo.) that were present as dry residues on the conducting surface made, e.g., of an aluminum foil (e.g., Reynold's Aluminum Foil®). Metal foil types suitable for use with the invention are not limited. Solutions were either used directly or diluted with methanol prior to analysis. A small drop (5 μL) was placed on the aluminum foil blank a few millimeters (mm) from the entrance to the mass spectrometer. The acids were analyzed from off the conducting surface with the UV-LED source described previously in reference to
a is a mass spectrum from analyses of a 1 μg surface residue of trinitrotoluene (TNT) that was ionized from a metal foil surface (e.g., aluminum foil) with the UV-LED ionization source (
In other exemplary tests, both liquid samples and dry residues of TNT were ionized by proton abstraction, whereas low-volatile, dry residues of RDX and PETN ionized by adduct formation. Trinitrotoluene (500 ppm) was found to be measured at nanogram quantities off conducting metal surfaces. Results demonstrate that the UV-LED source (
The UV-LED photoemission ionization source (see
The following examples provide a further understanding of the invention in one or more aspects.
Ambient air was analyzed using a UV-LED source (
In one test, background ions were generated in air at ambient temperature using a single UV-LED illuminating the conducting surface composed of oxidized aluminum. A background ion mixture resulting from UV light (280 nm) from a UV-LED source striking the conducting surface located near the sample cone was determined with an API 5000 atmospheric-sampling mass analyzer situated adjacent to UV-LED source. The sample cone was set to a potential of −7V. UV light from the UV-LED was set to strike the sample cone ˜0.5 cm from the MS inlet. The oxidized surface with the thin oxide film substantially lowered the work function of the material to 4.0 eV (compared to 4.23 eV for a non-oxidized aluminum), which occurs naturally at atmospheric pressure in room air. Work function for generation of photoelectrons on the non-oxidized surface is at the minimum necessary value to permit photoemission generation when exposed to 280 nm UV light.
In another test, background ions were generated in sufficient quantity in air when a single UV-LED illuminated a conducting surface made of oxidized stainless steel at ambient temperature. Background ion mixture resulting from UV light (280 nm) from a UV-LED source striking a conducting surface located near a sample cone was determined with an API 5000 atmospheric-sampling mass analyzer situated adjacent to UV-LED source. Background ion mixture resulting from UV light (280 nm) striking a stainless steel metal surface was determined using an API 5000 atmospheric-sampling mass analyzer coupled to the UV-LED source. The sample cone was set to a potential of −7V. UV light from the UV-LED was set to strike the sample cone −0.5 cm from the MS inlet. UV light from UV-LED source was set to strike the sample cone just below the inlet to the mass analyzer. The stainless steel metal surface has a work function that is just sufficient (4.4 eV) to permit photoemission generation when exposed to 280 nm light.
In another test, background ions were generated in sufficient quantity in air when a single UV-LED illuminated a conducting surface made of aluminum, coated with a thin layer of non-stick coating, composed of polysiloxane and other minor components (e.g., acrylic acid, ethyl acetate and butyl acetate) at ambient temperature. Background ion mixture resulting from UV light (280 nm) from a UV-LED source striking a conducting surface located near a sample cone, which was determined with an API 5000 atmospheric-sampling mass analyzer situated adjacent to UV-LED source. Background ion mixture resulting from UV light (280 nm) striking an stainless steel metal surface was determined using an API 5000 atmospheric-sampling mass analyzer coupled to the UV-LED source. The sample cone was set to a potential of −7V. UV light from the UV-LED was set to strike the sample cone −0.5 cm from the MS inlet. UV light from UV-LED source was set to strike the sample cone just below the inlet to mass analyzer. The polymer-coated aluminum surface generated similar ion intensities and types of ions as the non-coated aluminum surface.
In other experiments, analyte ions were generated using UV-LED photoemission ionization source at ambient pressure by sampling the headspace of several gas-phase organic compounds. Ions observed result from either reaction with an atmospheric reagent molecule or electron capture. In one test, Dichloromethane (CH2Cl2) (CAS #75-09-2) (ionization potential=11.32 eV) (Sigma Aldrich, St. Louis, Mo., USA) was analyzed. A Q-tip cotton swab moist with dichloromethane was placed an inch away from the UV-LED source. The ion signature was monitored by sampling the headspace above the aluminum metal surface. The headspace volume containing the ionized dichloromethane analyte was analyzed in an API 5000 mass spectrometer. Results showed the dominant ion (Cl−) for dichloromethane was a result of electron induced dissociation (EID), whereas most larger organic solvents resulted in electron attachment, charge transfer, proton abstraction (M-H)− or other fragmentation reactions.
While exemplary embodiments of the present invention have been shown and described herein, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its true scope and broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the spirit and scope of the invention.
