JP2008282595A - Device and method for mass spectrometry - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform mass spectrometry by raising dissociation efficiency of ions and dissociating even ions having a high molecular weight hardly dissociated conventionally. <P>SOLUTION: After selecting precursor ions by capturing the ions in an ion trap 1, ethylene gas is introduced into the ion trap 1 from an introducing part 23 for gas to be excited, and is irradiated with laser beam having a prescribed wavelength (10.6 μm) oscillating and exciting the gas molecules from an exciting laser irradiating source 22. The gas molecules absorb photons and are oscillated and excited to have high oscillation energy, and when the gas molecules in this oscillated and excited state are brought into contact with the precursor ions captured in a capturing region A, the oscillation energy is moved to the precursor ions. Every time the precursor ions are brought into contact with the gas molecules in the oscillated and excited state, the oscillation energy of the precursor ions is increased, and when the energy reaches sufficient size to dissociate, the precursor ions dissociate to generate product ions. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a mass spectrometry method and a mass spectrometer using an ion trap that traps ions by an electric field, and more specifically, dissociates (cleaves) the ions trapped in the ion trap and masses the ions generated thereby. The present invention relates to a mass spectrometry method and a mass spectrometer for analysis.

  In mass spectrometry, an ion having a specific mass is selected as a precursor ion, and then dissociation (fragmentation) of the precursor ion is promoted, and various product ions (fragment ions) generated thereby are mass analyzed. Methods of analysis (also called tandem analysis) are well known. One of the most widely used methods for dissociating precursor ions is Collision Induced Dissociation (CID).

  In general, in collision-induced dissociation, an excitation signal having a frequency corresponding to the mass of the target ion is given to the electrodes constituting the ion trap, and the translational motion of the target ion is caused by the action of an electric field formed in the ion trap. The ions are accelerated so as to increase. Then, the target ions are caused to collide with a collision gas (mainly rare gas neutral atoms such as He and Ar are used) introduced into the ion trap. Due to this collision, a part of the translational kinetic energy (Translational Energy) of the target ion is converted into vibration energy (Vibrational Energy) of the ion (T → V transfer). Each time collision with the collision gas occurs, the vibration energy of the target ion increases, and when the energy reaches a sufficient energy to cause dissociation, the target ion is cleaved by Unimolecular Dissociation.

  In addition, when strong infrared light is directly irradiated to ions in the gas phase, the ions continuously absorb a large number of photons, thereby exciting the vibrational state of the ions (increasing vibrational energy) and dissociating the ions. Happens. A method of applying such a phenomenon to ion fragmentation in mass spectrometry is known as infrared multiphoton dissociation (IRMPD) (see Non-Patent Document 1, etc.).

  Conventionally, in an ion trap mass spectrometer using a three-dimensional quadrupole ion trap or the like, the above-described collision-induced dissociation or infrared multiphoton dissociation is used to dissociate target ions. However, with such conventional dissociation techniques, ions having a large molecular weight are difficult to dissociate, and the sensitivity and accuracy of MS / MS analysis of such ions are not necessarily high. In addition, in the case of collision-induced dissociation, there is a problem that it is difficult to obtain product ions having a low mass in principle. On the other hand, although there is no such problem in the case of infrared multiphoton dissociation, the dissociated ions are irradiated with infrared light and secondary dissociation easily occurs, and the signal intensity of the primary dissociated ions to be analyzed is lowered. There is a problem of tending.

L. Sleno and one other, "Ion activation methods for tandem mass spectrometry", Journal of Mass Spectrometry ), 39 (2004), pp.1091-1112

  The present invention has been made to solve the above-mentioned problems, and the main purpose thereof is a mass spectrometry method using a new dissociation technique capable of dissociating ions that are difficult to dissociate by conventional dissociation methods. And providing a mass spectrometer.

A mass spectrometry method according to the present invention made to solve the above problems is a mass spectrometry method for dissociating ions in an ion trap composed of a plurality of electrodes and mass-analyzing the ions generated thereby,
a) an ion trapping step of trapping target ions in the ion trap by applying an alternating voltage to at least one electrode constituting the ion trap;
b) a gas introduction step for introducing predetermined gas molecules into the ion trap;
c) a gas molecule vibration excitation step for oscillating and exciting the gas molecules by irradiating light of a predetermined wavelength into the ion trap;
The target ions are dissociated by applying vibration energy to the target ions by bringing the vibrationally excited gas molecules into contact with the target ions trapped in the ion trap. .

Further, a mass spectrometer according to the present invention is an apparatus for carrying out the mass spectrometry method according to the above invention, and includes an ion trap that captures ions in a space surrounded by a plurality of electrodes. In a mass spectrometer for performing mass analysis of ions generated by dissociating ions,
a) voltage applying means for applying an alternating voltage to at least one electrode constituting the ion trap so as to trap target ions in the ion trap;
b) gas introduction means for introducing predetermined gas molecules into the ion trap;
c) a light irradiation means for irradiating the ion trap with light having a predetermined wavelength for vibrationally exciting the gas molecules;
The target ions are dissociated by applying vibration energy to the target ions by bringing the vibrationally excited gas molecules into contact with the target ions trapped in the ion trap.

In the mass spectrometer according to the present invention, first, ions for analysis are captured as precursor ions in an ion trap. The selection of precursor ions may be performed either inside or outside the ion trap. The gas introduction means introduces predetermined gas molecules into the ion trap. Here, as the predetermined gas molecule, one that is vibrationally excited at the wavelength of light irradiated by the light irradiation means (usually the wavelength in the infrared region) is selected. In general, it can be said that the higher the light absorbance, the easier the vibration excitation occurs. Therefore, the measurement of the absorption spectrum, the calculation of the absorption, etc., or the appropriate gas molecule and light wavelength using the database accumulated by such measurement / calculation And can be selected. For example, when a CO ₂ laser having a wavelength of 10.6 μm is used as the light irradiating means, the gas molecules that are vibrationally excited by this are C ₂ H ₄ (ethylene), SF ₆ (sulfur hexafluoride), BCl. ₃ (boron trichloride).

  When light of a predetermined wavelength is irradiated from the light irradiation means in a state where the gas molecules are introduced into the ion trap, the gas molecules absorb vibrations and are excited and have high vibration energy. When the precursor ion trapped in the ion trap comes into contact with this highly excited vibrational gas molecule, vibration energy is transferred from the gas molecule to the precursor ion (V → V transfer), and the precursor ion is vibrationally excited. The Each time the precursor ion comes into contact with a gas molecule in a vibrationally excited state, the vibration energy of the precursor ion increases. When the precursor ion reaches a sufficient energy to cause dissociation, dissociation occurs.

  The transfer efficiency of vibration energy from gas molecules to ions varies depending on the combination of the types of gas molecules and ions, but in general, the conversion efficiency of vibration energy → vibration energy (V → V transfer) is more collision-induced dissociation. Is known to be much higher than the conversion efficiency of translational energy → vibration energy (T → V transfer). Therefore, according to the dissociation method as described above, a higher dissociation efficiency than that of the conventional collision-induced dissociation can be obtained, and a larger amount of product ions can be generated. In addition, by introducing gas molecules into the ion trap at a relatively high concentration, it is possible to increase the opportunity to transfer vibration energy to the precursor ions, and thus ions that have been difficult to dissociate in the past are easily dissociated.

  In addition, vibrational excitation of gas molecules occurs not by the action of the electric field but by the absorption of photons as in the case of infrared multiphoton dissociation, so there is no restriction on the mass of the product ion due to excitation of ions by the action of the electric field. Analysis is possible up to mass product ions.

  In addition, in the case of infrared multiphoton dissociation, it is necessary to irradiate light directly to the captured precursor ions, so that the product ions will also be exposed to light. Since the irradiation light only needs to be able to vibrationally excite the gas molecules introduced into the ion trap, it is possible to avoid the light from hitting the product ions as much as possible and to prevent secondary dissociation. Thereby, it is possible to prevent a decrease in signal intensity of product ions in MS / MS analysis.

  In particular, in order to prevent the secondary dissociation of the product ions generated by the primary dissociation as described above, light for oscillating and exciting the gas molecules is irradiated to a region deviated from the center of the trapping region of the ion trap. It is good to do so. Normally, precursor ions and product ions are concentrated near the center of the trapping region due to the action of an alternating electric field for trapping, but gas molecules are distributed independently of the electric field, so light is avoided by avoiding the center of the trapping region. , It is difficult for light to hit product ions, and secondary dissociation can be suppressed.

  In addition, when an excitation signal is applied to at least one electrode constituting the ion trap to excite target ions trapped in the ion trap by the action of an electric field, a dissociation effect due to collision-induced dissociation is added. Can be further improved.

  For example, when the ion trap is a three-dimensional quadrupole ion trap composed of one ring electrode and two end cap electrodes, it is usually captured by applying an AC voltage for ion capture to the ring electrode. An electric field may be formed and an excitation signal may be applied between both end cap electrodes.

  Hereinafter, an ion trap time-of-flight mass spectrometer that is an embodiment of a mass spectrometer that performs a mass spectrometry method according to the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of an ion trap time-of-flight mass spectrometer according to this embodiment. Inside a vacuum chamber (not shown) to be evacuated, an annular ring electrode 11 whose inner surface has a rotating one-leaf hyperboloid shape is opposed to and sandwiched between them (in FIG. 1, left and right). A three-dimensional quadrupole ion trap 1 is provided which is composed of a pair of end cap electrodes 12 and 13 whose inner side surface has a rotating two-leaf hyperboloid shape. A trapping region A that traps ions by a trapping electric field is formed in a space surrounded by the electrodes 11, 12, and 13.

  A MALDI (Matrix Assisted Laser Desorption Ionization) ion source 2 is disposed outside the ion entrance 14 formed in the inlet end cap electrode 12 of the ion trap 1, while the outlet end cap electrode 13 A time-of-flight mass spectrometer 3 including a flight space 4 for separating ions according to mass and an ion detector 5 is disposed outside the ion emission port 15 formed in the tube. However, the ion source 2 is not limited to MALDI, and can be replaced with various known forms of ion sources. On the other hand, instead of the time-of-flight mass spectrometer 3, a mass spectrometer of another form may be used, and the ion trap 1 itself is used as a mass analyzer and only the ion detector is arranged outside the ion emission port 15. You can also

A laser irradiation hole 16 is formed in the center of the ring electrode 11 along the axis (R axis) direction of the ring electrode 11, and laser as excitation light emitted from the excitation laser irradiation source 22 through the laser irradiation hole 16. Light is irradiated so as to pass through the trapping region A of the ion trap 1. In this example, the excitation laser irradiation source 22 is an infrared CO ₂ laser having a wavelength of 10.6 μm. Further, ethylene (C ₂ H ₄ ) gas, which is an excited gas, is supplied into the ion trap 1 from the excited gas introducing portion 23.

  A trapped voltage generator 20 is connected to the ring electrode 11, and an excitation signal generator 21 is connected to both end cap electrodes 12 and 13. The trapped voltage generator 20 and the excitation signal generator 21 are controlled by a control signal supplied from the controller 24 so as to generate an alternating voltage having a predetermined frequency and a predetermined amplitude, respectively. However, a DC voltage may be superimposed on the AC voltage and applied as necessary. The control unit 24 includes a CPU, a ROM, a RAM, and the like, and controls the trapped voltage generation unit 20 and the excitation signal generation unit 21 according to a preset control program, as well as the MALDI ion source 2 and the excitation laser irradiation. The operations of the source 22 and the excited gas introduction unit 23 are also controlled.

  A characteristic operation when performing MS / MS analysis in this ion trap time-of-flight mass spectrometer will be described with reference to the flowchart of FIG.

  First, under the control of the control unit 24, ethylene gas is introduced into the ion trap 1 as a cooling gas from the excited gas introduction unit 23 in a pulsed manner and filled, and a predetermined trapping is performed on the ring electrode 11 from the trapping voltage generation unit 20 By applying a voltage, a quadrupole electric field for trapping ions is formed in the space inside the ion trap 1. In this state, ions are generated from the sample to be analyzed in the MALDI ion source 2, and the ions are introduced into the ion trap 1 through the ion incident port 14 (step S1). The introduced ions collide with ethylene gas functioning as a cooling gas and lose kinetic energy, that is, are cooled, trapped by the quadrupole electric field, and collected near the center of the trapping region A (step S2). Here, an inert gas such as He can be used as the cooling gas instead of the excited gas.

  After trapping various ions derived from the sample in the trapping region A in the ion trap 1, an excitation signal that greatly excites ions other than the precursor ions in order to leave only the target precursor ions in the ion trap 1. It is generated by the excitation signal generator 21 and applied between the end cap electrodes 12 and 13. Thereby, undesired ions other than the target precursor ion are discharged to the outside of the ion trap 1 through the ion incident port 14 and the ion emission port 15 (step S3).

After the precursor ions are thus selected, ethylene gas is introduced as an excited gas into the ion trap 1 by the excited gas introducing portion 23 (step S4). The reason why ethylene gas is used here is that the ethylene molecules (C ₂ H ₄ ) absorb the infrared light having the above wavelength (10.6 μm) well and are efficiently vibrationally excited. Actually, in addition to C ₂ H ₄ , gas molecules such as SF ₆ and BCl ₃ are also vibrationally excited with respect to the infrared light having the above wavelength. May be dissociated, and therefore it is desirable to avoid gas molecules containing halogen atoms having toxicity. For this reason, C ₂ H ₄ is selected here, but in principle, it is not particularly limited as long as it is a molecule that can be vibrationally excited by light irradiation.

  After introducing ethylene gas into the ion trap 1, the excitation laser irradiation source 22 is driven to oscillate and excite the gas molecules, and the ion trap 1 is irradiated with infrared laser light (step S5). As a result, the gas molecules absorb photons and are vibrationally excited resonantly, and have high vibration energy. Since the precursor ions are trapped in the trapping region A, gas molecules having high vibration energy come into contact with the precursor ions and give vibration energy to the precursor ions. That is, vibration energy is transferred from gas molecules to precursor ions, and the precursor ions themselves have vibration energy. Since the precursor ion receives vibrational energy every time it comes into contact with gas molecules in a vibrationally excited state, the vibrational energy gradually increases and dissociates when reaching sufficient energy for dissociation (step S6). Also, since some of the precursor ions are directly irradiated with infrared laser light, some of the precursor ions are dissociated by infrared multiphoton dissociation to generate product ions.

  Since the product ions generated as described above are also captured by the action of the trapping electric field, they are accumulated in the trapping region A without being dissipated. Then, after the precursor ions are dissociated as described above for a predetermined time, a voltage that discharges the product ions trapped in the ion trap 1 is applied from the excitation signal generator 21 to the end cap electrodes 12 and 13. By applying this, initial kinetic energy is imparted to the ions, and the ions are emitted simultaneously from the ion emission port 15 and introduced into the time-of-flight mass spectrometer 3 for mass analysis. Then, an MS / MS spectrum is created by processing the detection signal from the ion detector 5 by a data processing unit (not shown) (step S7).

  As described above, according to the ion trap time-of-flight mass spectrometer of the present embodiment, the precursor ions are efficiently dissociated by a mechanism different from conventional collision-induced dissociation and infrared multiphoton dissociation, and the ion ions are generated thereby. Product ions can be mass analyzed.

  Next, an ion trap time-of-flight mass spectrometer according to another embodiment of the present invention will be described with reference to FIG. In FIG. 3, the same or corresponding components as those in FIG. 1 are denoted by the same reference numerals. The configuration of FIG. 3 is different from the configuration of FIG. 1 in that the excitation laser beam is irradiated not on the center of the trapping region A of the ion trap 1 but on a position intentionally removed from the laser irradiation hole 17. Is provided at a position shifted from the central axis of the ring electrode 11.

  Both the selected precursor ions and the product ions generated by the dissociation by contact with the gas molecules in the vibrationally excited state as described above are trapped by the trapping electric field and exist at a high density in the center of the trapping region A. Therefore, as shown in FIG. 1, when the infrared laser beam is irradiated to the center of the capture region A, the precursor ions absorb the photons, and the number of photons absorbed by the gas molecules decreases, thereby improving the efficiency of vibration excitation of the gas molecules. It may get worse. In addition, the product ions that have undergone primary dissociation may undergo secondary dissociation due to infrared multiphoton dissociation, which may reduce the signal intensity of the target ions. On the other hand, in the configuration of FIG. 3, since the infrared laser beam is out of the center of the trapping region A where ions exist at a high density, the above-described undesirable phenomenon is unlikely to occur. Since the gas molecules to be vibrationally excited are distributed uniformly over a relatively wide range regardless of the trapped electric field, shifting the irradiation position of the infrared laser light as described above will not affect the gas molecule excitation. High dissociation efficiency can be ensured.

  Further, in the above embodiment, when the precursor ion is dissociated by irradiating the excited gas introduced into the ion trap 1 with infrared laser light to excite the gas molecules to vibrate, the trapping electric field is contained in the ion trap 1. Just trying to form. On the other hand, like the collision-induced dissociation, an excitation signal having a frequency corresponding to the mass of the precursor ion is applied between the end cap electrodes 12 and 13 to excite the captured precursor ion. Good. Thus, the precursor ion has a translational kinetic energy, and when this precursor ion comes into contact with a gas molecule in a vibrationally excited state, both the movement of the vibration energy from the gas molecule and the conversion from the translational kinetic energy to the vibration energy are performed. Due to the action, the efficiency of increase of vibration energy is improved. As a result, the dissociation efficiency is improved, and a larger amount of product ions can be subjected to mass spectrometry.

In the above embodiment, the wavelength of the infrared laser light to be irradiated is 10.6 μm, and the wavelength is not limited to this, but the wavelength of light that can vibrate and excite gas molecules is usually in the infrared region. . In addition to the CO ₂ laser, various types of lasers can be used. In particular, the use of a semiconductor (solid) laser is very advantageous for reducing the size and weight of the apparatus.

  Further, as described above, the type of gas molecule of the excited gas depends on the wavelength of the excitation laser beam, but if this wavelength is variable or selectable to some extent, it can be used accordingly. The types of gas molecules in the gas will also expand. According to this, for example, it becomes easy to select an excited gas having higher dissociation efficiency according to the type (mass) of the precursor ion.

  The above-described embodiment is an example of the present invention, and it is a matter of course that changes, modifications, and additions within the scope of the present invention are included in the scope of the claims of the present application.

  For example, in all of the above embodiments, a three-dimensional quadrupole ion trap is used as the ion trap. For example, four (or more) rod electrodes whose inner surfaces are hyperboloids or cylindrical surfaces are arranged in parallel. The present invention can also be applied to a mass spectrometry method and a mass spectrometer using a linear ion trap that forms a trapping region in a space surrounded by these rod electrodes.

1 is a schematic configuration diagram of an ion trap time-of-flight mass spectrometer according to an embodiment of the present invention. The flowchart which shows the procedure of the MS / MS analysis in the ion trap time-of-flight mass spectrometer of an Example. The schematic block diagram of the ion trap time-of-flight mass spectrometer by another Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Ion trap 11 ... Ring electrode 12 ... Inlet side end cap electrode 13 ... Outlet side end cap electrode 14 ... Ion entrance port 15 ... Ion exit port 16, 17 ... Laser irradiation hole 2 ... MALDI ion source 3 ... Time-of-flight mass Analyzer 4 ... Flight space 5 ... Ion detector 20 ... Capture voltage generator 21 ... Excitation signal generator 22 ... Excitation laser irradiation source 23 ... Excited gas introduction part 24 ... Control part A ... Capture region

Claims (6)

A mass spectrometry method for dissociating ions in an ion trap consisting of a plurality of electrodes and mass-analyzing the ions generated thereby,
a) an ion trapping step of trapping target ions in the ion trap by applying an alternating voltage to at least one electrode constituting the ion trap;
b) a gas introduction step for introducing predetermined gas molecules into the ion trap;
c) a gas molecule vibration excitation step for oscillating and exciting the gas molecules by irradiating light of a predetermined wavelength into the ion trap;
The target ions are dissociated by giving vibration energy to the target ions by bringing the vibrationally excited gas molecules into contact with the target ions trapped in the ion trap. Mass spectrometry method.   2. The mass spectrometric method according to claim 1, wherein, in the gas molecule vibration excitation step, light for vibrating and exciting the gas molecules is irradiated to a region off the center of the trapping region of the ion trap.   3. The mass spectrometric method according to claim 1, wherein an excitation signal is applied to at least one electrode constituting the ion trap to excite target ions trapped in the ion trap by an electric field. . In a mass spectrometer that includes an ion trap that traps ions in a space surrounded by a plurality of electrodes, dissociates the ions in the ion trap, and mass-analyzes the ions generated thereby.
a) voltage applying means for applying an alternating voltage to at least one electrode constituting the ion trap so as to trap target ions in the ion trap;
b) gas introduction means for introducing predetermined gas molecules into the ion trap;
c) a light irradiation means for irradiating the ion trap with light having a predetermined wavelength for vibrationally exciting the gas molecules;
A mass characterized by dissociating the target ions by applying vibration energy to the target ions by bringing the vibrationally excited gas molecules into contact with the target ions trapped in the ion trap. Analysis equipment.   5. The mass spectrometer according to claim 4, wherein the light irradiation unit irradiates light for vibrating and exciting gas molecules in a region off the center of the trapping region of the ion trap.   6. The excitation signal supply means for supplying an excitation signal to at least one electrode constituting the ion trap so as to excite target ions trapped in the ion trap. The mass spectrometer as described.

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