(12) Unrted States Patent
`(16) Patent N0.:
`US 6,661,178 B1
`
`Bertrand et al.
`(45) Date of Patent:
`*Dec. 9, 2003
`
`U5006661178B1
`
`(54) METASTABLE ATOM BOMBARDMENT
`SOURCE
`
`(75)
`
`Inventors: Michel J. Bertrand, Verdun (CA);
`.
`.
`.
`,
`Olmer Peraldl’ Montreal (CA)
`.
`.
`.
`.
`(73) ASSlgnee‘ Umvers‘te ‘19 MontreaL Montreal (CA)
`( * ) Notice:
`Subject to any disclaimer, the term 0f this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 122 days.
`
`This patent is subject to a terminal dis-
`claimer.
`
`.
`(21) Appl. NO" 09/723’221
`s
`22
`Filed:
`NOV. 28 2000
`
`Related US. Application Data
`
`(63)
`
`Continuation—in—part of application No. PCT/CA99/00502,
`filed on Jun. 1, 1999, which is a continuation of application
`No. 09/088,079, filed on Jun. 1, 1998, now Pat. No. 6,124,
`675
`7
`
`Int. Cl.
`(51)
`(52) US. Cl.
`
`................................................... H01J 7/24
`............................. 315/111.91; 315/111.81;
`315/111.21; 250/426; 250/427; 313/359.1
`(58) Field of Search ....................... 315/111.81, 111.21,
`315/11171, 111.91; 250/281, 423 R, 424,
`426, 427; 313/3591, 361.1
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`....... 250/423 R
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`11/1971 Cook et a1.
`................. 250/283
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`8/1975 Young .............. 250/287
`3,902,064 A
`
`
`11/1977 Walters ~~~~~~
`219/1214
`4,060,708 A
`
`8/1983 LeVeson ~~~~~~~~~~~ 324/465
`43989152 A
`10/1983 Meuzelaar "
`250/288
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`11/1984 Kaufman et al.
`...... 156/34539
`4,481,062 A
`10/1985 Tsuchiya et al.
`............ 250/288
`4,546,253 A
`.. 250/423 R
`4,782,235 A * 11/1988 Lejeune et al.
`
`4/1989 Conzemius ................. 250/287
`4,818,862 A
`
`
`
`4,948,962 A
`............... 250/288
`8/1990 Mitsui et al.
`5,083,061 A *
`1/1992 Koshiishietal.
`.
`250/423 R
`5,086,226 A
`2/1992 Marcus
`250/288
`52233132451 2 * 13133431 $11; It~~~~~~
`gig/3::
`,
`,
`c u z ......
`5,485,016 A
`1/1996 Irie et al.
`250/288
`..........
`5,594,243 A
`1/1997 Weinberger et al.
`........ 250/288
`5,896,196 A *
`4/1999 Pinnaduwage .............. 250/288
`*
`6,124,675 A
`9/2000 Bertrand et al.
`............ 250/426
`OTHER PUBLICATIONS
`.
`.
`N. Leymar1e, M. Bertrand, J.C. Mathurm, A. Bruno, & J.C.
`Tabet “To adapt a Metastable Atom Beam Source to a
`SATURN III Ion Trap”, 46th ASMS Conference on Mass
`Spectrometry and Allied Topics, Orlando, FL, May 23—Jun.
`4, 1998.
`A. Vuica, D. Faubert, M. Evans & M.J. Bertrand, “Analysis
`g
`g
`y
`y
`of Ion
`strai ht h drocarbons chains b GC—MAB—MS”,
`46th ASMS Conference on Mass Spectrometry and Allied
`Topics, Orlando, FL, May 23—Jun. 4, 1998.
`
`(List continued on next page.)
`
`Primary Examiner—Don Wong
`Assistant Examiner—Ephrem Alemu
`
`(57)
`
`ABSTRACT
`
`The metastable atom bombardment source provides a
`charged particle free beam of metastable Species that can be
`used to bombard and ionize organic and inorganic sub-
`stances in a gas phase. The metastable atoms are produced
`by inducing a discharge in a gas (rare gases or small
`molecules). The discharge is curved between the cathode
`and anode, with the cathode located in a medium pressure
`zone and the anode located off-axis in a low pressure zone.
`A nozzle located between the cathode and the anode pro-
`vides a collimated beam of metastable atoms of low kinetic
`energy that is directed at an ion volume containing the
`substances to be analyzed. By selecting the energy of the
`metastable state, selective fragmentation of molecules, par-
`ticularly large molecular weight molecules, can be carried
`out
`'
`
`4 Claims, 6 Drawing Sheets
`
`CATHODE
`125
`
`NOZZLE
`124
`
`ANODE
`150
`
`1.90
`
`MASS
`
`140
`
`SPECTROMETER
`
`
`
`
`
`
`RARE GAS
`115 J
`
`—l
`
`1.70
`
`k
`
`m
`120-_
`s\\
`
`.............
`
`[6‘0
`DEFLECTOR
`
`GILLETTE 121 3
`
`GILLETTE 1213
`
`

`

`US 6,661,178 B1
`
`Page 2
`
`OTHER PUBLICATIONS
`
`Denis Faubert, H. Pakdel, M. Mousselmal & M.J. Bertrand,
`“Thermal analysis of a pyrolytic oil in direct combination
`with the metastable atom bombardment (MAB) source”,
`46th ASMS Conference on Mass Spectrometry and Allied
`Topics, Orlando, FL, May 23—Jun. 4, 1998.
`Simon Letarte, Moussa Mousselmal, Denis Faubert &
`Michel J. Bertrand, “Use of MAB—MS for the Character-
`ization of Bacteria”, 46th ASMS Conference on Mass Spec-
`trometry and Allied Topics, Orlando, FL, May 23—Jun. 4,
`1998.
`Jon G. Wilkes, Thomas M. Heinze, James P. Freeman et al.,
`“Use of Probe Sample Introduction with E1 or MAB Ion-
`ization for Rapid Bacterial Chemotaxonomy”, 46th ASMS
`Conference on Mass Spectrometry and Allied Topics,
`Orlando, FL, May 23—Jun. 4, 1998.
`Jon G. Wilkes, Manuel Holcomb, Fatemeh Rafii et al.,
`“Probe Introduction/MAB/MS for Rapid Bacterial Chemo-
`taxonomy”, 46th ASMS Conference on Mass Spectrometry
`and Allied Topics, Orlando, FL, May 23—Jun. 4, 1998.
`N. Leymarie, M. Bertrand, & M. Mousselmal, “Negative Ion
`Formation in a Metastable Atom Bombardment (MAB) Ion
`Source”, 45‘” ASMS Conference on Mass Spectrometry and
`Allied Topics, Palm Springs, CA, Jun. 1—5, 1997.
`Denis Faubert, Moussa Mousselmal, Andreea Vuica & M.J.
`Bertrand, “User of Nitrogen as a Gas for Metastable Atom
`Bombardment (MABTM)”, 45th ASMS Conference on Mass
`Spectrometry and Allied Topics, Palm Springs, CA, Jun.
`1—5, 1997.
`Jonathan M. Curtis & Denis Faubert,“Metastable Atom
`Bombardment (MAB)/Hybrid Sector—TOF for quantitative
`GC/MS Analyses”, 45th ASMS Conference on Mass Spec-
`trometry and Allied Topics, Palm Springs, CA, Jun. 1—5,
`1997.
`
`Pascal Mireault, Denis Faubert, Gary J.C. Paul et al., “LC/
`MAB/MS: A new Ionization Techniques for LC/MS”, 415‘
`Int’l Conference on Analytical Sciences and Spectroscopy,
`Ontario, Canada, Aug. 14—16, 1995.
`Denis Faubert, Pascal Mireault & Michel J. Bertrand,
`“MAB: A Novel Ionization Source Providing Selective
`Ionization and Fragmentation”, 41“ Int’l Conference on
`Analytical Sciences and Spectroscopy, Ontario, Canada,
`Aug. 14—16, 1995.
`Denis Faubert, Pascal Mireault & Michel J. Bertrand, “Ana-
`lytical Applications of the MAB Source for the Analysis of
`Organic Compounds”, 3’“ Int’l Symposium on Applied
`Mass Spectrometry in the Health Sciences/European Tan-
`dem Mass Spectrometry Conference, Barcelona, Spain, Jul.
`9—13, 1995.
`Denis Faubert, Alain Carrier, Pascal Mireault & Michel J.
`Bertrand, “LC/MAB/MS: A New Ionization Technique for
`LC/MS”, 3’“ Int’l Symposium on Applied Mass Spectrom-
`etry in the Health Sciences/European Tandem Mass Spec-
`trometry Conference, Barcelona, Spain, Jul. 9—13, 1995.
`
`Denis Faubert, Moussa Mousselmal, Marc Cyr & Michel J.
`Bertrand, “Pyrolysis Analysis in Direct Combination with
`the Metastable Atom Bombardment (MAB) Source”, 14th
`Int’l Mass Spectrometry Conference, Tampere, Finland,
`Aug. 25—29, 1997.
`Denis Faubert, Moussa Mousselmal, Andreea Vuica et al.,
`“Characteristics of the MAB Source as a Common Ion
`
`Source for Mass Spectrometry”, 14th Int’l Mass Spectrom-
`etry Conference, Tampere, Finland, Aug. 25—29, 1997.
`D. Faubert, G.J.C. Paul, J. GirouX & M.J. Bertrand, “Selec-
`tive fragmentation and ionization of orgainc compouds
`using an energy—tunable rare—gas metastable beam source”,
`14‘” Int’l Mass Spectrometry Conference, Tampere, Finland,
`Aug. 25—29, 1997.
`
`D. Faubert, P. Mireault & M.J. Bertrand, “Analytical Poten-
`tial of the MAB source for routine analysis of organic
`compouds”, 43rd ASMS Conference on Mass Spectrometry
`and Allied Topics, Atlanta, GA, May 21—26, 1995 .
`M. Mousselmal, D. Faubert, J.J. Evans & M.J. Bertrand,
`“Comparison of El and MAB ionization for exact mass
`measurement”, 44th ASMS Conference on Mass Spectrom-
`etry and Allied Topics, Portland, OR, May 12—16, 1996.
`P. Mireault, D. Faubert, A. Carrier et al., “Evaluation of
`MAB as a selective Ion Source for Chromatography/Mass
`Spectrometry Techniques”, 44‘” ASMS Conference on Mass
`Spectrometry and Allied Topics, Portland, OR, May 12—16,
`1996.
`
`D. Faubert, M. Mousselmal, S.G. Roussis & M.J. Bertrand,
`“Comparison of MAB and EI for petroleum mass spectrom-
`etry”, 44th ASMS Conference on Mass Spectrometry and
`Allied Topics, Portland, OR, May 12—16, 1996.
`
`M. Cyr, D. Faubert, M. Mousslemal et al., “Analysis of the
`emanations
`from heated
`polyurethane
`foam using
`MAB—MS”, 44th ASMS Conference on Mass Spectrometry
`and Allied Topics, Portland, OR, May 12—16, 1996.
`
`R.J. Slobodrian, J. GirouX, R. Labrie et al., “Highly
`polarised He(23S) thermal metastable atom source”, J. Phys.
`E: Sci. Instrum., vol. 16, 1983, Great Britain.
`
`D. Faubert, G.J.C. Paul, J. GirouX & M.J. Bertrand, “Selec-
`tive fragmentation and ionization of organic compouds
`using an energy—tunable rare—gas metastable beam source”,
`Int’l Journal of Mass Spectometry and Ion Processes, 124
`(1992) 69—77 Elsevier Science Publishers B.V., Amsterdam.
`Michel J. Bertrand, D. Faubert, M. Mousselmal & O.
`Peraldi, “MAB: Metastable Atom Bombardment: A new
`Ionisation Technique for Analytical Mass Spectrometry and
`Tandem Mass Stepctrometry of Organic Compounds”, Cen-
`tre D’Etudes Du Bouchet and Universite Pierre Et Marie
`Curie, Essone, France, Mar. 11—13, 1998.
`
`* cited by examiner
`
`

`

`US. Patent
`
`Dec. 9, 2003
`
`Sheet 1 0f 6
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`US 6,661,178 B1
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`US 6,661,178 B1
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`US. Patent
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`Dec. 9, 2003
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`Sheet 6 0f 6
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`US 6,661,178 B1
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`High Voltage Zone
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`

`

`US 6,661,178 B1
`
`1
`METASTABLE ATOM BOMBARDMENT
`SOURCE
`
`The present application is a continuation-in-part of PCT/
`CA99/00502 filed Jun. 1, 1999 designating the United States
`which is a continuation of US. patent application Ser. No.
`09/088,079 filed Jun. 1, 1998, now US. Pat. No. 6,124,675.
`
`FIELD OF THE INVENTION
`
`The present invention is directed to an apparatus and
`method for producing a beam of metastable atoms or
`molecules, and in particular, a system and method for
`producing a beam of metastable species for use in ionizing
`sample substances undergoing analysis by mass spectros-
`copy or other devices requiring ionization or excitation of
`substances.
`
`BACKGROUND OF THE INVENTION
`
`Mass spectrometers are well known systems used for the
`detection and identification of chemical structures and quan-
`titative elemental analysis of substances. In all known mass
`spectrometry methods, atoms or molecules to be sampled
`are excited and ionized, so as to create an ion beam. The ion
`beam is then accelerated through electric and magnetic fields
`to an ion collector, with the ion collector typically attached
`to an electrometer. The electrometer then translates signals
`received from the ion collector into a mass spectrum, which
`serves to indicate what elements (or radicals or fragments)
`are contained within the sample.
`Many techniques have been suggested to excite and ionize
`the sample molecules and to fragment the ions from these
`molecules. These include the use of electrons to bombard
`
`species present in the gas phase, such as electron ionization;
`proton transfer reactions, such as those used in chemical
`ionization; or photoionization with lasers or other intense
`light sources. More recently, ionization has been accom-
`plished by the use of metastable atom bombardment, in a
`which a neutral metastable species is used to bombard the
`sample molecules and fragment ions from these molecules.
`The use of metastable atom bombardment in ionizing the
`sample molecules has allowed the possibility of performing
`selective ionization, and control over the fragmentation of
`particles from the sample molecules. However, in order to
`perform metastable atom bombardment which consistently
`ionizes the sample material, a reaction mechanism is needed
`to produce a consistent source of metastable atoms, which is
`high in its intensity, charge free and low velocity.
`A reaction system which produces a beam of metastable
`atoms is known in the art, and includes a reaction vessel
`having a source of rare gas at one end of the vessel, a cathode
`positioned inside the vessel and a small sonic nozzle placed
`at
`the other end of the vessel. Outside the vessel
`is a
`
`generally cone shaped anode referred to as a “skimmer” and
`which further includes an aperture at the apex of the cone.
`Behind the skimmer is a set of plates which serve as a
`deflector. In operation, the gas is injected at one end of the
`vessel and passes through the nozzle at the opposite end. The
`cathode within the vessel and the anode outside of the vessel
`
`are charged by a DC supply, such that a plasma arc is created
`between the cathode and anode. The atoms of gas which are
`injected through the discharge are energized to a metastable
`state, with some of the gas atoms being energized to the
`point of ionization, thus releasing free ions and electrons
`into the metastable gas stream. The metastable gas, the free
`ions and electrons then pass through the aperture in the apex
`of the skimmer into a set of charged deflector plates, where
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`the free ions/electrons are attracted to the deflector plates,
`leaving the relatively charge free, metastable gas particles to
`pass through the deflector plates where it is used to bombard
`the sample substance to be analyzed by the mass spectros-
`copy apparatus.
`A known disadvantage of this prior art device is that it
`does not always produce a consistent stream of metastable
`particles, and sometimes creates a stream of metastable
`particles mixed with ions/electrons. This occurs because the
`electric field which surrounds the cathode and anode is
`
`to a longitudinal axis passing
`symmetric with respect
`through the cathode and anode. As a result of this symmetric
`electrical field, the forces applied to the ions/electrons and
`ionized atoms created by the discharge is such that these
`particles are forced towards this longitudinal axis. Since this
`longitudinal axis also coincides with the axis of flow, the
`ions/electrons tend to remain in the flow path along with the
`metastable gas particles. Although the deflector does remove
`some of these ionized particles, the forces applied by the
`symmetric electric field work against the forces applied by
`the deflector, and thus ions tend to remain within the particle
`flow. Thus, the prior art apparatus does not produce a beam
`of purely metastable atoms, and produces spurious, unpre-
`dictable results when such a beam is used to ionize the
`
`sample to be tested by spectroscopy. The use of a skimmer
`and deflector plates also results in a larger assembly that
`causes a loss of metastable atoms. Because of the advantages
`of using metastable atom bombardment for selective ion-
`ization of the sample material, a need exists to improve the
`metastable atom bombardment system so that the beam of
`metastable atoms projected against the sample material only
`contains metastable atoms with a high density.
`SUMMARY OF THE INVENTION
`
`It is a feature of the present invention to provide an
`apparatus which efficiently produces a be am of metastable
`species having a good purity.
`It is an other feature of the present invention to provide a
`method of generating a beam of purely metastable species
`for use in spectroscopy applications.
`According to a first aspect of the present invention, the
`electric arc used to generate metastable gas follows a curved
`path.
`According to a second aspect of the present invention, the
`gas subjected to the electric arc passes from a low pressure
`chamber through a nozzle to a lower pressure chamber to
`form a jet of gas, in which the jet of gas is subjected to fields
`for removing ionized gas from the jet of gas prior to a
`substantial portion of the jet exiting the lower pressure
`chamber as a pure metastable jet into a reaction chamber of
`a mass spectrometer. The intensity of the arc may be selected
`to generate a higher concentration of ionized and metastable
`species, while the jet exiting the lower pressure chamber
`comprises substantially only metastable species of the gas.
`According to a third aspect of the present invention, the
`arc has a greater portion of its length in a higher pressure
`chamber than in the lower pressure chamber on the other
`side of the nozzle communicating between the higher and
`lower pressure chambers, so as to expend more energy in the
`higher pressure chamber.
`According to one embodiment of the present invention,
`there is provided an apparatus for generating a beam of
`metastable species for use in Penning ionization, compris-
`ing:
`first chamber having a gas inlet and a nozzle outlet, said
`inlet being connected to a substantially low pressure
`
`

`

`US 6,661,178 B1
`
`3
`source of gas suitable for being energized to a meta-
`stable state and inducing Penning ionization and Pen-
`ning energy transfer;
`a cathode arranged in said first chamber;
`a second chamber communicating with said nozzle and
`having a beam outlet substantially in line with said
`cathode and said nozzle, said second chamber being in
`communication with a substantially rough vacuum;
`an anode arranged in said second chamber to one side of
`a line extending substantially between said cathode,
`said nozzle and said beam outlet, wherein an electrical
`discharge formed between said cathode and said anode
`passes through said nozzle and then deviates from said
`nozzle to said anode, and an electric field between said
`cathode and said anode is asymmetric,
`whereby a jet of said gas emitted from said nozzle
`containing metastable and ionized species is projected
`to said beam outlet while ionized species are diverted
`from said beam outlet and a beam of said gas emitted
`from said beam outlet has an improved concentration of
`metastable species.
`The invention also provides method of generating a beam
`of metastable atoms for use in Penning ionization,
`comprising the steps of:
`providing a jet of gas suitable for being energized by
`electrical discharge to a metastable state and inducing
`Penning ionization;
`forming a curved electrical discharge arc co-extensive
`with a portion of the jet and deviating from the jet to
`one electrode to excite the gas to a metastable state; and
`communicating a downstream portion of the jet with a
`beam outlet.
`
`The invention further provides a method of ionizing and
`fragmenting a molecule,
`the method comprising the
`steps of:
`selecting a gas having an energy of a metastable state
`sufficient to cause ionization in the molecule and to
`break at least one desired bond in the molecule;
`generating a beam of the gas excited to the metastable
`state, the beam being substantially free from ions;
`providing the molecule is a gaseous state in an ionization
`reaction chamber; and
`directing the beam into the reaction chamber to cause
`ionization and selective fragmentation of the molecule.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention will be better understood by way of the
`following detailed description of a preferred embodiment
`with reference to the appended drawings, in which:
`FIG. 1 discloses a prior art system for generating a beam
`of metastable atoms from a source of rare gas;
`FIG. 2 is a diagram illustrating the known mechanism of
`ionization using a metastable atom source;
`FIG. 3 is a schematic diagram of the apparatus according
`to the preferred embodiment;
`FIG. 4 is a cross-sectional view of the apparatus according
`to the preferred embodiment;
`FIG. 5 is a schematic block diagram of the power supply
`electronic unit according to the preferred embodiment; and
`FIG. 6 is a schematic diagram of the circuit board used in
`the power supply electronic unit according to the preferred
`embodiment.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`FIG. 1 discloses a prior art system 10 for generating a
`beam of metastable atoms from a source of rare gas 15. The
`
`4
`source of rare gas 15 is projected into a chamber 20 having
`a pressure gradient from its entry to the beam exit at 50
`(anode). Within the chamber 20 is placed an energized
`cathode 25, while an energized anode 50 is set just outside
`the chamber 20. Due to the energy applied to the energized
`cathode and anode, an electric discharge is generated from
`the cathode to the anode, extending through the aperture or
`nozzle 40 in the chamber 20. The rare gas projected into the
`chamber 20 is driven by the pressure gradient
`into the
`discharge between the cathode and anode. The discharge in
`turn energizes the atoms of the rare gas into a mixture of
`ions/electrons and metastable atoms in which the electrons
`of these atoms are raised to higher energy levels.
`The stream of metastable atoms, ionized atoms and elec-
`trons then pass through a charged deflector 60, which
`removes some of the ions/electrons from the stream of
`particles. However, because the cathode and anode are in
`direct axial alignment with one another, a uniform and
`symmetric electric field is generated around the discharge
`generated between these two structures. This symmetric
`electric field in turn generates forces on the charged particles
`in the stream, namely, the ionized atoms/electrons but not
`the energized metastable atoms.
`The metastable atoms are not charged since they retain
`their electrons and are not ionized. However,
`the forces
`applied on the ions and electrons tends to force these
`particles towards the longitudinal axis extending between
`the cathode and anode. As a result,
`the forces of the
`symmetric electric field tend to force the charged particles
`towards the longitudinal axis of the stream, counteracting
`the effect of the deflector to remove these particles away
`from the stream and interfering with the passage of the
`metastable atoms. The net result is that the deflector 60 is not
`
`completely effective in removing the charged particles from
`the particle stream, and the particle stream applied against
`the sample material is not a stream of purely metastable
`atoms., Furthermore,
`the production rate of metastable
`atoms is relatively poor.
`When metastable atoms interact with neutral molecules, a
`process referred to as Penning ionization results. As illus-
`trated in the diagram of FIG. 2, a metastable species A*
`collides with a neutral molecule BC in the gas phase. An
`electron from the molecular orbitals of BC attacks the vacant
`
`orbital of the metastable species A* and an electron is
`ejected into the continuum (gamma) leading to ionization as
`illustrated. The ejected electron can take a range of kinetic
`energies that is defined by the species involved in the gas
`phase collision. As illustrated, the result may simply ionize
`BC, fragment BC into B+and C (or B and C“), or create
`ABC“.
`
`The excitation energies of various noble gases change
`with atomic weight. For example, the 381 and 1SO: similarly
`3P2 3P0 and states of He are 19.82 eV and 20.61 eV
`respectively, the 3P2 and 3P0 states of Ar are 11.55 eV and
`11.72 eV, and the 3P2 and 3P0 states of Xe are 8.32 eV and
`9.45 eV. For nitrogen gas, some more metastable states are
`in the range of 8.52 eV to 11.88 eV. In this specification,
`reference is often made to rare or noble gases and atoms as
`being the gases yielding metastable species. It
`is to be
`understood that other gases, preferably small molecules such
`as nitrogen, may also be suitable. It is important to choose
`a gas that
`is substantially inert when subjected to the
`discharge and then mixed with the substance to be ionized,
`and which provides a suitable excitation energy for ionizing
`and/or fragmenting the substance to be analyzed.
`FIG. 3 illustrates a preferred embodiment of the
`invention, which overcomes the problems created by sym-
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`US 6,661,178 B1
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`5
`metric electric fields in the particle stream path. The pre-
`ferred embodiment 100 includes a first chamber 120 con-
`
`taining a cathode 125, a first inlet 115 through which the rare
`gas (or other suitable gas) is supplied at a predetermined
`pressure and a nozzle orifice 124. Asecond chamber 122 has
`an anode 150 positioned off-axis. The first chamber 120 is
`maintained at higher pressure than the second chamber 122
`such that a jet of gas is created. First and second outlets 128
`and 140 respectively in the second chamber 122 are
`provided, and the pressure in chamber 122 is maintained at
`about 0.1 Torr. The second outlet 140 is in turn connected to
`the reaction chamber 170. The reaction chamber 170
`
`includes an inlet 175 for the injection of the sample to be
`tested, and an outlet 180 communicating with a mass spec-
`trometer 190 which is kept near vacuum pressure.
`The first chamber 120 has an inlet 115 for a noble gas and
`an outlet 124. Chamber 122 is maintained at a reduced
`
`pressure of preferably about 0.1 Torr. and has at the right end
`of the chamber outlet 128, which is less than the pressure of
`the chamber 120 where the noble gas is injected. This creates
`a pressure gradient across nozzle 124, so that a gas jet is
`created in the direction of outlet 140. Inserted into the
`chambers 120 and 122 are cathode 125 and anode 150
`
`respectively. The cathode 125 and anode 150 are energized
`so as to create a discharge 130 between the cathode and
`anode. The discharge 130 has a linear part in chamber 120
`and a curved part in chamber 122. The gas receives energy
`from the discharge 130 mostly in its linear part. As the gas
`atoms are ejected through nozzle 124, charged particles feel
`the effect of anode 150 and are deflected.
`
`Unlike the prior art device, the electric field generated by
`the anode 150 and cathode 125 is asymmetric. This is due to
`the fact that the cathode 125 and anode 150 are placed along
`axes that are radially separated from one another. The radial
`separation creates an asymmetric electric field which tends
`to force the ions away from the path of the neutral, meta-
`stable atoms. Thus, when the stream of gas approaches the
`separation plates 160 and orifice 162, the charged particles
`are already well separated from the stream of metastable
`atoms, and the separation plates are more effective at remov-
`ing these charged particles from the gas stream. It would be
`possible to reverse the direction of current
`flow from
`between the electrodes, however,
`it
`is preferred for the
`cathode, to be inside the first chamber, and for the anode to
`be a flat electrode. While a flat anode works well, a curved
`semi-cylindrical anode can also be used which allows for a
`greater surface to attract the charged particles.
`The resultant gas which passes into the chamber 170 is
`thus substantially a beam of purely metastable atoms. This
`beam is then bombarded against
`the sample molecules
`injected into the reaction chamber 170 at inlet 175. Depend-
`ing on the energy of the metastable atoms, they are able to
`ionize the sample up to a certain ionization energy by
`interaction, as described hereinabove. The ionized sample is
`then passed on to the mass spectrometer 190 through outlet
`180, where it is analyzed accordingly.
`The system of the preferred embodiment herein produces
`a stream or beam of metastable atoms which is collimated,
`low kinetic energy, charged particle free and high concen-
`tration (i.e. >10A15 atoms/sec/str). Such a beam is very
`efficient for performing the metastable atom ionization for
`mass spectrometry.
`When using rare gases or small molecules, such as N2, it
`is possible in a metastable atom bombardment source to
`have precisely known ionization energies in the range of
`8—20 eV. The use of Xe (8.32 eV), Kr (9.55 eV) or N2 (8.52
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`eV) for generating the metastable gas will lead to very soft
`ionization and essentially non fragmentation because the
`ionization energies of the compounds formed during pyroly-
`sis are of the order of 8 eV. Hence, all the available energy
`in the metastable species is used for ionization and ions are
`formed with low internal energies and cannot fragment as in
`electron ionization.
`
`While the invention may be used in a manner to avoid
`fragmentation, it may likewise be put into practice with the
`intent of selective fragmentation. The energy available for
`fragmentation is the energy remaining after ionization,
`namely the energy of the metastable state of the metastable
`gas less the ionization energy of the order of 8 eV. By using
`metastable atom energies greater than 8 eV,
`the present
`invention allows the high quality metastable atom beam to
`be used to selectively fragment high molecular weight
`organic molecules as a function of the particular bond or
`bonds to be broken in the organic molecules.
`The construction of the apparatus according to the pre-
`ferred embodiment is better shown in detail in FIG. 4. The
`
`cathode 125 includes a narrow diameter cylindrical tip with
`a tapered point, while the anode 150 is planar and located
`off-axis immediately after the nozzle. A curved discharge is
`created in which the electrons are removed from the center
`
`of the gas-flow that contains the metastable species that are
`not affected by the electrical field. The use of a planar
`electrode for the anode increases the stability of the dis-
`charge (greater surface to collect electrons) and reduces the
`electrical field in that region of the apparatus. The use of a
`planar electrode also allows the design to be very compact,
`thus, reducing the voltage necessary to maintain the dis-
`charge. The greater collection area for electrons and the
`reduced voltage combine to locally reduce the heat transfer
`of the anode thus avoiding overheating and anode erosion.
`This leads to greater stability of operation.
`A distance between the cathode and the nozzle is shown
`to be about three times the distance between the nozzle and
`
`the anode. This distance ratio may be between 1.5 to 4.0 (or
`more), and provides for a good portion of the energy to be
`expended inside the first chamber.
`Different shapes and materials have been studied for the
`cathode and the best results were obtained with a simple
`sharp needle made of pure Copper (without 02). The cath-
`ode is a sharp needle (or an assembly of sharp needles)
`mounted on a cylindrical body. This body can be machined
`with flats as shown in FIG. 4, or it can be drilled with tiny
`holes, knurled, (diagonal, straight, diamond pattern), or can
`be threaded (single or multiple helix). These configurations
`insure the flow of the rare gas through the body and recenter
`the cathode in the axis of the orifice. This configuration has
`also the advantage of pre-heating the rare gas before enter-
`ing the plasma, conferring more stability to the discharge. It
`also allows the cathode to be cooled, thus increasing stabil-
`ity. Finally, the cathode is equipped with an internal thread
`or an external thread (as shown in FIG. 4) to insure proper
`positioning in the gun-assembly, easy disassembly and good
`electrical contact with the electrical supply.
`The nozzle 124 which is located between the cathode and
`
`the anode is used to create a pressure drop in the gun-
`assembly which leads to the formation of a gas jet. The
`pressure in the first chamber 120 is of the order of 10—100
`torr while the pressure in the bottom end second chamber
`122 which is differentially pumped is less than one torr. The
`nozzle is machined in LavaTM material (Grade A, unfired)
`then the part is fired at 1100° C. for 30 minutes to crystallize
`the material into a ceramic (expansion factor of 2%). The
`
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`US 6,661,178 B1
`
`7
`diameter of the nozzle varies between 120 to 180 pm for
`optimum operating conditions with gases such as helium,
`neon, argon, krypton, xenon en (N2). A chamber is provided
`for aligning the gun on a centering plate as shown in FIG.
`4. Alip at the base of the orifice 124 is used to seal the nozzle
`on the body with an O-ring (or any other suitable sealing
`means) and maintain the seal. The nozzle is maintained in
`position by the polyimide cap screwed directly onto the
`body (an internal thread or screws through the cap). The cap
`can support the anode and the deflector or can be used as
`feedthrough for the deflector and the anode contacts as
`shown in FIG. 4 or any combination of these two configu-
`rations depending on the instrument. This design insulates
`the cathode from the seal and the apparatus body. These
`critical parts, namely the body and seals, are pro