JP2018073703A - Mass spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect ions discharged from a linear ion trap (LIT) while being mass separated by resonant excitation action, without waste.SOLUTION: Conversion dynodes (CD) 31, 32 are arranged, respectively, on the outside of ion discharge ports 21a, 22a arranged oppositely across the medial axis C of a LIT 2, and a shield plate 34 having an ion passage port 34a is provided between the CDs 31, 32 and the LIT 2. A voltage slightly lower than the voltage applied to the CDs 31, 32 is applied to the shield plate 34. When the ions held in the LIT 2 are discharged by resonant excitation, the ions are accelerated by an electric field formed between the LIT 2 and the shield plate 34 while bending the orbit, before reaching the CDs 31, 32 by passing through the ion passage port 34a. Electrons ejected from the CDs 31, 32 by receiving ion injection are pushed back by the shield plate 34 and reach an electron multiplier tube 33 and detected. Since the CDs 31, 32 may be made of inexpensive Al, or the like, ions are detected without waste while reducing the cost, and sensitivity can be improved.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a mass spectrometer, and more particularly to a linear ion trap type mass spectrometer that retains ions inside a linear ion trap and detects ions ejected from the linear ion trap with a detector.

  As one of mass spectrometers, a mass spectrometer using an ion trap that holds ions by the action of an electric field is known. Ion traps are roughly classified into three-dimensional quadrupole ion traps and linear ion traps. A general linear ion trap has four rod-like electrodes arranged substantially parallel to each other so as to surround a central axis, and a pair of end electrodes (ends) respectively arranged on both outer sides of the rod-like electrodes. Cap electrode) (see Patent Documents 1 and 2). When ions are held in the internal space of the linear ion trap, a predetermined high-frequency voltage is applied to each of the four rod-shaped electrodes, and a DC voltage having the same polarity as the ions is applied to the pair of end electrodes. Thereby, in the linear ion trap, ions are trapped in a relatively large space extending in the direction along the central axis.

  In such a linear ion trap, in many cases, ions are introduced into a space surrounded by rod-shaped electrodes through openings formed in one or both end electrodes and held in the internal space. When detecting ions held in the internal space while separating them according to the mass-to-charge ratio, the specific mass-to-charge ratio that is the detection target is controlled by controlling the frequency or amplitude of the high-frequency voltage applied to the rod-shaped electrode. Ions are resonantly excited. The excited ions are discharged to the outside through an ion discharge port formed in the rod-shaped electrode. An ion detector is disposed outside the ion discharge port, and the ion detector generates a detection signal corresponding to the number of ions that have reached.

  When general dipole resonance excitation is performed at the time of ion ejection, ions are greatly excited in both directions substantially perpendicular to the central axis of the linear ion trap. For this reason, in order to detect resonance-excited ions without waste, ion discharge ports are provided at opposite positions across the central axis of the rod-shaped electrode, and ions are converted to electrons outside the two ion discharge ports. It is necessary to dispose an ion detector in which a conversion dynode to be combined with an electron multiplier tube for multiplying and detecting the electron is detected. The mass spectrometer described in Patent Document 1 employs such a configuration. In this configuration, high detection sensitivity can be obtained by detecting ions without waste. However, since ion detectors, particularly electron multipliers, are expensive, providing two sets of ion detectors increases the cost of the apparatus.

  On the other hand, in the mass spectrometer described in Patent Document 2, an ion discharge port is provided only at one position in the rod-shaped electrode, and an ion detector is disposed outside the one ion discharge port. In this configuration, since only one ion detector is required, the cost can be reduced. However, since about half of the resonance-excited ions are not used for detection and are wasted, it is considerably disadvantageous in terms of detection sensitivity compared to a configuration using two sets of ion detectors.

US Pat. No. 6,797,950 JP 2012-184975 A

  That is, in the conventional linear ion trap mass spectrometer, the detection sensitivity must be sacrificed in order to reduce the cost of the apparatus. The present invention has been made to solve these problems, and an object of the present invention is to provide a linear ion trap mass spectrometer capable of achieving high detection sensitivity while reducing the cost of the apparatus. It is to be.

A mass spectrometer according to the present invention made to solve the above problems is as follows.
a) Linear ions including a plurality of rod-shaped electrodes arranged substantially parallel to each other around the central axis, and having a plurality of ion discharge ports for discharging ions held in an internal space surrounded by the rod-shaped electrodes to the outside Trap and
b) a plurality of conversion dynodes provided corresponding to a plurality of ion discharge ports in the linear ion trap, respectively, for receiving ions discharged through each ion discharge port and emitting electrons;
c) a shielding plate provided between the linear ion trap and the plurality of conversion dynodes and having a plurality of ion passage openings through which ions discharged through the plurality of ion discharge ports in the linear ion trap respectively pass;
d) a common detection unit that is located on the same side as the plurality of conversion dynodes as viewed from the shielding plate, receives electrons emitted from the plurality of conversion dynodes, and outputs a detection signal according to the amount of the electrons;
e) A voltage for attracting ions ejected through a plurality of ion outlets in the linear ion trap is applied to the shielding plate, and a voltage equal to or higher than an applied voltage to the shielding plate is applied to the plurality of conversion dynodes. A voltage application unit that applies a voltage higher than the voltage applied to the plurality of conversion dynodes to the detection unit;
It is characterized by having.

  In the mass spectrometer according to the present invention, the rod-shaped electrode constituting the linear ion trap is provided with a plurality of ion discharge ports for discharging ions held inside to the outside by resonance excitation. Typically, similarly to the mass spectrometer described in Patent Document 1, two ion discharge ports are provided, one on each side across the central axis on a straight line orthogonal to the central axis of the linear ion trap. Just do it. Corresponding to each of the plurality of ion discharge ports, the same number of conversion dynodes are arranged outside the linear ion trap. On the other hand, the detection unit that detects the electrons emitted from the conversion dynode upon receiving the ion is commonly provided for the plurality of conversion dynodes, that is, only one is provided.

  However, if a common detection unit is provided for a plurality of conversion dynodes that are separated from each other, it is difficult to bring each conversion dynode close to the electron entrance surface of the detection unit, and electrons emitted from the conversion dynode are detected. The efficiency of reaching the electron incident surface in the part tends to deteriorate. In particular, a voltage lower than that of the linear ion trap is applied to the conversion dynode, for example, when the analysis target is a positive ion so as to attract ions discharged from the linear ion trap to the conversion dynode. Therefore, the electrons emitted from the conversion dynode are subjected to a force that travels toward the linear ion trap, contrary to the ions. On the other hand, in the mass spectrometer according to the present invention, a shielding plate is provided between the linear ion trap and the conversion dynode, and the shielding plate is equal to or lower than the voltage applied to the conversion dynode (has a negative polarity). (The absolute value of the voltage is large). In addition, a voltage higher than the voltage applied to the conversion dynode, that is, a voltage that attracts electrons is applied to the detection unit.

  Due to the voltage applied to the shielding plate as described above, an electric field for accelerating the ions ejected from the linear ion trap toward the shielding plate is formed between the shielding plate and the linear ion trap. Many of the ions accelerated by this electric field pass through an ion passage opening formed at an appropriate position of the shielding plate and reach the conversion dynode. An electric field for moving electrons emitted from the conversion dynode toward the detection unit is formed between each conversion dynode and the detection unit. On the other hand, the electric field formed between each conversion dynode and the shielding plate does not have an effect of moving electrons emitted from the conversion dynode toward the shielding plate, or conversely, pushes back electrons toward the conversion dynode. Therefore, the electrons emitted from the conversion dynode upon receiving the ions are directed to the detection unit and efficiently reach the detection unit and are detected.

  In this manner, in the mass spectrometer according to the present invention, the distance between the conversion dynode that converts the ions discharged from the internal space of the linear ion trap through the plurality of ion discharge ports into electrons and the common detection unit is to some extent. Even if they are separated from each other, the electrons emitted from the conversion dynode can be efficiently collected and detected by the common detection unit.

  In the mass spectrometer according to the present invention, it is preferable that the voltage application unit apply a voltage higher than the voltage applied to the shielding plate to each of the plurality of conversion dynodes.

  According to this configuration, the electrons emitted from the conversion dynode in the direction of the shielding plate can be pushed back by the action of the electric field, so that the efficiency of the electrons reaching the detection unit can be further increased. However, if there is a large difference between the potential of the shielding plate and the potential of the conversion dynode, ions directed to the conversion dynode through the opening of the shielding plate due to the action of the electric field formed between them are directed to the shielding plate. There is a risk of being pushed back. Therefore, it is desirable to set the difference between the potential of the shielding plate and the potential of the conversion dynode small enough to push back electrons having a light mass but hardly affect ions having a large mass compared to electrons. .

  In the mass spectrometer according to the present invention, preferably, the plurality of ion discharge ports are provided to face each other across the central axis of the linear ion trap, and the plurality of ion passage openings provided in the shielding plate, The plurality of conversion dynodes and the common detector may be arranged at positions that are plane-symmetric with respect to a symmetry plane of the plurality of ion discharge ports including a central axis of the linear ion trap.

  According to this configuration, since the electric field is symmetric in the space on both sides of the symmetry plane of the plurality of ion discharge ports, the trajectory until the ions emitted from the ion discharge ports reach the detection unit is also almost symmetric. Therefore, it is only necessary to optimize the arrangement and applied voltage of each element in the space on one side so that the other space has the same arrangement and applied voltage. This facilitates device design and reduces manufacturing costs. Is also connected.

  According to the mass spectrometer of the present invention, although a plurality of conversion dynodes are required, the number of ion detectors that are much more complicated and expensive than the conversion dynodes can be reduced. Thereby, the cost of the apparatus can be suppressed as compared with the case where a plurality of combinations of conversion dynodes and ion detectors are provided. On the other hand, almost all of the ions resonance-excited by the linear ion trap are discharged through a plurality of ion discharge ports and detected without waste, so that high detection sensitivity can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS The schematic block diagram of the linear ion trap type | mold mass spectrometer which is one Example of this invention. The schematic plan view of the linear ion trap in FIG. The conceptual diagram which shows the state of the electric potential of each part between a linear ion trap and an electron multiplier. The figure which shows the simulation result of the track | orbit of ion and an electron.

  A linear ion trap mass spectrometer which is an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic configuration diagram of the linear ion trap mass spectrometer of the present embodiment, and FIG. 2 is a schematic plan view of the linear ion trap in FIG. 1 (a plan view in which the X axis is taken in a direction perpendicular to the paper surface). .

  In FIG. 1, description of an ion source that generates ions to be analyzed is omitted. The linear ion trap mass spectrometer of the present embodiment includes an ion source (not shown), a linear ion trap 2, an ion detection unit 3, a control unit 4, an ion trap power supply unit 5, a shielding plate power supply unit 6, A conversion dynode (CD) power supply unit 7 and a detector power supply unit 8 are included.

  The linear ion trap 2 includes four rod-shaped electrodes 21 whose inner surfaces are parallel to each other around a central axis C extending in the Z-axis direction (direction orthogonal to the paper surface) in FIG. 22, 23, 24. In FIG. 1, the linear ion trap 2 is shown as an end view in which the rod-shaped electrodes 21, 22, 23, and 24 are cut along a plane (XY plane) orthogonal to the central axis C. As shown in FIG. 2, elongate ion discharge ports 21a and 22a extending in the Z-axis direction are formed in the two rod-shaped electrodes 21 and 22 facing each other across the central axis C. Outside the ends of both rod-shaped electrodes 21, 22, 23, 24, substantially circular end electrodes 25, 26 are arranged so as to sandwich the rod-shaped electrodes 21, 22, 23, 24 from both sides, In the end electrodes 25 and 26, circular ion introduction ports 25a and 26a centering on the central axis C are formed.

  The ion detection unit 3 includes conversion dynodes 31 and 32 provided corresponding to the two ion discharge ports 21a and 22a of the linear ion trap 2, and a common electron increase for receiving electrons emitted from the conversion dynodes 31 and 32, respectively. A double tube 33 and a shielding plate 34 disposed between the conversion dynodes 31 and 32 and the linear ion trap 2 are included. Although the shielding plate 34 is a flat conductive member, an ion passage opening 34a through which ions directed from the ion discharge openings 21a and 22a to the conversion dynodes 31 and 32 can pass is provided at an appropriate position. . As shown in FIG. 1, two ion passages 34 a provided in the shielding plate 34, two conversion dynodes 31 and 32, and one common electron multiplier 33 are arranged on the central axis of the linear ion trap 2. It arrange | positions so that it may become plane symmetrical with respect to the symmetry plane of the two ion discharge ports 21a and 22a containing C. The conversion dynodes 31 and 32 are made of a conductive member such as aluminum. Aluminum is used because aluminum oxide formed on the aluminum surface has the second highest conversion efficiency from ions to electrons after alkali metals, and is inexpensive and easy to handle.

Under the control of the control unit 4, the ion trap power supply unit 5 applies predetermined voltages to the rod-shaped electrodes 21, 22, 23, 24 and the end electrodes 25, 26, respectively. applying a predetermined DC voltage V _2, the respective conversion dynode power supply unit 7 to the conversion dynode 31 and 32 by applying a predetermined DC voltage V _3, the detector power supply unit 8 is predetermined DC voltage to the photomultiplier tube 33 V ₄ is applied.

The mass analysis operation in the mass spectrometer of the present embodiment will be described by taking as an example the case where ions to be analyzed are positive.
Ions generated by an ion source (not shown) are introduced into an internal space surrounded by the rod-shaped electrodes 21, 22, 23, 24 through one or both of the ion introduction ports 25 a, 26 a, and are supplied from the ion trap power supply unit 5 to the rod-shaped electrode 21. , 22, 23, 24 are captured by a quadrupole electric field formed by a high frequency voltage applied. When the ions are trapped after being introduced into the internal space, a DC voltage is applied to the end electrodes 25 and 26 so as to push back the ions, so that the ions are surrounded by the rod-shaped electrodes 21, 22, 23 and 24. It is confined in the internal space (space elongated in the Z-axis direction).

When an ion having a specific mass-to-charge ratio M is detected separately from other ions, the control unit 4 applies a predetermined high-frequency voltage corresponding to the mass-to-charge ratio M to the rod-shaped electrodes 21, 22, 23, 24. The ion trap power supply unit 5 is controlled to be applied. Then, only the ions having the mass-to-charge ratio M among the various ions trapped in the internal space greatly vibrate in the direction along the X axis by resonance excitation and are discharged through the ion discharge ports 21a and 22a. At this time, the DC voltage V ₁ applied to each rod-shaped electrode 21, 22, 23, 24 of the linear ion trap 2 is set to 0V, for example. At this time, the DC voltage V ₂ applied to the shielding plate 34, the DC voltage V ₃ applied to the conversion dynodes 31 and 32, and the DC voltage V ₄ applied to the electron multiplier 33 are shown in FIG. It becomes a relationship like this.

That is, a voltage V ₃ that attracts ions is applied to the conversion dynodes 31 and 32 as in the prior art, but here a voltage V ₂ that is slightly lower than that is applied to the shielding plate 34. Also, the electron multiplier 33 (indicated as “EMT” in FIG. 3) has a voltage higher than the voltage V ₃ applied to the conversion dynodes 31 and 32 so as to attract electrons emitted from the conversion dynodes 31 and 32. V ₄ is applied. By applying such a voltage, ions discharged from the ion discharge ports 21a and 22a are placed in the space between the vicinity of the ion discharge ports 21a and 22a of the linear ion trap 2 and the shield plate 34 in the direction of the shield plate 34. An electric field that attracts By the action of this electric field, ions ejected substantially parallel to the X axis are accelerated while gradually bending the trajectory as shown by the thick dotted line in FIG. The accelerated ions pass through the ion passage port 34a.

As described above, since the voltage V ₂ applied to the shielding plate 34 is lower than the voltage V ₃ applied to the conversion dynodes 31 and 32, as shown in FIG. 3, conversion to ions that have passed through the ion passage 34 a is performed. A force that pushes back ions from the dynodes 31 and 32 side to the shielding plate 34 side acts. However, since the potential difference between V ₂ and V ₃ is small, the force to push back the ions is small, and the ions that have passed through the ion passage 34a are sufficiently accelerated, so that the conversion dynodes 31 and 32 have almost no trouble. To reach. When ions collide with the conversion dynodes 31 and 32, electrons are emitted instead. This electrons are emitted in various directions, by the applied voltage V ₄ to the applied voltage V ₃ and the electron multiplier 33 to the conversion dynode 31 and 32, from the conversion dynode 31 and 32 to the photomultiplier tube 33 An electric field that accelerates electrons is formed. On the other hand, between the conversion dynodes 31 and 32 and the shielding plate 34, a weak (gradient potential gradient) electric field having a force to push back electrons emitted from the conversion dynodes 31 and 32 toward the conversion dynodes 31 and 32 is formed. Has been. Therefore, most of the electrons emitted from the conversion dynodes 31 and 32 are directed toward the electron multiplier 33 as shown by the thick arrows in FIG. 1 and efficiently reach the electron multiplier 33 to be detected. The

  The shielding plate 34 hardly affects the electric field on the conversion dynodes 31 and 32 side and the electric field on the linear ion trap 2 side. That is, the shielding plate 34 also has a function of shielding the electric field.

  FIG. 4 is a diagram showing simulation results of ion and electron trajectories. Here, in order to simplify the configuration, the conversion dynodes 31 and 32 and the shielding plate 34 are integrated and set to the same potential (−8 kV). Further, another shielding plate 36 having a potential set to 0 V is inserted between the shielding plate 34 and the linear ion trap 2. This is to avoid the influence of the electric field formed by the potential of the shielding plate 34 from reaching the linear ion trap 2. Furthermore, a shielding plate having an L-shaped cross section is provided on the opposite side of the shielding plate 34 across the central axis of the ion discharging ports 21a and 22a outside the ion discharging ports 21a and 22a so as to surround the ion discharging ports 21a and 22a. 35 is inserted. The shielding plate 35 is for avoiding diffusion of ions emitted from the ion discharge ports 21a and 22a to the side opposite to the shielding plate 34. Furthermore, a shielding plate 37 is disposed so as to surround the electron multiplier tube 33. The shielding plate 37 limits the range in which electrons can enter the incident surface of the electron multiplier 33. Thereby, it is possible to suppress the incidence of electrons (or other charged particles) other than the electrons emitted from the conversion dynodes 31 and 32 to the incident surface of the electron multiplier 33, and to reduce noise. Although these shielding plates 35, 36 and 37 are not essential elements, they are practically quite effective.

  As shown in FIG. 4, the ions discharged from the ion discharge ports 21a and 22a of the linear ion trap 2 are gradually bent in the trajectory immediately after the discharge, pass through the ion passage port 34a, and the conversion dynodes 31 and 32. Has reached. That is, target ions ejected from the linear ion trap 2 by resonance excitation reach the conversion dynodes 31 and 32 without waste. Further, the electrons emitted from the conversion dynodes 31 and 32 reach the incident surface of the electron multiplier 33 located substantially equidistant from both of them without waste. From this result, it can be confirmed that the detection signal corresponding to the target ion discharged from the linear ion trap 2 can be obtained with high sensitivity by appropriately determining the arrangement, shape, applied voltage, and the like of each part.

The above-described embodiment is an example of the present invention, and it is a matter of course that modifications, corrections, and additions may be appropriately made within the scope of the present invention, and included in the scope of the claims of the present application.
For example, the arrangement and shape of the shielding plate 34, the conversion dynode, the electron multiplier 33, and the like can be changed as appropriate. Further, as described above, arbitrary components such as the shielding plates 35, 36, and 37 that are set to predetermined potentials may be added.

2 ... Linear ion traps 21, 22, 23, 24 ... Rod-shaped electrodes 21a, 22a ... Ion discharge ports 25, 26 ... End electrodes 25a, 26a ... Ion introduction ports 3 ... Ion detectors 31, 32 ... Conversion dynodes 33 ... Electron multipliers 34, 35, 36, 37 ... shielding plate 34a ... ion passage 4 ... control unit 5 ... ion trap power supply unit 6 ... shielding plate power supply unit 7 ... conversion dynode power supply unit 8 ... detector power supply unit C ... center axis

Claims (3)

a) Linear ions including a plurality of rod-shaped electrodes arranged substantially parallel to each other around the central axis, and having a plurality of ion discharge ports for discharging ions held in an internal space surrounded by the rod-shaped electrodes to the outside Trap and
b) a plurality of conversion dynodes provided corresponding to a plurality of ion discharge ports in the linear ion trap, respectively, for receiving ions discharged through each ion discharge port and emitting electrons;
c) a shielding plate provided between the linear ion trap and the plurality of conversion dynodes and having a plurality of ion passage openings through which ions discharged through the plurality of ion discharge ports in the linear ion trap respectively pass;
d) a common detection unit that is located on the same side as the plurality of conversion dynodes as viewed from the shielding plate, receives electrons emitted from the plurality of conversion dynodes, and outputs a detection signal according to the amount of the electrons;
e) A voltage for attracting ions ejected through a plurality of ion outlets in the linear ion trap is applied to the shielding plate, and a voltage equal to or higher than an applied voltage to the shielding plate is applied to the plurality of conversion dynodes. A voltage application unit that further applies a voltage higher than the voltage applied to the plurality of conversion dynodes to the detection unit;
A mass spectrometer comprising: The mass spectrometer according to claim 1,
The mass spectrometer according to claim 1, wherein the voltage application unit applies a voltage higher than a voltage applied to the shielding plate to each of the plurality of conversion dynodes. The mass spectrometer according to claim 1 or 2,
The plurality of ion discharge ports are provided to face each other across the central axis of the linear ion trap, and the plurality of ion passage openings provided in the shielding plate, the plurality of conversion dynodes, and the common detector include The mass spectrometer is arranged at a position that is plane-symmetric with respect to the symmetry plane of the plurality of ion discharge ports including the central axis of the linear ion trap.

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