Patent Application
20210242008
The present invention relates to a time-of-flight mass spectrometer (hereinafter, referred to as “TOFMS” if appropriate), and more particularly relates to a TOFMS that transports ions, derived from a sample component generated by an ion source, to an ion ejecting unit through an ion guide using a radio-frequency electric field and ejects the ions into a flight space from the ion ejecting unit for performing mass spectrometry. The TOFMS according to the present invention is particularly suitable for an orthogonal acceleration TOFMS (hereinafter, referred to as “OA-TOFMS” if appropriate).
A quadrupole-time-of-flight (hereinafter referred to as “Q-TOF”) mass spectrometer has been known as one type of mass spectrometer. A Q-TOF mass spectrometer disclosed in Patent Literature 1, for example, includes: an ion source that performs ionization by electrospray ionization (ESI); a quadrupole mass filter for selecting ions with a certain mass-to-charge ratio m/z; a collision cell that dissociates the selected ions by collision-induced dissociation (CID); and an OA-TOFMS including an orthogonal acceleration unit. The Q-TOF mass spectrometer dissociates ions derived from components in a sample, and acquires mass spectrum with high accuracy and high resolution for product ions generated through the dissociation of the ions derived from the components in the sample. Thus, MS/MS (=MS2) analysis is performed.
In the above Q-TOF mass spectrometer, an ion guide utilizing an ion focusing effect provided by a radio-frequency electric field is used, so that the ions generated under substantially atmospheric pressure can be efficiently transported to the quadrupole mass filter located in a vacuum chamber. The collision cell also incorporates an ion guide utilizing the ion focusing effect provided by a radio-frequency electric field. Furthermore, when the Q-TOF mass spectrometer performs normal mass spectrometry (what is known as MS1 analysis) without dissociating ions in the collision cell, instead of the MS/MS analysis, the quadrupole mass filter simply functions as the ion guide utilizing the ion focusing effect provided by a radio-frequency electric field.
As described above, when the normal mass spectrometry is performed with the Q-TOF mass spectrometer, ions generated by the ion source are introduced into the orthogonal acceleration unit of the OA-TOFMS via a plurality of ion guides, and the mass spectrometry is performed with the OA-TOFMS. In the ion guide utilizing a radio-frequency electric field, the ions travel while oscillating due to the interaction between the electric charge of the ions and the electric field. The magnitude of the oscillation depends on the mass-to-charge ratio of the ions and the magnitude (amplitude) of radio-frequency voltage applied to the ion guide. Therefore, a range of the mass-to-charge ratio of the ions that can pass through the ion guide depends on the magnitude of the radio-frequency voltage applied to the ion guide. Thus, if the amplitude of the radio-frequency voltage applied to the ion guide is fixed, the mass-to-charge ratio range of the ions that can pass through the ion guide is limited, meaning that a mass spectrum over a wide mass-to-charge ratio range is difficult to achieve.
In this respect, the following method has been known for obtaining a mass spectrum (MS1 spectrum) over a wide mass-to-charge ratio range. Specifically, measurement is executed for a plurality of times while changing the magnitude of the radio-frequency voltage applied to the ion guide, and mass spectra corresponding to different mass-to-charge ratio ranges obtained by the plurality of times of measurement are summarized.
Patent Literature 1: WO 2018/020600 A
Patent Literature 2: JP 2013-247000 A
However, the conventional mass spectrum summarizing processing method described above has a problem in that the intensity of peaks of a low mass-to-charge ratio region becomes much lower when the mass range is wide, compared with a case where the mass range is narrow. For example, as illustrated in
The present invention has been made to solve the problem described above, and an object of the present invention is to provide a TOFMS that repeats the measurement while changing the radio-frequency voltage applied to one or a plurality of ion guides and summarizes partial mass spectra obtained by the respective measurements to generate a mass spectrum of a wider mass-to-charge ratio range, with which lowering of the intensity of the peak in the low mass-to-charge ratio region due to a widened mass range can be suppressed.
The present inventors have elucidated the cause of the above problem in the conventional mass spectrum summarizing processing through experiments and studies. An ion guide that transports ions while focusing them by means of a radio-frequency electric field involves a phenomenon known as low-mass cut-off. This is a phenomenon in which ions with a low mass-to-charge ratio are less likely to be stably transported when a depth of a confining potential is increased for the purpose of more effective ion focusing.
When the measurement is repeated while changing the setting m/z as illustrated in
A time-of-flight mass spectrometer according to the present invention made for solving the problem described above and including an ion source for ionizing a sample component, a time-of-flight mass spectrometry unit including an ion ejecting unit for ejecting ions generated by the ion source or other ions derived from the generated ions into a flight space, and one or a plurality of ion transport optical elements for transporting the ions while focusing the ions by means of a radio-frequency electric field, the one or plurality of ion transport optical elements being provided between the ion source and the ion ejecting unit, further includes:
a) a voltage generating unit for applying radio-frequency voltage for forming a radio-frequency electric field to each of the one or plurality of ion transport optical elements;
b) a controlling unit for determining, in response to a mass range designated as an observation target, a radio-frequency voltage applied to each of the one or plurality of ion transport optical elements in each of a plurality of times of measurement using different measurement target mass-to-charge ratio ranges for acquiring a mass spectrum entirely covering a mass-to-charge ratio range of the mass range, and for controlling components to perform the plurality of times of measurement while changing the radio-frequency voltage applied to each of the one or plurality of ion transport optical elements by the voltage generating unit, the controlling unit determining the radio-frequency voltage for each of the plurality of times of measurement to maintain a ratio of times of measurement in which radio-frequency voltage is set so that ion passage efficiency in a low mass-to-charge ratio region is relatively high among the plurality of times of measurement to be substantially constant regardless of a width of the mass range; and
c) a mass spectrum summarizing processing unit for obtaining a mass spectrum corresponding to the mass range by summarizing mass spectrum data pieces obtained by the plurality of respective times of measurement under control by the controlling unit.
In the present invention, typically, a user designates a mass range as an observation target based on the purpose of the analysis or according to the type of an object of the analysis. When the mass range is designated, the controlling unit determines radio-frequency voltage applied to each ion transport optical element in each of a plurality of times of measurement with different mass-to-charge ratio ranges of a measurement target, for obtaining a mass spectrum entirely covering the mass-to-charge ratio range of the mass range. As described before, the mass-to-charge ratio range of ions that can be stably focused changes when the magnitude (amplitude) of the radio-frequency voltage changes. Thus, the radio-frequency voltage needs to be appropriately determined based on the mass-to-charge ratio range of the measurement target. The controlling unit determines the radio-frequency voltage to maintain, among a plurality of times of measurement, the ratio of times of measurement in which radio-frequency voltage is set so that ion passage efficiency in the low mass-to-charge ratio region is relatively high to be substantially the same between a case where the upper limit of the mass range is high and a case where the upper limit of the mass range is low.
As described above, in the conventional mass spectrum summarizing processing method, a higher upper limit of the mass range leads to a lower sensitivity in a low mass-to-charge region due to the low mass cutoff phenomenon, resulting in the region being underestimated. Thus, the intensity of ions in this region is less likely to be involved in the mass spectrum after the summarizing. On the other hand, in the present invention, as described above, when the upper limit of the mass range is high, that is, when the mass range is wide, the ratio of measurement in a state with a relatively high ion intensity in the low mass-to-charge ratio region (that is, not cut off by the low-mass cut-off phenomenon, or not significantly cut off by the low-mass cut-off phenomenon) is high. Thus, the ion intensity in the low mass-to-charge ratio region is more likely to be involved in the mass spectrum after the summarizing than in the conventional case. Thus, the deterioration of the sensitivity in the low mass-to-charge ratio region in the mass spectrum after the summarizing by the mass spectrum summarizing processing unit is suppressed.
It is in a sense appropriate for a user to perform a plurality of times of measurement with the setting m/z in the mass-to-charge ratio range of the mass range changed linearly, that is, at an equal mass-to-charge ratio interval as in
Thus, one preferable aspect of the present invention may be a configuration in which the controlling unit determines the radio-frequency voltage based on a table or a formula with which a relationship between a position on an axis between upper and lower limits of the mass range and the radio-frequency voltage is substantially the same between different mass ranges.
With this configuration, a change in the radio-frequency voltage becomes more moderate with a wider mass range, when a plurality of times of measurement are performed while changing the setting m/z by the same mass-to-charge ratio at a time within the mass-to-charge ratio range of the mass range. Thereby, a wide mass range results in a larger weight of the ion intensity in the low mass-to-charge ratio region, whereby the deterioration of sensitivity in the low mass-to-charge ratio region in the mass spectrum after summarizing can be suppressed.
Furthermore, the present invention is, for example, a Q-TOF mass spectrometer.
One embodiment of the present invention may be a configuration in which a quadrupole mass filter capable of selectively allowing ions with a certain mass-to-charge ratio to pass through and a collision cell for dissociating the ions are provided between the ion source and the ion ejecting unit, and the quadrupole mass filter operating without selecting the ions and an ion guide provided in the collision cell respectively serve as one of the one or plurality of ion transport optical elements.
Furthermore, in this configuration, one or a plurality of ion guides provided between the ion source and the quadrupole mass filter may serve as the one or plurality of ion transport optical elements.
With the present invention, the deterioration of the peak intensity in the low mass-to-charge ratio region, when the observation target mass range is widened, can be suppressed. This results in a higher similarity between peak patterns corresponding to the same mass-to-charge ratio range in the mass spectra with different mass ranges. Thus, comparison between the mass spectra obtained with different mass ranges is facilitated. Furthermore, a failure to find a peak or peaks to be observed in a low mass-to-charge ratio region can be prevented, or the content of a compound the peak of which is observed in the low mass-to-charge ratio region can be prevented from being underestimated.
A Q-TOF mass spectrometer, which is an embodiment of the present invention will be described with reference to the accompanying drawings.
The Q-TOF mass spectrometer of the present embodiment has a configuration of a multi-stage differential evacuation system with a chamber 1 containing an ionization chamber 2 at a substantially atmospheric atmosphere and a second analysis chamber 6 with the highest degree of vacuum, as well as a first intermediate vacuum chamber 3, a second intermediate vacuum chamber 4, and a first analysis chamber 5 with their degree of vacuum increasing in this order from the ionization chamber 2 to the second analysis chamber 6.
The ionization chamber 2 is provided with an electrospray ionization (ESI) spray 7 for performing ionization by ESI. When a sample solution including a target compound is supplied to the ESI spray 7, it is nebulized in charged droplets from the distal end of the spray 7. As the charged droplets split and the solvent evaporates, ions derived from the target compound are generated. The ionization is not limited to this, and other types of ionization such as atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) may be employed.
The various ions generated in the ionization chamber 2 are sent to the first intermediate vacuum chamber 3 through a heating capillary 8, focused by an array type ion guide 9 provided in the first intermediate vacuum chamber 3, and are sent to the second intermediate vacuum chamber 4 through a skimmer 10. The ions are further focused by a multipole type ion guide 11 provided in the second intermediate vacuum chamber 4, and are then sent to the first analysis chamber 5. The first analysis chamber 5 contains a quadrupole mass filter 12 and a collision cell 13 incorporating an ion guide 14 of a multipole type.
The various ions derived from the sample are introduced into the quadrupole mass filter 12. When MS/MS analysis is performed, ions having a certain mass-to-charge ratio corresponding to the voltage applied to the quadrupole mass filter 12 pass through the quadrupole mass filter 12. The ions are introduced into the collision cell 13 as precursor ions. The precursor ions are dissociated by coming into contact with the collision gas supplied into the collision cell 13. Thus, various product ions are generated. On the other hand, at the time of normal mass spectrometry (MS1 analysis) not involving the ion dissociation, the ions derived from the sample component pass through the quadrupole mass filter 12 almost intact (still, the mass-to-charge ratio range enabling the passage is actually limited as described later) to be introduced into the collision cell 13. Then, the ions come into contact with the collision gas supplied into the collision cell 13 so that their energy is reduced (i.e. cooled).
In the collision cell 13, the ions derived from the sample components are transported while being focused. The ions ejected from the collision cell 13 are introduced into the second analysis chamber 6 through an ion passage port 15 while being guided by an ion transport optical system 16. The second analysis chamber 6 contains an orthogonal acceleration unit 17 serving as an ion ejection portion, a flight space 18 provided with a reflector 19, and an ion detector 20. The ions introduced into the orthogonal acceleration unit 17 in the X axis direction along an ion optical axis C are accelerated in the Z axis direction at a predetermined timing to be ejected. The ejected ions fly freely in the flight space 18 as indicated by the two-dot chain line in
A data processing unit 30 includes functional blocks of a data collection unit 31 and a mass spectrum summarizing processing unit 32. The data collection unit 31 receives detection signal from the ion detector 20 and digitizes and accumulates the signal. The mass spectrum summarizing processing unit 32 generates a time-of-flight spectrum based on the collected data, and converts the time of flight into a mass-to-charge ratio to generate a mass spectrum. The mass spectrum summarizing processing unit 32 summarizes partial mass spectra each obtained by one of a plurality of times of measurement, as described later, to generate a mass spectrum of a wider mass-to-charge ratio range.
First to fourth voltage generating units 21 to 24 apply predetermined radio-frequency voltage respectively to the array type ion guide 9, the multipole type ion guide 11, the quadrupole mass filter 12, and the ion guide 14. Furthermore, appropriate DC voltages are also applied to components including these electrodes, but the description of a voltage generating unit for generating such voltages is omitted. Furthermore, predetermined DC voltages are applied other components (such as, for example, the heating capillary 8 and the skimmer 10) but the description of the other components is omitted. Thus, in this document, a description is given only on the components for applying the radio-frequency voltage to ion optical elements having the function of transporting ions while focusing them by means of a radio-frequency electric field, related to the characteristic operation of the apparatus of the present embodiment.
The first to the fourth voltage generating units 21 to 24 are each controlled by an analysis controlling unit 40. Functional blocks of the analysis controlling unit 40 include a normal analysis execution controlling unit 41 and a radio-frequency voltage scanning table storage unit 42. An input unit 44 receives an operation by the user, and a main controlling unit 43 is in charge of overall control of a user interface and the apparatus as a whole.
The analysis controlling unit 40, the main controlling unit 43, and the data processing unit 30 are entirely or partially implemented with a personal computer, which is a physical entity, executing dedicated control/processing software installed in the computer.
In the Q-TOF mass spectrometer of the present embodiment, the MS/MS analysis can be performed by dissociating the ions in the collision cell 13. Furthermore, as described above, the MS1 analysis without the dissociation of the ions in the collision cell 13 can also be performed. The Q-TOF mass spectrometer of the present embodiment performs characteristic control and processing when performing the normal MS1 analysis. Operations in the characteristic control and processing operations will be described below with reference to
When the normal analysis is performed, the quadrupole mass filter 12 transports the ions to the subsequent stage while focusing the ions by means of a radio-frequency electric field. Thus, like the ion guides 9, 11 and 14, the quadrupole mass filter 12 substantially serves as an ion guide using a radio-frequency electric field. Therefore, the ions derived from the sample component generated by the ion source pass through the four ion guides (the array type ion guide 9, the multipole type ion guide 11, the quadrupole mass filter 12, and the ion guide 14) to be introduced into the orthogonal acceleration unit 17. Any ion guide that uses a radio-frequency electric field imposes a limit to the mass-to-charge ratio range of the ions passing through. For the sake of simplicity of explanation, the array type ion guide 9 and the multipole type ion guide 11 are collectively regarded as one ion guide A, the quadrupole mass filter 12 is regarded as an ion guide B, and the ion guide 14 is regarded as an ion guide C.
Now, a case will be described where a summarized mass spectrum is acquired for two mass ranges with m/z 10 to 600 and with m/z 10 to 2000. As illustrated in
On the other hand, in the radio-frequency voltage scanning table in the Q-TOF mass spectrometer of the present embodiment, as illustrated in
Before the normal mass spectrometry is implemented, the user designates the observation target mass range using the input unit 44. Although there are only two types of mass ranges in
Thus, the normal analysis execution controlling unit 41 sets the setting m/z to be increased by a predetermined mass-to-charge ratio (for example, m/z 50 at a time as in the example illustrated in
Ions derived from sample components are measured while the radio-frequency voltage corresponding to one setting m/z being applied to each of the ion guides A, B, and C, and the data collection unit 31 stores mass spectrum data obtained by the measurement. The normal analysis execution controlling unit 41 repeats the same measurement while changing the setting m/z step by step, that is, while changing the radio-frequency voltage applied to the ion guides A, B, and C. Then, measurement is performed for each of a plurality of settings m/z over the entire mass-to-charge ratio range of the mass range, and then the measurement is terminated.
When the setting m/z is changed by m/z 50 at a time as illustrated in
As described above, with the Q-TOF mass spectrometer according to the present embodiment, a mass spectrum with sufficiently high sensitivity in the low mass-to-charge ratio region can be obtained even when the mass range is wide. Furthermore, the peak pattern corresponding to the same mass-to-charge ratio range is less likely to be different between cases where the mass range is wide and narrow. This facilitates comparison between mass spectra obtained with different mass ranges.
It should be noted that the numerical values used in the above embodiment, such as the numerical values in the table illustrated in
Further, the above described embodiment is about applying the present invention to the Q-TOF mass spectrometer capable of implementing MS/MS analysis. The present invention can also be applied to mass spectrometers such as an OA-TOFMS capable of implementing normal mass spectrometry only.
The embodiments described above are examples of the present invention, and thus modification, correction, addition, and the like to the embodiments without departing from the gist of the present invention are apparently included in the scope of the claims of the present application.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/018536 | 5/14/2018 | WO | 00 |
