Abstract:
An ion trap mass spectrometer is provided, including: an electron emitter; an ion trap storing ions generated by ionization resulting from an impact with electrons emitted from the electron emitter; a secondary ion filter for blocking out secondary ions generated due to ions selectively released by the ion trap; and a detector detecting ions selectively released from the ion trap, wherein the electron emitter, the ion trap, the secondary ion filter, and the ion detector are arranged on the same axis, so that a pure mass spectrum can be measured by excluding the secondary ions which are causes of background noise signals in the procedure of detection of the ions by the ion trap mass spectrometer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2012-0143703 filed in the Korean Intellectual Property Office on Dec. 11, 2012, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to an ion trap mass spectrometer, and more particularly, to an ion trap mass spectrometer capable of improving mass resolution and detection sensitivity by detecting only ions sequentially released from an ion trap and excluding secondary ions generated out of the ion trap to measure a mass spectrum in the procedure of detection of the ions. 
     (b) Description of the Related Art 
     In general, an ion trap mass spectrometer is composed of a donut-shaped ring electrode and two end cap electrodes covering upper and lower portions of the ring electrode. 
     When an AC voltage is applied between the ring electrode and the two end cap electrodes covering the upper and lower portions of the ring electrode, a quadrupole is formed in the center inside since the two end cap electrodes are connected to each other at the same potential. 
     As for the simple principle of the ion trap mass spectrometer, a gas sample molecule is ionized by an electron beam, and then the ions are trapped in the thus formed quadrupole. When the AC voltage is increased to change the ion storage conditions, the lighter ions are first released in sequence, and the ion detector measures the released ions, thereby obtaining a mass spectrum showing components and a compositional ratio of the gas sample. 
     In order to allow the ions released from the ion trap to reach the ion detector, the ions are accelerated at a voltage of about 2000 V to impact on a surface of the ion detector. Here, electrons generated are amplified and then recorded as a current signal. 
     The accelerated ions impact with other molecules present on a path on which they reach the ion detector to form secondary ions, and secondary electrons are again reversely accelerated to cause another ionization. These procedures are repeated and thus an ion congestion phenomenon occurs. 
     Since the secondary ions are not ions released from the ion trap but are random ions generated on the path, they cause a difficulty in analyzing the contents of gas components, which are targeted by the mass spectrometer. 
     In order to remove the secondary ion noise signal, the ion congestion phenomenon is reduced by forming a middle electrode for ion warping between an outlet of the ion trap and the ion detector to thereby allow the secondary ions to deviate from the path, or the background ion noise signal is reduced by forming an ion lens in the middle and applying a pulse type of voltage thereto. However, these methods are not significantly useful. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to provide an ion trap mass spectrometer having advantages of measuring a pure mass spectrum free from background noise signals, by forming a quadrupole potential well between an ion trap and an ion detector to prevent secondary ions, which are newly generated on a path out of an ion trap, from reaching the ion detector and allowing only ions, which are released from the ion trap by mass scanning, to reach the ion detector. 
     An exemplary embodiment of the present invention provides an ion trap mass spectrometer, including: an electron emitter; an ion trap storing ions generated by ionization resulting from an impact with electrons emitted from the electron emitter; a secondary ion filter for blocking out secondary ions generated due to ions selectively released by the ion trap; and a detector detecting ions selectively released from the ion trap, wherein the electron emitter, the ion trap, the secondary ion filter, and the ion detector are arranged on the same axis. 
     According to an embodiment of the present invention, a pure mass spectrum can be measured by excluding the secondary ions which are causes of background noise signals in the procedure of detection of the ions by the ion trap mass spectrometer. 
     According to an embodiment of the present invention, the mass resolution can be improved by preventing an ion congestion phenomenon resulting from secondary ionization to prevent the ion signal peak from being widened. 
     Further, since the background noise signals due to the secondary ionization are excluded, a trace amount of pure ions can be detected and thus the signal detection range (dynamic range) of the mass spectrum can be widened. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view illustrating a structure of an ion trap mass spectrometer according to an exemplary embodiment of the present invention; 
         FIG. 2  is a schematic perspective view illustrating an external appearance of an ion trap mass spectrometer according to an exemplary embodiment of the present invention; 
         FIG. 3A  and  FIG. 3B  are conceptual views illustrating an operational principle of a secondary ion filter included in an ion trap mass spectrometer according to an exemplary embodiment of the present invention, and are obtained by simulating and computing moving paths in the secondary ion filter of ions, which are generated due to secondary ionization occurring between an ion tap and an ion detector by a voltage of a secondary ion filtering ring electrode, and ions, which are released due to AC scanning in the ion trap to form a mass spectrum; 
         FIG. 4  is a potential distribution diagram of an ion trap mass spectrometer according to an exemplary embodiment of the present invention; and 
         FIG. 5  is a flowchart illustrating a secondary ion excluding method of an ion trap mass spectrometer according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present invention will be described more fully with reference to the accompanying drawings. First, concerning the designations of reference numerals, it should be noted that the same reference numerals are used throughout the different drawings to designate the same or similar components. Further, in the description of the present invention, when it is considered that detailed descriptions of related known constitutions or functions may obscure the gist of the present invention, such detailed descriptions are omitted. 
       FIG. 1  is a schematic cross-sectional view illustrating a structure of an ion trap mass spectrometer according to an exemplary embodiment of the present invention, and  FIG. 2  is a schematic perspective view illustrating an external appearance of an ion trap mass spectrometer according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 1 , an ion trap mass spectrometer  100  according to an embodiment of the present invention includes an electron emitter  110 , an ion trap  130 , a secondary ion filter  150 , and an ion detector  170 , which are disposed on the same axis. 
     The electron emitter  110  may be a hot filament that is heated by a current supplied from a battery, although not shown, to emit hot electrons. The emitted hot electrons pass through an electron focusing lens  120  disposed between the electron emitter  110  and the ion trap  130 , and then enters the ion trap  130 . 
     The ion trap  130  consists of a pair of plate-type ring electrodes  131  and  132  and a pair of plate-type end cap electrodes  133  and  134 . The plate-type ring electrodes  131  and  132  are spaced apart from each other at a predetermined interval to face each other, and the plate-type end cap electrodes  133  and  134  are respectively disposed at one side of each of the pair of plate-type ring electrodes  131  and  132  and spaced apart from each other at a predetermined interval to face each other. 
     The pair of plate-type ring electrodes  131  and  132  and the pair of plate-type end cap electrodes  133  and  134  are formed to be planar such that their facing opposite surfaces confront each other. A first aperture  133   a  is formed in the center of a first end cap electrode  133  of the pair of plate-type end cap electrodes  133  and  134 . The first aperture  133   a  is an inlet through which hot electrons emitted from the electron emitter  110  enter the ion trap  130 . 
     A second aperture  134   a  is formed in the center of a second end cap electrode  134  of the pair of plate-type end cap electrodes  134  and  134 . The first aperture  133   a  and the second aperture  134   a  are disposed on the same axis and have the same diameter. The second aperture  134   a  is an outlet through which the ions separated from the first aperture  130   a  of the ion trap  130  emit. 
     The secondary ion filter  150  is disposed between the ion trap  130  and the ion detector  170 . The secondary ion filter  150  consists of a plate-type ion filtering ring electrode  151  facing the second end cap electrode  134  of the ion trap  130  and a plate-type ion filtering end cap electrode  153  facing the plate-type ion filtering ring electrode  151 . 
     The second end cap electrode  134  of a plate type serves to form a quadrupole  151   a  together with the plate-type ion filtering ring electrode  151  and the plate-type ion filtering end cap electrode  153  of the secondary ion filter  150 . The second aperture  134   a  formed in the center of the second end cap  134  is used as both an outlet from the ion trap  130  and an inlet through which ions flow to the secondary ion filter  150 . 
     Of ions coming out from the ion trap  130 , secondary ions are filtered by the secondary ion filter  150 . 
     For achieving this, the ion filtering end cap electrode  153  of the secondary ion filter  150  is provided with a third aperture  153   a  in the center thereof. The third aperture  153   a  has a larger diameter than the second aperture  134   a  formed in the second end cap electrode  134 . 
     As such, the ion trap mass spectrometer  100  according to an exemplary embodiment of the present invention can have a slim and compact design since both the ion trap  130  and the secondary ion filter  150  are formed as a plate type. 
     In addition, a diaphragm  160  for controlling the diameter of the third aperture  153   a  is further provided between the ion filtering end cap electrode  153  and the ion detector  170 , so that the signal detection range (dynamic range) of a mass spectrum can be diversified even without changing the voltage applied to the ion filtering ring electrode  151 . 
     Now, referring to  FIGS. 3A to 4 , an operational principle of the ion trap mass spectrometer according to an exemplary embodiment of the present invention will be described. 
       FIG. 3A  and  FIG. 3B  are conceptual views illustrating an operational principle of a secondary ion filter included in an ion trap mass spectrometer according to an exemplary embodiment of the present invention, and are obtained by simulating and computing moving paths in the secondary ion filter of ions which are generated due to a secondary ionization occurring between an ion tap and an ion detector by a voltage of an ion filtering ring electrode, and ions which are released due to AC scanning in the ion trap to form a mass spectrum.  FIG. 4  is a potential distribution diagram of an ion trap mass spectrometer according to an exemplary embodiment of the present invention.  FIG. 5  is a flowchart illustrating a secondary ion excluding method of an ion trap mass spectrometer according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 3A , electrons emitted from the electron emitter  110  are focused by the electron focusing lens  120  to enter the ion trap  130  through the first aperture  133   a  of the first end cap electrode  133 , and then impact with and ionize the gas present in a space in the ion trap  130  (impact ionization). As the RF (radio frequency) voltage applied to the pair of plate-type ring electrodes  131  and  132  is increased, the ionized materials are sequentially discharged through the second aperture  134   a  of the second end cap electrode  134 , from the lighter ions to the heavier ions. 
     Here, a quadrupole is formed inside the secondary ion filter  150  by applying a first voltage to the plate-type ion filtering end cap electrode  153  of the secondary ion filter  150 , which is further disposed between the ion trap  130  and the ion detector  170 , and applying a second voltage to the plate-type ion filtering ring electrode  151  of the secondary ion filter  150 , the first voltage being equal to the voltage applied to the second end cap electrode  134  of the ion trap  130 , the second voltage being lower than the first voltage. 
     The voltage applied to the plate-type ion filtering ring electrode  151  may be a negative (−) voltage. 
     Between the second end cap electrode  134  of the ion trap  130  and the ion filtering end cap electrode  153  of the secondary ion filter  150 , the secondary ions generated due to an impact with ions emitted through the second aperture  134   a  of the second end cap electrode  134  are pulled toward the ion filtering ring electrode  151 , and then discharged out of the mass spectrometer instead of being moved to the detector, as shown in  FIG. 3A . The ions released from AC scanning of the ion trap move to the detector as shown in  FIG. 3B . Therefore, a mass spectrum excluding noise signals and having improved resolution can be recorded. 
     For achieving this, a ground unit  155  for grounding the secondary ions pulled toward the ion filtering ring electrode  151  may be further provided at the ion filtering ring electrode  151 . 
     The reason is that, since the diameter of the third aperture  153   a  of the ion filtering end cap electrode  153  is slightly larger than the diameter of the second aperture  134   a  of the second end cap electrode  134  while the second end cap electrode  134  and the ion filtering end cap electrode  153  have the same potential, the potential at the center axis of an outlet of the third aperture  153   a  of the ion filtering end cap electrode  153  is slightly lower than the potential at the center axis of an outlet of the second aperture  134   a  of the second end cap electrode  134 , as shown in  FIG. 4 . 
     Therefore, as shown in  FIG. 3B , the ions leaking out from the ion trap  130  through the second aperture  134   a  of the second end cap electrode  134  are accelerated toward a center portion of the ion filter  150  along the potential slope of the quadrupole  150   a  of the ion filter  150 , and are decelerated after the center portion and then pass through the secondary ion filter  150  through the third aperture  153   a  of the ion filtering end cap electrode  153 . However, the secondary ions generated due to the impact with ions released through the second aperture  134   a  of the second end cap electrode  134  are generated inside of the ion filter  150 , that is, at the site of which the potential is low, and thus cannot go over the potential at the center axis of the outlet of third aperture  153   a  of the ion filtering end cap electrode  153 . 
     Hereinafter, a secondary ion excluding method of an ion trap mass spectrometer according to an exemplary embodiment of the present invention will be described with reference to  FIG. 5 . 
     The secondary ion excluding method of an ion trap mass spectrometer according to an exemplary embodiment of the present invention includes a step of installing a quadrupole secondary ion filter between an ion trap and an ion detector of an ion trap mass spectrometer having a quadrupole ion trap (S 110 ). 
     The quadrupole secondary ion filter  150  may consist of a plate-type ion filtering ring electrode  151  and a plate-type ion filtering end cap electrode  153 . 
     A first voltage is applied to the plate-type ion filtering end cap electrode of the quadrupole secondary ion filter  150 , the first voltage being equal to that of the end cap electrode of the ion trap (S 120 ). 
     The first voltage is a DC voltage. 
     A second voltage is applied to the plate-type ion filtering ring electrode  151  of the quadrupole secondary ion filter  150  to form a quadrupole inside the secondary ion filter  150 , the second voltage being lower than the first voltage (S 130 ). The second voltage may be a negative voltage. 
     The secondary ion excluding method of an ion trap mass spectrometer according to an exemplary embodiment of the present invention includes a step of changing voltages of an inlet and an outlet of the quadrupole secondary ion filter  150  (S 140 ). 
     As described above, in the step of changing the voltages of the inlet and the outlet of the quadrupole secondary ion filter  150 , the difference between the voltages may be decreased by differentiating the diameters of the inlet and the outlet of the quadrupole secondary ion filter  150 . 
     Therefore, the ion trap mass spectrometer  100  according to an exemplary embodiment of the present invention can measure a pure mass spectrum since the secondary ions resulting in the background noise signal are excluded by the second ion filter  150 . 
     Further, the mass resolution can be improved by preventing an ion congestion phenomenon resulting from secondary ionization and thus preventing the ion signal peak from being widened. 
     Further, since the background noise signals due to the secondary ionization are excluded, a trace amount of pure ions can be detected and thus the signal detection range (dynamic range) of the mass spectrum can be widened in spite of a small and slim constitution. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 &lt;Description of Symbols&gt; 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 110: electron emitter 
                 120: electron focusing lens 
               
               
                   
                 130: ion trap mass spectrometer 
                 150: ion filter 
               
               
                   
                 160: diaphragm 
                 170: ion detector