Patent Grant
10541122
A mass spectrometer measures the masses within a molecular sample to analyze the composition of the sample. A residual gas analyzer (RGA) is a relatively small mass spectrometer that measures the composition of a gas by ionizing components of the gas to create a charge, and determining the mass-to-charge ratios of those components. RGAs are commonly used to check for gas composition and contamination, and may operate in an evacuated environment at lower pressure than the source of the gas being analyzed. The main components of a residual gas analyzer are an ion source, mass analyzer (mass filter), detector and associated electronics. The ion source ionizes molecules of the gas, the mass analyzer selects the ions by their mass-to-charge ratio, and the detector determines the amounts of the selected ions.
RGA ion sources are generally one of two types: open or closed. An open ion source is usually mounted in a vacuum chamber with its components exposed to sample gas from a process environment, directly. The sample gas molecules in the vacuum chamber can move through the ion source from many directions—there is no pressure difference within the ion source and around it. When the pressure of the gases is too high for the RGA to operate properly, a pressure-reducing gas-sampling vacuum system is used to bring a sample of the gas to be analyzed down to an acceptable pressure. In such applications, an open ion source suffers from drawbacks, such as interference from the gases in the residual vacuum of the sampling system (e.g., hydrogen, water, carbon monoxide, oils).
A closed ion source is preferred, generally, when using an RGA to analyze gas with a pressure-reducing gas-sampling system. A closed ion source provides an ionization chamber operated at, or below the pressure of the sample gas, but higher than can be tolerated by the whole RGA. This chamber has restricted gas exit conductance with only small openings for entrance and exit of gases, electrons, and ions. Electrons are directed into the chamber to form ions of the sample gas at the relatively high pressure in the chamber. The sample gas is at higher pressure than could be tolerated with an open ion source, so the signal from the sample gas is correspondingly higher than the signal from the residual vacuum of the pressure reducing system, providing a higher fidelity analysis of the sample gas. Because critical electrode surfaces of the closed ion source are exposed to the sample gas at a higher pressure than an open ion source, the closed ion source is susceptible to degradation much faster because the sample gas can contaminate those surfaces. Additionally, the electron source is typically located close to the hole where electrons are introduced into the ionization chamber, and is thus exposed to the sample gas at a pressure much higher than the average pressure of the mass spectrometer. Thus, closed ion sources have higher analytical fidelity but are susceptible to higher degradation rates, while open ion sources have lower degradation rates but provide lower analytical fidelity.
Prior approaches to this degradation problem used in other (non RGA) systems include cross beam ionizers and dynamically adjusted ion sources with extra control surfaces. However, extra control surfaces increase cost and complexity, often require frequent adjustment procedures, and have limited effectiveness with extreme contamination. Cross beam ion sources have low sensitivity for the amount of gas consumed as they analyze a collimated gas stream from a small portion of the sampled gas using multistage pumping systems to strip off a majority of the sampled gas. This results in either a small sample gas signal, or the need for large, expensive pumping systems that consume high flows of sample gas.
The disclosed embodiments provide good sample gas analysis fidelity with extended life and improved analytical stability of mass spectrometers in the presence of contaminating gases. One example embodiment is an ion source that includes a gas source, nozzle, electron source, and electrodes. As used herein, the term nozzle means a gas flow delivering element with a relatively small outlet. The nozzle may be a tube or similar structure of any length, even zero. If the length of the nozzle is zero, then the nozzle can be in the form of an aperture in a surface. The gas source delivers gas through the nozzle to an evacuated ionization volume, and is at a substantially higher pressure than that of the evacuated ionization volume. Gas passing from the source through the nozzle freely expands in an ionization region of the ionization volume, the gas pressure quickly decreasing as the gas expands away from the outlet of the nozzle. The electron source emits electrons that pass close to the nozzle through the expanding gas in the ionization region to ionize at least a portion of the expanding gas. The electrodes create electrical fields for ion flow from the ionization region to a mass filter of a mass spectrometer and are located at distances from and, orientations to, the nozzle to limit direct exposure of the electrodes to the gas.
Another example embodiment is a mass spectrometer system that includes a vacuum pump, mass filter, detector, and ion source. The ion source includes a gas source, nozzle, electron source, and electrodes, as described above, where the electrodes of the ion source create electrical fields for ion flow from the ionization region to the mass filter. The nozzle of the ion source may be oriented to direct the gas from the gas source toward the vacuum pump.
In many embodiments, at least 20% of the gas molecules from the nozzle pass through the ionization region. In some embodiments, the electron source can be a heated filament. In such (or other) embodiments, the electron source can be arranged on an opposite side, with respect to the ionization region, of a first electrode. In such embodiments, electrons produced by the electron source travel through an aperture of the first electrode and toward the ionization region, resulting in an electron beam traveling through the expanding gas in the ionization region. In such embodiments, a second electrode can be arranged opposite the first electrode. The second electrode can include an aperture. Electrons travel through the ionization region and toward the second electrode, many of which may travel through the aperture, if included.
A trap electrode may be arranged opposite the first electrode with respect to the ionization region, and can measure at least a portion of the electron beam current flowing through the ionization region. In embodiments including the second electrode with aperture, the trap electrode can be arranged outside the second electrode with respect to the ionization region. A second electron source, which in some embodiments may be configured to function as a trap electrode, can be arranged outside the aperture in the second electrode. In some embodiments, the first electron source may be used as a trap electrode, for example, when operating a second electron source.
In many embodiments, the electrodes include first and second electrodes arranged on opposite sides of the ionization region, where the surfaces of the first and second electrodes are substantially parallel to a primary direction of gas flow from the nozzle through the ionization region. In such (or other) embodiments, a repelling electrode may repel ions from the ionization region toward the mass filter, and in such (or other) embodiments, an ion exit electrode having an aperture may direct the ion flow from the ionization region to the mass filter. The voltages applied to the various electrodes may be independently controllable.
In some embodiments, the outlet opening of the nozzle may have an area of five square millimeters or less. The area of the outlet opening of the nozzle can relate inversely to the pressure of the gas source for a desired gas flow, such that the area of the outlet opening of the nozzle can be much smaller if the gas source pressure is very high. In such (or other) embodiments, the cross-sectional area of the electron beam at the ionization region may be twenty square millimeters or less. In such (or other) embodiments, the electrodes may be located at least five millimeters from the nozzle center.
Another example embodiment is a method of producing ions for a mass spectrometer having a mass filter. The method includes delivering gas from a gas source to an evacuated ionization volume through a nozzle. The gas source is at a substantially higher pressure than that of the evacuated ionization volume, and gas passing through nozzle freely expands in an ionization region of the ionization volume. The method further includes emitting electrons close to the nozzle and through the expanding gas in the ionization region to ionize at least a portion of the expanding gas, and directing ions formed in the ionization region to the mass filter.
In some embodiments, directing the ions can be accomplished using electric fields created by electrodes, in which case delivering the gas to the evacuated ionization volume includes delivering the gas at distances from the electrodes to limit direct exposure of the electrodes to the gas. In such (or other) embodiments, directing the ions can include repelling the ions from the ionization region toward the mass filter and can include focusing the ions from the ionization region through an aperture to the mass filter. In such (or other) embodiments, emitting the electrons can include emitting electrons from a heated filament, and can include emitting electrons through an aperture of a first electrode on one side of the ionization region, through the expanding gas in the ionization region, and through an aperture of a second electrode on an opposite side of the ionization region.
The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments.
A description of example embodiments follows.
The disclosed apparatus (e.g., ion source), systems (e.g., residual gas analyzer), and methods provide extended life and improved analytical stability in the presence of contaminating gases, especially gases that deposit surface coatings, while achieving substantial preferential ionization of sampled gases over internal background gases. The disclosed apparatus, systems, and methods provide performance like a closed ion source, but without the short lifetime and unstable gas species sensitivities due to ion source contamination and surface charging. Thus, improved service intervals and operating costs and improved results without excessive recalibration are achieved.
According to an example embodiment, sample gas is introduced directly into an ionizer region of a mass spectrometer (e.g., residual gas analyzer) in its vacuum chamber via a nozzle, such as, for example, a small diameter tube, the length of which can be from arbitrarily long to zero (an aperture). The sample gas freely expands into the vacuum chamber. The tip of the nozzle is positioned close (e.g., adjacent or near adjacent), to an electron beam, where ions of the sample gas are formed near an entrance aperture of a mass filter (e.g., quadrupole). The end of the nozzle can be relatively small to limit interaction with the electron beam. Critical ionizer electrode surfaces are not directly in the dominant path of the expanding gas; thus, there is minimal exposure of those surfaces to the gas and any contaminants it might contain. Any surfaces that do receive direct gas exposure are sufficiently off axis of the gas path and/or relatively far from the point of the gas expansion such that the density of the gas at these surfaces is less than, for example, 1/30th the density of the gas when in the nozzle. This reduces the rate of any surface film formation and any subsequent surface charging that can degrade effectiveness of the ion source. To further reduce the amount of sample gas that reaches any critical surfaces, the sample gas can be introduced in a direction toward the chamber's vacuum pump.
In the example ion source 100, the electron source 115 is a heated filament located outside of the ionization region 130 on the other side of electrode 120a and connected to electric leads 155a,b. The filament may be straight as shown, coiled, or have other forms as appropriate for the desired electron focusing. Electrons 135 produced by the filament 115 travel through an aperture 145a in electrode 120a, through the ionization region 130, and onto electrode 120b, as well as through aperture 145b in electrode 120b, on the other side of the ionization region 130. Electrodes 120a and 120b are arranged so that their surfaces are substantially parallel to a primary direction of gas flow 160 from the nozzle through the ionization region, which reduces the amount of gas that may be deposited on the electrodes 120a,b. While the primary direction of gas flow 160 is illustrated in
The voltages applied to electrodes 120a-d, 165, 170 and the nozzle 110 can be independently controlled to tune the performance of the ion source. The following describes example values and ranges of values for various components of the ion source 100. Electrode 120a (electron entrance) can have a voltage of +10V (within an example range of −20V to +25V). Electrode 120b (electron exit) can have a voltage of +10V (within an example range of 0V to +25V). The repelling electrode 120c can have a voltage of +12 V (within an example range of +5 V to +30V). The ion exit electrode 120d can have a voltage of +10V (within an example range of 0V to +25V). The nozzle 110 can have a voltage of +6V (within an example range of 1V to +20V). The extract lens electrode 165 can have a voltage of −37V (within an example range of −20V to −90V). The trap electrode 170 can have a voltage of +10V (within an example range of −110V to +30V). The filament 115 can have a voltage of −60V (within an example range of −10V to −110V), resulting in an example electron current 135 of 0.5 mA (within an example range of 0.005 mA to 3 mA). These example values and ranges are provided for illustrative purposes only, and not in a limiting fashion.
The ionization region can be considered a volume where the electrons pass through the sample gas that is freely expanding into the ionization volume, unconstrained by electrodes or other structures, and from which the generated ions are directed into the mass filter. Thus the shape of the ionization region is substantially defined in two dimensions by the cross-sectional height and width of the electron beam. In the third dimension, along a length of the electron beam, the ionization region can be limited by action of the focusing electric field around the nozzle, created by the electrodes, such that only those ions formed near the nozzle are transmitted efficiently through apertures 150 and 175. Electrons will encounter and ionize gas outside of the region determined by the electrodes, but the resultant ions are from lower density gas and not wanted in the mass filter. In one embodiment, the concentration of sample gas is at least two (preferably more) times that of the average concentration of all gases outside of the ionization region.
The ion source can be optimized for ionization of a sample gas as it flows into an ionization volume from a pressure (typically greater than 1E-4 Torr) that is higher than that in the ionization volume (typically less than 2E-5 Torr). In general, pressure in the ionization volume will be less than ⅕th of the pressure at the outlet of the nozzle, and preferably much less, for example less than 1/100th of the of the pressure at the outlet of the nozzle. The ion source can optimize ion formation in, and ion extraction from, the relatively small ionization region where the electron beam passes through the sample gas as it is expanding freely from, and close to, an aperture (nozzle) delivering the higher pressure sample gas to the ionization region of the ionization volume. It is preferred that the electron beam pass as close as reasonable to the nozzle without contacting the nozzle. With the closest edge of the ionization region being very close to the nozzle, preferably within five millimeters and more preferably closer than a millimeter for example, volumetric density of sample gas in the ionization region is higher than the average pressure in the ionization volume, and should generally be at least two times higher, and preferably greater than ten times or more in many circumstances, thereby creating more ions of sample gas molecules in the ionization region versus ionization of gas molecules in other areas of the ionization volume. The critical surfaces (e.g., electrodes) of the ion source that define the voltage fields for ion formation and extraction can be deployed off of the main axis of gas expansion, thereby reducing direct exposure to the sample gas. Minimizing such direct contact with the majority of the expanding sample gas reduces electrode contamination from the sample gas that can degrade ion source performance over time. This configuration also provides an ion stream for mass spectrometry that is predominately from the sampled gas, before it has interacted with any ion source surface, and has, therefore, little change due to surface reactions. Furthermore, as the sample gas is freely expanding from higher to lower pressure, there is minimal formation of ion-molecule species that would occur with ionization at higher pressure, such as, for example, in a conductance-limited ionization chamber. Therefore, a significant benefit of the disclosed ion source is the production of an ion stream that represents the sample gas with high fidelity, while minimizing performance degradation due to contamination from the sample gas. This is valuable for analyzing gases that are labile and that can form deposits on ion source surfaces.
Unlike a traditional open ion source, the beam of electrons provides ionization in a relatively small, select volume at the point of sample gas introduction. The disclosed ion source differs from traditional open ion sources, which are designed for ion formation and extraction from all gas in the ion source without preference for sample gas from a higher pressure before it has interacted with surfaces in the ion source. Operating at low pressure, a traditional open ion source can have a relatively low rate of degradation from sample interaction, but provides an ion stream with relatively low fidelity for the sample gas.
Unlike a closed ion source, the amount of sample gas that reaches critical surfaces is greatly reduced. A closed ion source has an ionization chamber with restricted exit conductance to hold the sample gas at a pressure higher than the average pressure in the mass spectrometer system. The disclosed ion source differs from closed ion sources, which are optimized for ion formation and extraction from sample gas at high pressure in a relatively closed volume, not freely expanding, and with a high degree of interaction with the ion source surfaces as well as ion-molecule formation. The disclosed ion source does not have a restricted-conductance ionization chamber to hold the sample gas at an elevated pressure, instead allowing the sample gas to expand, unrestricted. The ion stream from a closed ion source can provide a higher fidelity representation of the sample gas than that from an open ion source, but a closed ion source is susceptible to higher rates of degradation from sample gas interactions.
Unlike a cross-beam ion source, the entire sample gas flow is admitted through a nozzle for ionization in a higher pressure, free expanding region through which an electron beam passes close to the nozzle. This ion source differs from a cross-beam ion source, which ionizes from a collimated portion of a sample gas stream that is located far from a nozzle and higher gas pressure region, and which requires additional stages of pumping and collimation. The ion stream from a cross-beam ion source can have good sample gas fidelity and reduced surface contamination, but only as part of a larger, more complex analysis system with high gas pumping speeds. In contrast, the disclosed ion source uses a large portion of a much smaller flow of sample gas without the need for collimation, and is, therefore, much simpler and more compact, with lower cost.
In a specific example embodiment, sample gas can be admitted at about the same mass flow rate as for a closed source system (e.g., approximately 5E-4 Torr-liters/second), and the vacuum chamber pressure may be less than 2E-5 Torr. The pressure of the sample gas at, for example one millimeter, from the tip of the nozzle may be about three millitorr (typically between 0.1 and 30 millitorr), dropping as it expands away from the nozzle. Electron emission can be collimated into a focused beam, so that a much larger share of the current is engaged in useful ionization and primarily at the point of relatively high sample gas pressure near the nozzle. The pressure of the expanding gas at the center of the ionization region can be at least 5E-5 Torr, and the pressure of the gas when it reaches a critical surface can be at most 20% of that pressure. A typical mass filter (e.g., quadrupole), detector, and electronics can be used. In some embodiments, the active surfaces can be controlled independently to permit optimizing the tuning of the ion source to extend its operating life with respect to long-term contamination. To provide a relatively high local pressure in the ionization region with a total gas flow that can be accommodated by commonly available small turbomolecular vacuum pumps used to provide ionizer evacuation (typically less than 1E-2 Torr-liters/second), the gas emitter aperture (nozzle) can have an area less than five square millimeters, for example, with smaller values corresponding to high nozzle gas pressures. To minimize sample gas pressure throughout the ionizer, the sample gas stream can be directed toward the vacuum pump used for ionizer evacuation. To achieve useful operating life extension when sampling contaminating gases, the distances from the center of the gas nozzle to the nearest points of the electrode (or other) surfaces can be at least five millimeters, for example. To provide improved ionization of sample gas versus residual background gases, the cross sectional area of the electron beam can be well aligned between the apertures of the electrodes and be less than five times the area of the gas nozzle. To provide improved performance from the lowest pressure of sample gas from the gas source, the flow conductance of the gas path in the gas source can be greater than that of the area of the gas nozzle. To permit optimizing performance over the maximum operating life when sampling contaminating gases, the voltages on the electrodes may be independently and dynamically controllable, though improved performance relative to a closed ion source can usually be achieved with some of the electrodes being electrically preset and/or in common.
While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments encompassed by the appended claims. For example, the gas source may take different forms from that disclosed herein, and the nozzle may be of different shapes or dimensions than shown and described herein. The electron source may be any suitable electron source for generating electrons to travel though the ionization region containing freely expanding sample gas close to the nozzle. The electrodes may be of a different number, shape, or arrangement than shown and described herein, so long as the electrodes are largely out of the path of the expanding sample gas and direct the ions formed in the ionization region to a mass filter component. It will be understood by those skilled in the art that dimensions, areas, flows, and pressures of the various components can be outside the specific example ranges provided herein, and can depend on the particular application of the ion source.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2938116 | Benson et al. | May 1960 | A |
| 3505518 | Mitani | Apr 1970 | A |
| 4016421 | Hull | Apr 1977 | A |
| 4039828 | Pokar et al. | Aug 1977 | A |
| 4166952 | Colby | Sep 1979 | A |
| 4433241 | Boesl et al. | Feb 1984 | A |
| 4960991 | Goodley | Oct 1990 | A |
| 6777671 | Doroshenko | Aug 2004 | B2 |
| 6794644 | Park | Sep 2004 | B2 |
| 7034292 | Whitehouse et al. | Apr 2006 | B1 |
| 7091477 | Jolliffe et al. | Aug 2006 | B2 |
| 7495212 | Kim et al. | Feb 2009 | B2 |
| 7750312 | Corr et al. | Jul 2010 | B2 |
| 8084736 | Schneider et al. | Dec 2011 | B2 |
| 8232520 | Cristoni | Jul 2012 | B2 |
| 8296090 | Aoki et al. | Oct 2012 | B2 |
| 8450681 | Shaw et al. | May 2013 | B2 |
| 8536518 | Kozole | Sep 2013 | B2 |
| 8642946 | Splendore et al. | Feb 2014 | B2 |
| 8642949 | Makarov et al. | Feb 2014 | B2 |
| 8953145 | Kraus et al. | Feb 2015 | B2 |
| 8969795 | Wollnik et al. | Mar 2015 | B2 |
| 9006679 | Morokuma et al. | Apr 2015 | B2 |
| 9305759 | McEwen et al. | Apr 2016 | B2 |
| 9305762 | Covey et al. | Apr 2016 | B2 |
| 9343280 | Cousins et al. | May 2016 | B2 |
| 9502226 | Brown et al. | Nov 2016 | B2 |
| 9552973 | McEwen et al. | Jan 2017 | B2 |
| 20040026614 | Bateman et al. | Feb 2004 | A1 |
| 20140374589 | Javaheri et al. | Dec 2014 | A1 |
| 20150144779 | Verenchikov | May 2015 | A1 |
| 20160163528 | Splendore et al. | Jun 2016 | A1 |
| 20160247669 | Tateishi | Aug 2016 | A1 |
| 20160293392 | Berkout | Oct 2016 | A1 |
| Number | Date | Country |
|---|---|---|
| 1364387 | Jan 2016 | EP |
| 3059756 | Aug 2016 | EP |
| 2012024468 | Feb 2012 | WO |
| Entry |
|---|
| Gardner, et al., “Infrared Multiphoton Dissociation of Peptide Cations in a Dual Pressure Linear Ion Trap Mass Spectrometer”, Dept. of Chemistry and Biochemistry, The University of Texas at Austin, Anal. Chem., 2009, 81 (19), pp. 8109-8118. |
| Neidholdt, et al., “Compact Ambient Pressure Pyroelectric Ion Source for Mass Spectrometry”, Arthur Amos Noyes Laboratory of Chemical Physics and the Beckman Institute, Anal. Chem., 2007, 79 (10), pp. 3945-3948. |
| Ionoptika, Ion Beam Technology “IOG C60-10 Ion Beam System”, 2015, pp. 1-3, http://www.ionoptika.com/products/products/ion-beams/c60/iog-c60-10-ion-beam-system. |
| The University of Manchester, School of Chemistry, “Commercialisation of a ‘gentle’ ion beam—new analysis for biolabs”, pp. 1-4, retrieved Jun. 29, 2017. http://www.chemistry.manchester.ac.uk/our-research/research-impact/sims/. |
| XPS Technical Bulletin, “PHI Model 06-C60 Sputter Ion Gun”, 2011, Physical Electronics USA, Inc., pp. 1-12. |
| International Search Report and Written Opinion for International Application No. PCT/US2018/036523, entitled “Robust Ion Source”, dated Aug. 27, 2018. |
| Helm et al., “Electron-impact ionization of Ar2, ArKr, Kr2, KrXe and Xe2”, Physical Review A, American Physical Society, vol. 19, No. 6, Jun. 1, 1979, pp. 2154-2160. |
| Crawford, “Mass Spectrometer Ion Source for Ionication Cross-Section Measurement”, Journal of Vacuum Science and Technology, No. 5, No. 5, Sep. 1, 1968, pp. 131-140. |
| Hendricks et al., “Anion solvation at the microscopic level: Photoelectron spectroscopy of the solvated anion clusters, N0-(Y)n, where Y=Ar, Kr, Xe, N20, H2S, NH3, H20, and C2H4(0H)2”, Journal of Chemical Physics, vol. 116, No. 18, May 8, 2002, pp. 7926-1938. |
| Number | Date | Country | |
|---|---|---|---|
| 20180358217 A1 | Dec 2018 | US |
