A non-radioactive atmospheric pressure device for ionization of analytes comprises an atmospheric pressure chamber having an inlet for carrier gas, a first electrode at one end, and a counter-electrode at the other end of the chamber for creating an electrical discharge in the carrier gas thus creating metastable neutral excited-state species. Optionally, a grid is provided to generate electrons or ions by contact with the excited-state species. The carrier gas containing the excited-state species or the electrons generated therefrom is directed at an analyte at atmospheric pressure near ground potential to form analyte ions.
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0. 56. A non-radioactive atmospheric pressure device for ionization of analytes comprising:
an atmospheric pressure chamber having an inlet for carrier gas, an exit and means for creating one or more of ions, electrons, and metastable neutral excited-state species in the carrier gas; and
a grounded or charged grid electrode to direct the one or more of ions, electrons, and metastable neutral excited-state species via the exit to the analyte at atmospheric pressure near ground potential for ionization.
0. 58. A method of producing analyte, analyte fragment and/or analyte adduct ions for spectrographic analysis comprising the steps of:
introducing a carrier gas into an atmospheric pressure chamber and creating ions electrons and metastable neutral excited-state species in the carrier gas; and
directing ions, electrons, and metastable neutral excited-state species through a grounded or charged grid electrode into contact with the analyte maintained at atmospheric pressure and near ground potential to form analyte ions, analyte fragment ions, and/or analyte adduct ions.
0. 54. A method of ionizing an analyte with a non-radioactive atmospheric pressure device comprising the following steps:
introducing a carrier gas through a gas inlet into a chamber with an exit including a grounded or charged grid electrode;
energizing a first electrode relative to a second electrode spaced apart from the first electrode located in the chamber to provide current between the first and second electrodes to generate a glow discharge;
generating ions, electrons, and excited state species of the carrier gas; and
directing the ions, electrons, excited state species through the grounded or charged grid electrode at the analyte.
0. 52. A non-radioactive atmospheric pressure device for ionization of an analyte comprising:
a chamber including an exit, a first electrode and a second electrode spaced apart from the first electrode, where at least a portion of each of the first and second electrodes are positioned in the chamber;
at least one power supply configured to energize the first and second electrode to provide at least about 5 mA of current between the first and second electrodes to generate a glow discharge;
a gas inlet in the chamber configured to introduce carrier gas to the glow discharge to generate ions, electrons, and excited state species; and
a grounded or charged grid electrode to direct the ions, electrons, excited state species via the exit to the analyte for ionization.
0. 36. A non-radioactive atmospheric pressure device for ionization of an analyte comprising:
a chamber including an exit, a first electrode and a second electrode spaced apart from the first electrode, where at least a portion of each of the first and second electrodes are positioned in the chamber;
at least one power supply configured to energize the first and second electrode to provide current between the first and second electrodes to generate a glow discharge;
a gas inlet in the chamber configured to introduce carrier gas to the glow discharge to generate one or more of ions, electrons, hot atoms, hot molecules, radicals and neutral excited state species; and
a grounded or charged grid electrode to direct the neutral excited state species via the exit to the analyte at or near ground potential for ionization.
0. 1. A non-radioactive atmospheric pressure device for ionization of analytes comprising:
a first atmospheric pressure chamber having an inlet for carrier gas, a first electrode at one end, and a counter-electrode at the other end for creating an electrical discharge in the carrier gas creating metastable neutral excited-state species;
a second atmospheric pressure chamber adjacent the first chamber and having a port into the first chamber at one end and having an electrode at the other end and an outlet port for the carrier gas, the ports being sized to restrict flow, said first electrode and ports being substantially aligned; and
means for contacting gas containing excited-state species flowing out of the outlet port with an analyte at atmospheric pressure near ground potential.
0. 2. A non-radioactive atmospheric pressure device for ionization of analytes comprising:
a first atmospheric pressure chamber having an inlet for carrier gas, a first electrode therein, and a counter-electrode for creating an electrical discharge in the carrier gas creating metastable neutral excited-state species;
a second atmospheric pressure chamber adjacent the first chamber and having a port into the first chamber at one end and an electrode at the other end;
a third atmospheric pressure chamber adjacent the second chamber and having a port into the second chamber and an outlet port for the carrier gas, said first electrode, and ports being more or less aligned; and
means for contacting gas containing excited-state species flowing out of the outlet port with an analyte at atmospheric pressure near ground potential.
0. 3. A non-radioactive atmospheric pressure device for ionization of analytes comprising:
a first atmospheric pressure chamber having an inlet for carrier gas, a first electrode at one end, and a counter-electrode at the other end for creating an electrical discharge in the carrier gas creating metastable neutral excited-state species;
a second atmospheric pressure chamber adjacent the first chamber and having a port into the first chamber at one end and having an electrode at the other end, and an outlet port for the carrier gas, the ports being sized to restrict flow; and
a grounded or charged grid electrode at the output port for emission of charged particles upon contact with an excited-state species, said first electrode and ports being substantially aligned.
0. 4. A non-radioactive atmospheric pressure device for ionization of analytes comprising:
a first atmospheric pressure chamber having an inlet for carrier gas, a first electrode at one end, and a counter-electrode at the other end for creating an electrical discharge in the carrier gas creating metastable neutral excited-state species;
a second atmospheric pressure chamber adjacent the first chamber and having a port into the first chamber at one end and having an electrode at the other end, and an outlet port for the carrier gas, the ports being sized to restrict flow; and
a grounded or negatively charged grid electrode at the output port for emission of electrons upon contact with excited-state species, said first electrode and ports being substantially aligned.
0. 5. A non-radioactive atmospheric pressure device for ionization of analytes comprising:
a first atmospheric pressure chamber having an inlet for carrier gas, a first electrode therein, and a counter-electrode for creating an electrical discharge in the carrier gas creating metastable neutral excited-state species;
a second atmospheric pressure chamber adjacent the first chamber and having a port into the first chamber at one end and an electrode at the other end;
a third atmospheric pressure chamber adjacent the second chamber and having a port into the second chamber and an outlet port for the carrier gas; and
a grounded or negatively charged grid electrode at the output port for emission of electrons upon contact with excited-state species, said first electrode and ports being more or less aligned.
0. 6. A non-radioactive atmospheric pressure device for ionization of analytes comprising:
a first atmospheric pressure chamber having an inlet and exhaust for carrier gas, a first electrode therein, and a counter-electrode for creating an electrical discharge in the carrier gas creating metastable neutral excited-state species;
a second atmospheric pressure chamber adjacent the first chamber and having a flow restricting port into the first chamber at one end and an electrode at the other end, and having an inlet and outlet for optional cooling of reactant gases;
a third atmospheric pressure chamber adjacent the second chamber and having a flow restricting port into the second chamber and having an inlet and outlet for analyte gas or vapor; and
an outlet port for ionized products of the interaction of the carrier gas and the analyte gas or vapor, said first electrode and ports being more or less aligned.
0. 7. A non-radioactive atmospheric pressure device for ionization of analytes comprising:
a first atmospheric pressure chamber having an inlet for carrier gas, a first electrode therein, and a counter-electrode for creating an electrical discharge in the carrier gas creating metastable neutral excited-state species;
at least one intermediate atmospheric pressure chamber adjacent the first chamber and one of said intermediate chambers having a flow restricting port into the first chamber and having an inlet for optional cooling of reactant gases;
a final atmospheric pressure chamber adjacent one of said intermediate chambers and having a port into an intermediate chamber, and having an inlet for analyte gas or vapor; and
an outlet port for ionized products of the interaction of the carrier gas and the analyte gas or vapor, said first electrode and ports being substantially aligned.
0. 8. The device according to any one of
0. 9. The device according to any one of
0. 10. The device according to any one of
0. 11. The device according to any one of
0. 12. The device according to any one of
0. 13. The device according to any one of
0. 14. The device according to any one of
0. 15. The device according to any one of
0. 16. The device according to any one of
0. 17. A method of producing analyte, analyte fragment, and/or analyte adduct ions for spectrographic analysis comprising the steps of:
introducing a carrier gas into a first atmospheric pressure chamber having a first electrode at one end and a counter-electrode at the other end and applying a potential to the first electrode and counter-electrodes to cause an electrical discharge in the carrier gas creating metastable neutral excited-state species;
introducing the carrier gas and metastable species into a second atmospheric pressure chamber wherein the metastable species interacts with the carrier gas to produce positive ions and thermalized electrons; and
directing the carrier gas from the second chamber into contact with the analyte maintained at atmospheric pressure and near ground potential to form analyte ions, analyte fragment ions, and/or analyte adduct ions.
0. 18. The method according to
0. 19. The method according to
0. 20. The method according to
0. 21. The method according to
0. 22. A method of producing analyte, analyte fragment, and/or analyte adduct ions for spectrographic analysis comprising the steps of:
introducing a carrier gas into a first atmospheric pressure chamber having a first electrode at one end and a counter-electrode at the other end and applying a potential to the first electrode and counter-electrodes to cause an electrical discharge in the carrier gas creating metastable neutral excited-state species;
introducing the carrier gas and metastable species into a second atmospheric pressure chamber wherein the metastable species interacts with the carrier gas to produce positive ions and thermalized electrons;
introducing the carrier gas, positive ions, and/or thermalized electrons into a third atmospheric pressure chamber; and
introducing a gaseous or vaporized analyte into the third chamber to form analyte ions, analyte fragment ions, and/or analyte adduct ions.
0. 23. The method according to
0. 24. The method according to
0. 25. The method according to
0. 26. The method according to
0. 27. A method of producing analyte, analyte fragment, and/or analyte adduct ions for spectrographic analysis comprising the steps of:
introducing a carrier gas into a first atmospheric pressure chamber having a first electrode at one end and a counter-electrode at the other end and applying a potential to the first electrode, and counter-electrodes to cause an electrical discharge in the carrier gas creating metastable neutral excited-state species;
directing the carrier gas and metastable species at a grid electrode biased to generate electrons or ions; and then
directing the carrier gas into contact with the analyte at atmospheric pressure to form analyte ions, analyte fragment ions, and/or analyte adduct ions.
0. 28. A method of producing analyte, analyte fragment, and/or analyte adduct ions for spectrographic analysis of analytes on a surface at near atmospheric temperature and ground potential comprising the steps of:
introducing a carrier gas into a first atmospheric pressure chamber having a first electrode at one end and a counter-electrode at the other end and applying a potential to the first electrode and counter-electrodes to cause an electrical discharge in the carrier gas creating metastable neutral excited-state species;
directing the carrier gas and metastable species at a grid electrode biased at a negative potential to cause emission of electrons when struck by a metastable species; and then
directing the carrier gas and emitted electrons against a surface at atmospheric pressure and near ground potential to form analyte ions, analyte fragment ions, and/or analyte adduct ions.
0. 29. A method of replacing a radioactive source in a charged particle detector comprising:
removing the radioactive source and replacing it with a non-radioactive atmospheric pressure device for ionization of analytes comprising:
a first atmospheric pressure chamber having an inlet for carrier gas, a first electrode therein, and a counter-electrode for creating an electrical discharge in the carrier gas creating metastable neutral excited-state species; and
at least one intermediate atmospheric pressure chamber in communication with the first chamber and having a flow restricting port into the first chamber and having an inlet for analyte gas or vapor, and an outlet port in communication with the charged particle detector.
0. 30. A device according to
0. 31. A method of producing analyte, analyte fragment, and/or analyte adduct ions for spectrographic analysis of analytes on a surface at near atmospheric temperature and ground potential comprising the steps of simultaneously using more than one non-radioactive atmospheric pressure ionization device as described in
0. 32. A non-radioactive atmospheric pressure device for ionization of analytes comprising:
an atmospheric pressure chamber having an inlet for carrier gas and means for creating metastable neutral excited-state species in the carrier gas; and
means for contacting gas containing excited-state species with an analyte at atmospheric pressure near ground potential.
0. 33. A non-radioactive atmospheric pressure device for ionization of analytes comprising:
an atmospheric pressure chamber having an inlet for carrier gas and means for creating metastable neutral excited-state species in the carrier gas;
a grounded or charged grid electrode for emission of charged particles upon contact with an excited-state species; and
means for contacting gas containing said charged particles with an analyte at atmospheric pressure near ground potential.
0. 34. A method of producing analyte, analyte fragment and/or analyte adduct ions for spectrographic analysis comprising the steps of:
introducing a carrier gas into an atmospheric pressure chamber and creating metastable neutral excited-state species in said carrier gas; and
directing the carrier gas from the chamber into contact with the analyte maintained at atmospheric pressure and near ground potential to form analyte ions, analyte fragment ions, and/or analyte adduct ions.
0. 35. A method of producing analyte, analyte fragment, and/or analyte adduct ions for spectrographic analysis comprising the steps of:
introducing a carrier gas into an atmospheric pressure chamber for creating metastable neutral excited-state species;
directing the carrier gas and metastable species at a grid electrode biased to generate electrons or ions; and then
directing the carrier gas into contact with the analyte at atmospheric pressure to form analyte ions, analyte fragment ions, and/or analyte adduct ions.
0. 37. The device of claim 36, where the analyte is one or more of a vapor, a liquid or a solid.
0. 38. The device of claim 37, where the analyte is sampled directly without swabbing or solvent washing of a surface.
0. 39. The device of claim 37, where the analyte is sampled directly without the introduction of the analyte into the chamber.
0. 40. The device of claim 37, where the analyte is analyzed without the addition of a solvent into the chamber.
0. 41. The device of claim 37, where the analyte is analyzed without the introduction of air into the chamber.
0. 42. The device of claim 36, where the analyte is analyzed without the injection of air into the chamber.
0. 43. The device of claim 36, where the supply of carrier gas is a supply of helium.
0. 44. The device of claim 43, where the supply of helium is configured to introduce helium to the glow discharge to create neutral excited state species of helium.
0. 45. The device of claim 36, further comprising positioning one or both the first and the second electrode relative to the exit to direct the neutral excited state species at the analyte.
0. 46. The device of claim 36, further comprising a tube, where the neutral excited state species are directed to the analyte through the tube.
0. 47. The device of claim 36, where the grid electrode is a mesh.
0. 48. The method of claim 36, where the gas inlet in the chamber configured to introduce carrier gas to the glow discharge generates ions.
0. 49. The method of claim 48, where the grounded or charged grid electrode directs the ions via the exit to the analyte at or near ground potential for ionization.
0. 50. The method of claim 36, where the gas inlet in the chamber configured to introduce carrier gas to the glow discharge generates electrons.
0. 51. The method of claim 50, where the grounded or charged grid electrode directs the electrons via the exit to the analyte at or near ground potential for ionization.
0. 53. The device of claim 52, where the grid electrode is a mesh.
0. 55. The method of claim 54, where the grid electrode is a mesh.
0. 57. The device of claim 56, where the grid electrode is a mesh.
0. 59. The method of claim 58, where the grid electrode is a mesh.
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2. Description of Related Art
Regulatory and safety issues related to the use of radioactive materials, such as 63Ni, 241Am, and 3H, among others, have led to a search for non-radioactive ion sources for analytical instruments, such as ion mobility spectrometers. (See Turner et al. U.S. Pat. No. 6,225,623 entitled “Corona Discharge Ion Source for Analytical Instruments” and Doring U.S. Patent Application Publication No. 2002/0185593 entitled “Ion Mobility Spectrometer with Non-Radioactive Ion Source”.)
Certain available corona discharge ion sources for atmospheric pressure ionization (API) mass spectrometers or ion mobility spectrometers (IMS) or chemical agent monitors (CAM) introduce the analyte (including solvent, air, and other contaminants) into the region containing a discharge needle. This leads to several problems:
1. The presence of oxygen or other contaminants in the air leads to degradation of the electrodes.
2. It can be difficult to maintain the discharge in the presence of contaminants, requiring a high electrical potential or pulsed potentials.
3. A corona discharge in air leads to the formation of species, such as NO2−, NO3−, and related cluster ions. These ions can cause a loss of sensitivity for analyte ions (C. A. Hill and C. L. P. Thomas, Analyst, 2003, 128, pp. 55-60) and can interfere with the detection of NO2− and NO3− produced from analytes containing nitro functional groups, such as nitro explosives or in the case of chloride ion interference with chlorate propellants and rocket motors or phosphate interference with chemical warfare-related compounds.
4. Introducing air and analyte into the discharge region limits the possibilities for controlling the nature of the chemical background to control the ion-formation chemistry.
Taylor et al. U. S. Pat. No. 5,684,300 entitled “Corona Discharge Ionization Source” and Turner et al. U.S. Pat. No. 6,225,623 B1 entitled “Corona Discharge Ion Source for Analytical Instruments” describe corona discharge ion sources, but do not describe a means for separating the region where the discharge occurs from the region where the analyte is introduced. See also Zhao et al. entitled “Liquid Sample Injection Using Atmospheric Pressure Direct Current Glow Discharge Ionization Source,” Anal. Chem., 64, pp.1426-1433, 1992.
Bertrand et al. U.S. Pat. No. 6,124,675 entitled “Metastable Atom Bombardment Source” discloses a metastable atom source operating at reduced pressure for generating ions in a mass spectrometer. The device described requires substantially reduced pressures and does not describe means for using metastable atoms for atmospheric pressure ionization mass spectrometry or ion mobility spectrometers.
Tsuchiya et al. U.S. Pat. No. 4,546,253 entitled “Apparatus for Producing Sample Ions” describes a method for using metastable atoms to produce ions from a sample introduced at the tip of an emitter needle downstream from the corona discharge. This technique requires that the sample be placed on or near an intense electric field emitter needle. See also Otsuka et al. entitled “An Interface for Liquid Chromatograph/Liquid Ionization Mass Spectrometer,” Analytical Sciences, Vol. 4, Oct. 1988. The present invention avoids use of an emitter needle at high electrical potential placed downstream of the corona discharge source. Further, the present invention provides a means of sampling neutral analyte molecules without the restriction of relocating the analyte from the surfaces on which they are attached. For example, cocaine from cash currency, and chemical/biological warfare agents from surfaces of military interest can be sampled directly and in situ without swabbing or solvent washing the surface. Each time sample is relocated, analyte molecules are lost (30 to 100% for trace-level concentrations). Therefore, direct surface sampling is always preferred.
Briefly, according to this invention, there is provided an atmospheric pressure ionization source or interface comprising: a first atmospheric pressure chamber having an inlet for carrier gas, a first electrode, and a counter-electrode for creating a corona or glow electric discharge in the carrier gas causing the formation of neutral excited-state metastable species; a second atmospheric pressure chamber having a port in communication with the first chamber; and an optional third atmospheric pressure chamber having a port in communication with the second chamber, there being a lens electrode about said port between the second and third chambers, the third chamber having an outlet port for the carrier gas and an optional electrode at the outlet port. The discharge is confined to the first chamber. Preferably, the first electrode and ports are substantially aligned. A power supply is provided for maintaining selected potentials on each electrode. There may be a conductive grid at the outlet of the second or third chamber. The third chamber may advantageously be an elongated glass tube that is removably inserted into a socket in the second chamber.
The atmospheric pressure source or interface can be used to form positive or negative ions for use with spectrometers or other instruments which operate in the positive or negative ion mode. Typically, negative and positive ions are both formed when the analyte is brought into contact with the excited state species. Some analytes are electrophilic and tend to capture electrons to produce negatively charged analyte ions that allow detection and identification of these analytes. Others have a greater affinity for protons or positive ions and become ionized by picking up a proton [M+H]+,for example. This will guide the selection of an instrument in a positive or negative ion mode.
Preferably, the power supply permits the lens electrode and the electrode at the outlet port to switch polarity without switching the polarity of the first electrode and the counter-electrode. This will enable rapid selection of the ionization source between positive and negative ion modes. The first electrode and counter-electrode must be maintained at potentials sufficient to induce an electrical discharge. The counter-electrode also serves to filter ionized species. The potential difference between the first electrode and counter-electrode necessary for the formation of a discharge depends on the carrier gas and the shape of the first electrode, and is usually several hundreds of volts, say 400 or 1,200. But for small electron structures such as those used in flat-screen plasma TV's, a few volts is sufficient. The first electrode, for example, a needle electrode, may have either a positive or negative potential. The counter-electrode is normally grounded or of polarity opposite to the needle electrode. This is the case whether operating in the positive ion or negative ion mode. In the positive ion mode, the lens electrode may be between ground potential and a few hundred positive volts to filter out negative ions in the carrier gas. Also, in the negative ion mode, the lens electrode may be between ground and minus a few hundred volts to filter out positive ions in the carrier gas.
According to a first embodiment of this invention, the apparatus described in the preceding paragraph is placed with the outlet of the third chamber close to the entrance of a charged particle detector in a positive ion mode, such as a mass spectrometer, an ion mobility spectrometer, or a chemical agent monitor. The electrodes are placed in the positive ion mode. The gas containing excited-state species emerging from the outlet port of the third chamber is directed through or at an analyte positioned near the entrance to the detector operated in the positive ion mode. The metastable species in the carrier gas react with the analyte to form positive ions for analysis. Analyte molecules undergo ion molecule reactions to form species such as [M+H]+. The form of the analyte may be a vapor from an open vial, in solid form on a surface, or in the form of an aspirated liquid, for example.
According to a second embodiment of this invention, the apparatus described above is placed with the outlet of the third chamber close to the entrance to a charged particle detector. The electrodes are placed in the negative ion mode. The gas containing exited-state species emerging from the outlet port of the third chamber is directed through or at an analyte positioned near the entrance to the detector operated in the negative ion mode. The metastable species in the carrier gas react with the analyte to form negative ions for analysis. The form of the analyte may be a vapor from an open vial, in solid form on a surface, or in the form of an aspirated material, for example.
According to a third embodiment of this invention, the apparatus described above is used in a “sniffer” mode with the outlet of the third chamber close to the entrance of a charged particle detector. The electrode at the outlet may comprise a grid and is maintained at ground or negative potential or at an AC potential offset by a DC voltage to induce dissociation of the reactive species to provide a source of electrons as the metastable species collide with the grid. The resulting electrons are rapidly cooled (slowed) to thermal energies by collisions with the gas molecules. The electrodes are placed in the negative ion mode. The gas containing electrons emerging from the outlet port of the third chamber is directed through or at an atmospheric pressure analyte not in an intense electric field and positioned near the entrance to a charged particle detector, such as a mass spectrometer or ion mobility spectrometer, either of which is operated in the negative ion mode. The electrons in the carrier gas are captured by the analyte to form negative ions that are cooled by gas collisions. The form of the analyte may be a vapor from an open vial, in solid form on a surface, or in the form of a static or aspirated liquid, for example.
A fourth embodiment is similar to the third “sniffer” mode embodiment except that the grid electrode is maintained positive and the gas emerging from the outlet forms positive ions with the analyte for analysis in the positive ion mode.
According to a fifth embodiment of this invention, there is provided an atmospheric pressure ionization source or interface comprising: a first atmospheric pressure chamber having an inlet for carrier gas, an electrode therein, and a counter-electrode for creating a corona or glow discharge in the carrier gas creating metastable species, ions, electrons, hot atoms and molecules, and radicals; a second atmospheric pressure chamber adjacent the first chamber having a port into the first chamber and having an optional inlet and optional outlet for cooling or reactant gases; and a third atmospheric pressure chamber adjacent the second chamber having a port into the second chamber and having an inlet for analyte gas and an outlet port for ionized products of the interaction of the metastable species and the analyte gas, the electrode and ports being substantially aligned.
According to a sixth embodiment of this invention, there is provided an atmospheric pressure ionization source or interface comprising: a first chamber having an inlet for carrier gas, an electrode therein, and a counter-electrode for creating a glow or corona discharge in the carrier gas creating metastable species; and a second chamber adjacent the first chamber having an outlet port for electrons and/or metastable species, the electrode and ports being substantially aligned.
By atmospheric pressure in this specification and the appended claims is meant pressures near ambient pressures, say 400 to 1,400 Torr. This would include pressurized aircraft and submerged submarines. For laboratory use, ambient pressures may fall within the range 700 to 800 Torr.
The carrier gas may be heated prior to introduction into the interface or while in the interface to facilitate vaporization or desorption of the analyte into the gas phase from surfaces. It is preferable to provide an adjustable regulator for adjusting the gas pressure to control the speed of ionizing electron energy since electrons embedded in the gas stream will be carried along and accelerated by changing gas pressures. Energy resolved spectra may be achieved in this fashion.
Further features and other objects and advantages will become clear from the following detailed description made with reference to the drawings in which:
A generalized implementation of the present invention is shown schematically in
In one current implementation, carrier gas is introduced from a gas cylinder into chamber C1 at a positive pressure. This causes flow of metastable excited-state atoms into chambers C2 and C3. In this implementation, the chambers have a volume of about one cubic centimeter. The orifices between chambers are about 3 mm in diameter and the flow through the orifices is on the order of a few milliliters per minute.
The carrier gases that have been used by the Applicants are helium and nitrogen. P-10 gas (90% argon +10% methane) and He/Ne mixtures are potential carrier gases. Also under consideration are argon and krypton. Indeed, any gas or mixture of gases with a metastable state lying higher than a state of the analyte is a potential carrier gas.
The corona or glow electrical discharge occurs in chamber C1. Chamber C2 provides an optional buffer region between chambers C1 and C3, and provides an option for introducing a separate cooling gas or a reactant gas. Cooling gases comprise, for example, gases that would be ionized by metastable atoms to produce a positive ion and electron. The electron would be thermalized by further collisions. Carbon dioxide, methane, and air are examples of cooling gases. Reactive gases are those that favor distinctive ion peaks by ion-molecule reactions. Typically, a small amount of reactive gas, such as ammonia (to promote ammonium ion attachment for positive ions), or a gas that produces chloride ions (e.g., methylene chloride, chloroform, or carbon tetrachloride for negative ions), can be added to the cooling gas. Chloride ion addition has been shown to drastically enhance the detection of polynitro explosives by several orders of magnitude. The analyte may be introduced into chamber C3 and ions of analyte are extracted through a port in electrode E2 into the mass spectrometer atmospheric pressure interface or into the ion mobility spectrometer drift region. The device, or any part of it, can be heated to facilitate the analysis of compounds with low vapor pressures and to reduce sample carryover.
The carrier gases with which Applicants have practiced the invention are helium and nitrogen. Both have high first electron ionization potentials and are not reactive with other elements or compounds at room temperature and pressure. Other noble gases, such as argon, krypton, and xenon, are suitable carrier gases for this reason.
The discharge according to the present invention is either a corona discharge or a glow discharge. It is understood that in electrical discharges, electrons are accelerated into the atoms and molecules of the carrier gas causing additional electrons to be freed and accelerated in a cascading fashion. Collisions in addition to freeing electrons and creating positive ions transfer energy to atoms and molecules to create metastable excited-state species. A glow discharge is a luminous electrical discharge without sparks through a gas. A corona is a faint glow adjacent to the surface of an electrical conductor at high voltage. Typically, glow discharges require a large potential to initiate but a lower voltage to be sustained following “break down”. The internal resistance of the power supply for the needle electrode and other factors limit the current in the discharge. Higher currents that might result in sputtering or arcing would not be according to the present invention.
The device shown schematically in
TABLE 1
Nominal Mass
Composition
26
CN−
35
35Cl−
37
37Cl−
42
CNO−
45
HCO2−
46
NO2−
59
C2H3O2−
60
CO3−
61
HCO3−
62
NO3−
A benefit of the present invention is shown in
If an electrophilic analyte is introduced into inlet G3, characteristic ions can be observed. These ions can result from direct ionization and fragmentation of the analyte, as shown in
In the negative-ion mode, the background ions shown in
As shown in Equation 1, the metastable atoms produce electrons by Penning ionization and the resulting electrons are rapidly cooled to thermal energies by collisions with gas molecules at atmospheric pressure within a few nanoseconds. These electrons can undergo capture by electrophilic analytes to produce analyte ions. The analyte ions can undergo further reactions with species in chamber C3 to produce the resulting mass spectrum. The analytes do not need to be introduced into chamber C3 via port G3. The analytes can be remotely sampled just by aiming the gas stream at the analyte on the surface of a dollar bill, an agricultural leaf, a human fingertip, concrete, asphalt, or an airline ticket, for example.
If the ion source is biased to a more negative potential than the orifice of the mass spectrometer interface, negative ions will be attracted to the orifice and the signal intensity is more than 10 times higher.
The excellent selectivity of the present invention is illustrated for the detection of nitromethane. If one injects air into the corona discharge region of a prior art source, large amounts of NO2−, NO3− and related cluster ions are formed. This is shown in
Positive ions can be observed by switching the mass spectrometer polarity.
Positive-ion mass spectra for nitromethane (
Other modes of operation are possible. The electrons produced by the discharge in chamber C1 can be introduced onto chamber C2 and cooled to thermal energies for electron capture by analyte molecules in chamber C3. A related experiment was reported by Leymarie and coworkers (N. Leymarie, J.-C. Tabet, and M. Bertrand, presented at the Annual Meeting of the American Society of Mass Spectrometry, 2000) for a metastable atom ion source operated at subambient pressures and connected to a conventional high-vacuum mass spectrometer ion source. However, this report required a reduced-pressure source and did not describe the use of the ion source at atmospheric pressure for combination with an API mass spectrometer or an ion mobility mass spectrometer. The present invention makes use of the superior electron cooling efficiency of an atmospheric pressure cooling chamber C2. In one implementation, a gas, for example, CO2, that can be ionized by the metastable atoms is introduced into chamber C2 where the emitted electrons are further cooled.
In the “sniffer” mode, air is always present. The spectra shown in
The negative ion spectra for TNT is shown in
Referring now to
Referring now to
Referring to
Referring to
The atmospheric pressure ion source described herein is useful for the introduction of ions into mass spectrometers and ion mobility spectrometers for the detection and identification of analytes of interest, such as drugs, explosives, chemical weapons, toxic industrial materials, and the like. It is non-radioactive and provides rapid sampling of gas and vapor in headspace sampling. It also permits rapid and direct sampling of chemicals on surfaces. This feature makes the ion source described herein a very useful replacement for a radioactive source on IMS detectors.
It can be useful to simultaneously use more than one ion source or device as described herein. For example, Applicants have conducted experiments wherein two ion sources were simultaneously used to provide ions to a mass spectrometer. In one case, acetone was analyzed in the positive ion mode with two ion sources. The ion current using both sources was approximately the total of the ion currents using either source individually. In another experiment, oxygen ions were detected in the negative ion mode. Again, the ion current using both sources was approximately the total of the ion currents using either source individually.
As used herein, an “atmospheric ionization source” is one that does not require a vacuum pump. Of course, the analyzer (mass spectrometer) may require vacuum pumps, but the ions are formed at pressures somewhat above and below atmospheric pressure.
Having thus described our invention in the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.
Cody, Robert B., Laramee, James A.
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