MASS SPECTROMETER

Abstract

Conversion dynodes (CDs) 31 and 32 are respectively provided for ion-ejection ports 21a and 22a facing each other across the central axis C of a linear ion trap (LIT) 2. A shield plate 34 having ion-passage openings 34a is provided between LIT and CDs. A voltage slightly lower than the voltage applied to CDs is applied to the shield plate. Ions ejected from LIT by resonant excitation are accelerated by an electric field between LIT and the shield plate, having their trajectories gradually curved, to eventually reach CDs through the ion-passage openings. Upon receiving the ions, CDs emit electrons. Some electrons may initially move toward the shielding plate, but will be repelled to and detected by an electron multiplier tube 33. CDs can be made of aluminum or similar inexpensive materials, which reduces the cost as well as eliminates the loss of the ions and improves detection sensitivity.

Claims

1. A mass spectrometer comprising: a linear ion trap including a plurality of rod electrodes positioned substantially parallel to each other around a central axis, the linear ion trap having a plurality of ion-ejection ports for ejecting ions from an internal space surrounded by the rod electrodes to an outside; a plurality of conversion dynodes positioned for the plurality of ion-ejection ports in the linear ion trap, respectively, such that each of the conversion dynodes is configured to emit electrons upon receiving ions ejected through a respective one of the ion-ejection ports; a shield plate positioned between the linear ion trap and the plurality of conversion dynodes and having a plurality of ion-passage openings for allowing ions ejected through the plurality of ion-ejection ports of the linear ion trap to pass through; a common detector section positioned on a same side as the plurality of conversion dynodes with respect to the shield plate and configured to receive electrons emitted from the plurality of conversion dynodes and produce a detection signal corresponding to an amount of the electrons received from the plurality of conversion dynodes; and a voltage-applying section comprising circuitry configured to apply, to the shield plate, a voltage which attracts ions ejected through the plurality of ion-ejection ports of the linear ion trap, to apply, to the plurality of conversion dynodes, a voltage equal to or higher than the voltage applied to the shield plate, and to apply, to the common detector section, a voltage higher than the voltage applied to the plurality of conversion dynodes.

2. The mass spectrometer according to claim 1, wherein the voltage-applying section is configured to apply, to each of the plurality of conversion dynodes, a voltage which is higher than the voltage applied to the shield plate.

3. The mass spectrometer according to claim 1, wherein the plurality of ion-ejection ports are positioned to face each other across the central axis of the linear ion trap, and the plurality of ion-passage openings in the shield plate, the plurality of conversion dynodes, and the common detector section are positioned plane-symmetrical with respect to a symmetry plane of the plurality of ion-ejection ports, with the symmetry plane containing the central axis of the linear ion trap.

4. The mass spectrometer according to claim 2, wherein the plurality of ion-ejection ports are positioned to face each other across the central axis of the linear ion trap, and the plurality of ion-passage openings in the shield plate, the plurality of conversion dynodes, and the common detector section are positioned plane-symmetrical with respect to a symmetry plane of the plurality of ion-ejection ports, with the symmetry plane containing the central axis of the linear ion trap.

5. The mass spectrometer according to claim 1, wherein the circuitry of the voltage applying section includes a shield plate power supply configured to apply, to the shield plate, the voltage which attracts the ions ejected through the plurality of ion-ejection ports of the linear ion trap, a conversion dynode power supply configured to apply, to the plurality of conversion dynodes, the voltage equal to or higher than the voltage applied to the shield plate, a detector power supply configured to apply, to the common detector section, the voltage higher than the voltage applied to the plurality of conversion dynodes, and a controller comprising control circuitry configured to control the shield plate power supply, the conversion dynode power supply, and the detector power supply.

6. The mass spectrometer according to claim 1, wherein the circuitry of the voltage applying section includes an ion trap power supply configured to apply, to the plurality of rod electrodes, a voltage, the linear ion trap a shield plate power supply configured to apply, to the shield plate, the voltage which attracts the ions ejected through the plurality of ion-ejection ports of the linear ion trap, a conversion dynode power supply configured to apply, to the plurality of conversion dynodes, the voltage equal to or higher than the voltage applied to the shield plate, a detector power supply configured to apply, to the common detector section, the voltage higher than the voltage applied to the plurality of conversion dynodes, and a controller comprising control circuitry configured to control the ion trap power supply, the shield plate power supply, the conversion dynode power supply, and the detector power supply.

7. The mass spectrometer according to claim 1, wherein the common detector section comprises an electron multiplier tube configured to receive the electrons emitted from the plurality of conversion dynodes and produce the detection signal corresponding to the amount of the electrons received from the plurality of conversion dynodes.

8. The mass spectrometer according to claim 2, wherein the circuitry of the voltage applying section includes a shield plate power supply configured to apply, to the shield plate, the voltage which attracts the ions ejected through the plurality of ion-ejection ports of the linear ion trap, a conversion dynode power supply configured to apply, to the plurality of conversion dynodes, the voltage equal to or higher than the voltage applied to the shield plate, a detector power supply configured to apply, to the common detector section, the voltage higher than the voltage applied to the plurality of conversion dynodes, and a controller comprising control circuitry configured to control the shield plate power supply, the conversion dynode power supply, and the detector power supply.

9. The mass spectrometer according to claim 2, wherein the circuitry of the voltage applying section includes an ion trap power supply configured to apply, to the plurality of rod electrodes, a voltage, the linear ion trap a shield plate power supply configured to apply, to the shield plate, the voltage which attracts the ions ejected through the plurality of ion-ejection ports of the linear ion trap, a conversion dynode power supply configured to apply, to the plurality of conversion dynodes, the voltage equal to or higher than the voltage applied to the shield plate, a detector power supply configured to apply, to the common detector section, the voltage higher than the voltage applied to the plurality of conversion dynodes, and a controller comprising control circuitry configured to control the ion trap power supply, the shield plate power supply, the conversion dynode power supply, and the detector power supply.

10. The mass spectrometer according to claim 2, wherein the common detector section comprises an electron multiplier tube configured to receive the electrons emitted from the plurality of conversion dynodes and produce the detection signal corresponding to the amount of the electrons received from the plurality of conversion dynodes.

11. The mass spectrometer according to claim 3, wherein the circuitry of the voltage applying section includes a shield plate power supply configured to apply, to the shield plate, the voltage which attracts the ions ejected through the plurality of ion-ejection ports of the linear ion trap, a conversion dynode power supply configured to apply, to the plurality of conversion dynodes, the voltage equal to or higher than the voltage applied to the shield plate, a detector power supply configured to apply, to the common detector section, the voltage higher than the voltage applied to the plurality of conversion dynodes, and a controller comprising control circuitry configured to control the shield plate power supply, the conversion dynode power supply, and the detector power supply.

12. The mass spectrometer according to claim 3, wherein the circuitry of the voltage applying section includes an ion trap power supply configured to apply, to the plurality of rod electrodes, a voltage, the linear ion trap a shield plate power supply configured to apply, to the shield plate, the voltage which attracts the ions ejected through the plurality of ion-ejection ports of the linear ion trap, a conversion dynode power supply configured to apply, to the plurality of conversion dynodes, the voltage equal to or higher than the voltage applied to the shield plate, a detector power supply configured to apply, to the common detector section, the voltage higher than the voltage applied to the plurality of conversion dynodes, and a controller comprising control circuitry configured to control the ion trap power supply, the shield plate power supply, the conversion dynode power supply, and the detector power supply.

13. The mass spectrometer according to claim 3, wherein the common detector section comprises an electron multiplier tube configured to receive the electrons emitted from the plurality of conversion dynodes and produce the detection signal corresponding to the amount of the electrons received from the plurality of conversion dynodes.

14. The mass spectrometer according to claim 4, wherein the circuitry of the voltage applying section includes a shield plate power supply configured to apply, to the shield plate, the voltage which attracts the ions ejected through the plurality of ion-ejection ports of the linear ion trap, a conversion dynode power supply configured to apply, to the plurality of conversion dynodes, the voltage equal to or higher than the voltage applied to the shield plate, a detector power supply configured to apply, to the common detector section, the voltage higher than the voltage applied to the plurality of conversion dynodes, and a controller comprising control circuitry configured to control the shield plate power supply, the conversion dynode power supply, and the detector power supply.

15. The mass spectrometer according to claim 4, wherein the circuitry of the voltage applying section includes an ion trap power supply configured to apply, to the plurality of rod electrodes, a voltage, the linear ion trap a shield plate power supply configured to apply, to the shield plate, the voltage which attracts the ions ejected through the plurality of ion-ejection ports of the linear ion trap, a conversion dynode power supply configured to apply, to the plurality of conversion dynodes, the voltage equal to or higher than the voltage applied to the shield plate, a detector power supply configured to apply, to the common detector section, the voltage higher than the voltage applied to the plurality of conversion dynodes, and a controller comprising control circuitry configured to control the ion trap power supply, the shield plate power supply, the conversion dynode power supply, and the detector power supply.

16. The mass spectrometer according to claim 4, wherein the common detector section comprises an electron multiplier tube configured to receive the electrons emitted from the plurality of conversion dynodes and produce the detection signal corresponding to the amount of the electrons received from the plurality of conversion dynodes.

17. The mass spectrometer according to claim 5, wherein the common detector section comprises an electron multiplier tube configured to receive the electrons emitted from the plurality of conversion dynodes and produce the detection signal corresponding to the amount of the electrons received from the plurality of conversion dynodes.

18. The mass spectrometer according to claim 6, wherein the common detector section comprises an electron multiplier tube configured to receive the electrons emitted from the plurality of conversion dynodes and produce the detection signal corresponding to the amount of the electrons received from the plurality of conversion dynodes.

19. The mass spectrometer according to claim 1, wherein the circuitry of the voltage applying section includes a shield plate power supply configured to apply, to the shield plate, the voltage which attracts the ions ejected through the plurality of ion-ejection ports of the linear ion trap, a conversion dynode power supply configured to apply, to the plurality of conversion dynodes, the voltage higher than the voltage applied to the shield plate, a detector power supply configured to apply, to the common detector section, the voltage higher than the voltage applied to the plurality of conversion dynodes, and a controller comprising control circuitry configured to control the shield plate power supply, the conversion dynode power supply, and the detector power supply.

20. The mass spectrometer according to claim 2, wherein the circuitry of the voltage applying section includes a shield plate power supply configured to apply, to the shield plate, the voltage which attracts the ions ejected through the plurality of ion-ejection ports of the linear ion trap, a conversion dynode power supply configured to apply, to the plurality of conversion dynodes, the voltage higher than the voltage applied to the shield plate, a detector power supply configured to apply, to the common detector section, the voltage higher than the voltage applied to the plurality of conversion dynodes, and a controller comprising control circuitry configured to control the shield plate power supply, the conversion dynode power supply, and the detector power supply.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0024] FIG. 1 is a schematic configuration diagram of a linear ion trap mass spectrometer as one embodiment of the present invention.

[0025] FIG. 2 is a schematic plan view of the linear ion trap in FIG. 1.

[0026] FIG. 3 is a conceptual diagram showing the state of the potential at relevant sections between the linear ion trap and an electron multiplier tube.

[0027] FIG. 4 is a graphic image showing the result of a simulation of the trajectories of ions and electrons.

DESCRIPTION OF EMBODIMENTS

[0028] A linear ion trap mass spectrometer as one embodiment of the present invention is hereinafter described with reference to the attached drawings. FIG. 1 is a schematic configuration diagram of the linear ion trap mass spectrometer according to the present embodiment. FIG. 2 is a schematic plan view of the linear ion trap in FIG. 1 (with the X axis orthogonally directed to the plane of the drawing).

[0029] The ion source for generating ions to be analyzed is not shown in FIG. 1. The linear ion trap mass spectrometer according to the present embodiment includes the ion source (not shown), a linear ion trap 2, an ion detector section 3, a controller 4, an ion trap power supply 5, a shield plate power supply 6, a conversion dynode power supply 7 and a detector power supply 8.

[0030] The linear ion trap 2 includes four rod electrodes 21, 22, 23 and 24 arranged parallel to each other around a central axis C extending in the Z-axis direction in FIG. 1 (i.e. the direction which is orthogonal to the plane of the drawing), with their inner surfaces being hyperbolic in cross-section. In FIG. 1, the linear ion trap 2 is shown by an end view of the rod electrodes 21, 22, 23 and 24 sectioned at a plane perpendicular to the central axis C (X-Y plane). As shown in FIG. 2, the two rod electrodes 21 and 22 facing one another across the central axis C have slit-like ion-ejection ports 21a and 22a extending in the Z-axis direction, respectively. Two end electrodes 25 and 26 having a substantially circular shape are located on the outside of the two ends of the rod electrodes 21, 22, 23 and 24 in such a manner that the rod electrodes 21, 22, 23 and 24 are sandwiched between the end electrodes. Ion-introduction ports 25a and 26a in the form of a circle centered on the central axis C are formed in the end electrodes 25 and 26, respectively.

[0031] The ion detector section 3 includes: two conversion dynodes 31 and 32 provided for the two ion-ejection ports 21a and 22a of the linear ion trap 2, respectively; a common electron multiplier tube 33 for receiving electrons emitted from the conversion dynodes 31 and 32; and a shield plate 34 located between the linear ion trap 2 and the conversion dynodes 31 and 32. The shield plate 34 is a plate-shaped electrically conductive member, in which ion-passage openings 34a are formed at appropriate positions to allow for the passage of ions traveling from the ion-ejection ports 21a and 22a toward the corresponding conversion dynodes 31 and 32. As shown in FIG. 1, the two ion-passage openings 34a formed in the shield plate 34, the two conversion dynodes 31 and 32 as well as the single common electrode multiplier tube 33 are plane-symmetrically arranged with respect to the symmetry plane of the two ion-ejection ports 21a and 22a containing the central axis C of the linear ion trap 2. The conversion dynodes 31 and 32 are made of an electrically conductive material, such as aluminum. Aluminum is commonly used because the aluminum oxide formed on the aluminum surface yields a high level of ion-to-electron conversion efficiency which is second only to the alkaline metal, and also because aluminum is inexpensive as well as easy to handle.

[0032] Under the control of the controller 4, the ion trap power supply 5 applies predetermined voltages to the rod electrodes 21, 22, 23 and 24 as well as the end electrodes 25 and 26, respectively. The shield plate power supply 6 applies a predetermined direct voltage V.sub.2 to the shield plate 34. The conversion dynode power supply 7 applies a predetermined direct voltage V.sub.3 to each of the conversion dynodes 31 and 32. The detector power supply 8 applies a predetermined direct voltage V.sub.4 to the electron multiplier tube 33.

[0033] A mass spectrometric operation in the mass spectrometer according to the present embodiment is hereinafter described with reference to an example in which the ions to be analyzed are positive ions

[0034] Ions generated by the ion source (not shown) are introduced through one or both of the ion-introduction ports 25a and 26a into the inner space surrounded by the rod electrodes 21, 22, 23 and 24. The introduced ions are captured by a quadrupole electric field formed by the radio-frequency voltages applied from the ion trap power supply 5 to the rod electrodes 21, 22, 23 and 24. When the ions introduced into the inner space are to be captured, a direct voltage for repelling the ions is applied to the end electrodes 25 and 26, whereby the ions are confined within the inner space (an elongated space extending in the Z-axis direction) surrounded by the rod electrodes 21, 22, 23 and 24.

[0035] When an ion having a specific mass-to-charge ratio M is to be separated from the other ions and detected, the controller 4 operates the ion trap power supply 5 to apply a specific radio-frequency voltage corresponding to the mass-to-charge ratio M to the rod electrodes 21, 22, 23 and 24. Then, only the ions having the mass-to-charge ratio M among the various ions captured within the inner space are made to significantly oscillate in a direction along the X axis due to the resonant excitation, to be eventually ejected through the ion-ejection ports 21a and 22a. During this operation, the direct voltage V.sub.1 applied to the rod electrodes 21, 22, 23 and 24 of the linear ion trap 2 is set at 0 V, for example. The direct voltage V.sub.2 applied to the shield plate 34, the direct voltage V.sub.3 applied to the conversion dynodes 31 and 32, as well as the direct voltage V.sub.4 applied to the electron multiplier tube 33 are related to each other as shown in FIG. 3.

[0036] That is to say, as in the conventional case, a voltage V.sub.3 for attracting ions is applied to the conversion dynodes 31 and 32. Additionally, a slightly lower voltage V.sub.2 is applied to the shield plate 34. A voltage V.sub.4 which is higher than the voltage V.sub.3 applied to the conversion dynodes 31 and 32 is applied to the electron multiplier tube 33 in order to attract electrons emitted from the conversion dynodes 31 and 32. When the voltages are applied in this manner, an electric field which attracts ions ejected from the ion-ejection ports 21a and 22a toward the shield plate 34 is created within the space between the shield plate 34 and an area near the exit end of the ion-ejection ports 21a and 22a of the linear ion trap 2. Due to the effect of this electric field, the ions ejected substantially parallel to the X axis are accelerated having their trajectories gradually curved as shown by the thick broken lines in FIG. 1. The accelerated ions pass through the ion-passage openings 34a.

[0037] As just described, the voltage V.sub.2 applied to the shield plate 34 is lower than the voltage V.sub.3 applied to the conversion dynodes 31 and 32. Therefore, as shown in FIG. 3, a force which repels ions from the conversion dynodes 34 toward the shield plate 34 acts on the ions which have passed through the ion-passage openings 34a. However, since the potential difference between V.sub.2 and V.sub.3 is small, the ion-repelling force is also small, so that the ions which have been sufficiently accelerated by the point in time of the exit from the ion-passage openings 34a have almost no difficulty in reaching the conversion dynodes 31 and 32. Upon being hit by the ions, the conversion dynodes 31 and 32 emit electrons in exchange for the ions. Those electrons are emitted in various directions. Meanwhile, an electric field for accelerating electrons from the conversion dynodes 31 and 32 toward the electron multiplier tube 33 is formed by the voltage V.sub.3 applied to the conversion dynodes 31 and 32 as well as the voltage V.sub.4 applied to the electron multiplier tube 33. Additionally, a weak electric field (with a low potential gradient) producing a force which pushes the electrons emitted from the conversion dynodes 31 and 32 back to the same conversion dynodes 31 and 32 is created within the space between the shield plate 34 and the conversion dynodes 31 and 32. Therefore, most of the electrons emitted from the conversion dynodes 31 and 32 will travel toward the electron multiplier tube 33, as indicated by the thick arrows in FIG. 3. Thus, a high percentage of the electrons will reach and be detected by the electron multiplier tube 33.

[0038] The presence of the shield plate 34 also means that the electric field on the side where the conversion dynodes 31 and 32 are located insignificantly affects the electric field on the side where the linear ion trap 2 is located, and vice versa. In other words, the shield plate 34 has the function of blocking the electric fields.

[0039] FIG. 4 shows the result of a simulation of the trajectories of ions and electrons. For simplicity of the configuration, the conversion dynodes 31 and 32 in the simulation model are integrated with the shield plate 34 to form a single part, which is maintained at the same potential (8 kV). Another shield plate 36 having a predetermined potential of 0 V is located between the shield plate 34 and the linear ion trap 2. This shield plate 36 is intended to shield the linear ion trap 2 from the influence of the electric field created by the potential of the shield plate 34. Still another shield plate 35 having an L-shaped cross section surrounding the ion-ejection port 21a or 22a is located on the outside of each of the ion-ejection ports 21a and 22a within the space on the opposite side from the shield plate 34 across the central axis of the ion-ejection ports 21a and 22a. This shield plate 35 is intended to prevent ions ejected from the ion-ejection port 21a or 22a from diffusing toward the side which is opposite from the shield plate 34. There is yet another shield plate 37 surrounding the electron multiplier tube 33. This shield plate 37 limits the area through which electrons can hit the entrance surface of the electron multiplier tube 33. This decreases the amount of electrons (and other kinds of charged particles) different from those emitted from the conversion dynodes 31 and 32 from reaching the entrance surface of the electron multiplier tube 33. Consequently, the amount of noise will be reduced. These shield plates 35, 36 and 37 are dispensable elements, although they are considerably effective in practical situations.

[0040] As shown in FIG. 4, the ions ejected from the ion-ejection ports 21a and 22a of the linear ion trap 2 have their trajectories gradually curved immediately after their ejection, and eventually reached the conversion dynodes 31 and 32 after passing through the ion-passage openings 34a. As such, the target ions ejected from the linear ion trap 2 by resonant excitation have successfully reach the conversion dynodes 31 and 32 with no significant loss. The electrons emitted from the conversion dynodes 31 and 32 have also successfully reached the entrance surface of the electron multiplier tube 33, which is almost equidistant from both conversion dynodes 31 and 32, with no significant loss. These results confirm that a detection signal corresponding to a target ion ejected from the linear ion trap 2 can be obtained with a high level of sensitivity by appropriately determining the arrangement and shape of each relevant element, the level of the applied voltages as well as other factors.

[0041] It should be noted that the previous embodiment is a mere example of the present invention, and any change, modification or addition appropriately made within the spirit of the present invention will naturally fall within the scope of claims of the present application.

[0042] For example, the arrangement of the shield plate 34, conversion dynodes 31 and 32, electron multiplier tube 33 and other elements as well as their respective shapes can be appropriately changed. Furthermore, as noted earlier, any component which should be set at their respective predetermined potentials may be added, as with the shield plates 35, 36 and 37.

REFERENCE SIGNS LIST

[0043] 2 . . . Linear Ion Trap [0044] 21, 22, 23, 24 . . . Rod Electrode [0045] 21a, 22a . . . Ion-Ejection Port [0046] 25, 26 . . . End Electrode [0047] 25a, 26a . . . Ion-Introduction Port [0048] 3 . . . Ion Detector [0049] 31, 32 . . . Conversion Dynode [0050] 33 . . . Electron Multiplier Tube [0051] 34, 35, 36, 37 . . . Shield Plate [0052] 34a . . . Ion-Passage Opening [0053] 4 . . . Controller [0054] 5 . . . Ion Trap Power Supply [0055] 6 . . . Shield Plate Power Supply [0056] 7 . . . Conversion Dynode Power Supply [0057] 8 . . . Detector Power Supply [0058] C . . . Central Axis