Ionization apparatus

Abstract

In an ion source 3 in which a repeller electrode 32 for forming a repelling electric field that repels ions toward an ion emission port 311 is provided inside of an ionization chamber 31, ion focusing electrodes 36 and 37 are respectively arranged between an electron introduction port 312 and a filament 34 and between an electron discharge port 313 and a counter filament 35. An electric field formed by applying a predetermined voltage to each of the ion focusing electrodes 36 and 37 intrudes into the ionization chamber 31 through the electron introduction port 312 and the electron discharge port 313, and becomes a focusing electric field that pushes the ions in an ion optical axis C direction. Ions at positions off a central part of the ionization chamber 31 receive the combined force of the force of the repelling electric field and the force of the focusing electric field, and move toward the ion emission port 311 while approaching the ion optical axis C. Accordingly, the amount of ions sent out from the ion emission port increases. Further, even if a charge-up phenomenon occurs, the ion trajectories less easily change, and the stability of the sensitivity can be enhanced.

Claims

1. An ionization apparatus for ionizing predetermined sample molecules or atoms, the ionization apparatus comprising: a) an ionization chamber having: an electron introduction port for introducing thermal electrons to an internal space of the ionization chamber; an electron discharge port for discharging thermal electrons that have passed through the internal space; and an ion emission port for emitting sample-derived ions produced in the internal space to an outside; b) a thermal electron source for producing the thermal electrons, the thermal electron source being arranged on an outer side of the electron introduction port; c) an electron trapping unit for trapping the thermal electrons discharged through the electron discharge port, the electron trapping unit being arranged on an outer side of the electron discharge port; d) a repeller electrode for forming, in the ionization chamber, a repelling electric field that repels the sample-derived ions produced in the ionization chamber toward the ion emission port, the repeller electrode being arranged inside of the ionization chamber so as to be opposed to the ion emission port; and e) an ion focusing electrode for forming, in the ionization chamber, a focusing electric field that focuses the sample-derived ions produced in the ionization chamber around a central axis of an ion flow formed by repelling the sample-derived ions by the repelling electric field, the ion focusing electrode being arranged any one or both of between the thermal electron source and the electron introduction port and between the electron discharge port and the electron trapping unit.

2. The ionization apparatus according to claim 1, wherein the ion emission port is provided to the ionization chamber such that the ions are emitted in a direction substantially orthogonal to a direction in which the thermal electrons are introduced into the ionization chamber through the electron introduction port.

3. The ionization apparatus according to claim 2, further comprising a voltage applying unit for applying a DC voltage Vr having a same polarity as that of the sample-derived ions, to the repeller electrode and for applying a DC voltage Vs having a same polarity as that of the sample-derived ions, to the ion focusing electrode, wherein the DC voltage Vr is 1 and 20 [V], and the DC voltage Vs is 5 and 50 [V].

4. The ionization apparatus according to claim 1, further comprising a voltage applying unit for applying a DC voltage Vr having a same polarity as that of the sample-derived ions, to the repeller electrode and for applying a DC voltage Vs having a same polarity as that of the sample-derived ions, to the ion focusing electrode, wherein the DC voltage Vr is 1 and 20 [V], and the DC voltage Vs is 5 and 50 [V].

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic configuration diagram of a mass spectrometer using an ion source according to an embodiment of the present invention.

(2) FIG. 2 is a configuration diagram of the ion source according to the present embodiment.

(3) FIG. 3A and FIG. 3B are diagrams each illustrating a simulation result of ion trajectories at the time of using a repeller electrode (repelling mode) in a conventional ion source, in which FIG. 3A illustrates the case without charge-up and FIG. 3B illustrates the case with charge-up.

(4) FIG. 4A and FIG. 4B are diagrams each illustrating a simulation result of ion trajectories at the time of using an extraction electrode (extracting mode) in a conventional ion source, in which FIG. 4A illustrates the case without charge-up and FIG. 4B illustrates the case with charge-up.

(5) FIG. 5A and FIG. 5B are diagrams each illustrating a simulation result of ion trajectories in an ion source according to the present invention, in which FIG. 5A illustrates the case without charge-up and FIG. 5B illustrates the case with charge-up.

(6) FIG. 6 is a configuration diagram of the conventional ion source (repelling mode).

(7) FIG. 7 is a configuration diagram of the conventional ion source (extracting mode).

DESCRIPTION OF EMBODIMENTS

(8) An ion source according to an embodiment of the present invention is described with reference to the attached drawings. FIG. 1 is a schematic configuration diagram of a mass spectrometer using the ion source according to the present embodiment, and FIG. 2 is a configuration diagram of the ion source according to the present embodiment. The same constituent elements as those in the conventional ion sources that have already been described with reference to FIG. 6 and FIG. 7 are denoted by the same reference signs.

(9) First, the mass spectrometer using the ion source of the present embodiment is described with reference to FIG. 1. An ion source 3, an ion transport optical system 4, a quadrupole mass filter 5 as a mass analyzer, and an ion detector 6 are arranged inside of a chamber 1 that is evacuated by a vacuum pump 2. For example, a sample gas that flows out from a column of a gas chromatograph (not illustrated) is communicated with a sample introduction port 314 of an ionization chamber 31, and sample molecules or atoms contained in the sample gas continuously supplied to the ionization chamber 31 are ionized upon contacting thermal electrons produced from a filament 34. As described later, the sample-derived ions thus produced are sent out from the ionization chamber 31 through an ion emission port 311, are focused by the ion transport optical system 4, and are introduced to a space in the long axis direction of the quadrupole mass filter 5. A voltage in which a DC voltage and an RF voltage are superposed is applied to the quadrupole mass filter 5 from a power source (not illustrated), and only ions having a mass-to-charge ratio m/z corresponding to the applied voltage pass through the space in the long axis direction and reach the ion detector 6 to be detected by the ion detector 6. The other unnecessary ionic species cannot pass through the space in the long axis direction of the quadrupole mass filter 5, and thus diverge and disappear on the way.

(10) As illustrated in FIG. 1 and FIG. 2, in the ion source 3 of the present embodiment, similarly to the conventional ion source illustrated in FIG. 7, a repeller electrode 32 is arranged at a position opposed to the ion emission port 311 in the ionization chamber 31, and a predetermined DC voltage is applied to the repeller electrode 32 from a repeller voltage source 73. Ion focusing electrodes 36 and 37 are respectively arranged between an electron introduction port 312 and a filament chamber 341 and between an electron discharge port 313 and a filament chamber 351. Each of the ion focusing electrodes 36 and 37 is, for example, a ring-like conductor including an electron pass opening having an inner diameter that is substantially the same as or slightly smaller than the inner diameter of each of the electron introduction port 312 and the electron discharge port 313. A predetermined DC voltage is applied to the ion focusing electrode 36 from a first ion focusing voltage source 71, and a predetermined DC voltage is applied to the ion focusing electrode 37 from another second ion focusing voltage source 72. In other words, independent voltages can be respectively applied to the two ion focusing electrodes 36 and 37.

(11) In the case where positive ions are to be analyzed, the repeller voltage source 73 applies a DC voltage of Vr=1 and 20 [V] to the repeller electrode 32. The first and second ion focusing voltage sources 71 and 72 respectively apply a DC voltage of Vs=5 and 50 [V] to the ion focusing electrodes 36 and 37. The applied voltages Vr and Vs are different depending on the size of the ionization chamber 31, the sizes of the electron introduction port 312 and the electron discharge port 313, the shapes of the ion focusing electrodes 36 and 37, distances from the electron introduction port 312 and the electron discharge port 313, and other factors. For example, appropriate values for the applied voltages Vr and Vs may be determined in advance based on simulations and experiments.

(12) The thermal electrons produced from the filament 34 enter the inside of the ionization chamber 31 through the electron introduction port 312, and move toward the electron discharge port 313 while each flying on a spiral trajectory due to an action of a magnetic field formed by a pair of magnets 38. When the thermal electrons contact sample molecules or atoms on the way, the sample molecules or atoms are ionized. The ionization chamber 31 is grounded, and the positive DC voltage Vr of approximately 1 and 20 [V] is applied to the repeller electrode 32 as described above. Hence, a repelling electric field having a force of repelling the ions in the z-axis positive direction (rightward in FIG. 2) from the repeller electrode 32 toward the ion emission port 311 is formed in the ionization chamber 31. This is similar to the conventional ion source illustrated in FIG. 7.

(13) The thermal electrons each having a negative charge exist in an elongated region in the y-axis direction in the ionization chamber 31. Due to a space-charge effect produced by the electrons, the sample-derived ions each having the polarity opposite to that of the electrons tend to spread in the y-axis direction. To deal with this, in the ion source 3 of the present embodiment, the positive DC voltage Vs of approximately 5 and 50 [V] is applied to the ion focusing electrodes 36 and 37 respectively closely arranged on the outer sides of the electron introduction port 312 and the electron discharge port 313 as described above. Hence, an electric field is formed by a potential difference between the ion focusing electrodes 36 and 37 and the ionization chamber 31, and the electric field intrudes into the ionization chamber 31 through the electron introduction port 312 and the electron discharge port 313. This focusing electric field acts to push the ions in the y-axis negative direction (downward in FIG. 2) around the electron introduction port 312, and acts to push the ions in the y-axis positive direction (upward in FIG. 2) around the electron discharge port 313. Namely, this focusing electric field acts to confine the ions spreading in the y-axis positive-negative direction to a central part of the ionization chamber 31.

(14) In actuality, because the combined force of the repelling force of the repelling electric field and the confining (focusing) force of the focusing electric field acts on the ions, the ions existing around the center of the ionization chamber 31 are pushed in the z-axis positive direction, and the ions existing at positions closer to the electron introduction port 312 and the electron discharge port 313 than the ions existing around the center are pushed toward the ion emission port 311 while approaching an ion optical axis C as the central axis of the ion flow. Hence, collisions of ions against a wall surface of the ionization chamber 31 around the ion emission port 311 as in the simple repelling mode can be avoided, and these ions can be sent out from the ion emission port 311. In other words, the amount of ions that can be sent out from the ion emission port 311, in other words, can be used for mass spectrometry is larger than that in the conventional repelling mode, and this leads to enhancement of the analysis sensitivity.

(15) FIG. 3A to FIG. 5B each illustrate a calculation result of ion trajectories according to a computer simulation, in order to verify a difference in ion trajectories between the ion source 3 of the present embodiment and the conventional ion sources (the repelling mode and the extracting mode). FIG. 3A and FIG. 3B are diagrams each illustrating a simulation result of ion trajectories at the time of using the repeller electrode (repelling mode) in the conventional ion source, FIG. 4A and FIG. 4B are diagrams each illustrating a simulation result of ion trajectories at the time of using the extraction electrode (extracting mode) in the conventional ion source, and FIG. 5A and FIG. 5B are diagrams each illustrating a simulation result of ion trajectories in the ion source 3 of the present embodiment. FIG. 3A, FIG. 4A, and FIG. 5A each illustrate the case without a charge-up phenomenon, and FIG. 3B, FIG. 4B, and FIG. 5B each illustrate the case where a charge-up phenomenon occurs on the surface of the ionization chamber 31. Moreover, in FIG. 3A to FIG. 5B, constituent elements colored in solid black are constituent elements to which a positive DC voltage having the same polarity as that of the ions is applied. Further, in FIG. 3A to FIG. 5B, an upper half range illustrates the trajectories of ions that can reach the mass filter (not illustrated), and a lower half range illustrates the trajectories of ions that disappear by colliding against the electrode or the like on the way.

(16) As illustrated in FIG. 3A and FIG. 3B, in the conventional repelling mode, only the ions produced around the center of the ionization chamber 31 are transported to the subsequent stage, and most of the ions produced at positions off the center disappear in the ionization chamber 31 or upon contacting the electrode. This is almost the same even in the state where the charge-up phenomenon occurs.

(17) In comparison, as illustrated in FIG. 4A, in the conventional extracting mode, not only the ions produced around the center of the ionization chamber 31 but also the ions produced at positions significantly off the center are extracted from the inside of the ionization chamber 31 and are transported to the subsequent stage. However, even the ions existing around the center of the ionization chamber 31 partially disappear. This means that the electric field around the center of the ionization chamber 31 is relatively lower (the potential gradient is gentler) than that in the repelling mode. As a result, as illustrated in FIG. 4B, if the charge-up phenomenon occurs, the ion trajectories extremely change, and ions that are not transported to the subsequent stage increase. In other words, from the simulation results, it is easily presumed that the sensitivity of the extracting mode is higher when the charge-up phenomenon does not occur, but the sensitivity of the extracting mode more extremely decreases when the charge-up phenomenon occurs, compared with the repelling mode.

(18) As is apparent by comparing FIG. 5A and FIG. 3A, in the ion source 3 of the present embodiment, even the ions at positions off the central part of the ionization chamber 31, most of which are not transported to the subsequent stage in the simple repelling mode, are led to the ion emission port 311 and transported to the subsequent stage, and few ions disappear upon contacting the inner wall surface of the ionization chamber 31. This is achieved by an effect that the electric field formed by the ion focusing electrodes 36 and 37 intrudes into the ionization chamber 31 and pushes the ions toward the central axis of the ion flow. Consequently, a large amount of ions can be transported to the subsequent stage, and high analysis sensitivity can be achieved.

(19) As illustrated in FIG. 5B, the ion trajectories in the ion source 3 of the present embodiment are almost the same even in the state where the charge-up phenomenon occurs. This means that both the ion repelling action of the repelling electric field and the ion focusing action of the focusing electric field are hardly influenced by the charge-up phenomenon. From this fact, it can be understood that high analysis sensitivity can be maintained even if the charge-up phenomenon occurs along with long-term use of the apparatus.

(20) As described above, from the simulation results of the ion trajectories, it can be verified that both high sensitivity and high stability of the sensitivity can be achieved in the ion source 3 of the present embodiment.

(21) The trajectories of the thermal electrons produced from the filament 34 are also obtained together with the simulations of the ion trajectories. As a result, because the acceleration of the thermal electrons is high, it is verified that the trajectories of the thermal electrons are hardly influenced if the voltage Vs applied to each of the ion focusing electrodes 36 and 37 is within the above-mentioned range.

(22) In the above-mentioned embodiment, the ion focusing electrodes 36 and 37 are respectively provided on the outer sides of the electron introduction port 312 and the electron discharge port 313, and this is desirable in terms of an ion focusing effect. Alternatively, the ion focusing electrode may be provided on the outer side of only any one of the two ports. Moreover, although different voltages can be applied to the two ion focusing electrodes 36 and 37, it is sufficient to apply the same voltage to the two electrodes in a normal operation.

(23) Although the ion source of the above-mentioned embodiment is an EI ion source, the present invention can also be applied to a CI ion source. Moreover, not limited to the ion source of the mass spectrometer, the present invention can also be used as ion sources of other apparatuses using ions such as an ion implantation apparatus.

(24) The above-mentioned embodiment is merely an example of the present invention, and it is obvious that any adjustments, changes, and additions to be made appropriately within the gist of the present invention, as well as the above-mentioned modifications, are embraced in the scope of the claims of the present application.

REFERENCE SIGNS LIST

(25) 1 . . . Chamber 2 . . . Vacuum Pump 3 . . . Ion Source 31 . . . Ionization Chamber 311 . . . Ion Emission Port 312 . . . Electron Introduction Port 313 . . . Electron Discharge Port 314 . . . Sample Introduction Port 32 . . . Repeller Electrode 34 . . . Filament 35 . . . Counter Filament (Trap Electrode) 341, 351 . . . Filament Chamber 36, 37 . . . Ion Focusing Electrode 38 . . . Magnet 4 . . . Ion Transport Optical System 41 . . . Extraction Electrode 5 . . . Quadrupole Mass Filter 6 . . . Ion Detector 71 . . . First Ion Focusing Voltage Source 72 . . . Second Ion Focusing Voltage Source 73 . . . Repeller Voltage Source