Patent Publication Number: US-9412576-B2

Title: Ion trap mass spectrometer using cold electron source

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 2013-0150883, filed on Dec. 5, 2013, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an ion trap mass spectrometer, and more particularly, to an ion trap mass spectrometer using a cold electron source, in which cold electrons are produced at room temperature using an ultraviolet light emitting diode (UV LED), a microchannel plate (MCP) electron multiplier plate, and a channeltron electron multiplier (CEM), without using a thermionic source using a filament, and are applied to the mass spectrometer. 
     2. Discussion of Related Art 
     Generally, in a mass spectrometer, a process of ionizing gaseous molecules is required first to separate molecular ions according to masses of molecular ions and analyze components. 
     A method of ionizing gaseous molecules by bombarding with an electron beam is most frequently used. To produce the electron beam, a device for heating a filament at a high temperature to induce thermionic emission is most widely used. 
     The filament may be heated at a high temperature by flowing high current through a high-temperature metal such as tungsten or rhenium. However, due to high power consumption, battery power is rapidly consumed in a portable mass spectrometer. Further, a reaction of electron emission caused by a high temperature increase is slow, and thus the device using the filament is difficult to control in a mass spectrometer which is suitable to produce a continuous output electron beam and requires pulse ionization within a short time. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to providing a mass spectrometer using a cold electron source, in a production of a portable mass spectrometer, in which a microchannel plate (MCP) electron multiplier plate is used, a front surface of the MCP electron multiplier plate is injected with ultraviolet photons emitted from an ultraviolet diode to induce initial electron emission, electron beams amplified from the electrons are amplified using a channeltron electron multiplier (CEM), the amplified electron beams are accurately adjusted and injected into an ion trap, and thus an amplification rate increases, since a quadrupole field is used as an ion filter, initially injected electrons return to the inside of the ion trap mass separator, and thus an ionization rate increases. 
     According to an aspect of the present invention, there is provided an ion trap mass spectrometer using a cold electron source, which uses a device configured to acquire an ionization source using a microchannel plate (MCP) and a channeltron electron multiplier (CEM), in which ultraviolet photons radiated from an inside of a mass spectrometer vacuum chamber in a high vacuum state induce initial electron emission, and gaseous molecules are ionized through an electron beam obtained by amplifying the electrons and ions are detected, the ion trap mass spectrometer including an ultraviolet diode which emits ultraviolet rays to the inside of the mass spectrometer vacuum chamber; an MCP module which induces initial electron emission of ultraviolet photons emitted from the ultraviolet diode, amplifies the emitted electrons, and obtains electron beams from a back plate; a CEM module which amplifies the electron beam emitted from the MCP module, and obtains electron beams in quantity; an electron focusing lens which focuses the electron beam amplified through the CEM module; an ion trap mass separator which ionizes the gaseous sample molecules and traps the gaseous sample molecules in a certain space using the electron beams injected through the electron focusing lens; an ion filter which prevents a loss of electrons when electron beams injected through the electron focusing lens pass through the ion trap mass separator and proceed; and an ion detector which detects ions separated from the ion trap mass separator based on a mass spectrum. 
     The MCP module injects ultraviolet photons emitted in quantity from the ultraviolet diode to a front plate of the MCP, the ultraviolet photons induce initial electron emission in quantity, the CEM module is configured to include an ionization source CEM front electrode and an ionization source CEM back electrode, and obtains highly-amplified electron beams by injecting the electron beam amplified at a back plate of the MCP. 
     The ion trap mass separator is injected with ionization sources including an ionization source to ionize the gaseous sample, ionized ions are trapped by a trapping RF voltage, and sequentially includes a mass separator front electrode, a mass separator RF electrode, and a mass separator back electrode. 
     The ion filter is configured to include a mass separator back electrode of the ion trap mass separator, a quadrupole field ion filter electrode, and an exit electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  is a circuit configuration diagram of an ion trap mass spectrometer using a cold electron source according to an embodiment of the present invention; 
         FIG. 2  is a configuration diagram of a source module only formed as an ionization source of a cold electron source in  FIG. 1 ; and 
         FIG. 3  is a waveform diagram of an RF signal applied to a mass separator RF electrode in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Exemplary embodiments of the present invention will be described in detail below with reference to the accompanying drawings. While the present invention is shown and described in connection with exemplary embodiments thereof, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. 
     An ion trap mass spectrometer using a cold electron source according to an embodiment of the present invention will be described in conjunction with the accompanying drawings as follow. 
       FIG. 1  is a configuration diagram of ion trap mass spectrometer using a cold electron source according to an embodiment of the present invention, including an ultraviolet diode  100  which emits ultraviolet rays by supplying a power source, an MCP module  101  whose back plate obtains electron beams in quantity by inducing initial electron emission of ultraviolet photons from the ultraviolet diode  100  and amplifying the emitted electrons, a funnel-shaped CEM module  102  which obtains electron beams in quantity by amplifying electron beams passing through the MCP module  101 , an entrance electrode  103  which focuses amplified electron beams input from the CEM module  102  and injects ions, an electron focusing lens  104  which focuses the injected electrons, ion trap mass separators  105 ,  106 ,  107 , and  108  which ionize gaseous sample molecules using electron beams injected through the electron focusing lens  104 , an ion detector  120  which detects ions separated from the ion trap mass separators  105 ,  106 ,  107 , and  108  based on a mass spectrum, and a preamplifier  131  which amplifies a current signal detected through an ion signal detection electrode of the ion detector  120 . 
     The ultraviolet diode  100  radiates photons after receiving a pulse voltage having a constant current value through a voltage source V 1 . 
     The MCP module  101  is configured such that a voltage in a range of −2800 to −4000 V V 2  is applied to an MCP front plate  101   a  and ultraviolet photons radiated from the ultraviolet diode  100  are irradiated, an identical direct current (DC) voltage in a range of −2000 to −3000 V V 3  is applied to an MCP back plate  101   b  together with an ionization source CEM front electrode  111  of a CEM module  102  to amplify ultraviolet photons radiated from the MCP front plate  101   a  of the MCP module  101 . 
     The CEM module  102  includes the ionization source CEM front electrode  111  and an ionization source CEM back electrode  112 . 
     The ion trap mass separators  105  to  108  sequentially include a mass separator front electrode  105 , mass separator RF electrodes  106  and  107 , and a mass separator back electrode  108  from a back end of the electron focusing lens  104 . 
     The ion filters  108  to  110  include the mass separator back electrode  108  of the ion trap mass separators  105  to  108 , a quadrupole field ion filter electrode  109 , and an exit electrode  110 , and the mass separator back electrode  108  is shared in the ion filters. 
     The ion detector  120  is formed as a channeltron electron multiplier (CEM) module in which ions passing through the ion filters  108  to  110  are detected and amplified, and includes a CEM front electrode  121  for detecting ions, a CEM back electrode  122  for detecting ions, and an ion signal detection electrode  123 . 
     The ion filters  108  to  110  include quadrupole field ion filters  108 ,  109 , and  110  which serve to return initially injected ions to the ion trap mass separator without passing through the quadrupole field ion filter electrode  109  after passing through the ion trap mass separators  105 ,  106 ,  107 , and  108 . 
     Each of the components  100  to  123  of the mass spectrometer operates in a vacuum chamber  130  having a pressure in a range of 10 −4  to 10 −10  Torr. 
     With regard to an action of the above-described ion trap mass spectrometer using a cold electron source according to the embodiment of the present invention, detailed descriptions are described with reference to  FIGS. 1 to 3  as follows. 
     In the embodiment of the present invention, ultraviolet photons induce initial electron emission at the ultraviolet diode first, the emitted electrons are amplified to radiate electron beams, the radiated electron beams are focused by the electron focusing lens, and then gaseous sample molecules are ionized in the ion trap mass analyzer and the separated ions are detected by the ion detector. 
       FIG. 1  is a circuit configuration diagram of the ion trap mass spectrometer using a cold electron source according to an embodiment of the present invention, and  FIG. 2  is a separate configuration diagram of a cold electron ionization source. 
     In the vacuum chamber  130 , the MCP front plate  101   a  of the MCP module  101  is injected with ultraviolet rays emitted from the ultraviolet diode  100 , the ultraviolet photons induce initial electron emission in quantity at the MCP front and back plates  101   a  and  101   b.    
     Initially emitted electrons generated in quantity when the ultraviolet rays pass through the MCP front and back plates  101   a  and  101   b  are injected into a funnel-shaped inlet in the CEM module  102 , and thereby a further highly-amplified electron beam may be obtained. 
     Here, a negative voltage in a range of −2800 to −4000 V V 2  is applied to the MCP front plate  101   a , a negative voltage in a range of −2000 to −3000 V V 3  is applied to the MCP back plate  101   b  in conjunction with the CEM electrode  111 , a voltage in a range of −200 to 0 V V 4  is applied to the CEM electrode  112 , and thereby, highly amplifying the injected ultraviolet rays. 
     The electron beams amplified by the CEM module  102  are injected without loss by a voltage in a range of −100 to 0 V V 5  which is applied to the entrance electrode  103 , and focused in one direction by the electron focusing lens  104 , and then injected into the ion trap mass separators  105 ,  106 ,  107 , and  108  to ionize the gaseous sample molecules. 
     Here, the ionization is adjusted by an ultraviolet emission time and an amount of current of the ultraviolet diode  100 . That is, the ionization is adjusted by an on/off pulse signal of a voltage source V 1  driving the ultraviolet diode  100 . When the on pulse signal is applied for a long time, a large quantity of the ultraviolet ray is emitted. When the on pulse signal is applied for a short time, a small quantity of the ultraviolet ray is emitted. 
     Further, to adjust an intensity of the ultraviolet diode  100 , an amount of the emitted ultraviolet photons is adjusted by adjusting an amount of current of the ultraviolet diode, and thereby accurately and momentarily obtaining an electron current required for gas ionization in the mass spectrometer. 
     A negative voltage V 6  is applied to the electron focusing lens  104  to focus ultraviolet photons emitted from the cold electron ionization modules  100 ,  101 , and  102  which include the ultraviolet diode  100 , the MCP module  101 , and the CEM module  102 . A voltage higher than the negative voltage V 3  applied to the MCP back plate  101   b  of the MCP module  101  is applied to the electron focusing lens  104 , the same voltage V 3  as that of the ionization source CEM front electrode  111  is applied to the MCP back plate  101   b  of the MCP module  101  V 3 , and a voltage lower than that applied to the ionization source CEM back electrode  112  is applied to the MCP back plate  101   b  of the MCP module  101 . 
     The ion trap mass separators  105  to  108  are injected with the ionization sources including the ionization source, and ions which are ionized while colliding with electrons are trapped by a trapping RF voltage. 
     More specifically, the ion trap mass separators  105  to  108  separate gaseous samples into ions using electron beams passing through the electron focusing lens  104 , and the ion detector  120  detects the ions generated in the ion trap mass separators  105  to  108 , and the detected ions are detected as signals based on a principle of the ion trap mass analyzer. 
     To transmit ions generated in the ion trap mass separators  105  to  108  to the ion detector  120 , an RF voltage V 8  is applied to the mass separator RF electrodes  106  and  107 . 
     As the RF voltage applied to the RF electrodes  106  and  107  increases, ions are separated from the ion trap mass separator and detected by the ion detector corresponding to the RF voltage proportional to a mass value. 
     Here, as shown in  FIG. 3 , the RE voltage V 8  is a high frequency signal having a certain voltage to trap ions, and the voltage which gradually increases is applied to detect ions. 
     An interaction equation of the voltage and mass is the following Equation 1. When a frequency (Ω) is a fixed value, and a voltage value which is proportional to a mass value increases, ions having the corresponding mass value are detected at the outside of the ion trap mass separators  105  to  108 . 
     
       
         
           
             
               
                 
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                         8 
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                         ⁢ 
                         eV 
                       
                       
                         
                           q 
                           z 
                         
                         ⁢ 
                         
                           r 
                           2 
                         
                         ⁢ 
                         
                           Ω 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   Equation1 
                   ] 
                 
               
             
           
         
       
     
     Each electrode of the ion filters  108 ,  109 , and  110  which serve as quadrupole field ion filters is present at the back end of the ion trap mass separators  105  to  108 , and returns the electrons initially injected by the electrodes into the inside of the ion trap mass separator without escaping to the outside, thus increasing the ionization rate. 
     That is, the ion filter electrode  109  is formed as a quadrupole field ion filter electrode, which prevents a secondary ionization at the outside of the ion trap mass separator when electrons proceed after passing through the ion trap mass separators  105  to  108 . 
     For the ion filters  108  to  110  to serve as quadrupole field ion filters, the mass separator back electrode  108  and exit electrode  110  are grounded V 9  and V 11 , and the ion filter electrode  109  has a negative voltage value V 10 . 
     The ion detector  120  is formed as a CEM electron multiplier, for a normal operation of the CEM, a voltage in a range of −2000 to −300 V V 12  is applied to the CEM front electrode  121  for detecting ions, a voltage in a range of −300 to 0 V V 13  is applied to the CEM back electrode  122  to amplify the detected ions, and thereby an ion signal is obtained through the ion signal detection electrode  123 . 
     A current signal sensed by the ion signal detection electrode  123  is amplified to have the analyzable signal intensity through the preamplifier  131 , and thereby an ion signal is detected. 
     As described above, the device for acquiring an ionization source of a mass spectrometer using an MCP, a CEM according to the embodiment of the present invention can be applied to a low temperature electron gun required for portable small devices, low power devices, or devices in which a low temperature is maintained, or to devices generating and using electron beams 
     As described above, the ion trap mass spectrometer using a cold electron source can provide electron beams to ionize gaseous molecules at a low temperature, without using a high temperature and high current. The ion trap mass spectrometer provides a necessary amount of electron beams when needed, and thus a size and weight of the mass spectrometer can be decreased when applied to a small mass spectrometer, and since a battery power can be saved, the ion trap mass spectrometer can be effectively applied as a portable mass spectrometer. Further, thin electron beams can be emitted, and thus the electron beams can be easily focused. Furthermore, the quadrupole field ion trap mass spectrometer using the cold electron source can improve a performance of analyzing mass by including the quadrupole field ion filter. 
     Although specific embodiments of the present invention have been described here, it will be apparent to those skilled in the art that various modifications can be made to the above-described ion trap mass spectrometer using a cold electron source without departing from the spirit or scope of the invention. 
     Thus, it is intended that the present invention covers all such modifications provided they come within the scope of the appended claims and their equivalents.