JP2010244903A - Mass spectrometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance the convergency of ions of same m/z, when emitting pulse-likely the ions in an ion source by an EI method. <P>SOLUTION: A gate electrode 15 is arranged between a filament 12 for generation a thermoelectron and a thermoelectron introducing port 102 of an ionization chamber 10. The thermoelectron is shielded by impressing a prescribed voltage to the gate electrode 15, after a sample molecule is ionized by introducing the thermoelectron into the ionization chamber 10. An ion emission voltage is impressed to a repeller electrode 14 after the thermoelectron introduced into the ionization chamber 10 just before is passed through the ionization chamber 10, and initial kinetic energy is imparted to the ions in the ionization chamber 10, to emit the ions pulse-likely from the ionization chamber 10. An electric field is not disturbed at that time by the thermoelectron, and a behavior of the ion is not affected by the thermoelectron, because no thermoelectron exists substantially in the ionization chamber 10. A noise is also restrained from being generated by collision of the thermoelectron with the repeller electrode 14. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a mass spectrometer, and more particularly, to an ion source that generates ions using electron ionization (EI) in a mass spectrometer.

  2. Description of the Related Art Conventionally, an ion source using an electron ionization method (hereinafter referred to as EI method) is known as an ion source for ionizing a gaseous sample in a mass spectrometer. In the EI method, thermoelectrons generated by heating a filament are accelerated by the action of an electric field and collide with gaseous sample molecules (or atoms). Then, electrons jump out from the sample molecules, and sample molecule ions are generated.

  When using the EI ion source as an ion source of a time-of-flight mass spectrometer (TOF-MS), or when introducing ions generated by the EI ion source intermittently into a three-dimensional quadrupole ion trap or the like Needs to accelerate the ions generated by the ion source in a pulsed manner and send them out in a packet state (a group of states). In the TOF-MS described in Non-Patent Document 1, ions generated by the EI method are pulsedly emitted from an ion source and used for mass spectrometry as follows.

  That is, when the thermoelectrons generated in the filament are accelerated and continuously introduced into the ionization chamber, the thermoelectrons collide with the sample molecules introduced into the ionization chamber, thereby ionizing the sample molecules. Since the thermionic current is continuously supplied into the ionization chamber, if a gaseous sample is continuously supplied into the ionization chamber, ions are also generated almost continuously. When ions are sent out from the ionization chamber, a voltage having the same polarity as the ions is applied in a pulse manner to a repeller electrode disposed in the ionization chamber. As a result, an electric field in which ions are repelled is generated in the ionization chamber, so that ions are given initial kinetic energy by the action of this electric field and are emitted in a pulse manner through an ion emission port provided in the ionization chamber. The ions emitted from the ionization chamber as described above are further accelerated by one or more stages of acceleration electrodes, introduced into the flight space, and separated in accordance with the mass-to-charge ratio in the process of flying in the flight space.

  In order to achieve high mass accuracy and mass resolution with TOF-MS as described above, ions having the same mass-to-charge ratio (m / z) converge in time when ions reach the detector. Need to be. That is, it is preferable that the time-of-flight spectrum peak width of ions having the same mass-to-charge ratio is as narrow as possible. However, the conventional EI ion source has a problem that time convergence is not always sufficient. The following can be considered as the cause.

  That is, since the thermionic current is continuously introduced into the ionization chamber, when a voltage (voltage for ion emission) is applied to the repeller electrode in a pulsed manner to emit ions in the ionization chamber, electrons are added in addition to the ions. Groups are also present in the ionization chamber. The presence of this electron group disturbs the electric field in the ionization chamber when the ion extraction voltage is applied to the repeller electrode, affecting the behavior of the ions, and affecting the convergence of ions emitted from the ionization chamber. Can be considered. In the case of positive ion measurement, since a positive ion emission voltage is applied to the repeller electrode, electrons existing in the ionization chamber are attracted to the repeller electrode and collide with the electrode when the voltage is applied. In this way, the electron group collides with the repeller electrode, which causes noise.

  In addition, in order to sufficiently converge ion packets having the same mass-to-charge ratio, the initial position (spatial initial distribution) of ions in the ionization chamber at the time when the ion extraction voltage is applied to the repeller electrode is important. is there. In a conventional EI ion source, sample molecules are ionized one after another by constantly introducing a thermionic current into the ionization chamber, and ions are spatially expanded in the ionization chamber due to the Coulomb repulsive force between these many ions. . For this reason, when the ion extraction voltage is applied to the repeller electrode, the position of the starting point of each ion is different, and the convergence of ions is poor due to variations in kinetic energy to which each ion is applied or variations in flight distance. It is considered to be.

WC Wiley and 1 other, “Time-of-Flight Mass Spectrometer with Improved Resolution”, Review of Science Review of Scientific Instruments, Volume 26, 1995, p. 1150-1157

  The present invention has been made in view of the above problems, and the object of the present invention is to provide ions having the same mass-to-charge ratio when, for example, ions generated from an ion source are subjected to mass analysis by a time-of-flight mass analyzer. It is to provide a mass spectrometer capable of improving mass resolution and mass accuracy by performing the convergence of the above.

The present invention made to solve the above problems has a thermoelectron generator that generates thermoelectrons, and a thermoelectron inlet that introduces thermoelectrons generated by the thermoelectron generator into the interior. In a mass spectrometer comprising an ion source including: an ionization chamber that ionizes sample molecules or atoms using thermal electrons in
a) a gate electrode that is disposed between the thermoelectron generator and the thermoelectron inlet of the ionization chamber, and passes or blocks thermoelectrons;
b) an ion extraction electrode for applying kinetic energy to the ions generated in the ionization chamber by an electric field and emitting them from the ionization chamber;
c) first voltage applying means for applying a voltage for controlling passage / blocking of the thermoelectrons to the gate electrode;
d) a second voltage applying means for applying a voltage for ejecting ions from the ionization chamber to the ion ejection electrode;
e) Ion emission to generate ions in the ionization chamber in a state where thermionic electrons can pass through the gate electrode, and to emit ions from the ionization chamber after transitioning to a state in which the thermoelectrons are blocked by the gate electrode. Control means for controlling the first and second voltage application means so as to apply a voltage to the working electrode;
It is characterized by having.

  The thermoelectron generator is, for example, a filament that generates thermoelectrons by heating. The ion extraction electrode is disposed inside the ionization chamber, and is disposed on the outside of the ionization chamber by the action of an electric field. One of the extraction electrodes that draws out the ions to the outside.

  In the mass spectrometer according to the present invention, the thermoelectrons freely passing through the gate electrode are fed into the ionization chamber under the control of the control means. In the ionization chamber, the thermal electrons come into contact with the sample molecules, and the sample molecules are ionized. Thus, after ionization for a predetermined time, a predetermined voltage is applied to the gate electrode from the first voltage applying means, and the passage of the thermal electrons is obstructed by the action of the electric field formed thereby. Specifically, the thermoelectrons may be rebounded by applying a negative voltage to the gate electrode with respect to the thermoelectron generator. When the hot electrons are not supplied into the ionization chamber, there are almost no thermoelectrons in the ionization chamber in a short time. In this state, a predetermined ion emission voltage is applied from the second voltage application means to the ion emission electrode, and ions in the ionization chamber are sent out by the action of the electric field formed thereby.

  In this manner, according to the mass spectrometer of the present invention, ions generated immediately before the ionization chamber can be emitted from the ionization chamber with almost no thermoelectrons in the ionization chamber. Therefore, since the electric field in the ionization chamber at the time of ion emission is not affected by the electron group, for example, when introducing ions emitted from the ionization chamber into the time-of-flight mass analyzer, the same mass charge at the time of reaching the detector The convergence of ions having a ratio is improved. Thereby, mass resolution and mass accuracy can be improved. Further, even when a positive voltage is applied to the ion emission electrode in order to measure positive ions, collision of electrons with the electrode is reduced, so that generation of noise due to this collision can be suppressed.

  As a preferred aspect of the mass spectrometer according to the present invention, the control means controls the application of voltage to the gate electrode so that the thermoelectrons pass through the gate electrode in a pulsed manner, and the thermoelectrons The first and second voltage applying means may be controlled so that an ion emission voltage is applied to the ion emission electrode so that ions are emitted from the ionization chamber immediately after the transition to the blocked state.

  According to this configuration, a thermionic current is introduced into the ionization chamber for a short time in a pulsed manner, and then an electric field for ion extraction is quickly formed in the ionization chamber, so that kinetic energy is imparted to ions from the start of ion generation. It takes less time to start. As a result, ions can be ejected from the ionization chamber under the condition that the spatial expansion due to the Coulomb repulsive force between the ions hardly occurs. Thereby, the variation of the ion starting position and the variation of the applied kinetic energy can be reduced, and the convergence of the same mass-to-charge ratio can be further enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS The whole block diagram of the mass spectrometer which is one Example of this invention. The detailed block diagram of the ion source in the mass spectrometer of a present Example. The timing diagram of the voltage application to the gate electrode and repeller electrode in the mass spectrometer of a present Example. The example of actual measurement of the time-of-flight spectrum peak by the mass spectrometer of a present Example. The timing diagram of the voltage application to the gate electrode and repeller electrode by another Example.

  A mass spectrometer which is one embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is an overall configuration diagram of a mass spectrometer according to the present embodiment, and FIG. 2 is a detailed configuration diagram of an ion source.

  In FIG. 1, the inside of the vacuum vessel 6 is evacuated by a vacuum pump (not shown) 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 from the sample introduction tube 7 into the ionization chamber 10 of the ion source 1 through an appropriate interface. A thermoelectron flow from the filament 12 toward the trap electrode 13 is formed in the ionization chamber 10, and the molecules are ionized when the thermoelectrons contact the sample molecules introduced into the ionization chamber 10.

  The generated various ions are pulsedly emitted from the ionization chamber 10 by the action of an electric field to be described later (right side in FIG. 1), and are further accelerated by the second stage acceleration electrode 2 and formed in the flight tube 3. 4 is introduced. Various ions that have started from the ion source 1 almost simultaneously fly in the flight space at a speed corresponding to the mass-to-charge ratio. Therefore, ions having a larger mass-to-charge ratio are delayed while flying in the flight space 4, and reach the detector 5 with a time difference. A data processing unit (not shown) that has received the detection signal from the detector 5 converts the flight time into a mass-to-charge ratio using previously obtained calibration information, and creates, for example, a mass spectrum.

  As shown in detail in FIG. 2, a filament chamber 11 having a filament 12 is disposed outside the thermoelectron introduction port 102 opened in the wall surface of the ionization chamber 10. When a heating current is supplied, the temperature of the filament 12 rises and thermoelectrons are emitted. On the other hand, in the ionization chamber 10, a thermoelectron emission port 103 is provided at a position facing the thermoelectron introduction port 102, and a trap electrode 13 is disposed outside the thermoelectron emission port 103. Between the filament chamber 11 and the thermoelectron introduction port 102, a gate electrode 15 having an opening through which thermoelectrons pass is disposed. A flat repeller electrode 14 is disposed in the ionization chamber 10 on the side opposite to the ion emission port 101 formed on the wall surface of the ionization chamber 10.

  A magnet 16 and a yoke 17 are disposed on the outer side of the filament chamber 11 and the trap electrode 13. The magnets rotate spirally by the action of a magnetic field, and the space of the thermoelectrons in the ionization chamber 10. This is to suppress general spread. As a result, the space in which the thermoelectrons and the sample molecules can contact, that is, the ion generation region can be narrowed, and variations in the starting position of the ion flight can be reduced.

  For example, a direct current voltage V1 of −70 [V] is applied to the filament 12, and a direct current voltage V2 of −73 [V], for example, slightly lower than that is applied to the filament chamber 11, and the trap electrode 13 and the ionization chamber 10 are Grounded (potential 0 [V]). The gate electrode 15 has a predetermined voltage, which will be described later, from the electronic control voltage generator 21 corresponding to the first voltage application means in the present invention, and the repeller electrode 14 has an emission control voltage generator corresponding to the second voltage application means in the present invention. A predetermined voltage, which will be described later, is applied from 22, and the operations of the electronic control voltage generator 21 and the emission control voltage generator 22 are controlled by a controller 20 including a CPU. The potential of the trap electrode 13 is ideally about +10 [V], which is higher than the potential of the ionization chamber 10, but here it was set to 0 [V] to facilitate the experiment.

  FIG. 3 is added to FIGS. 1 and 2, and a characteristic ion generation operation in the mass spectrometer of the present embodiment will be described. FIG. 3 is a timing chart of voltage application to the gate electrode 15 and the repeller electrode 14.

  Under the control of the control unit 20, when the electronic control voltage generation unit 21 applies a voltage of 0 [V], which is the same as the potential of the ionization chamber 10, to the gate electrode 15, the thermoelectrons generated in the filament 12 are transferred to the gate electrode 15. It passes freely through the opening, and further passes through the thermal electron inlet 102 and enters the ionization chamber 10. That is, an electron flow (electron beam) is formed by thermionic electrons from the top to the bottom as shown in FIG. 2 (strictly, it advances spirally by the action of the magnetic field described above). The sample molecules introduced into the ionization chamber 10 are ionized in contact with the thermoelectrons. At this time, a voltage of 0 [V] (that is, the same potential as that of the ionization chamber 10) is applied to the repeller electrode 14 from the extraction control voltage generator 22, so that the generated positive ions accumulate in the ionization chamber 10.

  At time t1, the electronic control voltage generator 21 applies a predetermined voltage (for example, −100 [V]) lower than V1 and V2 to the gate electrode 15. Then, since a high potential barrier against electrons is formed between the gate electrode 15 and the filament chamber 11, the thermoelectrons generated in the filament 12 are rebounded and cannot enter the ionization chamber 10. The thermoelectrons that have passed through the gate electrode 15 immediately before the time t1 enter the ionization chamber 10, but thereafter, the incidence of the thermoelectrons into the ionization chamber 10 stops, so the density of the thermoelectrons in the ionization chamber 10 is rapid. In the period from the time t1 until a certain time elapses, all the thermoelectrons pass through the ionization chamber 10. As will be described later, since the speed of the electrons is very high, almost all the thermoelectrons pass through the ionization chamber 10 in a very short time, and there are almost no thermoelectrons in the ionization chamber 10.

  At the time t <b> 2 when such a state is reached, the emission control voltage generator 22 applies a predetermined positive voltage (for example, 1400 [V]) to the repeller electrode 14. Then, the positive ions accumulated in the ionization chamber 10 are given initial kinetic energy toward the right in FIG. 2 by the action of the electric field of the same polarity formed in the ionization chamber 10 by the voltage applied to the repeller electrode 14. Is done. Thereby, the ions derived from the sample molecules start to move almost simultaneously and are emitted out of the ionization chamber 10 through the ion emission port 101. These ions are further accelerated by the electric field formed by the voltage applied to the second stage acceleration electrode 2 and introduced into the flight space 4.

  As described above, since there are almost no thermoelectrons in the ionization chamber 10 at the time of applying the ion emission voltage to the repeller electrode 14 (time t2), the positive electric field formed by the applied voltage to the repeller electrode 14 is It is hardly affected by thermionic electrons. Therefore, the behavior of ions emitted from the ionization chamber 10 is not disturbed by thermal electrons, and ions having the same mass-to-charge ratio exhibit high convergence. Further, even if a positive voltage is applied to the repeller electrode 14, no thermal electrons collide with the repeller electrode 14, and thus noise caused by such electron collision can be suppressed.

  Next, actual measurement results using the ion source 1 having the configuration shown in FIG. 2 will be described. In this measurement, nitrogen gas was used as a sample. The potential difference between the repeller electrode 14 and the ionization chamber 10 was about 1400 [V], and the potential difference between the ionization chamber 10 and the second stage acceleration electrode 2 was about 6900 [V]. Also, the time difference t between t1 and t2 shown in FIG. This is because the actual potential of the repeller electrode 14 does not rise steeply as shown in FIG. 3, and about 30 [from the time when the output control voltage generator 22 starts increasing the voltage until the voltage of the repeller electrode 14 rises. This is because it takes nsec]. Therefore, when the rise of the voltage of the repeller electrode 14 is fast, it is better to provide a time difference t of several to several tens [nsec].

  FIG. 4 shows a signal waveform observed by the detector (MCP) 5 disposed at the position of the convergence plane P, that is, a time-of-flight spectrum showing the relationship between the time of flight and the signal intensity. The ion convergence characteristic can be shown by using the time width (half-value width) of the spectrum peak. In this example, the half-value width is 1.8 [nsec], which indicates that the ion is sufficiently converged.

  Next, a mass spectrometer according to another embodiment of the present invention will be described. The basic configuration of this mass spectrometer is the same as that of the above embodiment, but under the control of the control unit 20, the electronic control voltage generation unit 21 and the emission control voltage generation unit 22 are connected to the gate electrode 15 and the repeller electrode 14, respectively. The timing for changing the applied voltage is different. FIG. 5 is a timing diagram of voltage application to the gate electrode 15 and the repeller electrode 14.

  In this embodiment, as shown in FIG. 5, a voltage is applied from the electronic control voltage generator 21 to the gate electrode 15 so as to allow the passage of thermoelectrons for a short time in a pulsed manner, and immediately thereafter, to the repeller electrode 14. Ion extraction voltage is applied. As described above, since the sample molecules are ionized only in the ionization chamber 10 when the thermoelectrons are introduced, the ion generation is started at the time t3, and the ion generation is performed at the time t4. The ion extraction voltage is applied to the repeller electrode 14 at time t5.

  Since the ions generated in the ionization chamber 10 have the same polarity, a large number of ions that were initially densely packed in a narrow region spread spatially over time due to the Coulomb repulsive force. On the other hand, in the apparatus according to this embodiment, as described above, since the time from the ion generation start time to the ion extraction voltage application time is short, there is almost no spatial expansion of ions due to the Coulomb repulsion as described above. Thus, it is possible to accelerate by applying initial kinetic energy to the ions. As a result, the starting point of the ions falls within a comparatively narrow region, and the variation in the flight distance of each ion is suppressed, and the variation in the applied initial kinetic energy is also reduced. As a result, the convergence of ions having the same mass-to-charge ratio can be improved.

There is a large difference in speed between electrons and ions. For example, the velocity of the electron accelerated at 70 [eV] is 5 × 10 ⁶ [m / sec] and passes through a length of 50 [mm] (this is the order of the length of the ionization region in the ionization chamber 10). The time required for this is only 10 [nsec]. On the other hand, the velocity of nitrogen ions at 400 [K] is 6 × 10 ² [m / sec], and the distance that the ions move in a time of 10 [nsec] is only 6 [μm]. Therefore, even when the thermoelectrons completely pass through the ionization region in the ionization chamber 10 and the ion extraction voltage is applied to the repeller electrode 14 in a pulsed manner, the ions hardly spread spatially during that time. The effects as described above can be achieved.

  In any of the above-described embodiments, ions emitted from the ion source in a pulse manner are introduced into the time-of-flight mass analyzer to perform mass analysis. However, the ions emitted from the ion source in a pulse manner are, for example, three-dimensional. It can also be introduced into a quadrupole ion trap and once trapped, and then ions can be emitted from the ion trap for mass analysis. That is, in the mass spectrometer according to the present invention, the mass spectrometry method is not particularly limited.

  In addition, it is obvious that any modifications, corrections, and additions as appropriate within the scope of the present invention are included in the scope of the claims of the present application.

DESCRIPTION OF SYMBOLS 1 ... Ion source 10 ... Ionization chamber 101 ... Ion emission port 102 ... Thermionic introduction port 103 ... Thermionic emission port 11 ... Filament chamber 12 ... Filament 13 ... Trap electrode 14 ... Repeller electrode 15 ... Gate electrode 16 ... Magnet 17 ... Yoke 2 ... 2nd stage acceleration electrode 3 ... Flight tube 4 ... Flight space 5 ... Detector 6 ... Vacuum vessel 7 ... Sample introduction tube 20 ... Control part 21 ... Electronic control voltage generation part 22 ... Output control voltage generation part

Claims (2)

A thermoelectron generator that generates thermoelectrons, and a thermoelectron inlet that introduces thermoelectrons generated by the thermoelectron generator into the inside, and ionizes sample molecules or atoms using the thermoelectrons inside the thermoelectrons A mass spectrometer comprising an ion source including:
a) a gate electrode that is disposed between the thermoelectron generator and the thermoelectron inlet of the ionization chamber, and passes or blocks thermoelectrons;
b) an ion extraction electrode for applying kinetic energy to the ions generated in the ionization chamber by an electric field and emitting them from the ionization chamber;
c) first voltage applying means for applying a voltage for controlling passage / blocking of the thermoelectrons to the gate electrode;
d) a second voltage applying means for applying a voltage for ejecting ions from the ionization chamber to the ion ejection electrode;
e) Ion emission to generate ions in the ionization chamber in a state where thermionic electrons can pass through the gate electrode, and to emit ions from the ionization chamber after transitioning to a state in which the thermoelectrons are blocked by the gate electrode. Control means for controlling the first and second voltage applying means so as to apply a voltage to the working electrode;
A mass spectrometer comprising: The mass spectrometer according to claim 1,
The control means controls the application of voltage to the gate electrode so that thermionic electrons pass through the gate electrode in a pulsed manner, and immediately after the transition to the state where the thermionic electrons are blocked by the gate electrode, from the ionization chamber. A mass spectrometer characterized by controlling first and second voltage applying means so as to apply an ion extraction voltage to the ion extraction electrode to emit ions.

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