Patent Publication Number: US-10790132-B2

Title: Time-of-flight mass spectrometer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2017/002598 filed Jan. 25, 2017. 
     TECHNICAL FIELD 
     The present invention relates to a time-of-flight mass spectrometer (hereinafter abbreviated as “TOFMS”). 
     BACKGROUND ART 
     Generally, a TOFMS gives a predetermined kinetic energy to an ion derived from a sample component to make the ion fly through a space by a predetermined distance, and measures the time required for the flight, thereby calculating the mass-to-charge ratio of the ion from the flight time. 
     The TOFMS performs various processes including operations such as temporarily trapping ions generated, selecting only ions within a predetermined narrow mass-to-charge ratio range, and dissociating the ions. A TOF unit in a succeeding stage separates ions with high accuracy in accordance with their mass-to-charge ratios (m/z ratios). In order to enhance the feature of high accuracy separation, one equipped with a reflection for extending an ion flight distance is often used as the TOF unit in the succeeding stage. 
     As an example of such a TOFMS.  FIG. 7  shows a schematic configuration of a tandem mass spectrometer (Patent Literature 1). The tandem mass spectrometer has, in a vacuum vessel  18  an ion source  11 , a quadrupole mass filter  12 , a collision cell  13  incorporating an ion guide  14 , an ion trap  15 , a time-of-flight mass separator  16  of a reflectron type, and an ion detector  17 . Usually, ion optical elements such as an ion guide and an ion lens for efficiently transporting ions to a subsequent stage are provided between the ion source  11  and the quadrupole mass filter  12  or at other appropriate positions. However, a description of such elements will be omitted here. Referring to  FIG. 7 , the ion trap  15  has a three-dimensional quadrupole type configuration in which a pair of end cap electrodes  152  and  153  are provided, with a ring electrode  151  being disposed between them. However, the ion trap  15  may have any configuration for storing ions, and is sometimes replaced with a linear ion trap or the like. 
     The time-of-tlight mass separator  16  has an orthogonal acceleration type ion acceleration unit including an expulsion electrode  161  and a grid electrode  162  for accelerating ions that have traveled from the ion source  11  in the preceding stage to the ion trap  15  in a direction orthogonal to their traveling direction. A reflectron  164  composed of a number of plate-shaped electrodes is disposed at the rear end (lower end in  FIG. 7 ) of a TOFMS flight space  163  in the succeeding stage which extends orthogonal to the ion flight axis of the preceding stage. 
     The ion source  11  in the preceding stage ionizes various compounds contained in the sample. The quadrupole mass filter  12  passes only precursor ions having a designated specific mass-to-charge ratio. The precursor ions are dissociated inside the collision cell  13  and produce various fragments (product ions and neutral losses). Product ions generated by the dissociation, and precursor ions not dissociated, are introduced into and trapped by the ion trap  15 . The ion trap  15  temporarily captures the ions, and ejects the ions in a packet form to send to an ion acceleration unit of the time-of-flight mass separator  16 . 
     Applying a predetermined voltage between the expulsion electrode  161  and the grid electrode  162  at the timing when the ion packet arrives at the ion acceleration unit, each ion in the ion packet is given an initial kinetic energy, and accelerated in a direction substantially orthogonal to the initial traveling direction. The accelerated ions are introduced into the flight space  163 , are made to fly back by the action of a reflection electric field formed by the reflectron  164 , and lastly reach the ion detector  17 . 
     The TOFMS using the reflection can implement highly accurate analysis for the following reasons in addition to a reason that the flight distance of an ion is extended as described above. 
     The TOFMS applies a predetermined acceleration energy to an ion derived from a target component to make the ion fly through a space by a predetermined distance, and measures the length of time required for the flight, thereby calculating the mass-to-charge ratio of the ion from the time of flight. Even if ions have the same mass-to-charge ratio, when the initial kinetic energy of individual ions in the direction of acceleration varies before acceleration, the variation brings about a difference in flight velocity, and time differences develop when the ions reach the ion detector. The time differences lead to a decrease in mass resolution. Therefore, in order to achieve high mass resolution in the TOFMS, it is important to reduce the influence of the variation of initial kinetic energy of ions. 
     In order to avoid differences in the time of flight of ions having the same mass-to-charge ratio arising from variations in the initial kinetic energy, the reflectron that reverses the flight trajectory of the ions by the reflection electric field is effective. That is, when ions enter a reflection electric field formed by the reflection, ions having a larger energy advance farther before being reflected even if they have the same mass-to-charge ratio. Therefore, ions with a larger energy and larger flight velocity have longer practical flight distances, which compensates for the differences in the time of flight. This makes it possible to improve the time convergence (or energy convergence) of ions having the same mass-to-charge ratio in a TOFMS with a reflectron and to improve mass resolution. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2014-165053 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In an actual product of the TOFMS having the above configuration, the front-stage units, the TOF unit, and other units are housed in the vacuum vessel  18 . The completed overall product is fixed on a chassis  19 . The TOFMS thus assembled and manufactured in a factory is transported to a place (hereinafter referred to as an installation site) where a user uses it by a truck and other means. In the meantime, the TOFMS is transported first to a loading/unloading site near the installation site by a truck and other means. After being unloaded from the truck and other means, the TOFMS is moved to the installation site by using casters  191  attached to the bottom surface of the chassis. Alternatively, the TOFMS is mounted on a carriage with casters (without providing casters for the chassis) and moved to the installation site. After the movement, the TOFMS is fixed with stoppers  192 . 
     However, when the user actually uses the TOFMS that has been transported to the installation site in this way, it sometimes occurs that the same degree of accuracy as that established (built-in) in the product at the time of production cannot be obtained. 
     It is an object of the present invention to provide a TOFMS taken measures for preventing such a deterioration in accuracy caused at the time of transportation to an installation site. 
     Solution to Problem 
     According to the present invention made to solve the above-mentioned problems, a time-of-flight mass spectrometer for performing mass separation based on a time of flight of an ion flying in a flight space includes: 
     a) an ion transportation unit configured to transport an ion; 
     b) an acceleration unit configured to receive the ion transported by the ion transportation unit and accelerate the ion to introduce the ion into the flight space; 
     c) a flight unit incorporating the flight space; 
     d) a first vacuum vessel enclosing the ion transportation unit, the acceleration unit, and at least a part of the flight unit; 
     e) a chassis on which the first vacuum vessel is placed; and 
     f) a reflector unit to which a reflector and a second vacuum vessel are fixed, the reflector being configured to reverse a flight trajectory of the ion accelerated by the acceleration unit and introduced into the flight space, and the second vacuum vessel being attachable to an end of the first vacuum ressel and enclosing the reflector. 
     The flight unit may include various devices, such as a quadrupole mass filter, internally having a space in which ions generated by the ion source fly horizontally. 
     As a result of investigations to solve the above-mentioned problems, the present inventor has found that the reflectron in a TOFMS in particular is a cause of the deterioration in accuracy. That is, the reflectron is constituted by a number of doughnut-shaped flat-plate electrodes arrayed in parallel with each other with their central axes being aligned. As described above, in reflecting ions, high accuracy is required for the placement of each electrode plate to form an electric field so as to compensate for variations in initial kinetic energy. However, even if a reflectron is produced with high accuracy in a factory, vibrations during transportation of the TOFMS sometimes cause the displacement of the electrode plates, resulting in a deterioration in accuracy of the TOFMS as a whole. 
     In the TOFMS according to the present invention, the reflector unit is separated from the first vacuum vessel and the part of the flight unit accommodated in the first vacuum vessel. The folio ing describes how to transport the TOFMS from the factory where the TOFMS is completed to an installation site where the TOFMS is used, and then to install the TOFMS at the installation site. 
     (1) First, the finished TOFMS is separated into a reflector unit, and an ion transportation unit, an acceleration unit, (at least a part of) a flight unit, a first vacuum vessel, and a chassis on which these components are mounted (hereinafter referred to collectively as a main body unit). 
     (2) The main body unit and the reflector unit are transported by transportation means such as a truck to a loading/unloading site near the installation site. Here, at least the reflector unit is transported by a method with special care in order to prevent vibrations from giving to the reflector unit. 
     (3) The main body unit and the reflector unit are unloaded from the transportation means at the loading/unloading, site. The main body unit has casters disposed on the chassis or is mounted on a carriage with casters, and is moved to the installation site by the casters. At this time, although vibrations are generated accompanying the rotation of the casters, the reflector unit is not affected by the vibrations since the reflector unit is not fixed to the main body unit 
     (4) The reflector unit is moved from the loadinalunloading, site to the installation site by means and a method with less vibration as compared with the movement using the casters. This movement may be achieved in such a manner that the reflector unit is made to slide on rails installed on the floor or is transported by human power. 
     (5) At the installation site, the reflector of the reflector unit is attached to the end of the flight unit of the main body unit which has been moved first and fixed there, and the second vacuum vessel is attached and fixed to the end of the first vacuum vessel. The assembly of the TOFMS is thus completed at the installation site, and the TOFMS becomes usable. 
     In order to facilitate the transportation of the main body unit, desirably, the chassis has casters as described above. 
     In the TOFMS according to the present invention, the second vacuum vessel may be fixed on a sub-chassis via a damper for absorbing vibrations, and the sub-chassis may be fixed to the chassis. This makes is possible to more reliably fix the main body unit and the reflector unit to each other. 
     This sub-chassis may also have casters (sub-chassis casters). This facilitates the movement of the reflector unit. In this case, the damper described above reduces the influence of vibrations at the time of movement on the reflector. If appropriate countermeasures are taken against vibrations during the movement using the sub-chassis casters, the second vacuum vessel may be fixed on the sub-chassis without the damper, or may have the sub-chassis casters. 
     Desirably, the chassis has, in its bottom surface, a notch to receive the reflector unit, the notch being formed at a portion of the bottom surface immediately below a position (attachment position) where the second vacuum vessel is attached to the first vacuum vessel. With this structure, moving the reflector unit and placing it into the notch allows the reflector unit to be easily loaded to immediately below the attachment position, thus facilitating attaching work. In particular, when the reflector unit has sub-chassis casters, the reflector unit is loaded to immediately below the attachment position only by the movement using the sub-chassis casters. This further facilitates attaching work. 
     Advantageous Effects of Invention 
     In the TOFMS according to the present invention, the reflector unit is separated from the first vacuum vessel and the part of the flight unit accommodated in the first vacuum vessel. Accordingly, when the TOFMS is transported from the factory where the TOFMS is completed to the installation site, particularly when the TOFMS is transported from the loading/unloading site near the installation site to the installation site, the main body unit is easily moved by the casters provided for the chassis or the carriage on which the main body unit is mounted. Meanwhile, the reflector unit is moved to the installation site without being affected by the vibrations caused by the movement of the main body unit using the casters. Therefore, the high assembly accuracy of the reflector completed in the factory is maintained in the process of transporting and moving the TOFMS to the installation site. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a TOFMS according to an embodiment of the present invention. 
         FIG. 2  is a schematic configuration diagram of a reflector unit in the TOFMS according to this embodiment. 
         FIG. 3  is a schematic configuration diagram of the reflector unit and the other parts that are separated from each other in the TOFMS according to this embodiment. 
         FIG. 4  is a top view of a chassis and a sub chassis in the TOFMS according to this embodiment. 
         FIGS. 5A to 5D  are schematic configuration diagrams of modifications of the reflector unit in the TOFMS according to this embodiment. 
         FIG. 6  is a schematic configuration diagram of a modification of the TOFMS according to the present invention. 
         FIG. 7  is a schematic configuration diagram of an example of a conventional TOFMS. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     With reference to  FIGS. 1 to 6 , a TOFMS according to an embodiment of the present invention will be described. 
     As shown in  FIG. 1 , like the conventional TOFMS described above, a TOFMS  10  according to this embodiment includes an ion source  11 , a quadrupole mass filter  12 , a collision cell  13 , an ion guide  14 , an ion trap  15 , a time-of-flight mass separator  16 , and an ion detector  17 . As described above, the time-of-flight mass separator  16  includes an expulsion electrode  161 , a grid electrode  162 , a TOFMS flight space  163 , and a reflectron (reflector)  164 . A portion from immediately after the ion source  11  to immediately before the time-of-flight mass separator  16  causes ions to fly almost horizontally, and the combination of the expulsion electrode  161  and the grid electrode  162  in the time-of-flight mass separator  16  accelerates the ions to make them fly downward. As described above, the TOFMS  10  according to this embodiment is an orthogonal acceleration type TOFMS that accelerates ions in a direction orthogonal to the incident direction of the ion beam. 
     The TOFMS  10  also includes a first vacuum vessel (upper vacuum vessel)  18 A accommodating the ion source  11 , the quadrupole mass filter  12 , the collision cell  13 , the ion guide  14 , the ion trap  15 , the expulsion electrode  161 , the grid electrode  162 , the ion detector  17 , and an upper TOFMS flight space  163 A that is a part of the TOFMS flight space  163 . The first vacuum vessel  18 A has, in longitudinal section view, such an L shape that one end of a transverse space extending in the transverse direction is connected to the upper end of a longitudinal space extending in the longitudinal direction. The ion source  11 , the quadrupole mass filter  12 , the collision cell  13 , the ion guide  14 , and the ion trap  15  are accommodated in the transverse space, and the TOFMS flight space  163  is formed in the longitudinal space. The quadrupole mass filter  12 , the ion nuide  14 , and the ion trap  15  correspond to the above-described ion transportation unit. The expulsion electrode  161 , the grid electrode  162 , and the ion detector  17  are disposed in a portion where the transverse space and the longitudinal space intersect. In the case of the first vacuum vessel  18 A alone, the lower end of the longitudinal space is open. 
     The first vacuum vessel  18 A is mounted and fixed on a chassis  19 . As in the case of the conventional TOFMS, casters  191  and stoppers  192  are attached to the lower surface of the chassis  19 . 
     The TOFMS  10  includes a second vacuum vessel (lower vacuum vessel)  28  accommodating the reflection  164  and a lower TOFMS flight space  163 B that is the remaining part of the TOFMS flight space  163 . In the case of the second vacuum vessel  28  alone, the upper end of the second vacuum vessel  28  is open. The lower end of the longitudinal space of the first vacuum vessel  18 A and the upper end of the second vacuum vessel  28  are fastened with bolts, and a vacuum seal (not shown) for maintaining airtightness is disposed between the two vessels. This integrates the first vacuum vessel  18 A with the second vacuum vessel  28  to form a vacuum space where ions fly. 
     The second vacuum vessel  28  is fixed to a sub-chassis  21 . Sub-chassis casters  22  are attached to the lower surface of the sub-chassis  21 . A damper  23  for absorbing vibrations is disposed between the sub-chassis  21  and the second vacuum vessel  28 . The sub-chassis  21  is placed on the chassis  19  and fixed to the chassis  19  with bolts. In this state, the sub-chassis casters  22  are floating in the air. Note that the sub-chassis  21  may be fixed on a side portion of the chassis  19 . In either case, fixing the sub-chassis  21  to the chassis  19  integrates the sub-chassis  21  with the chassis  19  (enables the sub-chassis  21  to serve as a part of the chassis  19 ), thereby increasing the strength of the chassis  19 . The damper  23  may be disposed between the wall of the second vacuum vessel  28  and the reflection  164  from the viewpoint of not giving vibrations to the reflection  164 . That is, the damper  23  may be disposed in the second vacuum vessel  28 . However, the damper  23  generates a gas to cause a reduction in degree of vacuum in the second vacuum vessel  28 . Hence, the damper  23  is desirably disposed between the second vacuum vessel  28  and the sub-chassis  21  located outside the second vacuum vessel  28 . 
     The combination of the reflectron  164 , the second vacuum vessel  28 , the sub-chassis  21 , the sub-chassis casters  22 , and the damper  23  constitutes a reflector unit  20  (see  FIG. 2 ). 
     The operation of the TOFMS  10  accordingto this embodiment at the time of mass spectrometry is similar to that of the conventional TOFMS; therefore, the description thereof is omitted. The following describes the operation to be performed in transporting the TOFMS  10  from the factory and then installing the TOFMS  10  in the installation site. 
     First, the finished TOFMS  10  is separated into the reflector unit  20  and the other parts ( FIG. 3 ) in the factory. The parts other than the reflector unit  20  are moved by the casters  191  after releasing of the stoppers  192 , and mounted on transportation means such as a truck. At that time, the vibrations received from the floor surface through the casters  191  are transmitted to the parts. However, since the reflectron  164  is separated from the parts, the reflection  164  is not affected by the vibrations. Meanwhile, the reflector unit  20  including the reflection  164  is moved to the transportation means as carefully as possible so as not to give vibrations to the reflector unit  20 . At that time, the sub-chassis casters  22  may be used on the flat floor of the route to the transportation means since the damper  23  absorbs the vibrations. On the other hand, the reflector unit  20  is lifted and moved on the uneven road surface so as not to give vibrations to the reflectron  164  since the damper  23  may fail to sufficiently absorb the vibrations. Alternatively, the reflector unit  20  may be moved in such a manner that the reflector unit  20  is made to slide on rails placed on the floor surface. 
     Next, the reflector unit  20  and the other parts are transported to a loadinglunloading site near the installation site by the transportation means. At that time, at least the reflector unit  20  is transported by a method with special attention being paid not to give vibrations to the reflector unit  20  as much as possible, for example, using a truck equipped with an air suspension that absorbs vibrations or mounting the reflector unit  20  on a damping base. 
     After arriving at the loading/unloading site, the reflector unit  20  and the other parts are unloaded from the transportation means. Subsequently, as in the case of movement from the factory to the transportation means, the parts other than the reflector unit  20  are moved to the installation site by the casters  191  after releasing of the stoppers  192 . In addition, as in the case of movement from the factory to the transportation means, with regard to the route to the installation site, the reflector unit  20  is moved on the flat floor by the casters  22 , is moved on the uneven road surface while being lifted, or is moved by the rails placed on the floor surface. 
     At the installation site, first, the parts other than the reflector unit  20  are moved to the installation position of the TOFMS  10  and fixed at the installation position with the stoppers  192 . Next, the reflector unit  20  is moved below the first vacuum vessel  18 A, and the second vacuum vessel  28  and the first vacuum vessel  18 A are fastened with bolts. Further, the sub-chassis  21  and the chassis  19  are fixed. The installation of the TOFMS  10  in the installation site is thus completed. 
     As shown in the top view of  FIG. 4 , the bottom surface of the chassis  19  has a notch  193  located immediately below the first vacuum vessel  18 A and formed to receive the reflector unit  20 . The notch  193  allows the sub-chassis casters  22  of the reflector unit  20  to easily move the reflector unit to immediately below the attachment position. Although this notch reduces the strength of the chassis  19 , fixing the chassis  19  to the sub-chassis  21  of the reflector unit integrates the sub-chassis  21  with the chassis  19 , thereby increasing the strength of the chassis  19 . 
     In the TOFMS  10  according to this embodiment, the reflector unit  20  is separated from the other parts. Therefore, the reflector unit  20  is moved with the influence of vibrations suppressed at the time of transportation. The other parts are easily moved by the casters  191 . Accordingly, the high assembly accuracy of the reflectron  164  completed in the factory is maintained in the process of transporting and moving the TOFMS  10  to the installation site. 
     In the TOFMS  10  according to this embodiment, since the reflector unit  20  includes the sub-chassis casters  22  and the damper  23 , the sub-chassis casters  22  facilitates the movement of the reflector unit  20  on a flat floor surface. In use of the TOFMS  10 , moreover, the damper  23  inhibits the vibrations generated due to a vacuum pump (not shown) or the like evacuating the interior of the vacuum vessel from being transmitted to the reflectron  164 . This also contributes to maintaining high assembly accuracy of the reflectron  164 . 
     The TOFMS according to this embodiment may be variously modified. 
     In the above embodiment, the damper  23  is disposed between the sub-chassis  21  and the second vacuum vessel  28 . Alternatively, the damper  23  may be omitted as in the case of a reflector unit  20 A shown in  FIG. 5A . According to this configuration, in transporting the reflector unit  20 A, the sub-chassis casters  22  are not used, and the reflector unit  20 A is lifted and moved such that vibrations from the floor surface are not transmitted to the reflectron  164 . However, in the situation of work at the installation site, when the floor surface is flat, the sub-chassis casters  22  may be used for a small distance to move the reflector unit  20 A using the sub-chassis casters  22 . This facilitates work at the installation site. Alternatively, the sub-chassis casters  22  may be omitted as in the case of a reflector unit  20 B shown in  FIG. 5B , or the sub-chassis  21  may be omitted as in the case of a reflector unit  20 C shown in  FIG. 5C , In addition, as in the case of a reflector unit  2013  shown in  FIG. 5D , the sub-chassis  21  may be omitted and casters  22 A may be disposed on the lower surface of the second vacuum vessel  28 . 
     In the above embodiment, the casters  191  are disposed on the lower surface of the chassis  19 . Alternatively, the casters  191  may be omitted. In this case, the chassis  19  may be mounted on a carriage having casters and moved to an installation site. 
     In the above embodiment, the acceleration unit is of the orthogonal acceleration type that accelerates ions in a direction orthogonal to the incident direction of the ion beam. Alternatively, the ion trap  15  may be used to accelerate the ions in the same direction as the incident direction of the ion beam.  FIG. 6  shows such an example. In this example, the ion trap  15  is provided such that ions traveling in the horizontal direction are incident. In addition, a TOFMS flight space  1631  in which ions fly in the horizontal direction and a reflection  1641  for reflecting the ions are disposed at the subsequent stage of the ion trap  15 . The ion trap  15 , each component on the preceding stage, the detector  17 , and a part of the TOFMS flight space  1631  are accommodated in a first vacuum vessel  18 B. The reflection  1641  and the remaining part of the TOFMS flight space  1631  are accommodated in a second vacuum vessel  28 B. The first vacuum vessel  1813  and the second vacuum vessel  28 B are provided such that their openings face each other at the same height and both are fastened so that the openings communicate with each other at the installation site. The second vacuum vessel  28 B is disposed on a sub-chassis  21 B via support columns, and the sub-chassis  21 B has, on its lower surface, sub-chassis casters  22 B. The reflectron  1641 , a part of the TOFMS flight space  1631 , the second vacuum vessel  28 B, the sub-chassis  21 B, and the sub-chassis casters  22 B constitute a reflector unit  20 E. 
     Obviously, the present invention is not limited to the above embodiments and the above modifications, and various modifications can be made. 
     REFERENCE SIGNS LIST 
     
         
           10  . . . TOFMS 
           11  . . . Ion Source 
           12  . . . Quadrupole Mass Filter 
           13  . . . Collision Cell 
           14  . . . Ion Guide 
           15  . . . Ion Trap 
           151  . . . Ring Electrode 
           152  . . . End Cap Electrode 
           16  . . . Time-of-flight Mass Separator 
           161  . . . Expulsion Electrode 
           162  . . . Grid Electrode 
           163 ,  1631  . . . TOFMS Flight Space 
           163 A . . . Upper TOFMS Flight Space 
           163 B . . . Lower TOFMS Flight Space 
           164 ,  1641  . . . Reflectron 
           17  . . . Ion Detector 
           18  . . . Vacuum Vessel 
           18 A,  18 B . . . First Vacuum Vessel 
           28 ,  28 B . . . Second Vacuum Vessel 
           19  . . . Chassis 
           191  . . . Caster 
           192  . . . Stopper 
           20 ,  20 A,  20 B,  20 C,  20 D,  20 E . . . Reflector Unit 
           21 ,  211  . . . Sub-chassis 
           22 ,  22 B . . . Sub-chassis Caster 
           22 A . . . Caster 
           23  . . . Damper