Abstract:
A time-of-flight mass spectrometer capable of cutting out a major portion of carrier gas-derived ions ahead of the ion reservoir. The ion source is of the electron impact type and has source magnets for deflecting some of the produced ions away from the center axis of the ion reservoir. Electrostatic lenses for promoting the deflection of the ions caused by the source magnets and a differentially pumped slit for cutting off the deflected ions are mounted downstream of the ion source.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a high-performance gas chromatograph/mass spectrometer used for quantitative analysis and simultaneous qualitative analysis of trace organic compounds and for structural analysis of sample ions. 
   2. Description of Related Art 
   Gas chromatograph/mass spectrometer (GC-MS) instruments fitted with electron impact (EI) ion sources chiefly include magnetic-sector and quadrupole mass spectrometers. 
   In a magnetic-sector mass spectrometer, a high ion acceleration voltage of the order of kilovolts is usually applied to ions produced in an EI ion source to make them travel toward the mass analyzer region. The ions pass through a free space or a lens system, such as Q lenses, and then are deflected in the mass analyzer region based on the following relation:
 
 m/z=K·B   2   /V   a 
 
where m/z is the mass-to-charge ratio of the ions, B is the magnetic flux density of the magnetic field, V a  is the ion acceleration voltage, and K is a constant.
 
   Accordingly, if the magnetic flux density B is kept at a certain value, and if only ions having a certain mass-to-charge ratio are allowed to reach the detector, then a quantitative analysis of the ions can be performed. Furthermore, if the magnetic flux density B is swept, a mass spectrum can be obtained. Hence, simultaneous qualitative analysis can be performed. 
   On the other hand, in a quadrupole mass spectrometer, ions produced in an EI ion source are made to travel under a low acceleration voltage of about tens of volts. The mass analyzer region of the quadrupole mass spectrometer literally consists of four parallel metal rods which may have hyperbolic or cylindrical surfaces. A superimposition of an RF AC voltage and a DC voltage is applied to each rod to perform mass separation. Based on the following relation, only ions having a certain mass-to-charge ratio pass through the mass analyzer region:
 
 m/z=K·V   f /f 2  
 
where m/z is the mass-to-charge ratio of the ions, V f  is the RF amplitude voltage, f is the RF frequency, and K is a constant.
 
   In each of the magnetic-sector and quadrupole mass spectrometers, the EI ion source consists of an ionization chamber, source magnets, a filament, and an Einzel lens system made up of plural electrodes. Each of the Einzel lens electrodes is made of a unitary structure provided with a circular opening about the axis of the ion trajectory. See Japanese Patent Laid-Open No. S62-168328. 
   In recent years, a gas chromatograph/time-of-flight mass spectrometer fitted with an EI ion source has been developed. This instrument has an analyzer region made of an orthogonal acceleration time-of-flight (oa-TOF) mass spectrometer. A time-of-flight mass spectrometer performs mass separation by making use of the fact that the flight speed differs according to the ion mass when a given acceleration voltage is applied to ions. A mass spectrum is recorded according to differences in arrival time at the ion detector. 
   As this ion detector, a microchannel plate (MCP) detector having high time resolution is often used. In the case of an oa-TOFMS instrument, ions introduced into the ion acceleration region all reach the ion detector regardless of their mass-to-charge ratio. 
   Therefore, if a carrier gas, such as helium gas, that flows in at a high flow rate of 1 to 2 ml per min. from a gas chromatograph is ionized in large quantity by electron impact, the amount of the resulting ion current reaches 100 to 1,000 times the amount of ion current of the sample ions. 
   When this large amount of carrier gas ions reaches the MCP, a dead time of tens of microseconds to several milliseconds is produced due to saturation of the MCP. As a result, mass spectral data is lost. In addition, other problems, such as shortening of the life of the MCP, take place. 
   In conventional magnetic-sector and quadrupole mass spectrometers, only ions having a certain mass-to-charge ratio are allowed to reach the detector, and mass separation is effected. Because of this mechanism, it has been possible to prevent the ions of the carrier gas of the gas chromatograph from reaching the detector by appropriately setting the analytical conditions. Therefore, it can be understood that these problems are unique to TOFMS instruments. Furthermore, where ions are treated under low acceleration conditions, the effects of charging due to adhesion of contaminants to the electrodes cannot be neglected. 
   For example,  FIG. 1  shows the configuration of an orthogonal acceleration time-of-flight mass spectrometer. This instrument has an external ion source  1 , a differentially pumped (evacuated) chamber  10  formed by first and second partition walls and by a vacuum pump (not shown), an intermediate chamber  11 , and a measuring chamber  13 . A first orifice  2  is formed in the first partition wall of the differentially pumped chamber  10 . A ring lens  3  is placed in the differentially pumped chamber  10 . A second orifice  4  is formed in the second partition wall of the differentially pumped chamber  10 . Ion guides  5  are placed in the intermediate chamber  11 . Ion optics including a set of lenses  6  made up of focusing lenses and a deflector, a launcher  7  consisting of an ion repeller plate and accelerating lenses (grids), an ion reflector  8  for reflecting ions, and an ion detector  9  are fitted in the measuring chamber  13 . 
   In this configuration, ions produced from a sample in the external ion source  1  are first introduced into the differentially pumped chamber  10  through the first orifice  2 . The ions that tend to diffuse themselves in the differentially pumped chamber  10  are focused by the ring lens  3  located inside the chamber  10  and admitted into the intermediate chamber  11  through the second orifice  4 . The ions are then decreased in kinetic energy in the intermediate chamber  11 . The diameter of the ion beam is reduced by the RF electric field produced by the ion guides  5  and guided into the high-vacuum measuring chamber  13 . A third orifice  12  is formed in the partition wall that partitions the intermediate chamber  11  and measuring chamber  13  from each other. The ions guided in from the ion guides  5  are shaped into a round ion beam having a certain diameter by the third orifice  12  and introduced into the measuring chamber  13 . 
   The set of lenses  6  consisting of the focusing lenses and deflector is installed at the entrance of the measuring chamber  13 . The ion beam entering the measuring chamber  13  is corrected in terms of diffusion and deflection by the lenses  6  and then introduced into the launcher  7 . As shown in  FIG. 2 , an ion reservoir and accelerating lenses arrayed perpendicularly to the axis of the ion reservoir are mounted in the launcher  7 . In the ion reservoir, an ion repeller electrode  14  and grid  15  are placed opposite to each other. 
   The ion beam  18  is at first in a very low energy state of 20 to 50 eV and is introduced parallel toward the ion reservoir  17  surrounded by the ion repeller electrode  14 , grids  15 , and accelerating lenses  16  as shown in  FIG. 2 . The ion beam  18  having a given length and moving parallel through the ion reservoir  17  is pulsed and accelerated in a direction (Z-axis direction) perpendicular to the direction of entrance (X-axis direction) of the ion beam  18  by applying a pulsed accelerating voltage on the order of kV having the same polarity as the ions to the repeller plate  14  as shown in  FIG. 2 . As a result, an ion pulse  19  that starts to travel toward a reflector  8  located opposite to the ion reservoir  17  is formed. 
   The ions accelerated in the vertical direction have a velocity that is the sum of the X-axis direction velocity assumed when they are introduced into the measuring chamber  13  and the Z-axis direction velocity that is given perpendicularly to the X-axis direction by the ion repeller electrode, grids, and accelerating lenses. Consequently, the ions travel in a direction slightly deviating from the Z-axis direction and are reflected into the ion detector  9  by the reflector  8 . 
   In the oa-TOFMS instrument, ions must be introduced into the ion reservoir  17  with a quite low accelerating energy. Because of this principle, it is advantageous to set the ion acceleration voltage at the ion source as low as possible. If such ion introduction at low velocity is carried out using a large amount of carrier gas ions, the ions come into contact with the ion repeller electrode  14  and grids  15  because the ions have a spatial spread. This promotes charging of these electrodes. 
   In the case of an oa-TOFMS instrument, mass separation is performed based on variations in arrival time at the ion detector. Therefore, the instrumental sensitivity and resolution depend on the initial position of the ions in the ion reservoir and on the uniformity of the initial kinetic energy. Accordingly, if there is nonuniform charging in the ion reservoir, the ions introduced with low acceleration have nonuniform initial position in the ion reservoir and nonuniform initial kinetic energy. Consequently, the instrumental sensitivity and resolution are deteriorated severely. Furthermore, they will age with time. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a time-of-flight mass spectrometer which has an ion source acting to massively ionize carrier gas-derived ions introduced in large quantity from a gas chromatograph and which is capable of cutting out a major portion of the carrier gas-derived ions ahead of the ion reservoir. 
   This object is achieved by a time-of-flight mass spectrometer which is built in accordance with the present invention and comprises: an electron impact ion source fitted with source magnets for enhancing the ionization efficiency of a sample gas; an ion reservoir for making ions produced by the ion source stay therein; an ion repeller plate and grids disposed on opposite sides of the ion reservoir to impulsively accelerate the ions out of the ion reservoir; a time-of-flight mass analyzer region for mass separating the ions extracted from the ion reservoir via the grids; and an ion detector for detecting the mass separated ions. The mass spectrometer is characterized in that it further includes first deflection means mounted between the ion source and the ion reservoir and cutoff means located between the first deflection means and the ion reservoir. The first deflection means can deflect the ions in the same direction as the direction of deflection that the ions traveling from the ion source to the ion reservoir undergo from the magnetic field produced by the source magnets. The cutoff means cuts off the deflected ions. 
   In one feature of the present invention, the cutoff means is a rectangular slit. 
   In another feature of the present invention, the cutoff means is a differentially pumped slit. 
   In a further feature of the present invention, the cutoff means is so constructed that a voltage can be applied to the means. 
   In still another feature of the present invention, second deflection means for correcting deflection of the ions induced by the source magnets and first deflection means is mounted downstream of the cutoff means. 
   In yet another feature of the present invention, an entrance slit for limiting the spread of the ion beam is mounted between the second deflection means and the ion reservoir. 
   In an additional feature of the present invention, the entrance slit has a heating mechanism. 
   In yet a further feature of the present invention, the sample gas is a gas supplied from a gas chromatograph. 
   In still a further feature of the present invention, the ions cut off are ions of a carrier gas supplied from a gas chromatograph. 
   Other objects and features of the present invention will appear in the course of the description thereof, which follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a conventional time-of-flight (TOF) mass spectrometer; 
       FIG. 2  is a diagram illustrating the vicinities of the ion reservoir of the conventional TOF mass spectrometer; 
       FIGS. 3(   a ) and  3 ( b ) are diagrams illustrating one embodiment of the vicinities of an ion reservoir incorporated in a TOF mass spectrometer according to the present invention; and 
       FIGS. 4(   a ),  4 ( b ),  4 ( c ), and  4 ( d ) show mass spectra measured using the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention is hereinafter described with reference to the accompanying drawings.  FIGS. 3(   a ) and  3 ( b ) show one embodiment of the portion of a time-of-flight (TOF) mass spectrometer according to the invention that ranges from its ion source to the vicinity of its ion reservoir.  FIG. 3(   a ) shows the portion from the ion source in the vicinity of the ion reservoir, as viewed from the Y-axis direction, and  FIG. 3(   b ) shows the same portion, as viewed from the Z-axis direction. 
   The mass spectrometer has an ionization chamber  20  consisting of an EI ion source. In this chamber, a sample gas introduced from a gas chromatograph is ionized by thermionic emission emitted from a filament (not shown) toward the Y-axis direction. A pair of source magnets  21  is disposed on the outer wall of the ionization chamber  20  and located opposite to each other in the Y-axis direction. The main role of the source magnets  21  is to suppress the spread of the thermionic emission produced from the filament (not shown) to enhance the ionization efficiency of the sample gas. The magnets also act to deflect the produced ions in the Z-axis direction. 
   An electrostatic lens system  22  consisting of plural electrodes is disposed behind the ionization chamber  20 .  FIGS. 3(   a ) and  3 ( b ) show an example in which the lens system is made up of three electrodes. At least one of the electrodes is split into two parts at the center. The two parts are located opposite to each other in the Z-axis direction and on the opposite sides of the optical axis (X-axis) of the ions. In the example of  FIGS. 3(   a ) and  3 ( b ), the second electrode as counted from the side of the ionization chamber  20  is split into two. The other electrodes are each centrally provided with an opening on the X-axis. 
   A differentially pumped slit  23  (baffle) is placed behind the system of electrostatic lenses  22  and in the partition wall that partitions the chamber in which the system of electrostatic lenses  22  is placed from the chamber in which the ion reservoir  26  is placed. A circular opening on the X-axis, a rectangular opening that is long in the Y-axis direction and short in the Z-axis direction, or an elliptical opening that is long in the Y-axis direction and short in the Z-axis direction is formed in the center of the differentially pumped slit  23 . 
   The opening in the differentially pumped slit  23  is elongated in the Y-axis direction such that the opening extends in the same Y-axis direction along which the thermionic emission is emitted in the ion source  20 . Furthermore, the opening in the slit  23  having a reduced dimension in the Z-axis direction is intended to facilitate cutting off the ions deflected in the Z-axis direction by the source magnets  21  and the split electrodes of the electrostatic lens system  22 . Slits having such features are herein collectively referred to as rectangular slits. 
   Electrostatic deflectors  24  consisting of at least one pair of electrodes are disposed behind the differentially pumped slit  23  and opposite to each other in the Z-axis direction to correct the deflection of the ions caused in the Z-axis direction by the source magnets  21  and system of electrostatic lenses  22 . 
   An entrance slit  25  for limiting the spread of the ion beam and the ion reservoir  26  are disposed behind the electrostatic deflectors  24 . A circular opening on the X-axis, a rectangular opening that is long in the Y-axis direction and short in the Z-axis direction, or an elliptical opening that is long in the Y-axis direction and short in the Z-axis direction is formed in the center of the entrance slit  25 . The ion reservoir  26  stores the ions passed through the entrance slit  25  and accelerates the ions toward the time-of-flight mass analyzer region (not shown) on the downstream side. 
   The instrument of the present embodiment constructed as described so far operates as follows. First, the sample gas supplied from the gas chromatograph is ionized by the thermionic emission produced in the Y-axis direction from the filament (not shown) mounted in the ionization chamber  20 . Spread of the thermionic emission is suppressed by the magnetic field produced by the source magnets  21  that are located opposite to each other in the Y-axis direction on the opposite sides of the ionization chamber  20 . 
   The produced sample ions are extracted from the ionization chamber  20  by the potential difference between the ionization chamber  20  and the system of electrostatic lenses  22 . Helium ions are derived from the carrier gas in the gas chromatograph and contained in the sample ions. Ions having small masses including these helium ions are deflected to a greater extent in the Z-axis direction at this time by the magnetic field produced by the source magnets  21  than other sample ions having larger masses. 
   A potential difference is produced between split electrodes  22   a  and  22   b  of the system of electrostatic lenses  22 , the electrodes  22   a  and  22   b  being located opposite to each other in the Z-axis direction on the opposite sides of the optical axis (X-axis) of the ions. This promotes the deflection in the same direction as the direction of deflection produced by the magnetic field of the source magnets  21 . 
   For this purpose, in a case where the produced ions are positive ions and the axis of the ion beam is deflected toward the split electrode  22   a  by the source magnets  21 , the split electrode  22   a  is placed at a lower potential than the split electrode  22   b.    
   Furthermore, in a case where the produced ions are positive ions and the axis of the ion beam is deflected toward the split electrode  22   b  by the source magnets  21 , the split electrode  22   b  is placed at a lower potential than the split electrode  22   a.    
   Where the produced ions are negative ions and the axis of the ion beam is deflected toward the split electrode  22   a  by the source magnets  21 , the split electrode  22   a  is placed at a higher potential than the split electrode  22   b.    
   Where the produced ions are negative ions and the axis of the ion beam is deflected toward the split electrode  22   b  by the source magnets  21 , the split electrode  22   b  is placed at a higher potential than the split electrode  22   a.    
   Of the ions deflected by the source magnets  21  and the split electrodes  22   a  and  22   b  of the system of electrostatic lenses  22 , helium ions and other ions having small masses are deflected to a greater extent. These lighter ions collide against the wall of the differentially pumped slit  23  and thus are cut off. Hence, these lighter ions cannot reach the downstream ion reservoir  26 . 
   On the other hand, those of the sample ions deflected by the split electrodes  22   a  and  22   b  of the system of electrostatic lenses  22  which are deflected to a lesser extent and have larger masses pass through the opening in the differentially pumped slit  23  and then are corrected in terms of deflection by the downstream electrostatic deflectors  24  consisting of at least one pair of electrodes located opposite to each other in the Z-axis direction such that the Z-axis direction deflection of the ion beam caused by the source magnets  21  and split electrodes  22   a ,  22   b  of the electrostatic lenses  22  agrees in direction with the optical axis (X-axis) of the ion reservoir  26 . 
   The sample ions corrected in terms of ion beam axis are restricted in ion spread by the entrance slit  25  and introduced into the ion reservoir  26 . Then, the ions are accelerated toward the grids  28  (i.e., in the Z-axis direction) by applying a high pulsed voltage of hundreds of volts to kilovolts having the same polarity as the ions to the ion repeller plate  27 . The ions travel through the time-of-flight mass analyzer region (not shown) and reach the ion detector (not shown), where they are detected. 
     FIGS. 4(   a ),  4 ( b ),  4 ( c ), and  4 ( d ) are mass spectra observed in a case where the same value of voltage is applied to the electrostatic lens electrodes  22   a  and  22   b  bisected in the Z-axis direction such that no potential difference is produced between them and in another case where different values of voltage are applied to the electrodes to produce a potential difference between them, respectively. 
     FIGS. 4(   a ) and  4 ( b ) show the mass spectra obtained where the same value of voltage is applied to the electrostatic lens electrodes  22   a  and  22   b  bisected in the Z-axis direction such that no potential difference is produced between them.  FIGS. 4(   c ) and  4 ( d ) show the mass spectra obtained where different values of voltage are applied to the electrodes to produce a potential difference between them. 
   Also in  FIGS. 4(   a ) and  4 ( c ) are the mass spectra derived by noting helium ions (m/z =4).  FIGS. 4(   b ) and  4 ( d ) are mass spectra derived by noting the peak (m/z=281) derived from the column&#39;s stationary phase. (Hereafter, the mass spectra are referred to as (a), (b), (c), and (d).) 
   With respect to these mass spectra, the MCP (detector) voltage that determines the sensitivity of the detector (MCP) has different values of 2.1 kV, 2.6 kV, 2.5 kV, and 2.6 kV for spectra (a), (b), (c), and (d), respectively. Therefore, the sensitivity is not uniform. The relation between the MCP voltage and sensitivity (ion intensity) is given by 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
                 
             
             
                 
               MCP voltage 
               Sensitivity 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               2.1 kV 
               1 
             
             
                 
               2.5 kV 
               22 
             
             
                 
               2.6 kV 
               37 
             
             
                 
                 
             
           
        
       
     
   
   In each of the mass spectra (a) and (c), the left end peak indicates helium ions. In these mass spectra, the peak intensities of the helium ions are
         441×10 3  (MCP voltage: 2.1 kV)   347×10 3  (MCP voltage: 2.5 kV), respectively.
 
By taking account of the difference in MCP sensitivity, a potential difference is produced between the electrostatic lens electrodes  22   a  and  22   b . As a result, 96% of the helium ions can be cut out.
       

   Comparison of the mass spectra (b) and (d) reveals that the mass resolution has been improved by a factor of about 1.5 compared with the conventional TOFMS instrument. 
   It is to be noted that various changes and modifications are possible in the present invention. For example, the differentially pumped slit  23  may be made of an electrode to which a voltage can be applied. This can cancel the effects of charging that would normally be produced by causing a collision of helium ions and cutting them off. 
   Furthermore, the entrance slit  25  may be provided with a heating mechanism. Adhesion of contaminants to the entrance slit  25  is prevented by heating the slit  25 . Consequently, the slit can be operated stably for a long time. 
   The present invention makes it possible to cut out a major portion of carrier gas-derived ions ahead of the ion reservoir, it being noted that the carrier gas-derived ions are introduced in large quantity from a gas chromatograph and will be ionized in large quantity in the ion source. 
   Having thus described our 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.