JP2007194094A - Mass spectroscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem wherein, when electrons generated by an ion source pass through an ion optical system along with ions and enter into a quadrupolar mass filter, they may cause background noise. <P>SOLUTION: A magnetic field formation means 11 for superposing a deflection magnetic field is arranged in an electrostatic field of the ion optical system 7 for transporting ions emitted from an ionization chamber 4 being the ion source to the quadrupolar mass filter 9. Lorentz force acts on ions and electrons passing through the deflection magnetic field, and, whereas the electrons dramatically small in mass as compared with ions can not pass through the ion optical system 7 by bending their orbits, the ions can reach the quadrupolar mass filter 9 without being influenced by the magnetic field. Thereby, unnecessary electrons can be eliminated before the quadrupolar mass filter 9 without impairing transport efficiency of the ions. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a mass spectrometer, and more particularly to an apparatus suitable for a mass spectrometer using an ion source using electrons for ionization, such as an electron impact ionization method and a chemical ionization method.

  In a mass spectrometer, various ionization methods are used to ionize molecules and atoms of a sample to be analyzed. Among them, the electron impact ionization method is one of typical ionization methods. In an ion source using an electron impact ionization method, thermoelectrons generated by a filament are accelerated by an electric field and brought into contact with a sample molecule (or atom), and the sample molecule is ionized by the contact action (see Patent Document 1, for example).

  In this electron impact ionization ion source, there is a possibility that the thermoelectrons generated from the filament are accelerated in the direction of the ion optical axis by the interaction with the sample or a carrier gas such as helium. Specifically, electrons generated in the ion source may have kinetic energy up to an acceleration voltage (usually about 70 eV). Therefore, in principle, electrons that can reach the mass analysis unit exist unless a region having a potential equal to or lower than that of the filament exists in the space from the ion source to the mass analysis unit.

  When these electrons reach the quadrupole mass filter, which is the mass analyzer, the speed is so high that they pass through the quadrupole mass filter almost unaffected by the high-frequency electric field of the quadrupole mass filter. . Even if the electrons introduced into the quadrupole mass filter do not pass through the filter due to the influence of a direct current electric field or the like, if the electrons collide with the quadrupole, there is a risk of knocking out secondary electrons or X-rays. is there. When these electrons or X-rays are introduced into the ion detector, they contribute to background noise.

No. 2000-067808

  Conventionally, the influence of the incidence of electrons on the mass spectrometer as described above has hardly been taken into consideration. However, it is expected that the above problems will become more apparent as the analysis accuracy increases with various improvements. The present invention has been made in view of such a point, and the object of the present invention is to positively exclude electrons incident on the mass analysis unit, thereby reducing background noise caused by the electrons. An object of the present invention is to provide a mass spectrometer that can be reduced.

  In order to solve the above-mentioned problems, the present invention provides a mass spectrometer having an ion optical system for transporting ions starting from an ion source to a mass analyzer, on a path of ions transported by the ion optical system. Magnetic field forming means for forming a deflection magnetic field is provided, and electrons introduced into the mass analysis unit are excluded by the deflection magnetic field.

  In the mass spectrometer according to the present invention, the ion source is not necessarily a means for ionizing the sample component itself. For example, the ion source temporarily accumulates ions generated in another place such as an ion trap. A kinetic energy may be imparted to various ions at a predetermined timing to start toward the mass analyzer. However, the present invention is particularly useful when a large number of electrons are incident with ions toward the mass spectrometric section, and therefore it may be applied to a configuration that is an ion source by an electron impact ionization method. Further, the mass spectrometric unit is not particularly limited as long as it separates ions according to the mass number, but a quadrupole mass filter can be given as an example.

  When the electrons are accelerated together with the ions starting from the ion source and travel toward the mass analyzer, the electrons pass through the deflection magnetic field formed by the magnetic field forming means. Since electrons are a kind of charged particles, when passing through a magnetic field, they are subjected to Lorentz force in a direction orthogonal to both the direction of the magnetic field and the direction of travel of the electrons. Similarly, an ion, which is a kind of charged particle, receives the same Lorentz force, but the electron has a much smaller mass than the ion with the smallest mass, and the smaller the mass, the greater the amount of deflection in the direction perpendicular to the traveling direction. Since the electrons are large, their orbits are greatly bent by the time they reach the mass analyzer and do not enter the mass analyzer. On the other hand, ions whose mass is much larger than that of electrons proceed almost without being affected by the magnetic field, reach the mass analyzer, and are separated for each mass number.

  As described above, according to the mass spectrometer according to the present invention, unnecessary electrons are surely eliminated before entering a mass analysis unit such as a quadrupole mass filter without impairing the transport efficiency of ions to be analyzed. can do. This not only prevents electrons entering the mass analyzer from passing through and reaching the detector, but also the electrons and X-rays that are secondarily generated inside the mass analyzer. It is possible to prevent the incident. Therefore, background noise can be further reduced than before, and analysis sensitivity and analysis accuracy can be improved.

  Hereinafter, a mass spectrometer according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of a main part of the mass spectrometer of the present embodiment.

  The inside of the vacuum vessel 1 is evacuated by a vacuum pump 2 and maintained at a predetermined degree of vacuum. For example, sample gas flowing out from a column of a gas chromatograph (GC) (not shown) is supplied into the ionization chamber 4 from the connection pipe 3 through an appropriate interface. A thermoelectron flow from the filament 5 toward the trap electrode 6 is formed in the ionization chamber 4, and the thermoelectrons come into contact with the sample molecules introduced into the ionization chamber 4 so that the molecules are ionized. The various ions generated are extracted from the ionization chamber 4 and introduced into the space in the long axis direction of the quadrupole mass filter 9 through the ion optical system 7 centering on the cylindrical electrostatic lens electrode 8. The quadrupole mass filter 9 is applied with a voltage obtained by superimposing a DC voltage and a high-frequency voltage from a power source (not shown), and only ions having a mass number corresponding to the applied voltage pass through the space in the major axis direction. It reaches the detector 10 and is detected. Other unnecessary ion species cannot pass through the space in the long axis direction of the quadrupole mass filter 9 and diverge and disappear in the middle.

  In the space in the vacuum vessel 1, three-dimensional coordinates of an X axis, a Y axis, and a Z axis that are orthogonal to each other are considered as illustrated. The ion optical axis C, which is the central axis of the ion flow, is the Z-axis direction.

  The ion source including the ionization chamber 4, the filament 5, the trap electrode 6 and the like is an ion source by a so-called electron impact ionization method. In this ion source, some of the thermoelectrons introduced into the ionization chamber 4 are given acceleration energy in the Z-axis direction similar to that of ions by the action of the sample components and the carrier gas flowing from the connection tube 3, and the ion optical system. 7 may be incident. If this electron is introduced into the quadrupole mass filter 9 as it is, it may become background noise as described above. Therefore, as a characteristic configuration in the mass spectrometer of this embodiment, the electrostatic lens electrode 8 is sandwiched. Thus, the magnetic field forming means 11 composed of two flat magnetic poles 11a and 11b (one is an S pole and the other is an N pole) separated from each other in the X-axis direction and parallel to each other is arranged.

  FIG. 2 is a schematic view of the electrostatic lens electrode 8 and the magnetic field forming unit 11 as viewed from the direction of ion incidence. A deflection magnetic field B is formed between the two plate magnetic poles 11a and 11b sandwiching the electrostatic lens electrode 8, and the direction of the lines of magnetic force in the magnetic field B is the X-axis direction.

In general, the equation of motion of a charged particle flying in an electromagnetic field is as follows: Note that vectors in X, Y, and Z three-dimensional coordinates are described with ↑. That is, a vector is indicated by a ↑.
m (d ² r ↑ / dt ² ) = q [E ↑ + (dr ↑ / dt) · B ↑] = q · E ↑ + q · (dr ↑ / dt) · B ↑] (1)
Here, m is the mass of the charged particle, q is the charge, r is the position coordinate, t is the time, E is the electric field, and B is the magnetic field.

  When there is no magnetic field forming means 11 as in the conventional configuration, it is not necessary to consider the magnetic field B, so the second term on the right side of equation (1) can be ignored. Therefore, if the initial energies of the charged particles are the same, the trajectories are the same regardless of the mass. Ions and electrons are both charged particles, and the mass of electrons is much smaller than the smallest mass of ions, but they can take the same orbit.

  On the other hand, when the deflection magnetic field B is formed by the magnetic field forming unit 11 as described above, it is necessary to consider the second term on the right side of the equation (1). This is the Lorentz force acting on the charged particles when passing through the deflection magnetic field B, and the direction of the force is both in the direction of movement of the charged particles (Z-axis direction) and the direction of the magnetic field lines of the magnetic field (X-axis direction). The direction is orthogonal, that is, the Y-axis direction. That is, when electrons and ions that are charged particles pass through the deflection magnetic field B, they receive a force that causes their orbit to bend in the Y-axis direction. However, the deflection amount at that time depends on the mass, and the smaller the mass, the larger the deflection amount. Therefore, even if the ions are hardly deflected, the electrons are largely deflected.

  FIG. 3 is a schematic diagram showing the state of this deflection. The ions incident in the deflection magnetic field B formed by the plate magnetic poles 11a and 11b travel almost straight without being deflected (but vibrate in the case of a high-frequency electric field), but the electrons are in the Y-axis direction. A greatly bent trajectory P2 is taken. Accordingly, ions are incident on the quadrupole mass filter 9 and are subject to mass analysis in substantially the same manner as when there is no deflection magnetic field B, but electrons are excluded before entering the quadrupole mass filter 9 and are quadrupole. It does not enter the mass filter 9. As a result, the problem caused by electrons entering the quadrupole mass filter 9 and reaching the ion detector 10 can be solved.

  Next, the trajectory simulation results performed to verify the electron exclusion effect in the mass spectrometer of the present embodiment will be described. 4A and 4B are diagrams showing the trajectory simulation results when the deflection magnetic field is not superimposed on the electrostatic field generated by the electrostatic lens electrode 8. FIG. 4A shows the trajectory of ions having a mass number of 500, and FIG. Ion trajectory of the generated electrons. It can be seen that almost all of the ions can pass through the ion optical system and enter the quadrupole mass filter, but almost all of the unnecessary electrons also pass through the ion optical system 7.

  FIG. 5 is a diagram showing a trajectory simulation result when the deflection magnetic field B as described above is superimposed on the electrostatic field generated by the electrostatic lens electrode 8, and (a) is a trajectory of ions having a mass number of 500 as in FIG. (B) is an ion orbit of electrons. At this time, the orbit of ions hardly changes as shown in FIG. 4A despite the superposition of the deflection magnetic field, and all of them can pass through the ion optical system 7 and reach the quadrupole mass filter. On the other hand, the trajectory of the electrons is bent by the deflection magnetic field, and almost all of them collide with the emission side electrode of the ion optical system 7 and cannot reach the quadrupole mass filter. As can be seen from this result, in the mass spectrometer of this example, it is possible to efficiently eliminate only unnecessary electrons in front of the quadrupole mass filter without affecting the ion transport efficiency. Thereby, the background noise can be reduced.

  In the above embodiment, the magnetic field forming means 11 is the parallel plate magnetic poles 11a and 11b, but actually the direction in which electrons are deflected is any direction except the Z-axis direction, preferably any direction perpendicular to the Z-axis. Therefore, various shapes can be used without using parallel plate magnetic poles. FIG. 6 is an example of a structure in which the electrostatic lens electrode and the deflecting electric field generating magnetic pole are shared, and is a schematic view as seen from the incident direction of ions as in FIG. In this configuration, the semi-cylindrical electrodes 12a and 12b having S-pole and N-pole magnets are arranged to face each other with a predetermined distance therebetween. According to this, the structure is simple and the cost can be reduced.

  In the above embodiment, the mass analyzer is a quadrupole mass filter. However, the present invention is not limited to this, and the time-of-flight mass spectrometer, sector-type mass spectrometer, or other incidents of electrons adversely affect performance. The present invention can be applied to general mass spectrometers having a mass analysis unit that may affect the mass. The ion source is not limited to the electron impact ion source, but is particularly useful for an ion source that uses electrons for ionization, for example, an ion source using a chemical ionization method.

The schematic block diagram of the principal part of the mass spectrometer by one Example of this invention. The schematic diagram which looked at the electrostatic lens electrode and the magnetic field formation means from the incident direction of ions in the mass spectrometer according to the present embodiment. The schematic diagram which shows the trajectory of the electron and ion in a deflection magnetic field. It is a figure which shows the trajectory simulation result when a deflection magnetic field is not superimposed on the electrostatic field by an electrostatic lens electrode, (a) is the trajectory of the ion of mass number 500, (b) is the electron emitted in the same direction as the ion. Ion orbit. It is a figure which shows the trajectory simulation result at the time of superimposing a deflection magnetic field on the electrostatic field by an electrostatic lens electrode, (a) is the trajectory of the ion of mass number 500, (b) is the electron emitted in the same direction as the ion. Ion orbit. The figure which shows the structure which shared the electrostatic lens electrode and the magnetic pole for deflection | deviation electric field generation | occurrence | production of the mass spectrometer by other Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Vacuum container 2 ... Vacuum pump 3 ... Connection pipe 4 ... Ionization chamber 5 ... Filament 6 ... Trap electrode 7 ... Ion optical system 8 ... Electrostatic lens electrode 9 ... Quadrupole mass filter 10 ... Ion detector 11 ... Magnetic field formation Means 11a, 11b ... Flat magnetic pole B ... Deflection magnetic field C ... Ion optical axis

Claims (2)

  A mass spectrometer having an ion optical system for transporting ions starting from an ion source to a mass analyzer, comprising: a magnetic field forming means for forming a deflection magnetic field on a path of ions transported by the ion optical system; The mass spectroscope is characterized in that electrons introduced into the mass spectrometric section are excluded by the above. The mass spectrometer according to claim 1, wherein the ion source is an ion source based on an electron impact ionization method.

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