Time-of-flight mass spectrometer

Abstract

A shielding plate 6 having a forward-side slit opening 61 and return-side slit opening 62 is placed in a free flight space 3 with no electric field. Ions which significantly deviate from a reference path are removed by the shielding plate 6 on both the forward-side and return-side paths. The opening width of the forward-side slit opening 61 is smaller than that of the return-side slit opening 62. Those opening widths and the placement position of the shielding plate 6 are determined based on the result of an ion trajectory calculation by accurate simulation. As compared to a conventional device with a shielding plate placed only on the return side, the present configuration allows for an increase in the opening width of the return-side slit opening while achieving the same level of resolving power. The ion transmission ratio is thereby improved, and the analytical sensitivity is enhanced.

Claims

1. A reflectron time-of-flight mass spectrometer including: a time-of-flight mass separator including a free flight space with no electric field and an ion reflector for reflecting an ion; an ion accelerator for accelerating an ion into the free flight space; and a detector for detecting an ion returning from a round trip in which the ion initially flies through the free flight space and once more flies through the free flight space after being reflected by the ion reflector, wherein: a shielding plate is placed in the free flight space, the shielding plate having a forward-side opening located on a forward-side ion path directed from the ion accelerator to the ion reflector and a return-side opening located on a return-side ion path directed from the ion reflector and the detector; and a width of the forward-side opening of the shielding plate in an extending direction of a plane on which both a central axis of the forward-side ion path and a central axis of the return-side ion path lie is smaller than a width of the return-side opening in the same direction.

2. The reflectron time-of-flight mass spectrometer according to claim 1, further comprising an ion-converging lens for converging a packet of ions.

3. The reflectron time-of-flight mass spectrometer according to claim 1, wherein the ion reflector comprises a plurality of reflecting electrodes arrayed along a central line at predetermined intervals.

4. The reflectron time-of-flight mass spectrometer according to claim 3, wherein the shielding plate is placed so that a center axis of the shielding plate aligns with the central line.

5. A reflectron time-of-flight mass spectrometer including: a time-of-flight mass separator including a free flight space with no electric field and an ion reflector for reflecting an ion; an ion accelerator for accelerating an ion into the free flight space; and a detector for detecting an ion returning from a round trip in which the ion initially flies through the free flight space and once more flies through the free flight space after being reflected by the ion reflector, wherein: a shielding plate is placed in the free flight space, the shielding plate having a single asymmetrical opening through which ions received from a forward-side ion path directed from the ion accelerator to the ion reflector pass and through which ions returning from the reflector to a return-side ion path directed from the ion reflector and the detector pass, the single asymmetrical opening being asymmetrical with respect to a central line axis of reflecting electrodes of the ion reflector.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic configuration diagram of a reflectron TOFMS as one embodiment of the present invention.

(2) FIGS. 2A and 2B are plan views of shielding plates in the reflectron TOFMS in the present embodiment.

(3) FIGS. 3A and 3B are time-of-flight spectra obtained by a simulation calculation in the reflectron TOFMS in the present embodiment and a conventional reflectron TOFMS.

DESCRIPTION OF EMBODIMENTS

(4) A reflectron TOFMS as one embodiment of the present invention is hereinafter described with reference to the attached drawings.

(5) FIG. 1 is a schematic configuration diagram of the reflectron TOFMS in the present embodiment.

(6) The reflectron TOFMS in the present embodiment is an orthogonal acceleration TOFMS. It includes: an orthogonal accelerator 1 including a plate-shaped push-out electrode 11 and a grid electrode 12; an ion-converging lens 2, which is an Einzel lens; a free flight space 3 with no electric field; an ion reflector 4 including a plurality of reflecting electrodes 41 and a back plate 42; and a detector 5 for detecting ions. Though not shown, the free flight space 3 and the ion reflector 4 are placed within a drift tube maintained in a high vacuum state.

(7) An example of the reflecting electrode 41 is a plate-shaped electrode with a rectangular opening, as in the device described in Patent Literature 1. A plurality of reflecting electrodes 41 are arrayed along the central line C at predetermined intervals of space (there are a total of 14 electrodes in FIG. 1, but there is no restriction on this number). The back plate 42 with no opening is located at the farthest position as viewed from the side from which ions come. Different DC voltages are respectively applied from a voltage source (not shown) to the reflecting electrodes 41 and the back plate 42. By these voltages, a reflecting electric field for initially decelerating incident ions and subsequently reflecting and accelerating those ions is created within the rectangular columnar space formed by virtually connecting the rectangular openings of the reflecting electrodes 41.

(8) An example of the detector 5 is an ion detector using a microchannel plate or secondary electron multiplier. It has a wide detection surface to efficiently detect a cluster of ions which are spatially spread to a certain extent when arriving at the detector.

(9) In the reflectron TOFMS in the present embodiment, a shielding plate 6 having a characteristic structure is placed in the free flight space 3 in order that ions which may cause a decrease in the resolving power due to a variation in the initial energy or initial position at the time of their acceleration will be removed in the middle of their flight. FIG. 2A is a plan view of the shielding plate 6 used in the TOFMS in the present embodiment. This shielding plate 6 is a plate-shaped member 63 in which two rectangular slit openings 61 and 62 having different opening widths L1 and L2 are asymmetrically formed with respect to the axis P. The slit opening 61 with opening width L1 corresponds to the forward-side opening in the present invention, while the slit opening 62 with opening width L2 corresponds to the return-side opening in the present invention. That is to say, the forward-side slit opening 61 is smaller in opening width than the return-side slit opening 62. This shielding plate 6 is placed so that the axis P coincides with the central line C of the reflecting electrodes 41.

(10) The opening widths L1 and L2 of the slit openings 61 and 62 in the shielding plate 6 are determined based on an ion trajectory calculation by a simulation using a computer. In a calculation of ion trajectories by simulation, a considerably accurate calculation can be made for an ion flying along a specific path from a specific initial position with a specific amount of initial energy, to determine what amount of temporal difference from an ion traveling along the reference path the ion concerned will have in arriving at the detector. Accordingly, the initial position, initial energy, mass-to-charge-ratio range and other necessary conditions related to the ion packet are defined based on rough estimates determined from the results of a preliminary experiment or preliminary simulation calculation, and accurate ion trajectories are calculated under those conditions. Then, based on the calculated results, the placement position of the shielding plate 6 as well as the opening widths L1 and L2 of the slit openings 61 and 62 are determined so that the path of an ion whose temporal difference from the time of flight of an ion which follows the reference path is equal to or greater than a predetermined value will be blocked while an ion whose temporal difference is smaller than the predetermine value will pass through the slit openings and eventually reach the detector 5. The permissible value of the temporal difference of the time of flight can be set according to the desired level of the resolving power.

(11) In the reflectron TOFMS in the present embodiment, ions which have entered the orthogonal accelerator 1 as indicated by the thick arrow in FIG. 1 are almost simultaneously accelerated in a direction substantially orthogonal to the axis of incidence of the ions by the DC electric field created by the DC voltages respectively applied to the push-out electrode 11 and the grid electrode 12. Thus, an ion packet including ions which fall within a predetermined mass-to-charge-ratio range is ejected from the orthogonal accelerator 1. After being converged by the ion-converging lens 2, the ion packet is sent into the free flight space 3 and arrives at the shielding plate 6. If there is an ion whose initial energy at the time of acceleration is significantly different from the reference value of the initial energy, or an ion whose initial position is significantly displaced from the reference point of the initial position, the path of the ion deviates from the reference path in the early phase of the flight. Accordingly, such an ion cannot pass through the forward-side slit opening 61 with the relatively small opening width L1 and is thereby removed.

(12) The ion packet whose spatial spread has been limited through the forward-side slit opening 61 enters the ion reflector 4 and is returned by the reflecting electric field created within the reflector. In this process, an ion having the same mass-to-charge ratio as other ions yet having a greater amount of energy reaches a deeper point before being reflected. The reflected ions once more fly in the free flight space 3 and arrive at the shielding plate 6. Any ion which may possibly cause a decrease in the resolving power due to its path being deviated from the reference path cannot pass through the return-side slit opening 62 and is thereby removed.

(13) If no removal of ions is performed on the forward-side path, there may be an ion which follows a path significantly deviated from the reference ion path on the forward side before entering the ion reflector 4 and then travels toward the detector 5 along a path which is corrected as a result of the reflection. In order to remove such an ion by a shielding plate placed on the return side, the slit opening on the return side must be small in width. By comparison, in the reflectron TOFMS in the present embodiment, ions which take eccentric paths which may possibly cause a decrease in the resolving power can be removed to a certain extent by the forward-side slit opening 61. This allows the return-side slit opening 62 to have a relatively large opening width L2. The transmission efficiency of the ions which will practically cause no decrease in the resolving power is thereby enhanced, so that a greater number of ions will be introduced into the detector 5, and a high level of analytical sensitivity will be achieved.

(14) The present inventor has conducted a simulation calculation to confirm the difference in analytical sensitivity between the reflectron TOFMS in the present embodiment and a conventional reflectron TOFMS. FIGS. 3A and 3B are time-of-flight spectra obtained by the simulation calculation. The horizontal axis in the spectra indicates the time of flight, and the vertical axis indicates the number of ions which arrived at the detector 5.

(15) FIG. 3B is the time-of-flight spectrum obtained in the case where a shielding plate having slit openings which were symmetrical with respect to the axis P (i.e. L1=L2) was used in place of a shielding plate having asymmetrical slit openings as shown in FIG. 2A. The shape of this shielding plate having the symmetrical slit openings was determined based on the result of an ion trajectory calculation by simulation so that a resolving power of 21000 would be achieved. According to the calculation, the ion transmission ratio in the present case was 35.8%. The resolving power was R=21700.

(16) FIG. 3A is the time-of-flight spectrum in the reflectron TOFMS in the present embodiment. According to the calculation, the ion transmission ratio in the present case was 46.7%. The resolving power was R=21400. Thus, as compared to the case where the shielding plate having the symmetrical slit openings was used, the reflectron TOFMS in the present embodiment achieved an approximately 30% increase in the ion transmission ratio while maintaining almost the same level of resolving power. A difference in the ion transmission ratio almost directly turns into a difference in analytical sensitivity. Accordingly, it is possible to consider that the reflectron TOFMS in the present embodiment has the effect of enhancing the analytical sensitivity by approximately 30% as compared to the conventional device.

(17) In the reflectron TOFMS in the previous embodiment, the forward-side slit opening 61 and the return-side slit opening 62 in the shielding plate 6 are completely separated from each other. It is possible to use a shielding plate in which the slit openings on both sides are connected to be a single asymmetrical slit opening 65 as shown in FIG. 2B.

(18) It should be noted that the previous embodiment is one 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.

(19) For example, although the previous embodiment is an orthogonal acceleration TOFMS, it is evident that the present invention is also applicable in a TOFMS in which ions temporarily stored in an ion trap are ejected from the ion trap and made to fly, or a TOFMS in which ions generated from a sample by an ion source employing the MALDI or similar technique are extracted from an area near the sample and accelerated into flight motion.

REFERENCE SIGNS LIST

(20) 1 . . . Orthogonal Accelerator 11 . . . Push-Out Electrode 12 . . . Grid Electrode 2 . . . Ion-Converging Electrode 3 . . . Free Flight Space 4 . . . Ion Reflector 41 . . . Reflecting Electrode 42 . . . Back Plate 5 . . . Detector 6 . . . Shielding Plate 61 . . . Forward-Side Slit Opening 62 . . . Return-Side Slit Opening 63 . . . Plate-Shaped Member 65 . . . Slit Opening C . . . Central Line