Patent Publication Number: US-6707034-B1

Title: Mass spectrometer and ion detector used therein

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mass spectrometer and an ion detector used therein. 
     2. Description of the Related Art 
     U.S. Pat. No. 6,091,068 discloses an ion detector that includes a Faraday cup and a tube-shaped continuous-dynode electron multiplier. (Details of a tube-shaped continuous-dynode electron multiplier are disclosed in U.S. Pat. No. 5,866,901.) In a Faraday cup mode of operation, the Faraday cup is connected to the input of an electrometer. The incoming ion beam formed from positively charged ions impinges on the collector plate of the Faraday cup. The ions are neutralized upon striking the collector plate, drawing a current as a signal output to the electrometer. 
     The continuous-dynode electron multiplier in U.S. Pat. No. 6,091,068 includes a conical entrance opening. A grid shield is positioned adjacent to the conical entrance opening. During an electron multiplier mode of the ion detector, a high electrical potential is established at the grid shield so that incoming ions are drawn into the. conical entrance opening. At this time, readings are taken from the output of the continuous-dynode electron multiplier. 
     SUMMARY OF THE INVENTION 
     Continuous-dynode electron multipliers cannot be used with a heavy current, so have a limited dynamic range of 0.1 FA to 100 nA. As shown in FIG. 1, Faraday cups have a dynamic range of only about 1 mA to 1 μA. Therefore, there is a range Y where the ion detector of U.S. Pat. No. 6,091,068 cannot take accurate readings. 
     Also, continuous-dynode electron multipliers only have a small secondary electron emissive surface for multiplying electrons. The surface area of the secondary electron emissive surface is limited by the inner surface of the channel running through the tube. The channel is an approximately 1 mm diameter hole, so the electron density per unit surface area is great. Therefore, a large burden is placed on the secondary electron emissive surface in the channel so that the continuous-dynode electron multiplier has a short life. 
     It is an objective of the present invention to overcome the above-described problems and provide an ion detector with a broad dynamic range and with a long use life. 
     In order to achieve the above-described objectives, an ion detector according to the present invention includes an ion input face, a Faraday cup, an ion-to-electron converter dynode, two ion deflection electrodes, an electron multiplier portion, and an anode. The ion input face is formed with an ion input opening. The Faraday cup has an ion collection surface that confronts the ion input opening. The ion-to-electron converter dynode is disposed to one side with respect to the Faraday cup and the ion input opening and has a conversion surface that converts impinging ions into electrons. The two ion deflection electrodes generate an electron lens that attracts and focuses ions from the ion input opening toward the conversion surface of the ion-to-electron converter dynode. The electron multiplier portion receives and multiplies the electrons from the ion-to-electron converter dynode, and includes a plurality of dynodes that multiply electrons one after the other. The plurality of dynodes are juxtaposed in an arc-shape around the Faraday cup. The anode receives electrons from the electron multiplier portion and outputs a signal that corresponds to the amount of input ions. 
     A mass spectrometer according to the present invention includes the above-described ion detector, an ionization portion, and a mass separator. The ionization portion converts molecules of a sample into ions. The mass separator separates desired ions from other ions from the ionization portion. The ion input face confronts the mass separator and the ion collection surface of the Faraday cup confronts the mass separator through the ion input opening. 
     According to another aspect of the present invention an ion detector includes an ion input face, a Faraday cup, an ion-to-electron converter dynode, an ion deflection electrode, an electron multiplier portion, and an anode. The ion input face is formed with an ion input opening. The Faraday cup has an ion collection surface that confronts the ion input opening. The Faraday cup is connected to ground. The ion-to-electron converter dynode is disposed to one side with respect to the Faraday cup and the ion input opening. The ion-to-electron converter dynode is applied with a high voltage and has a conversion surface that converts impinging ions into electrons. The ion deflection electrode generates, with the Faraday cup and the ion-to-electron converter dynode, an electron lens that attracts and focuses ions from the ion input opening toward the conversion surface of the is ion-to-electron converter dynode. The electron multiplier portion receives and multiplies the electrons from the ion-to-electron converter dynode. The electron multiplier portion includes a plurality of dynodes that multiply electrons one after the other. The plurality of dynodes are juxtaposed in an arc-shape around the Faraday cup. The anode receives electrons from the electron multiplier portion and outputs a signal that corresponds to the amount of input ions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the embodiment taken in connection with the accompanying drawings in which: 
     FIG. 1 is a chart showing dynamic ranges of a Faraday cup and a continuous-dynode electron multiplier of a conventional ion detector; 
     FIG. 2 is a block diagram showing components of a mass spectrometer according to an embodiment of the present invention; 
     FIG. 3 is a side view showing a mass separator and an ion detector of the mass spectrometer; 
     FIG. 4 is a cross-sectional view taken along line IV—IV of FIG. 3; 
     FIG. 5 is a perspective view showing external configuration of the ion detector; 
     FIG. 6 is a schematic view showing operation of an electron multiplier portion of the ion detector; 
     FIG. 7 is a chart showing dynamic ranges of the electron multiplier portion and a Faraday cup of the ion detector of FIG. 4; and 
     FIG. 8 is a schematic view showing a modification of the embodiment of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     Next, a mass spectrometer  100  including an ion detector  1  according to an embodiment of the present invention will be described. As shown in FIG. 2, the mass spectrometer  100  includes a gas chromatographer  110 , a stainless steel envelope  120 , and a data processing unit  130 . The gas chromatographer  110  includes a sampler injection port (not shown) through which liquid samples are injected The envelope  120  houses an ionization portion  121 , a mass separator  122 , and the ion detector  1  within a vacuum chamber  120   a . The ionization portion  121  includes a filament (not shown) for generating heat that converts molecules in the sample into positive or negative polarity ions. As shown in FIG. 3, the mass separator  122  includes cylindrical quadruple (Q-) pole electrodes  122   a  that are arranged in parallel around an imaginary axis X and that are electrically connected to the data processing unit  130 . Four Q-pole electrodes  122   a  are provided, although only two are shown in the drawings. 
     Returning to FIG. 2, the data processing unit  130  controls application of voltage to the filament of the ionization portion  121  and to the Q-pole electrodes  122   a  and also to a single high-voltage connector  40   a  of the ion detector  1  as will be described later. The data processing unit  130  further receives and analyses electric signals from the ion detector  1  to determine various information about the liquid sample injected into the gas chromatographer  110 . 
     As shown in FIG. 3, the ion detector  1  includes two confronting ceramic walls  70 ,  71 , an electron multiplier portion  50 , a Faraday cup connector  30   a , the high-voltage connector  40   a , and an anode connector  60   b . As will be described later, the ceramic walls  70 ,  71  support the electron multiplier portion  50  therebetween. The Faraday cup connector  30   a , the high-voltage connector  40   a , and the anode connector  60   b  are connected to the data processing unit  130  through pins  131 ,  132 ,  133 , respectively. 
     Referring to FIG. 4, the ion detector  1  further includes a stainless steel shield  10 , a Faraday cup  30 , a deflection electrode  40 , and an anode  60 . The shield  10  is formed from a single sheet of stainless steel bent into a substantial C-shape and includes an input face  11 , a rear support  12 , and a base  13 . The shield  10  is connected to ground. The input face  11  is formed with an ion input opening  1   a  that is aligned on the imaginary axis X. The shield  10 , in particular the rear support  12 , is located at a position closer to the anode  60  than to the Faraday cup  30 , the ion deflection electrode  40 , and an ion-to-electron converter dynode  51  of the electron multiplier portion  50 . It should be noted that as shown in FIG. 4, no stainless shield is provided at the side nearest the ion-to-electron converter dynode  51 . 
     The Faraday cup  30  is disposed adjacent to and in confrontation with the input opening  11   a . The Faraday cup  30  includes an integral ion deflector portion  31  and an ion collection surface  32 , both of which are constantly connected to ground through the Faraday cup connector  30   a  and the data processing unit  130 , and so are maintained at a constant voltage of 0 V. The ion collection surface  32  is aligned on the imaginary axis X so as to confront the ion input opening  11   a  and mass separator  122  through the ion input opening  11   a . The ion deflector portion  31  extends from the ion collection surface  32  in the general direction of the ion input opening  11   a  and the ion deflection electrode  40 . 
     The ion deflection electrode  40  is disposed to one side of the imaginary axis X at a location between a non-open portion of the input face  11  and the Faraday cup  30 . The ion deflection electrode  40  is bent in a substantial Z shape so that one end of the electrode is closer to the opening  11   a . The ion deflection electrode  40  is electrically connected to the high-voltage connector  40   a.    
     The electron multiplier portion  50  includes the ion-to-electron converter dynode  51 , inner dynodes  52 , and outer dynodes  53 . The ion-to-electron converter dynode  51  is disposed to one side of the Faraday cup  30  and the ion deflection electrode  40  with respect to the imaginary axis X. The ion-to-electron conversion dynode  51  includes a conversion surface  51   a  and is electrically connected to the ion deflection electrode  40  by a line  41 . The inner dynodes  52  and the outer dynodes  53  are juxtaposed in an arc-shape around the Faraday cup  30 . Each of the inner dynodes  52  and the outer dynodes  53  has a secondary electron emissive surface aligned to receive and multiply electrons from the preceding dynode of the electron multiplier portion  50 , starting with electrons generated by the ion-to-electron converter dynode  51 . The outer dynodes  53  are juxtaposed on an imaginary arc farther from the Faraday cup  30  than the inner dynodes  52  and each has a larger secondary electron emissive surface than do each of the inner dynodes  53 . 
     The anode  60  is disposed in confrontation with the secondary electron emissive surface of the last dynode  53  of the electron multiplier portion  50  and is electrically connected to the data processing unit  130  through the anode connector  60   b.    
     External configuration of the ion detector  1  is shown in more detail in FIG.  5 . The ceramic walls  70 ,  71  are each formed with two holes  74  (only one hole  74  of the wall  71  is shown in FIG.  5 ). The rear support  12  of the shield  10  has four crimped sections  12   a  (only one is shown in FIG.  4 ), which are bent into corresponding holes  74  in the ceramic walls  70 ,  71  to support the ceramic walls  70 ,  71  in place. 
     The ceramic walls  70 ,  71  are further formed with a plurality of slits  76 ,  80 ,  81 , which are elongated through hole passing completely through the ceramic walls  70 ,  71 . Plural slits  76  are formed at positions corresponding to positions of the dynodes  51 ,  52 ,  53 . Connection terminals  54  of the dynodes  51 ,  52 ,  53  protrude through the slits  76 . A circuit pattern  78  is formed on the ceramic wall  71 . The circuit pattern  78  is electrically connected to the high-voltage connection  40   a  and includes resistance for determining voltage that is applied to the dynodes  51 ,  52 ,  53  through connection terminals  54  of the dynodes  51 ,  52 ,  53 . Because the circuit pattern  78  is formed on the surface of the insulating substrate wall  71 , the ion detector  1  overall can be made more compact. The connection terminals  54  are electrically connected to the circuit pattern  78  at their outermost tips through the tips of wires  78   a . The ceramic walls  70 ,  71  are formed with three slits  80  (only one is shown in FIG.  5 ): two in the ceramic wall  71  and one in the ceramic wall  70 . The high-voltage connector  40   a , the anode connector  60   b , and the Faraday cup connector  30   a  protrude through the slits  80 . The slit  81  is formed completely through the ceramic wall  71  at a position between the Faraday cup  30  and the first one of the inner dynodes  52  as shown in dotted line in FIG.  4 . 
     Next, operation of the mass spectrometer  100  will be described. First, the power of the mass spectrometer  100  is turned ON. Then, the operator of the mass spectrometer  100  injects a liquid sample into the sampler injection port of the gas chromatographer  110 . The ionization portion  121  converts molecules in the sample into positive or negative polarity ions (positive in this example). At this time, the data processing unit  130  generates a voltage by superimposing a constant voltage and an AC voltage with a predetermined frequency and applies the voltage to the Q-pole electrodes  122   a . Of the ions generated by the ionization portion  121 , only ions with a mass that corresponds to the predetermined frequency are guided through the Q-pole electrodes  122   a  to the ion input opening  11   a  of the ion detector  1  and so are separated from the ions with other mass. 
     The ion detector  1  converts the amount of ions from the mass separator  122  into an electric signal using the electron multiplier portion  50  or the Faraday cup  30 , depending on the mode of the mass spectrometer  100 . Initially the mass spectrometer  100  is in its electron multiplier mode at the start of operations. 
     During the electron multiplier mode, the data processing unit  130  applies a high voltage of −1,000 V to the high-voltage connection  40   a . Because the high-voltage connection  40   a  is electrically connected to the ion deflection electrode  40  and, through the connecting line  41 , to the ion-to-electron conversion dynode  51 , a voltage of 1,000 V is developed at the ion deflection electrode  40  and to the ion-to-electron conversion dynode  51 . As a result, an electric field develops between the Faraday cup  30  (particularly the electrode wall  31  thereof), the ion deflection electrode  40 , and the ion-to-electrode converter dynode  51 . The electric field functions as an electron lens to, as shown in FIG. 6, draw ions  95  that pass from the mass separator  122  through the ion input opening  11   a , through a single focal point and toward the conversion surface  51   a  of the ion-to-electron converter dynode  51 . The shapes of, the positions of, and voltages applied to the Faraday cup  30 , the ion deflection electrode  40 , and the electron multiplier portion  50  determine the effects of the electron lens. For example, because the ion deflection electrode  40  is bent in a substantial Z shape and one end is closer to the opening  11   a , ions are more strongly pulled toward the ion-to-electron converter dynode  51 . 
     It should be noted that at this time an electric short-circuit between the high-voltage ion-to-electron converter dynode  51  and the shield  10  is prevented because the shield  10 , in particular the rear support  12 , is located at a position closer to the anode  60  than to the Faraday cup  30 , the ion deflection electrode  40 , and the ion-to-electron converter dynode  51  of the electron multiplier portion  50 . 
     The ion-to-electron conversion dynode  51  converts ions that impinge on the conversion surface  51   a  into electrons. The circuit pattern  78  is also applied with the 1,000 V voltage from the high-voltage connection  40   a . The resistance of the circuit pattern  78  on the ceramic wall  71  regulates voltage developed at the other dynodes  52 ,  53 . For example, a −900 V voltage is developed at the first inner dynode  52 . It should be noted that at this time, the slit  81  prevents an electric discharge from occurring by current flowing across the surface of the ceramic wall  70  from the first of the inner dynodes  52  (−900 volts) to the Faraday cup  30  (ground). Such a discharge would be undesirable because the light generated by the discharge could be picked up by the electron multiplier portion  50 . 
     The electrons from the ion-to-electrode conversion dynode  51  are deflected toward the secondary emission surface of the first inner dynode  52 . The other dynodes  52 ,  53  multiply the electrons one after the other as shown in FIG. 6 until the multiplied electrons  97  reach the anode  60 . The anode  60  receives electrons from the electron multiplier portion  50  and outputs a signal to the data processing unit  130  through the anode connector  60   b . The signal corresponds to the amount of ions input through the ion input opening  11   a . During this time, the Faraday cup  30  physically blocks light (photons) from entering the electron multiplier portion  50  from the direction of the ion emission source. Such light can be a source of undesirable noise. Also, the electron multiplier portion  50  is electrically shielded by the shield  10 . 
     The data processing unit  130  monitors the signal from the anode connector  60   b  and determines whether the signal exceeds a predetermined threshold. The data processing unit  130  maintains the electron multiplier mode as long as the signal is equal to or less than the predetermined threshold. However, if the data processing unit  130  judges that the amount of ions output from the anode  60  exceeds the predetermined threshold, then the data processing unit  130  switches to the Faraday cup mode. In the present embodiment, the threshold is 10 μA or greater. 
     During the Faraday cup mode, the data processing unit  130  stops application of voltage to the high-voltage connection  40   a  and connects the high-voltage connection  40   a  to ground. As a result, ions input from the mass separator  122  through the ion input opening  11   a  impinge on the ion collection surface  32 . Each time an ion from the mass separator  122  impinges on the ion collection surface  32 , an electron travels through the Faraday cup connector  30   a , either to or from ground depending on the polarity of the ion. The data processing unit  130  reads the resultant electric signal on the Faraday cup connector  30   a  to determine ion amount. 
     Because the electron multiplier portion  50  includes a plurality of dynodes  51 ,  52 ,  53 , it can be applied with a heavy current compared with continuous-dynode electron multipliers. Therefore, the ion detector of the present invention has a broader dynamic range. As shown in FIG. 7, the dynamic range of the Faraday cup  30  and the electron multiplier portion  50  properly overlap, so that readings are accurate over an overall broader range. Further, because the electron multiplier portion  50  has a larger secondary electron emissive surface than do continuous-dynode electron multipliers, the electron multiplier portion  50 , and consequently the ion detector  1 , has a comparatively long life. 
     Because the Faraday cup  30  (particularly the electrode wall  31  thereof), the ion deflection electrode  40 , and the ion-to-electrode converter dynode  51  generate an electron lens, ions  95  that pass from the mass separator  122  through the ion input opening  11   a can be reliably drawn through a single focal point and toward the conversion surface  51   a  of the ion-to-electron converter, dynode  51 . Because the ion deflector portion  31  is used as one of the electrodes to form the electron lens, the ion detector  1  is easier to produce, and can be made more compact, than if a separate electrode were provided. Further, the ion deflector portion  31  enhances the function of the Faraday cup  30  of blocking ions. 
     FIG. 8 shows an ion detector according to a modification of the embodiment. In this modification, the deflection electrode  40  is replaced with a deflection electrode  40 ′. The deflection electrode  40 ′ includes an extension  41 ′ that is welded directly to the ion-to-electron conversion dynode  51 . With this configuration, production of the ion detector is much easier. 
     While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims. 
     For example, the embodiment described the electrode and the first dynode are connected to the same power source However, an independent voltage source could be used instead. 
     Further, the operation of switching from the electron multiplier mode to the Faraday cup mode could be performed using a physical switch instead of switching by processes of the data processing unit  130 .