JP2019053852A - Mass spectroscope and mass spectrometry - Google Patents

Abstract

To provide a mass spectroscope by which mass spectra high in both of SN ratio and mass resolution can be obtained in a short time.SOLUTION: A mass spectroscope 1 comprises: a laser light source 10; an intensity regulating part 20; a TOF tube 30; a sample holder-driving part 40; a detector part 50; and a processing part 60. A mixture of a sample targeted for analysis and a matrix is placed on the sample holder 31 in the TOF tube 30. The detector part 50 outputs a time-waveform signal representing an arrival time distribution of ions concerned and outputs an ion image signal representing a quantity of an arriving ion image for each shot of pulse laser light. The processing part 60 determines whether or not the ion image quantity is larger than a threshold based on ion image signals for each shot of pulse laser light. The processing part 60 integrates other time-waveform signals to determine mass spectra of the sample, except a time-waveform signal when it is determined that an ion image exceeds the threshold.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a mass spectrometer and a mass spectrometry method.

  As one method for ionizing sample molecules, a matrix-assisted laser desorption / ionization (MALDI) method is known. In the MALDI method, a mixture of a sample and a matrix is irradiated with pulsed laser light, and heat is released from the matrix that has absorbed the energy of the pulsed laser light. The sample and the matrix are vaporized by this heat, and charge is exchanged between the sample molecules and the matrix molecules to ionize the sample molecules. As the matrix, an organic compound having a low molecular weight that is easily ionized by irradiation with pulsed laser light is preferably used.

  On the other hand, a laser desorption / ionization (LDI) method is also known in which a sample is directly irradiated with pulsed laser light without being mixed with a matrix to ionize molecules of the sample. In the LDI method, the sample may be damaged, whereas in the MALDI method, the sample can be prevented from being damaged by mixing with the matrix. Therefore, the MALDI method is suitable for ionization of biomolecules having a high molecular weight that are easily destroyed (for example, proteins and polysaccharides).

  Further, ions generated by the MALDI method are subjected to mass spectrometry by a time-of-flight mass spectrometry (TOF-MS) method. That is, in the TOF-MS method, ions generated by the MALDI method are accelerated by the electric field and fly to the detection unit. At this time, as the ratio of the mass m to the charge z (mass-to-charge ratio m / z) increases, the flight speed of the ion decreases, so that the ion reaches the detection unit later. Therefore, the mass spectrum of the sample can be obtained based on the time waveform signal indicating the ion arrival time distribution (ion flight time distribution) in the detection unit.

  Non-Patent Document 1 describes a mass spectrometry automation technique that combines a MALDI method and a TOF-MS method. The mass spectrometry automation technology described in this document uses a fuzzy logic engine to evaluate the average ion intensity and mass resolution of the mass spectrum of the sample detected by the detector, and based on the evaluation results, the pulse laser beam Feedback control of the fluence. By this control, each of the average ion intensity and mass resolution of the mass spectrum is maintained within a predetermined range. Conventionally, a skilled person adjusts the fluence by looking at the shape of the mass spectrum. According to this automation technique, an analysis result comparable to that obtained by the skilled person is obtained. The fluence is light energy per unit light irradiation area. The mass resolution can be defined by the ratio (m / Δm) between the mass m at the peak position in the mass spectrum and the full width at half maximum Δm.

Ole N. Jensen, et al., "Automationof Matrix-Assisted Laser Desorption / Ionization Mass Spectrometry Using Fuzzy Logic Feedback Control," Analytical Chemistry, Vol.69, No.9, pp.1706-1714 (1997).

  In the conventional technique, the fluence of the pulse laser beam is set to a low fluence region close to the ion generation threshold. Also in the technique described in Non-Patent Document 1, feedback control is performed on the fluence in a low fluence region. This is because high mass resolution can be obtained in a low fluence region. However, when the fluence of the pulse laser beam is low, the number of ions generated per shot of the pulse laser beam is small. In order to obtain a mass spectrum with a high S / N ratio, it is necessary to increase the number of shots of the pulse laser beam and increase the number of accumulated time waveform signals. Therefore, it takes a long time to obtain a mass spectrum with a high S / N ratio.

  On the other hand, when the fluence of the pulse laser beam is high, the number of ions generated per shot of the pulse laser beam is large. However, in this case, in the time waveform signal, the peak value of the ion of the sample to be analyzed becomes small and the full width at half maximum Δm becomes large, so that the mass resolution of the ion of the sample to be analyzed may be lowered. This is thought to be due to the fact that most of the energy of the pulsed laser light is absorbed by the matrix. Hereinafter, such a state is referred to as “matrix mode”. On the other hand, the state in which the ion peak value is larger than that of the “matrix mode” and a high mass resolution can be obtained for the ions of the sample to be analyzed is referred to as a “sample molecule mode”.

  When the fluence of the pulse laser beam is high, either the sample molecule mode or the matrix mode is set for each shot of the pulse laser beam. In particular, in the saturated fluence region where the number of ions generated per shot hardly increases even when the fluence is increased, the frequency of the matrix mode is high. The mechanism by which this phenomenon occurs has not yet been elucidated.

  From the above circumstances, it has been difficult to obtain a mass spectrum with high S / N ratio and mass resolution in a short time with the conventional technique.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a mass spectrometer and a mass spectrometry method capable of obtaining a mass spectrum having a high SN ratio and mass resolution in a short time. And

  The mass spectrometer of the present invention comprises: (1) a laser light source that outputs a pulsed laser beam to be irradiated to a mixture of a sample to be analyzed and a matrix; and (2) a pulsed laser beam is applied to the mixture to irradiate the mixture. An acceleration unit that accelerates and flies the generated ions by an electric field, and (3) a time waveform signal that represents the ion arrival time distribution for each shot of pulsed laser light, provided at a position where ions flying from the acceleration unit arrive And (4) for each shot of the pulsed laser beam, the size of the ion image is less than the threshold value based on the ion image signal. A processing unit for determining whether the ion image is larger than the threshold value, excluding the time waveform signal, and integrating the other time waveform signal to obtain the mass spectrum of the sample; Is provided.

  The mass spectrometric method of the present invention includes (1) an irradiation step of irradiating a mixture of a sample to be analyzed and a matrix with pulsed laser light output from a laser light source, and (2) irradiating the mixture with pulsed laser light. The acceleration step of accelerating and flying the ions generated from the mixture by an electric field in the acceleration unit, and (3) the detection unit provided at the position where the ions flying from the acceleration unit reach each shot of the pulse laser beam, A detection step for outputting a time waveform signal representing the ion arrival time distribution and also outputting an ion image signal representing the size of the reached ion image, and (4) an ion image signal for each shot of pulsed laser light. Based on this, it is determined whether the size of the ion image is larger than the threshold value, the time waveform signal when the ion image is determined to be larger than the threshold value is excluded, and the other time Processing steps for integrating the waveform signals to obtain the mass spectrum of the sample.

  In the mass spectrometer or the mass spectrometry method of the present invention, the detection unit generates a first signal by receiving electrons output from the microchannel plate and a microchannel plate that generates electrons and multiplies the electrons upon arrival of ions. A first anode electrode that outputs, and a second anode electrode that is provided separately from the first anode electrode and that receives electrons output from the microchannel plate and outputs a second signal. It is preferable to output one of the signals as a time waveform signal and the other as an ion image signal.

  Further, in the mass spectrometer or the mass spectrometry method of the present invention, the detection unit generates an electron upon arrival of ions and multiplies the electrons, and receives an electron output from the microchannel plate and receives an ion image. It is also preferable to include a fluorescent plate that forms a fluorescent image according to the time and outputs a time waveform signal, and an imaging unit that outputs imaging data obtained by imaging the fluorescent image formed on the fluorescent plate as an ion image signal It is.

  In the mass spectrometer of the present invention, it is preferable that the processing unit controls the fluence of the pulsed laser light applied to the mixture based on the time waveform signal. In the mass spectrometry method of the present invention, it is preferable to control the fluence of the pulsed laser light irradiated to the mixture based on the time waveform signal in the processing step.

  According to the present invention, a mass spectrum having both a high SN ratio and a high mass resolution can be obtained in a short time.

FIG. 1 is a diagram showing the configuration of the mass spectrometer 1. FIG. 2 is a diagram illustrating a first configuration example of the detection unit 50. FIG. 2A shows the arrangement of the first anode electrode 71 and the second anode electrode 72. FIG. 2B shows a cross section. FIG. 3 is a diagram illustrating an example of a time waveform signal. FIG. 3A shows a time waveform signal in the matrix mode. FIG. 3B shows a time waveform signal in the sample molecule mode. FIG. 4 is a diagram illustrating an example of an ion image reaching the MCP 70 (an electron image output from the MCP 70). FIG. 4A shows an ion image in the matrix mode. FIG. 4B shows an ion image in the sample molecule mode. FIG. 5 is a diagram illustrating the waveforms of the first signal (time waveform signal) output from the first anode electrode 71 and the second signal (ion image signal) output from the second anode electrode 72. FIG. 6 is a diagram illustrating a second configuration example of the detection unit 50. FIG. 7 is a diagram illustrating an example of a change in fluence over time. FIG. 8 is a diagram illustrating another example of the time variation of the fluence. FIG. 9 is a graph showing the relationship between the cumulative number of ions obtained from each irradiation spot and the fluence. FIG. 10 is a graph showing the relationship between the number of ions per shot obtained from each irradiation spot and the number of shots. FIG. 11 is a graph showing the cumulative number of ions obtained from each irradiation spot. FIG. 12 is a diagram illustrating a configuration of the mass spectrometer 1A. FIG. 13 is a diagram illustrating an example of a change in fluence over time.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. The present invention is not limited to these exemplifications, but is defined by the scope of the claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of the claims.

  FIG. 1 is a diagram showing the configuration of the mass spectrometer 1. The mass spectrometer 1 includes a laser light source 10, an intensity adjustment unit 20, a TOF tube 30, a sample stage drive unit 40, a detection unit 50, and a processing unit 60. The mixture of the sample to be analyzed and the matrix is placed on the sample stage 31 in the TOF tube 30.

  The laser light source 10 outputs pulsed laser light to be irradiated on the mixture placed on the sample stage 31. For example, the laser light source 10 is a nitrogen laser light source that outputs pulsed laser light having a wavelength of 337 nm. The intensity adjusting unit 20 receives the pulse laser beam output from the laser light source 10 and adjusts and outputs the intensity of the pulse laser beam. The intensity adjusting unit 20 includes, for example, an acousto-optic modulator (Acousto-Optic Modulator, AOM) or an ND filter that can modulate the light intensity according to the input voltage.

  The mirror M reflects the pulsed laser beam output from the intensity adjusting unit 20 and irradiates the mixture placed on the sample stage 31 with the pulsed laser beam. The mirror M may be an off-axis parabolic mirror that focuses and irradiates the mixture with pulsed laser light. Alternatively, a condensing lens for condensing and irradiating the mixture with pulsed laser light may be provided on the optical path.

  An extraction electrode 32 is provided in the TOF tube 30 so as to face the sample table 31. A voltage is applied between the sample stage 31 and the extraction electrode 32. The sample stage 31 and the extraction electrode 32 to which such a voltage is applied function as an accelerating unit that accelerates and flies ions generated from the mixture by an electric field when the mixture is irradiated with pulsed laser light. The sample stage drive unit 40 can move the sample stage 31, thereby scanning the pulse laser light irradiation position in the mixture.

  The detection part 50 is provided in the position where the ion which flew from the acceleration part arrives. The detection unit 50 outputs a time waveform signal representing the ion arrival time distribution for each shot of the pulse laser light and also outputs an ion image signal representing the size of the reached ion image. A configuration example of the detection unit 50 will be described later.

  The processing unit 60 inputs the time waveform signal and the ion image signal output from the detection unit 50. The processing unit 60 determines whether or not the size of the ion image is larger than the threshold based on the ion image signal for each shot of pulsed laser light. Then, the processing unit 60 excludes the time waveform signal when it is determined that the ion image is larger than the threshold value, and integrates other time waveform signals to obtain the mass spectrum of the sample.

  The processing unit 60 controls the intensity adjustment of the pulse laser beam by the intensity adjusting unit 20. Further, the processing unit 60 controls one-dimensional or two-dimensional scanning of the pulse laser light irradiation position in the mixture by the sample stage driving unit 40. The processing unit 60 includes an input unit that receives inputs such as analysis conditions, an analysis start instruction, and an analysis end instruction. The processing unit 60 includes a display unit that displays analysis results and the like. For example, the processing unit 60 is a computer having a keyboard or the like as an input unit and a display as a display unit.

  The mass spectrometry method using this mass spectrometer 1 is as follows. The intensity of the pulse laser beam output from the laser light source 10 is adjusted by the intensity adjusting unit 20 and irradiated to the mixture placed on the sample stage 31 (irradiation step). Ions generated from the mixture by irradiating the mixture with pulsed laser light are accelerated by an electric field in an accelerating portion composed of the sample stage 31 and the extraction electrode 32 (acceleration step). When the ions flying from the acceleration unit reach the detection unit 50, the detection unit 50 outputs a time waveform signal representing the ion arrival time distribution for each shot of the pulsed laser light, and the ion image of the reached ion image An ion image signal representing the magnitude is also output (detection step).

  Then, for each shot of pulsed laser light, the processing unit 60 determines whether or not the size of the ion image is larger than the threshold based on the ion image signal, and when the ion image is determined to be equal to or less than the threshold. The time waveform signals are integrated to obtain the mass spectrum of the sample (processing step). Moreover, the mass spectrum of the sample at each position of the mixture can be obtained by scanning the irradiation position of the pulse laser beam in the mixture by the sample stage driving unit 40.

  FIG. 2 is a diagram illustrating a first configuration example of the detection unit 50. FIG. 2A shows the arrangement of the first anode electrode 71 and the second anode electrode 72. FIG. 2B shows a cross section. The detection unit 50A of the first configuration example includes a micro-channel plate (MCP) 70, a first anode electrode 71, and a second anode electrode 72.

  The MCP 70 generates electrons upon arrival of ions and multiplies the electrons. The MCP 70 has a plate-like structure in which a large number of glass capillaries (channels) are bundled. Each channel has an inner diameter of several μm to 20 μm, and has an inner wall as a resistor and a secondary electron emitter. When ions reach one surface of the MCP 70 having a plate-like structure and enter the channel, and the ions collide with the inner wall of the channel, electrons are emitted therefrom. When the electrons further collide with the inner wall of the channel, secondary electrons are emitted. Thus, each channel has an electron multiplying function. The ion image that has reached one surface of the MCP 70 having a plate-like structure is output as an electronic image from the other surface of the MCP 70.

  The first anode electrode 71 and the second anode electrode 72 are separately provided on the electronic image output side of the MCP 70. The first anode electrode 71 receives electrons output from the MCP 70 and outputs a first signal. The second anode electrode 72 receives electrons output from the MCP 70 and outputs a second signal. The detection unit 50A outputs one of the first signal and the second signal as a time waveform signal, and outputs the other as an ion image signal. The processing unit 60 inputs the first signal and the second signal.

  The first anode electrode 71 and the second anode electrode 72 are preferably arranged as shown in FIG. That is, it is preferable that the ring-shaped plate-shaped second anode electrode 72 is disposed so as to surround the circular plate-shaped first anode electrode 71. In this case, the first signal output from the first anode electrode 71 is a time waveform signal, and the second signal output from the second anode electrode 72 is an ion image signal. This will be described with reference to FIGS.

  FIG. 3 is a diagram illustrating an example of a time waveform signal. FIG. 3A shows a time waveform signal in the matrix mode. FIG. 3B shows a time waveform signal in the sample molecule mode. In each figure, the lower waveform is an enlarged part of the upper waveform (near the mass of the sample molecule to be analyzed). In the time waveform signal in the matrix mode (FIG. 3A), the peak value of the ion of the matrix is large, the peak value of the ion of the sample to be analyzed is small, and the full width at half maximum of the peak of the sample ion is Since it is large, the mass resolution is low. On the other hand, in the time waveform signal in the sample molecule mode (FIG. 3B), the peak value of the ion of the matrix is small while the peak value of the ion of the sample to be analyzed is large. Since the full width at half maximum of the peak is small, the mass resolution is high.

  FIG. 4 is a diagram illustrating an example of an ion image reaching the MCP 70 (an electron image output from the MCP 70). FIG. 4A shows an ion image in the matrix mode. FIG. 4B shows an ion image in the sample molecule mode. Comparing the sizes of both ion images, the ion image in the matrix mode (FIG. 4A) is larger than the ion image in the sample molecule mode (FIG. 4B). In both the matrix mode and the sample molecule mode, the image of all ions including both the matrix ions and the sample ions to be analyzed and the image of the sample ions to be analyzed are substantially the same size. .

  In the present embodiment, this is utilized to determine whether the mode is the matrix mode or the sample molecule mode based on the size of the ion image. In the configuration shown in FIG. 2, the value of the second signal (ion image signal) output from the second anode electrode 72 is larger in the matrix mode than in the sample molecule mode. Therefore, the processing unit 60 can determine that the mode is the matrix mode when the value of the second signal is larger than a certain threshold value. This threshold may be 0. When the value of the second signal is less than or equal to the threshold value, the processing unit 60 determines that the sample molecule mode is in effect, integrates the first signal (time waveform signal) output from the first anode electrode 71, and Is obtained.

  FIG. 5 is a diagram illustrating the waveforms of the first signal (time waveform signal) output from the first anode electrode 71 and the second signal (ion image signal) output from the second anode electrode 72. This figure shows a first signal (time waveform signal) and a second signal (ion image signal) for three shots of pulsed laser light. In the example schematically shown in this figure, shot No. 2 is determined to be in the matrix mode because the value of the second signal (ion image signal) is large. In contrast, shot No. 1 and shot No. 3 are determined to be in the sample molecule mode because the value of the second signal (ion image signal) is small, and the first signal (time waveform signal) is integrated. Is done.

  Note that the shape and arrangement of the first anode electrode 71 and the second anode electrode 72 are not limited to those shown in FIG. If the size of the ion image is determined from at least one of the first signal output from the first anode electrode 71 and the second signal output from the second anode electrode 72, and a time waveform signal is obtained from at least the other As long as that is the case, the shape and arrangement of the first anode electrode 71 and the second anode electrode 72 are arbitrary. Based on the difference or ratio between the values of the first signal and the second signal, the size of the ion image may be determined to determine whether the mode is the matrix mode or the sample molecule mode. The sum of the first signal and the second signal may be a time waveform signal.

  The detection unit 50 may have other configurations. FIG. 6 is a diagram illustrating a second configuration example of the detection unit 50. The detection unit 50B of the second configuration example includes an MCP 70, a fluorescent plate 73, an imaging unit 74, a resistor 75, a constant voltage source 76, and a capacitor 77.

  The fluorescent plate 73 is provided on the electronic image output side of the MCP 70. The fluorescent plate 73 is connected to a constant voltage source 76 via a resistor 75, and given a constant voltage. The fluorescent plate 73 is connected to the processing unit 60 via a capacitor 77. The fluorescent plate 73 receives the electrons output from the MCP 70 and forms a fluorescent image corresponding to the ion image, and outputs a time waveform signal to the processing unit 60 via the capacitor 77.

  The imaging unit 74 outputs imaging data obtained by imaging a fluorescent image formed on the fluorescent plate 73 as an ion image signal. The imaging unit 74 is, for example, a CCD camera or a CMOS camera. The imaging unit 74 images the fluorescent image formed on the fluorescent plate 73 by forming an image on an imaging device using an imaging optical system.

  The imaging unit 74 may have an imaging element (area sensor) in which a plurality of pixels are two-dimensionally arranged and output two-dimensional image data. In this case, the processing unit 60 can determine the size of the ion image by analyzing the two-dimensional image data.

  The imaging unit 74 may include a high-speed frame rate sensor (profile sensor) S9132 for obtaining two-dimensional projection data manufactured by Hamamatsu Photonics as a light receiving element. This profile sensor is a high-performance CMOS area sensor specialized for acquiring projection data in the X and Y directions on the light receiving element. Since the projection profile in each of the X direction and the Y direction has a small amount of data, it is possible to detect the position and size of the spot light at a higher speed than a normal area sensor. In this case, the processing unit 60 can determine the size of the ion image by analyzing the projection profiles in the X direction and the Y direction.

  In the present embodiment, even when the fluence of the pulse laser beam is increased to increase the number of ions generated per shot (that is, when the matrix mode is likely to be generated), the matrix mode causes a decrease in mass resolution. While excluding the time waveform signal at the time of, the time waveform signal at the time of the sample molecule mode with high mass resolution can be integrated. Therefore, a mass spectrum having both a high S / N ratio and mass resolution can be obtained in a short time. The present embodiment can also be applied to a low fluence region (region where the frequency of becoming a matrix mode is low) close to the ion generation threshold.

  Next, fluence control will be described. In the present embodiment, the fluence of each shot of the pulsed laser light applied to the sample may be constant, but the fluence of each shot may be changed by the intensity adjusting unit 20 for each irradiation position.

  When the intensity adjustment unit 20 includes an AOM, the processing unit 60 can adjust the fluence by changing the voltage applied to the AOM. When a time waveform of 0 to 1 V, for example, is input at a voltage of a TTL level, the AOM can modulate the intensity of output light according to the time waveform. If the repetition frequency of the pulsed laser beam is 10 kHz, the shot interval is 100 μs. The fluence can be controlled by allowing the pulse laser beam to pass through the AOM and inputting a control signal of a pattern prepared in advance into the AOM. For example, when 100 shots are irradiated to each irradiation spot and the movement time to the next irradiation spot by the sample stage driving unit 40 is 100 ms, an example of a fluence time waveform is as shown in FIG. FIG. 7 is a diagram illustrating an example of a change in fluence over time. During the period (100 ms) required to change the irradiation spot position, the input voltage to the AOM is set to 0 so that the pulse laser beam is not irradiated onto the mixture. A plurality of control signal patterns for the AOM may be prepared, and an optimum pattern may be selected for each sample or measurement condition.

  When the intensity adjustment unit 20 includes an AOM, the processing unit 60 is also preferably configured to adjust the input voltage to the AOM based on the time waveform signal and feedback control the fluence of the pulsed laser light irradiated to the mixture. . For example, the processing unit 60 integrates the time waveform signal output from the detection unit 50 with pulse laser light (10,000 shots per second) with a repetition frequency of 10 kHz every 100 shots (10 ms), and based on the integration result Then, the peak intensity of the analysis target ion to be fed back is obtained. In this integration, the matrix mode time waveform signal is excluded. The processing unit 60 controls the fluence by adjusting the input voltage to the AOM so that the obtained peak intensity of the analysis target ion becomes the target peak intensity. FIG. 8 shows an example of a fluence time waveform when irradiating 1000 shots of pulse laser light with a repetition frequency of 10 kHz to each irradiation spot and performing fluence feedback control. FIG. 8 is a diagram illustrating an example of a change in fluence over time. In this case, the fluence value is updated 10 times in total every 10 ms.

FIG. 9 is a graph showing the relationship between the cumulative number of ions obtained from each irradiation spot and the fluence. Here, bradykinin 50 pmol / μL, which is a peptide sample, was used as a sample to be analyzed, and α-CHCA (α-Cyano-4-hydroxycinnamic Acid) was used as a matrix. Further, a laser light source 10 having an output wavelength of 355 nm and a repetition frequency of 20 Hz was used. The size of the irradiation spot in the mixture was 147 μm × 275 μm. As shown in this figure, in a region where the fluence is small, if the fluence is increased, the number of integrated ions increases accordingly. However, in a region where the fluence is large (saturated fluence region), even if the fluence is increased, an increase in the number of integrated ions corresponding thereto is not recognized. In the example shown in this figure, when the fluence exceeds approximately 30 J / m ² , the saturation fluence region is reached.

  The mass spectrometer 1 and the mass spectrometry method of the present embodiment can be applied even in a low fluence region close to the ion generation threshold, and can also be applied in a saturated fluence region. Particularly in the latter case, in the present embodiment, a mass spectrum having both a high SN ratio and a high mass resolution can be obtained in a short time.

  FIG. 10 is a graph showing the relationship between the number of ions per shot obtained from each irradiation spot and the number of shots. This figure shows the case where the fluence of each shot of the pulsed laser light applied to the sample is not adjusted and the case where the fluence of each shot is feedback controlled. The measurement conditions are the same as the conditions in FIG.

As shown in this figure, when the fluence of each shot of the pulsed laser light applied to the sample is not adjusted and is constant at 28 J / m ² , the number of ions per shot increases as the number of shots increases when the number of shots is small However, when the number of shots is exceeded, the number of ions per shot decreases as the number of shots increases.

On the other hand, when feedback control is performed for the fluence of each shot starting from the initial value of 28 J / m ² , a decrease in the number of ions per shot is suppressed even if the number of shots increases. Thus, even in the saturated fluence region or a region close thereto, a mass spectrum having both a high S / N ratio and mass resolution can be obtained in a short time.

FIG. 11 is a graph showing the cumulative number of ions obtained from each irradiation spot. This figure shows the case where the fluence of each shot of the pulsed laser light applied to the sample is not adjusted and the initial value is kept constant, and the case where the fluence of each shot is feedback controlled starting from the initial value. Yes. The initial values were 28 J / m ² , 42.9 J / m ² , 61.8 J / m ² , and 102.4 J / m ² . Other measurement conditions are the same as those in FIG. As shown in this figure, when the fluence of each shot is not adjusted and the initial value is kept constant, but when the fluence of each shot is feedback controlled starting from the initial value, the total number of ions is 1.6. It increased by a factor of 2.5 to 2.5.

  When feedback control of the fluence of each shot is started from the initial value, the initial value of the fluence may be a value of a low fluence region close to the ion generation threshold, or a value of a saturation fluence region or a region close thereto. May be. The fluence initial value may be a value in the range of 50% to 90% with respect to the fluence at which the maximum ionic strength is obtained. The initial fluence value is preferably somewhat smaller than the saturated fluence region.

  Next, another embodiment will be described. FIG. 12 is a diagram illustrating a configuration of the mass spectrometer 1A. A mass spectrometer 1A shown in FIG. 12 includes an intensity adjusting unit 20A instead of the intensity adjusting unit 20 in the configuration shown in FIG. The intensity adjustment unit 20 </ b> A includes a first rotation ND filter 21, a second rotation ND filter 22, and a shutter 23. These arrangement orders are arbitrary.

  Each of the first rotating ND filter 21 and the second rotating ND filter 22 has a rotatable disk shape, has a transmittance distribution along the circumferential direction, and emits a pulse laser beam with a transmittance according to the rotation direction. Can be transmitted. The shutter 23 can select whether to pass or block the pulse laser beam. Therefore, the intensity adjusting unit 20A can set the fluence to a value of 0 when the shutter 23 is in the off (blocking) state, and the first rotating ND filter 21 and the shutter 23 when the shutter 23 is in the on (passing) state. A fluence can be set in accordance with the product of the transmittance of each of the second rotary ND filters 22.

  FIG. 13 is a diagram illustrating an example of a change in the fluence over time in the configuration including the strength adjustment unit 20A. If the repetition frequency of the pulsed laser beam is 10 kHz, the shot interval is 100 μs. When each shot spot is irradiated with 1000 shots of pulsed laser light, the shutter 23 is turned on for a measurement period of 100 ms. When the period during which the irradiation spot is moved is 100 ms, the shutter 23 is turned off over the movement period of 100 ms.

  The rotation of the second rotating ND filter 22 is controlled by the processing unit 60 so as to rotate once in the measurement period of 100 ms. The second rotation ND filter 22 can make a change in fluence over time in a desired pattern by making one rotation in a measurement period of 100 ms in which the shutter 23 is in an on state. It is preferable that the fluence gradually increases from the initial value immediately after the shutter 23 is turned on. The first rotation ND filter 21 is used when changing the fluence initial value.

  DESCRIPTION OF SYMBOLS 1,1A ... Mass spectrometer, 10 ... Laser light source, 20, 20A ... Intensity adjustment part, 21 ... 1st rotation ND filter, 22 ... 2nd rotation ND filter, 23 ... Shutter, 30 ... TOF tube, 31 ... Sample stand 32 ... extraction electrode, 40 ... sample stage drive unit, 50, 50A, 50B ... detection unit, 60 ... processing unit, 70 ... microchannel plate (MCP), 71 ... first anode electrode, 72 ... second anode electrode, 73: fluorescent plate, 74: imaging unit, 75: resistor, 76: constant voltage source, 77: capacitor.

Claims (8)

A laser light source that outputs a pulsed laser beam to be irradiated onto the mixture of the sample to be analyzed and the matrix;
An accelerating unit for accelerating and flying ions generated from the mixture by an electric field by irradiating the pulsed laser light on the mixture;
Ion which is provided at a position where ions flying from the acceleration unit arrive, outputs a time waveform signal representing the ion arrival time distribution for each shot of the pulsed laser light, and represents the size of the reached ion image A detector that also outputs an image signal;
For each shot of the pulsed laser beam, it is determined whether or not the size of the ion image is larger than a threshold based on the ion image signal, and the time waveform signal when the ion image is determined to be larger than the threshold A processing unit for integrating the other time waveform signals to obtain a mass spectrum of the sample, and
A mass spectrometer comprising: The detector includes a microchannel plate that generates electrons and multiplies electrons upon arrival of the ions, a first anode electrode that receives electrons output from the microchannel plate and outputs a first signal, and A second anode electrode that is provided separately from the first anode electrode and receives electrons output from the microchannel plate and outputs a second signal, and outputs one of the first signal and the second signal. Output as the time waveform signal, the other as the ion image signal,
The mass spectrometer according to claim 1. The detection unit generates electrons upon arrival of the ions and multiplies the electrons, and forms a fluorescent image corresponding to the ion image by receiving electrons output from the microchannel plate and the time A fluorescent plate that outputs a waveform signal; and an imaging unit that outputs imaging data obtained by imaging a fluorescent image formed on the fluorescent plate as the ion image signal.
The mass spectrometer according to claim 1. The processing unit controls the fluence of pulsed laser light irradiated on the mixture based on the time waveform signal.
The mass spectrometer of any one of Claims 1-3. An irradiation step of irradiating a mixture of a sample to be analyzed and a matrix with pulsed laser light output from a laser light source;
An acceleration step of accelerating and flying ions generated from the mixture by irradiating the pulsed laser light to the mixture by an electric field in an accelerating unit;
The detection unit provided at the position where the ions flying from the acceleration unit arrives outputs a time waveform signal representing the ion arrival time distribution for each shot of the pulsed laser light, and the size of the reached ion image A detection step of outputting an ion image signal representing
For each shot of the pulsed laser beam, it is determined whether or not the size of the ion image is larger than a threshold based on the ion image signal, and the time waveform signal when the ion image is determined to be larger than the threshold A step of integrating the other time waveform signals to obtain a mass spectrum of the sample,
A mass spectrometry method comprising: In the detection step, the detection unit generates a electron upon arrival of the ions and multiplies the electron, and outputs a first signal upon receiving the electron output from the microchannel plate. An anode electrode; and a second anode electrode that is provided separately from the first anode electrode and that receives electrons output from the microchannel plate and outputs a second signal. The first signal and the second electrode One of the signals is output as the time waveform signal, and the other is output as the ion image signal.
The mass spectrometric method according to claim 5. In the detection step, the detection unit generates a electron by the arrival of the ions and multiplies the electrons, and a fluorescence image corresponding to the ion image by receiving the electrons output from the microchannel plate. A fluorescent plate that forms and outputs the time waveform signal; and an imaging unit that outputs imaging data obtained by imaging a fluorescent image formed on the fluorescent plate as the ion image signal.
The mass spectrometric method according to claim 5. In the processing step, based on the time waveform signal, the fluence of pulsed laser light irradiated on the mixture is controlled.
The mass spectrometry method of any one of Claims 5-7.