Ion detection device and mass spectrometer

Abstract

An ion detector (4) includes a shield electrode (42) between an aperture plate (41) and a conversion dynode (43). The shield electrode (42) has a rectilinearly-moving particle block wall (42a) positioned on an extension line (C′) extending from the central axis (C) of a quadrupole mass filter (3), and an ion attracting electric field adjustment wall (42b) inclined by a predetermined angle θ (acute angle) with respect to the extension line (C′). In the ion attracting electric field adjustment wall (42b) is provided an ion passing aperture (42c). The rectilinearly-moving particles, such as neutral particles, which are ejected from the quadrupole mass filter (3), are blocked by the rectilinearly-moving particle block wall (42a), thereby reducing noises caused by the rectilinearly-moving particles. Meanwhile, the potential of the ion attracting electric field adjustment wall (42b) corresponds to equipotential surfaces in a strong electric field formed by the conversion dynode (43), and thus the condition of the strong electric field is not remarkably changed from the state where no shield electrode (42) is provided. Therefore, the effect of drawing ions is exhibited, thereby maintaining the high ion-detection efficiency.

Claims

1. An ion detection device for detecting: an ion that has passed through an ion separator which separates ions according to masses or mobilities of the ions; or an ion ejected from the ion separator, the ion detection device comprising: a) a conversion dynode disposed at a position out of an extension line extending from a central axis of a flow of injected ions, for converting, to an electron, the ion drawn by an electric field formed by the conversion dynode itself; b) an electron detector disposed opposite to the conversion dynode across the extension line of the central axis of the flow of the injected ions, for detecting the electron ejected from the conversion dynode; c) a shield electrode disposed between an injection position of the flow of the injected ions, and the conversion dynode as well as the electron detector, the shield electrode having: c1) a block wall disposed on the extension line extending from the central axis of the flow of the injected ions, configured to prevent a particle from passing, and c2) an electric field adjustment wall that extends from the block wall, formed in one of: a flat plane inclined at an acute angle with the central axis towards an ion collision face of the conversion dynode; a curved plane containing a curved line approximating the curved plane, and a multi-facet plane approximating the curved plane, and has an aperture or a cut portion configured to allow the ion moving to the conversion dynode to pass through; and d) a voltage applying section configured to apply a predetermined direct-current voltage to the shield electrode.

2. The ion detection device according to claim 1, further comprising an aperture electrode configured to shield an electric field caused by the ion separator while allowing the ion to pass through, at the injection position of the flow of the ions ejected from the ion separator, wherein the shield electrode is disposed between the aperture electrode and the conversion dynode as well as the electron detector.

3. The ion detection device according to claim 2, wherein the electric field adjustment wall has a wall provided with the aperture through which the ion moving toward the conversion dynode passes.

4. The ion detection device according to claim 3, wherein the aperture provided in the electric field adjustment wall is positioned out of a cylindrical space virtually formed by moving an aperture of the aperture electrode, through which the ion pass, in a direction extending from the central axis of the flow of the injected ions.

5. The ion detection device according to claim 3, wherein the block wall is parallel to a plane substantially perpendicular to the central axis of the flow of the injected ions, and the shield electrode has an auxiliary electric field adjustment wall that is parallel to the block wall and extends from the electric field adjustment wall on a side of the electric field adjustment wall opposite to the block wall.

6. The ion detection device according to claim 1, wherein the electric field adjustment wall is a flat plane approximating a curved equipotential plane around a position where the shield electrode is located, in the electrical field formed by the conversion dynode in a state where no shield electrode is provided.

7. A mass spectrometer comprising: the ion detection device according to claim 1, an ion source configured to ionize a compound in a sample; and a quadrupole mass filter configured to selectively allow an ion having a specified mass-to-charge ratio to pass, among ions generated in the ion source, wherein the ion that has passed through the quadrupole mass filter is introduced in the ion detection device so as to be detected.

8. A mass spectrometer comprising: the ion detection device according to claim 1, an ion source configured to ionize a compound in a sample; a previous-stage quadrupole mass filter configured to selectively allow an ion having a specified mass-to-charge ratio to pass, among ions generated in the ion source; an ion dissociation section configured to dissociate the ion that has passed through the previous-stage quadrupole mass filter; and a later-stage quadrupole mass filter configured to selectively allow an ion having a specified mass-to-charge ratio to pass, among product ions generated by dissociation in the ion dissociation section, wherein the ion that has passed through the later-stage quadrupole mass filter is introduced in the ion detection device so as to be detected.

9. A mass spectrometer comprising: the ion detection device according to claim 1, an ion source configured to ionize a compound in a sample; and an ion trap configured to: first trap ions generated in the ion source or other ions derived from the ions generated in the ion source; separate the ions according to mass-to-charge ratios of the ions; and sequentially eject the ions, wherein the ions ejected from the ion trap are introduced in the ion detection device so as to be detected.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic diagram showing the entire configuration of a mass spectrometer including an ion detector according to an embodiment of the present invention.

(2) FIG. 2 illustrates simulation results of the trajectories of ions in the ion detector according to the present embodiment.

(3) FIG. 3 is an explanatory diagram showing how to define the shape of a shield electrode based on the simulation results of equipotential planes in an electric field formed by the conversion dynode in the ion detector according to the present embodiment.

(4) FIG. 4 is a perspective view of the exterior of the shield electrode in the ion detector according to the present embodiment.

(5) FIGS. 5A and 5B are graphs respectively showing the effect of improvement in the SN ratio and the effect of reduction in a noise level in the ion detector according to the present embodiment.

(6) FIG. 6 is a perspective view of the exterior of a modified example of the shield electrode.

(7) FIGS. 7A to 7D are schematic plan views showing another modified example of the shield electrode.

(8) FIG. 8 is a schematic diagram showing the entire configuration of another example of the mass spectrometer including the ion detector according to the embodiment of the present invention.

(9) FIG. 9 is a schematic configuration diagram of an ion detector in a conventional quadrupole mass spectrometer.

DESCRIPTION OF EMBODIMENTS

(10) The mass spectrometer including the ion detector according to an embodiment of the present invention is described, with reference to the drawings.

(11) FIG. 1 is a schematic diagram of the entire configuration of the mass spectrometer. FIG. 2 illustrates simulation results of the trajectories of ions in an ion detector 4 shown in FIG. 1. FIG. 3 is an explanatory diagram showing how to define the shape of a shield electrode based on the simulation results of the equipotential planes in an electric field formed by a conversion dynode in the ion detector 4. FIG. 4 is a perspective view of the exterior of a shield electrode 42 in the ion detector 4. The mass spectrometer performs mass spectrometry by ionizing compounds contained in a liquid sample, and a liquid chromatograph is typically connected in the previous stage of the mass spectrometer.

(12) As shown in FIG. 1, an ionization chamber 11, a first intermediate vacuum chamber 12, a second intermediate vacuum chamber 13, and a high vacuum chamber 14 are provided in a chamber 10. The ionization chamber 11 is kept, in its interior, at approximate atmospheric-pressure. The degree of vacuum increases in a stepwise manner from the ionization chamber 11 to the high vacuum chamber 14, as a multistage differential discharge system. A liquid sample is sprayed from an electrospray ionization nozzle 21 to the interior of the ionization chamber 11. Compounds in electrified droplets generated by the spray are ionized during the process in which the droplets are broken up and a solvent is evaporated. The various ions that are generated are sent to the first intermediate vacuum chamber 12 via a heated capillary 22, converged by an ion guide 23, and then sent to the second intermediate vacuum chamber 13 through a skimmer 24. The ions are converged by an ion guide 25, and sent to the high vacuum chamber 14, so as to be introduced into the quadrupole mass filter 3.

(13) The predetermined voltage (the voltage obtained by totaling a direct-current voltage with a radio-frequency voltage) is applied to four rod electrodes that constitute the quadrupole mass filter 3. Only ions having the mass-to-charge ratio corresponding to the applied voltage pass through the quadrupole mass filter 3, and are introduced into the ion detector 4. The ion detector 4 creates detection signals according to the amount of the introduced ions. Here, the central axis C of the quadrupole mass filter 3 is the optical axis (central axis) of the flow of ions that pass through the quadrupole mass filter 3.

(14) The ion detector 4 includes an aperture electrode 41, a shield electrode 42, a conversion dynode 43, and a secondary electron multiplier tube 44. The aperture electrode 41 is located in the very vicinity of an ejection port of the quadrupole mass filter 3, has substantially a disc shape, and is provided with a circular aperture having its center on the central axis C of the quadrupole mass filter 3. The conversion dynode 43 has a substantially disc-shaped ion collision face 43a, and is located so that the central axis B of the ion collision face 43a is substantially perpendicular to an extension line C′ extending from the central axis C of the quadrupole mass filter 3. The secondary electron multiplier tube 44 is disposed at a position substantially opposite to the ion collision face 43a of the conversion dynode 43 across the extension line C′ extending from the central axis C of quadrupole mass filter 3.

(15) The aperture electrode 41 is grounded, and the predetermined direct-current voltage is applied to each of the shield electrode 42, the conversion dynode 43, and the secondary electron multiplier tube 44, from an SE power source 6, a CD power source 7, and an SEM power source 8. These voltages are controlled by a controller 5. Although it is natural that the predetermined voltage is also applied to each of the quadrupole mass filter 3, and the ion guides 23 and 25, the description of circuit blocks for applying the voltage to the respective structural elements other than the ion detector 4 is omitted.

(16) For the convenience of the description, a direction extending from the central axis C of the quadrupole mass filter 3 (the horizontal direction in FIGS. 1 to 3) is set to the Z direction; a direction orthogonal to the Z direction and extending from the central axis of the ion collision face 43a of the conversion dynode 43 (the vertical direction in FIGS. 1 to 3) is set to the Y direction, and a direction orthogonal to the Z direction as well as the Y direction is set to the X direction (the direction perpendicular to the drawing sheet).

(17) In the ion detector 4, the aperture electrode 41, the conversion dynode 43, and the secondary electron multiplier tube 44 are basically the same as those of conventional ion detectors as shown in FIG. 8. The distinguishing structural element of the ion detector 4 is the shield electrode 42 disposed between the aperture electrode 41 and the conversion dynode 43.

(18) As shown in FIG. 4, the shield electrode 42 is formed, for example, by bending a single metallic plate member (or an electrically conductive plate member other than the metallic plate member) at two lines both extending in the X direction, and includes a rectilinearly-moving particle block wall 42a, an ion attracting electric field adjustment wall 42b, and an auxiliary electric field adjustment wall 42d, all of which extend in this order. The rectilinearly-moving particle block wall 42a and the auxiliary electric field adjustment wall 42d both are parallel to the X-Y plane. The ion attracting electric field adjustment wall 42b is inclined by the predetermined angle θ (here, θ is an acute angle) with respect to the X-Z plane. The plane includes a straight line perpendicular to the rectilinearly-moving particle block wall 42a (in FIG. 4, the straight line corresponds to the central axis C of the ion flow or the extension line C′ extending from the central axis C). A circular-shaped ion passing aperture 42c is drilled at the predetermined position of the ion attracting electric field adjustment wall 42d.

(19) As shown in FIGS. 1 to 3, the shield electrode 42 having such a shape is disposed in such a manner that: the rectilinearly-moving particle block wall 42a is orthogonal to the central axis C of the quadrupole mass filter 3; the auxiliary electric field adjustment wall 42d is closer to the aperture electrode 41 than the rectilinearly-moving particle block wall 42a is; and the auxiliary electric field adjustment wall 42d is located between the aperture electrode 41 and the conversion dynode 43. Hereinafter, the description is given to the way of determining the inclination angle θ of the ion attracting electric field adjustment wall 42b and the voltage to be applied to the shield electrode 42.

(20) FIG. 3 shows that equipotential planes of the electric field (strictly, the equipotential lines at the section including the central axis C) which is formed by a voltage (here, the voltage is −10 kV) applied to the conversion dynode 43, when no shield electrode 42 is provided. The equipotential lines between the conversion dynode 43 and the aperture electrode 41 have the curved shape as shown in FIG. 3. The trajectories of ions which are ejected from the quadrupole mass filter 3 and move in the Z direction, are gradually bent due to the potential gradient in accordance with the equipotential planes, so that the ions reach the ion collision face 43a of the conversion dynode 43.

(21) In order to keep the efficiency in detecting ions when the shield electrode 42 is provided between the aperture electrode 41 and the conversion dynode 43, it is preferable that the trajectories of the ions from the quadrupole mass filter 3 to the conversion dynode 43 are changed as little as possible from the state where no shield electrode 42 is provided. In view of this, it is preferable that the electric field in the ion-passing region, i.e., the condition of the equipotential planes, changes as little as possible. Accordingly, the curved equipotential lines in the electric field near the ion-passing region, as shown in FIG. 3, are approximated to a straight line, so as to determine the inclination angle θ of the ion attracting electric field adjustment wall 42b of the shield electrode 42 based on the angle of the straight line obtained by the approximation with respect to the central axis C.

(22) In the example shown in FIG. 3, the shape of the shield electrode, which is indicated by the reference numeral 420 in FIG. 3, is calculated based on the straight line obtained by the approximation of the equipotential lines in the region indicated by the reference sign A in FIG. 3. A voltage to be applied to the shield electrode 42 is determined from the potential of the equipotential lines around the intersection of the center of the ion trajectories and the ion attracting electric field adjustment wall 42b of the shield electrode 42. Here, even if the equipotential planes are calculated by the simulation as shown in FIG. 3, discrepancy is unavoidable in equipotential planes in an actual apparatus. Furthermore, there are ions and rectilinearly-moving particles which do not show the ideal action. In addition, the actions of ions are slightly different from one another depending on the mass-to-charge ratio of ions to be observed. Accordingly, it is preferable to find the optimal state by adjusting the shape of the shield electrode and the voltage to be applied, for obtaining the highest efficiency in detecting ions.

(23) FIG. 2 shows the simulation result of the trajectories of ions and electrons. As seen from FIG. 2, ions that have passed through the aperture electrode 41 pass through the ion passing aperture 42c almost without colliding with the ion attracting electric field adjustment wall 42b of the shield electrode 42. In contrast, most of the rectilinearly-moving particles, such as neutral particles, collide with the rectilinearly-moving particle block wall 42a and rebound, so as to be discharged to the outside by the evacuation. Accordingly, few rectilinearly-moving particles enter a space between the conversion dynode 43 and the secondary electron multiplier tube 44, thereby significantly inhibiting noises caused by the rectilinearly-moving particles. Meanwhile, ions receive little influence caused by the shield electrode 42, thereby achieving high efficiency in the detection of ions.

(24) FIG. 5 shows the results of experimental search on the SN ratio and the level of noises caused by the rectilinearly-moving particles, under the conditions with and without the shield electrode. As seen from the results, if the aforementioned shield electrode is provided, the rectilinearly-moving particles are blocked, thereby lowering the level of noise caused by the rectilinearly-moving particles, as well as improving the SN ratio. Thus, the effectiveness of the shield electrode is confirmed.

(25) The shape of the shield electrode is not limited to the one shown in FIG. 4. What is important is to block the rectilinearly-moving particles, and to prevent the significant change in the state of the electric field between the aperture electrode 41 and the conversion dynode 43, from the state where no shield electrode is provided. For the former one, the rectilinearly-moving particle block wall 42a is necessary. For the latter one, the ion attracting electric field adjustment wall 42b extending from the rectilinearly-moving particle block wall 42a is necessary. Here, the ion attracting electric field adjustment wall 42b may have a short length. For example, as shown in FIG. 6, the ion attracting electric field adjustment wall 42b may have a short length up to a position where the ion passing aperture 42c is provided in the shield electrode 42, as shown in FIG. 4.

(26) FIGS. 7A to 7D show examples of the shield electrode in different shapes. FIGS. 7A to 7D are side views of shield electrodes. FIG. 7A is the shield electrode 42 shown in FIG. 4, and FIG. 7B is the shield electrode 42B shown in FIG. 6. In these shield electrodes 42 and 42B, the ion attracting electric field adjustment wall 42b has the planar shape. Meanwhile, in a shield electrode 42C shown in FIG. 7C, the ion attracting electric field adjustment wall 42b is bent at its midway portion. In a shield electrode 42D shown in FIG. 7D, the ion attracting electric field adjustment wall 42b is shaped in a curved face. It is apparent that these configurations can also provide the same effects as those provided by the ion detector 4 in the previous embodiment.

(27) Furthermore, the rectilinearly-moving particle block wall 42a may not be completely orthogonal to the extension line C′ extending from the central axis C of the quadrupole mass filter 3. The same is applied to the auxiliary electric field adjustment wall 42d.

(28) Next, the description is given to the case where the ion detector 4 in the aforementioned embodiment is used in a mass spectrometer in which compounds in a sample gas are ionized to be subjected to mass spectrometry. FIG. 8 is a schematic diagram of the entire configuration of such a mass spectrometer, and structural elements which are the same as or correspond to those in the mass spectrometer in FIG. 1 are allocated with the same reference signs, and the detailed description of these elements is omitted. A gas chromatograph is often connected to the mass spectrometer in the previous stage.

(29) In the mass spectrometer, an ion source 110, a lens electrode 120, the quadrupole mass spectrometer 3, and the ion detector 4 are provided inside a chamber 100 that is evacuated by a vacuum pump (not shown). Here, the ion source 110 is prepared by the EI method, and includes an ionization chamber 111, a filament 112 for generating thermal electrons, a trap electrode 113 for trapping the thermal electrons, and a sample-gas introduction tube 114 for introducing sample gas into the ionization chamber 111. In addition, a repeller electrode is provided inside the ionization chamber 111 (not shown).

(30) The sample gas is introduced into the ionization chamber 111 through the sample gas introduction tube 114, and compounds in the sample gas are ionized by being in contact with the thermal electrons that are generated by the filament 112 and move toward the trap electrode 113. The generated ions are pushed out of the ionization chamber 111 by the electric field formed by the repeller electrode, or drawn out of the ionization chamber 111 by the electric field formed by the lens electrode 120, so as to be introduced into the quadrupole mass filter 3, while being converged by the lens electrode 120. The actions of the ions after being introduced into the quadrupole mass filter 3 are the same as those described with reference FIGS. 1 to 4. In this mass spectrometer, the majority of the sample gas is a carrier gas to be used in the gas chromatograph in the previous stage. The molecules of the carrier gas or metastable molecules that are meta-stabled carrier-gas molecules are easily introduced, as neutral particles, into the quadrupole mass spectrometer 3. The rectilinearly-moving particles, which are such neutral particles, are blocked by the rectilinearly-moving particle block wall 42a of the shield electrode 42, as mentioned earlier, so as to be prevented from being the noise source.

(31) When the ion source prepared by the CI method, as opposed to the EI method, is used as the ion source 110, a reagent gas for the ionization is introduced into the ionization chamber, and this reagent gas also becomes the rectilinearly-moving particles. Such rectilinearly-moving particles that are neutral particles are also blocked by the rectilinearly-moving particle block wall 42a of the shield electrode 42, as mentioned earlier, so as to be prevented from being the noise source.

(32) Although the mass spectrometers shown in FIGS. 1 and 8 each are a single quadrupole mass spectrometer, the ion detector 4 in the embodiment may be used as an ion detector of a triple quadrupole mass spectrometer. In addition, the ion detector 4 can also be used in an ion trap mass spectrometer. In such a case, the ion trap mass spectrometer is either a linear mass spectrometer or a three-dimensional quadrupole mass spectrometer. It is only required that the ion detector 4 be disposed so that the aperture electrode 41 is located at the outside of an ion ejection port from which ions are ejected from the ion trap.

(33) In the embodiment described earlier, the aperture electrode 41 is not necessarily provided in the ion detector 4. However, if the aperture electrode 41 is not provided, it is necessary for the ion detector 4 to be disposed away from the quadrupole mass filter 3 (or the ion trap). In such a configuration, however, the loss of the ions sent from the quadrupole mass filter 3 increases, causing the disadvantage of the efficiency in the ion detection. Accordingly, it is preferable that the aperture electrode 41 be practically provided, though it is not indispensable.

(34) The aforementioned embodiment and various modified embodiments of the embodiment are an example of the present invention. It is apparent that any modification, correction, or addition within the concept of the present invention is included in the scope of claims of the present application.

REFERENCE SIGNS LIST

(35) 10 . . . Chamber 11 . . . Ionization Chamber 12 . . . First Intermediate Vacuum Chamber 13 . . . Second Intermediate Vacuum Chamber 14 . . . High Vacuum Chamber 21 . . . Electrospray Ionization Nozzle 22 . . . Heated Capillary 23, 25 . . . Ion Guide 24 . . . Skimmer 3 . . . Quadrupole Mass Filter 4 . . . Ion Detector 41 . . . Aperture Electrode 42, 42b, 42c, 42d . . . Shield Electrode 42a . . . Rectilinearly-Moving Particle Block Wall 42b . . . Ion Attracting Electric Field Adjustment Wall 42c . . . Ion Passing Aperture 42d . . . Auxiliary Electric Field Adjustment Wall 43 . . . Conversion Dynode 43a . . . Ion Collision Face 44 . . . Secondary Electron Multiplier Tube 5 . . . Controller 6 . . . SE Power Source 7 . . . CD Power Source 8 . . . SEM Power Source 110 . . . Ion Source 111 . . . Ionization Chamber 112 . . . Filament 113 . . . Trap Electrode 114 . . . Sample Gas Introduction Tube 120 . . . Lens Electrode