Abstract:
An orthogonal acceleration time-of-flight (oa-TOF) mass spectrometer has an ion source producing ions, a first region of a low degree of vacuum, an ion reservoir, and a second region of a high degree of vacuum. A space for transporting the ions produced by the ion source is placed in the first region. The ion reservoir accelerates the ions transported in from the first region in a pulsed manner and extracts the ions. A time-of-flight mass analyzer for mass separating the ions extracted from the ion reservoir is disposed in the second region, together with the ion reservoir. An isolation valve is mounted in a hole that places the first and second regions in communication with each other to permit the first and second regions to be isolated from each other.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an orthogonal acceleration time-of-flight (oa-TOF) mass spectrometer and, more particularly, to an oa-TOF mass spectrometer which, if the ion source is contaminated with a sample, can remove the contaminant in a short time and resume measurements. 
     2. Description of Related Art 
     A mass spectrometer is an instrument in which ions created from a sample are made to travel through a vacuum. During the process of the flight, ions having different masses are separated and recorded as a spectrum. Known types of mass spectrometers include: magnetic mass spectrometer in which ions are dispersed according to mass using a sector magnetic field; quadrupole mass spectrometer (QMS) for sorting ions (filtering) according to mass using quadrupole electrodes; and time-of-fight mass spectrometer (TOFMS) for separating ions by making use of variations in time of flight due to different masses. 
     Of these mass spectrometers, magnetic mass spectrometer and QMS are adapted for ion sources that create ions continuously. On the other hand, TOFMS is suitable for ion sources that create pulsed ions. Accordingly, if one attempts to use a continuous ion source for TOFMS, some contrivance is necessary for utilization of the ion source. The orthogonal acceleration time-of-flight mass spectrometer (oa-TOFMS) is one example of TOFMS designed to emit pulsed ions from a continuous ion source. 
     A typical configuration of oa-TOFMS is shown in  FIG. 1A . This instrument has a continuous atmospheric-pressure ion source  1  (such as electrospray ionization (ESI) ion source or inductively coupled plasma (ICP) ion source), differentially pumped walls  10   a ,  10   b  consisting of first and second partition walls and a vacuum pump (not shown), a first orifice  2  formed in the first partition wall of the differentially pumped walls  10   a ,  10   b , a ring lens  3  placed within the differentially pumped walls  10   a ,  10   b , a second orifice  4  formed in the second partition wall forming the differentially pumped walls  10   a ,  10   b , an intermediate chamber  11  of a somewhat low degree of vacuum where an ion guide  5  is placed, lenses  6  consisting of focusing lenses and deflectors, a launcher  7  consisting of an ion repeller plate  7   a  and accelerating lenses (grids)  7   b , a reflector  8  for reflecting ions, and a measuring chamber  13  of a high degree of vacuum where components forming the ion optics, such as an ion detector  9 , are placed. 
     The various portions have the following degrees of vacuum. The degree of vacuum of the atmospheric-pressure ion source  1  is 0.1 MPa (atmospheric pressure). The degree of vacuum of the intermediate chamber  11  is in a range from 10 −1  to 10 −4  Pa. The degree of vacuum of the measuring chamber  13  is of the order of 10 −5  Pa or better. 
     In this configuration, ions generated from the sample in the atmospheric-pressure ion source  1  are first introduced into the differentially pumped walls  10   a ,  10   b  through the first orifice  2 . The ions tending to diffuse within the differentially pumped walls  10   a ,  10   b  are focused by the ring lens  3  in the walls  10 . Then, the ions are admitted through the second orifice  4  into the intermediate chamber  11 , where the ions are made uniform in kinetic energy. The ion beam diameter is reduced by an RF electric field produced by the ion guide  5 . The ions are then guided into the high-vacuum measuring chamber  13 . The partition wall that partitions the intermediate chamber  11  and the measuring chamber  13  from each other is provided with a third orifice  12  that places both chambers in communication with each other. This third orifice  12  shapes the ions that are guided in by the ion guide  5  into an ion beam of a given diameter (e.g., about 0.3 mm). The ion beam is then passed into the measuring chamber  13 . 
     On the other hand, as shown in  FIG. 1B , in an oa-TOFMS instrument having the continuous ion source  1  (such as electron impact (EI) ion source, chemical ionization (CI) ion source, field desorption (FD) ion source, or fast atom bombardment (FAB) ion source) in its intermediate chamber  11 , ions produced in the ion source  1  pass through a focus lens  1 ′ and an orifice  12  and are introduced into the measurement chamber  13 . 
     These various ion sources roughly have the following degrees of vacuum. The degrees of vacuum of EI ion sources are 10 −2  to 10 −3  Pa. The degrees of vacuum of CI ion sources are 5×10 −2  to 5×10 −3  Pa. The degrees of vacuum of FD ion sources are of the order of 10 −4  Pa. The degrees of vacuum of FAB ion sources are of the order of 10 −3  Pa. 
     The lenses  6  consisting of the focusing lenses and deflectors are installed at the entrance of the measuring chamber  13 . The ion beam entering the measuring chamber  13  is corrected for diffusion and deflection by the lenses  6  and introduced into the launcher  7 . Installed inside the launcher  7  are the ion reservoir and accelerating lenses arrayed in a direction orthogonal to the axis of the ion reservoir. In this ion reservoir, an ion repeller plate is disposed opposite to the grids. 
     The ion beam first travels straight toward the ion reservoir  17  that is located among the repeller plate  14 , grids  15 , and accelerating lenses  16  as shown in  FIG. 2 . The ion beam  18  moving straight through the ion reservoir  17  and having a given length is accelerated in a pulsed manner in a direction (X-axis direction) vertical to the direction (Y-axis direction) along which the ion beam  18  enters, by applying a pulsed accelerating voltage to the repeller plate  14 . This forms pulsed ions  19  which begin to travel toward a reflector (not shown) mounted opposite to the ion reservoir  17 . 
     The ions accelerated in the vertical direction travel in a slightly oblique direction slightly deviating from the X-axis direction because the velocity in the Y-axis direction assumed on entering the measuring chamber  13  and the velocity in the X-axis direction orthogonal to the Y-axis direction are combined. The latter velocity is given by the repeller plate, grids, and accelerating lenses. The ions are reflected by the reflector  8  and arrive at the ion detector  9 . 
     When the ions are being accelerated, the same potential difference acts on every ion regardless of the masses of the individual ions. Therefore, lighter ions have greater velocities and vice versa. As a consequence, variations in ion mass appear as variations in arrival time taken to reach the ion detector  9 . Variations in ion mass can be separated as variations in ion flight time. 
     In this way, the continuous ion source can be applied to TOFMS adapted for a pulsed ion source by accelerating the ion beam created from the continuous ion source  1  in a pulsed manner by the launcher  7  consisting of the repeller plate, grids, and accelerating lenses. 
     In oa-TOFMS, the kinetic energy of ions made to enter the ion reservoir is normally set to a very small value of less than 50 eV. Therefore, oa-TOFMS is affected much more by charging of the electrodes than the magnetic mass spectrometer. As a result, if an area ranging from the external ion source to the ion reservoir is charged at all, the ion beam introduced into the ion reservoir is deflected and tilted as shown in  FIG. 3 . This deteriorates the resolution and sensitivity of oa-TOFMS. 
     Such charging can occur quite easily by adhesion of organics to the surfaces of the electrodes, the organics being residues of the sample ions. Especially, this phenomenon occurs quite easily in measurements using cold-spray ionization mass spectrometry that is one method of ESI (see Japanese Patent No. 3137953) or inductively coupled plasma-mass spectrometry (ICP-MS) because the concentration of the sample is very high and the components around the ion trajectory within the vacuum region on the side of the ion source are often contaminated in a short time. 
     A conventional measure for eliminating this problem consists of halting the operation of the oa-TOFMS, breaking the vacuum, taking components located around the ion trajectory within the vacuum region on the side of the ion source into the atmosphere, and cleaning the components. 
     If the cleaning is done by this method and then the components are mounted again, a waiting time of from about half to full day is necessary until the degree of vacuum of the oa-TOFMS is recovered. During this time interval, the instrument cannot be used. 
     On the other hand, in a normal gas chromatograph-mass spectrometer (GC-MS), a system having an isolation valve and a preliminary evacuation chamber has been already put into practical use such that contamination of the ion source can be removed while the vacuum in the mass spectrometer is maintained (see Japanese Patent Laid-Open No. 2004-134321 and Japanese Patent Laid-Open No. 2004-139911). 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide an orthogonal acceleration time-of-flight mass spectrometer (oa-TOFMS) whose operation can be resumed in a short time if components around the ion trajectory within the vacuum region on the side of the ion source are taken into the atmosphere and cleaned or otherwise operated and which permits stable measurements if an isolation valve is mounted. 
     An oa-TOFMS instrument for achieving this object in accordance with the present invention has an ion source for producing ions, a first region of a low degree of vacuum, a space for transporting the produced ions, the space being placed in the first region, an ion reservoir for accelerating the ions transported in from the first region in a pulsed manner and extracting the ions, a second region of a high degree of vacuum, a flight time mass analyzer for mass separating the ions extracted from the ion reservoir, the ion reservoir and the mass analyzer being placed in the second region, and a hole for placing the first and second regions in communication with each other. A partition valve is mounted in the hole such that the regions can be isolated from each other. 
     In one feature of the present invention, the ion source is an atmospheric-pressure ion source consisting of an ESI ion source or ICP ion source. 
     In another feature of the present invention, the degree of vacuum of the first region is in a range from 10 −1  Pa to 10 −4  Pa. 
     In a further feature of the present invention, the degree of vacuum of the second region is of the order of 10 −5  Pa or better. 
     In still another feature of the present invention, the partition valve is a gate valve or rotary valve through which an optical axis extends. 
     In yet another feature of the present invention, the partition valve is made of a gate valve fitted with an O-ring having a sealing surface facing away from the direction in which the ion beam travels. 
     In an additional feature of the present invention, an ion guide is placed in the space through which the ions produced by the ion source are transported, and an orifice plate having an orifice is placed between the ion guide and the partition valve. 
     In still another feature of the present invention, the orifice plate is mounted to the end of the ion guide that is on the side of the mass analyzer. 
     In yet another feature of the present invention, the orifice has a diameter slightly larger than the diameter of the ion beam being transported. 
     If components located around the ion trajectory in the vacuum region at a side of the ion source are taken into the atmosphere and cleaned, the instrument can be started to be used again in a short time. 
     The present invention also provides an oa-TOFMS instrument having an ion source for producing ions, a first region of a low degree of vacuum, a space for transporting the produced ions, the space being placed in the first region, an ion reservoir for accelerating the ions transported in from the first region in a pulsed manner and extracting the ions, a second region of a high degree of vacuum, a flight time mass analyzer for mass separating the ions extracted from the ion reservoir, the ion reservoir and the mass analyzer being placed in the second region, and a hole for placing the first and second regions in communication with each other. The ion source is one selected from the group consisting of EI ion source, CI ion source, FD ion source, and FAB ion source. A partition valve made of a gate valve is mounted in the hole. The gate valve is fitted with an O-ring having a sealing surface facing away from the direction in which the ion beam travels. 
     Other objects and features of the invention will appear in the course of the description thereof, which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate the prior art oa-TOFMS instrument; 
         FIG. 2  illustrates the vicinities of the ion reservoir of the prior art oa-TOFMS instrument; 
         FIG. 3  illustrates the vicinities of the ion reservoir of the prior art oa-TOFMS instrument, showing a state different from the state shown in  FIG. 2 ; 
         FIG. 4  is a diagram of an oa-TOFMS instrument according to the present invention; 
         FIG. 5  illustrates the vicinities of the ion reservoir of the oa-TOFMS instrument shown in  FIG. 4 ; 
         FIG. 6  is a diagram illustrating voltages applied to the ion repeller electrode of the instrument shown in  FIGS. 4 and 5 ; and 
         FIG. 7  illustrates the vicinities of the partition valve of an oa-TOFMS instrument according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention are hereinafter described with reference to the accompanying drawings.  FIG. 4  shows an orthogonal acceleration time-of-flight (oa-TOF) mass spectrometer according to one embodiment of the present invention. This instrument comprises an external continuous ion source  20  (such as an electrospray ionization (ESI) ion source (known as a cold-spray ion source) or ICP ion source which tends to be easily contaminated with a sample), an ion transport portion  21  having a degree of vacuum of about 10 −1  to 10 −4  Pa (more preferably, about 10 −2  to 10 −3  Pa) for transporting the ions created ion source  20  toward an oa-TOF mass analyzer  22  located behind the ion transport portion  21 . The analyzer  22  is placed in a degree of vacuum of the order of 10 −5  Pa or better. An ion guide (not shown) is installed in the ion transport portion  21  to transport the ions efficiently from the ion source  20  to the mass analyzer  22 . 
     The ion transport portion  21  and the oa-TOF mass analyzer  22  are partitioned by a partition wall provided with a hole to place these two portions  21  and  22  in communication with each other and to permit passage of ions. An isolation valve  23  is mounted in this hole. The two portions  21  and  22  can be separated spatially by the isolation valve  23 . A gate valve is used as the isolation valve  23  and designed such that the optical axis extends through the hole and that the hole is plugged up by linear motion of the valve body. A rotary valve having a valve body that plugs up the hole when the valve body is rotated may also be used, if the optical axis extends through the hole. 
     The ion beam first moves straight in a low-energy state of 20 to 50 eV toward an ion reservoir  26  as shown in  FIG. 5 . The reservoir  26  is sandwiched between an ion repeller electrode  24  and grids  25 . The ion beam (a) has a given length and moves within the ion reservoir  26  parallel to it. A pulsed accelerating voltage of the order of kilovolts and having the same polarity as the ions as shown in  FIG. 6  is applied to the ion repeller electrode  24 . As a result, the ions are accelerated in a pulsed manner in a direction (X-axis direction) orthogonal to the direction (Y-axis direction) of an axis along which the ion beam (a) enters. This creates a TOF ion beam (b), which starts to fly toward a reflector  27  mounted in a position opposite to the ion reservoir  26 . 
     The ions accelerated in the vertical direction fly in a slightly oblique direction slightly deviating from the X-axis direction, because the velocity in the Y-axis direction assumed on entering the oa-TOF mass analyzer  22  and the velocity in the X-axis direction orthogonal to the Y-axis direction are combined. The latter velocity is given by the ion repeller plate  24  and grids  25 . The ions are reflected by the reflector  27  and arrive at an ion detector  28 . 
     During the process of acceleration of the ions, the same potential difference acts on the ions regardless of their masses. Therefore, lighter ions fly faster and heavier ions fly slower. As a result, variations in ion mass appear as variations in arrival time to the ion detector  28 . Variations in ion mass can be separated as variations in ion flight time. 
       FIG. 7  is an enlarged view of the vicinities of an isolation valve of an oa-TOFMS according to the present invention. An ion guide  29  is mounted to guide ions created at atmospheric pressure (0.1 MPa) into a vacuum such that the ions can be transported without ion intensity loss from an intermediate chamber region of a somewhat low degree of vacuum (10 −2  to 10 −3  Pa) to a measuring chamber region of a high degree of vacuum (of the order of 10 −5  Pa or better). In the illustrated embodiment, an RF power supply (not shown) is connected with quadrupole electrodes to operate them as the ion guide. 
     An orifice plate  30  provided with an orifice having a diameter that is set such that the ion beam spatially restricted by the ion guide  29  and guided can narrowly pass through the orifice in the orifice plate  30 . Because of the effect of this orifice diameter, the degree of vacuum of the mass analyzer  31  can be maintained in a state higher than the degree of vacuum of the ion transport portion  32  by two or three orders of magnitude. In particular, the diameter of the ion beam is 0.3 mm. The diameter of the orifice in the orifice plate  30  is about 1 to 20 times the diameter of the ion beam, i.e., about 0.3 to 6 mm, more preferably about 1.7 times, i.e., about 0.5 mm. 
     The orifice plate  30  is attached to the end of the ion guide  29  at the side of the mass analyzer. If the ion guide  29  is taken into the atmosphere, the orifice plate  30  can be taken into the atmosphere at the same time. 
     An isolation valve  33  is mounted behind the orifice plate  30 . The isolation valve is made of a gate valve such that it is accommodated within a narrow space as viewed along the optical axis of the ions. The isolation valve  33  is opened and closed by a knob  34 . Since the valve is made of the gate valve, the diameter of the partition portion can be made large. Consequently, the member forming the partition portion is located remotely from the optical axis of the ions. Hence, ions undergoing low acceleration energy are less affected by asymmetrical structure of the partition portion and by contaminant such as grease adhering to the surface. That is, the ions are less bent out of their trajectory. Furthermore, since a gate valve is used as the isolation valve, the thickness can be reduced if the diameter is increased compared with other types of isolation valves. As a consequence, the distance from the orifice in the orifice plate  30  to the mass analyzer  31  can be made small. Thus, the ion beam can be introduced into the mass analyzer  31  while reducing the effects of ion beam broadening. 
     The isolation valve  33  is fitted with an O-ring having a sealing surface  35 . During the operation of the isolation valve  33 , the sealing surface  35  is exposed to the ion beam and so tends to be contaminated with electrically charged substances. Therefore, the sealing surface  35  faces away from the direction of travel of the ion beam (Y-axis direction). Consequently, it is unlikely that the ion beam directly hits the sealing surface. The sealing surface is not easily electrically charged. 
     If the inside of the vacuum chamber is contaminated with the sample or the like, the isolation valve  33  is first closed. The vacuum pump (not shown) is stopped only on the side of the ion transport portion  32 . The inside is placed at the atmospheric pressure. Then, a flange (not shown) is opened. The contaminated part is removed and cleaned. If the ion guide  29  is removed, the orifice plate  30  can be pulled out together. Consequently, the orifice plate  30  that tends to be contaminated can be cleaned together. 
     The cleaned part is mounted again. The vacuum pump (not shown) on the side of the ion transport portion  32  is restarted. The power supply of the ion source is turned ON to restart it. After a lapse of about 15 minutes, the isolation valve  33  is opened. Thus, preparations are made for remeasurements. In the time when the isolation valve  33  was not available, evacuation was required for from about half to full day until measurements could be resumed. Hence, a long waiting time was necessary. Hence, the time shortening effect is quite advantageous. 
     In the present embodiment, an orthogonal acceleration time-of-flight (oa-TOF) mass spectrometer equipped with an ESI ion source or ICP ion source has been described. Obviously, the invention can also be applied to an oa-TOF mass spectrometer equipped with an EI ion source, CI ion source, FD ion source, or FAB ion source. 
     The present invention can be used in a wide range of oa-TOF mass spectrometers. 
     Having thus described my invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.