BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a mass spectrometer using plasma as anion source, and more particularly, to amass spectrometer having an ion deflector lens. 
         [0003]    2. Description of the Related Art 
         [0004]    In a mass spectrometer using plasma as an ion source, such as an inductively coupled plasma mass spectrometer (ICP-MS) or a microwave plasma mass spectrometer (MIP-MS), it has been required that photons and neutral particles with high energy, which cause background noises, be separated from ion beams (for example, refer to JP 10-97838 A (FIG. 10), JP 2004-71470 A (FIG. 1), JP 8-7829 A (Paragraph [0009]), JP 2002-525821 A (FIG. 1), JP 61-107650 A (FIGS. 1 and 2), JP 2000-67805 A (FIG. 1), and JP 2000-100375 A (FIG. 2)). 
         [0005]    Conventionally, photons and neutral particles which cause the background noises are separated from ion beams by an aid of a tilted or bent ion guide (for example, refer to JP 10-97838 A). Hereinafter, the tilted or bent ion guide is generally called “nonlinear ion guide.” Photons and neutral particles are also separated from ion beams by an aid of a cylindrical ion deflector lens. The cylindrical ion deflector lenses are of a configuration symmetrical with respect to a point (for example, refer to JP 2004-71470 A) or of a configuration asymmetrical with respect to a point so as to provide an angle between before and after the ion deflector lens in an ion flying direction (for example, refer to JP 8-7829 A). Herein, the point asymmetrical means asymmetrical with respect to a point. Photons and neutral particles are also separated from ion beams by means of an ion mirror (for example, refer to JP 2002-525821 A and JP 2000-67805 A). 
         [0006]    When the nonlinear ion guide is used for separation of photons and neutral particles, ion beams cannot be precipitously deflected by the nonlinear ion guide. Therefore, when an interval between an aperture arranged in front of the nonlinear ion guide and an aperture arranged at the other end of the ion guide is narrow, because the deviation quantity of ion beam becomes small, blocking of photons and neutral particles in the nonlinear ion guide is not sufficient. In addition, an input part and an output part of the tilted ion guide as well as a bent portion of the bent ion guide are low in the transmittance of ion. 
         [0007]    Further, in the case of using an ion deflector lens for separation of photons and neutral particles, when the deviation quantity of ion beam is reduced, it is necessary to also reduce the diameter of the ion deflector lens. As a result, an aberration of the deflected ion beam is increased, and the transmittance of ions is reduced. Besides, the ion deflector lens that provides an angle between upstream and downstream of an ion flying direction is a factor for increasing the difficulty in the manufacture of the front and rear ion optical systems, and also a factor for increasing the size of a mass spectrometer. 
         [0008]    Further, in the case of using an ion mirror for separation of photons and neutral particles, a deflector mechanism for the ion beam is complicated and large in size. Further, in order to set the transmittance of ion beam to a practical value, a plurality of electrode voltages are required to be adjusted. 
       SUMMARY OF THE INVENTION 
       [0009]    In view of the above-mentioned circumstances, They are required (1) to provide a plasma ion source mass spectrometer with an ion deflector lens having an improved removal ratio of photons and neutral particles as compared with the conventional art while the ion transmittance is maintained, and (2) to downsize the ion deflector lens and the mass spectrometer. 
         [0010]    In order to achieve the above-mentioned object, a mass spectrometer which uses plasma as an ion source, includes an ion deflector lens, in which the ion deflector lens includes: an input side plate-like electrode having one aperture; an output side plate-like electrode having one aperture; and at least one tubular electrode disposed between the input side plate-like electrode and the output side plate-like electrode, in which the input side plate-like electrode and the output side plate-like electrode face each other so that axes of the respective apertures are displaced from each other, and in which the tubular electrode is of a point asymmetric shape, and arranged so that a center axis of the tubular electrode is closer to an axis of travel of ions upstream of the input side plate-like electrode than an axis of travel of ions downstream of the output side plate-like electrode. It should be noted that the tubular electrode may have such a potential and configuration that the axis of travel of ions downstream of the output side plate-like electrode is substantially in parallel to the axis of travel of ions upstream of the input side plate-like electrode. Further, the tubular electrode may have a configuration in which a portion including at least a part of an input side end of the tubular electrode is removed from a complete cylinder. Further, the tubular electrode may have a configuration in which only the portion including the at least a part of an input side end of the tubular electrode is removed from the complete cylinder. Further, as a desirable configuration for the asymmetrical tubular electrode, there may be provided a configuration in which the complete cylinder is equally divided into four pieces by a virtual plane including the center axis of the complete cylinder and a virtual plane perpendicular to the center axis, and one of the four pieces, which is on an input side, is removed from the complete cylinder. It should be noted that the complete cylinder means a cylinder whose cross-sectional shapes taken along planes perpendicular to the central axis at any positions in the central axial direction of the cylinder are identical with each other. 
         [0011]    The tubular electrode may be formed of a single cylindrical member or a plurality of cylindrical members. When the tubular electrode is formed of the plurality of cylindrical members, the cylindrical members may be formed by dividing the tubular electrode by planes perpendicular to the axis. Further, a cross section perpendicular to the axis of the tubular electrode may be circular, elliptical, or other line-symmetric shapes. The tubular electrode having a circular cross section is preferable from the viewpoint of ease of manufacture. 
         [0012]    As the ion source, there may be provided ion sources such as inductively coupled plasma (ICP) or microwave induced plasma (MIP). In particular, high-frequency inductively coupled plasma, which is generated at atmospheric pressure, is preferable. 
         [0013]    The ion deflector lens may be disposed upstream of a mass filtermass filter and downstream of another ion optical system having a negative potential. Alternatively, the ion deflector lens may be arranged immediately upstream of the mass filtermass filter. 
         [0014]    According to the present invention, the tubular electrode of the ion deflector lens is of a point asymmetric shape, and the central axis of the tubular electrode is arranged closer to the axis of travel of ions upstream of the input side plate-like electrode than the axis of travel of ions downstream of the output side plate-like electrode. Therefore, the removal ratio of photons and neutral particles in the ion deflector lens becomes higher than that of the conventional art while the ion transmittance in the ion deflector lens is equal to or higher than that in the conventional art. 
         [0015]    Further, according to the present invention, the ion deflector lens is arranged upstream of the mass filter, and also arranged downstream of the another ion optical system having a negative potential, or immediately upstream of the mass filter. As a result, the signal sensitivity of the mass spectrometer is increased, and the voltage adjustment for the another ion optical system is easily performed, as compared with the conventional art. This is because, in the ion optical system positioned upstream of the ion deflector lens, it is unnecessary to suppress the generation of neutral particles, and there is no limit of applied voltage of the ion optical system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    In the accompanying drawings: 
           [0017]      FIG. 1  is a diagram illustrating a configuration of an inductively coupled plasma mass spectrometer according to a first embodiment of the present invention; 
           [0018]      FIGS. 2A and 2B  are perspective views illustrating a configuration of an ion deflector lens that is a characteristic portion of the first embodiment of the present invention; 
           [0019]      FIG. 3  is a cross-sectional view illustrating the configuration of the ion deflector lens; 
           [0020]      FIG. 4  is a diagram illustrating a potential distribution and an ion trajectory within the ion deflector lens; 
           [0021]      FIG. 5  is a cross-sectional view illustrating a configuration of an ion deflector lens according to a first modified example of the ion deflector lens; 
           [0022]      FIG. 6  is a cross-sectional view illustrating a configuration of an ion deflector lens according to a second modified example of the ion deflector lens; and 
           [0023]      FIG. 7  is a cross-sectional view illustrating a configuration of an ion deflector lens according to a third modified example of the ion deflector lens. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    Hereinafter, a description is given of a preferred embodiment of the present invention with reference to the accompanying drawings. In this example,  FIGS. 1 to 4  are referred to.  FIG. 1  is a diagram illustrating a partial schematic configuration of an inductively coupled plasma mass spectrometer according to a first embodiment of the present invention. Hereinafter, the inductively coupled plasma mass spectrometer is simply referred to as “mass spectrometer”. Referring to  FIG. 1 , the configuration of the mass spectrometer  10  is illustrated in a cross-sectional view, but in actuality, the mass spectrometer  10  has a substantially tubular stereoscopic configuration which extends in the axial direction.  FIGS. 2A and 2B  are perspective views illustrating an ion deflector lens  100  which is a characteristic portion of the first embodiment. In  FIGS. 2A and 2B ,  FIG. 2A  is a view illustrating an entire ion deflector lens  100 , and  FIG. 2B  is a view illustrating a part of the ion deflector lens  100 .  FIG. 3  is a cross-sectional view taken along a line A-A′ of  FIG. 2A .  FIG. 4  is a diagram illustrating a result of simulating a potential distribution in the ion deflector lens  100  illustrated in  FIG. 3 , and orbits of ions that pass through the ion deflector lens  100 . 
         [0025]    First, the entirety of the mass spectrometer  10  is described. The mass spectrometer  10  includes a plasma torch  20 , an interface unit  40 , a extraction electrode unit  60 , a cell  80 , an ion deflector lens  100 , a mass filter  91 , and a detector  92 . 
         [0026]    The plasma torch  20  includes a coil  21  for generating a high frequency electromagnetic field in the vicinity of a leading end thereof, and is located under the atmospheric pressure. The coil  21  is connected to an RF power supply (not shown). Within the plasma torch  20 , a high frequency inductively coupled plasma  30  is generated due to a high frequency electromagnetic field developed by the coil  21 . Within the plasma torch  20 , an atomized sample (not shown) is introduced into the plasma  30  from the front of the plasma torch  20 . The introduced sample (not shown) is evaporated and decomposed by action of the plasma  30 , and finally converted into ions in the case of the majority of elements. The ionized sample (not shown) is contained in the plasma  30 . In addition, a gas stream occurs from a trailing end at left side toward a leading end at right side within the plasma torch  20 , and thus the plasma  30  extends toward a sampling cone  41 . 
         [0027]    The interface unit  40  includes a sampling cone  41  and a skimmer cone  43 . A part of the plasma  30  that has passed through an aperture  42  of the sampling cone  41  directly facing the plasma  30  reaches the skimmer cone  43  that is arranged downstream of the sampling cone  41 . Thereafter, the part of the plasma  30  passes though an aperture  44  of the skimmer cone  43 , and reaches the rear of the skimmer cone  43 . A vacuum chamber  51  is exhausted by an oil-sealed rotary pump RP. Accordingly, gas molecules (containing neutralized ions) that do not pass through the skimmer cone  43  are exhausted from the vacuum chamber  51  through an outlet  54 . 
         [0028]    The extraction electrode unit  60  is applied with a negative potential, and therefore only positive ions are extracted in the form of an ion beam from the plasma  30  that has passed through the aperture  44  in the vicinity of an aperture  61  of the extraction electrode unit  60 . The extraction electrode unit  60  is configured by one electrode in  FIG. 1 , but is not limited to this configuration. The extraction electrode unit  60  may be configured by two or more electrodes, for example, as described in JP 2001-185073 A. 
         [0029]    An ion beam  200  extracted in the extraction electrode unit  60  is guided into the cell  80  through a gate valve  72  disposed in a partition  71 . The gate valve  72  is disposed for maintaining airtightness of a high vacuum unit when the operation of the mass spectrometer  10  is stopped. The cell  80  is equipped with a multipole electrode  81 . The ion beam  200  guided into the cell  80  is guided downstream along an orbit determined by an electric field generated by the multipole electrode  81 . The multipole electrode  81  is, for example, of an eight-pole structure. Further, into the cell  80 , a collision and/or reaction gas is introduced from an inlet  82 . The molecules of introduced gas collide with various ions contained in the ion beam  200  or react with charge transfer, and act so as to decompose polyatomic interference ions containing argon atoms being a carrier gas or a plasma gas for desorption. The ion beam  200  extracted from the cell  80  is introduced into the mass filter  91  through the ion deflector lens  100  and an aperture  75  of a partition  74 . The ion deflector lens  100  is fitted to the partition  74  through an insulator  73 . The structural feature of the present invention resides in the ion deflector lens  100 , and details thereof are described later. The vacuum chamber  52  is exhausted by a turbo molecule pump (TMP1). Accordingly, molecules resulting from neutralizing molecular ions contained in the plasma  30  by the extraction electrode unit  60 , and molecules of collision and/or reaction gas which have been introduced into the cell  80  are exhausted from the vacuum chamber  52  through an outlet  55 . The vacuum chamber  52  is higher in vacuum than the vacuum chamber  51 . 
         [0030]    The mass filter  91  is configured by a multipole electrode with a prefilter (not shown). The prefilter (not shown) and the multipole electrode (not shown) of the mass filter  91  are typically of a four-pole structure. Ions contained in the ion beam  200  which have been guided into the mass filter  91  are separated on the basis of a ratio of mass and charges (m/z value) in the mass filter  91 , and then guided into the detector  92 . The detector  92  detects the introduced ions, and outputs an electric signal according to the detection result. A vacuum chamber  53  is exhausted by a turbo molecule pump (TMP2) through the outlet  56 . The vacuum chamber  53  is higher in vacuum than the vacuum chamber  52 . 
         [0031]    Next, the ion deflector lens  100  is described in detail. The ion deflector lens  100  is formed of a pair of plate-like electrodes  110  and  130 , and one tubular electrode  120  disposed between the plate-like electrode  110  and the plate-like electrode  130 . The input side plate-like electrode  110  has an aperture  140 . The aperture  140  is of a circle 2 mm in diameter. An axis  160  of the aperture  140  passes through the center of the aperture  140 , and is perpendicular to a plane of the plate-like electrode  110 . In addition, the axis  160  is identical with an optical axis of the ion optical system upstream of the plate-like electrode  110 , that is, an axis of travel of ions upstream of the plate-like electrode  110 . The output side plate-like electrode  130  includes an aperture  150 . The aperture  150  is substantially oval 1.5 mm in short axis and 2.4 mm in long axis. The direction of the long axis is identical with the longitudinal direction in  FIG. 1 . An axis  170  of the aperture  150  passes through the center of the aperture  150  and is perpendicular to the plane of the plate-like electrode  130 . Further, the axis  170  is identical with an optical axis of the ion optical system downstream of the plate-like electrode  130 , that is, the axis of travel of ions downstream of the plate-like electrode  130 . The plate-like electrode  130  is arranged in parallel to the plate-like electrode  110  so that the axis  160  is displaced from the axis  170 . The plate-like electrode  130  faces the plate-like electrode  110 . A distance between the axis  160  and a center axis  180  is 0.6 mm, and a distance between the center axis  180  and the axis  170  is 1.9 mm. A distance between the axis  160  and the axis  170  is 2.5 mm. That is, the center axis  180 , the axis  160 , and the axis  170  of the tubular electrode  120  are contained on an identical virtual plane. The tubular electrode  120  is configured so that input-side one of four pieces into which the complete cylinder is equally divided by a virtual plane including the center axis of the complete cylinder and a virtual plane perpendicular to the center axis is removed from the complete cylinder. The inner diameter of the tubular electrode  120  is 8 mm, and the overall length of the tubular electrode  120  is 5 mm. Accordingly, a cut portion corresponds to a half cylinder of 2.5 mm in overall length and 8 mm in diameter. The plate-like electrode  110  and the plate-like electrode  130  are applied with a DC voltage of −30 V. The tubular electrode  120  is applied with a DC voltage of +15 V which is higher than the voltage that is applied to the plate-like electrode  110  and the plate-like electrode  130 . 
         [0032]    The path of ions guided into the ion deflector lens  100  is bent due to action of an electric field developed within the ion deflector lens  100 , and the ions pass through the aperture  150 . In this case, the axis of travel of ions downstream of the plate-like electrode  130  is substantially parallel to the axis of travel of ions upstream of the plate-like electrode  110 . That is, ions downstream of the plate-like electrode  130  are advanced in a direction which the ions are capable of entering an ion optical system (mass filter  91  in this embodiment) having an optical axis parallel to the axis (optical axial direction of the multipole electrode  81  in this embodiment) of travel of ions upstream of the plate-like electrode  110 , which is an ion optical system downstream of the plate-like electrode  130 . The ions that have passed through the aperture  150  are separated by the mass filter  91 , and detected by the detector  92 . On the other hand, photons and neutral particles of high energy which have been guided into the ion deflector lens  100  together with the ions go straight as indicated by a solid line  300  of  FIG. 1  without changing the path, and collide with the plate-like electrode  130 , and stop. 
         [0033]    As a result, photons and neutral particles may be prevented from entering the mass filter  91  and the detector  92 , thereby eliminating background noise caused by the photons and the neutral particles. On the other hand, as is apparent from  FIG. 4 , most of ions input to the ion deflector lens  100  are output from the ion deflector lens  100 , and the ion transmittance is kept equal to or higher than that of the conventional art. 
         [0034]    The mass spectrometer  10  according to the first embodiment may be deformed as follows. For example, in the first embodiment, the axis  160  is apart from the center axis  180 , but the axis  160  maybe identical with the center axis  180 . In addition, in the first embodiment, the cross section of the tubular electrode  120  is circular, but may be oval, rectangular, or have other line-symmetric shapes. In that case, the ion beam convergence property in two axial directions that are perpendicular to the cross section may be appropriately changed depending on the cross-section shape and the drive voltage. Further, in the first embodiment, the voltage applied to the plate-like electrode  110  and the voltage applied to the plate-like electrode  130  are identical with each other, which may be made different from each other. For example, it is possible that a DC voltage of −30 V is applied to the plate-like electrode  110 , a DC voltage of −50 V is applied to the plate-like electrode  130 , and a DC voltage of +10 V is applied to the tubular electrode  120 . 
         [0035]    Besides, in the first embodiment, the tubular electrode  120  is configured so that input-side one of four pieces into which the complete cylinder is equally divided by the virtual plane including the center axis of the complete cylinder and the virtual plane perpendicular to the center axis is removed from the complete cylinder. However, the tubular electrode  120  is not limited to the above-mentioned configuration. For example, the ion deflector lens  100  may be replaced with an ion deflector lens  400  illustrated in  FIG. 5 . The ion deflector lens  400  is formed of an input side plate-like electrode  410  having the same configuration as that of the plate-like electrode  110 , an output side plate-like electrode  430  having the same configuration as that of the plate-like electrode  130 , and a tubular electrode  420  disposed between the plate-like electrode  410  and the plate-like electrode  430 . The plate-like electrode  410  and the plate-like electrode  430  face each other in parallel so that axes of the respective apertures thereof are displaced from each other. The tubular electrode  420  is of a cylindrical shape having an input side end cut by a virtual plane inclined with respect to a center axis  480 . The tubular electrode  420  may be of a cylindrical shape having an output side end further cut by a virtual plane inclined with respect to the center axis  480 . 
         [0036]    Further, for example, the ion deflector lens  100  may be replaced with an ion deflector lens  500  illustrated in  FIG. 6 . The ion deflector lens  500  is formed of an input side plate-like electrode  510  having the same configuration as that of the plate-like electrode  110 , an output side plate-like electrode  530  having the same configuration as that of the plate-like electrode  130 , and a tubular electrode  520  disposed between the plate-like electrode  510  and the plate-like electrode  530 . The plate-like electrodes  510  and  530  face each other in parallel so that axes of the respective apertures thereof are displaced from each other. The tubular electrode  520  is configured so that relatively small input-side one of four pieces into which the complete cylinder is equally divided by the virtual plane apart from the center axis  580  of the complete cylinder and parallel to the center axis  580 , and the virtual plane perpendicular to the center axis  580  is removed from the complete cylinder. 
         [0037]    Further, for example, the ion deflector lens  100  may be replaced with an ion deflector lens  600  illustrated in  FIG. 7 . The ion deflector lens  600  is formed of an input side plate-like electrode  610  having the same configuration as that of the plate-like electrode  110 , an output side plate-like electrode  630  having the same configuration as that of the plate-like electrode  130 , and a tubular electrode  620  disposed between the plate-like electrode  610  and the plate-like electrode  630 . The plate-like electrodes  610  and  630  face each other in parallel so that axes of the respective apertures thereof are displaced from each other. The tubular electrode  620  is configured so that output-side one of four pieces into which the complete cylinder is equally divided by the virtual plane including the center axis  680  of the complete cylinder and the virtual plane perpendicular to the center axis  680  is removed from the tubular electrode  120 . The portion cut on the input side is mainly located on a side different from that of the portion cut on the output side with respect to the virtual plane including the center axis  680  of the complete cylinder. The tubular electrode of the ion deflector lens according to the present invention may have a shape other than the above-mentioned shape. 
         [0038]    Further, in the respective drawings, the ion beam is deflected upward in the ion deflector lens, but the deflection direction is not limited thereto. For example, the ion beam may be deflected downward. In this case, at least shapes of the tubular electrodes and locations of the apertures of the plate-like electrodes are reversed upside down. Naturally, the location of the ion optical system located upstream or downstream of the ion deflector lens will be changed. As described above, the shape of the tubular electrode and the like need to be appropriately changed in accordance with the defection direction of the ion beam, which may be easily performed by a person skilled in the art.
Provided is a plasma ion source mass spectrometer with an ion deflector lens having an improved removal ratio of photons and neutral particles as compared with the conventional art while an ion transmittance is maintained. The ion deflector includes an input side plate-like electrode, an output side plate-like electrode, and a tubular electrode disposed between the input side plate-like electrode and the output side plate-like electrode. The tubular electrode is of a point asymmetrical configuration. The tubular electrode is arranged so that a center axis of the tubular electrode is closer to an axis of travel of ions upstream of the input side plate-like electrode than an axis of travel of ions downstream of the output side plate-like electrode.