JP2007121178A - Mass analyzer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mass analyzer capable of enhancing the analyzing sensitivity or precision of target product ions by enhancing the selectivity of precursor ions. <P>SOLUTION: When a CID gas is introduced into an ion trap, the frequency of an exciting signal is determined in consideration of that the secular vibration frequency of the ions trapped by the collision with the CID gas lowers slightly. That is, the gas pressure in a vacuum chamber 1 in which the ion trap is mounted is detected by a pressure sensor 4 and a control part 20 corrects the frequency of the exciting signal on the basis of the frequency shift quantity data calculated corresponding to this gas pressure. Then, when the secular vibration frequency of the ions in the ion trap 10 lowers by an increase in the introducing amount of the CID gas, the detected gas pressure rises and the frequency of the exciting signal is lowered. Accordingly, target ions are accurately excited to cause dissociation with high frequency. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a mass spectrometer equipped with an ion trap, and more particularly, to a mass spectrometer that selectively collides and induces dissociation of ions trapped in an ion trap and mass-analyzes product ions generated thereby.

  As one of mass spectrometers, an ion trap that captures and accumulates ions by an electric field formed in a space surrounded by a plurality of electrodes, and a mass that separates and detects ions discharged from the ion trap for each mass number A mass spectrometer that combines an analyzer and an analyzer is known. As the mass spectrometer, a time-of-flight mass spectrometer or a quadrupole mass spectrometer is often used. In addition, as the ion trap, a three-dimensional quadrupole ion trap composed of one annular ring electrode and a pair of end cap electrodes arranged to face each other with the ring electrode interposed therebetween is used. Often.

In recent years, substances to be analyzed by mass spectrometry tend to be increasingly diversified, and in particular, there is an increasing demand for analysis of substances having a complicated chemical structure and a large mass number. In general, a mass spectrometer has an upper limit on the mass number that can be analyzed, and it is difficult to analyze a substance having an extremely large mass number. Therefore, the importance of MS ⁿ analysis in which molecular ions are dissociated and decomposed into ions having a smaller mass number and each dissociated ion is subjected to mass spectrometry is further increased.

In the mass spectrometer described above, the dissociation of ions trapped in the ion trap is promoted by introducing a collision-induced dissociation (CID) gas into the ion trap, and the product thus produced ^MSn analysis is possible by discharging various ions including ions from the ion trap and performing mass analysis. As a method for dissociating a target single mass number ion in an ion trap, Patent Document 1 discloses a specification corresponding to the frequency by applying an AC voltage having a specific frequency to the end cap electrode. It is described that only ions having a mass number are selectively resonantly oscillated, and the ions oscillated by resonance are dissociated by collision with a CID gas introduced into an ion trap.

  Since the relationship between the mass number of the ion to be selected as the precursor ion and the frequency of the excitation signal to be applied to the end cap electrode in order to cause the ion to oscillate resonantly can be obtained in advance by calculation, In this case, the frequency of the excitation signal should be determined in accordance with the mass number of the target ion using the relationship between the mass number and the frequency thus obtained in advance. However, in practice, even if the excitation signal of the single frequency component obtained as described above is applied to the end cap electrode, ions of the target mass number are not sufficiently excited, and the mass number is slightly smaller than that. In some cases, undesired ions that have shifted may be excited. As a result, the signal intensity of the product ions generated by the dissociation of the target ions cannot be sufficiently obtained, and the analysis sensitivity and analysis accuracy are lowered.

Japanese Patent No. 3470671 (paragraph 0003 and FIG. 6)

  The present invention has been made to solve the above-mentioned problems, and the object of the present invention is to select a precursor ion when it is desired to dissociate a single mass number ion as a precursor ion in an ion trap. An object of the present invention is to provide a mass spectrometer capable of enhancing the analysis sensitivity and analysis accuracy of target product ions by improving the properties.

  According to the study of the present inventor, the main cause of the deviation of the mass number of ions that are resonantly oscillated with respect to the excitation signal of a certain single frequency component as described above is the CID gas and ions introduced into the ion trap. The secular frequency of various ions trapped in the ion trap and oscillating with the secular frequency corresponding to the mass number of each ion is slightly shifted to the lower side due to resistance due to collision with It is believed that there is. Since the degree of resistance due to the collision with the CID gas depends on the molecular density of the CID gas, the shift amount of the mass number is influenced by the introduction flow rate of the CID gas and the evacuation ability of the vacuum atmosphere in which the ion trap is arranged. Receive. Therefore, the mass spectrometer according to the present invention, which has been made to solve the above-mentioned problems, changes the frequency of the excitation signal in consideration of the molecular density of the CID gas in the ion trap, that is, the degree of vacuum. This is to correct the deviation of the number.

That is, the present invention includes a mass spectrometer that includes an ion trap that traps ions in a space surrounded by a plurality of electrodes, dissociates ions in the ion trap, and mass-analyzes the ions generated thereby.
a) generating at least an excitation signal having a single frequency component and a predetermined amplitude so as to selectively resonate and vibrate ions having a target mass number in the ion trap; Excitation signal generating means applied to one electrode;
b) a gas introduction means for introducing a collision-induced dissociation gas (CID gas) into the ion trap;
c) a gas pressure monitoring means for detecting or estimating a gas pressure in the ion trap into which the collision-induced dissociation gas is introduced;
d) correction processing means for correcting the frequency component of the excitation signal according to the gas pressure obtained by the gas pressure monitoring means;
It is characterized by having.

  For example, when the ion trap is composed of a ring electrode and a pair of end cap electrodes as described above, the excitation signal is usually applied to the pair of end cap electrodes.

  As described above, the change in the secular frequency of various ions trapped in the ion trap is mainly due to the collision with the CID gas. Therefore, the higher the density of the CID gas, that is, the gas pressure monitoring means. The higher the gas pressure, the greater the decrease in secular frequency. When an ion having an arbitrary single mass number is selectively resonantly oscillated, it is necessary to form an electric field having a frequency component that matches the secular frequency. Therefore, the correction processing means increases the excitation signal as the gas pressure increases. The frequency component is corrected to be lower than the theoretically obtained value. As a result, the deviation between the resonance frequency of the ion to be selected and the frequency of the excitation signal is almost eliminated, and the target ion is resonantly oscillated and dissociated by collision with the CID gas.

  As described above, according to the mass spectrometer according to the present invention, when ions of a single mass number are selectively dissociated and product ions are analyzed, the target ions are dissociated with high selectivity in the ion trap. In addition, since the dissociation of other undesired ions is suppressed, the amount of product ions to be analyzed is increased, and the analysis sensitivity and analysis accuracy can be improved.

  Hereinafter, a mass spectrometer according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of the mass spectrometer of the present embodiment.

  Inside the vacuum chamber 1 evacuated by an evacuation means (not shown), one annular ring electrode 11 whose inner surface has a rotating one-leaf hyperboloid shape is sandwiched between them (left and right in FIG. 1). An ion trap 10 constituted by a pair of end cap electrodes 12 and 13 which are provided so as to face each other and whose inner side surface has a rotating two-leaf hyperboloid shape is disposed. An ion trap region for capturing ions is formed in a space surrounded by the electrodes 11, 12, and 23.

  An ion source 2 such as MALDI is disposed outside the entrance formed in the entrance end cap electrode 12 of the ion trap 10, while the exit end cap 13 is provided in the ion trap 10. For example, a time-of-flight mass spectrometer 3 is disposed outside the emission port, and a detection signal from a detector of the mass spectrometer 3 is input to the data processing unit 24. The inside of the ion trap 10 is supplied with CID gas from a gas supply source 6 through a gas supply pipe 5. The gas pressure (vacuum degree) in the vacuum chamber 1 is a pressure sensor (vacuum gauge). ) 4.

  A main RF voltage generator 14 is connected to the ring electrode 11, and an auxiliary RF voltage generator 15 is connected to the end cap electrodes 12 and 13. The main RF voltage generation unit 14 and the auxiliary RF voltage generation unit 15 are controlled by a control signal supplied from the control unit 20 so as to generate an AC voltage having a predetermined frequency and a predetermined amplitude, respectively. The control unit 20 includes a CPU, a ROM, a RAM, and the like, and a mass number-frequency data table 22 and a gas pressure-frequency correction data table 23 are connected to select ions as will be described later. Yes. The mass number-frequency data table 22 stores data indicating correspondence between the mass number of ions excited in the ion trap 10 and the frequency of the excitation signal. For example, when the mass number is input, the frequency data of the excitation signal is stored. Can be derived. On the other hand, the gas pressure-frequency correction data table 23 stores data indicating the correspondence between the gas pressure in the vacuum chamber 1 and the displacement amount of the excitation signal. For example, when the gas pressure is input, the frequency of the excitation signal is stored. Deviation data can be derived. Such data can be obtained in advance by theoretical calculation, but may be data obtained experimentally. Further, as long as a correction value corresponding to the pressure value can be derived, data can be stored in an appropriate format such as a calculation formula instead of a table format.

  In this mass spectrometer, a case will be described where ions having a specific single mass number are dissociated and product ions generated thereby are subjected to mass analysis. When the operator inputs and sets the mass number of the precursor ion to be analyzed through the input unit 21, the control unit 20 obtains the frequency of the excitation signal corresponding to the mass number of the precursor ion from the mass number-frequency data table 22. The frequency at this time is based on the condition that the degree of vacuum in the vacuum chamber 1 is assumed to be extremely high, or when the gas pressure in the vacuum chamber 1 is under a predetermined reference condition.

  When the analysis is started, a quadrupole electric field is first formed in the space inside the ion trap 10 by the voltage applied from the main RF voltage generator 14 to the ring electrode 11 under the control of the controller 20, and the ion source 2, various ions generated from the sample to be analyzed are confined in the ion trap region. At this time, ions having various mass numbers in the ion trap region vibrate with their own specific frequencies, that is, secular frequencies.

  Next, the control unit 20 sends the frequency information of the previously obtained excitation signal to the auxiliary RF voltage generation unit 15, and the auxiliary RF voltage generation unit 15 generates an excitation signal of this single frequency component to generate the end cap electrode 12. , 13. At the same time or at the same time, the valve 7 is opened to start introducing the CID gas into the ion trap 10. Excitation may be started after CID gas introduction.

  Due to the alternating electric field formed in the ion trap 10 by the excitation signal applied to the end cap electrodes 12 and 13, ions having that frequency as the resonance frequency start to resonate. When there is no influence of CID gas, ions excited at this time are ions having a mass number input and set by the operator. However, when the CID gas is introduced, the ions trapped in the ion trap 10 collide with the CID gas and receive resistance, so that not only the type of ions but the secular frequency decreases slightly as a whole. Therefore, the control unit 20 monitors the gas pressure in the vacuum chamber 1 by the pressure sensor 4 and refers to the gas pressure-frequency correction data table 23 to obtain frequency shift data corresponding to the gas pressure at that time. Even if the evacuation capacity is the same, the gas pressure in the vacuum chamber 1 is relatively high if the introduction flow rate of the CID gas is large, and conversely, if the introduction flow rate of the CID gas is small, the gas pressure in the vacuum chamber 1 is relatively low. . That is, the detected value of the gas pressure by the pressure sensor 4 reflects the density of the CID gas in the ion trap 10.

  When the control unit 20 obtains the frequency shift data, the control unit 20 corrects the frequency information of the excitation signal sent to the auxiliary RF voltage generation unit 15 based on the data. Specifically, as shown in FIG. 5, the shift is made to the low frequency side so that the shift amount Δf from the original frequency becomes larger as the gas pressure is higher. As a result, the frequency of the electric field formed in the ion trap 10 is lowered by the excitation signal applied to the end cap electrodes 12 and 13, but the amount of reduction is the resonance frequency of the target ion due to the collision with the CID gas as described above. It becomes almost the same as the deviation. Accordingly, target ions are selectively excited in the ion trap 10 and are efficiently dissociated by vigorously colliding with the CID gas. For example, even when the gas pressure in the vacuum chamber 1 fluctuates due to fluctuations in the introduction flow rate of CID gas, the frequency of the excitation signal is corrected from moment to moment according to the gas pressure detected by the pressure sensor 4, The target ion having a single mass number can be excited and dissociated with high efficiency.

  Since various product ions generated by the collision-induced dissociation are also held in the ion trap 10, after the target ions are excited for a predetermined time as described above to promote the dissociation, the ring electrode 11, end The voltage applied to the cap electrodes 12 and 13 is switched to discharge the ions accumulated in the ion trap 10 and introduce them into the time-of-flight mass spectrometer 3. Then, various ions including product ions are detected separately for each mass number by the time-of-flight mass spectrometer 3, and a mass spectrum is created by the data processing unit 24. Although the ion trap 10 itself has a mass separation function, generally, the mass number resolution of the ion trap is not so good. Therefore, it is better to use an external mass analyzer for analysis requiring high mass resolution.

  In the above configuration, the mass number resolution of ions excited in the ion trap 10 greatly depends on the amplitude of the excitation signal applied to the end cap electrodes 12 and 13. That is, if the amplitude of the excitation signal is large, not only ions having a desired mass number but also ions having adjacent mass numbers are likely to be excited simultaneously. On the other hand, if the amplitude of the excitation signal is small, ions having adjacent mass numbers are not excited, but ions having a desired mass number may not be sufficiently excited. Therefore, here, due to collision-induced dissociation, the intensity of the peak P1 of the target single mass number precursor ion is attenuated by 50% or more, and the intensity of the peaks P2 and P3 of ions having adjacent mass numbers is 80% or more. The amplitude of the excitation signal is adjusted so as to remain (see FIG. 4).

  An example of the actual measurement of the mass spectrum acquired by the above apparatus is shown in FIG. This example shows a change in mass spectrum when a single mass number (m / z = 1552.8) peak is selectively excited using a Glu-Fibrinopeptide sample, (a) Is the state before applying the excitation signal, and (b) is the result after applying the excitation signal. It can be seen that the peak intensity of the corresponding precursor ion is greatly reduced because only ions having a single mass number are selectively excited by application of the excitation signal and lost due to collision-induced dissociation. By selectively exciting only a single mass number ion species in this way, as shown in FIG. 3, several fragment ion species generated by collision-induced dissociation of this ion species can be observed. . The molecular structure of the target molecular ion selected as the precursor ion can be analyzed from the mass number of the fragment ion species and its peak intensity.

  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 the configuration of the above embodiment, the ion source is provided outside the ion trap, but ionization can also be performed inside the ion trap. Further, as described above, an ion trap may be used as the mass analyzer of the mass spectrometer.

1 is an overall configuration diagram of a mass spectrometer according to an embodiment of the present invention. The figure which shows the example of an actual measurement of the mass spectrum acquired in the mass spectrometer of a present Example. The figure which shows the example of the mass spectrum of the fragment ion seed | species produced | generated by excitation and collision induced dissociation of a precursor ion. The schematic diagram which shows the change state of the peak intensity of the target precursor ion at the time of adjusting the amplitude of an excitation signal, and the peak intensity of an adjacent ion. The schematic diagram for demonstrating the operation | movement of the frequency shift of the excitation signal according to the detection result of gas pressure.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Vacuum chamber 2 ... Ion source 3 ... Time-of-flight mass spectrometer 4 ... Pressure sensor 5 ... Gas supply pipe 6 ... Gas supply source 7 ... Valve 10 ... Ion trap 11 ... Ring electrodes 12, 13 ... End cap electrode 14 ... Main RF voltage generator 15 ... auxiliary RF voltage generator 20 ... control unit 21 ... input unit 22 ... mass number-frequency data table 23 ... gas pressure-frequency correction data table 24 ... data processing unit

Claims (1)

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) generating at least one excitation signal having a single frequency component and a predetermined amplitude so as to selectively resonate and vibrate ions of a target mass number in the ion trap, and at least configure the ion trap Excitation signal generating means applied to one electrode;
b) a gas introduction means for introducing a gas for collision-induced dissociation into the ion trap;
c) a gas pressure monitoring means for detecting or estimating a gas pressure in the ion trap into which the collision-induced dissociation gas is introduced;
d) correction processing means for correcting the frequency component of the excitation signal according to the gas pressure obtained by the gas pressure monitoring means;
A mass spectrometer comprising:

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