JP5333072B2 - Ion trap device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make it possible to adjust the number of ions after introduction of ions to an ion trap at neither the time of ion generation nor ion introduction into the ion trap, so as to reduce a space charging effect inside the ion trap and enhance an analyzing performance. <P>SOLUTION: After capturing ions inside an ion trap 2, an electric current is supplied to a solenoid coil 26 to form a direct current magnetic field B and moreover to release a capturing high-frequency electric field, and an opposite polar voltage is applied on end-cap electrodes 22, 24. Approximately a half of the captured ions advances toward the intake-side end-cap electrode 22 along a field line and disappear. The remaining half of the ions advance toward the exit side end-cap electrode 24 along the field line and is reflected by an electric field. The direct current magnetic field B is released and a capturing electric field is revived before the reflected ions reach the intake-side end-cap electrode 22, and the ions can be reduced to approximately a half. The number of ions can be reduced further by repetition of this operation. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an ion trap apparatus that traps ions by the action of a high-frequency electric field.

  In a mass spectrometer, an ion trap captures and confines ions by the action of a high-frequency electric field, selects ions with a specific mass-to-charge ratio m / z, and cleaves the selected ions. (Refer to patent document 1 etc.). A typical ion trap has one ring electrode whose inner surface is a rotating one-leaf hyperboloid shape, and a pair of end cap electrodes whose inner surfaces are opposed to each other with the ring electrode in between. This is a three-dimensional quadrupole configuration consisting of In addition, a linear ion trap composed of four rod electrodes arranged in parallel is also known. In this specification, for the sake of convenience, the ion trap will be described by taking “three-dimensional quadrupole type” as an example.

  In the conventional general ion trap, a high frequency electric voltage is usually applied to the ring electrode to form a high frequency electric field in a space surrounded by the ring electrode and the end cap electrode, and the high frequency electric field confines ions while vibrating. . After that, for example, a high frequency low voltage of a predetermined frequency is applied between the opposed end cap electrodes, ions having a specific mass-to-charge ratio corresponding to the frequency are excited and discharged from the ion trap, and the detection installed outside the ion trap Detect with instrument. By scanning the frequency of the high frequency and low voltage applied between the end cap electrodes according to a predetermined sequence, the mass-to-charge ratio of ions ejected from the ion trap is scanned, and a mass spectrum is created based on the detection signal obtained accordingly. can do.

  Alternatively, after ions are trapped in the ion trap as described above, a predetermined DC voltage is applied to the end cap electrode, initial energy is given to the trapped ions, and the ions are discharged simultaneously from the ion trap. And ion is introduce | transduced into the time-of-flight mass spectrometer installed in the outer side, and it isolate | separates according to mass-to-charge ratio in this analyzer, and detects. In general, in such a configuration using a time-of-flight mass analyzer, a mass spectrum can be created with higher resolution.

  The performance of the mass spectrometer using the ion trap as described above is greatly influenced by the density of ions trapped in the ion trap. That is, when the ion density in the ion trap increases and exceeds a certain threshold, the mass resolution and the mass accuracy decrease. This is because the space charge formed by the ions trapped in the ion trap affects the spatial distribution and orbit of the ions. Therefore, in order to improve mass resolution and mass accuracy, it is preferable to quantitatively adjust the number of ions trapped in the ion trap so as to be an appropriate value.

  In the mass spectrometer provided with the electrospray ion source described in Patent Document 2, ionization of the sample is continuously performed. Therefore, the number of ions to be introduced, that is, the charge density in the ion trap is adjusted by controlling the introduction time width for introducing ions into the ion trap. However, it takes longer for the ions emitted from the ion source to reach the ion introduction port of the ion trap, so that the ions with a larger mass-to-charge ratio take longer, so the delayed ions do not reach the ion trap within the set introduction time, It is not introduced into the ion trap. As a result, there is a problem that the mass range of ions trapped in the ion trap is narrowed.

  On the other hand, in a mass spectrometer using a MALDI ion source, ionization is intermittently performed by irradiating a sample with laser light in a pulsed manner. Therefore, the amount of ions generated can be adjusted by adjusting the intensity of the irradiated laser or the amount of the sample. It is conceivable to adjust the number of ions trapped in the ion trap. However, adjustment of the sample amount is a very troublesome and time-consuming operation. In general, since there is a large variation in the relationship between the laser intensity to be irradiated and the ion generation amount, it is difficult to accurately adjust the ion generation amount even if the laser intensity is adjusted.

U.S. Pat. No. 2,939,952 JP-T-2005-500662

  The present invention has been made to solve the above-mentioned problems, and the object of the present invention is to reduce the number of the ions appropriately after introducing ions into the ion trap to alleviate the space charge effect, and thus mass resolution and the like. It is an object to provide an ion trap apparatus that can improve the performance.

The present invention made in order to solve the above-mentioned problem consists of a plurality of electrodes, and in an ion trap apparatus for trapping ions in a space surrounded by these electrodes,
a) high-frequency voltage applying means for applying a high-frequency voltage to at least one of the plurality of electrodes in order to form a high-frequency electric field for ion trapping in the space inside the ion trap;
b) magnetic field forming means for forming a DC magnetic field in the ion trap space;
c) In a state where the formation of the high-frequency electric field is stopped or the electric field intensity is reduced and a DC magnetic field is formed by the magnetic field forming means, ions are repelled on one of the pair of electrodes intersecting the magnetic field lines of the DC magnetic field. An auxiliary voltage applying means for applying a voltage for generating an electric field to be applied, and applying a voltage for generating an electric field for attracting ions to the other of the pair of electrodes;
It is characterized by having.

  The ion trap apparatus according to the present invention can be applied to both a three-dimensional quadrupole ion trap and a linear ion trap. In the case of a three-dimensional quadrupole ion trap having a ring electrode and a pair of end cap electrodes disposed opposite to each other across the ring electrode, the high-frequency voltage application means applies a high-frequency voltage to the ring electrode, The auxiliary voltage applying means may apply different voltages to the pair of end cap electrodes.

  When the space charge due to the ions trapped in the ion trap is approximately 0 V, the auxiliary voltage applying means applies a voltage having the same polarity as the ions to one of the pair of electrodes intersecting the magnetic field lines of the DC magnetic field, By applying a voltage having a polarity opposite to that of ions, an electric field that repels ions and an electric field that attracts ions can be generated as described above.

  In the ion trap apparatus according to the present invention, ions are first introduced into the ion trap and captured by the action of a high-frequency electric field. Thereafter, the formation of the high-frequency electric field is stopped or the strength of the high-frequency electric field is reduced and a DC magnetic field is formed by the magnetic field forming means, and further, one of the pair of electrodes intersecting the magnetic field lines of the DC magnetic field by the auxiliary voltage applying means. A voltage that generates an electric field that repels ions is applied to the (first electrode), and a voltage that generates an electric field that attracts ions is applied to the other (second electrode). Ions, which are charged particles, move in a predetermined orbit in a high-frequency electric field, but when a high-frequency electric field is weakened (or disappears) while a DC magnetic field is formed, the ions follow magnetic field lines due to Lorentz force acting on the ions. Move. The direction of movement along the magnetic field lines is determined by the initial energy of the ions when the DC magnetic field is formed. If the distribution of ions in the ion trap space is ideal, approximately half of the trapped ions become magnetic field lines. Proceed in opposite directions along.

  Ions directed to the first electrode are reflected by an electric field generated by a voltage applied to the electrode in the vicinity of the first electrode. On the other hand, the ions directed to the second electrode become neutral particles (that is, the ions disappear) due to the transfer of charges with the second electrode. Therefore, by returning the high-frequency electric field to the original ion-capturing state before the ions reflected in the vicinity of the first electrode reach the second electrode and disappear, the number of ions in the ion trap becomes the second number. It will decrease by the amount that disappears at the electrode. Thereby, the number of ions in the space in the ion trap can be reduced, and the influence of the space charge effect can be reduced.

  By repeating the ion trapping and the above-described ion number reduction operation, the number of ions in the ion trap space can be reduced stepwise.

  The ion trap apparatus according to the present invention reduces the number of ions once introduced into the ion trap, rather than the time when ions are generated or the time when ions are introduced into the ion trap. Can be reduced. Thereby, high mass resolution and mass accuracy in mass spectrometry can be ensured. In addition, since the number of ions is not adjusted by adjusting the ion introduction time width to the ion trap, the number of ions can be reduced almost independently of the mass-to-charge ratio of the ions, and the mass range of ions used for mass analysis is secured. can do. Furthermore, since the number of ions can be reliably reduced with high reproducibility, the number of ions unintentionally decreases and sensitivity decreases, and conversely the space charge effect reduces the number of ions as intended. The problem that can't be ignored is unlikely to occur.

The block diagram of the principal part of the ion trap mass spectrometer by one Example of this invention. The schematic timing diagram at the time of ion reduction operation in the ion trap mass spectrometer of the present embodiment. The principle explanatory drawing of ion reduction operation in the ion trap mass spectrometer of a present Example.

Hereinafter, an ion trap mass spectrometer according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.
In FIG. 1, an ionization unit 1, an ion trap 2, and a detection unit 3 are disposed inside a vacuum chamber (not shown). The ionization unit 1 is a MALDI ion source, and a laser beam emitted in a pulse form from the laser irradiation unit 11 is irradiated to the sample S placed on the sample plate 12. Ions generated from the sample S are extracted by the extraction electrode 13 and sent toward the ion trap 2 through the ion transport optical system 14.

  The ion trap 2 is a three-dimensional quadrupole ion trap including one annular ring electrode 21 and a pair of end cap electrodes 22 and 24 provided so as to sandwich the electrode 21. An ion incident port 23 is formed at substantially the center of the inlet end cap electrode 22, and an ion emission port 25 is formed at substantially the center of the outlet end cap electrode 24 substantially in line with the ion incident port 23. . Further, as a characteristic configuration, a solenoid coil 26 is embedded in the ring electrode 21, and a direct current is supplied from the excitation drive unit 7 to the solenoid coil 26, so that a direct current magnetic field is generated in the internal space of the ion trap 2. Is formed.

  The detector 3 includes a conversion dynode 31 and an electron multiplier 32, detects ions discharged through the ion emission port 25 of the ion trap 2, and outputs a detection signal corresponding to the number of ions.

  The main power supply unit 5 is connected to the ring electrode 21 of the ion trap 2, and the auxiliary power supply unit 6 is connected to the end cap electrodes 22 and 24. The main power supply unit 5 includes, for example, a high frequency (RF) high voltage generation unit using an LC resonance circuit. The auxiliary power supply unit 6 includes a DC voltage generation unit and a high frequency low voltage generation unit, and these voltages are switched and applied to the end cap electrodes 22 and 24. The control unit 8 mainly composed of the CPU controls operations of the main power supply unit 5, the auxiliary power supply unit 6, the excitation drive unit 7, the laser irradiation unit 11, and the like according to a predetermined sequence.

  Although not shown, a cooling gas such as helium or argon is appropriately introduced into the ion trap 2, and the ions introduced into the ion trap 2 are brought into contact with the cooling gas, so that ions are generated. You can take away the kinetic energy you have (that is, cool).

  In the ion trap mass spectrometer of the present embodiment, ions generated by irradiating the sample S with laser light in the ionization unit 1 are introduced into the space inside the ion trap 2 through the ion incident port 23. The introduced ions are captured by the action of a high-frequency electric field formed in the space inside the ion trap 2 by a high-frequency high voltage applied to the ring electrode 21 from the main power supply unit 5. The ions once trapped are discharged from the ion trap 2 while being mass-separated by applying excitation high-frequency low voltage to the end cap electrodes 22 and 24, detected by the detection unit 3, and detected by the data processing unit 4. A mass spectrum is created by processing.

  In the ion trap mass spectrometer of the present embodiment, it is possible to reduce the number of ions to be trapped by eliminating a part of the ions trapped in the internal space of the ion trap 2. Next, the operation for reducing the number of ions will be described in detail. FIG. 3 is a diagram for explaining the principle of ion reduction operation in the ion trap mass spectrometer of the present embodiment, and FIG. 2 is a schematic timing chart at the time of ion reduction operation in the ion trap mass spectrometer of the present embodiment.

Now, in a state where the electric field and magnetic field in the ion trap 2 can be considered to be zero, a direct current is supplied to the solenoid coil 26, and thereby a uniform magnetic field line shown in FIG. 3 can be drawn in the ion trap 2. Consider a case where a DC magnetic field B is formed. Since ions are charged particles and a Lorentz force acts by the DC magnetic field B, the ions trapped in the ion trap 2 immediately before the DC magnetic field B is generated have a swirling trajectory centered on the magnetic field lines of the DC magnetic field B. . The orbit radius r _L is expressed by the following equation (1).
_{r L = (M i k b} T i) 1/2 / eqB ... (1)
Here, M _i represents the mass of the ion, k _b represents the Boltzmann coefficient, T _i represents the temperature of the ion, and q represents the number of charges of the ion.
On the other hand, the component v ′ in the direction of the magnetic field of the DC magnetic field B at the average ion velocity v is expressed by the following equation (2).
v ′ = (k _b T _i / M _i ) ^1/2 (2)

In the method of reducing the number of ions in the present embodiment, a DC magnetic field B is formed in the ion trap 2 after introducing and trapping ions in the ion trap 2. The ions are repelled in the vicinity of one of the pair of electrodes intersecting the magnetic field lines of the DC magnetic field B, that is, an electric field having the same polarity as the ions is formed, and the ions are attracted in the vicinity of the other electrode, that is, the ions Forms an electric field of the opposite polarity. In order to form such an electric field, for example, when the ions are positive ions, a positive voltage V _A is applied to the outlet end cap electrode 24 and a negative voltage V _B is applied to the inlet end cap electrode 22. .

  It can be considered that the velocity distribution of ions in a sufficiently cooled state in the ion trap 2 follows the Maxwell distribution along a line (center axis) C connecting the ion entrance port 23 and the ion exit port 25. When the ion velocity distribution follows the Maxwell distribution, the ion trap 2 includes the center point and is perpendicular to the central axis C, and is formed between the inlet end cap electrode 22 side and the outlet end cap electrode 24 side. It can be considered that the number of ions present is the same (see FIG. 3B). Theoretically, ions existing on the inlet side end cap electrode 22 side from the plane orthogonal to the central axis C are directed to the inlet side end cap electrode 22 side by the DC magnetic field B, and the outlet side end cap electrode 24 side. The ions present at the end are directed toward the outlet end cap electrode 24 by the DC magnetic field B. That is, 50% of the ions originally trapped go to the inlet end cap electrode 22 and the remaining 50% go to the outlet end cap electrode 24.

It is considered that R _r [%] given by the following equation (3) among the same ions directed toward the exit-side end cap electrode 24 is reflected by the electric field having the same polarity as the ions and returns to the center direction of the ion trap 2. It is done. On the other hand, most of the ions that have reached the end cap electrode 22 on the inlet side become neutral particles by interaction with the electrode surface (contact with the electrode or transfer of charge to and from the electrode). Being neutral particles means annihilation as ions.
R _r = [1-exp (−2 eqV _A / k _b T _i )] × 100 (3)

Assuming that the shortest distance between the end cap electrodes 22 and 24 is L _trap , among the ions trapped in the ion trap 2, the ion with the smallest mass (mass: M _light ) is reflected near the exit end cap electrode 24. The distance flies at least 1.5 × L _trap before reaching the inlet end cap electrode 22. The time required for this is
T _light = 1.5L _trap / v ' _light … (4)
It becomes. The maximum mass M _{heavy of} ions annihilated in the vicinity of the inlet end cap electrode 22 within this time T _light is _calculated from the equations (2) and (4):
M _heavy = 9 × M _light … (5)
It is obtained. That is, if R _{r is} almost 100%, the total number of ions included in the mass range from the mass M _light to 9 × M _light among the ions trapped in the ion trap 2 can be reduced to about 50%. it can.

However, the behavior of the above-mentioned ions is based on the premise that all ions move along a magnetic field line caused by the DC magnetic field B (taken around a turning trajectory centered on the magnetic field line). That is, it is necessary to eliminate as much as possible the possibility that ions come into contact with the ring electrode 21 and disappear. For that purpose, it is necessary to set the intensity of the DC magnetic field B so that the condition of the following equation (6) is satisfied with respect to the mass-to-charge ratio of ions to be trapped in the ion trap 2.
r _L <R _trap
Here, R _trap is an inscribed circle radius of the ion trap 2.

In the ion trap mass spectrometer of the present embodiment, specific control of each part when performing the ion number reduction operation based on the above principle will be described with reference to FIG.
When ions are introduced into the ion trap 2 through the ion entrance 23, a positive DC voltage is applied to the outlet end cap electrode 24 from the auxiliary power source 6, and a negative DC voltage is applied to the inlet end cap electrode 22. Apply. As a result, the positive ions pass smoothly through the ion entrance port 23 and are reflected to return to the center side of the ion trap 2 when traveling to the vicinity of the exit-side end cap electrode 24, so that the ions are directly discharged through the ion exit port 25. Can be avoided. After introducing ions into the ion trap 2 in this way, a high frequency high voltage is applied from the main power source 5 to the ring electrode 21 to trap the ions. At this time, as described above, a cooling gas can be introduced into the ion trap 2 to cool the ions.

  Thereafter, a current is supplied from the excitation drive unit 7 to the solenoid coil 26 to form a DC magnetic field in the ion trap 2. At this time, the voltage applied to the ring electrode 21 is set to zero, or the frequency of the high frequency high voltage is increased in order to reduce the Coulomb barrier against ions and facilitate movement. Further, a positive DC voltage is applied to the outlet end cap electrode 24 and a negative DC voltage is applied to the inlet end cap electrode 22. By this, as described above, about half of the ions that were captured immediately before disappeared, the DC magnetic field disappeared before the rest disappeared, and the high frequency electric field for capture was restored, Ensure that the remaining ions are captured.

For example, the size of the ion trap 2 is the inscribed circle radius: 10 mm, the shortest distance L _trap between the end cap electrodes 22 and 24: 10 mm, and the range of the mass-to-charge ratio of ions trapped in the ion trap 2 is (5) Assume 120-1000 Da that satisfies the equation. In this case, from the equations (1) and (6), the magnetic flux density of the DC magnetic field B needs to be about 500 Gauss. Further, from the equation (4), the time for the ion number reducing operation in FIG. 2 is about 100 μsec.

  In addition, it is possible to repeat the above ion number reduction operation, and can further reduce the number of ions trapped thereby.

  In the above embodiment, assuming that the space potential is 0 [V], a voltage having a polarity opposite to that of the ions and a voltage having the same polarity are applied to the end cap electrodes 22 and 24, respectively. Then, the electrode 24 repels ions. On the other hand, when the space potential in the ion trap 2 is + Vs [V] instead of 0 [V], a value obtained by subtracting Vs from the voltage value applied to the end cap electrodes 22 and 24 is opposite in polarity to the ions. By setting the voltage and the voltage of the same polarity, an electric field that attracts ions and an electric field that repels ions can be generated, respectively.

  In the above embodiment, the present invention is applied to a three-dimensional quadrupole ion trap, but it is obvious that the number of ions to be captured can be reduced by a similar method even in a linear ion trap.

  Moreover, the said Example is one Example of this invention, and even if it corrects, changes, an addition etc. suitably in the range of the meaning of this invention, it is clear that it is included in the claim of this application. For example, the configurations of the ionization unit 1 and the detection unit 3 are not limited to those of the above-described embodiments, and a mass separation unit such as a time-of-flight mass analyzer is provided outside without performing mass separation by the ion trap 2 itself. There may be.

DESCRIPTION OF SYMBOLS 1 ... Ionization part 11 ... Laser irradiation part 12 ... Sample plate 13 ... Extraction electrode 14 ... Ion transport optical system 2 ... Ion trap 21 ... Ring electrode 22 ... Inlet end cap electrode 23 ... Ion entrance 24 ... Outlet end cap electrode 25 ... Ion exit 26 ... Solenoid coil 3 ... Detection unit 31 ... Conversion dynode 32 ... Electron multiplier 4 ... Data processing unit 5 ... Main power supply unit 6 ... Auxiliary power supply unit 7 ... Excitation drive unit 8 ... Control unit

Claims (2)

In an ion trap device that consists of a plurality of electrodes and traps ions in a space surrounded by the electrodes,
a) high-frequency voltage applying means for applying a high-frequency voltage to at least one of the plurality of electrodes in order to form a high-frequency electric field for ion trapping in the space inside the ion trap;
b) magnetic field forming means for forming a DC magnetic field in the ion trap space;
c) In the state where the formation of the high-frequency electric field is stopped or the electric field intensity is weakened and a DC magnetic field is formed by the magnetic field forming means, ions are attracted to one of a pair of electrodes intersecting the magnetic field lines of the DC magnetic field. An auxiliary voltage applying means for applying a voltage for generating an electric field and applying a voltage for generating an electric field for repelling ions to the other of the pair of electrodes;
An ion trap apparatus comprising: The ion trap apparatus according to claim 1,
A ring electrode and a pair of end cap electrodes disposed opposite to each other with the ring electrode interposed therebetween, wherein the high frequency voltage applying means applies a high frequency voltage to the ring electrode, and the auxiliary voltage applying means is applied to the pair of end cap electrodes. An ion trap device that applies voltage to each.

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