Means for reducing the contamination of mass spectrometer leak detector ion sources

Abstract: In a mass spectrometer leak detector an ion source assembly is disposed in a cavity of a vacuum envelope and is surrounded by a thin removable liner which has means for positioning this liner within the cavity. The liner positioning means limit the thermal contact of the liner with the vacuum envelope and due to heat-radiation relationship of the liner with the ion source the temperature of the ion source region increases and contamination of the ion source decreases.

Description: Field of the Invention The present invention relates to a mass spectrometer leak detector, and more particularly to means for reducing the contamination of the ion sources used in mass spectrometer leak detectors. Background of the Invention Mass spectrometer leak detection is a well developed technique useful in a wide range of applications. The leak detector consists of a mass spectrometer set to detect a tracer gas, the apparatus for maintaining a vacuum around the mass spectrometer and for conducting the tracer gas into the mass spectrometer, and appropriate electronics to provide control voltages and to detect and display the signal. Helium is typically used as the tracer gas. The leak detector is usually designed to produce a signal with a magnitude proportional to the leak rate of the tracer gas into the leak detector. The mass spectrometer used in leak detectors is commonly of the type known as a magnetic sector mass spectrometer. In this type of mass spectrometer, the ions derived from the tracer gas and other gases within the vacuum envelope are formed into a beam by acceleration through a fixed electric field. The ion beam is directed through a region of constant magnetic field which causes a deflection of the ions which is dependent on the ion mass. The electric and magnetic fields are so arranged that only one ion mass characteristic of the tracer gas reaches the ion detector. A leak detector mass spectrometer of this type is described in U.S. Patent 3,277,295 issued to W. E. Briggs on October 4, 1966. The production of ions for analysis by a mass spectrometer in a leak detector is commonly accomplished by passing a current of electrons through the gases within the leak detector vacuum envelope. The means for producing ions is called the ion source. Briggs gives a detailed description of a typical ion source now used in mass spectrometer leak detectors. The electrons needed for the production of ions are obtained by passing an electric current through a filament of refractory metal, such as rhenium, iridium, or tungsten, thereby heating it to incandescence. By means of appropriate electrical potentials applied to the various parts of the ion source, the electrons are accelerated and guided into a region where ion formation is desired. This region is designed to facilitate the extraction and collimation of the ions formed therein into a focused beam for mass analysis in the mass spectrometer. The ion source is the part of a mass spectrometer leak detector which commonly requires the most maintenance. In operation, the mass spectrometer leak detector measures the amount of tracer gas entering the vacuum enclosure from the system or component under test. Other vapors may also enter the vacuum enclosure and the ion source as well, either from the test system, or from the vacuum pumps of the leak detector, or from other sources. These extraneous vapors are subject to decomposition within the ion source by virtue of the presence of the incandescent filament, by the action of the ionizing electrons, and by interaction with ions formed from all of the gases and vapors within the ion source. The reaction of the decomposition products with themselves and with the undecomposed vapors produces deposits on the surfaces of the ion source and the surrounding vacuum enclosure. The action of ions and electrons on vapors adsorbed on the surface of the ion source components and on the surfaces of the vacuum envelope surrounding the ion source also causes the formation of deposits on these surfaces. When these deposits become thick enough, they begin to acquire electrical charges from the impact of ions and electrons. These charges affect the motion of the desired ions from the tracer gas and cause the sensitivity of the leak detector to decrease. The ion source region of a leak detector mass spectrometer is provided with a collimating aperture through which ions pass as they enter the mass analyzer. This aperture serves to prevent ions with incorrect trajectories from entering the mass analyzer and contributing to unwanted noise at the detector. The area around this ion beam limiting aperture is especially subject to contamination because of the high intensity of ion bombardment of the metal surface. When the ion source of a mass spectrometer leak detector becomes so contaminated that maintenance is required, it is common practice to replace the entire ion source assembly with a new or rebuilt unit in order to save time and labor costs. However, it is still necessary to disassemble the mass spectrometer and clean the portion of the interior of the vacuum envelope which surrounds the ion source. One or more hours may be required to restore the leak detector to full operation. It is desirable to reduce the frequency of the required ion source replacement and to reduce the time required to return the leak detector to operating condition. It is well known that the formation of deposits on surfaces in a mass spectrometer may be reduced by heating the surfaces to reduce the amount of vapors adsorbed. There are several approaches to providing the required heating, including the provision of a separate heater within or around the ion source vacuum enclosure. In leak detector mass spectrometers such as described in Briggs, the entire vacuum enclosure is machined from a single piece of metal, and heating the ion source unavoidably also heats the detector part of the mass spectrometer. Because elevated temperatures increase the amplitude of the noise and drift of the electrometer circuits commonly used in the detector stages of mass spectrometer leak detectors, any approach which applies additional heat to the ion source is of limited utility in this design. U. S. Patent No. 3,155,826 "Mass Spectrometer Leak Detector Including a Novel Repeller-Heater Assembly" describes an approach in which the heater is incorporated into one of the components of the ion source. The repeller is heated to about 370°C (700°F) to minimize contamination of the ion source chamber. U. S. Patent No. 4,816,685 describes a mass spectrometer ion source with a circular filament for producing electrons and a circular ring completely surrounding the filament. The ring is electrically connected to the filament to suppress unwanted secondary electron emission, and the ring absorbs heat from the filament. The heat absorbed by the ring may be conducted through the electrically insulated mounting structure to other components of the ion source to reduce contamination of these components. U. S. Patent No. 3,723,729 addresses the problem of mass spectrometer ion source contamination by disclosing an ion source which can be very rapidly replaced with out opening the mass spectrometer vacuum enclosure. The ion source described can be both heated and cooled as required. EPA-A-0 060 075 discloses an ionizer having an interchangeable ionization chamber so that the ion volume can be optimized for impact ionization or chemical ionization. The arrangement also makes it possible to maintain clean ionization volumes or chambers. Summary of the Invention It is an object of the present invention to provide a means of increasing the temperature of the ion source region of a mass spectrometer leak detector without adding additional heat to the mass spectrometer. It is another object of the present invention to reduce the contamination of the leak detector ion source. It is a further object of the present invention to reduce the maintenance time of the mass spectrometer leak detector. The present invention provides a mass spectrometer leak detector as set out in claim 1. The present invention also provides a method of reducing the contamination of an ion source region of a mass spectrometer leak detector as set out in claim 11 and a method of using a liner for this purpose as defined in claim 13. In one embodiment the liner is of the same shape as the interior of the vacuum envelope, but very slightly smaller in every dimension. Preferably the ion beam limiting aperture is an opening of the appropriate size, shape, and location in the liner. The construction of the liner is such that while it cannot move from its desired location inside the ion source vacuum enclosure, it is none the less in contact with the vacuum enclosure in a limited number of places and only over a minimal area. Much of the heat radiated by the incandescent filament and/or by an additional heater is captured by the liner. Because of the thermal isolation of the liner, it is heated to a high temperature (100°C-200°C) by the radiated heat. The high temperature of the liner reduces the contamination of the liner and the ion source region. If the ion beam limiting aperture is incorporated into the liner, then the contamination of the area around the aperture is reduced by the higher temperature. When replacement of the ion source finally becomes necessary, the contamination which would otherwise have been deposited on the vacuum system enclosure is now on the liner, which can be discarded and replaced along with the ion source. Embodiments of the invention will now be described with reference to the accompanying drawings in which:- Figure 1 is an exploded view of an ion source and a portion of the mass spectrometer vacuum envelope including the ion source housing liner according to the present invention. Figure 2 is a cross-sectional view of the ion source assembly inside the vacuum envelope according to the present invention. Figure 3 A through 3 C are views of three different embodiments of the present invention showing different means of preventing thermal contact between the liner and the vacuum enclosure of the ion source. In Figures 1 and 2, vacuum envelope 10 with cavity 17 of the leak detector mass spectrometer is machined from a solid block of metal, such as Al. Ion source assembly 11 is built onto multi pin tube header 18 and is sealed to vacuum envelope 10 with an elastomer O-ring (not shown) and held in place with retaining plate 14. A typical ion source of this construction is described more fully in US Patent 3,277,295 together with a typical leak detector mass spectrometer. In Figure 2, the ion source is shown in schematic form including two identical filaments 16, either of which emit electrons when heated by an appropriate electric current. The electrons enter a box formed by two shaped electrodes: repeller 19 and ion chamber box 14. Electrons from either of thermionic filaments 16 enter the ion source box along a path controlled by electrical bias voltages and/or magnetic fields derived from well known means not shown. Ion source liner 12 of the present invention together with ion beam limiting aperture 20 in ground plate 13 fit inside cavity 17. Ion source assembly 11 fits inside liner 12. The liner is preferably constructed of thin conducting or semi-conducting material. For example, liners have been successfully constructed from a 0.015mm (0.006 inch) thick sheet of stainless steel. The gap which separates the liner and vacuum enclosure is in the order of about 0.5 mm. The total area of contact between the liner and the vacuum envelope is less than 1% of total surface area of the liner. In one preferred embodiment, the plate containing the ion beam limiting aperture is attached to the end of liner 12 or is an integral part of the liner. Thus liner 12 and the ground plate containing the ion beam limiting aperture may be attached together or they may be combined into a single part with a shape similar to a straight-walled cup. Regardless of the details of the construction of the parts, in accordance with this invention, liner 12 and/or ground plate 13 are held in position by mechanical means which limit the thermal conductivity between liner 12 and/or ground plate 13 and the mass spectrometer vacuum envelope 10. Figures 3A through 3C show examples of mechanical means for positioning the inventive liner while limiting the thermal contact with the vacuum envelope. In Figure 3A, multiple, outward protruding, dimples pressed into the sheet of material from which the liner is formed allow contact between the liner and the vacuum envelope over a very small fraction of the liner surface area. In Figure 3B, either an outward extending crease or a piece of thin wire spiraled around the liner allow contact over only a small area. In Figure 3C, small sharp tabs formed with a simple cut from the thin sheet material of the liner support the liner at a small number of points of contact with the cylindrical vacuum envelope. While the inventive ion source vacuum envelope liner has been described here with reference to a cylindrical ion source vacuum envelope, a liner of any shape which surrounds the ion source inside the vacuum envelope, regardless of the shape of either the liner or the vacuum envelope, would be within the scope of the invention as defined by the appending claims. While the invention has been described in terms of a liner supported by contact with the ion source vacuum envelope, other means of support which position the liner around the ion source would be possible.

Claims: A mass spectrometer leak detector comprising a vacuum envelope (10) having a cavity (17) and an ion source (16) within said cavity characterized by a liner (12) for said envelope fitting in said cavity and substantially surrounding said ion source, the liner being mounted in said cavity such that thermal contact between the liner and the envelope is limited, and the liner being arranged such that it is heated by said ion source in use, for reducing contamination of the ion source region. A detector as claimed in claim 1 wherein said liner (12) is slidably supported within said cavity (17) of said envelope (10). A detector as claimed in claim 1 or 2 wherein the liner (12) is mounted in said cavity (17) of said envelope (10) by means comprising multiple dimples pressed into the wall of said liner (12), said dimples protruding outwardly towards said envelope (10). A detector as claimed in claim 1 or 2 wherein the liner (12) is mounted in said cavity (17) of said envelope (10) by means comprising at least one crease in said liner (12), said crease extending outwardly towards said envelope (10). A detector as claimed in claim 1 or 2 wherein the liner (12) is mounted in said cavity (17) of said envelope (10) by means comprising multiple sharp tabs formed on the liner (12) to minimise contact of said liner with said envelope (10). A detector as claimed in any one of claims 1 to 5 wherein said liner (12) comprises semiconductor material. A detector as claimed in any one of claims 1 to 5 wherein said liner (12) comprises conducting material. A detector as claimed in claim 7 wherein said liner (12) comprises stainless steel. A detector as claimed in any one of claims 1 to 8 wherein said liner (12) comprises a cylindrical wall with a bottom plate (13) attached to said wall, the bottom plate being formed with an exit slit (20) for ions from said source (16). A detector as claimed in any one of the preceding claims wherein said ion source (16) is replaceably mounted within said cavity (17) of said envelope (10). A method of reducing the contamination of an ion source region provided within a cavity of the vacuum envelope (10) of a mass spectrometer leak detector, the method comprising providing a liner (12) for the envelope which fits in said cavity and substantially surrounds an ion source (16), the liner being mounted in the cavity such that thermal contact between the liner and the envelope is limited, and the liner being arranged such that it is heated by said ion source in use. A method as claimed in claim 11, wherein the mass spectrometer leak detector is as claimed in any one of claims 1 to 10. Use of a liner (12) substantially surrounding the ion source (16) of a mass spectrometer leak detector, which liner is mountable in a cavity (17) of the vacuum envelope (10) of the mass spectrometer leak detector so as to fit in said cavity and to have limited thermal contact with the vacuum envelope (10) and which liner is arranged such that it is heated by the ion source in use, for reducing contamination of the ion source region. Use as claimed in claim 13, wherein the mass spectrometer leak detector is as claimed in any one of claims 1 to 10.

Inventors: BUTTRILL SIDNEY, BUTTRILL, SIDNEY
Applicants: VARIAN INC, VARIAN, INC.
Application year: 1995
Main classification: H01J49
IPC codes: H01J49, G01N27, G01M3