Mass spectrometer

A mass spectrometer that allows easy replacement of an MCP (microchannel plate) and is enabled to secure orthogonality between an incident surface of the MCP and an ion track at high accuracy is provided. A flight tube 2 where ions fly is arranged in a vacuum vessel composed of a vacuum flange 6 and a body 1, and an MCP group 4 is attached to a tail end of the flight tube 2 via an MCP-IN electrode 3. A vacuum flange 6 is attachably and detachably attached to the body 1, and the MCP group 4, by a spring 710 provided on a circuit board 7 for detection attached to the vacuum flange 6, is urged toward an end portion of the flight tube 2 so that its orthogonality with respect to an ion flight track is secured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a time-of-flight mass spectrometer (TOF-MS) used for detection of the molecular weight of a polymer and the like.

2. Related Background Art

In a TOF-MS, the mass of detecting ions is detected based on time required for the detecting ions to fly within a vacuum flight tube. An apparatus of a type disclosed in JP2007-87885A has been known as a charged-particle detecting apparatus to be used as a detector in such a TOF-MS.

This charged-particle detecting apparatus has a detecting section including a microchannel plate (MCP) arranged on a vacuum flange, and thus has a configuration that makes it easy to replace the MCP when the detector reaches its life end.

SUMMARY OF THE INVENTION

Meanwhile, in the TOF-MS, a mass detection accuracy of detecting ions depends on a detection accuracy of a time of flight, that is, a half-value width of an output signal to be output when the ions have reached an ion incident surface of the detector. Recently, a particularly high detection accuracy has been demanded, and a demanded half-value width of an output signal of ions is 1 ns or less. A flight track of ions in the flight tube is in a direction almost along the direction in which the flight tube extends, and orthogonality with respect to this direction of the ion incident surface of the detector is demanded. This is because, if the ion incident surface has an inclination, the length of a flight track differs depending on the position of the ion incident surface, which affects the detection accuracy of a time of flight. It is necessary in order to satisfy the half-value width condition of an output signal of ions described above to arrange the ion incident surface so that a difference in flight distance is within ±20 μm.

Because an incident surface of the MCP to serve as an ion incident surface is fixed to the flight tube via the vacuum flange in the technique described in the above-mentioned document, it is difficult to secure orthogonality between an ion track and the MCP incident surface.

It is therefore an object of the present invention to provide a mass spectrometer that allows easy replacement of an MCP and is enabled to secure orthogonality between an incident surface of the MCP and an ion track at high accuracy.

In order to achieve the above-mentioned object, a mass spectrometer according to the present invention is a mass spectrometer that, based on time required for ions emitted from a sample to fly within a flight tube being a vacuum vessel in an apparatus body, analyzes a mass of the ions, including: an MCP arranged in the vacuum vessel at an ion reaching side of the flight tube, for outputting electrons in response to reached ions, the MCP being directly fixed with the apparatus body by an input-side electrode electrically and physically connected to its ion reaching surface side; a flange portion attachably and detachably connected and fixed to an ion reaching-side end portion of the flight tube to form the vacuum vessel, and having a signal output terminal and a potential supply terminal exposed on an outer surface of the vacuum vessel; an anode portion fixed onto the flange portion to face the MCP, being input with electrons output from the MCP, and electrically connected to the signal output terminal; and output-side electrode urging means fixed to the flange portion to urge an output side surface of the microchannel plate for electrically connecting the output side electrode of the microchannel plate and the potential supply terminal to each other.

The MCP is preferably fixed to the flight tube via the input-side electrode. This MCP is preferably stacked up in a plurality of stages. It is preferable that the mass spectrometer further includes input-side electrode urging means fixed to the flange portion to urge the input-side electrode for electrically connecting the input-side electrode and an electric power input terminal provided on the flange portion to each other.

It may be preferable that the mass spectrometer further includes an electron multiplier section arranged on the MCP side further than the anode, and fixed to the flange portion. For the output-side electrode urging means, a spring, conductive rubber, a metal projection, and the like are preferably used.

In the mass spectrometer according to the present invention, because the MCP having an ion incident surface is directly fixed to a vacuum vessel body by the input-side electrode, it is easy to secure orthogonality between the ion incident surface and an ion track, and replacement of the MCP is also easy.

Further, if in a mode of fixation to a flight tube end portion, the accuracy of orthogonality between the ion incident surface and an ion track depends on the accuracy of orthogonality of the end portion in the flight tube, so that it becomes easy to secure the accuracy of the MCP.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. To facilitate the comprehension of the explanation, the same reference numerals denote the same parts, where possible, throughout the drawings, and a repeated explanation will be omitted.

FIG. 1is a view showing a structure of a flight tube end portion in a first embodiment of a mass spectrometer according to the present invention,FIG. 2is an enlarged view of a II part thereof,FIG. 3is a view showing a structure of a vacuum flange,FIG. 4is an enlarged view of a IV part thereof,FIG. 5are views showing a structure of a circuit board,FIG. 6shows an equivalent circuit thereof, andFIG. 7shows an assembled state.

The flight tube2is a cylindrical structure to be arranged in a body1of the mass spectrometer. At its end portion of a side that has not been illustrated, an ion source is arranged. On the other hand, at the illustrated end portion, two disk-like MCPs41and42(hereinafter, collectively referred to as an MCP group4) are arranged. The MCPs41and42are bonded to each other by a conductive thermoplastic adhesive, and further, an MCP-IN electrode3formed of an annular metal is bonded to an MCP41-side surface by the same conductive thermoplastic adhesive. Then, by arranging the MCP-IN electrode3on an end face20of the flight tube2, and inserting and fixing by screwing screws5passed through a plurality of holes (preferably, three or more holes, and four holes are arranged in the present embodiment) provided in the electrode3into screw holes22provided in the flight tube2, the electrode3is fixed to the flight tube2. Thus, the flight tube2and the MCP-IN electrode3are electrically and physically connected to each other.

The vacuum flange6, which is a disk-like metal member, is attached to an end portion of the body1surrounding a circular cylindrical portion of the flight tube2across a gasket65(seeFIG. 7) so as to be attachable and detachable with respect to the flight tube2. The body1and the vacuum flange6, which compose a vacuum vessel, keep the inside of a space to be thereby sealed in a vacuum so as to keep a portion including an ion flight track in the flight tube2in a vacuum. On a surface to be arranged inside of the vacuum vessel of the vacuum flange6, a substrate7retaining an anode75is arranged.

The substrate7, which is, for example, a rectangular plate made of polyimide, is provided with a screw hole700at an outer edge portion close to an intermediate portion of each side, and fixed to the vacuum flange6, across an insulating and circular cylindrical insulator701, by a screw702passing through the screw hole700. Thus, a space is secured between the substrate7and the vacuum flange6and both are electrically connected to each other so as to ground the substrate7.

The substrate7has a circular cutout72in its center, and is attached, at its rear surface (surface to be arranged on the vacuum flange6side), with the anode75formed of a plate-like metal. The anode75is electrically and physically connected to an anode terminal86to be described later by bonding using a conductive adhesive, resistance welding, or soldering, and fixedly fitted to the substrate7. Moreover, the substrate7is mounted thereon with a bleeder circuit formed of resistors83,84and capacitors82,85, and has an output terminal80, a power supply terminal81, and an anode terminal86as connection terminals of this circuit.

The power supply terminal81is connected to a high voltage terminal811passing through the vacuum flange6, and supplied with power from an external power supply815connected to the terminal811. On the other hand, the output terminal80is connected with an an SMA (Sub Miniature Type A) terminal801passing likewise through the vacuum flange6, and readout from a connected external device is enabled. On the substrate7surrounding the cutout72, an annular MCP-OUT electrode73formed by a copper foil pattern is provided, and on the MCP-OUT electrode73, four springs710are attached by resistance welding. When the vacuum flange6is attached, these springs710urge the MCP group4to apply a stress to these, and are electrically connected to the MCP group4to supply potential.

As a result of providing such a configuration, the MCP group4is pressed against an ion output-side end face of the flight tube2by the springs710, and therefore, it becomes easy to secure parallelism between an input surface of the MCP group4(more concretely, an incident surface of the input-side MCP41) and the output-side end face of the flight tube2at high accuracy. Accordingly, securing in advance orthogonality of the output-side end face of the flight tube2to an ion flight track in manufacturing makes it easy to secure orthogonality between the ion flight track and the incident surface of the MCP41at high accuracy. Concretely, it suffices to secure the accuracy of orthogonality of the end face with respect to a central axis of the flight tube2and make a contrivance to have a difference in flight distance within ±10 μm.

At the time of operation, a predetermined potential is applied, through the terminal811from the external power supply815, to both ends of the MCP group4and the anode75, and the vacuum flange6is provided at ground potential. At detection of cations, it suffices to apply a voltage of −5 kV from a power supply25of the flight tube2side and −2.9 kV from the power supply815of the vacuum flange6side. On the other hand, at detection of anions or electrons, it suffices to apply a voltage of 5 kV from the power supply25of the flight tube2side and 7.1 kV from the power supply815of the vacuum flange6side, respectively.

According to the present embodiment, because orthogonality of the incident surface of the MCP group4with respect to the ion flight track in the flight tube2can be secured at high accuracy, a narrow half-value width of an output signal of ions of 2 ns or less can be obtained. On the other hand, with regard to parallelism between an output surface of the MCP group4and the anode75, because the flying speed of electrons is sufficiently fast, as high an accuracy as the accuracy for orthogonality of the MCP group4is not required, and almost no effect occurs on the half-value width of an output signal of ions even at an accuracy of about ±100 μm. Accordingly, replacement of the MCP group4and detector can also be easily performed by attachment and detachment of the vacuum flange6.

The method for attaching the MCP group4is not limited to that of the above-mentioned embodiment. In the following, description will be given of other embodiments where the attaching method is different.

In the second embodiment shown inFIG. 8andFIG. 9, an insulator52being a circular cylindrical insulator is arranged on a through-hole for a screw provided in the MCP-IN electrode3, a hooked clamp51is thereon arranged, and by fixing the clamp51, the insulator52, and the MCP-IN electrode3with a screw50screwed in a screw hole of the flight tube2, the MCP group4is fixed. The screw50is an insulating screw formed of a PEEK (polyetheretherketone) resin or a Teflon resin, and the clamp51and the MCP group4are separated in potential from each other.

In the third embodiment shown inFIG. 10andFIG. 11, the MCP-IN electrode3is fixedly fitted at an end portion of the flight tube2by bonding, welding, or the like, and thereon attached via an arc-shaped insulator54is a fixing plate53formed of a metal plate which is likewise in an arc shape, by an adhesive or the like. The MCP group4is arranged inserted in a groove part formed between the fixing plate53and the MCP-IN electrode3. In this case as well, the fixing plate53and the MCP group4are separated in potential from each other.

It becomes possible also in these second and third embodiments as in the first embodiment to secure orthogonality of the incident surface of the MCP group4with respect to the ion flight track in the flight tube2at high accuracy. Moreover, in these embodiments, there is also an advantage that only the MCP group4can be easily replaced.

The configuration of the detector side is also not limited to that shown in the first embodiment. For example, as shown inFIG. 13toFIG. 15, a configuration arranging a metal channel dynode (MCD)90on a substrate7aon a vacuum flange6aand urging the MCP group4by a spring91may be adopted. In this case, a connection with an external device is performed by using terminals93aand92aconnected to an input terminal93and an output terminal92, respectively, that are connected to the MCD90.

The above embodiments can be appropriately modified. For example, in the first embodiment, the MCP group4has been urged by the springs710provided on the substrate7, however, springs may be provided between the substrate7and the vacuum flange6so as to urge the MCP group4indirectly by the substrate7urged by the springs or by another member.

Although, in the above, a description has been given of an example of supplying the MCP-IN electrode3with potential from the flight tube2side, it may be possible, as in the case of an MCP-OUT electrode, to secure a path to electrically perform a connection using conductive urging means or the like from the vacuum flange6side. In this case, there is an advantage that an electrical connection can be performed entirely on an exposed surface of the vacuum flange6.

Moreover, the fixing position of the input side of the MCP group4is not limited to the end face of the flight tube2, and for example, a form of fixation to an end face part of the body1surrounding the flight tube2may be adopted. Moreover, for the urging means, conductive rubber, a metal projection, and the like can be used besides the metal spring.