JP5445219B2 - Time-of-flight mass spectrometer - Google Patents

Description

  The present invention relates to a time-of-flight mass spectrometer, and more particularly to an ion source using an ionization method such as a matrix-assisted laser desorption ionization method in a time-of-flight mass spectrometer.

  In general, a time-of-flight mass spectrometer (hereinafter referred to as “TOFMS”) introduces ions accelerated by an electric field into a flight space that does not have an electric field and a magnetic field to allow free flight and reach the detector. According to the above, various ions are separated for each mass to charge ratio (m / z). In order to increase the mass resolution in TOFMS, it is necessary to increase the flight distance. Therefore, in addition to the linear type structure in which ions fly straight, a reflectron that makes ions fly back using an electric or magnetic field. A configuration of a mold and a configuration of a revolving type in which substantially the same closed track is revolved a plurality of times are also known.

  As an ion source for TOFMS, a MALDI ion source based on a matrix-assisted laser desorption / ionization (MALDI) method is widely used. In the MALDI method, a sample is prepared by, for example, mixing a solution of a substance to be measured with a matrix solution, further mixing another ionization aid if necessary, and applying it on a sample plate, and removing the solvent. The sample prepared in this way is in a state in which the substance to be measured is almost uniformly mixed with a large amount of matrix. When this sample is irradiated with laser light, the matrix absorbs the energy of the laser light and converts it into thermal energy. At this time, a part of the matrix is rapidly heated and vaporized together with the measurement target substance, and the measurement target substance is ionized in the process.

  In TOFMS using a MALDI ion source, as described above, various ions generated from a sample by laser light irradiation are extracted from the vicinity of the sample by the action of an electric field, accelerated, and sent into flight space. In order to obtain high mass resolution, it is necessary that the initial velocities of ions of the same species (having the same mass-to-charge ratio) are aligned when ions are introduced into the flight space. However, in the MALDI ion source, in general, the initial energy variation of the ions at the time of the ion generation is large, so that the initial velocity variation becomes large and the time convergence is deteriorated. Therefore, in order to avoid this problem, a technique called a delayed extraction method has been generally used (see Patent Document 1, Patent Document 2, etc.).

  FIG. 9 is a diagram for explaining an ion extraction operation by the delayed extraction method. As shown in FIG. 9A, the sample 2 mixed with the matrix is held on a conductive sample plate 1, and the sample 2 is irradiated with laser light for ionization. Ions are extracted and accelerated from the vicinity of the sample 2 by the action of the electric field formed by the voltage applied to the extraction electrode 3 and the base electrode 4c arranged facing the sample plate 1, and are sent to a flight space (not shown). It is done.

  More specifically, when the sample 2 is irradiated with laser light, the same voltage VE is applied to the sample plate 1 and the extraction electrode 3, and a predetermined base voltage VB is applied to the base electrode 4c. In general, VB = 0 (that is, ground potential), and it is assumed here that VB = 0. As a result, the potential distribution on the ion optical axis C is as shown in FIG. That is, since there is no potential gradient in the extraction region between the sample plate 1 and the extraction electrode 3 (substantially no electric field), ions generated from the sample 2 by laser light irradiation are not accelerated. For this reason, since ions having a large initial energy at the time of ion generation move away from the sample 2, when a predetermined time has elapsed since the generation of ions, ions having a large initial energy are closer to the extraction electrode 3 regardless of the mass-to-charge ratio. Exists.

  After a certain delay time (usually about several tens to several hundreds nsec) has elapsed since the laser light irradiation, the voltage applied to the sample plate 1 is increased stepwise from VE to VS. As a result, as shown in FIG. 9C, an electric field having a large downwardly inclined potential gradient from the sample plate 1 toward the extraction electrode 3 is formed. By this electric field, various ions existing in the extraction region are accelerated all at once. At this time, the ions closer to the sample plate 1, that is, the ions with lower initial energy, have a higher accelerating voltage, so the applied kinetic energy increases. Therefore, even for ions of the same type, the smaller the initial energy at the time of ion generation, the higher the speed, the higher the speed of the initial energy that is advanced during the flight, Catch up and finally reach the detector almost simultaneously. In this way, the influence of variations in initial energy in the same kind of ions is eliminated, and high time convergence can be achieved.

  However, these conventional delayed withdrawal methods have the following problems. The correction of the initial energy variation as described above is achieved by correcting the kinetic energy through the change in potential energy of each ion. The average value of the initial velocity (or initial energy) of ions generated from the sample 2 by laser light irradiation is substantially constant without depending on the mass-to-charge ratio. Therefore, the energy required for correction is proportional to the mass-to-charge ratio, and the voltage value required for correction (potential difference ΔV from VE in FIG. 9C) also depends on the mass-to-charge ratio. On the other hand, ions are generated in a very small space near the surface of the sample 2, and an electric field does not act on the space during the free flight period until the voltage is increased during execution of delayed extraction. The spatial distribution of ions when an acceleration voltage is applied later is independent of the mass-to-charge ratio.

  That is, in order to increase the time convergence of the same type of ions as much as possible in order to increase the mass resolution, that is, to appropriately correct the initial energy variation, an appropriate acceleration voltage (the above potential difference ΔV) is applied to the ions for each mass-to-charge ratio. It is necessary to apply. However, as described above, since the spatial distribution of ions when an acceleration voltage is applied does not depend on the mass-to-charge ratio, it is appropriate for ion species having a certain mass-to-charge ratio when the acceleration voltage is set to a certain value. Although correction can be performed, sufficient correction may not be performed for ion species having other mass-to-charge ratios. For this reason, the mass-to-charge ratio range in which the mass resolution is improved by the conventional delayed extraction method is considerably limited, and there is a problem that it is difficult to improve the mass resolution over a wide mass-to-charge ratio range to be measured. .

  Here, although the MALDI ion source is taken as an example as an ion source, an ion source by another ionization method used as an ion source of TOFMS, for example, a laser desorption ionization (LDI) method without using a matrix, Similar problems also occur in secondary ion mass spectrometry (SIMS), desorption electrospray ionization (DESI), plasma desorption ionization (PDI), and the like.

Japanese Patent Laid-Open No. 11-185697 ([0005]-[0008] and FIG. 7) JP 2009-52994 A ([0003]-[0004] and FIG. 4)

  The present invention has been made to solve the above-mentioned problems, and in a time-of-flight mass spectrometer equipped with an ion source that extracts and accelerates ions generated from a sample using a delayed extraction method, a wide mass-to-charge ratio range is provided. Accordingly, it is an object of the present invention to improve mass resolution by appropriately correcting variations in the initial energy of ions.

A first invention made to solve the above problems is a time-of-flight type in which ions generated from a sample are accelerated and introduced into a flight space, and ions are separated and detected in accordance with the mass-to-charge ratio in the flight space. In the mass spectrometer,
a) an extraction electrode disposed at a predetermined distance from the sample holding unit for holding the sample, and forming an electric field for extracting and accelerating ions from the sample surface in a space between the sample holding unit;
b) The spatial distribution of ions in the space between the sample surface and the extraction electrode when a predetermined delay time has elapsed from the start of ion generation, the closer the ions with a smaller mass-to-charge ratio, the closer to the extraction electrode. so that things, during the period from the ion generation starting time point to the predetermined delay time has elapsed, to form an electric field Before moving according to their mass to charge ratio ions toward the extraction electrode from the sample surface , than the potential of the extraction electrode potential of the sample holder, keeping high as the first potential difference corresponding to the predetermined delay time and the predetermined distance, the point in time and thereafter passed the predetermined delay time is The potential of the sample holder is set so that ions in the space between the sample holder and the extraction electrode are accelerated all at once and an electric field passing through the extraction electrode is formed. Serial than the potential of the extraction electrode, so as to maintain high as large second potential difference than the first potential difference, the potential setting means for setting the potential of the sample holder and the extraction electrode,
It is characterized by having.

  Here, as an ionization method for generating ions from a sample, a method using a laser beam such as MALDI or LDI, a method using an ion beam such as SIMS, a method using a charged spray flow such as DESI, or PDI A method using plasma such as is conceivable. The same applies to the second invention described later.

  In the conventional general delayed extraction method, the sample holding unit and the extraction electrode are maintained at substantially the same potential during a period from when the ion generation starts until a predetermined delay time elapses, and the sample holding unit and the extraction electrode A substantial electric field is not formed in the space between. For this reason, ions generated from the sample diffuse freely according to the initial energy. On the other hand, in the time-of-flight mass spectrometer according to the first invention, an electric field is formed in the space between the sample holder and the extraction electrode at the time of starting the ion generation to extract ions from the sample surface in the direction of the extraction electrode. ing. However, the potential difference (first potential difference) between the sample holder and the extraction electrode at this time is not so large as to accelerate all the generated ions simultaneously and with a large acceleration, and is formed by the potential difference. Various ions slowly move from the sample surface toward the extraction electrode by the action of the electric field.

  Since the ion velocity is inversely proportional to the size under a certain electric field, the smaller (generally, the mass to charge ratio) ions are closer to the extraction electrode, and conversely, the larger ions are present closer to the sample. Of course, the initial energy of each ion varies depending on the mass-to-charge ratio, and the moving speed is affected by this initial energy. For this reason, when a predetermined delay time elapses, each ion does not have an orderly distribution according to the mass-to-charge ratio, but has a spatial distribution depending on the mass-to-charge ratio as compared with a case where there is no electric field. That is, the spatial spread of ions having the same mass-to-charge ratio is reduced. When the potential difference between the sample holder and the extraction electrode is increased to accelerate the ions all at once after a predetermined delay time has elapsed, the same ion species exists at a relatively close position in space, so the acceleration is approximately the same. A voltage is given. Therefore, it is possible to give a more appropriate acceleration voltage corresponding to the mass-to-charge ratio to various ions, and the mass resolution can be improved over a wide mass-to-charge ratio range.

  In addition, among ions having the same mass-to-charge ratio, the smaller the initial energy, the closer to the sample holding portion, so that a relatively large acceleration voltage is applied. Therefore, it is possible to sufficiently obtain the effect of correcting the initial energy variation by the delayed extraction method.

A second invention made to solve the above problem is a time of flight in which ions generated from a sample are accelerated and introduced into a flight space, and ions are separated and detected in accordance with the mass-to-charge ratio in the flight space. Type mass spectrometer
a) an extraction electrode disposed at a predetermined distance from the sample holding unit for holding the sample, and forming an electric field for extracting and accelerating ions from the sample surface in a space between the sample holding unit;
b) After the start of ion generation, the gradient of the potential gradient that draws ions in the direction from the sample holder to the extraction electrode in the space between the sample holder and the extraction electrode has a predetermined same or different time. A potential setting means for stepwise increasing the relative potential of the sample holder with respect to the potential of the extraction electrode so that an electric field that increases stepwise is formed each time
It is characterized by having.

  In the time-of-flight mass spectrometer according to the second invention, the potential of the sample holder and the potential of the extraction electrode at the start of ion generation may be substantially the same, or the sample is held as in the first invention. The potential of the part may be set higher than the potential of the extraction electrode.

  In the time-of-flight mass spectrometer according to the second invention, the relative potential of the sample holder rises when a certain delay time elapses from the start of ion generation, and an electric field having a potential gradient is generated between the sample holder and the extraction electrode. Are formed in the space between the two, the ions existing in the space are accelerated and start moving toward the extraction electrode (first acceleration operation). At this time, since ions having a smaller mass-to-charge ratio are more likely to move, ions having a smaller mass-to-charge ratio become ions having a larger mass-to-charge ratio even when the spatial distribution of the ion immediately before that is not dependent on the mass-to-charge ratio. It tends to pass through the extraction electrode earlier than that. For this reason, when a certain delay time has elapsed since the first acceleration operation, ions remaining in the space between the sample holding portion and the extraction electrode have a large proportion of those having a relatively large mass-to-charge ratio. Become.

  Therefore, when the relative potential of the sample holder is further increased and the gradient of the potential gradient becomes steep, additional kinetic energy is imparted to the remaining ions having a relatively large mass-to-charge ratio, which accelerates the ions. Then, it moves toward the extraction electrode (second acceleration operation). This acceleration operation can be performed any number of times equal to or greater than two. For the acceleration operation, the potential of the sample holding part is increased while the potential of the extraction electrode is fixed, the potential of the extraction electrode is decreased while the potential of the sample holding part is fixed, or the potential of the sample holding part is decreased. Either raising or lowering the potential of the extraction electrode may be performed at the same time.

  By increasing the acceleration voltage stepwise in this way, it becomes possible to give a more appropriate acceleration voltage corresponding to the mass-to-charge ratio to various ions, as in the first invention. Of course, even in this case, even if the mass-to-charge ratio is the same, the position at the time of the acceleration operation varies depending on the difference in the initial energy of the ions, so that the acceleration is not completely separated for each mass-to-charge ratio. However, a clear mass resolution improvement effect can be obtained as compared with the conventional case where acceleration is performed without considering the mass-to-charge ratio.

  According to the time-of-flight mass spectrometers according to the first and second inventions, when ions generated from a sample are extracted and accelerated by the delayed extraction method in an ion source such as a MALDI ion source, the initial energy and the initial velocity are simply varied. Not only correction, but also correction by change of kinetic energy according to mass-to-charge ratio, so that mass resolution can be improved over a wide range of mass-to-charge ratio compared to conventional general delayed extraction method. it can.

The schematic block diagram of MALDI-TOFMS which is one Example of this invention. The figure which shows the simulation result for verifying the effect of MALDI-TOFMS by 1st Example. The figure for demonstrating the delay extraction operation | movement in MALDI-TOFMS by a 1st Example. The figure for demonstrating the delay extraction operation | movement in MALDI-TOFMS by a 1st Example. The figure for demonstrating the delay extraction operation | movement in MALDI-TOFMS by 2nd Example. The timing diagram which shows the voltage change at the time of delay drawing | extracting in MALDI-TOFMS by 2nd Example. The figure which shows the simulation result for verifying the effect of MALDI-TOFMS by 2nd Example. The figure for demonstrating the delay extraction operation | movement in MALDI-TOFMS by the modification of 2nd Example. The figure for demonstrating the conventional general delay drawer | drawing-out operation | movement.

[First embodiment]
MALDI-TOFMS, which is an embodiment of the present invention, will be described with reference to FIGS. FIG. 1 is a schematic configuration diagram of the MALDI-TOFMS of this embodiment, and FIGS. 3 and 4 are diagrams for explaining a delay extraction operation in the MALDI-TOFMS of this embodiment.

  In the MALDI-TOFMS of this embodiment, an extraction electrode 3, an ion optical system 4, a flight space 7, and a detector 8 are arranged along an ion optical axis C that is substantially orthogonal to the sample plate 1 that holds the sample 2. . Under the instruction of the control unit 11, the laser light emitted from the laser irradiation unit 5 is reflected by the mirror 6 and irradiated onto a small diameter region on the surface of the sample 2. The sample plate 1 is made of metal or conductive glass and is actually held by a stage (not shown), and a voltage is applied through the stage. In FIG. 1, for convenience, the sample plate 1 is directly applied to the sample plate 1. , So that a voltage is applied.

  The extraction voltage generator 12 applies predetermined DC voltages to the sample plate 1 and the extraction electrode 3 in accordance with instructions from the controller 11. The ion optical system 4 includes a plurality of electrodes including a base electrode 4c to which a predetermined potential (VB) is applied, and suppresses the spread of ions by a voltage applied to these electrodes from a power supply unit (not shown) near the ion optical axis C. Focus ions. Similarly to FIG. 9, it is assumed that the potential (VB) of the base electrode 4c is 0 in this example. The detector 8 is, for example, a photomultiplier tube, detects ions that are sequentially separated in accordance with the mass-to-charge ratio in the process of passing through the flight space 7, and sequentially processes the detection signal according to the amount of ions. Send to part 10. The signal processing unit 10 creates a flight time spectrum indicating the relationship between the flight time and the ion intensity based on the detection signal, and creates a mass spectrum by converting the flight time into a mass-to-charge ratio based on previously obtained calibration information. To do.

  Next, an analysis operation including a delay extraction operation characteristic of the MALDI-TOFMS of this embodiment will be described.

  When a start signal is sent from the control unit 11 to the laser irradiation unit 5, the laser irradiation unit 5 emits a laser beam having a predetermined pulse width correspondingly. This laser beam is reflected by the mirror 6 and irradiated onto the sample 2 on the sample plate 1. On the other hand, when the laser beam is emitted, a signal obtained by monitoring a small part of the laser beam is fed back from the laser irradiation unit 5 to the control unit 11, whereby the control unit 11 recognizes the laser emission. And the control part 11 considers that time as an ion generation start time, and starts the time measurement of an internal timer.

  Further, at an appropriate time before the laser beam is irradiated, the controller 11 sets the extraction voltage so that the applied voltage Ve to the extraction electrode 3 is VE and the applied voltage Vs to the sample plate 1 is VS higher than VE. The generator 12 is controlled. In the conventional general delay extraction method, VS = VE, whereas in this embodiment, VS> VE. However, the potential difference VS−VE at this time is much smaller than the potential difference V0−VE during ion acceleration described later. The reason will be described later. Since the potential VB of the base electrode 4c is 0, the potential distribution on the ion optical axis C is in the state shown in FIG. That is, in the space (extraction region) between the sample plate 1 and the extraction electrode 3, an electric field having a potential gradient gently inclined downward from the sample plate 1 toward the extraction electrode 3 is formed. In the space (acceleration region) between the electrodes 4c, an electric field having a potential gradient that steeply drops from the extraction electrode 3 toward the base electrode 4c is formed.

  When the sample 2 is irradiated with laser light, the matrix in the sample 2 and the target sample are vaporized together, and the target sample is ionized. Since the above-described electric field acts on various ions generated in a narrow space near the surface of the sample 2, the ions are attracted in the direction toward the extraction electrode 3 (rightward in FIG. 3A). At this time, the velocity of ions derived from the potential energy given by the electric field increases as the mass-to-charge ratio decreases. Therefore, ions with a smaller mass-to-charge ratio are closer to the extraction electrode 3.

  Of course, each ion has an initial energy that does not depend on the mass-to-charge ratio at the time of generation, and there is a velocity component thereby, so that the ions are not simply arranged in the order of the mass-to-charge ratio. However, when, for example, ions having different mass-to-charge ratios to which the same initial energy is applied at the time of ion generation are observed, ions having a small mass-to-charge ratio approach the extraction electrode 3 more quickly, so that overall FIG. As shown in FIG. 4, ions having a small mass-to-charge ratio (drawn in a small size in FIG. 4) precede, and ions having a large mass-to-charge ratio (drawn in a large size in FIG. 4) remain near the surface of the sample 2. . When a collection of ions having substantially the same mass-to-charge ratio is viewed in detail, ions having a larger initial energy are present closer to the extraction electrode 3 in advance. FIG. 4B is a diagram schematically showing the distribution of ions after a predetermined time has elapsed from the time of ion generation in the conventional delayed extraction method. In this case, since there is no electric field near the sample 2, each ion moves at a speed corresponding to the initial energy. Since the initial energy varies regardless of the mass to charge ratio, the spatial distribution of ions does not depend on the mass to charge ratio.

  As apparent from FIG. 3B, if the gradient of the potential gradient in the extraction region is too steep at the time of ion generation, each ion is accelerated immediately after generation and passes through the extraction electrode 3 in a short time. That is, this is not a substantial delayed withdrawal. Therefore, it is necessary to make the gradient of the potential gradient gentle so that ions extracted from the vicinity of the surface of the sample 2 are given kinetic energy to the extent that they do not pass through the extraction electrode 3 within a delay time described later. is there. That is, it is necessary to reduce the potential difference VS-VE in FIG. On the other hand, if the potential difference VS-VE is too small and the gradient of the potential gradient is too slow, the influence of the initial energy of the ions on the electric field is greater than the kinetic energy received by the ions, and the ions are not separated according to the mass-to-charge ratio. . Therefore, based on conditions such as the distance between the sample plate 1 and the extraction electrode 3 and the delay time, various ions are appropriately separated in the extraction region within the delay time according to the mass-to-charge ratio. It is desirable to determine the potential difference VS-VE appropriately. This appropriate potential difference can be determined by, for example, simulation calculation described later or an experiment using an actual apparatus.

  When a predetermined delay time t has elapsed since the start of the internal timer, the control unit 11 extracts the voltage generator 12 so that the voltage Vs applied to the sample plate 1 is stepped up from Vs up to V0. To control. On the other hand, the applied voltage VE to the extraction electrode 3 is maintained at the same voltage value as before. As a result, the potential distribution on the ion optical axis C changes to the state shown in FIG. That is, an electric field having a potential gradient that steeply slopes downward from the sample plate 1 toward the extraction electrode 3 is formed in the extraction region.

  As a result, an acceleration voltage of a maximum V0-VE is simultaneously applied to the ions existing in the extraction region immediately before that, and the ions are extracted toward the extraction electrode 3. Further, after ions enter the acceleration region, they are further accelerated by the potential difference VE-VB (= VE) between the potential of the extraction electrode 3 and the potential VB (= 0) of the base electrode 4c, and are sent to the flight space 7. The ions introduced into the flight space 7 are separated according to the mass-to-charge ratio during the flight and reach the detector 8. Since ions present at positions closer to the sample plate 1 in the extraction region are given higher acceleration energy, they are introduced into the flight space 7 at a higher speed. In other words, ions with a larger mass-to-charge ratio have a higher speed, and ions having a lower initial energy have a higher speed with the same mass-to-charge ratio.

  Ions having a certain mass-to-charge ratio existing in the extraction region near the sample plate 1 enter the flight space 7 later in time than ions having the same mass-to-charge ratio and closer to the extraction electrode 3. be introduced. However, since the flight speed is higher, it is possible to gradually catch up with the preceding ions during the flight and reach the detector 8 at almost the same time. That is, energy convergence of ions having the same mass-to-charge ratio can be performed.

  On the other hand, ions having a large mass-to-charge ratio are given higher acceleration energy than ions having a small mass-to-charge ratio. Therefore, an appropriate potential energy change can be given to ions of each mass-to-charge ratio, so that the difference in the effect of correcting the initial speed variation due to the mass-to-charge ratio can be reduced. Thereby, it is possible to achieve high mass resolution by reducing variations in initial speed over a wide range of mass-to-charge ratios without shifting to a specific mass-to-charge ratio.

  Next, a simulation for verifying the effect of the delayed extraction operation in the MALDI-TOFMS of this embodiment will be described. In this simulation, an axially symmetric ion transport system centered on the ion optical axis C as shown in FIG. 1 is assumed, and the distance between the sample plate 1 and the extraction electrode 3 is about 3.5 [mm], The distance of the free flight trajectory in the flight space 7 was about 1000 [mm]. The ion optical system 4 performs acceleration of ions and convergence of the trajectory, and gives an acceleration energy of about 18 [keV] before the ions are introduced into the flight space 7. In voltage control for delayed extraction, the voltage applied to the extraction electrode 3 is made constant, and the voltage applied to the sample plate 1 is increased stepwise when a predetermined delay time t elapses from the time of laser light irradiation. did.

  The execution conditions of the delayed extraction method (the method of the present invention) in the above-described embodiment are as follows: VS = 18000 [V], VE = 17730 [V], V0−VS = 830 [V], VB = 0, t = 900 [ ns]. On the other hand, the execution conditions of the conventional delayed extraction method (conventional method) are VS = 18000 [V], VE = 18160 [V], V0−VS = 950 [V], VB = 0, t = 900 [ns]. . These execution conditions are selected so that the simulation results are good as will be described later. In the conventional method, the applied voltage VE to the extraction electrode 3 is slightly (160 [V]) higher than the applied voltage VS of the sample plate 1, but this cancels the influence of the electric field by the ion optical system 4. As a result, the potential gradient along the ion optical axis C in the vicinity of the sample plate 1 becomes almost zero, and it can be regarded that there is no electric field in the extraction region.

Under the above execution conditions, the mass-to-charge ratio was changed by 250 [Da] in the mass-to-charge ratio range of 500 to 5000 [Da], and numerical calculation was performed for the arrival time of ions to the detector 8 to examine the resolution. In this calculation, the central value of the initial velocity of the ion is 600 [m / s], the half-value width of the initial velocity variation is 300 [m / s], and the velocity direction is 30 ° with respect to the symmetry axis (ion optical axis C). It is assumed that the angle is In addition, the initial position of ions is assumed to be random in a space within a range of 0.1 [mm ] from the ion optical axis C.

  FIG. 2 shows a simulation result. In the conventional method, the execution condition is adjusted so that the resolution becomes maximum at m / z 2000 [Da]. On the other hand, in the method of the present invention, the execution conditions are adjusted so that the mass-to-charge ratio range including m / z 2000 [Da] and the resolution exceeding 5000 is as wide as possible. As apparent from FIG. 2, the mass-to-charge ratio range in which the resolution exceeds 5000 in the conventional method is only about 1000 [Da] centering on 2000 [Da], whereas in the method of the present invention 1500 to 5000. The resolution exceeds 5000 over a wide range of [Da]. Therefore, according to the method of the present invention, it can be seen that the mass-to-charge ratio range in which high resolution can be obtained can be expanded 3.5 times compared with the conventional method.

[Second Embodiment]
MALDI-TOFMS, which is another embodiment (second embodiment) of the present invention, will be described with reference to FIGS. Since the configuration of the second embodiment is the same as that of the first embodiment, description thereof is omitted. The difference is in the control sequence of the control unit 11, and in addition, the circuit configuration of the extraction voltage generation unit 12 needs to be changed as necessary.

  The analysis operation including the delay extraction operation characteristic of the MALDI-TOFMS of the second embodiment will be described mainly with respect to differences from the first embodiment with reference to FIGS.

  The controller 11 controls the extraction voltage generator 12 so that the applied voltage Ve to the extraction electrode 3 and the applied voltage Vs to the sample plate 1 are both set to VE1 at an appropriate time before the laser light irradiation. . The potential distribution on the ion optical axis C is in the state shown in FIG. 5B, which is the same as in the conventional delayed extraction method. That is, there is no substantial electric field that attracts ions in the extraction region. The sample 2 is irradiated with laser light, and various ions generated in the vicinity of the surface of the sample 2 move at an initial speed based on their initial energies. Therefore, the spatial distribution of ions after the elapse of a predetermined time from the ion generation start time is as shown in FIG. 4B, and the ions are dispersed regardless of the mass-to-charge ratio.

  When the predetermined delay time t1 has elapsed from the start of the internal timer, the control unit 11 sets the extraction voltage generator 12 so that the applied voltage Ve to the extraction electrode 3 is stepped down from VE1 to VE2 so far. Control. At this time, the voltage VS applied to the sample plate is maintained at the same voltage value VE1 as before. As a result, the potential distribution on the ion optical axis C changes to the state shown in FIG. That is, an electric field having a potential gradient that gently slopes downward from the sample plate 1 toward the extraction electrode 3 is formed in the extraction region. Here, although the potential of the extraction electrode 3 is lowered by fixing the potential of the sample plate 1, the potential of the extraction plate 3 may be increased by fixing the potential of the extraction electrode 3.

  By forming a potential gradient in the extraction region, the ions receive energy from an acceleration voltage of maximum VE1-VE2. However, since the acceleration voltage at this time is relatively low and the energy is small, the ion velocity is relatively small. In addition, the ion velocity at this time becomes smaller as the mass-to-charge ratio becomes larger. Therefore, when ions at the same position, that is, ions with different mass-to-charge ratios subjected to the same acceleration energy are compared, the smaller the mass-to-charge ratio, the larger the velocity. have. Therefore, as a whole, ions having a smaller mass to charge ratio pass through the extraction electrode 3 and enter the acceleration region earlier, whereas ions having a larger mass to charge ratio tend to stay in the extraction region. In this way, an operation close to mass separation is performed in the drawer region.

  When the predetermined delay time t2 (t2> t1) has elapsed since the start of the internal timer, the control unit 11 maintains the voltage Ve applied to the extraction electrode 3 at VE2 and the voltage applied to the sample plate 1 this time. The extraction voltage generator 12 is controlled so as to increase Vs from VE1 to V0 stepwise. As a result, the potential distribution on the ion optical axis C changes to the state shown in FIG. That is, the gradient of the potential gradient from the sample plate 1 toward the extraction electrode 3 becomes steep. Here, although the potential of the extraction electrode 3 is fixed and the potential of the sample plate 1 is raised, the potential of the extraction plate 3 may be lowered by fixing the potential of the sample plate 1.

When an electric field in the extraction region is changed as described above, the maximum V0-VE 2 becomes an acceleration voltage to large ion mainly mass-to-charge ratio remained in extraction region at that time is given all at once, the ions extraction electrode It is drawn towards 3. The ions that have entered the acceleration region are further accelerated by the potential difference VE2-VB (= VE2) between the potential of the extraction electrode 3 and the potential VB (= 0) of the base electrode 4c, and are sent to the flight space 7. The ions introduced into the flight space 7 are separated according to the mass-to-charge ratio during the flight and reach the detector 8.

  As described above, in the MALDI-TOFMS of the second embodiment, the ions generated near the surface of the sample 2 freely spread at the initial velocity during the period from the start of ion generation until the delay time t1 elapses. Regardless of the mass-to-charge ratio, ions with a larger initial velocity (initial energy) are dispersed so as to approach the extraction electrode 3. During the period from the elapse of the delay time t1 until the delay time t2 elapses, ions having a small mass-to-charge ratio existing in the extraction region are extracted toward the extraction electrode 3 with relatively small acceleration energy. It is introduced into the acceleration area. At this time, among ions having the same mass-to-charge ratio, ions closer to the sample 2 are given higher acceleration energy, so energy convergence is achieved for ions having the same mass-to-charge ratio, and thus time convergence is achieved.

  Subsequently, when the delay time t2 elapses, mainly ions having a large mass-to-charge ratio existing in the extraction region are extracted in the direction of the extraction electrode 3 with a large acceleration energy and introduced into the acceleration region. At this time as well, ions having the same mass-to-charge ratio are closer to the sample 2 and are given higher acceleration energy, so energy convergence is achieved for ions having the same mass-to-charge ratio and thus time convergence is achieved. Is done.

  Various ions generated from the sample 2 are roughly separated according to the mass-to-charge ratio, and ions having a large mass-to-charge ratio are given higher acceleration energy than ions having a small mass-to-charge ratio. Therefore, as in the first embodiment, since an appropriate potential energy change according to the mass-to-charge ratio can be given to the ions, the difference in the effect of correcting the initial speed variation due to the mass-to-charge ratio can be reduced. Thereby, it is possible to achieve high mass resolution by reducing variations in initial speed over a wide range of mass-to-charge ratios without shifting to a specific mass-to-charge ratio.

  A simulation for verifying the effect of the delayed extraction operation in the MALDI-TOFMS of the second embodiment will be described. Basic simulation conditions other than the execution conditions for the delayed withdrawal are the same as in the first embodiment.

The execution conditions of the delayed extraction method (the method of the present invention) in the second embodiment are as follows. The initial applied voltage to the sample plate 1 = 18000 [V] and the initial application to the extraction electrode 3 are as shown in FIG. Applied voltage = 18160 [V], decrease width of applied voltage to extraction electrode 3 when t1 has elapsed (VE1−VE2 in FIG. 5C) = 850 [V], applied voltage to sample plate 1 when t2 has elapsed (V0−VE1 in FIG. 5D) = 220 [V], VB = 0, t1 = 700 [ns], t2 = 1500 [ns]. The execution conditions of the conventional delayed extraction method (conventional method) are the same as in the first embodiment. These execution conditions are selected so that the simulation results are good as will be described later. The reason why the more the initial application voltage to the extraction electrode 3 than initial applied voltage of the sample plate 1 in the present invention method is slightly higher is as described above.

  FIG. 7 shows a simulation result. In the conventional method, the execution condition is adjusted so that the resolution becomes maximum at m / z 2000 [Da]. On the other hand, in the method of the present invention, the execution conditions are adjusted so that the mass-to-charge ratio range including m / z 2000 [Da] and the resolution exceeding 5000 is as wide as possible. As is apparent from FIG. 7, the mass-to-charge ratio range in which the resolution exceeds 5000 in the conventional method is only about 1000 [Da] centering on 2000 [Da], whereas in the method of the present invention, 750 to 4500. The resolution exceeds 5000 over a wide range of [Da]. Therefore, it can be seen that the mass-to-charge ratio range in which high resolution can be obtained can be expanded to 3.5 times that of the conventional method also by the method of the present invention.

  In the description of the second embodiment, a substantial electric field is not formed in the vicinity of the sample 2 at the time of ion generation, and then the acceleration voltage is increased in two stages. The acceleration voltage may be increased in small increments. In the description of the second embodiment, the voltage applied to the extraction electrode 3 is lowered stepwise after the delay time t1 has elapsed, and then the voltage applied to the sample plate 1 is increased stepwise after the delay time t2 has elapsed. Although the gradient of the potential gradient from the plate 1 to the extraction electrode 3 is changed, either the voltage applied to the sample plate 1 or the voltage applied to the extraction electrode is fixed and the other is changed stepwise. Control can be performed. FIG. 8 is a diagram showing changes in the potential distribution when the voltage applied to the extraction electrode 3 is fixed and the voltage applied to the sample plate 1 is increased stepwise in several steps. When the voltage applied to the sample plate 1 is fixed, the voltage applied to the extraction electrode 3 may be decreased stepwise.

  Further, the feature point of the delayed extraction method in the first embodiment and the feature point of the delay extraction method in the second embodiment can be used in combination. In other words, an electric field having a gradual potential gradient that causes mass separation of ions in the extraction region at the time of ion generation is formed, and then the potential gradient is stepped every time a predetermined constant or non-constant delay time elapses. Control may be performed so as to increase the size.

  3 and 5, the relationship between the voltage Vs applied to the sample plate 1 (VS, V0, VE1), the extraction electrode Ve (VE, VE1, VE2), and the potential VB of the base electrode 4c is relative. Therefore, for example, even if the potential VB of the base electrode 4c is lowered by 10 [kV] (VB = −10 [kV]) and Vs and Ve are lowered in the same manner, the behavior control of ions as described above is performed. It is clear that this is possible.

  It should be noted that any of the above-described embodiments is an example of the present invention, and it is obvious that modifications, corrections, and additions may be made as appropriate without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 ... Sample plate 2 ... Sample 3 ... Extraction electrode 4 ... Ion optical system 4c ... Base electrode 5 ... Laser irradiation part 6 ... Mirror 7 ... Flight space 8 ... Detector 10 ... Signal processing part 11 ... Control part 12 ... Extraction voltage generation Part C ... Ion optical axis

Claims (3)

In a time-of-flight mass spectrometer that accelerates ions generated from a sample and introduces them into a flight space, and separates and detects ions according to the mass-to-charge ratio in the flight space,
a) an extraction electrode disposed at a predetermined distance from the sample holding unit for holding the sample, and forming an electric field for extracting and accelerating ions from the sample surface in a space between the sample holding unit;
b) The spatial distribution of ions in the space between the sample surface and the extraction electrode when a predetermined delay time has elapsed from the start of ion generation, the closer the ions with a smaller mass-to-charge ratio, the closer to the extraction electrode. so that things, during the period from the ion generation starting time point to the predetermined delay time has elapsed, to form an electric field Before moving according to their mass to charge ratio ions toward the extraction electrode from the sample surface , than the potential of the extraction electrode potential of the sample holder, keeping high as the first potential difference corresponding to the predetermined delay time and the predetermined distance, the point in time and thereafter passed the predetermined delay time is The potential of the sample holder is set so that ions in the space between the sample holder and the extraction electrode are accelerated all at once and an electric field passing through the extraction electrode is formed. Serial than the potential of the extraction electrode, so as to maintain high as large second potential difference than the first potential difference, the potential setting means for setting the potential of the sample holder and the extraction electrode,
A time-of-flight mass spectrometer.
In a time-of-flight mass spectrometer that accelerates ions generated from a sample and introduces them into a flight space, and separates and detects ions according to the mass-to-charge ratio in the flight space,
a) an extraction electrode disposed at a predetermined distance from the sample holding unit for holding the sample, and forming an electric field for extracting and accelerating ions from the sample surface in a space between the sample holding unit;
b) After the start of ion generation, the gradient of the potential gradient that draws ions in the direction from the sample holder to the extraction electrode in the space between the sample holder and the extraction electrode has a predetermined same or different time. A potential setting means for stepwise increasing the relative potential of the sample holder with respect to the potential of the extraction electrode so that an electric field that increases stepwise is formed each time
A time-of-flight mass spectrometer.   The time-of-flight mass spectrometer according to claim 1 or 2, wherein the ionization of the sample is performed by any one of the MALDI method, the LDI method, the DESI method, the PDI method, and the SIMS method. A time-of-flight mass spectrometer.

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