WO1999066536A2 - An apparatus for reduction of selected ion intensities in confined ion beams - Google Patents
An apparatus for reduction of selected ion intensities in confined ion beams Download PDFInfo
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- WO1999066536A2 WO1999066536A2 PCT/US1999/013517 US9913517W WO9966536A2 WO 1999066536 A2 WO1999066536 A2 WO 1999066536A2 US 9913517 W US9913517 W US 9913517W WO 9966536 A2 WO9966536 A2 WO 9966536A2
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- ions
- ion
- carrier gas
- gas
- reagent gas
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J49/00—Particle spectrometers or separator tubes
- H01J49/004—Combinations of spectrometers, tandem spectrometers, e.g. MS/MS, MSn
- H01J49/0045—Combinations of spectrometers, tandem spectrometers, e.g. MS/MS, MSn characterised by the fragmentation or other specific reaction
- H01J49/0077—Combinations of spectrometers, tandem spectrometers, e.g. MS/MS, MSn characterised by the fragmentation or other specific reaction specific reactions other than fragmentation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J49/00—Particle spectrometers or separator tubes
- H01J49/02—Details
- H01J49/10—Ion sources; Ion guns
- H01J49/14—Ion sources; Ion guns using particle bombardment, e.g. ionisation chambers
- H01J49/145—Ion sources; Ion guns using particle bombardment, e.g. ionisation chambers using chemical ionisation
Definitions
- the present invention relates generally to an apparatus for producing an ion beam having an increased proportion of analyte ions compared to carrier gas ions. More specifically, the apparatus is an ion source coupled to a collision cell or to an ion trap containing a selectively reactive gas.
- ion beams might occur in ion guns, ion implanters, ion thrusters for attitude control of satellites, laser ablation plumes, and various mass spectrometers (MS), including linear quadrupole MS, quadrupole ion trap MS (e.g. "Paul" trap), ion cyclotron resonance MS, time-of-flight MS, and electric and/or magnetic sector MS.
- MS mass spectrometers
- ion beams including electron impact, laser irradiation, electrospray, and variations thereof such as lonsprayTM, thermospray, inductively coupled plasma sources, glow discharges and hollow cathode discharges.
- Typical arrangements combine a sample with a carrier or support gas whereby the carrier gas is utilized to aid in transporting, ionizing, or both transporting and ionizing, the sample.
- analyte substances often occur in combination with other substances which can also be ionized and transported along with the analyte ions and carrier gas ions. These other substances will be collectively referred to as matrix substances, or in ionized form as matrix ions.
- matrix refers to all substances in a sample apart from the analyte.
- matrix ions refers to all ions apart from the analyte ions and thus, matrix ions includes plasma ions.
- plasma ions such as ArH + , ArO + , ArN + are formed.
- the matrix ions are typically an interference in chemical analysis applications or other utilizations of the plasma or ion beam.
- a sample is combined with the carrier gas in an electrical field, whereupon the sample and the carrier gas are ionized in a strong electric or magnetic field and later used in an analytical or other process.
- the carrier gas is first ionized in a strong electric or magnetic field whereupon the sample is then introduced into the ionized carrier gas.
- Ionized carrier gas contains carrier gas and carrier gas ions. Electric fields are generated by a variety of methods well known in the art including, but not limited to, capacitive and inductive coupling of radiofrequency (RF) and/or DC electrical energy.
- an RF voltage is applied to a coil of a conducting material, typically brass.
- a carrier gas such as argon
- a sample which may be any substance or mixture of substances.
- the analyte may be supplied in a variety of forms including but not limited to a gaseous form, as a liquid, as a droplet form, as an aerosol, or as a laser ablated plume.
- a large electrical field is generated within the coil. Within this field, any free electrons will initiate a chain reaction in the sample and the carrier gas causing a loss of electrons and thus ionization of the carrier gas and the sample.
- both the carrier gas and the sample in the plasma may be in the form of particles, atoms or molecules, or a mixture of particles, atoms and molecules, depending on the particular species selected for use as the carrier gas and the species and form selected for use as the sample.
- the carrier gas and the sample may be combined by a wide variety of methods well known in the art.
- the sample in an aerosol form is combined with the carrier gas and directed to the interior of a coil in an inductively coupled plasma.
- electrospray and variations thereof such as ionsprayTM
- a needle which receives a liquid sample from a source such as a liquid chromatograph.
- a carrier gas such as nitrogen as a high velocity atomizing carrier gas.
- Both the needle and the tube empty into a chamber. Upon discharge from the needle, the sample liquid is evaporated and atomized in the nitrogen carrier gas.
- Ions of both the evaporated liquid sample and the nitrogen carrier gas are produced by creating an electric field within the chamber.
- the electric field may be produced by creating a voltage difference between the needle and the chamber.
- a voltage difference may be created by applying a voltage to the needle and grounding the chamber.
- the resultant plasma generated by any of the foregoing methods is typically directed towards either an analytical apparatus or towards a reaction zone wherein the carrier gas and sample ions are analyzed or otherwise reacted or utilized in some fashion.
- the resultant plasma is typically directed by means of an electric or magnetic field, or by means of a pressure differential, or both. As the plasma is directed, the plasma is converted from a plasma to an ion beam.
- the term "ion beam” refers to a stream consisting primarily of positively charged and neutral species.
- the bulk of the negatively charged species in the plasma are typically electrons, which are rapidly dispersed as the plasma is directed by either electric or magnetic fields or by a pressure differential.
- the ion beam may not be completely void of negatively charged species.
- the free electrons due to their low mass relative to the positively charged ions, tend to disperse from the plasma, thus converting the plasma to an ion beam.
- the ion beam itself will tend to disperse due to several effects. Most prominent among these effects are free jet expansion and the repulsive forces of charged species within the ion beam.
- the effect of dispersion of the constituent species in the ion beam is charge separation among those species and is well known in the art.
- the resultant ion beam is thus typically characterized by high net positive charge density. Since the carrier gas is typically present in excess over the sample, this high positive charge density is primarily attributable to the relatively high abundance of positively charged carrier gas ions.
- the abundance of positively charged carrier gas ions, matrix ions and/or the resultant high charge density may be undesirable.
- the ion beam be focused through a small aperture, for example, if the sample ions were to be analyzed in a mass spectrometer.
- the high charge density will prescribe a space charge limit to the maximum on beam current that may be passed through a given aperture. Any beam current in excess of the space charge limit is unable to pass through the aperture and is thus lost.
- the portion of the beam which is lost includes analyte ions.
- a loss of a portion of the beam may result in a disproportionate loss of some or all of the analyte ions because the analyte ions may not be evenly distributed throughout the ion beam or may not respond to the various dispersing and directing forces in the same manner as the carrier gas or matrix ions.
- carrier gas or matrix ions are undesirable in a quadrupole ion trap mass spectrometer where the quadrupole ion trap has a limited ion storage capacity.
- the carrier gas and matrix ions compete with analyte ions for the limited storage capacity of the quadrupole ion trap.
- the storage capacity for analyte ions in the quadrupole ion trap is thereby increased.
- Carrier gas ions and plasma ions can be potent chemical ionization sources and can cause high levels of ionization of background gases in the trap.
- Such background ions can be formed in sufficient number that they interfere with the detection of analyte ions even if good vacuum practices and high vacuum conditions are maintained.
- removal of carrier gas ions and/or plasma ions also has the beneficial effect of reducing such background ionization.
- carrier gas or matrix ions are also undesirable in any application where the analyte ions are to be used in a process or reaction where the carrier gas or matrix ions might interfere with such process.
- ion beams may be directed towards a targeted material such as a silicon wafer to impart electrical or physical properties to the material.
- the desired properties are typically highly dependent on the specific ions directed at such materials.
- carrier gas or matrix ions may cause undesirable effects if implanted in the targeted materials.
- Apparati that have been used to carry out these processes may be characterized as an ion source coupled or connected to a mass analyzer.
- a collision cell defines a region of space containing a sufficiently high pressure of a gas.
- the gas may be reactive or non-reactive gas.
- the collision cell is provided with a guiding field (electric or magnetic to ensure that ions can traverse the ceil in spite of a large number of collisions with the gas.
- a typical guiding field may be formed using an RF/DC multipole which restricts ion motion transverse to the long axis of the multipole while allowing relatively unrestricted motion along the axis. Multipoles may be formed in a wide variety of ways well known in the art.
- a multipole is formed by arranging an even number of pole elements, typically circular cross section rods, symmetrically around a common axis with the radial separation of the rods constant along the length of the multipole.
- pole elements typically circular cross section rods
- an object of the invention in one of its aspects to provide an apparatus for producing an ion beam with increased proportion of analyte ions to carrier gas ions and/or matrix ions (if present) and a corresponding decreased proportion of carrier gas ions and/or matrix ions by neutralizing carrier gas ions and/or matrix ions while minimally removing or neutralizing the analyte ions.
- an ion source providing the ion beam at a desired kinetic energy and directing the ion beam either into an ion trap or through a collision cell having a volume of a reagent gas (selectively reactive gas) wherein the carrier gas ions and/or matrix ions selectively reacts with the reagent gas rendering the carrier gas ions and/or matrix ions a neutral species, or the carrier gas ions and/or matrix ions are incorporated into product ions with different masses than the carrier gas ions and/or matrix ions.
- a reagent gas selectively reacts with the reagent gas rendering the carrier gas ions and/or matrix ions a neutral species
- the resulting ion beam thus contains an increased proportion of analyte ions to carrier gas and/or matrix ions, and the product ions may be then selectively dispersed, leaving an ion beam having a greater fraction of analyte ions to total ions.
- the ion beam may be directed to a mass analyzer.
- aperture is used as understood in this art to be a solid element with a through hole, that can be a plate, cylinder or other geometric shape.
- the solid element e.g. plate and/or cylinder
- the solid element is the physical element defining the hole through which ions or gas pass.
- reaction refers to any pathway for reaction between reagent gas and carrier gas ions and/or matrix ions wherein the net effect is that carrier gas ions and/or matrix ions are rendered charge neutral or react so as to form product ions with mass-to-charge (m/z) ratio different from the carrier gas ions and/or matrix ions.
- Reactions may include but are not limited to charge transfer, atom transfer, and bond insertion.
- charge transfer refers to any pathway wherein the net effect is that charge is exchanged between a charged species and a neutral species.
- the pathway may involve steps which are not charge transfer reactions. Steps within the pathway may include but are not limited to chemical reaction(s), alone or in series, such as resonant charge transfer(s), electron transfer, proton transfer, and atom transfer.
- selectivity of a reaction refers to a ratio of the extent of reaction between the reagent gas and the carrier gas ions to the extent of reaction between the reagent gas and the non-carrier gas ions (e.g analyte ions and/or matrix ions). Preferably the ratio is at least 10, most preferably at least 1000.
- analyte ion refers to any ion generated by any means including but not limited to thermal ionization, ion beams, electron impact ionization, laser irradiation, electrospray, and variations thereof such as lonsprayTM, inductively coupled plasmas, microwave plasmas, glow discharges, arc/spark discharges and hollow cathode discharges.
- Analyte ions are distinguished from other ions (carrier gas ions, matrix ions, and background ions) in that analyte ions are desired to be detected or utilized in isolation from other ions. Thus, the occurrence of other ions is an interference or limitation in the detection or utilization of analyte ions.
- reagent gas refers to any gas suitable for selective reaction with carrier gas ions and/or matrix ions. Reagent gas may be provided by any means including but not limited to commercially available substances provided in gaseous form and mixtures thereof and gases, vapors, particles, or aerosols generated by evaporation or laser ablation of condensed substances.
- reagent gas as used herein may include neutral species of carrier ions, analyte ions, or matrix ions generated by any of the foregoing methods.
- the method of the present invention is not limited to systems containing a carrier gas per se.
- the gas species are an analyte, matrix, and a carrier gas.
- the method of the present invention will work equally well in any system having two or more ion species, even if none of the species were provided as a carrier gas or matrix.
- suitable reagents may be selected to remove or neutralize those daughter ions by charge transfer.
- a particular sample may contain
- Suitable reagents may be selected to remove or neutralize the separate interfering substance by selective reaction.
- isobaric interferences in elemental mass spectrometry can be resolved by causing selective reaction of one or more of the interfering elements.
- the carrier gas selected is argon and the reagent gas selected is hydrogen.
- ion source can be an elemental ion source including but not limited to inductively coupled plasma ion source, thermal ionization, ion beams, electron impact ionization, laser irradiation, inductively coupled plasmas, microwave plasmas, glow discharges, arc/spark discharges and hollow cathode discharges, and combinations thereof.
- An ion source may be a molecular ion source, including but not limited to electrospray ion source.
- RF multipoles may be formed in a wide variety of ways well known in the art.
- an RF multipole is formed by arranging an even number of pole elements, typically circular cross section rods, symmetrically around a common axis with the radial separation of the rods constant along the length of the multipole.
- the theory, design and performance of such RF multipoles has been described in great detail by Gerlich (Dieter Gerlich, in State-Selected and State-to- State Ion-Molecule Reaction Dynamics, Part 1: Experiment; in Advances in Chemical Physics Series Vol. LXXXII; Cheuk-Yiu Ng and Michael Baer, editors; John Wiley & Sons, 1992) and others.
- the pole elements need not be circular in cross section, the cross sectional shape need not be constant along the length of the pole elements, the radial separation of the pole elements need not be constant, while an even number of pole elements should be used, the pole elements may have different cross sectional shapes, the multipole may be "bent" by forming a bend in each pole element at some point along its length, the length of the pole elements may be comparable to the radial separation of the pole elements, although the length is typically many times greater than the radial separation of the pole elements.
- a linear RF multipole refers to any RF multipole that is formed using pole elements of uniform cross section along their length, arranged approximately symmetrically around a single, common axis.
- the separation of the rods in a linear RF multipole may be constant along the length of the multipole or not constant:
- non-linear RF multipole refers to RF multipoles formed in any other manner including but not limited to the use of non-circular cross section pole elements, the use of different cross sectional sizes or shapes for the pole elements within a single multipole, the use of bent pole elements to form a bent multipole.
- Non-linear is also known in the art as a description of the symmetry of an electric field wherein the restoring force experienced by an ion displaced from the center of the field (or from the minimum in the potential energy surface which defines the field) varies non-lineariy with the spatial displacement of the ion.
- a perfectly quadrupolar field is linear in this sense; all other multipole fields, including actual quadrupole fields, have some degree of non- linearity.
- any actual RF multipole will have a field whose symmetry is composed of linear components (i.e., quadrupolar) and non-linear components (i.e., hexapolar, octopolar, and higher order symmetries).
- linear components i.e., quadrupolar
- non-linear components i.e., hexapolar, octopolar, and higher order symmetries.
- an object of the invention in one of its aspects to provide an apparatus for selectively reducing the charge density of an ion beam by neutralizing the ions of a carrier gas (preferably argon), without eliminating or neutralizing the analyte ions.
- a carrier gas preferably argon
- This is accomplished by directing the ion beam from an ion source either into an ion trap or through a collision cell having a volume of selectively reactive gas (preferably hydrogen) at kinetic energies wherein the carrier gas selectively transfers charge to the selectively reactive gas.
- a carrier gas preferably argon
- Ar + in an Ar ICP leads to formation of molecular ions by charge transfer and other reactions of Ar + .
- ArC + , ArN + , ArO + , Ar 2 + have all been observed in conventional ICP mass spectra and are thought to form early in the plasma sampling process or in the ICP itself.
- Other molecular ions may be formed by reaction of certain vacuum system background gases (H 2 O, O 2) N 2 , CO, organics from pump oil, etc.) with Ar + or other plasma ions.
- molecular ions are nuisances in that they overlap isobarically with elemental ions of interest, e.g., 28 Si + and N 2 + , 55 Mn + and C 4 H 7 + 56 Fe + and ArO + , 80 Se + and Ar 2 + .
- These molecular ions may react with H 2 or other reagent gases in such a way as to be neutralized.
- Ar 2 + reacts with H 2 to form ArH + which reacts further with H 2 eventually forming H 2 + and/or H 3 + .
- Ar 2 + reacts with H 2 to form Ar 2 H + which reacts further with H 2 to form H 3 + .
- C H 7 + may be reduced concomittantly with the reduction in Ar + afforded by the present invention since one mechanism for the formation of hydrocarbon ions is chemical ionization by Ar + .
- the charged reagent gas may be removed by resonance ejection, the sudden (i.e., quick with respect to the period of the trapping RF waveform) change in m/z due to the ion-molecule reaction, loss due to instability by virtue of a m/z lower than the lowest m/z ion stably stored in the trap (i.e., the charged reagent gas m/z is lower than the so-called "low mass cut-off' of the ion trap).
- a method for removing ions selectively is by ion-molecule reaction where an ion which is desired to be removed from the ion trap reacts with a reagent gas thereby forming a charged reagent gas, wherein the charged reagent gas is more easily removed from the ion trap than the former ion.
- an ion trap the interior of which is pressurized with reagent gas and in which the trapped ions are selectively depleted in carrier gas ions by virtue of reaction between the reagent gas and the carrier gas ions. It is a further object of the invention to provide such an ion trap wherein the reagent gas is hydrogen and the carrier gas is argon.
- FIG. 1 is a schematic drawing of the apparatus used in the first preferred embodiment of the present invention.
- FIG. 2 is two mass spectra from experiments performed in the apparatus used in the first preferred embodiment of the present invention.
- FIG. 2a is the lower trace 110 of Fig. 2 at a finer vertical scale.
- FIG. 3 is a schematic drawing of the apparatus used in the second preferred embodiment of the present invention.
- FIG. 4 is a schematic drawing of the apparatus used in the third preferred embodiment of the present invention.
- FIG. 5 contains two mass spectra from experiments performed in the apparatus used in the third preferred embodiment of the present invention.
- FIG. 6 contains two mass spectra from experiments performed in the apparatus used in the third preferred embodiment of the present invention.
- FIG. 7 is a schematic drawing of the apparatus used in the fourth preferred embodiment of the present invention.
- FIG. 8 contains two mass spectra from experiments performed in the apparatus used in the fourth preferred embodiment of the present invention.
- FIG. 9 is a schematic drawing of the apparatus used in the fifth preferred embodiment of the present invention.
- ICP/MS inductively coupled plasma mass spectrometers
- An ICP/MS is a device wherein a plasma consisting of a carrier gas (typically argon) and a sample is generated in an inductively coupled plasma (ICP) and a mass spectrometer is employed to separate and distinguish constituent atoms and isotopes.
- ICP inductively coupled plasma
- the ICP is typically operated at atmospheric pressure.
- a lens stack 60 typically consists of a series of lens elements 70, 80, typically plates and/or cylindrical tubes which have potentials applied to them and which have apertures through which the ion beam is directed.
- the ion beam is directed through these lens elements 70, 80 which focus the ion beam into a narrow stream which is directed to a mass analyzer 10, or a linear quadrupole 200 (FIG. 3).
- mass analyzer or ion discriminating unit refers to any apparatus which separates charged species according to their m/z and/or kinetic energy.
- Ion discriminating units include but are not limited to a linear quadrupole, a quadrupole ion trap, a time-of-flight tube, a combination of a quadrupole ion trap and a time-of- flight tube, a magnetic sector, an electric sector, a combination of a magnetic sector and an electric sector, a lens stack, a DC voltage plate, an ion cyclotron resonance cell, and an rf multipole ion guide.
- Modified ICP/MS systems have been built which use a three-dimensional RF quadrupole ion trap, either alone or in combination with a linear RF quadrupole as an ion discriminating unit.
- the ion beam Upon exiting the lens stack, the ion beam is directed into the ion discriminating unit. Ions are selectively emitted from the ion discriminating unit according to their mass to charge ratio (m/z) and/or kinetic energy. These selectively emitted ions are then directed to a charged particle detector 50. In this manner, the ICP/MS is able to determine the presence of selected ions in an analyte according to their (m/z) and/or kinetic energy.
- ICP/MS systems typically employ apertures between approximately 0.5 mm to approximately 2 mm.
- FIG. 7 depicts a deployment of a collision cell 710.
- a collision cell may be between the first and second apertures 20, 30, between the second aperture 30 and the lens stack 60, between lens elements in the lens stack 60, or between the lens stack 60 and the mass analyzer 10 (e.g. quadrupole ion trap).
- an ion trap containing the reagent gas may be placed between the first aperture 20 and the charged particle detector 50.
- FIG. 1 depicts a deployment of a mass analyzer 10 that is a quadrupole ion trap.
- An ion trap may be between the apertures 20, 40, between the apertures 30, 40, between the differential aperture 40 and the charged particle detector 50. Yet other arrangements are possible, for example with an ion trap located between first aperture 20 and the charged particle detector 50, various lens stacks may be utilized between the aperture 20 and the ion trap and between the ion trap and the charged particle detector. Other mass analyzers may also be employed in combination with the ion trap and located between the first aperture 20 and the ion trap and/or between the ion trap and the charged particle detector 50.
- the reagent gas is introduced within an ion beam having a carrier gas and a sample to allow the carrier gas ions and/or matrix ions to react or to be neutralized, whereupon the product ions may be selectively dispersed from the ion beam.
- the extent of reaction will be driven by at least four factors. First, any two species selected will have an inherent rate of reaction which will affect the completeness of reaction over a given period of time, all other things held constant. Second, lower ion velocities will provide a longer residence time for ions in the reaction zone and thereby provide a greater extent of reaction. Third, there is a velocity dependence for the reaction cross section which is in general different for any given reacting species so that for any given reaction the optimum velocity may be low or high.
- the completeness of reaction in a given time period is increased as the probability of a collision between the ions and reagent gas species is increased. Therefore, the completeness of reaction is dependent upon the pressure of the reagent gas and the time that the two gases are in contact Ions must have sufficient opportunities to come into contact with the reagent gas, i.e., a long residence time must be employed if the reagent gas species is present at low concentration or pressure.
- the apparatus of the present invention may be advantageously applied in any system having a collision cell and/or an ion trap filled with a reagent gas into which a carrier gas and an analyte gas are introduced wherein the carrier gas ions are removed or neutralized.
- the ICP/MS system as well as the instruments described in the preferred embodiments which follow, both practice and are demonstrative of the present invention because they contain detection elements to verify the selective neutralization or removal of carrier gas ions.
- a conventional ICP/MS manufactured by VG Elemental, now Fisons (Winsford, Cheshire, England; model PQ-I) was modified by replacing the linear quadrupole and its associated electronics (not shown) with the mass analyzer 10 as a quadrupole ion trap and its associated electronics (not shown).
- the quadrupole ion trap was installed with the ion input and output ends reversed to maximize the ion transfer efficiency from the lens stack 60 into the quadrupole ion trap.
- the quadrupole ion trap used was removed from a quadrupole ion trap mass spectrometer (ITMSTM) manufactured by Finnigan MAT (San Jose, California).
- the electron gun (not shown) and injection gate electrode assembly (not shown) were removed to allow transfer of ions from the lens stack 60 into the quadrupole ion trap.
- the vacuum system was modified from a standard Fisons vacuum system and consisted of three vacuum regions separated by two apertures. These vacuum regions are evacuated by standard vacuum pumps (not shown).
- the first vacuum region 15 is contained in between a first aperture 20 and second aperture 30 and is typically operated at 0.1 to 10 Torr.
- the second vacuum region 25 is contained between the second aperture 30 and a differential aperture 40 and is typically operated at 10 "5 to 10 "3 Torr.
- the differential aperture 40 is located within the lens stack 60 at substantially the same position as employed in the standard Fisons ICP/MS.
- the third vacuum region 35 is separated from the second vacuum region 25 by the differential aperture 40.
- the third vacuum region 35 contains a portion of the lens stack 60, the quadrupole ion trap and a charged particle detector 50.
- the third vacuum region 35 is typically operated at 10 "8 to 10 ⁇ 3 Torr. Other arrangements of vacuum pumps and apertures are possible in making the transition from atmospheric pressure at the ion source to high (or ultrahigh) vacuum at the charged particle detector.
- FIG. 1 A series of experiments was performed utilizing the apparatus described in the first preferred embodiment.
- the configuration of the various components is shown in FIG. 1.
- the vacuum regions 15,25,35 were operated under conventional conditions as described above.
- the potentials applied to the lens stack 60 were within the ranges recommended by the manufacturer of the ICP/MS (Fisons).
- the first and second apertures 20,30 were both grounded.
- the differential aperture 40 was biased at a DC potential of about -120 V.
- the potentials on the lens elements 70,80 were optimized for maximum transfer efficiency of ions into the quadrupole ion trap and were different than the potentials used in conventional ICP/MS instruments. Ions are gated into the quadrupole ion trap by switching the potential on lens element 80 in the lens stack 60.
- lens element 80 described as lens element "L3" by the manufacturer (Fisons), were switched between a negative value used to admit ions into the quadrupole ion trap, in the range between about -10 V to about -500 V, preferably -35 V, and a positive value used to prevent ions from entering the quadrupole ion trap, in the range between about +10 V to about +500 V, preferably above +10 V, or the kinetic energy of the ions.
- the electronic gating control (not shown) used for switching the voltage on lens element 80 was provided by inverting the standard signal provided by the Finnigan MAT ITMS to gate electrons. This inversion was accomplished using an extra inverter (not shown) on the printed circuit board (not shown) that performs the gating.
- the quadrupole ion trap is manufactured with a port 90 typically used for introduction of a buffer gas such as helium.
- a buffer gas such as helium.
- Reagent gases were introduced into the quadrupole ion trap by adding the reagent gases to the helium.
- Typical helium buffer gas pressures were in the range between about 10 "5 and 10 ⁇ 3 Torr. Reagent to buffer gas pressure ratios ranged between about 0.01% to 100%. Experiments were performed in this instrument wherein Ar, H 2 , Xe, or Kr were introduced as reagent gases into the quadrupole ion trap.
- FIG. 2 Representative mass spectra showing the effects of added H 2 are shown in FIG. 2.
- the upper trace 100 in FIG. 2 was obtained using pure helium buffer gas and is offset from zero for the sake of clarity in FIG. 2.
- the lower trace 110 in FIG. 2 was obtained using about 5% H 2 and about 95% helium.
- the upper trace 100 shows the intensity of various peaks, most notably, H 2 0 + at m/z 18 102, H 3 O + at m/z 19 104, Ar + at m/z 40 106, ArH + at m/z 41 108.
- Fig. 2a shows the lower trace 110 of Fig. 2 at a finer vertical scale, i.e., "blown up".
- Ar + , ArH + , and H 2 O + several other ions commonly observed in ICP/MS spectra are notably absent in the lower trace 110 shown in Fig. 2a. These other ions include ArO + and Ar 2 + , among others.
- Atomic cations of the following elements have been tested for reaction with H in the apparati of the preferred embodiments described herein using argon as carrier gas: N, O, Na, Mg, Al, Si, K, Ar, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Kr, Rb, Sr, Y, Zr, Mo, Rh, Ag, Cd, In, Sb, I, Xe, Cs, Ba, TI, Pb, Bi, Th, and U.
- a conventional ICP/MS manufactured by VG Elemental, now Fisons (Winsford, Cheshire, England; model PQ-I) was modified by interposing a scaled mass analyzer 210, e.g. a scaled quadrupole ion trap between the linear quadrupole 200 and the charged particle detector 50.
- the electrodes (not shown) used in the scaled quadrupole ion trap were custom built to be scaled versions of the ITMSTM electrodes manufactured by Finnigan MAT (San Jose, California), to increase the ion storage capacity of the ion trap and to reduce the mass range of the standard instrument (10-650 AMU) by about a factor of 2.
- This scaled ion trap size is independent of the use of a reagent gas according to the present invention.
- the electrodes of the custom built quadrupole ion trap were 44% larger than the electrodes of the Finnigan MAT ITMS and were assembled in a pure quadrupole, or un-stretched geometry.
- the standard ITMSTM electronics package (not shown) manufactured by Finnigan MAT was used with the modifications as described in the first preferred embodiment using the voltages as described below.
- the lens stack 60 is operated at potentials recommended by the manufacturer.
- a second lens stack 250 is interposed between the quadrupole exit aperture 220 and the scaled quadrupole ion trap (scaled mass analyzer 210) in the fourth vacuum region 230.
- the second lens stack 250 consisted of three lens elements 252,254,256 taken from standard Fisons lens stacks, specifically two "L3" lens elements and an "L4" lens element.
- the second lens stack 250 was fabricated to provide high ion transport efficiency between the linear quadrupole 200 and the scaled quadrupole ion trap.
- a potential of between about -10 V and about -300 V, preferably about -30 V were applied to lens elements 252,256 at each end of the second lens stack 250.
- the center lens element 254 was used to gate ions into the scaled quadrupole ion trap and the potential applied was varied between about -180 V for the open potential and about +180 volts for the closed potential.
- the electronic gating control (not shown) used for the center lens element 254 of the second lens stack 250 was provided by inverting the standard signal provided by the Finnigan MAT ITMSTM to gate electrons into the quadrupole ion trap. This inversion was accomplished using an extra inverter (not shown) on the printed circuit board (not shown) that performs the gating.
- the vacuum system was the standard Fisons system consisting of four vacuum regions separated by three apertures with an additional pump on the fourth vacuum region 230. These vacuum regions are evacuated by standard vacuum pumps (not shown).
- the first vacuum region 15 is contained in between the first aperture 20 and second aperture 30 and is typically operated at 0.1 to 10 Torr.
- the second vacuum region 25 is contained between the second aperture 30 and a differential aperture 40 and is typically operated at 10 "5 to 10 "3 Torr.
- the differential aperture 40 is located within the lens stack 60.
- the third vacuum region 215 is contained between the differential aperture 40 and the quadrupole exit aperture 220 and is typically operated at 10 "8 to 10 "4 Torr.
- the third vacuum region 215 contains the linear quadrupole 200.
- the fourth vacuum region 230 is separated from the third vacuum region 215 by the quadrupole exit aperture 220.
- the fourth vacuum region 230 contains the scaled quadrupole ion trap and a charged particle detector 50.
- the fourth vacuum region 230 is typically operated at 10- 8 to 10- 3 Torr.
- a tube 260 was provided to allow the introduction of reagent gases into the second vacuum region 25 through two ports 280 provided in the housing 270 surrounding the first vacuum region 15.
- the tube 260 was fashioned into a shape so as to avoid electrical contact with the lens stack 60 and to position the end of the tube 260 approximately 1 cm behind the base of the second aperture 30 and approximately 1 cm from the central axis defined by the four apertures 20,30,40,220. In this way, reagent gases are introduced into the second vacuum region 25 as close to the second aperture 30 as possible without interfering with the gas dynamics of the sampled plasma and with minimal distortion of the electric field generated by the lens stack 60.
- the values in the second column under the heading "H 2 " show that as the H 2 pressure is increased, the Ar + ion intensity falls about 10 times faster than the ln + ion intensity, confirming the selective removal of carrier gas ions.
- the effect of H pressure on the analyte and other ion signals were observed by recording the mass spectrum in both the analog and pulse counting modes of operation of the ICP/MS as provided by the manufacturer.
- Two mass spectra recorded without addition of H 2 into the second vacuum region 25 are shown in FIG. 5.
- the upper trace 500 in FIG. 5 was obtained using the analog mode of operation.
- the lower trace 510 in FIG. 5 was obtained using the pulse counting mode of operation.
- H 2 The most dramatic effect of added H 2 is an approximately 200-fold increase in the intensity of the H 3 + peak 618. Addition of H 2 also causes an approximately 10- fold decrease in the intensity of the Ar + peak 612 and an approximately 2-fold increase in the intensity of the ArH + peak 614. These mass spectra show minimal reduction (less than 10%) in the intensity of the peaks for other analytes (not shown). These mass spectra thus show a selective removal of Ar + and an increase in H 3 + thereby confirming the mechanism of charge transfer in the reaction of H 2 with Ar + .
- H 2 was introduced as a reagent gas into the second vacuum region 25 via the vacuum port 400 provided by the manufacturer for pressure measurements. H 2 pressures ranged from about 0.1 mTorr to about 1 mTorr. The measured Ar + intensity was reduced by a factor of two with the introduction of the H 2 reagent gas, demonstrating that introduction of H 2 into the second vacuum region 25 of an unmodified ICP/MS can be used to reduce the Ar + ion intensity.
- Table II contains selected data from the experiments performed using the apparatus of the first, second, and third preferred embodiments described herein. Each row of the table gives reduction factors for Ar + and an analyte ion as well as the ratio of these reduction factors. The ratio is the selectivity with which the Ar + intensity in the mass spectrum is reduced relative to the intensity of the analyte ion.
- the entries in the first column in Table II list the preferred embodiment used to obtain the data given in each row.
- the second column in Table II lists the reagent gas used.
- the reagent gas was introduced into the quadrupole ion trap for the results shown in Table II for the first preferred embodiment above.
- the reagent gas was introduced in vacuum region 25 for the results shown in Table II for the second and third embodiments.
- the third row in Table II shows that the reaction of the carrier gas ion (Ar + ) leads to a 30-fold reduction in Ar + intensity under conditions that reduce the intensity of Sc + by a factor of two.
- the apparatus described in the first preferred embodiment was further modified by incorporating a collision cell 710.
- the collision cell 710 is formed by enclosing an octopole 720 with a metal tube 730, an entrance aperture 740, and an exit aperture 770.
- the lens stack 60 of the first preferred embodiment was disassembled so that the collision cell 710 could be inserted.
- the differential aperture 40 used in previous preferred embodiments was replaced with a entrance aperture 740 with a hole diameter is in the range 1/16" to 1/2", preferably 1/4".
- the collision cell 710 was inserted between the entrance aperture 740 and the exit aperture 770.
- the lens element 80 served as a gating electrode to control injection of ions into the trap, although other elements in the ion optics could be used for gating, for example, entrance aperture 740 or exit aperture 770.
- Installation of the new collision cell 710 and lens stack 750 was accommodated by mounting the lens stack 750 closer to the second aperture 30 (skimmer cone) than in previous embodiments and by mounting the quadrupole ion trap farther from the second aperture 30.
- the octopole 720 consisted of 2.0 mm diameter S.S.
- rods 11.8 cm long, mounted on an inscribed radius of 2.7 mm, although octopoles of other dimensions could be used as well as other RF multipoles, e.g., quadrupoles, hexapoies, dodecapoles, etc.
- Buffer and/or reagent gases are admitted into the quadrupole ion trap through a port 90. Buffer and/or reagent gases are admitted into the collision cell through a port 760.
- a series of experiments was performed utilizing an argon carrier gas and using H 2 as reagent gas introduced into the collision cell 710. Mass spectra were obtained for H 2 pressures in the third vacuum region 35 between zero and about 10 "3 Torr and are summarized below.
- FIG. 8 The mass spectra recorded with and without the addition of H 2 into the collision cell 710 are shown in FIG. 8.
- the upper trace 800 in FIG. 8 was obtained without H 2 introduced into the collision cell 710.
- the lower trace 810 in FIG. 8 was obtained with H 2 introduced into the collision cell 710.
- the upper trace 800 shows the intensity of various peaks, most notably, H 2 O + at m/z 18 802, H 3 O + at m/z 19 804, Ar + at m/z 40 806, and ArH + at m/z 41 808.
- Instrument LODs measured using the apparatus of the fourth preferred embodiment are in the range 0.3 pg/mL to 10 pg/mL. Method LODs are approximately ten times greater than this, i.e., 2 pg/mL to 50 pg/mL.
- the collision cell 920 is contained within a first vacuum region 930.
- the collision cell 920 confines ions in a region close to the aperture 910 through which the ions are introduced into the first vacuum region 930. In this manner, ions are directed from the ion source 900 to the collision cell 920 with minimum opportunity for ion dispersion.
- the first vacuum region 930 contains the collision cell 920 that is made to contain the optimal pressure of reagent gas which allows both ion transport through the collision cell 920 and sufficient charge transfer between the carrier gas ions and the reagent gas.
- the collision cell 920 also can be made to control the kinetic energy of the ions.
- the collision cell 920 can be used to increase the residence time the carrier gas ions are in contact with the reagent gas and thus to increase the extent of charge transfer.
- the collision cell 920 can be made to discriminate against, i.e., not transmit, slow ions by application of velocity or kinetic energy discriminating methods, such as the application of suitable DC electric fields. In this manner, charge exchange between fast carrier gas ions and slow reagent gas neutrals can be used to remove selected carrier gas ions from the ion beam.
- the kinetic energy of the ions in the collision cell 920 is maintained as high as possible so as to minimize space charge expansion of the ions, but low enough for a given pressure of reagent gas to allow sufficient charge transfer.
- the optimal pressure of the reagent gas will be limited by acceptable analyte ion scattering losses in the cell and practical considerations such as pumping requirements.
- the fifth preferred embodiment may be operated using argon as the carrier gas.
- the collision cell 920 may be provided as any apparatus suitable for confining the ions in the first vacuum region 930, including but not limited to, a quadrupole ion trap, a long flight tube, a lens stack or an RF multipole ion guide.
- the collision cell 920 may be operated to selectively disperse reagent gas ions from the ion beam and simultaneously contain and guide analyte ions.
- a reagent gas having a low mass such as H 2
- the RF multipole ion guide may be operated with a low mass cut-off greater than m/z 3. In this manner, H 2 + and H 3 + , which are formed as charge transfer products, are selectively dispersed from the ion beam by virtue of their low m/z.
- the resultant ion beam may then be utilized as one of any number of end uses including but not limited to an ion gun or an ion implanter. Further, the resultant beam may be analyzed in various apparatus including but not limited to an optical spectrometer, mass spectrometers (MS), including linear quadrupole MS, quadrupole ion trap MS, ion cyclotron resonance MS, time-of-flight MS, and magnetic and/or electric sector MS, and quadrupole ion trap time-of-flight MS.
- MS mass spectrometers
- the resultant ion beam may be directed through any electrical or magnetic ion focusing or ion directing apparatus, including but not limited to, a lens stack, a wire ion guide, an RF multipole ion guide, an electrostatic sector, or a magnetic sector.
- the resultant ion beam thus has an increased proportion of analyte ions compared to carrier gas ions.
- the resultant ion beam is directed through an aperture for which the original ion beam current (i.e., the ion beam current prior to selective charge transfer and separation of the charged reagent gas) would meet or exceed the space charge current limit for that aperture
- the increased proportion of analyte ions compared to carrier gas ions directed into the aperture will create an increase in the rate at which the analyte ions pass through the aperture.
- Such an increase in analyte current may also be realized without separating the charged reagent gas if the ion guiding fields that direct ions through the aperture act in a disproportionate fashion with respect to the transmission through the aperture of analyte ions versus other ions in the beam.
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- Analytical Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
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- Engineering & Computer Science (AREA)
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- Other Investigation Or Analysis Of Materials By Electrical Means (AREA)
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Abstract
Description
Claims
Priority Applications (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EP99931805A EP1088334A2 (en) | 1998-06-15 | 1999-06-15 | An apparatus for reduction of selected ion intensities in confined ion beams |
AU48234/99A AU4823499A (en) | 1998-06-15 | 1999-06-15 | An apparatus for reduction of selected ion intensities in confined ion beams |
CA002335108A CA2335108A1 (en) | 1998-06-15 | 1999-06-15 | An apparatus for reduction of selected ion intensities in confined ion beams |
JP2000555278A JP2002518810A (en) | 1998-06-15 | 1999-06-15 | Apparatus for reducing predetermined ion intensity in a limited ion beam |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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US09/097,995 | 1998-06-15 | ||
US09/097,995 US6259091B1 (en) | 1996-01-05 | 1998-06-15 | Apparatus for reduction of selected ion intensities in confined ion beams |
Publications (2)
Publication Number | Publication Date |
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WO1999066536A2 true WO1999066536A2 (en) | 1999-12-23 |
WO1999066536A3 WO1999066536A3 (en) | 2000-02-03 |
Family
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Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
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PCT/US1999/013517 WO1999066536A2 (en) | 1998-06-15 | 1999-06-15 | An apparatus for reduction of selected ion intensities in confined ion beams |
Country Status (6)
Country | Link |
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US (1) | US6259091B1 (en) |
EP (1) | EP1088334A2 (en) |
JP (1) | JP2002518810A (en) |
AU (1) | AU4823499A (en) |
CA (1) | CA2335108A1 (en) |
WO (1) | WO1999066536A2 (en) |
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US9916972B2 (en) | 2013-06-20 | 2018-03-13 | University Of Helsinki | Method and device for ionizing particles of a sample gas flow |
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US9406492B1 (en) | 2015-05-12 | 2016-08-02 | The University Of North Carolina At Chapel Hill | Electrospray ionization interface to high pressure mass spectrometry and related methods |
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JP7094752B2 (en) * | 2018-03-29 | 2022-07-04 | 株式会社ニューフレアテクノロジー | Charged particle beam irradiator |
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GB201904135D0 (en) | 2019-03-26 | 2019-05-08 | Thermo Fisher Scient Bremen Gmbh | Interference suppression in mass spectrometers |
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- 1999-06-15 AU AU48234/99A patent/AU4823499A/en not_active Abandoned
- 1999-06-15 EP EP99931805A patent/EP1088334A2/en not_active Withdrawn
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Also Published As
Publication number | Publication date |
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US6259091B1 (en) | 2001-07-10 |
EP1088334A2 (en) | 2001-04-04 |
CA2335108A1 (en) | 1999-12-23 |
AU4823499A (en) | 2000-01-05 |
JP2002518810A (en) | 2002-06-25 |
WO1999066536A3 (en) | 2000-02-03 |
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