Abstract:
The objective of the present invention is to improve the robustness, sensitivity, and maintainability of a mass spectrometer to which a liquid chromatograph can be connected. This mass spectrometer includes: an ion source; a mass spectrometry unit; a plurality of flat plates used as electrodes having openings through which ions can be introduced to the mass spectrometer from an orthogonal direction along an ion introduction axis; a porous member provided between the mass spectrometry unit and the plurality of flat plates; and a mechanism for feeding gas to the plurality of flat plates for the side opposite from the ion introduction direction; the plurality of flat plates being disposed under atmospheric pressure, and the plurality of electrodes being set at a temperature higher than room temperature.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a mass spectrometer to which a liquid chromatograph can be connected. 
       BACKGROUND ART 
       [0002]    A liquid chromatograph (LC) becomes an analysis device necessary for researching and developing organic chemistry and is introduced widely in laboratories at universities, laboratories in chemical companies and the like. In the natural world, materials of single components are little and most of materials are mixtures of complex components. The liquid chromatograph is a device that can separate the complex components into individual components, using a liquid as a measurement sample. The complex components are separated into the individual components, so that calculation of a content of each component and extraction and elimination of a specific component are enabled. When these components are detected, types of measured samples may be large and detectors for the liquid chromatograph may be diversified. Even as for detectors to which representative optical technology is applied, there are an ultraviolet detector, a visible detector, a diode array detector, a differential refractive index detector, an electric conductivity detector, a fluorescent detector, and a chemiluminescence detector. 
         [0003]    In addition, when molecular weights of the components separated by the liquid chromatograph are measured, a mass spectrometer (MS) is used. A device to which the liquid chromatograph and a mass spectrometry unit are coupled is called a liquid chromatograph/mass spectrometer (LC/MS). The mass spectrometer ionizes a sample solution and introduces the ions into the mass spectrometer in at vacuum state. The mass spectrometer separates the ions by electric and magnetic actions according to a mass/charge ratio (m/z) and detects the individual ions. A mass spectrum in which a horizontal axis shows the mass/charge ratio and a vertical axis shows detection strength is obtained. Because a charged component (ion) molecule is directly detected, sensitivity is high as compared with the detector using the optical technology. In addition, there is a superior characteristic in that a structure of the component can be determined because the molecular weight can be measured. For this reason, the liquid chromatograph/mass spectrometer is used in wide fields such as development and quality control of pharmaceutical products, environment measurement, and foods. Recently, measurement of a blood biomarker from the knowledge obtained from a biomarker search, measurement of protein generated from genetic information, and structure analysis of the protein after a modification in a cell attract attention. 
         [0004]    A use environment: of the liquid chromatograph/mass spectrometer also spreads from the university or company laboratories to clinical laboratories in hospitals and the liquid chromatograph/mass spectrometer changes from a device used by a specialist in mass spectrometry to a device used by a specialist in other field. For this reason, a device that has convenience and high durability as well as high sensitivity to be one of characteristics of the mass spectrometry and a device that can perform maintenance easily are demanded. 
         [0005]    The mass spectrometers are largely divided into devices mainly performing quantitative analysis and devices mainly performing qualitative analysis. As a representative mass spectrometer that mainly performs the quantitative analysis, there is a triple quadrupole mass spectrometer (hereinafter, referred to as the triple-QMS) that has a plurality of quadrupole mass spectrometry units provided inside. Because the triple-QMS has a characteristic that specific ions of a measurement sample can be continuously detected, quantitative analysis performance is high. Meanwhile, as a mass spectrometer that mainly performs the qualitative analysis, there is a flight time type mass spectrometry unit (hereinafter, referred to as the TOF/MS). Measurement ions are flown in vacuum, a time until the ions arrive at a detector is measured, and a molecular weight is measured. Because a width of a mass/charge ratio observed is large and a mass spectrum with high resolution is easily obtained, qualitative analysis performance becomes high. 
         [0006]    In the mass spectrometer, an internal state becomes a vacuum state, electrodes having various shapes are disposed, and ions introduced into the mass spectrometer are controlled and separated by an electric field. For example, the quadrupole mass spectrometry unit is also called a Q mass (QMF) or a mass filter and includes four cylindrical electrodes. The cylindrical electrodes are combined by positioning a center of a circle at a vertex of a square. A positive/negative direct current and a high frequency alternating-current voltage are applied to the electrodes adjacent to the fixed cylindrical electrode to be superimposed. When the ions having the charge pass through the electrodes, the ions pass through the electrodes while vibrating and only a certain ion vibrates stably according to the voltage and the frequency and passes through the electrodes. Meanwhile, the vibration of the other ions increases during passing through the electrodes and the ions collide the electrodes and cannot pass through the electrodes. The mass spectrum is obtained by changing the high frequency alternating-current voltage linearly while maintaining a ratio of the direct current and the alternating-current voltage constantly. 
         [0007]    A device in which one quadrupole mass spectrometry unit (QMF) is mounted in the mass spectrometer is called a single quadrupole mass spectrometer (hereinafter, referred to as the Single-QMS). Because the mass spectrometer has a small size, the mass spectrometer is relatively cheap and is used widely for gas analysis. Meanwhile, the resolution of the mass spectrum is not high as about a 1 mass-charge ratio width. In addition, when the mass spectrometer is used as the detector of the liquid chromatograph, sensitivity is deteriorated because a noise derived from the solvent is observed at a low mass-charge ratio. 
       CITATION LIST 
     Patent Literature 
       [0008]    PTL 1: Japanese Patent Application Laid-Open. No. 2011-258563 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    As representative mass spectrometers coupled for the quantitative analysis in the liquid chromatograph/mass spectrometer, there are the Single-QMS and the Triple-QMS. As a name suggests, a typical Triple-QMS mounts three QMFs inside, transmits only a specific ion (A) by the first QMF, decomposes (A′) the ion by the second QMF, and detects the decomposed ion (A′) by the third QMF. This measurement method is called an MSMS method. The Single-QMS mounts one QMF inside and detects a specific ion (A). (A plurality of ions can be detected by changing an observation ion temporally). One of differences of the Triple-QMS and the Single-QMS is whether the MSMS method is enabled or disabled. When a noise ion (B) having almost the same mass as the mass of the specific ion (A) selected by the first QMF is superimposed on the specific ion (A), the specific ion (A) and the noise ion (B) are added (A+B) and observed, due to low resolution of the QMF. However, in a lot of cases, structures of the specific ion (A) and the noise ion (B) are different from each other. For this reason, if the ions are decomposed, mass/charge ratios of the decomposed specific ion. (A′) and the decomposed noise ion (B′) are different from each other. Only the mass/charge ratio of the decomposed specific ion (A′) is observed, so that only the specific ion can be selectively decomposed. This is because the noise can be reduced by detecting only the specific ion (A′) after the decomposition, at the time of the quantitative analysis of the specific ion (A), and SN can be greatly improved. Because the MSMS analysis is disabled in the Single-QMS, the noise cannot be reduced and the sensitivity of the Single-QMS is disadvantageous as compared with the Triple-QMS. 
         [0010]    As representative noises observed in the liquid chromatograph/mass spectrometer, there are a solvent-derived ion of a liquid introduced into the mass spectrometer, a noise observed when a neutral particle not to be charged collides with the detector in the mass spectrometer, and a noise generated from the detector. An object of the invention is to reduce solvent-derived ions of an introduced liquid and realize high sensitivity, in a mass spectrometer. 
         [0011]    In addition, because the mass spectrometer operates ions by a magnetic field and an electric field in the mass spectrometer, an internal state of the mass spectrometer becomes a vacuum state. For example, when an inner portion of the mass spectrometer is polluted by a measurement sample, it is necessary to stop a vacuum and perform maintenance and a long time is necessary to stop the vacuum, perform the maintenance, and restart the vacuum in the mass spectrometer. The specialized knowledge is necessary for the maintenance and it is required to perform pollution control measures of the mass spectrometer at the atmospheric side (the vacuum is not stopped). 
       Solution to Problem 
       [0012]    To solve the problem, a mass spectrometer of the present invention includes: 
         [0013]    an ion source; 
         [0014]    a mass spectrometry unit; 
         [0015]    a plurality of flat plates which function as electrodes having openings through which ions are introduced in a direction orthogonal to an ion introduction axis for the mass spectrometer; 
         [0016]    a porous member which is provided between the mass spectrometry unit and the plurality of flat plates; and 
         [0017]    a mechanism which flows gas to the plurality of flat plates for the side opposite to the ion introduction direction, 
         [0018]    wherein the plurality of flat plates are disposed under atmospheric pressure and temperatures of the plurality of electrodes are set to temperatures higher than a room temperature. 
       Advantageous Effects of Invention 
       [0019]    According to the invention, in a mass spectrometer that can be connected to a liquid chromatograph, neutral particles not ionized at the time of ionizing and low molecular ions derived from a solvent used in the liquid chromatograph, which are main components of noise components, are reduced and sensitivity of the mass spectrometer is improved. At the same time, because pollution occurring when a liquid sample is measured by the mass spectrometer is detected at the atmospheric side, maintenance of the mass spectrometer can be performed under atmospheric pressure and the maintenance becomes easy. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  illustrates an example of a configuration of a mass spectrometer in the present invention. 
           [0021]      FIG. 2  illustrates the detail of a configuration of an atmospheric ion introduction electrode in the present invention. 
           [0022]      FIG. 3  illustrates an example of an RF voltage applied to the atmospheric ion introduction electrode in the present invention. 
           [0023]      FIG. 4  illustrates an operation of the atmospheric ion introduction electrode in the present invention and a mass spectrum difference according to ON/OFF. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]      FIG. 2  illustrates an example of a configuration of an atmospheric ion introduction electrode according to an embodiment of the present invention. 
       Embodiment 
       [0025]    A measurement sample fed by a liquid chromatograph  10  and the like is ionized by an ion source  100 . The ion source  100  is under atmospheric pressure. Ions generated from the ion source  100  are introduced into an atmospheric ion introduction electrode unit  115 . The atmospheric ion introduction electrode  115  is configured by a plurality of parallel flat plates. In this embodiment, three parallel flat plates are illustrated. The ions having passed through a slit of a counter electrode  110  pass through a slit in a front electrode  120  and are introduced into a gap between the front electrode  120  and a rear electrode  130  connected to an ion introduction opening of a mass spectrometer. At this time, absolute values of voltages applied to the counter electrode  110 , the front electrode  120 , and the rear electrode  130  are set as counter electrode  110 &gt;=front electrode  120 &gt;=rear electrode  130 , so that ion introduction efficiency in the mass spectrometer can be improved. The ions having passed through the gap between the front electrode  120  and the rear electrode  130  enter a first pore  140  of a mass spectrometry unit. 
         [0026]    The first pore  140  becomes a partition wall between an atmospheric side  190  and a vacuum side  200  and a vacuum is created between the first pore  140  and a second pore  150  by connecting ( 160 ) a vacuum pump such as a rotary pump. The ions having passed through the second pore  150  are introduced into a single quadrupole mass spectrometry unit (Single QMS)  170 , for example. The mass spectrometry unit  170  to execute mass separation of the ions may be a quadrupole mass spectrometry unit, an ion trap, a flight time type, a Fourier transform type, or the like. The mass spectrometry unit is connected to a PC  180  to execute control of the mass spectrometer and data processing and obtains data. 
         [0027]      FIG. 2  illustrates the detail of the atmospheric ion introduction electrode unit  115 . The atmospheric ion introduction electrode  115  is configured by a plurality of parallel flat plates. In this embodiment, three parallel flat plates are illustrated. The ions generated by the ion source  100  are introduced into the first pore  140  of the mass spectrometer along an ion flow  520 . Meanwhile, gas can be flown to the atmospheric ion introduction electrode  115  and a gas flow  510  is generated in a direction facing the ion flow  520 . A diameter of the first pore  140  of the mass spectrometer is about φ0.3. The gap between the front electrode  120  and the rear electrode  130  has a shape of a parallel flat plate, an interval  530  of the front electrode  120  and the rear electrode  130  is 2 mm or less, an area of the gap is larger than an area of the first pore  140 , and the gas easily flows. For this reason, most of the introduced gas flow  510  passes through the gap between the front electrode  120  and the rear electrode  130  and flows in a direction toward the counter electrode  510 . The gap of the front electrode  120  and the rear electrode  130  may be 1 mm or less. 
         [0028]    In the ion source  100 , the ions are basically generated. However, the sprayed gas includes neutral particles that cannot be ionized, droplets that have large particle diameters and the like in addition to the ions. The gas flow  510  discharged from the counter electrode  510  prevents the neutral particles, the large droplets and the like from entering the mass spectrometer. Meanwhile, a flow rate of the gas flowing between the front electrode  120  and the rear electrode  130  is about 2.0 L/min or less and is faster than a flow rate of the gas sprayed from the ion source  100 . Even though the neutral particles pass through the counter electrode  110 , the neutral particles cannot pass through the gap between the front electrode  120  and the rear electrode  130 . Because the ions are the charged particles as the name suggests, absolute values of voltages electrically applied to the counter electrode  110 , the front electrode  120 , and the rear electrode  130  are set as counter electrode  110 &gt;=front electrode  120 &gt;=rear electrode  130 , so that the ions are introduced into the side of the mass spectrometry unit  170 . 
         [0029]    In addition, the atmospheric ion introduction electrode  115  has an opening through which the ions can be introduced in a direction orthogonal to an ion introduction axis for the mass spectrometry unit  170 . The neutral particles having the large particle diameters collide with the rear electrode  130 , so that the neutral particles are prevented from being introduced into the mass spectrometry unit  170 . 
         [0030]    In addition, temperatures of the electrodes configuring the atmospheric ion introduction electrode unit  115  are set to be higher than a room temperature and to be equal to or lower than about 250° C., so that ionizing can be accelerated in the atmospheric ion introduction electrode  115 . 
         [0031]    As described above, most of the ions passing through the gap between the front electrode  120  and the rear electrode  130  have the charge, from the structure of the atmospheric ion introduction electrode unit  115 , the gas flow, and the temperature setting. For this reason, an electric operation is enabled in the atmospheric pressure (a mean free path is short). An RF component  1000  in which a positive component and a negative component are asymmetrical is applied to the rear electrode  130 ; and a direct-current voltage necessary for ion introduction is applied to the front electrode  120 . Then, the ions are operated within the gap of the shape of the parallel flat plate between the front electrode  120  and the rear electrode  130 . The RE signal may be applied to only the front electrode  120  or both the front electrode  120  and the rear electrode  130 . This is because a voltage of a direct-current component is superimposed on each RE signal, so that the ion operation and the ion introduction for the mass spectrometry unit  170  are enabled. In addition, the applied RF signal is not a sine wave and may be a rectangular wave  1100  and a triangular wave. Here, amplitude of the asymmetrical RF voltage can be set differently in a solvent used by the liquid chromatograph and a sample of a measurement target. 
         [0032]    A representative diameter of the first pore of the mass spectrometer is about φ0.3 to 0.5 mm. For this reason, the ions passing through the gap of the shape of the parallel flat plate between the front electrode  120  and the rear electrode  130  are slightly moved, so that the ions may be introduced or not introduced into the mass spectrometer. 
         [0033]    Meanwhile, in the liquid chromatograph, the measurement sample is the liquid and the used representative solvent is water, methanol, and acetonitrile. There is hardly any demand for measuring these solvents because they are originally “a s solvent”. If the solvent, is ionized and is introduced into the mass spectrometer, the ions become a noise source and causes sensitivity of the mass spectrometer to be deteriorated. For this reason, if an amount of low molecular ions, derived from the solvent, introduced into the mass spectrometry unit  170  is decreased, the noise decreases and the sensitivity of the mass spectrometer is improved. 
         [0034]    In a liquid chromatograph/mass spectrometry unit, there are various types of measurement targets. However, there are steroids such as testosterone as a representative measurement sample. The steroid has a benzene ring structure inside and the structure thereof is different from structures of linear molecules such as water, methanol, and acetonitrile to be the solvents. Because the behaviors of the ions passing through the gap of the shape of the parallel flat plate between the front electrode  120  and the rear electrode  130  are defined by mobility of the ions, the mobility is different according to the molecular structure. For this reason, the solvent and the measurement sample can be divided by the applied voltage. 
         [0035]    Therefore, the voltages are set such that the ions derived from the solvent introduced from the liquid chromatograph do not pass through the gap of the shape of the parallel flat plate between the front electrode  120  and the rear electrode  130  and the measurement ions pass through the gap, at the time of passing through the gap, so that the amount of low molecular ions, derived from the solvent, introduced into the mass spectrometer can be decreased. In  2000  and  2100  of  FIG. 4 , data when the RF is not applied to the atmospheric ion introduction electrode unit and data when the RF is applied to the atmospheric ion introduction electrode unit are illustrated, respectively. A horizontal axis shows a mass/charge ratio and a vertical axis shows signal strength. When the RF of the atmospheric ion introduction electrode unit is set to Off, a noise of about 5 counts is observed as a background. If the RF is set to On, a background component becomes 1 to 2 counts and the noise becomes about ½ to ⅕. Because signal strength of a target component is rarely changed and is observed, SN is improved about 2 to 5 times. 
         [0036]    When the solvent and the like are measured, the application of the RF voltage to the atmospheric ion introduction electrode unit can be set to Off. 
         [0037]    Because the atmospheric ion introduction electrode according to the present invention is connected to the atmospheric side of the mass spectrometry unit, the atmospheric ion introduction electrode can be adopted in the Single QMS, the Triple QMS, an ion trap, a flight time type, a Fourier transform type, or a complex type mass spectrometer. The sensitivity of the mass spectrometer is improved due to reduction of the noise. Particularly, the atmospheric ion introduction electrode is introduced into the Single QMS in which the MSMS analysis is disabled, so that a reduction effect of the noise is large, and maintenance is enabled under the atmospheric pressure. Therefore, even if there is not specialized knowledge, the present invention can be carried out. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  liquid chromatograph 
           100  ion source 
           110  counter electrode 
           115  atmospheric ion introduction electrode 
           120  front electrode 
           130  rear electrode 
           140  first pore 
           150  second pore 
           160  connection with vacuum pump 
           170  mass spectrometry unit 
           180  device control and data processing apparatus such as PC 
           190  atmospheric side 
           200  vacuum side 
           510  gas flow 
           520  ion flow 
           530  distance between front electrode  120  and rear electrode  130   
           540  ion introduction axis in mass spectrometer 
           550  ion introduction axis in mass spectrometer 
           1000  voltage setting example (sine wave shape) of rear electrode  130   
           1100  voltage setting example (rectangular wave shape) of rear electrode  130   
           2000  atmospheric ion introduction electrode unit, RF voltage OFF 
           2100  atmospheric ion introduction electrode unit, RF voltage ON