Classifications

• • 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
  • H01J49/0063—Combinations of spectrometers, tandem spectrometers, e.g. MS/MS, MSn characterised by the fragmentation or other specific reaction by applying a resonant excitation voltage
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
  • G01N27/622—Ion mobility spectrometry
  • G01N27/623—Ion mobility spectrometry combined with mass spectrometry
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/26—Mass spectrometers or separator tubes
  • H01J49/34—Dynamic spectrometers
  • H01J49/42—Stability-of-path spectrometers, e.g. monopole, quadrupole, multipole, farvitrons
  • H01J49/4205—Device types
  • H01J49/4245—Electrostatic ion traps
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/26—Mass spectrometers or separator tubes
  • H01J49/34—Dynamic spectrometers
  • H01J49/42—Stability-of-path spectrometers, e.g. monopole, quadrupole, multipole, farvitrons
  • H01J49/426—Methods for controlling ions
  • H01J49/427—Ejection and selection methods
  • H01J49/429—Scanning an electric parameter, e.g. voltage amplitude or frequency

Definitions

• Dynamic Traps such as for example the quadrupole ion traps (QIT) of Paul's design
• static traps such as the more recently developed electrostatic confinement traps.
• Electrostatic traps that are presently available, and used for mass spectrometry generally rely on harmonic potential trapping wells to trap ions into ion-energy- independent oscillations within the trap, with oscillation periods related only to the mass-to-charge ratio of the ions.
• Mass analysis in some modern electrostatic traps has been performed through the use of remote, inductive pick up and sensing electronics and Fast Fourier Transform (FFT) spectral deconvolution in Fourier transform mass spectrometry (FTMS).
• FFT Fast Fourier Transform
• Quadrupole radial confinement fields are used to constrain ion trajectories in a radial direction while electrostatic potentials wells are used to confine ions in the axial direction into substantially harmonic oscillatory motions. Resonant excitation of the ion motion in the axial direction is then used to effect mass-selective ion ejection.
• the PCT/US2007/023834 application by Ermakov et al. discloses an electrostatic ion trap that confines ions of different mass-to-charge ratios and kinetic energies within an anharmonic potential well. The ion trap is also provided with a small amplitude AC drive that excites confined ions.
• This ion trap was entirely cylindrically symmetric, with on-axis ionization of gas molecules and atoms by impact with electrons transmitted from a hot filament into the trap, AC excitation of the ions by application of a small amplitude RF potential to one of the cup electrodes, and detection of the mass-selectively ejected ions by an on-axis electron multiplier device.
• This design produced good quality spectra of high vacuum environments (pressures lower than 10 "6 Torr), but produced noisy spectra with substantial baseline offsets and loss of spectral resolution in higher pressure (10 ⁇ 4 -10 ⁇ 5 Torr) environments.
• an ion trap in some embodiments, includes an electrode structure, including a first and a second opposed mirror electrodes and a central lens therebetween, that produces an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, the confining potential being anharmonic.
• the ion trap also includes an AC excitation source having an excitation frequency /that excites confined ions at a frequency related to the natural oscillation frequency of the ions, the AC excitation frequency source preferably being connected to the central lens.
• the ion trap includes a scan control that controls the relationship between the AC excitation frequency and the natural oscillation frequency of the ions.
• the scan control can sweep the AC excitation frequency/at a sweep rate in a direction from an excitation frequency higher than twice the natural oscillation frequency of the ions to achieve autoresonance.
• the scan control can sweep the AC excitation frequency/at a sweep rate in a direction from an excitation frequency lower than twice the natural oscillation frequency of the ions.
• the scan control can sweep the AC excitation frequency/at a nonlinear sweep rate.
• the nonlinear sweep can be composed of concatenated linear sweeps.
• the sweep rate can be set such that d(l//")/dt is about equal to a constant and n is greater than zero. In one embodiment, n is approximately equal to 1. In another embodiment, n is approximately equal to 2.
• the first opposed mirror electrode of the electrode structure includes a first plate-shaped electrode with at least one aperture, located off-axis with respect to an axis of the opposed mirror electrode structure, and a second electrode shaped in the form of a cup, open towards the central lens, with a centrally located aperture.
• the second opposed mirror electrode of the electrode structure includes a first plate-shaped electrode with an axially located aperture and a second electrode shaped in the form of a cup, open towards the central lens, with a centrally located aperture.
• the central lens is plate-shaped and includes an axially located aperture.
• the ion trap can be configured as a mass spectrometer by further including an ion detector.
• the ion detector can be located on-axis relative to the electrode structure. In other embodiments, the ion detector can be located off-axis relative to the electrode structure.
• the ion detector can be an electron multiplier device. In other embodiments, the ion detector can detect ions by measuring the amount of RF power absorbed from the AC excitation source as the AC excitation source frequency varies. In yet other embodiments, the ion detector can detect ions by measuring the change in electrical impedance of the electrode structure as the AC excitation frequency varies. In still other embodiments, the ion detector can detect ions by measuring the current induced by image charges as the AC excitation frequency varies.
• the ion detector can detect ions by measuring the amount of RF power absorbed from the AC excitation source as the magnitude of the electrostatic potential varies. In yet other embodiments, the ion detector can detect ions by measuring the change in electrical impedance of the electrode structure as the magnitude of the electrostatic potential varies. In still other embodiments, the ion detector can detect ions by measuring the current induced by image charges as the magnitude of the electrostatic potential varies.
• an ion trap comprises an electrode structure, including first and second opposed mirror electrodes and a central lens therebetween, that produces an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, the confining potential being anharmonic, and a scan control that mass selectively reduces a frequency difference between an AC excitation frequency connected to the electrode structure and a multiple of the natural oscillation frequency of the ions from an excitation frequency lower than the multiple of the natural oscillation frequency of the ions.
• the scan control reduces the frequency difference by sweeping the AC excitation frequency at a sweep rate.
• the sweep rate can be a nonlinear sweep rate.
• the nonlinear sweep can be composed of concatenated linear sweeps.
• an ion trap includes an electron source that creates ions by electron impact ionization of a gaseous species, and a collector electrode that collects the ions to form a total pressure reading.
• the ion trap further includes an electrode structure that produces an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, and a mass analyzer that analyzes the gaseous species.
• the electrode structure includes a central lens. The electrostatic potential can be anharmonic.
• the ion trap can further include an AC excitation source having an excitation frequency that excites confined ions at a frequency of about twice the natural oscillation frequency of the ions, the AC excitation source being connected to the central lens.
• the electron source can be located off-axis relative to the electrode structure.
• the collector electrode is plate-shaped and includes an axially located aperture in line with the electrode structure.
• the collector electrode surrounds the electron source.
• the collector electrode is located inside the electrode structure.
• an ion trap in another embodiment, includes an electron source that creates ions by electron impact ionization of a gaseous species, and an electrode structure, including a central lens, that produces an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies.
• the ion trap further includes a gauge that measures total pressure, and a partial pressure analyzer that relates the densities of confined ions having specific natural oscillation frequencies to the total pressure.
• the electrostatic potential produced by the electrode structure can be anharmonic.
• the ion trap can include an AC excitation source having an excitation frequency that excites confined ions at a frequency of about twice the natural oscillation frequency of the ions, the AC excitation source being connected to the central lens.
• the total pressure gauge can include an ionization gauge.
• an ion trap includes an electrode structure, including first and second opposed mirror electrodes and a central lens therebetween, that produces an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, the confining potential being anharmonic, and an AC excitation source having an excitation frequency that excites confined ions at a frequency of about two times the natural oscillation frequency of the ions, the AC excitation source being connected to the central lens.
• the ion trap further includes a bias controller that biases one of the first and the second opposed mirror electrode structures sufficiently unequally such that substantially all the ions escape the trap and are collected by an ion detector to form a total pressure reading.
• an ion trap in still another embodiment, includes an electrode structure that produces an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, the confining potential being anharmonic, and an AC excitation source, connected to the electrode structure, having an excitation frequency that excites confined ions at a frequency that is about an integer multiple of natural oscillation frequency of the ions.
• the ion trap further includes nonvolatile memory storing control parameters, and control electronics operatively connected to the AC excitation source and to the electrode structure, the control electronics controlling the AC excitation source and the electrostatic potential using the control parameters. Control parameters can include configuration and calibration parameters and sensitivity factors.
• the nonvolatile memory and control electronics can be integrated with the electrode structure.
• Configuration parameters can include magnitudes of electrostatic potentials applied on the electrode structure that produce the electrostatic potential in which ions are confined, and amplitude and frequency settings for the AC excitation source, calibration parameters can include voltage and current input and output calibration parameters of the ion trap, and sensitivity factors can include a conversion factor from natural frequency of oscillation of ions to ion mass-to-charge (m/q) ratio.
• the excitation frequency excites confined ions at a frequency of about twice the natural oscillation frequency of the ions.
• an ion trap in yet another embodiment, includes an electrode structure that produces an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, the confining potential being anharmonic, and an AC excitation source, connected to the electrode structure, having an excitation frequency that excites confined ions at a frequency that is about an integer multiple of natural oscillation frequency of the ions.
• the ion trap further includes a scan control that sweeps the AC excitation frequency/ at a sweep rate that is set such that d(l//")/dt is equal to a constant and n is greater than zero.
• n is approximately equal to 1.
• n is approximately equal to 2.
• a method of trapping ions in an ion trap includes producing an anharmonic electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, in an electrode structure that includes first and second opposed mirror electrodes and a central lens therebetween, and exciting confined ions at a frequency of about twice the natural oscillation frequency of the ions with an AC excitation source having an excitation frequency/, the AC excitation source being connected to the central lens.
• the method includes the step of scanning the excitation frequency of the AC excitation source and mass selectively reducing a frequency difference between the AC excitation frequency and about twice the natural oscillation frequency of the ions.
• the step of scanning the excitation frequency can be performed at a sweep rate from an excitation frequency higher than about twice the natural oscillation frequency of the ions, to mass selectively achieve autoresonance as the frequency difference approaches zero.
• the step of scanning the excitation frequency can be performed at a sweep rate from an excitation frequency lower than about twice the natural oscillation frequency of the ions.
• the sweep rate can be a nonlinear sweep rate.
• the nonlinear sweep can be composed of concatenated linear sweeps.
• the sweep rate can be set such that d(l//")/dt is about equal to a constant and n is greater than zero. In one embodiment, n is approximately equal to 1. In another embodiment, n is approximately equal to 2.
• scanning the excitation frequency can include the step of sweeping a magnitude V of the electrostatic potential at a sweep rate in a direction such that twice the natural oscillation frequency of the ions changes from a frequency lower than the frequency of the AC excitation source.
• scanning the excitation frequency can include the step of sweeping a magnitude V of the electrostatic potential at a sweep rate in a direction such that twice the natural oscillation frequency of the ions changes from a frequency higher than the frequency of the AC excitation source.
• the sweep rate can be nonlinear.
• the first opposed mirror electrode structure and the second opposed mirror electrode structure can be biased unequally.
• the first and the second opposed mirror electrode structures each includes a first plate-shaped electrode with an axially located aperture and a second electrode shaped in the form of a cup, open towards the central lens, with a centrally located aperture, and the central lens is plate-shaped and includes an axially located aperture.
• the first opposed mirror electrode structure includes a first plate-shaped electrode with at least one aperture, located off-axis with respect to an axis of the opposed mirror electrode structure, and a second electrode shaped in the form of a cup, open towards the central lens, with a centrally located aperture.
• the second opposed mirror electrode structure includes a first plate-shaped electrode with an axially located aperture and a second electrode shaped in the form of a cup, open towards the central lens, with a centrally located aperture.
• the central lens is plate-shaped and includes an axially located aperture.
• the method can include using an ion source.
• the method can include using an ion detector, configured as a mass spectrometer.
• the ion source can include at least one electron emissive source that creates ions by electron impact ionization of a gaseous species.
• the ion source can include two electron emissive sources.
• the at least one electron emissive source can be a hot filament.
• the at least one electron emissive source can be a cold electron emissive source.
• the at least one electron emissive source can be located off-axis relative to the electrode structure. In these embodiments, electrons generated by the at least one electron emissive source can be injected at an angle of between about 20 degrees and about 30 degrees away from an axis normal to an axis along the electrode structure.
• the ion detector can be located on-axis relative to the electrode structure. In other embodiments, the ion detector can be located off-axis relative to the electrode structure.
• the ion detector can be an electron multiplier device. In other embodiments, the ion detector can detect ions by measuring the amount of RF power absorbed from the AC excitation source as the AC excitation source frequency varies. In yet other embodiments, the ion detector can detect ions by measuring the change in electrical impedance of the electrode structure as the AC excitation frequency varies. In still other embodiments, the ion detector can detect ions by measuring the current induced by image charges as the AC excitation frequency varies.
• the ion detector can detect ions by measuring the amount of RF power absorbed from the AC excitation source as the magnitude of the electrostatic potential varies. In yet other embodiments, the ion detector can detect ions by measuring the change in electrical impedance of the electrode structure as the magnitude of the electrostatic potential varies. In still other embodiments, the ion detector can detect ions by measuring the current induced by image charges as the magnitude of the electrostatic potential varies.
• a method of trapping ions in an ion trap includes producing an anharmonic electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, in an electrode structure that includes first and second opposed mirror electrodes and a central lens therebetween.
• the method further includes scanning an excitation frequency of an AC excitation source connected to the electrode structure and mass selectively reducing a frequency difference between the AC excitation frequency and a multiple of the natural oscillation frequency of the ions from an excitation frequency lower than the multiple of the natural oscillation frequency of the ions.
• the scan control reduces the frequency difference by sweeping the AC excitation frequency at a sweep rate.
• the scan control reduces the frequency difference by sweeping a magnitude V of the electrostatic potential at a sweep rate in a direction such that the multiple of the natural oscillation frequency of the ions changes from a frequency higher than the frequency of the AC excitation source.
• the sweep rate can be a nonlinear sweep rate.
• a method of trapping ions in an ion trap includes creating ions with an electron source by electron impact ionization of a gaseous species, collecting ions with a collector electrode to form a total pressure reading, producing an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies using an electrode structure, and providing a mass analyzer for analyzing the gaseous species.
• the electrostatic potential can be anharmonic.
• the method can further include providing an AC excitation source having an excitation frequency that excites confined ions at a frequency of about twice the natural oscillation frequency of the ions, the AC excitation source being connected to the central lens.
• the electrode structure can include a central lens.
• the electron source can be located off-axis relative to the electrode structure.
• the collector electrode is plate-shaped and includes an axially located aperture in line with the electrode structure.
• the collector electrode surrounds the electron source.
• the collector electrode is located inside the electrode structure.
• a method of measuring absolute partial pressure using an ion trap includes creating ions with an electron source by electron impact ionization of a gaseous species, producing an electrostatic potential with an electrode structure, including a central lens, in which ions are confined to trajectories at natural oscillation frequencies, measuring total pressure with a total pressure gauge, and relating the densities of confined ions having specific natural oscillation frequencies to the total pressure with a partial pressure analyzer.
• the electrostatic potential produced by the electrode structure can be anharmonic.
• the ion trap can include an AC excitation source having an excitation frequency that excites confined ions at a frequency of about twice the natural oscillation frequency of the ions, the AC excitation source being connected to the central lens.
• the total pressure gauge can include an ionization gauge.
• a method of measuring total pressure with an ion trap includes producing an anharmonic electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, in an electrode structure that includes a first and a second opposed mirror electrodes and a central lens therebetween, exciting confined ions at a frequency of about twice the natural oscillation frequency of the ions with an AC excitation source having an excitation frequency, the AC excitation source being connected to the central lens, and biasing one of the first and the second opposed mirror electrode structures with a bias controller sufficiently unequally such that substantially all the ions escape the trap and are collected by an ion detector.
• a method of trapping ions in an ion trap includes producing an anharmonic electrostatic potential in an electrode structure in which ions are confined to trajectories at natural oscillation frequencies, exciting confined ions at a frequency of about an integer multiple of the natural oscillation frequency of the ions with an AC excitation source connected to the electrode structure and having an excitation frequency, and storing configuration and calibration parameters and sensitivity factors in nonvolatile memory.
• a method of trapping ions in an ion trap includes producing an anharmonic electrostatic potential in an electrode structure in which ions are confined to trajectories at natural oscillation frequencies, exciting confined ions at a frequency of about an integer multiple of the natural oscillation frequency of the ions with an AC excitation source connected to the electrode structure and having an excitation frequency, and scanning the excitation frequency f at a sweep rate that is set such that d(l//")/dt is about equal to a constant and n is greater than zero.
• n is approximately equal to 1.
• n is approximately equal to 2.
• This invention has many advantages, such as improved quality of mass spectra at higher pressure ( 10 4 - 10 5 Torr) and reduced baseline in mass spectra throughout the operational pressure range of the ion trap (10 ⁇ 4 Torr to 10 ⁇ 10 Torr).
• FIG. 1 is a schematic diagram of an electrostatic ion trap.
• FIG. 2A is a drawing of an anharmonic potential well.
• FIG. 2B is a drawing of a portion of the anharmonic potential well of FIG. 2A at an exit plate voltage of 0 VDC and an exit plate voltage of -15 VDC.
• FIG. 2C is a drawing of a harmonic potential well and an anharmonic potential well.
• FIG. 3 is a drawing of mass spectra obtained by employing autoresonance and obtained by scanning the AC excitation frequency in the reverse direction.
• FIG. 4 is a drawing of AC excitation frequency as a function of sweep time for linear, log, XIf and Xlf frequency scans.
• FIG. 5 A is a drawing of a mass spectrum obtained by scanning the high voltage (HV) on the central lens at a fixed RF frequency of 389 kHz.
• FIG. 5B is a drawing of mass spectra obtained by employing on-axis ionization and scanning the RF frequency autoresonantly, and in the reverse direction.
• FIG. 5 C is a drawing of mass spectra obtained by employing off-axis ionization and scanning the RF frequency autoresonantly, and in the reverse direction.
• FIG. 6A is a schematic diagram of part of an electrostatic ion trap employing on-axis ionization and a drawing of the electron density as a function of distance.
• FIG. 7 is a drawing of an electrostatic ion trap with two electron emissive sources.
• FIG. 8 is a schematic diagram of an electrostatic ion trap employing a cold electron emissive source.
• FIG. 9 is a schematic diagram of an electrostatic ion trap employing on-axis ion detection.
• FIG. 10 is a schematic diagram of an electrostatic ion trap employing off- axis ion detection.
• FIG. 11 is a schematic diagram of an electrostatic ion trap used to detect ions by measuring the amount of RF power absorbed from the AC excitation source.
• FIG. 12 is a drawing of a mass spectrum obtained with the ion trap of FIG. 11 and using a fixed central lens voltage of -400 VDC.
• the ejection frequency for water was 654 kHz and the ejection frequency for argon was 437 kHz. Ions were detected by an electron multiplier detector.
• FIG. 13 is a drawing of a mass spectrum obtained with the ion trap of FIG. 11 by scanning the magnitude of the electrostatic potential, with a fixed RF frequency of 540 kHz. Water was ejected at -270 VDC, and argon was ejected at - 600 VDC. Ions were detected by an electron multiplier detector.
• FIG. 14 is a schematic diagram of the ion trap of FIG. 11, with RF coupled into the exit cup 7, and configured to mass selectively detect ions by measuring the amount of RF power absorbed as the magnitude of the electrostatic potential varies.
• FIG. 15 is a schematic diagram of the equivalent electric circuit for the electrostatic ion trap and circuit of FIG. 14.
• FIG. 16 is a drawing of a mass spectrum obtained by measuring the change in amplitude of the RF signal from a weakly driven oscillator source, as the magnitude of the electrostatic potential varies.
• FIG. 17 is a drawing of potential energy wells at two points during a scan of the magnitude of the electrostatic potential, indicating the energy of nitrogen ions oscillating at 445 kHz at -200 VDC and -275 VDC transition plate voltage.
• FIG. 18 is a drawing of electron energy as a function of distance in an electrostatic ion trap.
• FIG. 19 is a drawing of mass spectra obtained by using 50 eV, 60 eV, and 70 eV electrons to create ions and measuring the change in electrical impedance of the electrode structure as the magnitude of the electrostatic potential varies.
• FIG. 20 is a schematic diagram of an electrostatic ion trap used to detect ions by measuring the change in coupled RF amplitude as the magnitude of the electrostatic potential varies.
• FIG. 21 is a drawing of a mass spectrum obtained by measuring the drop in coupled RF amplitude as the magnitude of the electrostatic potential varies.
• FIG. 22 is a drawing of a mass spectrum with calculated and experimental ejection frequencies.
• FIG. 23 is a drawing of mass spectra at 3.5x10 7 Torr of essentially pure nitrogen and 7.5xlO "7 Torr of a 1 :1 mixture OfN 2 IAr by volume.
• FIG. 24 is a schematic diagram of an ion collector that surrounds an electron emissive source.
• FIG. 25 is a schematic diagram of an ion collector shaped as a ring electrode adjacent to an electron emissive source.
• FIG. 26 is a schematic diagram of an ion collector shaped as a ring electrode located outside the entry plate.
• FIG. 27 is a schematic diagram of an ion collector located inside the electrode structure of the electrostatic ion trap.
• FIG. 28 is a schematic diagram of a combination total pressure measurement and partial pressure measurement apparatus employing an electrostatic ion trap.
• FIG. 29 is a drawing of a graph of the autoresonance ejection threshold with increasing sweep rate.
• FIG. 34 is a derivation of the total pressure reported by an ionization gauge.
• FIG. 37 is a graph of partial pressure of nitrogen and noble gases measured by ART MS and an SRS RGA.
• FIG. 38 is an illustration of an ART MS system.
• FIG. 39 is an illustration of an ART MS standalone configuration where the front panel assembly is the Master.
• An ion trap comprises an electrode structure, including a first and a second opposed mirror electrodes and a central lens therebetween, that produces an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, the confining potential being anharmonic.
• the ion trap also includes an AC excitation source having an excitation frequency/ that excites confined ions at a frequency of about a multiple of the natural oscillation frequency of the ions, the AC excitation frequency source preferably being connected to the central lens.
• an ion trap 110 comprises an electrode structure made up of two plates 1 and 2, two cup-shaped electrodes 6 and 7, and a flat central lens 3.
• the cup-shaped electrodes 6 and 7 are arranged with the wide openings of the cups opposed to one another in an opposed mirror image arrangement, as shown in FIG. 1.
• the protrusion in plate 1 is away from the cup electrode 6.
• the spacing between plate 1 and cup electrode 6 is about 0.175", and the spacing between plate 2 and cup electrode 7 is about 0.25".
• the flat plates 1 and 2 are adjacent to the bottom of the cup electrodes 6 and 7, respectively.
• mass spectrometry or ion- beam sourcing performance is also less sensitive to unit-to-unit variations allowing more relaxed manufacturing requirements for an anharmonic resonant ion trap mass spectrometer (ART MS) compared to most other mass spectrometry technology.
• ART MS anharmonic resonant ion trap mass spectrometer
• the anharmonic potential depicted in the curve of FIG. 2A is clearly presented for reference only, and it will be understood by those skilled in the art that various changes in form and detail can be made to the anharmonic potential without departing from the scope of the present invention.
• the ion trap also includes an AC excitation source having an excitation frequency/ that excites confined ions at a frequency of about a multiple of the natural oscillation frequency of the ions.
• the ion trap is also provided with a scan control 100 shown in FIG. 1, which mass selectively reduces a frequency difference between the AC excitation frequency and twice the natural oscillation frequency of the ions.
• the scan control 100 sweeps the AC excitation frequency/ at a sweep rate in a direction from a frequency higher than twice the natural oscillation frequency of the ions towards a frequency lower than twice the natural oscillation frequency of the ions to achieve autoresonance.
• autoresonant excitation of a group of ions of given mass-to-charge ratio, m/q is achieved in the following fashion:
• An AC drive is connected to the system with an initial drive frequency, fa, above the natural oscillation frequency of the ions- fa > fM, or, alternatively, above a multiple, such as, for example, double the natural oscillation frequency of the ions- fa > 2fM.
• the autoresonant excitation process described above can be used to 1) excite ions causing them to undergo new chemical and physical processes while stored, and/or 2) eject ions from the trap in a mass selective fashion.
• Ion ejection can be used to operate pulsed ion sources, as well as to implement full mass spectrometry detection systems, in which case a detection method is required to detect the autoresonance events and/or the ejected ions.
• DDS direct- digital frequency synthesis
• DAC digital-to-analog converter
• scan control 100 shown in FIG. 1, can be used to sweep a magnitude V of the electrostatic potential at a sweep rate in a direction such that twice the natural oscillation frequency of the ions changes from a frequency lower than the frequency of the AC excitation source towards a frequency higher than the frequency of the AC excitation source.
• the bias on transition plate 3 sets the voltage at the bottom of the electrostatic potential well shown in FIG. 2 A.
• the natural frequency of oscillation of the ions is set by the depth of the trapping potential well. Any change in the transition plate bias voltage results in a shift in the natural frequency of oscillation of the ions.
• the roundtrip time for ions of a fixed mass-to-charge ratio is related to the square root of the trapping potential shown in FIG. 2A.
• the first and the second opposed mirror electrode structures each includes a first plate-shaped electrode (entry plate) 1, with an axially located aperture and a second electrode 6 shaped in the form of a cup, open towards the central lens, with a centrally located aperture, and the central lens is plate-shaped and includes an axially located aperture.
• the electron density produced by electron emissive source 16 is largest close to entry plate 1 , where the electron energy is also larger than further away from entry plate 1. Therefore, most of the ions are created close to the entry plate 1 by impact with relatively high energy electrons, and the high energy ions are more likely to escape the electrode structure before being trapped, resulting in a large baseline offset in the spectra.
• the first plate-shaped electrode (entry plate) 1 includes at least one aperture located off-axis with respect to an axis of the opposed mirror electrode structure.
• the electron density produced by electron emissive source 16 is localized deeper inside the trap, and away from entry plate 1, and therefore the ions created by impact with the electrons have lower energy and an increased probability of being trapped.
• Off-axis ionization also eliminates the generation of ESD ions in direct line-of-sight with the exit plate 2 of the trap as described in connection with on-axis ionization (i.e., FIG. 6A).
• ESD has a much diminished contribution to the baseline offset signal in the off-axis ionization scheme illustrated in FIG. 6B.
• Ions formed close to the entry plate 1 have sufficient energy to escape the trap without being trapped, and therefore those ions produce a baseline offset (increase) in the signal measured by an ion detector, that is independent of ion mass, and therefore is a substantial contribution to detector noise.
• Increased baseline noise compromises the lifetime of the ion detector, due to constant ion bombardment, requires additional signal processing for baseline subtraction, and increases (compromises) the detection limits of the sensor.
• the relative contribution of baseline offset to the output signal of the sensor also appears to increase as a function of the total pressure in the system when on-axis ionization is used.
• ions can be mass-selectively detected by measuring electrical characteristics of the ion trap as the AC excitation source frequency varies, or as the magnitude of the electrostatic potential varies.
• the electrical characteristics include (1) the amount of RF power absorbed from the AC excitation source, (2) the change in electrical impedance of the electrode structure, and/or (3) the currents induced by image charges that develop at the end plates as the ions phase lock with the AC excitation source and oscillate closer to the end plates.
• the electrostatic ion traps described herein do not include tight control of ion energies and/or injection times, so that ions of the same mass-to-charge ratio can have a wide range of oscillation frequencies and phases.
• Mass-selective ion detection can be performed by monitoring changes in the dissipation of RF power into the trap. As the frequency is scanned at fixed HV, or as the HV is scanned at fixed frequency, ions of different masses will come into autoresonance with the RF field and phase lock their oscillations with the RF field oscillations. The energy gained by the ions during autoresonant excitation is extracted from the RF field and it is possible to detect those abrupt changes in RF power consumption using dedicated circuits. For example, the RF power absorption can be detected and quantified with the help of "weakly-driven-oscillators" (WDO).
• WDO weakly-driven-oscillators
• the ejection frequencies were compared to those calculated for the same two ions (water and argon) using SIMION.
• Table 1 shows that SIMION provided a very accurate measurement of the ejection frequencies for different voltages, showing that a) SIMION can be used to calculate ejection frequencies to within a few percent accuracy, and b) the ejection frequencies correspond to the oscillation frequencies of ions located adjacent to the entry/exit plates.
• Frequency scans were obtained between -1000 VDC and -300 VDC trapping potential.
• FIG. 12 shows a representative spectrum obtained at -400 VDC central lens trapping potential.
• the same trap shown in FIG. 11 was then reconfigured to perform HV scans (i.e., fixed frequency scans) using a HV multiplier circuit based on an EMCO HV module, model CA20N-5 (EMCO, Sutter Creek, CA).
• HV on the central lens was scanned between approximately -200 VDC and -800 VDC while the AC excitation frequency was fixed at 540 kHz. Ion ejection was observed on both sides of the voltage sweep slope: autoresonant and reverse scans - i.e., ions were ejected whether the voltage was scanned upward or downward.
• the RF source was a FAWG, Agilent 33220 set to a frequency of 540 KHz with an amplitude of 120 mVpp.
• FIG. 11 shows the RF injection point for the RF.
• Table 1 Ar ions were expected to be ejected at a potential of approximately -600 VDC for the excitation frequency of 540 kHz.
• FIG. 13 shows the HV scan spectrum including water and Argon peaks, as detected by electron multiplier ion detector 17 (shown in FIG. 11). Argon was ejected at -600 VDC as predicted from Table 1. It is clear from FIGS. 12-13 that both frequency and HV scans can be used to generate mass spectra in this trap.
• FIG. 12-13 shows the HV scan spectrum including water and Argon peaks, as detected by electron multiplier ion detector 17 (shown in FIG. 11).
• FIGS. 12-13 both frequency and HV scans can be used to generate mass spectra in this trap.
• HV scan 13 is an example of a HV sweep in which the HV was scanned in the direction of increasing amplitude- i.e., an autoresonant scan.
• Argon was ejected at -600 VDC whether the voltage was scanned in a direction of amplitude going up or down, that is, HV scans can be used for both autoresonant or reverse scan ejection.
• the LC tank itself had a quality factor, Q ⁇ 100 at resonance, and provided a signal of 27 mV rms to the trap while the function generator output was set to 300 mV pp .
• the output of the WDO was monitored with a SR844 lock- in amplifier (Stanford Research Systems, Sunnyvale, CA).
• ChI (X) and Ch2 (Y) outputs of the lock- in amplifier were configured to display the amplitude (R) and phase ( ⁇ ) of the WDO signal and connected to separate inputs of a digital oscilloscope.
• the resonant tank delivered AC signal to the trap, providing detectable drops in RF amplitude every time energy was absorbed from the RF field (i.e., tank losses).
• the lock- in amplifier monitored the amplitude of the WDO signal using the output of the function generator as the reference signal.
• the trap could be electrically represented as an impedance load connected in parallel to the LC tank, shown as Z trap in FIG. 15.
• Measuring the RF signal in this phase-sensitive fashion provided two alternative ways to detect autoresonant excitation: 1) measuring power absorption from the trap by measuring drops in the amplitude of the WDO output, and 2) measuring the X and Y components of the RF signal, i.e., the change in the electrical impedance (Z trap shown in FIG.
• FIG. 16 shows an RF power absorption spectrum for air at 3.5E-7 Torr.
• Linear HV sweeps were generated with a high power HV amplifier (Trek Inc., Model 623B-L-CE) connected to the sawtooth output of a function generator.
• the AC excitation frequency was 446 kHz
• the scan rate was 30 Hz
• the HV was scanned from -100 VDC to -600 VDC.
• the trace showing an upward peak corresponds to the ion ejection signal, i.e., multiplied ion current detected by the electron multiplier 17 (shown in FIG. 11).
• the amplitude continues to decrease as the ions gain energy and ions of higher energy join them in an "energy bunching" process which is characteristic of autoresonant excitation in anharmonic electrostatic ion traps.
• the RF intensity decrease comes to an end as soon as the ions start leaving the trap and RF absorption can no longer take place. Ejection of nitrogen ions takes place at about -270 VDC (in agreement with
• SIMION was used to calculate the energy of the ions that first produce a detectable absorption signal as they phase-lock with the RF field. As calculated, using SIMION, for a -200 VDC trapping potential, nitrogen ions at the -18V equipotential oscillate with a natural oscillation frequency of 440 kHz, and are the first ones to provide detectable RF absorption levels for this detection scheme (see FIG. 17). It seems that using 70 eV electrons only produces significant (i.e., detectable) RF absorption signal for ions starting at the ⁇ -20 VDC equipotential.
• the electrons need at least 15 eV of KE to ionize nitrogen, no significant ion concentration is expected beyond (i.e., to the right of) the -55 VDC equipotential - i.e., the ionization volume starts at the entry plate grid and ends at the -55 VDC equipotential.
• the electrons have kinetic energies between 70 eV and 15 eV, which are all above the ionization threshold for nitrogen.
• ionization efficiency above threshold scales with electron energy and the density of ions formed by the electron impact ionization is expected to decrease as one moves away from the entry plate grid and into the trap (i.e., moving to the right in FIG. 18). Electrons with at least 50 eV of kinetic energy are needed in this trap to get a critical concentration of phase-locked ions that will provide a detectable drop in RF amplitude with this experimental setup.
• the width of the RF absorption band scales with the electron energy.
• the ionization volume also decreases and the range of energy of ions that can absorb RF power is reduced, causing the slope of the amplitude drop to be steeper as the electron energy is decreased from 70 eV to 50 eV, as shown in FIG. 19.
• the three curves in this plot were normalized in amplitude to highlight the difference in HV ranges. Changing the electron energy has no effect on the ejection voltage, but has a substantial effect on the range of voltage over which RF absorption takes place.
• the phase locked bunch of ions increases in population as the ions go up in energy, which is described as energy bunching.
• the increase in ion population is easily seen in FIG. 19 as a slow rise in RF absorption that starts at a HV amplitude much lower than the ionization energy voltage.
• the ions oscillate back and forth between the end plates with increasing oscillation amplitudes.
• a simple way to detect RF power absorption in a trap is to detect frequency dependent drops in "RF rms amplitude" at electrodes capacitively coupled to the RF source plate.
• RF power absorption inside the trap reduces the RF field intensity inside the trap and in turn diminishes the amplitude of RF capacitively coupled to adjacent electrode structures.
• the drop in RF amplitude can be interpreted as a change in the impedance of the trap that affects the amount of RF coupling into electrode plates adjacent to the RF source plate. Therefore, using the embodiment of the ion trap shown in FIG.
• the RF voltage applied to the entry cup was carefully monitored to make sure no dips in amplitude were observed as energy was absorbed from the FAWG by the phase- Io eked ions: the FAWG kept up with the power demands and no detectable changes in amplitude were observed at the entry plate. Then, the lock- in signal input was connected to the exit plate and a significant change in the amplitude of RF signal was observed while scanning at the frequency corresponding to the autoresonant excitation of ions. The amplitude changes observed at the exit plate are caused by the reduction of RF field intensity (i.e. or impedance changes) inside the trap. The signal is relatively easy to detect and measure, and follows the shape of the absorption signals collected with a WDO, as shown in FIG.
• the RF frequency was 600 kHz and the voltage was 100 mV pp connected into the entry cup.
• the nominal capacitively coupled RF amplitude on the exit plate was 0.5 mV with a +15 degree phase shift relative to the RF source (measured in the absence of gas load).
• the signal transient originated from 3.5xlO "7 Torr level of air, and both N 2 and O 2 signals are evident.
• the drop in RF amplitude at the exit plate is easily detected, and the shape of the curve shown in FIG. 21 is in good agreement with the power absorption curves described above, such as, for example, FIG. 19. In other words, the RF amplitude drops in association with the rate at which power is absorbed from the RF field.
• the decrease in the RF field intensity inside the trap as autoresonance takes place can also lead to interesting effects when a trap is operated with RF amplitudes very close to the ejection threshold. For example, mass peaks have been observed to disappear from a spectrum as the concentration of gas molecules corresponding to those peaks is increased and their RF power absorption inside the trap brings the RF field below the ejection threshold. In its most common manifestation, the main peak in the spectrum suddenly disappears from the spectrum as the gas concentration for that species increases, and the peak reappears as soon as the RF amplitude is increased bringing the RF field back above threshold. As described above, ART MS traps rapidly and simply identify the masses of the molecules present in a gas mixture.
• ART MS devices are superior in terms of mass axis calibration to quadrupole mass spectrometers because they only require single gas calibrations, due to the strict and deterministic linear relationship between ejection frequency and mass, as shown in FIG. 22.
• ART MS devices are also excellent ratiometric devices. The relative amounts, i.e., relative concentrations, of the different species present in a gas mixture are generally adequately represented by the ratio of peak amplitudes in a spectrum.
• ART MS devices are also free of the zero- blast effect that affects the operation of small quadrupole mass spectrometers at low masses, making ART MS sensors excellent candidates for isotope ratio mass spectrometers.
• the zero-blast signal corresponds to a mass independent signal that floods and overwhelms the ion detector of a quadrupole mass spectrometer at low masses, while the RF/DC fields are too low to stop all ions from reaching the detector.
• FIG. 23 shows an example of the charge density saturation effects that occur in ion traps.
• the front trace corresponds to a spectrum of air at a pressure of 3.5E-7 Torr.
• the rear trace corresponds to a spectrum obtained for the same air sample, but with an additional 4E-7 Torr of Argon added to the gas mixture.
• the first approach is to measure ion currents collected by appropriately biased ion collector surfaces located inside or outside the electrostatic ion trap structure.
• the total pressure measurements can take place in parallel with the partial pressure readings, or can require transient interruption of the partial pressure readings in order to collect total pressure readings.
• the external ion collector surface 76 can surround the electron emissive filament 16, wherein ions 89 formed by the electrons on their way into the trap are collected by the ion collector 76 formed of a surrounding shield or tube, to provide an ion current 90 that is proportional to the absolute total pressure.
• the ion collector 76 can be a ring or tube electrode, as shown in FIG. 25.
• the external ion collector surface 76 can also be located at the exit slit of entry plate 1 for the electron beam, as shown in FIG. 26, where the electron collector 72 is used to provide a measurement of the electron emission current.
• the ion collector surface 76 can be located inside entry cup electrode 6. In this mode of operation, the entry cup 6 and the transition plate 3 are momentarily biased at the same voltage, preferably +180 VDC, and the ion collector electrodes are preferably grounded (0 VDC) so that ions are effectively captured by the collection surfaces 76.
• a Bayard-Alpert ionization gauge can be connected in tandem to the trap.
• the total pressure measurement is external to the trap, and only interrupted momentarily as a fraction of the ions is transferred into the trap volume for partial pressure analysis.
• the bias voltage on ionization grid 92 is +180 VDC, same as for the entry cup 6.
• One or two grounded collectors 76 pick up the ions formed inside the grid by electron impact ionization providing an ion current that is proportional to the total pressure of gas in the system.
• the voltage on the entry cup is momentarily decreased from about +180 VDC to about +170 VDC to pull some grid ions into the trap, and then raised back up to +180 VDC to trap the injected ions and perform a partial pressure analysis using ion detector 17, operating the ART MS trap in pulsed mode.
• the electron emissive source 16 is located off-axis relative to the electrode structure as in a typical Bayard-Alpert ionization gauge.
• a tandem configuration such as that illustrated in FIG. 28, is an excellent example of the combination gauge sensor configurations that are possible for ART MS technology.
• This type of configuration is also a good example of an instrumentation setup in which ions are formed outside the trap volume and injected into the trap using electrostatic gating pulses.
• a similar setup can be envisioned to transfer ions exiting an ion mobility spectrometer (IMS) into an ART MS trap.
• IMS ion mobility spectrometer
• the ion collector surface 76 can be a plate, located inside the trap volume between the entry plate 1 and the entry cup 6, and includes an axially located aperture in line with the electrode structure. During a total pressure measurement, the ion collector surface 76 is biased at a voltage more negative than the bias of the filament 16, to attract and collect all ions formed between entry plate 1 and entry cup 6 while repelling any electrons from the filament.
• the second approach to measuring total pressure is to bias the exit plate sufficiently unequally such that substantially all the ions escape the trap and are collected by an ion detector to form a total pressure reading.
• the ion collector can be a simple electrode operated as a Faraday cup collector, or an electron multiplier, as shown in the preferred embodiment illustrated in FIG. 1. If a voltage of about -15 VDC is applied to exit plate 2 shown in FIG. 1, then the anharmonic potential shown in FIG. 2A is sufficiently asymmetric, as shown in greater detail in FIG. 2B, that substantially all ions of all m/q ratios escape the trap together and are collected by ion detector 17 shown in FIG. 1.
• auxiliary gauges simultaneously present in the vacuum system, using their independent readings to scale the ratiometric partial pressure measurements of ART MS to provide absolute partial pressure readings.
• a common implementation is to build analog input ports into the ART MS electronics control unit (ECU) and use the analog and digital output signals from the auxiliary gauges to provide reliable, accurate and real time pressure measurement readings to the ECU's microprocessor and its control software. Digital and analog input ports can be added to the ECU to acquire pressure readings from the digital and analog output ports of the auxiliary gauge controller.
• a flexible I/O interface is the key feature required to interface with the wide range of commercial gauge technologies presently available and compatible with the pressure range of ART MS traps. External pressure readings can also be used to protect the filament and electron multiplier of ion traps as well as to decide the proper time in a process to activate the trap operation. It is also possible to envision synergistic interactions between external total pressure gauges and ion traps in which ratiometric partial pressure information delivered from the trap is used by the total pressure gauge controller in order to adjust and correct its gas-species-dependent pressure readings.
• a partial pressure analyzer based on ART MS technology and including a total pressure measurement facility (internal or external) is capable of delivering total pressure measurements, ratiometric partial pressure concentrations and, with the proper computational capabilities built into the ECU, can also deliver absolute partial pressure readings.
• the calculations and algorithms required to derive absolute partial pressure readings from the combination of ratiometric partial pressure and absolute total pressure information can include varied levels of complexity and assumptions but are well understood in the art. The complexity level of the calculations involved depends on whether all ionic fragments, all molecular species and all ionization, extraction and detection efficiencies are known and/or considered for the multiple molecular species present in the vacuum environment.
• the first factor that needs to be considered when dealing with ART MS traps is that there is a limited charge capacity in the electrostatic trap - i.e., there is a limit to the number of ions that can be stored inside an ART MS trap. Any attempt to introduce new ions into a trap that is already at its charge saturation limit results in (1) the excess ions being ejected from the trap in order to make room for new ones and (2) a change in the chemical composition of the ion charge. This means that adding a new gas component into a gas mixture does not result in an increase in the amount of total charge inside the trap, but rather a shift in the relative concentration of ions for the species stored inside the trap. The total charge remains the same but the ratio of charge between the different species changes to reflect changes in gas composition.
• the charge capacity limit of an ART MS trap is a complex function of: (1) the physical and (2) the electrostatic characteristics of the trap. The net charge content during operation depends dynamically on multiple factors including: (1) the electron emission current, (2) the total pressure, (3) the scan rate, (4) the RF amplitude, etc.
• full charge i.e. charge saturation
• 1E-7 Torr i.e. assuming emission currents above 100 ⁇ A, ⁇ 40 mV RF Vpp, and typical 80 msec scan times).
• Gas molecules are ionized inside the trap in proportion to their partial pressures, but their relative contribution to the total charge is weighted by their relative ionization efficiencies. For example, for a 50/50 mixture of two gases, the gas with the larger ionization efficiency will contribute relatively more charge inside the trap - i.e. in proportion to the ratio of ionization efficiencies between the two species.
• FIG. 33 shows an example in which two gases are present in a vacuum chamber. Gas A is present in a partial pressure PP A and gas B is present in a partial pressure PP B . The two partial pressures add up together to the actual total pressure in the system which, as we will see below, is different from the total pressure reading reported from the ionization gauge.
• gas B is assumed to have an ionization efficiency that is X AB times larger than A.
• ionization efficiency that is X AB times larger than A.
• each gas ionizes without fragmentation (i.e. only a main peak in the spectrum due to the parent molecule).
• the ratio of charge between both gases is:
• X AB acts as a correction factor which adjusts for the dependence of the sensitivity factor of the gauge on the ionization efficiency for the two gases. This is a very reasonable assumption since ions formed inside the anode grid of an ionization gauge are generally collected with the same efficiency independent of mass, but ionize at different rates proportional to their ionization efficiencies.
• the mass dependent charge ejected from the trap during a scan is expected to be proportional to the amount of charge stored in the trap for each gas species.
• the amount of charge ejected for each mass can be calculated by integrating the mass dependent ion current as shown in FIG. 35. Notice that in this first example we assume a very simple case in which gases A and B result in a single peak (i.e. no fragmentation) and no spectral overlap exists between those two peaks. However, the same arguments can be extended if the entire fragmentation pattern package is considered in the calculations for each species.
• the amount of charge stored in the trap for species A and B is proportional to the charge ejected from the trap for species A and B.
• the charge ejected is calculated integrating the ion current vs. time for the mass peaks corresponding to species A and B.
• qA and q ⁇ are the charges ejected as part of the mass peaks in the spectrum corresponding to gases A and B.
• This very simple calculation provides the breakdown of the ionization gauge current into its gas dependent constituents. Once the constituents are identified (i.e. through gas fitting) and the relative ionization efficiency factors are applied, it is then possible to remove the effect of ionization efficiency on the total pressure readings provided by the ionization gauge. In other words, once A and B are identified, and their contributions to the ion current are identified, then their actual partial pressures, PP A and PP B , can be calculated and displayed. The "corrected"partial pressures can then be added up to provide a species-independent total pressure reading.
• the combination of an ART MS mass spectrometer sensor with a total pressure ionization gauge provides a very synergistic combination of sensors with ultimately leads to the ability to calculate absolute partial pressures and to report species-independent total pressures in real time.
• the charge ejection efficiency is not a strong function of mass under carefully selected frequency scan profiles. This assumption has not been strictly proven through focused experiments but seems to be validated by the accuracy of our absolute partial pressure calculations.
• the efficiency of ion detection is highly dependent on RF amplitude and on the scan profile selected. Strong mass dependence in ion ejection efficiency has been observed for linear sweeps and logarithmic sweeps.
• the ART MS trap operated with 1/f frequency sweep profiles seems to offer an ejection efficiency that is much less dependent on mass. Even if mass dependence were observed in the ejection efficiency, that could be included into the calculations as a mass dependent adjustment factor that could be easily calibrated.
• the number of ions of each gas species ejected from the trap is proportional to the number of ions of that species stored inside the trap - i.e. it is expected that the ratio of charge for ions ejected from the trap closely reflects the ratio of ion charges inside the trap.
• a fraction of the ions stored in mass selectively ejected after each RF sweep If continuous ionization is used, the trap is refilled during the rest of the mass scan since the trap is continuously loaded with new ions. Even though this assumption has not been strictly verified through experimentation, the accuracy of the absolute partial pressure measurement results supports this assumption.
• the collection efficiency of the ionization gauge is the same for all ions regardless of their mass.
• the correction factors needed to adjust sensitivity factors of ionization gauges for the other gases are related strictly to the ratios between ionization efficiencies for the different gases.
• the amount of charge that is ejected from the trap for each gas species needs to be measured in order to determine the amount of charge corresponding to each gas that is stored in the trap. If the gas molecule ionizes with no fragmentation, then its contribution to the total charge can be easily calculated, integrating the charge for its only mass peak over time for each scan. Since the ions are mass selectively ejected, and the ejected ion current is collected vs. time, this requires integrating the current under the peak vs. time in order to calculate the contribution of the peak to the total charge. This also means that it is necessary to integrate the ion currents generated during the scan for each peak detected in the spectrum in order to determine the contribution of each peak to the total charge.
• a mass spectrum is collected and stored in memory. This can be a single spectrum or an average spectrum, depending on speed and dynamic range requirements for the data.
• a peak finding algorithm is executed to identify all the mass peaks in the spectrum. Peak identification can be performed through a wide variety of peak finding algorithms and methodologies well documented in the literature. The exact way in which the peaks are detected and tagged is inconsequential to this methodology. 3. If peak overlaps are present at higher masses, then a peak deconvolution algorithm must be applied to break the broad peaks into individual components. For example, it is not unusual for small traps to provide unresolved isotopic envelopes for Xenon gas at approximately 130 amu.
• a peak deconvolution algorithm can be used to break the broad unresolved isotopic envelope peak into its individual integer mass components based on the known resolution of the device. Peak deconvolution algorithms are well known by mass spectroscopists and are part of many commercially available mass spectrometry analysis packages. 4. As shown in FIG. 35, the areas under the identified peaks are integrated against time to determine their contributions to total charge. The integration of ion current must be done over time to provide the measure of charge ejected from the trap at that particular mass. 5. All the identified peaks and their contributions to the total charge are then fed into a gas identification engine which assigns the peaks in the spectra to individual gases and resolves issues such as complex fragmentation patterns and spectral overlaps.
• Spectral identification relies on an accurate gas spectral library which includes the masses and abundances of the parent molecules and fragments for gases commonly found in vacuum systems. Most commercial libraries also allow user editing to include more rare gases that might be of interest to the user. Matching mass peaks to a spectral library can be performed through a variety of mathematical statistical procedures that are well known in the mass spectrometry industry. 6. The identified gasses and their individual relative contributions to the total charge are then used to calculate partial pressures.
• ionization efficiency factors associated to the gases identified are used to eliminate the gas dependence for the partial pressure readings and to provide a gas species independent partial pressure reading.
• peak identification peak deconvolution, spectral deconvolution and gas identification.
• this application does not adhere to or prefers any particular procedure. The process of identifying peaks, resolving peak overlaps at high masses, calculating their contribution to total charge and identifying gases and determining their contribution to total charge is described very generically in order to drive the point that the details of implementation are not that important to this application.
• One of the advantages of this general methodology is that it can provide accurate absolute partial pressure readings even if gas calibrants are not available. This is a big difference from quadrupole mass spectrometers where the unpredictable mass dependent throughput of quadrupole filters makes it impossible to calculate partial pressures without the aid of gas reference cylinders.
• An ART MS trap can provide accurate partial pressure numbers even if a calibrant is not available thanks to the uniformity of its mass ejection efficiency across the mass range and the cancellation of the ionization efficiency effects when the trap charge is combined with the ion current output of the ionization gauge.
• the exact details of the peak identification algorithm are not critical to this application. Examples include Gaussian fitting, and Wavelet Analysis.
• Peak deconvolution techniques can be applied in that case to fit the spectra and estimate the contribution of each isotope or gas to the total charge.
• the width of the peak provides a first indication that there might be a spectral overlap that needs to be resolved.
• FIG. 37 demonstrates some of the advantages of ART MS traps over quadrupole based residual gas analyzers in terms of absolute partial pressure calculations. It also demonstrates the accuracy of the methodology described above.
• the total pressure measurements were performed using a 390 ionization gauge module (Granville Philips, Longmont, CO) connected to the ART MS trap controller.
• the RGA data was obtained with a 200 amu range quadrupole residual gas analyzer (RGA) in Faraday cup (FC) mode of operation (Stanford Research Systems (SRS), Sunnyvale, CA).
• the system was pumped down to a base pressure of 5E-8 Torr, and the partial pressure for the peak at 28 amu was calculated with both the SRS RGA (peak intensity at 28 amu) and with the ART MS device (contribution to total charge from the 28 amu peak). Even though most of the signal at 28 amu in this case is due to CO, both the SRS RGA and the ART MS sensor provided very similar partial pressure results for the species responsible for the mass peak at 28 amu. Moving to the right, the system was exposed to the pure nitrogen gas source. The SRS RGA and the ART MS sensor provided similar partial pressure values for N 2 under a total pressure of 2.6E- 7 Torr clearly dominated by nitrogen gas.
• the system was exposed to a second source of gas containing both Kr and Xe.
• the addition of two more gases to the mixture increased the total pressure in the chamber to roughly 3.2E-7 Torr.
• the ART MS sensor showed no change in nitrogen levels as expected; however, the SRS RGA showed a small dip in the nitrogen signal at 28 amu as the new ions displaced some of the nitrogen ions from the ionizer.
• the krypton and xenon levels reported by the ART MS device were very close to the actual partial pressures in the vacuum system, while the SRS RGA completely underestimated its levels by as much as a decade.
• the nitrogen gas source was shut off. As expected, very small change was observed in the levels of Kr and Xe.
• Electron Energy, eV Electron Energy, eV
• Entry plate Bias, VDC (typically 0 VDC);
• the two cups preferably are AC coupled to the transition plate by high voltage (HV) capacitors.
• HV high voltage
• the use of capacitors to couple the RF from the transition to the cups can be optimized through experimentation by finding the values that provide the highest signal with the least amount of RF Vpp and the least amount of mass peak contributions from superharmonics.
• the trap illustrated in FIG. 1 has also been operated without coupling capacitors between the transition plate and its adjacent cups, although performance was not as efficient as compared to the preferred embodiment.
• the voltage on the cups is adjusted to assure maximum signal and resolution in the spectra.
• the voltage on the cups is usually a fixed fraction of the transition plate voltage. In fact, it is usually desirably to maintain a fixed ratio between the transition plate and cup voltages as the transition plate voltage is changed to a new value. Preserving a constant ratio between the transition plate and cup voltages is definitely required while performing HV scans over a large voltage range.
• the cup voltage is typically around 1/1 Oth of the transition bias voltage (assuming the entry plate is at ground).
• the proper voltage on the cups is generally tuned to assure stable ion trajectories and a large and stable signal. There is generally a narrow range of voltages that leads to proper trap operation. Generally the ideal voltage is selected by adjusting the cup potential until the maximum intensity is achieved for all signals in the mass spectrum. Cup voltage affects both intensity and resolution and in some cases, signal might be sacrificed for an increase in resolution.
• the entry cup voltage also affects the arch trajectory described by the electrons inside the trap and it might be required to readjust the electron energy when the entry cup voltage is adjusted. In general, there is no reason to adjust the cup voltage at any value other than the one that provides the maximum signal, unless higher resolution is needed.
• the depth of the trapping potential well also influences the minimum value of the RF Vp P that is required to eject ions.
• the RF V pp "threshold" is the minimum RF peak-to-peak amplitude required to eject ions from the trap.
• the RF Vp P threshold increases in amplitude as the potential well gets deeper, i.e., the ions need to be "kicked harder” in order to eject them from the trap.
• the amplitude of the RF V pp required to eject ions gets smaller.
• the scan times can also be reduced in the trap as the ions move faster.
• RF Waveform Both sine wave and square wave RF excitation are routinely used for ion excitation. Sine waves are preferred for practical reasons, but square waves can be very useful, because such a design does not require a dedicated direct digital synthesis source, and enables the use of pulse width modulation (PWM) output modules that are already built into standard microprocessor electronics boards, or field programmable gate arrays, or application specific integrated circuits, thus reducing cost, power consumption, complexity, and size.
• PWM pulse width modulation
• the AC excitation of the preferred embodiment is coupled into the transition plate using a balanced/unbalanced (BALUN) transformer, typically terminated into a 50 Ohm resistor.
• BALUN transformers are commonly used in Cable TV splitters, due to their low cost and large bandwidth. Coupling the RF into the center plate is preferred because it simplifies the electrical scheme required to distribute the RF through the trap and also has been shown to eject ions at twice the natural frequency of oscillation of the ions. Center plate excitation has also been shown to produce fewer spurious peaks due to superharmonic excitation in the preferred embodiment.
• the AC excitation amplitude needs to be kept above the autoresonance threshold for ions to be ejected.
• the ejection threshold depends on the scan speed the initial energy of the ions, the depth of the potential, the symmetry of the trapping potential, and the total pressure.
• the amplitude of the RF needs to be set above threshold in order to obtain any signal. Generally, operation close to threshold provides the highest mass resolution. As the RF V pp increases, the amplitude of the signal also increases until a plateau is achieved. At that point increasing the RF intensity causes the peaks to broaden (i.e., loss of resolution) without any significant intensity gains. Increasing the RF Vpp also usually leads to the appearance of spurious peaks due to excitation by higher harmonics.
• the RF Vpp adjustments are designed to control the interplay between resolution, intensity and the relative contribution from superharmonics to the spectrum.
• the RF Vpp can also be adjusted during a scan, for example, by increasing the RF V pp amplitude with decreasing frequency. Dynamic adjustment of the amplitude during frequency scans can better match ART MS spectra to those from other mass separation instruments such as quadrupole and magnetic sector mass spectrometers. As the pressure in the system increases, the signal amplitude and resolution is often improved by an increase in RF V pp voltage. The idea is to reduce the time required for the ions to be ejected from the trap before scattering collisions prevent them from being effectively extracted.
• Peaks corresponding to the ejection of ions at frequencies corresponding to multiples of the applied excitation frequency are called superharmonic peaks and generally appear at low masses. For example, under certain trap conditions, a peak at 4.5 amu may appear in the spectrum when a large peak at 18 amu (corresponding to H 2 O + ions) is also present. The peak at 4.5 amu corresponds to excitation at the second harmonic (i.e., 1.2 MHz superharmonic) of the main 18 amu excitation frequency (i.e., 600 kHz).
• superharmonic ejection of ions requires higher RF Vpp thresholds than ejection at the natural oscillation frequency, or twice the natural oscillation frequency if the RF excitation is applied to the transition plate.
• the factors that lead to the appearance and mitigation of superharmonics in the preferred embodiment are:
• RF V 1212 Superharmonic peaks appear when the RF V pp is increased above a certain threshold that allows the ions to be ejected by multiples of their natural oscillation frequency. In general the superharmonic peaks are the first to disappear from the spectrum as the RF V pp is reduced since they have the highest thresholds. In other words, since the threshold value for ejection of superharmonics is larger than for the ejection at the natural oscillation frequency (or twice the natural oscillation frequency for transition plate pumping), the superharmonic peaks are the first to disappear from the spectrum as RF V pp is reduced.
• the preferred embodiment operates with transition plate RF coupling.
• ions are naturally ejected at twice their natural oscillation frequency.
• One way to visualize the operation of an ion trap with transition plate RF coupling is to think of pumping a child on a swing by pulling on the chain rather than simply pushing the child from one side. In this case the most efficient pumping is achieved at twice the natural oscillation frequency of the swinging child.
• transition plate RF coupling is already producing ions ejected at twice their natural oscillation frequency, this is not technically described as superharmonic excitation.
• Mass selective ejection preferably employs RF scans in order to eject the ions from the trap.
• autoresonance means to scan across its natural oscillation frequency (or twice its natural oscillation frequency in the preferred embodiment) from high to low frequency values. In most cases, the frequency range is scanned to cover a very wide range of masses so that full mass spectral information can be obtained. It is also equally possible, however, to scan narrow frequency ranges that eject only one or a few masses from the trap.
• Mass selectively ejecting ions of specified mass to charge can be accomplished by tuning the RF to a frequency range that overlaps the frequency peak of the ions of interest.
• An FFT inversion algorithm can generate the wide spectrum RF that can then be applied to the trap. This might not lead to efficient ion ejection (since autoresonance is not involved when no actual scanning is done), but this mode can enable the trap to operate essentially as a filter that only allows ions of a specific mass to be ejected.
• the trap can be held at a specific mass and instantaneous changes in concentration can then be monitored.
• the ions are typically formed deep within the trap.
• the exact origin of the ions depends on the angular orientation of the filament relative to the entry plate's plane and the energy of the electrons. As the energy of the electrons increases the electrons reach further inside the trap (i.e., with lower potential energy values) and higher RF V pp is required to eject the ions from the trap in the same scan time (i.e., the ejection threshold increases). If the arch gets too short, the end of the electron arch can reach the back plane of the entry plate and form ions within line of sight of the oscillating beam.
• the typical procedure used to adjust the electron bias voltage is:
• the ejection of ESD ions by stray electrons that collide with the back surface of the entry plate can be minimized by applying specialized coatings to the back of the plate, such as, for example, gold and platinum coatings, which have been shown to reduce ESD ion levels compared to stainless steel.
• a natural consequence of off-axis ionization is that the section of the electron beam arch that produces ions in line with the oscillating ion beam does not have electrons at the maximum electron energy possible. At the turn around point of the arch line-of-sight exposed to the beam, the electron beam has lost the axial velocity component, but still retains the initial component of radial velocity. The voltage difference between the filament and the entry plate must not be used to calculate ionization potentials for different gases, because appearance potentials calculated in this way will always be over-estimates.
• Ions can be created in an ART MS ion trap either continuously or intermittently, in pulses.
• Pulsed filling is an alternative mode of operation in which ions are created inside, or loaded into the trap during pre-specified short periods of time.
• pulsed filling involves the generation of ions in the absence of any AC excitation: the ions are created and trapped under the influence of purely electrostatic trapping conditions, and then an RF frequency or trapping potential scan is triggered to produce mass selective ejection of the ions. The cycle can be repeated for each new ion pulse.
• An advantage of pulsed operation is improved control of space charge buildup inside the ion trap.
• ion concentrations inside a trap can cause peak broadening, resolution losses, dynamic range losses, peak position shifts, non-linear pressure dependent response, signal saturation, and increased background noise levels.
• Another advantage of pulsed operation is better control of the initial ionization conditions during mass selective storage, fragmentation, or dissociation experiments. For example, to completely clear undesirable ions from a trap, it is necessary to stop introducing new ions while a cleaning frequency sweep is on.
• Anharmonic electrostatic ion traps relying on electron impact ionization can include electron gates to turn the electron beam on and off, or, alternatively, rely on the fast turn on/off times of cold electron emitters based on field emission, such as, for example, electron generator arrays, to control the duty cycle of the electron fluxes entering the trap's ionization volume.
• Gated external ionization sources are well known in the art.
• the ionization duty cycle, or filling time, in pulsed filling operation can be determined through feedback arrangements.
• the total charge inside the trap can be integrated at the end of each scan and used to determine the filling conditions for the next scan cycle.
• Charge integration can be accomplished by (1) simply collecting all the ions in the trap with a dedicated charge collection electrode in a total pressure measurement as described above, or (2) integrating the total charge in the mass spectrum, or (3) using a representative measure of total ion charge, such as, for example, current flowing into an auxiliary electrode, to define the ionization duty cycle for the next scan.
• total charge can also be determined by measuring the number of ions formed outside the trap as the pressure increases.
• Ion filling times can also be adjusted based on the specific mass distributions or concentration profiles present in the previous mass spectrum, or based on the presence, identity, and relative concentrations of specific analyte molecules in the gas mixture, or based on target specifications for the mass spectrometer, such as, for example, mass resolution, sensitivity, signal dynamic range, or detection limits for particular species. Pulsed filling operation under electron impact conditions is shown in FIGS.
• the filament 16 and repeller 85 were biased at -70 VDC, while the grid and the gate electrodes were biased at -60 VDC, resulting in electrons reaching the entry cup's grid with 70 eV of energy- i.e., gate open.
• the gate is then closed by rapidly switching the gate bias 87 to ⁇ -85 VDC (i.e., more negative than the electron emitter bias), such that the electrons turn around and never reach the entry cup's mesh, as illustrated in FIG. 30B.
• Advantages of this simplistic electron gun design include fast response, with switching times in the range of nanoseconds, a constant electron extraction field, and a relatively compact design.
• the electron extraction field is constant because the grid bias, which is set by the voltage difference between the emitter's surface and the grid electrode, does not change when the gate is on or off, improving electron emission efficiency.
• the filament emission can be controlled with a feedback loop without risk of burning the filament when the gate is closed.
• Gated electron sources are well known in the art, and the example illustrated in FIGS. 3 OA-B is just one representative implementation.
• the timing diagram for a pulsed filling experiment is shown in FIG. 31.
• the top graph indicates the time during which the electron emission current is activated, which in this case corresponds to a 5 msec fill-time. This time can be in the range from tens of nanoseconds to several milliseconds.
• the emission current levels during the gate-on periods can be in the range from microamps to a few milliamps.
• the middle graph shows the time during which the RF frequency is scanned from high to low values. As shown in the graph, there is a small delay period between the time the gate is closed and the RF scan starts to allow ions that are not stored in the oscillating beam to leave the trap and to allow the detection electronics to respond to the new levels. This delay is necessary to insure that all background signal, caused by non-stored ions, dissipates from the trap as clearly shown in the bottom graph. In general, a delay of ⁇ 0.1 millisecond is sufficient for noise to dissipate, although longer delay times might be required if the detection electronics have limited bandwidth.
• Pulsed electron impact ionization is a convenient way to reduce baseline offset noise in ART MS traps using on-axis ionization. Once the electron gate is closed, no more ions are added to the trap by electron impact ionization. As ions are trapped in their oscillatory motion, they collide with neutral atoms and molecules as they oscillate from one cup to the other, and some ions will be lost after each cycle of oscillation due to scattering collisions. Therefore, it is important to adjust the scan time so that it does not exceed the residence time of the ions in the trap.
• the residence time of ions in a trap at ultra-high vacuum (UHV) pressures is typically in the range of about 10 milliseconds to about 100 milliseconds.
• FIG. 31 clearly demonstrates the ability to generate a useful spectrum at 10 "8 Torr with a scan time of 45 msec.
• the scan time has to be adjusted (i.e., reduced) to eject all the ions out of the trap before they are lost to ion-neutral scattering.
• scan times must be as fast as 1 millisecond, i.e., the residence time of ions in the trap, to make sure the ions are ejected before being lost.
• This method of operation improves the ion signal while still minimizing noise.
• the two peaks of interest, mass peaks 1 and 2 are preselected and ionization is timed so that the ions corresponding to mass peak 1 and mass peak 2 do not have time to be lost to scattering collisions at higher pressures.
• Fast frequency scans generally require higher RF V pp voltages that can lead to decreased resolution.
• Pulsed operation is also important for using the ART MS trap as an ion storage device.
• the trap can be filled with ions, and then RF excitation can be used to eject undesirable ions of specific masses out of the trap, or store or preconcentrate ions of preselected masses in the trap by ejecting all other masses.
• Pulsed operation is generally preferred when autoresonance excitation is used to chemically react the ions stored in the trap through collision- induced or electron-attachment dissociation.
• a total pressure measurement can be obtained by filling the trap with a short electron pulse, followed by measuring the decay of the ion signal with time after multiple frequency scans - the signal decay rate will be strictly related to pressure.
• a single ionization pulse can be followed by two scan cycles separated by a fixed time.
• the signal decay between the first and second scan will be directly related to the pressure inside the trap.
• two frequency scans can be performed one after the other, but with different delays between the ionization pulse and the start of the frequency scan.
• the difference in the rate of decay in signal measured for the longer delay scan compared to the rate of decay of the shorter delay scan can be an indication of total pressure inside the trap.
• the operational parameters of an ART MS trap need to be optimized to provide the best possible signal. As the pressure gets too high, it is also necessary to protect the filament and electron multiplier from operation under damaging conditions that can lead to reduced life or even catastrophic failure.
• Electron Emission current 1. Electron Emission current 2. Filament Repeller bias voltage
• Electron Emission Current Operation at Ultra High Vacuum (UHV) levels requires an increase in electron emission current relative to operation under standard High Vacuum (HV) levels in order to achieve high signal levels and increased SNRs.
• UHV levels the trap does not fully saturate with charge and the ion signals in the spectrum tend to track the total pressure in the system. As the pressure increases, the signal intensities in the mass spectrum eventually reach a plateau as the trap becomes saturated with charge (i.e. the trap is full).
• the electron emission current also has a role in the control of baseline offset signals (see below).
• the electron emission setting affects filament temperature and as such has an impact on filament lifetime and analyte decomposition at the filament. Reducing the emission current at high pressures also increases filament lifetime and reduces the production of byproducts through thermal decomposition at the filament surface.
• a repeller is often located behind the hot filament to focus the electron beam and improve the coupling of the electron beam into the trap volume.
• the use of repellers is very common in off axis ionization sources where tight coupling requirements are usually in place.
• the filament repeller voltage is a function of electron energy, filament bias, and electron emission current.
• the repeller voltage needs to be optimized at each new emission current in order to achieve optimal SNRs. Since emission current needs to be adjusted with pressure, so does the filament repeller voltage.
• Repeller voltage directly affects the coupling of electrons into the trap, and as such, it affects signal intensity, SNR, and peak shape.
• the RF V pp amplitude is a critical parameter in ART MS traps. Operation of an ART MS sensor with an RF V pp too close to threshold can lead to loss of signal as the ion population increases with increasing pressure, and the effective RF field inside the trap drops below threshold. In general the RF V pp amplitude required to obtain consistent SNR levels in an ART MS trap increases with pressure. An increase in RF V pp generally causes an increase in ion signal with a decrease in mass resolution. In general, the RF V pp values need to be adjusted if trap operation is adjusted at the 1-2 E-7 Torr value and pressure reaches close to IE- 5 Torr levels.
• Mass Axis calibration Parameter As the pressure in an ART MS trap increases it is not unusual to see changes in peak positions in the mass spectra. Subtle changes are caused by changes in the charge density inside the trap. Changes in mass axis calibration are also expected as the voltages inside the trap are modified. Users must have calibration parameters that match the trap operational parameters at each pressure or pressure range.
• Electron Multiplier (EM) Voltage As the pressure in the system increases it is possible to see increased noise from the electron multiplier due to ion feedback. Good quality electron multipliers with ion feedback compensation will be less susceptible. On the flipside, as the signal amplitude (i.e. trap ion output) decrease with pressure it might be necessary to increase multiplier gain to improve SNR. An increase in EM voltage generally leads to a reduction in EM lifetime, but might be required to preserve SNR levels.
• Scan Speed At higher pressures it often makes sense to scan at higher scan rates. Higher scan rates result in lower charge density inside the trap and can reduce the effects of noise and baseline offset on signal. Since higher scan rates increase the RF Vpp thresholds, this adjustment is usually followed by a comparable adjustment of RF signal levels. Higher scan rates at higher pressures are also consistent with the idea that ions have shorter residence times in a trap as the pressure increases. Protection Modes: As the pressure increases it might be useful to carefully control the rate at which bias voltage is increased in the Electron Multiplier and the Transition Plate when the unit is first turned on. In general, sudden increases in bias voltage are known to cause more arcs than slower increases at higher pressures. As the pressure increases it might be useful to set maximum pressure values- i.e.
• the user can also set time delayed triggers that shut down the ART MS sensor if the pressure remains above the threshold for a pre-specified amount of time. This functionality is useful in order to minimize sensor shutdowns in the presence of brief pressure transients that cannot damage the unit.
• Filament and EM protection modes and thresholds might also be species dependent since the filament will be more sensitive to certain species than others.
• the nature of the ART MS device is to rapidly fill with ions from a sample of gas to be analyzed, with each gas component oscillating at its own resonant frequency.
• the energy requirement to selectively eject a sequence of m/q ions is very small and can be done using low-power electronic signals. This enables the trap in FIG. 1 to complete a 300 amu range scan within 200 ms or a 100 amu range scan in less than 70 ms. This scan rate is much faster than the typical 1-2 second 100 amu scan speed associated with typical residual gas analyzers (RGA) based on quadrupole mass spectrometry.
• RAA residual gas analyzers
• This speed advantage can be used in two ways; (1) as an ultra- fast measurement instrument in closed-loop control system, and (2) for additional measurement averaging to improve signal-to-noise ratio for trace level contamination control.
• the combination of high sensitivity and high speeds makes ART MS devices ideally suited for fast closed-loop process control based on compositional analysis.
• quadrupole mass spectrometers capable of scan speeds comparable with ART MS.
• high-performance systems are very large, require bulky electronics and are very expensive to purchase and maintain.
• a well recognized advantage of ART MS sensors including total pressure measurement capabilities is their ability to deliver both total and partial pressure information in real time and from a simple and small package which does not take any more room in a vacuum port than a traditional ionization gauge.
• Electrostatic repulsion between ions in the oscillating beam leads to space charge limitations which ultimately fixes the density of charge that can be stored in the ART MS device, and is related to the length (L) and diameter (D) - a larger trap can store more ions and a smaller trap can store fewer ions.
• This property is largely pressure independent and the amount of charge stored within the trap is relatively constant over its usable range, and therefore the performance of the trap is more consistent over its usable range. Therefore, the speed and sensitivity advantage of the ART MS relative to a quadrupole MS increases with decreasing pressure because the quadrupole MS requires additional scan time to pass additional charge through the mass filter.
• An additional advantage of the typical ART MS device is that the small size has less surface area that is exposed to vacuum than most full range quadrupole devices, which minimizes vacuum surface and process memory effects.
• ART MS fast scanning speed Another characteristic of the ART MS fast scanning speed is that the sampled data better represents the gas components at the time of measurement if the gas components are rapidly changing.
• the fast scan rates provide a more accurate "point measurement" of the sample gas that enables the ART MS to better capture transient events, especially in the UHV pressure ranges.
• Surface science experiments and the detection of pressure bursts due to faulty vacuum valve operation are examples of applications that can take advantage of the "point measurement" capabilities of an ART MS device.
• ART MS devices store a fixed amount of charge
• the sensor is intrinsically a ratiometric device, where the maximum ion charge is a fixed 100% and the gas component partial pressures represent a portion of the 100%.
• ratiometric information is preferred over absolute partial pressure information and is a native output of the ART MS device.
• absolute partial pressure information is needed, the output of an ART MS trap can be easily scaled using total pressure information to provide partial pressure outputs.
• the low drive and operating power requirements of the ART MS device allows the sensor head to be operated at the end of a cable or integrated into a modular form (integrated electronics and sensor).
• quadrupole-based instruments require a close physical and electrical coupling of the RF drive electronics with the quadrupole sensor and are only available in the modular form (i.e., no cable between gauge and controller).
• a cable can be used to connect to the drive electronics. Combining the ability for remote cable operation, the small size of the ART MS device and the ability for a smaller electronics packaging of an ART MS based system provides additional flexibility for installation on crowded vacuum tools.
• the mass specific oscillation frequency of the ART MS device is largely dependent upon the physical dimensions of the ion trap and the amplitude of the trapping potential, and is not dependent on the drive electronics. Therefore, once the extraction conditions for a single m/q are known, then all other m/q can be calibrated to the single gas. This allows for a rapid and easy single point calibration based upon gauge manufacturing dimensional control or through a single gas calibration. For example, detecting a water peak (18 amu) in the ART MS device allows for full calibration of the entire 1-300 amu range. This is an important ease-of-use benefit for both newly manufactured and field supported equipment.
• ART MS ART MS, by design, does not have the zero blast effects and can fully resolve all the lower amu peaks at full sensor resolution. Therefore, a permanently installed ART MS device could easily be used as an in-situ helium leak detector, in portable leak detector applications and as an efficient isotopic ratio mass spectrometer.
• a very exciting opportunity provided by ART MS traps is the ability to monitor both atomic and molecular hydrogen at masses of one and two amu, respectively.
• ART MS sensors are often compared against RGAs based on quadrupole mass analyzers which typically operate in constant mass resolution mode.
• the throughput of quadrupole mass filters is mass dependent.
• ART MS stores all ions inside the trap and preserves the relative concentrations of the ions during ejection, providing a more accurate representation of gas concentrations for the sample gas.
• ART MS sensors are expected to provide a much more accurate representation of hydrogen levels in ultra high and extreme high vacuum experiments.
• ART MS sensors are expected to find applications as fullness detectors for cryogenic pumps indicating the need to regenerate the cold arrays based on subtle changes in the gas composition of the residual gases emerging from the cryogenically cooled surfaces.
• ART MS sensors do not technically have an upper mass limitation, or mass range.
• mass limitation or mass range.
• ART MS traps have been shown to provide mass information well above the 600 amu range.
• most volatile chemical substances have a molecular weight below 350 amu and the electronics and data acquisition software is typically limited to a mass range of 300 amu to support the most common gas sampling applications.
• ART MS devices with higher molecular weight ranges are expected to find use in more sophisticated applications including specialized ion sources or tandem mass spectrometry.
• ART MS technology has demonstrated immense potential for a large variety of research and industrial processing applications.
• the combination of (1) low power requirements, (2) small size, (3) ease-of-assembly, (4) ease-of-use and (5) high resolving power makes ART MS technology ideal for field applications relying on battery, solar and other-alternative power sources.
• ART MS devices packaged into self-contained, f ⁇ eld-dep lovable gas sampling units i.e., integrating low power high vacuum pumping systems
• ART MS trap designs Miniaturization of ART MS trap designs has the potential to provide the first truly palm-portable, fast gas analysis devices for both military and forensic applications featuring unprecedented operational times between battery charges.
• ART MS traps will very likely find applications in combination with separation techniques (such as gas and liquid chromatography) as well as competitive mass spectrometry techniques (such as time-of- flight systems, quadrupoles, magnetic sectors and even orbitraps) and ion mobility spectrometers.
• Combination sensors including multiple pressure gauges as well as an ART MS trap are expected to become the new standard for gas analysis and vacuum quality measurement in high vacuum (HV) and ultra-high vacuum
• ART MS trap into a MALDI source to separate matrix ions from the biological ions of interest and/or to preconcentrate biological molecules prior to delivery to a time-of- flight source.
• the high resolving power of ART MS traps at low masses is expected to result in the application of the technology to leak detectors as well as low-mass iso topic ratio mass spectrometry.
• ART MS traps combined with sensitive charge detectors are expected to become the new standard for field deployable isotopic ratio mass spectrometry.
• the fast speed of ART MS devices, combined with their ability to perform measurements at the point of interest, is expected to revolutionize the field of mass spectrometry for surface science and catalysis.
• ART MS sensors can be used to monitor base pressure conditions at unprecedented sampling rates and without significant space requirements.
• ART MS devices can be used to monitor base pressure conditions, diagnose pumpdown problems, perform leak detection procedures and characterize pumpdown rates.
• the relative low cost of ART MS devices is expected to justify the replacement of each and every ionization gauge presently installed in a high or ultrahigh vacuum system with a combination gauge including both total and ART MS partial pressure capabilities.
• the standard gauge/ECU architecture for ART MS devices used in our laboratory includes the gauge (transducer) connected to the controller (ECU) using a cable.
• the transducer connects to a standard vacuum port while the ECU is placed somewhere else in the system without interfering with other components attached to the vacuum chamber.
• Another preferred implementation includes an ART MS sensor of reduced dimensions with the ECU permanently attached to the transducer's envelope to provide a monolithic/modular device of small physical dimensions and simplified operation.
• the information provided by the mass spectrometer system can be displayed as raw mass spectrometry data on a remote computer display or, if available, the necessary information can be extracted from the spectra, processed according to user's scripts and displayed on a graphical front panel display incorporated directly into the ECU.
• the partial pressure information can also be linked to one or many different analog, digital and relay-closure I/O ports located on the back plane of the controller to provide real-time process control.
• ART MS sensors provide unprecedented sampling rates combined with unobtrusive mounting configurations and point-of-use capabilities. Use cases compatible with ART MS include: (1) fixed volume sampling, (2) package/hermeticity testing, (3) transient response detection, (4) concentration control, and (5) process fingerprinting.
• ART MS sensors are expected to find immediate application in all regions of modern semiconductor ion implantation tools where careful control of gas composition is not only critical to assure wafer cleanliness, but also to assure proper dose delivery.
• ART MS devices are expected to provide real-time prompts for preventative maintenance based on changes in gas composition, as well as post-preventive maintenance (post-PM) system readiness analysis.
• ART MS traps will find applications in modern implantation cluster sources providing the ability to separate labile parent cluster ions from their fragments before delivery. ART MS devices will also find application in semiconductor PVD processes where interwafer times can be significantly reduced if gas compositional analysis is available between wafer process steps and photoresist contamination can be a big problem if undetected. ART MS traps are also expected to find application in the fast cycling processes used for magnetic media disk manufacturing. Typical cycle times for disk manufacturing steps are as short as 3 seconds, with the pressure of the chambers oscillating between medium to high vacuum during each cycle. ART MS devices provide the first opportunity to perform real time analysis of gas composition simultaneously controlling (1) pump down rates, (2) gas mixture ratios and (3) cross contamination between chambers.
• ART MS ECU controllers will generally include multiple I/O options including: (1) digital I/O, (2) analog I/O and (3) relay closure I/O and well as (4) standard communication ports such as USB, Ethernet, Device Net and RS485.
• a test instrument provides multiple sets of information to multiple independent computer host applications by incorporating a multi-client connection hierarchy into the test instrument (i.e., Ethernet Transmission Control Protocol/Internet Protocol [TCP/IP] network, direct serial, universal serial bus, and/or other test instrument-to-host data connections).
• TCP/IP Ethernet Transmission Control Protocol/Internet Protocol
• test instrument can be connected to a primary host connection (i.e., a process tool) performing functions requested by the primary host, while a separate application (i.e., advanced process control software) can collect the same or a different set of data, and a third application (i.e., maintenance application software) can collect the same or a different set of data or instrument status.
• a primary host connection i.e., a process tool
• a separate application i.e., advanced process control software
• third application i.e., maintenance application software
• FIG. 38 shows the concept of the ART MS trap connected to a network via an Ethernet TCP/IP connection to three (or more) Host/Client connections.
• the ART MS is a high performance test instrument that provides total pressure and partial pressure measurement information/data made by the ART MS sensor (right hand side of FIG. 38) that is connected to a vacuum system or chamber.
• the ART MS test instrument has a Controller, Sensor and cable.
• the ART MS On the Controller there is a front panel User Interface (display and buttons, not shown) and on the back panel a variety of input and output electrical signal connections, setpoint relays, a Universal Serial Bus (USB) data connection and an Ethernet data connection.
• the ART MS has the ability to process the sensor's vacuum measurements into a plurality of measurement output formats that are accessed via the electrical signal connections, setpoint relays, or by external host connections to the USB and/or Ethernet data interfaces using an Application Programmer Interface (API) set of computer functions.
• API Application Programmer Interface
• the ART MS will include an Ethernet connector and one USB-2 connector.
• the ART MS will control (as Master) or access data (as Monitor) the ART MS via device-specific commands and replies.
• the data communications will be the payload of an Ethernet frame or USB data packets.
• the Ethernet and one USB-2 connector are located on the ART MS back panel.
• the ART MS will support multiple client connections as data Monitors, however only one device can be the ART MS Master.
• the ART MS has a user- level hierarchy:
• Administrator This is a special level of user (see below) that can preempt any Master (if an Administrator registers as Master), control all functions of the ART MS device through the standard and an extended API, and has access to reserved information. Note that it is possible to register as an Administrative Master or an Administrative Monitor. Only the Administrative Master registration will preempt an existing Master. Administrative Monitors can access restricted data, but not control the gauge. Master: This level of user can configure the sensor driver and data collection parameters and allow or lock out the front panel buttons and display. There is only one Master at a time. When a client tries to register as a Master and one already exists, the registration fails or the registration preempts the existing Master (forcing it to a Monitor registration).
• This level of user can retrieve collected data via the API, but can not change the way data is collected or the configuration of the ART MS. These user-levels allow one Master to drive the ART MS and many data and/or maintenance Monitors. The Administrator allows for maintenance and support access to ART MS functions independent of the prior hosts connections.
• the ART MS front panel assembly (FPA) consists of a graphic display and buttons and is connected to the ART MS via a dedicated front panel connector.
• the front panel can be mechanically attached to the ART MS.
• the FPA is a host connection and is the Master in the ART MS power-up or post reset state.
• the FPA can be the Master or a Monitor.
• the only exception is the IG OFF button which has priority over both a Master or Monitor host connections.
• the Front Panel Assembly controls all the functions, scanning parameters and input/output assignments on the ART MS.
• the FPA has two display modes:
• select variables e.g., selected relative partial pressures and/or partial pressures.
• the PC/Host is a monitor only to collect data or monitor the performance of the ART MS.

Landscapes

• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Electrochemistry (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
• General Physics & Mathematics (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Molecular Biology (AREA)
• Other Investigation Or Analysis Of Materials By Electrical Means (AREA)
• Electron Tubes For Measurement (AREA)

Priority Applications (6)

Applications Claiming Priority (8)

Related Child Applications (1)

Publications (3)

Family

ID=43050850

Family Applications (1)

Country Status (6)

Cited By (9)

Families Citing this family (61)

Citations (4)

Family Cites Families (21)

• 2010
  • 2010-05-05 EP EP10772778.6A patent/EP2430646B1/en not_active Not-in-force
  • 2010-05-05 JP JP2012509953A patent/JP5688494B2/ja not_active Expired - Fee Related
  • 2010-05-05 KR KR1020157032631A patent/KR101724389B1/ko not_active Expired - Fee Related
  • 2010-05-05 WO PCT/US2010/033750 patent/WO2010129690A2/en not_active Ceased
  • 2010-05-05 CN CN201510194511.6A patent/CN104779132B/zh not_active Expired - Fee Related
  • 2010-05-05 KR KR1020117028183A patent/KR101570652B1/ko not_active Expired - Fee Related
  • 2010-05-05 CN CN201080029456.0A patent/CN102648511B/zh not_active Expired - Fee Related
• 2011
  • 2011-11-04 US US13/289,142 patent/US8586918B2/en not_active Expired - Fee Related
• 2014
  • 2014-09-10 JP JP2014183890A patent/JP5918821B2/ja not_active Expired - Fee Related

Patent Citations (4)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events