Abstract:
An object of the present invention is to provide means for solving troubles. Examples of the troubles include sensitivity degradation and resolution degradation of a mass spectrometer, which are caused by an axis deviation of a component, particularly at least one orifice located between an ion source and a detector, to decrease the number of ions reaching the detector, and a variation in performance caused by exchange of components such as the orifice. 
     For example, the invention has the following configuration in order to solve the troubles. A mass spectrometer includes: an ion source; a detector that detects an ion; an orifice and a mass separator that are disposed between the ion source and the detector; and an axis adjusting mechanism that adjusts axis positions of the orifice and/or the mass separator such that an opening of the orifice and/or an incident port of the mass separator is disposed on a line connecting the ion source and an incident port of the detector.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mass spectrometer, particularly to miniaturization and weight reduction of the mass spectrometer. 
     2. Description of the Related Art 
     In a mass spectrometer, a molecule or an atom as an analytical target is ionized, and the ions are transported in vacuum to be subjected to mass separation by utilizing an electric field and a magnetic field. The separated ions are detected by a detector. When a degree of vacuum in a vacuum vessel of a mass spectrometer is low, the ion collides with a residual gas molecule in the vacuum vessel at a number of times, and loses its charge due to exchange of charges or changes its traveling direction due to collision, whereby the number of ions reaching the detector is decreased to hardly perform correct mass spectrometry. Therefore, the degree of vacuum is set to be about 10 −3  Pa or less in a spatial area of a vacuum chamber in which a mass separator such as a Q-mass filter and a detector such as a channeltron or a photomultiplier are disposed. For example, when a TOF (Time Of Flight) type mass spectrometer in which an ion reflector (reflector) and an MCP (multichannel plate) detector are combined is used under low vacuum, an adverse effect such as an interference between the ion and the residual gas molecule emerges for the same reason, and therefore the spatial area of the vacuum chamber in which the mass separator and the detector are disposed is set to be a high degree of vacuum. 
     Generally, in a mass spectrometer, a sample or an ionized sample is introduced to a vacuum side from an atmosphere, and the space in which a detector is disposed is maintained under high vacuum. Therefore, plural orifices are disposed between an ion source and the detector, and the space is evacuated in a differential pumping manner by a vacuum pump (for example, see Japanese Patent Application Laid-Open No. 2005-259483). 
     Recently, social concern with safety and security has been increasing in mainly security and food fields. Conventionally, a large-size mass spectrometer installed in an analytical laboratory has been used to sense a trace harmful substance. However, there is a need to rapidly measure the trace harmful substance on site, and miniaturization and weight reduction of the mass spectrometer have been attempted. 
     In order to miniaturize the mass spectrometer, it is necessary to miniaturize components constituting the mass spectrometer. A vacuum pump that is a component having a high structural ratio with respect to a size has been also miniaturized. Generally, with the miniaturization of the vacuum pump, a pumping rate is decreased to degrade the degree of vacuum of a vacuum vessel. When the degree of vacuum is degraded, as described above, the number of ions reaching the detector is decreased to hardly perform the mass spectrometry correctly. Therefore, a diameter of a fine hole of the orifice has been further reduced to decrease a flow rate in the vacuum vessel, thereby achieving the high degree of vacuum in the vacuum vessel. 
     Frequently a voltage is applied to the orifice so that the orifice extracts, accelerates, and focuses the ion beam, and the orifice is fixed to the vacuum vessel having a ground potential through an electric insulator such as alumina. An axis deviation of the orifice may be generated up to about 100 μm with respect to a correct center axis due to accumulation of machining tolerances such as deviations of a diameter of a hole in which the insulator of a vacuum chamber is attached, a diameter of the insulator, a diameter of a hole in which the insulator of the orifice is fitted, and a center axis of the fine hole of the orifice. Interference is generated between the ion beam and the orifice due to the axis deviation when the ion beam passes through the plural orifices, and the amount of ions reaching the detector is reduced to degrade apparatus performance such as the apparatus sensitivity and the resolution degradation. 
     By decreasing the mechanical tolerance of each component, the axis deviation amount can be decreased but the apparatus becomes expensive. It is necessary to adjust the axis deviation amount up to several tens of micrometers. Therefore, it is necessary to finely adjust the axis. When the component is exchanged for the purpose of orifice maintenance, the axis deviation amount after the re-assembly may be different from the axis deviation amount before the maintenance, and the amount of ion reaching the detector may vary. Therefore, the apparatus performances such as the apparatus sensitivity and resolution are changed and not stabilized. A sample gas adheres to a surface of the orifice to form an insulating film on the surface of the orifice, which results in a problem such that a drift of the ion beam is generated due to accumulation of charge. In order to prevent such a problem, sometimes the orifice is heated to a high temperature by a heater. In such a case, the orifice is thermally expanded. The temperature of the orifice changes depending on the time elapsed after the start-up of the apparatus, and a thermal expansion amount also changes, which results in a problem such that the axis deviation amount changes transiently. 
     An object of the present invention is to provide means for solving the problems of the related art. Examples of the problems include sensitivity degradation and resolution degradation of a mass spectrometer, which are caused by an axis deviation of a component, particularly at least one orifice, located between an ion source and a detector, to decrease the number of ions reaching the detector, and a variation in performances caused by exchange of components such as the orifice. 
     SUMMARY OF THE INVENTION 
     For example, the invention has the following configuration in order to solve the problems above. 
     A mass spectrometer includes: an ion source; a detector that detects an ion; an orifice and a mass separator that are disposed between the ion source and the detector; and an axis adjusting mechanism that adjusts axis positions of the orifice and/or the mass separator such that an opening of the orifice and/or an incident port of the mass separator is disposed on a line connecting the ion source and an incident port of the detector. 
     According to the invention, the center axis of the component located between the ion source and the detector, particularly the center axis of the orifice and an ion beam traveling axis connecting a beam outgoing axis of the ion source and an incident port axis of the detector can substantially be aligned with each other to minimize the axis deviation amount, so that the number of ions reaching the detector can be maximized. Therefore, the vacuum pump can be miniaturized, and the compact, light-weight, high-sensitivity, high-resolution mass spectrometer can be implemented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an entire configuration of a mass spectrometer according to an embodiment of the invention; 
         FIGS. 2A and 2B  illustrate relationships between an axis deviation amount and a passing beam current amount; 
         FIG. 3  illustrates an entire configuration of a mass spectrometer according to an embodiment of the invention in which APCI (Atmospheric Pressure Chemical Ionization) is used; 
         FIG. 4  illustrates a relationship between an output current value of a detector and an elapsed time; 
         FIG. 5  illustrates a relationship between a mass-to-charge ratio m/z and ion strength (relative value); 
         FIG. 6  illustrates an axis position adjusting mechanism of a first orifice; 
         FIG. 7  illustrates an axis position adjusting method; 
         FIG. 8  illustrates an entire configuration of a TOF (Time Of Flight) mass spectrometer according to an embodiment of the invention; 
         FIG. 9  illustrates an axis position adjusting mechanism of a first orifice; 
         FIGS. 10A to 10D  illustrate an axis position adjusting mechanism of a first orifice; and 
         FIG. 11  illustrates a change in signal amount by axis position adjustment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An exemplary embodiment of the invention will be described below with reference to the drawings. 
       FIG. 1  is a sectional view illustrating a conceptual configuration of a mass spectrometer according to an embodiment of the invention. 
     For example, electron ionization (EI), chemical ionization (CI), electron spray ionization (ESI), nano-electron spray ionization, atmospheric pressure chemical ionization (APCI), fast atom bombardment ionization (FAB), electric field ionization (FI), electric field desorption ionization (FD), matrix-assisted laser desorption ionization (MALDI), desorption electrospray ionization (DESI), desorption electrospray ionization (DART), or barrier discharge ionization is used for ionization of an ion source  1 . 
     An ion beam  2  is extracted from the ion source  1  by an extraction electric field that is applied between a first orifice  3  and an ion source electrode (not illustrated). Air containing the ion beam  2  flows through a fine hole of the first orifice  3  in a first differential pumping chamber  4  connected to a rough vacuum pump  5 . 
     Similarly, the air flows through a fine hole of a second orifice  6  in a second differential pumping chamber  7  connected to a second pumping port (low pumping speed side) of a main vacuum pump  18 . An octopole  8  is disposed in the second differential pumping chamber  7 . In the octopole  8 , eight multipole rod electrodes are disposed in an axially symmetric manner in parallel with one another, a potential having an identical phase is provided to the rod electrodes that are opposite to each other, and a potential having a constant phase difference is provided to the adjacent rod electrode. In the octopole  8 , an octopole high-frequency electric field is generated to form a potential that becomes convex on the axis, which allows the ion to be focused near the axis. 
     Potentials of tens volts are provided to the first orifice  3  and the second orifice  6  in order to extract the ion beam, and the ion is accelerated by a potential difference between the first orifice  3  and the second orifice  6 . 
     The air containing the ion beam  2  flows through a fine hole of a third orifice  9  in an analytical chamber  10 . The analytical chamber  10  is evacuated by connection with a first pumping port (high pumping speed side) of the main vacuum pump  18 . A background of the main vacuum pump  18  is evacuated by the rough vacuum pump  5 . 
     The analytical chamber  10  includes a quadrupole mass separator  11  and a detector  20 . The quadrupole mass separator  11  includes a front electrode  12 , a quadrupole rod  13 , a blade electrode  14 , a front wire  15 , a rear wire  16 , and a rear electrode  17 . In the quadrupole rod  13 , an identical AC voltage (identical amplitude and phase) is provided to the electrodes that are opposite each other, and an AC voltage whose phase is inverted is applied to the adjacent electrode. Generally the AC voltage ranges from several hundred volts to 5 kV and the frequency ranges from 500 kHz to 2 MHz. In a radial direction of the quadrupole rod  13 , a concave potential is formed in an axis center portion to focus the ion around the axis by the applied AC voltage. In an axial direction of the quadrupole rod  13 , an inclined DC potential is formed on a beam axis by mainly the front electrode  12  and the rear electrode  17 . The ion is trapped in the quadrupole mass separator  11  by the concave potential and the inclined DC potential. Accumulation and emission of the ion are sequentially performed by mainly changing the voltages at the front electrode  12  and the rear electrode  17 . 
     A mass spectrometry sequence will be described below. 
     MS analysis and MS n  analysis can be cited as an example of the mass spectrometry sequence. In the MS analysis, an amplitude of an AC voltage is changed to trap the ion, the ion is selectively ejected in an ion beam traveling axial direction, the ion is detected by the detector  20 , and a molecular structure and a molecular formula of the sample are fixed from a relationship between a mass-to-charge ratio m/z and a detected ion current strength (relative value). 
     In the MS n  analysis, a specific ion (precursor ion) is caused to selectively remain in the quadrupole mass separator  11 , collision induced dissociation (CID) of the precursor ion is generated to create a fragment ion, and mass scanning and mass separation of the fragment ion are performed to finely investigate the molecular structure of the sample. The MS n  analysis will be described in detail below. A filtered noise field (FNF) having frequencies except a specific frequency is provided to the blade electrode  14  to eject ions except the specific precursor ion to the outside of the quadrupole mass separator  11 , thereby selecting the specific precursor ion. The AC voltage having a resonant frequency of the precursor ion is applied to the precursor ion remaining in the quadrupole mass separator  11 . At this point, a gas (such as helium, nitrogen gas, or argon) for collision induced dissociation is caused to flow in the quadrupole mass separator  11  to collide with the precursor ion, and the precursor ion is dissociated to create a product ion. The ion scanning and the mass separation of the created product ion are performed by changing the amplitude of the AC voltage amplitude applied to the quadrupole rod  13  and the blade electrode  14 . At this point, only the product ion overcoming a potential barrier caused by a DC voltage applied to the front wire  15  is incident to the detector  20  by the extraction electric field of the rear wire  16 . A variation in ion energy flowing in the ion detector is reduced by the front wire  15  and the rear wire  16 , so that resolution can be improved. 
     Magnetic field (sector type) mass spectrometry, time of flight mass separation (TOFMS), ion trap mass spectrometry (ITMS), Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS) in which the mass separation is performed by utilizing ion rotation motion generated by the magnetic field, or orbitrap mass spectrometry in which the ion rotation motion generated by the electric field is utilized can be used as a mass separation method except the quadrupole mass separator in which the quadrupole rod is used. 
     The detector will be described below. 
     In  FIG. 1 , the detector  20  exhibits a secondary electron photomultiplier provided with a conversion dynode  21 . The ion is caused to collide with the conversion dynode  21  by the electric field that is generated by the voltage of several kilovolts applied to the conversion dynode  21 , and the generated secondary electron  28  is amplified to a degree of the sixth power of ten by a multi-stage dynode  22 . The amplified secondary electron  28  is taken out to the atmosphere using a current introduction terminal  25 , further amplified by an amplifier circuit  26 , and captured in a micro ammeter  27  to perform monitoring. For example, a Farady cup in which the ion is received by a cup-shaped electrode to measure an amount of generated secondary electron, a channeltron in which the electrode is not independently formed but constitutes a high-resistance pipe, a micro channeltron including channeltrons having diameters range from 10 to 20 micrometers and arrayed in plate, or a photomultiplier in which light is converted into a photoelectron by a photoelectric surface to amplify the generated secondary electron can be used as the ion detector. 
     The mass spectrometer includes an axis adjusting mechanism  30  on the ion traveling axis connecting a center axis of an ion beam outgoing port of the ion source  1  and a center axis of an incident port of the detector  20  such that center axes of the fine holes of the first orifice  3 , the second orifice  6 , and the third orifice  9  are aligned with one another. Therefore, in the mass spectrometer, the axis position adjustment can be performed at a micrometer level. The components, such as the octopole  8  and the quadrupole mass separator  11 , which are disposed between the ion source  1  and the detector  20  can be adjusted by an axis adjusting mechanism (not illustrated). For the octopole  8  and the quadrupole mass separator  11 , plural axis adjusting mechanisms  30  may be provided near the incident port and the outgoing port so as not to deviate (incline) the axis. 
       FIGS. 2A and 2B  illustrate positional relationships between a fine hole  35  of the second orifice and an ion beam  36  passing through the fine hole of the first orifice when the small and large axis deviations are generated between the first orifice and the second orifice (left side), and intensity distributions  38  of the ion beam passing through the first orifice on the second orifice surface and states of an ion beam  37  passing through the second orifice (right side). Because a diameter of the first orifice is larger than a diameter of the second orifice, the ion beam  36  incident to a surface of the second orifice through the first orifice does not interfere with the fine hole  35  of the second orifice in case of small axis deviation. On the other hand, part of the ion beam passing through the first orifice does not pass through the fine hole  35  of the second orifice in case of large axis deviation, and the ion beam current reaching the detector is decreased to generate troubles such as sensitivity degradation of an apparatus and resolution degradation. 
     Therefore, the axes (positions) of the first orifice and the second orifice are adjusted to align the center axes by means of the axis adjusting mechanism  30  such that the ion beam passing through the first orifice can pass through the second orifice. Although the relationship between the first orifice and the second orifice is adjusted in the embodiment, the axis position adjustment may be performed in the components disposed between the ion source and the detector. 
     The invention will be described below referring to embodiments applied to specific apparatuses. 
     First Embodiment 
       FIG. 3  illustrates an entire configuration of an apparatus in which APCI (Atmospheric Pressure Chemical Ionization) is used as the ion source in the apparatus of  FIG. 1 . In  FIG. 1 , the octopole  8  and the quadrupole mass separator  11  are illustrated in the perspective views. On the other hand, in  FIG. 3 , the octopole  8  and the quadrupole mass separator  11  are illustrated in a plan view. Hereinafter, the overlapping description is omitted. 
     Air  45  is taken in the ion source  1  by a suction pump  40 . At this point, TCP (trichlorophenol) that is of a standard sample  41  is heated and vaporized by a heater  42 . After a vaporized gas amount becomes constant while the standard sample  41  is maintained at a constant temperature, a flow rate of the air  45  is set through a filter  44  by a mass flow controller  43 . The heater  42  is wound around pipe  46  located on a downstream side such that adhesion of a vaporized component of TCP to the pipe  46  is suppressed as much as possible. A voltage of several kilovolts is applied to a discharge needle  50  through a power cable  51  and a holder  52 , which are connected to a power supply (not illustrated). A voltage lower than the voltage applied to the discharge needle  50  is applied to a counter electrode  53  that located several millimeters from a leading end of the discharge needle  50  (for positive ion). A corona discharge  55  is generated in the air by the potential difference. A voltage of several tens of volts is applied to the first orifice  3 . The ion beam is extracted toward the detector  20  by the differential voltage. As illustrated in  FIG. 3 , contrary to the ion beam extraction direction, the air  48  containing the TCP sample gas flows from the counter electrode  53  to the discharge needle  50 . The reason that the flow of the sample gas is set to the opposite direction to the ion beam extraction direction is that a reaction area where the desired ion reacts with radical and other ions is reduced to the minimum. The sample gas flows in the corona discharge area to generate the radical and other ions, which are the electrically neutral, in addition to the desired ion. The radical and other ions block the desired ionization to lower the desired ion current. Therefore, the flow of the sample gas is set to the opposite direction to the ion beam extraction direction in order to minimize the reaction area where the desired ion reacts with the radical and other ions. 
     The whole of ion source is heated to a high temperature by a heater (not illustrated). The first orifice  3  includes an elongated pipe in the center portion thereof. The elongated pipe has an inner diameter of about 100 micrometers and a length of 10 millimeters. The first differential pumping chamber  4  located on the downstream side of the first orifice  3  is connected to a diaphragm pump (not illustrated) having a pumping speed of several tens of liters per minute, and the degree of vacuum of the first differential pumping chamber  4  becomes about 1000 pascals. Because the air containing the sample gas is adiabatically expanded when flowing in the first orifice  3 , the temperature of the air containing the sample gas is lowered to generate clustering of the ion. When the clustering of the ion is generated, the mass spectrometry cannot correctly be performed. The sample gas adheres to the surface of the first orifice  3  to form an insulating film, and the charge is accumulated on the insulating film to generate a drift of the ion beam. Therefore, the first orifice  3  is heated to several hundreds of degrees Celsius by a heater (not illustrated) in order to prevent the drift from generating. 
     Similarly the second orifice  6  is heated by a heater (not illustrated). The first orifice  3  is fixed to a vacuum chamber  58  with an insulator  47  and a vacuum O-ring  59  interposed therebetween. The O-ring  59  is used to retain the vacuum. The ion is accelerated to enter the octopole  8  by the potential difference between the first orifice  3  and the second orifice  6 . A hole having a diameter of several hundreds of micrometers is made in the second orifice  6 . The second differential pumping chamber  7  located on the downstream side of the second orifice  6  is connected to a split-flow turbo molecular pump (not illustrated) having a pumping speed of several liters per second through a second pumping port. The air containing the sample gas flowing in the second differential pumping chamber  7  is restricted by a flow rate narrowing-down effect of the second orifice  6 , and the degree of vacuum of the second differential pumping chamber  7  becomes several pascals. The octopole  8  is disposed in the second differential pumping chamber  7 . The octopole  8  performs the above-described operation, and causes the ion beam to be focused and to pass through the fine hole of the third orifice  9 , so that the ion beam is incident to the analytical chamber  10 . The third orifice  9  has the hole diameter of about 1 millimeter. The pumping port of the analytical chamber  10  located on the downstream side of the third orifice  9  is connected to a split-flow turbo molecular pump (not illustrated) having a pumping speed of several tens of liters per second through a first pumping port. The analytical chamber  10  becomes the degree of vacuum of the minus third power of ten. The operation of the quadrupole mass separator  11  disposed in the analytical chamber  10  is described above. The scanned and separated ion having the mass-to-charge ratio m/z is incident to the detector  20 . 
     The output of the detector  20  is obtained as follows. 
       FIG. 4  illustrates a temporal change of the total ion current value that is the output of the detector  20  when the quadrupole mass separation is not performed.  FIG. 4  shows that the total ion current value has a variation of about plus or minus several percent. Although the total ion current value has the above-described variation when the apparatus runs normally, the total ion current value of the detector is largely decreased, when the amount of sample gas that is source material flowing in the ion source is decreased due to the adhesion of the sample on a cold spot on a pipe, or when an ion beam passage rate is decreased due to clogging of the orifice. 
       FIG. 5  illustrates a relationship between the mass-to-charge ratio m/z and the ion strength (relative value) when the quadrupole mass separation is performed at a time T 1  of  FIG. 4 . Because TCP is used as the standard sample, a peak is observed at the mass-to-charge ratio m/z of 195. 
     A specific configuration of the axis adjusting mechanism  30  will be described below. 
       FIG. 6  illustrates the axis adjusting mechanism between the first orifice and the second orifice as an example of the axis adjusting mechanism. A adjustment screw mounting plate  60  is fixed to the vacuum chamber  58 . Screw holes are made in the first orifice  3 , and adjustment screws  61  are threaded in the screw holes. An elastic member such as a spring  62  is fixed to a position opposite the adjustment screws  61 . The position of the first orifice  3  can be adjusted by a balance between a spring repulsive force  63  of the spring  62  and a pressing force  64  of the adjustment screw  61 . A trapezoidal disc spring as the spring  62  is used to generate the large repulsive force in the narrow area. The identical mechanism is provided in a direction orthogonal to the adjustment direction, and the identical adjustment can be performed. The adjustment can be performed in the two directions orthogonal to each other by the method. Alternatively, the position of the fine hole may be adjusted not two-dimensionally but three-dimensionally including a trolling angle by additionally providing an inclination mechanism (not illustrated). FOMBLIN having a sufficiently low saturated vapor pressure is applied to the O-ring  59  such that friction between the first orifice  3  and the vacuum chamber  58  is reduced to improve slippage and such that apparatus performance is not adversely affected. The first orifice  3  can be fixed using a fixing screw  66  after the axis adjustment. A distance of movement and adjustment of the first orifice  3  is several hundreds of micrometers. Similarly the second orifice  6  is fixed to the vacuum chamber  58  with the insulator  47  interposed therebetween. The ion beam  2  is extracted onto the detector side by the potential difference between the first orifice  3  and the second orifice  6 . When a fine screw having a screw pitch of 0.5 mm is used as the adjustment screw, because the screw travels by 0.5 mm per rotation of 360°, the movement and adjustment of about 10 μm can be performed by 7°. For the finer adjustment, a piezoelectric element, a servo motor and a ball screw, and a precisely direct acting stage may be used as a driving structure, to allow the adjustment to be performed at a nanometer level at the minimum.  FIG. 6  illustrates the axis position adjusting mechanism between the first orifice and the second orifice. Similarly the axis position adjusting mechanism (not illustrated) may be provided among the first orifice  3 , the quadrupole mass separator  11 , and the detector  20  to perform the axis adjustment. 
     Sometimes the vaporized gas of the lubricant agent is generated when the lubricant agent is used in the O-ring. In such cases, possibly the ionization of the sample is blocked to decrease the necessary ion current value. Also, a noise component is increased to possibly degrade an S/N ratio. On the other hand, when the lubricant agent is not used, the friction between the first orifice  3  and the O-ring  59  is increased to twist the O-ring  59 , which sometimes causes a leak of the vacuum chamber. 
     Therefore, as illustrated in  FIG. 9 , a mechanism that moves the first orifice  3  in the direction identical to that of the beam axis is provided to separate the first orifice  3  and the O-ring  59 , and the first orifice  3  is moved in the direction orthogonal to the axial direction. A dovetail groove (a sidewall of a groove in which the O-ring is accommodated is inclined) is provided in order that the generation of the twist of the O-ring  59  and the generation of the leak are prevented to lessen the motion of the O-ring  59  as much as possible. 
     At this point, the first orifice  3  is moved as illustrated in  FIGS. 10A to 10D . The first orifice  3  is moved from a state ( FIG. 10A ) in the beam axis direction to the upstream side (the side of the ion source  1 ) by a screw  67  ( FIG. 10B ). The first orifice  3  is moved in the direction orthogonal to the beam axis by the adjustment screw  61  ( FIG. 10C ). The first orifice  3  is moved in the beam axis direction to the downstream side (the side of the detector  20 ) by the screw  67  and fixed by the fixing screw  66  ( FIG. 10D ). 
     The mechanism is used in each orifice and each mass separator to adjust the axis position. 
     An axis adjusting method will be described below. 
       FIG. 7  illustrates a method for adjusting the axis deviation. The first orifice  3  is moved along an axis  1 - 1 ′. The right side in the upper stage of  FIG. 7  illustrates a transition of the beam current value when the first orifice  3  passes through the fine hole of the second orifice  6 . In  FIG. 7 , the first orifice  3  is moved in the direction of a→e. The output signal of the detector becomes the maximum at the position c. Then the adjustment is performed in the direction of  2 - 2 ′ illustrated in the lower stage of  FIG. 7 . First the first orifice is located in the position c. When the first orifice is moved in the direction of c→a*→b*, the detected current value is decreased. Therefore, the first orifice is returned and moved in the direction of b*→c*→d*. The right side in the lower stage of  FIG. 7  illustrates the change of the detected signal. The detected signals are connected by an approximate curved line to determine the first orifice position in which the detected signal is maximized, and the first orifice is adjusted to the position and fixed. Then the axis deviation adjusting work is ended. In the embodiment, the axis adjustment is less frequently performed. However, actually it is necessary to repeatedly perform the adjustment plural times. In the embodiment, the adjustment is manually performed. Alternatively, the adjustment may automatically be performed such that the current value of the detector becomes the maximum, when a combination of a motor (stepping motor) and a ball screw is used to drive the orifice, or when a combination of the piezoelectric element and precision stage is used to drive the orifice. 
     Because sizes of maintenance components such as the orifice vary within mechanical tolerances, it is necessary to perform the axis adjustment after the maintenance. Because the orifice is heated by the heater as described above, the center axis position of the fine hole changes in the transient state. Therefore, the adjustment is efficiently performed after the apparatus is thermally stabilized in the running state. Whether the apparatus is thermally stabilized can be determined based on whether the signal of the detector  20  in the ion beam detecting state is substantially kept constant (the variation falls within a predetermined range). It is necessary that the axis position adjustment is performed when the apparatus runs normally. The stability of the apparatus is confirmed by the variation in total ion current value that is a kind of the detector output and the mass-to-charge ratio m/z in which the peak of the ion intensity (relative value) is observed as illustrated in  FIGS. 4 and 5 . The variation in total ion current value and the mass-to-charge ratio m/z are monitored in performing the axis adjustment. When an abnormality is generated, if a warning is issued to an operator to stop the axis adjustment and the repair or maintenance of the apparatus is performed, operability, performance, and reliability of the apparatus are improved. 
       FIG. 11  illustrates an example of test result. In  FIG. 11 , a horizontal axis indicates a movement distance in the direction orthogonal to the beam axis, and a vertical axis indicates the total ion current value (TCP signal intensity). The change of maximum/minimum=about two times is generated by the axis adjustment, and the maximum performance can be exerted by the current correction using the axis adjustment mechanism. 
     Thus, the axis adjusting mechanism is used to effectively reduce the mechanical tolerance. 
     Second Embodiment 
       FIG. 8  illustrates a TOF (Time Of Flight) mass spectrometer provided with the axis adjusting mechanism. The ion is accelerated in the orthogonal direction by an acceleration electric field of several hundreds of volts to several kilovolts applied to a push-out electrode  71  and an acceleration pull-out electrode  72 , the ion deflects through the ion reflector  73  which is called a reflector reaches the detector, and the ion reaches the detector such as a multi channel plate  74 . The variation in initial energy of the ion is corrected to equalize a total flight time of the ions having the identical mass-to-charge ratio m/z using the reflector, so that mass resolution can be enhanced. 
     The miniaturization of the mass spectrometer can also be implemented by utilizing the axis adjusting mechanism  30  in each orifice. 
     Description of Reference Numerals 
     
         
           1  ion source 
           3  ion beam 
           3  first orifice 
           4  first differential pumping chamber 
           5  rough vacuum pump 
           6  second orifice 
           7  second differential pumping chamber 
           8  octopole 
           9  third orifice 
           10  analytical chamber 
           11  quadrupole mass separator 
           12  front electrode 
           13  quadrupole rod 
           14  blade electrode 
           15  front wire 
           16  rear wire 
           17  rear electrode 
           18  main vacuum pump 
           20 ,  23  detection unit 
           21  conversion dynode 
           22  dynode 
           25  current introduction terminal 
           26  amplifier circuit 
           27  micro ammeter 
           28  secondary electron 
           30  axis adjusting mechanism 
           33  adjustment direction 
           35  fine hole 
           36  ion beam that already passing through first orifice 
           37  ion beam that already passing through second orifice 
           38  intensity distribution 
           40  suction pump 
           41  standard sample 
           42  heater 
           43  mass flow controller 
           44  filter 
           45  air 
           46  pipe 
           47  insulator 
           48  air containing sample gas 
           50  discharge needle 
           51  power cable 
           52  holder 
           53  counter electrode 
           55  corona discharge 
           58  vacuum chamber 
           59  O-ring 
           60  adjustment screw mounting plate 
           61  adjustment screw 
           62  spring 
           63  spring repulsive force 
           64  screw pressing force 
           65  first fine hole 
           66  fixing screw 
           67  screw 
           71  push-out electrode 
           72  pull-out electrode 
           73  ion reflector (reflector) 
           74  multi channel plate 
           75  vacuum pump