# Patent

## Title
Ion trap mass spectrometer and ion trap mass spectrometry method

## Abstract

There are provided an ion trap mass spectrometer and an ion trap mass spectrometry method which can realize reduction of the number of times that a sample is ionized, and shortening of the measurement time. Ions corresponding to a plurality of peaks P11, P12 and P13 with the intensity or S/N ratio falling within a predetermined range L are detected as MS2 precursor ions based on the MS1 spectrum. A plurality of ions detected as the MS2 precursor ions are dissociated at a time in an ion trap and subjected to mass spectrometry to measure a MS2 spectrum.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ion trap mass spectrometer and an ion trap mass spectrometry method in which ions obtained by ionizing a sample are captured in an ion trap, and the ions are dissociated and subjected to mass spectrometry to perform MSn analysis (n is an integer of 2 or greater).
2. Description of the Related Art
An ion trap mass spectrometer provided with an ion trap is widely used in identification of a high-molecular compound such as a peptide from a mixture sample such as a biological sample (see, for example, Andrew N. Krutchinsky, Markus Kalkum, and Brian T. Chait, Automatic Identification of Proteins with a MALDI-Quadrupole Ion Trap Mass Spectrometer, Anal. Chem., 2001, 73(21), 5066-5077). In this type of mass spectrometer, for example, a sample is vaporized in vacuum together with a matrix by MALDI (matrix-assisted laser desorption-ionization), and the sample is ionized by delivery of protons between the sample and the matrix. Ions obtained by ionizing the sample can be then captured in an ion trap and subjected to mass spectrometry.
FIG. 9 is a flow chart showing one example of process when mass spectrometry is performed by a conventional ion trap mass spectrometer. In this example, ions captured in an ion trap are dissociated by so called CID (collision-induced dissociation) and subjected to mass spectrometry to perform MSn analysis.
First, mass spectrometry (MS1 analysis) of an ionized sample is performed to measure a MS1 spectrum (step S501). The MS1 spectrum is then analyzed to detect an ion corresponding to a peak that satisfies a predetermined criterion as a MS2 precursor ion (steps S502 and S503).
When the MS2 precursor ion is detected (Yes in step S504), ions obtained by ionizing the sample are captured in an ion trap, ions detected as MS2 precursor ions are left in the ion trap and dissociated one by one, and subjected to mass spectrometry (MS2 analysis) to measure a MS2 spectrum (step S505). Thereafter, the MS2 spectrum is analyzed to detect an ion corresponding to a peak that satisfies a predetermined criterion as a MS3 precursor ion (steps S506 and S503).
In this manner, MSn analysis is performed by repeating the processes in steps S503 to S506 until the MSn precursor ion (n is an integer of 2 or greater) is no longer detected (No in step S504). Sample components can be identified based on the MSn spectrum obtained by the MSn analysis.
In the conventional ion trap mass spectrometer described above, one MS2 precursor ion is usually selected from each peak and MS2 analysis is performed when the MS1 spectrum has a plurality of peaks that satisfy a predetermined criterion. That is, an attempt has not been made to identify a plurality of peptides by performing measurement for a plurality of MS2 precursor ions in parallel.
Therefore, every time MS2 analysis is performed for a MS2 precursor ion corresponding to each peak in a MS1 spectrum, a sample is ionized to reduce the amount thereof, so that the sample may be exhausted before all the components are identified. The measurement time is increased, so that a matrix may be sublimed in vacuum, thus making it impossible to continue measurement. Particularly, DHB (2,5-dihydroxybenzoic acid), a typical compound of a matrix is easily sublimed in vacuum.
The present invention has been devised in view of the above-described situations, and an object of the present invention is to provide an ion trap mass spectrometer and an ion trap mass spectrometry method which can realize reduction of the number of times that a sample is ionized, and shortening of the measurement time.
SUMMARY OF THE INVENTION
An ion trap mass spectrometer of the present invention is an ion trap mass spectrometer in which ions obtained by ionizing a sample are captured in an ion trap, and the ions are dissociated and subjected to mass spectrometry to perform MSn analysis (n is an integer of 2 or greater), the ion trap mass spectrometer including a MS1 measurement processing section, a precursor ion detection processing section and a MS2 measurement processing section. The MS1 measurement processing section is configured to measure a MS1 spectrum by performing mass spectrometry of the ionized sample. The precursor ion detection processing section is configured to detect, as MS2 precursor ions, ions corresponding to a plurality of peaks with the intensity or S/N ratio falling within a predetermined range, based on the MS1 spectrum. The MS2 measurement processing section is configured to measure a MS2 spectrum by dissociation of a plurality of ions, which are detected as MS2 precursor ions, at a time in the ion trap and subjecting the ions to mass spectrometry.
According to this configuration, ions corresponding to a plurality of peaks with the intensity or S/N ratio falling within a predetermined range are detected as MS2 precursor ions based on a MS1 spectrum, and the plurality of ions are dissociated at a time in an ion trap and subjected to mass spectrometry, whereby a MS2 spectrum can be measured. When based on the MS2 spectrum thus obtained, components corresponding to a plurality of peaks are identified at a time, the number of measurements is reduced, so that the number of times that a sample is ionized can be reduced, and the measurement time can be shortened.
The ion trap mass spectrometer may further include a MS3 measurement processing section. In this case, the MS3 measurement processing section may be configured to measure a MS3 spectrum in the following manner: among product ions produced through the dissociation treatment in measurement of the MS2 spectrum, only an ion corresponding to a peak at a predetermined mass-to-charge ratio is dissociated and subjected to mass spectrometry.
In the ion trap mass spectrometer, when a fragment ion generated due to a neutral loss by a releasable modified molecule and an adducts exist in the MS1 spectrum, MS2 analysis can be performed while one of ion peaks adjacent with a mass difference in mass-to-charge ratio of an ion corresponding to a known neutral loss is left and the rest is excluded from precursor ions to be subjected to MS2 analysis. A situation can be hereby prevented in which a plurality of peptide-derived product ions sharing a partial structure are superimposed due to a neutral loss, so that it becomes difficult to analyze a structure of a part that is not shared.
The ion trap mass spectrometer may further include a MS2 remeasurement processing section. In this case, the MS2 remeasurement processing section may be configured to perform a process by the MS2 measurement processing section again for an ion corresponding to a component that cannot be identified when a component that cannot be identified exists in a plurality of ions detected as the MS2 precursor ions.
According to this configuration, even when a component that cannot be identified exists in a plurality of ions detected as MS2 precursor ions due to a difference in product ion production efficiency between components, etc., a MS2 spectrum can be measured again for an ion corresponding to the component that cannot be identified. When identification is performed again based on the MS2 spectrum thus obtained, measurement can be performed while a difference in product ion production efficiency between components is taken into consideration.
The ion trap mass spectrometer may further include an on-target separation processing section and a MS1 remeasurement processing section. In this case, the on-target separation processing section may be configured to perform a process in which a sample on a target is separated on the target when a component that cannot be identified exists in a plurality of ions detected as the MS2 precursor ions. Further, the MS1 remeasurement processing section may be configured to perform a process by the MS1 measurement processing section again for the sample treated by the on-target separation processing section.
According to this configuration, even when a component that cannot be identified exists in a plurality of ions detected as MS2 precursor ions, the component may be capable of being identified by performing a process by the MS1 measurement processing section again for the sample treated by the on-target separation processing section. Further, a component that does not appear as a peak in the MS1 spectrum before the sample is treated by the on-target separation processing section may appear as a peak when the sample is treated by the on-target separation processing section. Therefore, by performing a process by the MS1 measurement processing section again for the sample treated by the on-target separation processing section, a larger number of components can be identified.
An ion trap mass spectrometry method of the present invention is an ion trap mass spectrometry method in which ions obtained by ionizing a sample are captured in an ion trap, and the ions are dissociated and subjected to mass spectrometry to perform MSn analysis (n is an integer of 2 or greater), the method including a MS1 measurement step, a precursor ion detection step and a MS2 measurement step. The MS1 measurement step is a step of measuring a MS1 spectrum by performing mass spectrometry of the ionized sample. The precursor ion detection step is a step of detecting, as MS2 precursor ions, ions corresponding to a plurality of peaks with the intensity or S/N ratio falling within a predetermined range, based on the MS1 spectrum. The MS2 measurement step is a step of measuring a MS2 spectrum by dissociation of a plurality of ions, which are detected as MS2 precursor ions, in the ion trap and subjecting the ions to mass spectrometry.
According to the present invention, components corresponding to a plurality of peaks with the intensity or S/N ratio falling within a predetermined range can be identified at a time, and therefore the number of measurements is reduced, so that the number of times that a sample is ionized can be reduced, and the measurement time can be shortened.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing an example of a configuration of an ion trap mass spectrometer according to one embodiment of the present invention;
FIG. 2 is a block diagram showing one example of a control unit and a memory unit;
FIG. 3 is a schematic view showing one example of a MS1 spectrum and MS2 spectrum;
FIG. 4 is a flow chart showing one example of process by the control unit at the time of performing MSn analysis;
FIG. 5 is a schematic view showing one example of a MS1 spectrum and MS2 spectrum when an ion that is easily released to a MS2 precursor ion is included;
FIGS. 6A and 6B are flow charts each partially showing a first modification of a process by the control unit at the time of performing MSn analysis;
FIG. 7 is a flow chart partially showing a second modification of a process by the control unit at the time of performing MSn analysis;
FIG. 8 is a flow chart partially showing a third modification of a process by the control unit at the time of performing MSn analysis; and
FIG. 9 is a flowchart showing one example of process when mass spectrometry is performed by a conventional ion trap mass spectrometer.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a schematic view showing an example of a configuration of an ion trap mass spectrometer according to one embodiment of the present invention. The ion trap mass spectrometer (hereinafter, referred to simply as a “mass spectrometer”) according to this embodiment can be used in identification of a high-molecular compound such as a peptide from a mixture sample such as a biological samples, and includes a mass spectrometry unit 1, a control unit 2, a memory unit 3, and so on.
The mass spectrometry unit 1 includes, for example, an ionization unit 11, an ion trap 12 and a TOFMS (time of flight mass spectrometer) 13. In this embodiment, a matrix-assisted laser desorption-ionization ion trap time of flight mass spectrometer (MALDI-IT-TOFMS) is described as one example of the mass spectrometer.
The ionization unit 11 ionizes a sample, and supplies the obtained ions to the ion trap 12. In this example, by irradiating the sample with a laser beam using MALDI (matrix-assisted laser desorption-ionization), a sample is vaporized in vacuum together with a matrix, and the sample is ionized by delivery of protons between the sample and the matrix. The sample is provided in a concentrated state, for example, on a target 111 formed of a plate, and set in the ionization unit 11 in a vacuum state together with the target 111 at the time of analysis.
The ion trap 12 is, for example, of a three-dimensional quadrupole type, and can capture ions obtained in the ionization unit 11, and selectively leave some of the captured ions in the ion trap 12 and dissociate the ions by CID (collision-induced dissociation). The ions dissociated in this manner are supplied to the TOFMS 13 from the ion trap 12.
In the TOFMS 13, ions flying in a flight space 131 are detected by an ion detector 132. Specifically, ions accelerated by an electric field formed in the flight space 131 are temporally separated according to a mass-to-charge ratio while flying in the flight space 131, and sequentially detected by the ion detector 132. A relationship between a mass-to-charge ratio and a detection intensity in the ion detector 132 is hereby measured as a spectrum to realize mass spectrometry.
In this embodiment, by repeatedly performing a series of operations in which ions are dissociated in the ion trap 12 and subjected to mass spectrometry by the TOFMS 13, MSn analysis (n is an integer of 2 or greater) can be performed to measure a MSn spectrum. Sample components can be identified by performing database search using MSn spectra obtained as described above.
The control unit 2 controls the operations of the mass spectrometry unit 1, and processes a MSn spectrum obtained by mass spectrometry. The memory unit 3 includes, for example, a RAM (Random Access Memory), a ROM (Read Only Memory), a hard disk and so on, and stores data used for process in the control unit 2, data generated by process in the control unit 2, and so on. The control unit 2 and the memory unit 3 may be formed integrally with or separately from the mass spectrometry unit 1.
FIG. 2 is a block diagram showing one example of the control unit 2 and the memory unit 3. For example, the control unit 2 includes a CPU (Central processing Unit), and functions as a MSn measurement processing section 21, a precursor ion detection processing section 22, an identification processing section 23, an on-target separation processing section 24 and so on, as the CPU runs a program.
The MSn measurement processing section 21 performs a process for measuring a MSn spectrum in the mass spectrometry unit 1. The measured MSn spectrum is stored in a spectrum storage region 31 assigned to the memory unit 3. In the MSn analysis, a MS1 spectrum, a MS2 spectrum, a MS3 spectrum . . . are sequentially measured, and each is stored in the spectrum storage region 31.
The precursor ion detection processing section 22 detects, based on a MSn-1 spectrum, an ion (MSn precursor ion) that is a target at the time of measuring a MSn spectrum. In MSn analysis, mass spectrometry (MS1 analysis) of a sample ionized in the ionization unit 11 is first performed in the TOFMS 13 to measure a MS1 spectrum. At this time, the MSn measurement processing section 21 functions as a MS1 measurement processing section. The precursor ion detection processing section 22 then detects a MS2 precursor ion based on the measured MS1 spectrum.
Thereafter, MS2 analysis is performed for the MS2 precursor ion. Specifically, ions obtained by ionizing a sample in the ionization unit 11 are captured in the ion trap 12, and only an ion detected as a MS2 precursor ion is separated in the ion trap 12. The ion left in the ion trap 12 is dissociated by CID, and subjected to mass spectrometry (MS2 analysis) in the TOFMS 13 to measure a MS2 spectrum. At this time, the MSn measurement processing section 21 functions as a MS2 measurement processing section.
The identification processing section 23 performs a process for identifying a sample component based on the measured MSn spectrum. In this example, a database for identification is assigned to a database region 32 that is a part of the memory unit 3. Sample components can be identified by calculating a degree of coincidence between data of the mass-to-charge ratio for various sample components, which is included in the database for identification, and the mass-to-charge ratio of each peak included in the MSn spectrum. The identification process may be configured to be automatically performed, or may be configured to be manually performed by a user.
For example, in identification of sample components after MS2 analysis, database search is performed using a database for identification based on the mass-to-charge ratio of a peak corresponding to the MS2 precursor ion in the MS1 spectrum and the mass-to-charge ratio of each peak in the MS2 spectrum. As a result, when a component that cannot be identified exists, the MSn measurement processing section 21 performs a process for measuring a MS3 spectrum. It is to be noted that the database for identification is not necessarily configured to be assigned to the memory unit 3 of the mass spectrometer, and for example, a database connected to the mass spectrometer through a network can be used.
An on-target separation processing section 24 performs a process in which a sample concentrated on the target 111 is separated on the target 111 (on-target separation) for the mass spectrometry unit 1. In on-target separation, for example, a phosphorylated peptide on the target 111 can be flushed with a phosphate solution and separated using a known method (see, for example, Analytical Chemistry, 2011, No. 83, pages 761-766). In this embodiment, on-target separation can be performed when a component that cannot be identified in the identification processing section 23 exists.
FIG. 3 is a schematic view showing one example of a MS1 spectrum and MS2 spectrum. Here, FIG. 3A is a schematic view of a MS1 spectrum obtained by subjecting a sample to MS1 analysis. FIG. 3B is a schematic view of a MS2 spectrum obtained by performing MS2 analysis for a MS2 precursor ion detected based on the MS1 spectrum in FIG. 3A.
In this embodiment, ions corresponding to a plurality of peaks are detected at the time of detecting a MS2 precursor ion based on the MS1 spectrum. In the example in FIG. 3A, ions corresponding to a plurality of peaks P11, P12 and P13 with the intensity or S/N ratio falling within a predetermined range L in the MS1 spectrum are detected as MS2 precursor ions. The predetermined range L may be predefined, or may be arbitrarily settable.
The predetermined range L is a range defined by a lower limit value and an upper limit value. Therefore, ions corresponding to peaks with the intensity or S/N ratio being below the lower limit value (P14 and P15) and above the upper limit value (P16) are not detected as MS2 precursor ions. In this way, only ions corresponding to a plurality of peaks P11, P12 and P13, which are relatively close in intensity or S/N ratio, can be detected as MS2 precursor ions.
In MS2 analysis, ions obtained by ionizing a sample in the ionization unit 11 are captured in the ion trap 12, and a plurality of ions detected as MS2 precursor ions are then separated in the ion trap 12. The plurality of ions left in the ion trap 12 are dissociated at a time by CID, and MS2 analysis is performed to obtain a MS2 spectrum as shown in FIG. 3B.
FIG. 4 is a flow chart showing one example of process by the control unit 2 at the time of performing MSn analysis. For performing MSn analysis, mass spectrometry (MS1 analysis) of an ionized sample is first performed to measure a MS1 spectrum (step S101: MS1 measurement step). The MS1 spectrum is then analyzed to detect, as MS2 precursor ions, ions corresponding to a plurality of peaks with the intensity or S/N ratio falling within a predetermined range (steps S102 and S103: precursor ion detection steps).
When the MS2 precursor ions are detected (Yes in step S104), ions obtained by ionizing the sample are captured in the ion trap 12, the plurality of ions detected as MS2 precursor ions are left in the ion trap 12 and dissociated at a time, and subjected to mass spectrometry (MS2 analysis) to measure a MS2 spectrum (step S105: MS2 measurement step). An identification process is performed based on the measured MS2 spectrum to identify sample components (step S106: identification step).
Thereafter, the MS2 spectrum is analyzed to detect, as MS3 precursor ions, ions corresponding to a plurality of peaks with the intensity or S/N ratio falling within a predetermined range (steps S107 and S103: precursor ion detection steps). At this time, the range of the intensity or S/N ratio of peaks corresponding to ions detected as MS3 precursor ions may be identical to or different from the range of the intensity or S/N ratio of peaks corresponding to ions detected as MS2 precursor ions.
In this manner, MSn analysis is performed by repeating the processes in steps S103 to S107 until the MS1 precursor ion is no longer detected (No in step S104).
As described above, in this embodiment, ions corresponding to a plurality of peaks with the intensity or S/N ratio falling within a predetermined range L are detected as MS2 precursor ions based on the MS1 spectrum, and the plurality of ions are dissociated at a time in the ion trap 12 and subjected to mass spectrometry, whereby a MS2 spectrum can be measured. When based on the MS2 spectrum thus obtained, components corresponding to a plurality of peaks are identified at a time, the number of measurements is reduced, so that the number of times that a sample is ionized can be reduced, and the measurement time can be shortened.
In the embodiment described above, a configuration has been described in which ions corresponding to a plurality of peaks are detected as MS2 precursor ions based only on the condition of whether or not the intensity or S/N ratio falls within the predetermined range L, but the present invention is not limited to this configuration, and other conditions may be included. For example, a configuration may be employed in which ions corresponding to a plurality of peaks with the intensity or S/N ratio falling within the predetermined range L among a plurality of peaks with the mass-to-charge ratio falling within a predetermined range are detected as MS2 precursor ions. In this case, a configuration may be employed in which by setting a plurality of ranges of the mass-charge ratio and performing measurement for each range, measurement is performed with a measurable range of the mass-to-charge ratio divided into a plurality of sections.
FIG. 5 is a schematic view showing one example of a MS1 spectrum and MS2 spectrum when an ion that is easily released to a MS2 precursor ion is included. Here, FIG. 5A is a schematic view of a MS1 spectrum obtained by subjecting a sample to MS1 analysis. FIG. 5B is a schematic view of a MS2 spectrum obtained by performing MS2 analysis for a MS2 precursor ion detected based on the MS1 spectrum in FIG. 5A.
As in the case of FIG. 3, ions corresponding to a plurality of peaks are detected at the time of detecting a MS2 precursor ion based on MS1 spectrum. In the example in FIG. 5A, ions P21, P22 and P23, which are left after excluding one of ions P23 and P27 adjacent in terms of a mass difference at a predetermined mass-to-charge ratio Δmz (ion P27 in the lower mass range in this example), among ions corresponding to a plurality of peaks P21, P22, P23 and P27 with the intensity or S/N ratio falling within the predetermined range L in the MS1 spectrum are detected as MS2 precursor ions. On the other hand, ions corresponding to peaks P24, P25 and P26 with the intensity or S/N ratio falling out of the predetermined range L are not detected as MS2 precursor ions.
In MS2 analysis, ions obtained by ionizing a sample in the ionization unit 11 are captured in the ion trap 12, and a plurality of ions detected as MS2 precursor ions are then separated in the ion trap 12. The plurality of ions left in the ion trap 12 are dissociated at a time by CID, and MS2 analysis is performed to obtain a MS2 spectrum as shown in FIG. 5B.
In this example, a plurality of ions detected as MS2 precursor ions include ions that are easily released, and therefore a high-intensity peak 27′ appears in the lower mass range at a predetermined mass-to-charge ratio Δmz with respect to the mass-to-charge ratio mz1 of the precursor ion P23 in product ions obtained by measuring the MS2 spectrum.
In this embodiment, the MSn measurement processing section 21 dissociates only an ion corresponding to the peak P27′ and subjects the ion to mass spectrometry (MS3 analysis) to measure a MS3 spectrum in the case where a peptide cannot be identified from the MS2 spectrum when the peak P27′ appears at a predetermined mass-to-charge ratio calculated from a known mass difference Δmz as described above. At this time, the MSn measurement processing section 21 functions as a MS3 measurement processing section.
FIGS. 6A and 6B are flow charts each partially showing a first modification of a process by the control unit 2 at the time of performing MSn analysis. The process shown in FIG. 6A can be performed at the time of selecting a MS2 precursor (step S103 in FIG. 4), and the process shown in FIG. 6B can be performed after the identification process based on the MS2 spectrum at the time of MS2 analysis (after step S106 in FIG. 4).
When peaks P23 and P27 adjacent with a mass difference in a predetermined mass-to-charge ratio Δmz (Yes in step S211) among a plurality of peaks P21, P22, P23 and P27 with the intensity or S/N ratio falling within the predetermined range L in the MS1 spectrum at the time of selecting a MS2 precursor ion, a MS2 precursor ion left after excluding one of the adjacent peaks is selected (step S212) as shown in FIG. 6A. At this time, the peak P27 in the lower mass range may be excluded as in the example in FIG. 5.
When as a result of the identification process, a component that cannot be identified exists in a plurality of ions detected as MS2 precursor ions (Yes in step S221), whether or not there is a peak P27′ having a mass-to-charge ratio identical to that of the peak excluded in step S212 in FIG. 6A is determined (step S222) as shown in FIG. 6B. When as a result, the measured MS2 spectrum has an ion peak P27′ corresponding to the peak P27 excluded from MS2 precursor candidates as a peak adjacent in terms of a predetermined mass difference Δmz at a mass-to-charge ratio of a known modified molecule (Yes in step S222), only an ion corresponding to the peak P27′ is dissociated among ions in the ion trap 12, which are dissociated at the time of measuring the MS2 spectrum. The dissociated ion is then subjected to mass spectrometry (MS3 analysis) with respect to the dissociated ions to measure a MS3 spectrum (step S203: MS3 measurement step).
Thus, in the modification in FIG. 6, when a fragment ion generated due to a neutral loss by a releasable modified molecule and an adducts exist in the MS1 spectrum, MS2 analysis can be performed while one of ion peaks adjacent with a mass difference Δmz in mass-to-charge ratio of an ion corresponding to a known neutral loss is excluded from precursor ions to be subjected to MS2 analysis. A situation can be hereby prevented in which a plurality of peptide-derived product ions sharing a partial structure are superimposed, so that it becomes difficult to analyze a structure of a part that is not shared.
FIG. 7 is a flow chart partially showing a second modification of a process by the control unit 2 at the time of performing MSn analysis. The process shown in FIG. 7 can be performed after the identification process based on the MS2 spectrum at the time of MS2 analysis (after step S106 in FIG. 4).
Specifically, when as a result of the identification process, a component that cannot be identified exists in a plurality of ions detected as MS2 precursor ions (Yes in step S301), the MSn measurement processing section 21 performs MS2 analysis again for an ion corresponding to the component that cannot be identified. That is, ions obtained by ionizing the sample are captured in the ion trap 12, and only an ion corresponding to the component that cannot be identified is left in the ion trap 12 and dissociated, and subjected to mass spectrometry to measure a MS2 spectrum again (step S302: MS2 remeasurement step). At this time, the MSn measurement processing section 21 functions as a MS2 remeasurement processing section.
An identification process is performed based on the measured MS2 spectrum to identify sample components (step S303: identification step). In the modification in FIG. 7, even when a component that cannot be identified exists in a plurality of ions detected as MS2 precursor ions due to a difference in product ion production efficiency between components, etc., a MS2 spectrum can be measured again for an ion corresponding to the component that cannot be identified. When identification is performed again based on the MS2 spectrum thus obtained, measurement can be performed while a difference in product ion production efficiency between components is taken into consideration.
Remeasurement of the MS2 spectrum may be performed under conditions identical to or different from those for the first measurement of the MS2 spectrum. For example, when conditions such as the cumulated number of laser irradiation to a sample and a laser intensity are changed, a sample that is hardly ionized may be properly identified. The processes in steps S301 to S303 in FIG. 7 may be repeatedly performed multiple times.
FIG. 8 is a flow chart partially showing a third modification of a process by the control unit 2 at the time of performing MSn analysis. The process shown in FIG. 8 can be performed after the identification process based on the MS2 spectrum at the time of MS2 analysis (after step S106 in FIG. 4).
Specifically, when as a result of the identification process, a component that cannot be identified exists in a plurality of ions detected as MS2 precursor ions (Yes in step S401), the on-target separation processing section 24 performs a process for subjecting a sample concentrated on the target 111 to on-target separation (step S402: on-target separation step). For the sample subjected to on-target separation, the MSn measurement processing section 21 performs MS1 analysis again to measure a MS1 spectrum (step S403: MS1 remeasurement step). At this time, the MSn measurement processing section 21 functions as a MS1 remeasurement processing section.
In the modification in FIG. 8, even when a component that cannot be identified exists in a plurality of ions detected as MS2 precursor ions, the component may be capable of being identified by performing MS1 analysis again for the sample subjected to on-target separation. Further, a component that does not appear as a peak in the MS1 spectrum before the sample is subjected to on-target separation may appear as a peak when the sample is subjected to on-target separation. Therefore, by performing MS1 analysis again for the sample subjected to on-target separation, a larger number of components can be identified.
After the process in FIG. 8 is performed, a next process can be started at step S102 in FIG. 4. The processes in steps S401 to S403 in FIG. 8 may be repeatedly performed multiple times.
In the embodiment described above, the mass spectrometer is a MALDI-IT-TOFMS. However, the present invention is not limited to the above-mentioned configuration, and for example, a configuration may be employed in which the ionization unit 11 ionizes a sample using an ionization method using laser irradiation, other than MALDI.
The mass spectrometer is not limited to the TOFMS 13, and a configuration may be employed in which mass spectrometry is performed using other mass spectrometers such as a magnetic sector type mass spectrometer, a quadrupole mass spectrometer and a Fourier transform ion cyclotron resonance mass spectrometer, or a configuration may be employed in which mass spectrometry is performed using the mass separation function of the ion trap 12 itself.

## Claims

1. An ion trap mass spectrometer in which ions obtained by ionizing a sample are captured in an ion trap, and the ions are dissociated and subjected to mass spectrometry to perform MSn analysis (n is an integer of 2 or greater), the ion trap mass spectrometer comprising:
a MS1 measurement processing section configured to measure a MS1 spectrum by performing mass spectrometry of the ionized sample;
a precursor ion detection processing section configured to detect, as MS2 precursor ions, ions corresponding to a plurality of peaks with the intensity or S/N ratio falling within a predetermined range, based on the MS1 spectrum; and
a MS2 measurement processing section configured to measure a MS2 spectrum by dissociation of a plurality of ions, which are detected as MS2 precursor ions, at a time in the ion trap and subjecting the ions to mass spectrometry.

2. The ion trap mass spectrometer according to claim 1, wherein the precursor ion detection processing section is configured to leave one of peaks adjacent with a predetermined mass-to-charge ratio as a mass difference among a plurality of ions detected as the MS2 precursor ions and exclude the rest.

3. The ion trap mass spectrometer according to claim 1, further comprising a MS3 measurement processing section configured to measure a MS3 spectrum by dissociation of an ion, which is positioned in the lower mass range with a predetermined mass-to-charge ratio or more as a mass difference from a mass-to-charge ratio of one of the MS2 precursor ions, among product ions obtained by measuring the MS2 spectrum, and subjecting the ion to mass spectrometry.

4. The ion trap mass spectrometer according to claim 1, further comprising a MS2 remeasurement processing section configured to perform a process by the MS2 measurement processing section again for an ion corresponding to a component that cannot be identified when a component that cannot be identified exists in a plurality of ions detected as the MS2 precursor ions.

5. The ion trap mass spectrometer according to claim 1, further comprising: an on-target separation processing section configured to perform a process in which a sample on a target is separated on the target when a component that cannot be identified exists in a plurality of ions detected as the MS2 precursor ions; and
a MS1 remeasurement processing section configured to perform a process by the MS1 measurement processing section again for the sample treated by the on-target separation processing section.

6. An ion trap mass spectrometry method in which ions obtained by ionizing a sample are captured in an ion trap, and the ions are dissociated and subjected to mass spectrometry to perform MS' analysis (n is an integer of 2 or greater), the method comprising:
a MS1 measurement step of measuring a MS1 spectrum by performing mass spectrometry of the ionized sample;
a precursor ion detection step of detecting, as MS2 precursor ions, ions corresponding to a plurality of peaks with the intensity or S/N ratio falling within a predetermined range, based on the MS1 spectrum; and
a MS2 measurement step of measuring a MS2 spectrum by dissociation of a plurality of ions, which are detected as MS2 precursor ions, in the ion trap and subjecting the ions to mass spectrometry.

7. The ion trap mass spectrometry method according to claim 6, wherein one of peaks adjacent with a predetermined mass-to-charge ratio as a mass difference among a plurality of ions detected as the MS2 precursor ions is left and the rest is excluded in the precursor ion detection step.

8. The ion trap mass spectrometry method according to claim 6, further comprising a MS3 measurement step of measuring a MS3 spectrum by dissociation of an ion, which is positioned in the lower mass range with a predetermined mass-to-charge ratio or more as a mass difference from a mass-to-charge ratio of one of the MS2 precursor ions, among product ions obtained by measuring the MS2 spectrum, and subjecting the ion to mass spectrometry.

9. The ion trap mass spectrometry method according to claim 6, further comprising a MS2 remeasurement step of performing a process by the MS2 measurement step again for an ion corresponding to a component that cannot be identified when a component that cannot be identified exists in a plurality of ions detected as the MS2 precursor ions.

10. The ion trap mass spectrometry method according to claim 6, further comprising: an on-target separation step of performing a process in which a sample on a target is separated on the target when a component that cannot be identified exists in a plurality of ions detected as the MS2 precursor ions; and
a MS1 remeasurement step of performing a process of the MS1 measurement step again for the sample treated by the on-target separation step.