Abstract:
A compact high-performance mass spectrometer includes an ion source, an ion filter, a collision cell, a fragment filter, and an ion detector, along with one or more ion deflectors and one or more gas removal rings. An ion deflector allows a straight ion filter and a straight collision cell to be coupled in a folded configuration to make a compact design without the loss of performance associated with the use of curved quadrupole components. A gas removal ring, located proximate to an ion path aperture of the collision cell, allows an ion path aperture to be large for high sensitivity while minimizing performance degradation associated with the tendency of collision cell gas to escape via the collision cell ion path apertures to enter the high vacuum region and the detector.

Description:
FIELD OF THE INVENTION 
     The present invention relates to mass spectrometers that use electrodynamic assemblies as mass filters, and in particular to tandem mass spectrometers that use multiple quadrupole mass filters. 
     BACKGROUND OF THE INVENTION 
     A traditional tandem mass spectrometer that uses multiple quadrupole mass filters comprises an ion source, a first mass analyzer, a collision cell, a second mass analyzer, and ion detector, typically laid out in a straight line. Since the quadrupole mass filters are generally 0.2 to 2 cm in diameter and 5 to 30 cm long, this straight line arrangement tends to produce an elongated mass spectrometer of one or more meters in length. 
     Mass spectrometry is an analytical technique for determining the composition of compounds present in a sample. In a single stage mass spectrometer using a quadrupole mass filter, compounds in a sample are ionized, accelerated and focused to form a stream or beam of ions that enters a first quadrupole mass filter. Appropriate adjustment of the alternating and constant voltages applied to the first or only quadrupole mass filter allows the user to select which ionic species are transmitted through the filter. Ions emerging from the filter are detected and converted to electrical impulses or current by known means such as an electron multiplier. Rapid scanning of these voltages further allows the user to produce a spectrum of the ionic species corresponding to the sample compounds. 
     For some purposes, it is useful to select a first ionic species in a first filter, fragment the ions of the first ionic species to produce ionized fragments, and then analyze the ionized fragments with a second quadrupole mass filter. The ionized fragments are typically detected by known means such as an electron multiplier. For example, analyzing these fragments can aid in elucidating the structure of an unknown molecule. One method of producing such fragments is through a technique of accelerating the selected ion to an energy between 2-100 eV and inducing collisions between sample ions and inert molecules in a collision cell. 
     Collision cells are well known in the art. A typical collision cell comprises a housing at an elevated pressure (10 −4  to 10 −2  Torr of Argon or Xenon) containing a set of parallel rods to which alternating electric potentials are applied. These potentials serve only to help contain and focus the sample and fragment ions. Sample ions enter at one end of the collision cell and fragment ions, and any remaining sample ions, emerge from the other. 
     Mass filters and collision cells typically employ electrodynamic assemblies to impose alternating electric fields on ions and fragments. These electrodynamic assemblies typically comprise an even number of electrodes arranged about a central axis. Ions travel about the axis where the ions and/or fragments are subjected to electric fields. Electrodynamic assemblies which employ four electrodes are known in the art as quadrupoles. Although cylindrical electrodes are common, the electrodes may assume a variety of shapes. Quadrupole assemblies are generally described by Paul et al. in U.S. Pat. No. 2,939,952. 
     In a quadrupole mass filter and in a collision cell, each electrode is typically 0.2 to 2.0 cm in diameter and 5 to 30 cm long. One common configuration of a quadrupole mass filter based mass spectrometer employs three quadrupole assemblies, each arranged in line about a common central axis to allow ions to travel a substantially straight path. The arrangement, simple in execution, requires a substantial amount of linear space. A linear space of one or more meters in length is frequently required. 
     An example of a prior art tandem mass spectrometer is the PE-Sciex model API 300, manufactured by PE-Sciex, Thornhill, Ontario, Canada. FIGS. 15A and 15B provide a schematic view and a cut away elevation view, respectively, of this instrument. The ion path elements of the API 300 mass spectrometer are laid out in a straight line. As illustrated in FIG. 15A, the ion path elements of the model API 300 include an ion source  101 , an ion filter  102 , a collision cell  103 , a fragment filter  104  and an ion detector  105 . The carriage assembly which supports most of the ion optics components is illustrated in FIG.  15 B. The carriage assembly is enclosed in a high vacuum chamber (not shown). A detailed description of a tandem mass spectrometer of this type is provided in U.S. Pat. No. 4,234,791, issued Nov. 18, 1980, to Enke et al. 
     It is desirable to make mass spectrometers that are more compact and that have higher performance. Many prior art mass spectrometers are elongated and occupy too much space on a typical laboratory bench. Some attempts to design a compact mass spectrometer have been made. Bear Instruments, Inc., of Santa Clara, Calif. used a curved first quadrupole filter (analyzer), a curved collision cell, and a curved second quadrupole filter (analyzer). A tandem mass spectrometer using this approach is disclosed in U.S. Pat. No. 5,559,327 to Steiner. The instrument was marketed by Bear Instruments, Inc. as “Bear Cub 800”. However, the introduction of curvature into a quadrupole filter was found to adversely affect stability of the quadrupole filter, thereby limiting the resolution of the mass spectrometer. Subsequently, Bear Instruments introduced the “Kodiak 1200 Quadrupole Mass Spectrometer”. This instrument uses straight filters in conjunction with a curved collision cell. The curved collision cell turns the ion beam through an angle of 180°. Although the curvature of a collision cell has less adverse effect on performance than curvature of a quadrupole filter the effect on the performance of the instrument and the costs of manufacture and alignment are not negligible. Accordingly, there is still a need for a compact high-performance mass spectrometer. 
     SUMMARY OF INVENTION 
     The present invention provides a compact high-performance mass spectrometer. To achieve a compact design with high performance, a preferred embodiment includes an ion deflector and a gas removal ring. The ion deflector allows a straight ion filter and a straight collision cell to be coupled in a folded configuration to make a compact design without the loss of performance associated with the use of curved quadrupole components. The gas removal ring, located proximate to an ion path aperture of the collision cell, allows an ion path aperture to be large for high sensitivity while minimizing performance degradation associated with the tendency of collision cell gas to escape via the collision cell ion path apertures to enter the high vacuum region and the detector. 
     As used herein, “compact” refers to occupying a small area of laboratory bench with emphasis on the mass spectrometer&#39;s, longest dimension. The longest dimension of a mass spectrometer is usually the sum of the lengths of the aligned components in the ion path trajectory. The longest components are typically the quadrupole components, i.e., the two ion filters and the collision cell. As used herein, “performance” refers to a combination of sensitivity and resolution. 
     The preferred embodiment of the mass spectrometer of the present invention includes an ion source, an ion filter, an ion deflector, a collision cell having a gas removal ring at each of its ends, a fragment deflector, a fragment filter and an ion detector. The collision cell includes a gas enclosure having an ion entry aperture and a fragment exit aperture. The ion source produces a stream of ions, each ion having a mass to charge ratio in accordance with its structure. The ion filter accepts ions from the ion source and selectively passes ions according to mass to charge ratio. Ions leaving the ion filter enter an ion deflector which deflects them through a first angle into the collision cell. In the collision cell, ions are fragmented to produce fragments. Fragments leaving the collision cell enter the fragment deflector which deflects them through a second angle into the fragment filter. The fragment filter selectively passes fragments according to mass to charge ratio. Fragments leaving the fragment filter enter the ion detector. 
     The preferred embodiment further includes an enclosure assembly defining an ion-path chamber, an ion source chamber and a components chamber. A first vacuum pump, having a high vacuum flange and a low vacuum flange, is mounted within the components chamber. The high vacuum flange is coupled to the ion-path chamber. The low vacuum flange is coupled to the ion-source chamber. 
     The preferred embodiment further includes a second vacuum pump, having a high vacuum flange and a low vacuum flange. The second vacuum pump is mounted within the components chamber. The high vacuum flange is coupled to the ion-path chamber. The low vacuum flange is coupled to the gas removal ring. 
     In the preferred embodiment the ion deflector and the fragment deflector each include an ion lens and an ion mirror, the ion lens located on the ion trajectory proximate to the ion mirror. 
     In the preferred embodiment the first angle and the second angle are both approximately 90°. 
     In alternative embodiments, either the ion deflector or the fragment deflector or both may include an energy analyzer tuned to effect a change in ion trajectory. In alternative embodiments, the first angle may be one angle and the second angle may be the same as the first angle, or a different angle. Either angle may be approximately 90° or approximately 180° or any angle between 90° and 180°. Less advantageously, either angle may be between 0° and 90°. 
     Another alternative embodiment of the mass spectrometer of the present invention includes an ion source, an ion filter, a first gas removal ring, a collision cell, a second gas removal ring, a fragment filter and an ion detector. The collision cell has a gas enclosure with an ion entry aperture and a fragment exit aperture. The first gas removal ring is proximate to the ion entry aperture. The second gas removal ring is proximate to the fragment exit aperture. Each gas removal ring is positioned to remove gas from a portion of ion trajectory proximate to an aperture of the collision cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of the preferred embodiment of a mass spectrometer according to the present invention. 
     FIG. 2 is a cross section elevation view of the embodiment of FIG. 1 across A—A. 
     FIG. 3 is a cross section plan view of the embodiment of FIG. 1 across B—B. 
     FIG. 4 is a partially cut away perspective view of the embodiment of FIG.  1 . 
     FIG.  5 . is a perspective view of the ion deflector of the embodiment of FIG.  1 . 
     FIG.  6 . is a plan view of the ion deflector assembly of the embodiment of FIG.  1 . 
     FIG. 7 is a perspective view of the ion lens of the embodiment of FIG.  1 . 
     FIG. 8A is a perspective view of a stand-alone gas removal ring. 
     FIG. 8B is a cross section front elevation view, across AA—AA and BB—BB in FIG. 8A, of the gas removal ring. 
     FIG. 8C is a cross section side elevation view, across AA—AA and CC—CC in FIG. 8A, of the gas removal ring. 
     FIG. 8D is a schematic representation of a portion of an alternative embodiment that includes the stand-alone gas removal rings of FIG.  8 A. 
     FIG. 8E is a schematic representation showing a portion of the mass spectrometer of the embodiment of FIG. 1, illustrating the integral gas removal rings of the collision cell. 
     FIGS. 9A and 9B show a first alternative embodiment of a mass spectrometer according to the present invention. 
     FIGS. 10A and 10B show a second alternative embodiment of a mass spectrometer according to the present invention. 
     FIGS. 11A and 11B show a third alternative embodiment of a mass spectrometer according to the present invention. 
     FIGS. 12A and 12B show a fourth alternative embodiment of a mass spectrometer according to the present invention. 
     FIG. 13 shows a fifth alternative embodiment of a mass spectrometer according to the present invention. 
     FIG. 14 shows a sixth alternative embodiment of a mass spectrometer according to the present invention. 
     FIGS. 15A and 15B are a schematic view and a cut away elevation view, respectively, of a prior art mass spectrometer having functionality similar to that of the present invention. 
     FIG. 16A is a schematic view of a spherical analyzer that may be used as an ion deflector in the present invention. 
     FIG. 16B is a cut away, perspective view of a prior art spherical mass analyzer that may be used as an ion deflector in the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a preferred embodiment of a mass spectrometer according to the present invention. In particular, it shows the arrangement of ion path components within the housing. For purposes of clarity, details of the supporting structure, the vacuum enclosure, vacuum components and electronic components are not shown. 
     Referring to FIG. 1, tandem mass spectrometer  10  is housed in an enclosure assembly  11 . Ion path components include ion source  14 , ion filter  15 , ion lens  47 , ion deflector  16 , collision cell  17  (with its first integral gas removal ring  31  and second integral gas removal ring  32 ), fragment deflector  23 , ion lens  48 , fragment filter  18 , and ion detector  19 . Note that all components except the ion source are in the high vacuum ion path chamber  13 , at approximately 4×10 −5  Torr. The low vacuum ion source chamber  21  is at approximately 8×10 −3  Torr. 
     Trajectory  12  is shown dotted, starting at ion source  14  and ending at ion detector  19 . The preferred trajectory passes along ion filter axis  37 , cell axis  38  and fragment filter axis  39 . Ions leaving ion source  14  enter ion filter  15  at ion filter entry end  24  and exit ion filter  15  at ion filter exit end  25  to pass through ion lens  47 . Ions deflected by ion deflector  16 , enter collision cell  17  (with its first integral gas removal ring  31  and second integral gas removal ring  32 ) at ion entry aperture  52 . In the collision cell large ions are induced to collide to form smaller ions referred to as fragments. These fragments exit collision cell  17  at second integral gas removal ring  32 , fragment exit aperture  53  before being deflected by fragment deflector  23 . Deflected fragments then pass through ion lens  48  and enter fragment filter  18  at fragment filter entry end  28 . Filtered fragments leave at fragment filter exit end  29  to be captured and detected by ion detector  19 . 
     Ion filter  15  is a quadrupole filter having four quadrupole rods. Two of these rods, quadrupole rods  34  and  35 , are shown in FIG.  1 . Fragment filter  18  is also a quadrupole filter having four quadrupole rods, as is collision cell  17 . Although collision cell  17  is in the high vacuum of ion path chamber  13 , the collision cell includes gas enclosure  51  which contains a gas suitable for collision-induced dissociation (e.g. a gas such as xenon or argon) at a pressure of approximately 1×10 −2  Torr. If this gas were to escape in significant quantities via ion entry aperture  52  and fragment exit aperture  53 , shooting out in both directions along the line of the ion trajectory, it would have an adverse effect on resolution and/or sensitivity. An advantage of the ion and fragment deflectors, as used in the present invention, is that the gas does not shoot directly into ion filter  15 , or directly into fragment filter  18 . Also, the adverse affect of escaping gas is further reduced by first gas removal ring  31  (shown in FIG.  1  and FIG. 8E) and second integral gas removal ring  32  (shown in FIG. 1 only). The use of a gas removal ring permits the use of a larger aperture which provides increased sensitivity. 
     FIG. 2 is a cross section elevation view of the preferred embodiment of tandem mass spectrometer  10 , across A—A, in FIG.  1 . FIG. 2 shows details of the supporting structure, the vacuum enclosure, the vacuum pumps and the vacuum delivery manifold of the preferred embodiment. Enclosure assembly  11  includes baseplate  61 , top cover  62  and bottom cover  63 . Baseplate  61  and top cover  62  define the high vacuum ion path chamber  13  and the low vacuum ion source chamber  21 . Ion source chamber  21  is further defined by first wall  72  and second wall  73 . Ion path components that can be seen in the ion path chamber, as shown in FIG. 2, are ion source  14 , collision cell  17  and ion detector  19 . Components chamber  54  contains first split-flow vacuum pump  55 , and second split-flow vacuum pump  56 . The first split-flow vacuum pump  55  and the second split-flow vacuum pump  56  are preferably turbomolecular drag pumps. Components chamber  54  also contains vacuum delivery manifold  57  and an electronics unit  58 . Delivery manifold  57  is coupled to the two gas removal rings in the assembled mass spectrometer. Preferably, top cover  62  and bottom cover  63  are castings. Suitable split-flow vacuum pumps, such as Pfeiffer turbomolecular drag pump model TMH 261-150-005, are available from Pfeiffer Vacuum Technology, Inc., Nashua, N.H. 
     Rigid baseplate  61  defines the orientation and alignment of all ion path components by means of a set of precision pins, holes and stops. The pins, holes and stops (not shown) are used to precisely locate the ion optics elements, including the deflectors, one to another so as to achieve a desired trajectory. 
     FIG. 3 is a cross section plan view of the components chamber of the preferred embodiment. It is a cross section view across B—B in FIG. 2 looking down. FIG. 3 shows the location of first split-flow vacuum pump  55 , second split-flow vacuum pump  56  and vacuum delivery manifold  57 . Each of split-flow vacuum pumps  55  and  56  has a high vacuum coupler and a low vacuum coupler. First high vacuum coupler  80  and first low vacuum coupler  81  of first split-flow vacuum pump  55  are coupled respectively to the high vacuum ion path chamber  13  and to the comparatively low vacuum of ion source chamber  21  (chambers  13  and  21  are both shown in FIG.  2 ). Second high vacuum coupler  82  is also coupled to the high vacuum ion path chamber  13  to augment the pumping of first split-flow vacuum pump  55 . Vacuum delivery manifold  57 , via low vacuum coupler  83  and low vacuum coupler  84 , pumps escaping gas from ion entry aperture  52  and fragment exit aperture  53 , respectively. 
     The mechanical structure of the housing of the preferred embodiment is shown in the partially cut away perspective view of FIG.  4 . It can be seen from FIG. 4 that baseplate  61  and top cover  62  define vacuum chamber  13 . Ion source chamber  21  is defined by baseplate  61 , top cover  62 , first wall  72 , and second wall  73 . It can also be seen that baseplate  61 , bottom cover  63  define components chamber  54 . Note ion exit aperture  74  in second wall  73 . Baseplate  61  has a groove shaped to accept vacuum gasket  65  and its ion source enclosure portion  66 . This gasket, in contact with the lower surface of top cover  62 , helps to maintain the high vacuum in ion path chamber  13  and the low vacuum in ion source chamber  21 . The gasket is held under mechanical pressure by nuts  89  on bolts  88 , the bolts passing through clamping bosses such as  71 ,  78  and  79 . First high vacuum aperture  67  and first low vacuum aperture  68  are vacuum couplers coupling the high vacuum (from first high vacuum coupler  80 ) and the low vacuum (from first low vacuum coupler  81 ) of first split-flow vacuum pump  55  (shown in FIG. 3) to ion path chamber  13  and ion source chamber  21 , respectively. Three holes in baseplate  61  of FIG. 4 are vacuum pass-through holes. The three holes are high vacuum pass-through hole  69  (associated with the ion path chamber) and the two low vacuum pass-through holes  70 . High vacuum pass-through hole  69  passes high vacuum from second high vacuum coupler  82  to exhaust ion path chamber  13 . Holes  70  pass low vacuum from a second low vacuum coupler ( not shown) of second split-flow vacuum pump  56  via vacuum delivery manifold  57  to exhaust the two integral gas removal rings  31  and  32  of FIG.  1 . 
     Ion deflector  16  of the preferred embodiment is shown in detail in FIG.  5 . The deflector includes a grid  45  and a stainless steel repeller plate  46 , the grid mounted to the plate by insulating standoffs. In use the grid and the plate are maintained at different electrical potentials, V 1  and V 2 , respectively. In the preferred embodiment V 1 =0 volts, i.e. the grid is grounded. V 2  must be greater than the incident ion energy measured in electron volts for singly charged ions. In the preferred embodiment, with incident ion energy in the range 1-5 eV, V 2  is set to 10 volts. The grid is preferably #MN-20 nickel mesh 90.1 lpi 90.3% transmission, available from Buckbee-Mears, Inc. of St. Paul, Minn. FIG. 6 is a plan view of ion deflector assembly  50 . FIG. 6 shows ion deflector  16  and ion lens  47  both mounted to ion deflector base  44 . The grid and the plate are typically separated by a distance of approximately 3 mm. 
     FIG. 7 is a perspective view of ion lens  47 , the main component of which is electrically conducting tube  49 . Preferably, tube  49  is made of stainless steel. 
     FIG. 8A is a perspective view of an embodiment of a stand-alone gas removal ring  122 . FIG. 8B is a cross section front elevation view of the gas removal ring, viewed across AA—AA and BB—BB of FIG.  8 A. FIG. 8C is a cross section side elevation view of the gas removal ring viewed across AA—AA and CC—CC of FIG.  8 A. The gas removal ring is formed of two half-rings  90 , each having a central circular ion-pass-thru hole  91 . A cylindrical distribution cavity  94 , coaxial with the two ion-pass-thru holes  91 , is formed by cylindrical cavities in the inner face of each of the two half-rings  90 . Distribution cavity  94  (shown in FIG. 8C) forms a pumping passageway to gas exhaust hole  93  and gas exhaust coupler  92  for gas molecules entering an ion-pass-thru hole  91  from the collision cell. For precise mounting of the gas removal ring to baseplate  61  (shown in FIG.  4 ), half-rings  90  are mounted to a common base  96 . FIG. 8D is a schematic representation of a portion of an alternative embodiment that includes the stand-alone gas removal rings  122  of FIG.  8 A. 
     In the preferred embodiment of FIG. 1, the collision cell includes an integral gas removal ring at each end. This principle is illustrated in FIG. 8E which shows the ion entry end of a collision cell  17  having a first integral gas removal ring  31 . A similar integral gas removal ring (shown as  32  in FIG. 1) is provided at the ion exit end of the collision cell. Integral gas removal ring  31  is shown in FIG. 8E as being formed as a hollow wall having an outer enclosure portion  124  which is essentially an extension of collision cell gas enclosure  123 . The hollow wall has first and second walls  125  and  126  respectively and a transverse axis corresponding to trajectory  12 . Enclosure portion  124  and the two walls define a cylindrical distribution cavity  127 . Each of walls  125  and  126  has a hole at its center. Ion entry aperture  52  and ion-pass-thru hole  129  of integral gas removal ring  31  constitute the ion entry aperture of the collision cell. Outer enclosure portion  124  includes gas exhaust coupler  130 . 
     Additional detail of how to make and use a mass spectrometer of this general type can be found in U.S. Pat. No. 4,234,791, issued Nov. 18, 1980, to Enke et al. 
     A first alternative embodiment of a mass spectrometer according to the present invention is shown in FIGS. 8A and 8D. This embodiment includes stand-alone gas removal rings. 
     A second first alternative embodiment of a mass spectrometer according to the present invention is shown in FIGS. 9A and 9B. This embodiment includes an ion source, an ion filter, an ion deflector, a collision cell, and an ion detector. In this embodiment, the ion deflector turns the ions through an angle of approximately 180°. 
     A third alternative embodiment (not shown) replaces the ion filter, ion deflector and collision cell of FIGS. 9A and 9B with a collision cell, fragment deflector and fragment filter, respectively to provide a mass spectrometer having a compact form similar to that of FIGS. 9A and 9B. This embodiment includes an ion source, a collision cell, a fragment deflector, a fragment filter, and an ion detector. In this embodiment, the fragment deflector turns the fragments through an angle of approximately 180°. 
     A fourth alternative embodiment of a tandem mass spectrometer according to the present invention is shown in FIGS. 10A and 10B. This embodiment includes an ion source, an ion filter, an ion deflector, a collision cell, a fragment filter, and an ion detector. The ion deflector turns the ions through an angle of approximately 180°. 
     Fifth and sixth alternative embodiments of a tandem mass spectrometer are shown in FIGS. 11A &amp; 11B and FIGS. 12A &amp; 12B, respectively. These embodiments both include an ion source, an ion filter, an ion deflector, a collision cell, a fragment deflector, a fragment filter, and an ion detector. In each embodiment ion deflector  40  turns the ions through an angle of approximately 180°, and fragment deflector  41  turns the fragments through an angle of approximately 180°. 
     Seventh and eighth alternative embodiments of a tandem mass spectrometer are shown in FIGS. 13 and 14, respectively. In both of these embodiments both the ion deflector ( 111  and  113 , respectively) and the fragment deflector ( 112  and  114 , respectively) deflect through an angle between 90° and 180°. 
     Other embodiments of a tandem mass spectrometer may use an energy analyzer, such as a spherical or radial cylindrical analyzer, as an ion deflector or as a fragment deflector. FIG. 16A is a schematic view of a first spherical analyzer (prior art) that may be used. FIG. 16B is a cut away, perspective view of a second spherical mass analyzer (prior art) that may be used. Details of the operation and construction of energy analyzers is found in “Building Scientific Apparatus—A Practical Guide to Design and Construction” at page 309-312. (“Building Scientific Apparatus”, 1983, John H. Moore, Addison-Wesley Publishing Company, Inc., Reading, Mass.).