JP2009164034A - Laser desorption ionization method, laser desorption ionization device, and mass spectroscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve a high ion generation efficiency and secure a sufficient detection sensitivity even when a beam convergence diameter of laser beam is reduced to near the diffraction limit. <P>SOLUTION: The beam diameter of the laser beam emitted from a laser light source 1 is expanded by a beam expander 2, and immediately after that, it is made to pass through a circumferential light polarizer 3. Then, near an optical axis C, light energy is mutually offset by interference and energy distribution is formed into a doughnut-shape beam. The light is collected by a lens 4 and irradiated to a sample 5. The energy distribution of light spots on the sample 5 is doughnut-shaped, and even when light is collected to near the diffraction limit, the energy distribution shape does not change. Accordingly, two conditions are fulfilled that energy distribution stays within a limited range and there is a region having spatially low density in the energy distribution, and even in a mass analysis in a minute range, the high ion generation efficiency can be achieved, and the sufficient detection sensitivity can be secured. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a laser desorption / ionization method for ionizing a sample, a laser desorption / ionization apparatus for performing the method, and a mass spectrometer using the laser desorption / ionization apparatus.

  Laser desorption / ionization is a method in which a sample is irradiated with laser light and ionization is performed by accelerating the movement of charges inside the substance that has absorbed the laser light. Matrix Assisted Laser Desorption / Ionization (MALDI) absorbs laser light to analyze samples that are difficult to absorb laser light and samples that are easily damaged by laser light, such as proteins. A material that is easy to ionize and is easily ionized is mixed in advance in the sample as a matrix, and the sample is ionized by irradiating the sample with laser light. In particular, a mass spectrometer using MALDI, for example, MALDI-TOFMS in which MALDI and a time-of-flight mass spectrometer (TOFMS = Time of Flight Mass Spectrometer) are combined, analyzes a polymer compound having a large molecular weight without much cleavage. In recent years, it has been widely used in fields such as life science. In this specification, the general ionization method including MALDI and ionizing sample components by directly or indirectly irradiating the sample with laser light is referred to as a laser desorption ionization method (abbreviated as LDI). .

  In the mass spectrometer using the above-mentioned LDI, the spot diameter of the irradiation laser beam is reduced, and the irradiation position is moved two-dimensionally on the sample, and mass spectrometry is performed at each position. An image representing the intensity distribution (two-dimensional substance distribution) of ions having a certain mass (strictly speaking, the mass-to-charge ratio m / z) can be obtained. Such an apparatus is known as a mass spectrometry microscope, a microscopic mass spectrometer, an imaging mass spectrometer, or the like. In particular, in the biochemistry field, the medical field, and the like, an application for obtaining distribution information of proteins contained in living cells. Is expected (see, for example, Non-Patent Document 1, Patent Document 1, etc.).

  When obtaining the two-dimensional substance distribution image as described above, it is desirable that the spatial resolution is high in order to obtain useful knowledge about the sample in various fields of use. For this purpose, it is necessary to reduce the focusing diameter of the laser light to, for example, about several μm or less. In conventional general LDI, a Gaussian beam emitted from a laser light source is condensed by a lens optical system and irradiated onto a sample. In this case, the energy (light intensity) distribution of the laser beam on the sample has a Gaussian distribution with the largest energy near the optical axis, and the area of the region having an energy density effective for LDI is small. Only low detection sensitivity can be obtained with low ion generation.

  On the other hand, mainly in order to increase ion generation efficiency, Non-Patent Document 2 attempts to form a plurality of light spots that are appropriately spaced on a sample by shaping laser light emitted from a laser light source. Yes. For this purpose, optical elements such as multimode fibers, microlens arrays, light diffusion foils, and perforated masks can be used. However, even if such an optical element is used, since the electromagnetic field around the optical axis of the original laser beam does not change greatly, the energy distribution of the entire spot has a simple Gaussian distribution. Therefore, if the laser beam is narrowed down to a size close to the diffraction limit, the ion generation efficiency is poor and it is difficult to obtain a satisfactory detection sensitivity.

US Patent No. 5808300 Yasuhide Naito, “Mass Microscope for Biological Samples”, J. Mass Spectrom. Soc. Jpn., Vol. 53, No. 3, 2005, pp. 125-132. Armin Holle and three others, “Optimizing UV laser focus profiles for improved MALDI performance”, Journal of Journal of Mass Spectrometry, 2006, 41, p.705-716

  The present invention has been made in view of the above problems, and its main purpose is a laser that can achieve high detection sensitivity even when the condensing diameter of laser light is reduced in order to achieve high spatial resolution. To provide a desorption ionization method, a laser desorption ionization apparatus, and a mass spectrometer.

According to the above-mentioned Non-Patent Document 2, it is shown that the following two conditions are required to efficiently perform ionization by LDI, that is, to increase detection sensitivity.
(1) The light energy is within a finite range in the vicinity of the sample.
(2) A region having a small light energy density exists spatially.
Non-Patent Document 2 uses an optical system that disperses a plurality of spot points of a laser beam in a lattice shape in order to satisfy the above condition. However, since the energy distribution at each point is a Gaussian distribution, it is difficult to obtain an optimum energy distribution for LDI when the light is condensed to a degree close to the diffraction limit. Therefore, the inventor of the present application paid attention to a laser beam having a donut-shaped energy distribution such as a Laguerre-Gaussian beam as a laser beam that satisfies the above-described conditions and has an energy distribution that is not a simple Gaussian distribution.

  That is, a first invention made to solve the above problems is a laser desorption ionization method in which a sample is irradiated with laser light to ionize components contained in the sample, and the beam is perpendicular to the optical axis. It is characterized by condensing laser light having a circular cross-sectional shape and irradiating the sample.

  The second invention made to solve the above-mentioned problems is an apparatus for carrying out the laser desorption ionization method according to the first invention, and the components contained in the sample by irradiating the sample with laser light. Is a laser desorption ionization apparatus that ionizes a laser beam, a laser irradiation unit that emits laser light whose beam cross-sectional shape perpendicular to the optical axis is annular, and a condensing unit that condenses the laser light and irradiates the sample It is characterized by providing these.

  In the laser desorption ion method according to the first invention carried out by the laser desorption ionization apparatus according to the second invention, when the energy distribution is viewed in a plane perpendicular to the optical axis, the laser desorption ionization method has a substantially circular shape around the optical axis. The sample is irradiated with laser light that has zero energy and has an annular (doughnut-shaped) energy around it. This cross-sectional shape does not depend on the distance from the light collecting means, and maintains the same cross-sectional shape (however, the sizes are different) at an arbitrary position along the optical axis. That is, it is a laser beam called a Laguerre-Gaussian beam.

  At this time, even if the converging diameter of the laser beam is reduced to near the diffraction limit by the condensing means, the outer diameter does not spread extremely greatly. When such a laser beam is irradiated onto the sample, the vicinity of the center (optical axis) of the laser beam becomes a cavity having a low energy density, and this cavity extends in the height direction perpendicular to the sample. In addition, in the donut-shaped spot, the portion where the light energy is maximized has a circular shape, and therefore, a condition equivalent to that a large number of spots are continuous is satisfied. That is, the two conditions mentioned above, that is, that the light energy is in a finite range in the vicinity of the light spot, and that a region having a low energy density exists in the light spot, Can be realized with a beam having a circular beam cross-sectional shape. Therefore, ions can be generated efficiently and high detection sensitivity can be achieved.

  In the laser desorption / ionization apparatus according to the second aspect of the present invention, the laser irradiation means can make the beam cross-sectional shape perpendicular to the optical axis circular by various methods, but the apparatus is small and light, and the energy distribution is stable. In consideration of the characteristics (for example, small temporal variation), the laser irradiation means is configured so that the laser light source and the laser light emitted from the laser light source have an annular shape in a beam cross section perpendicular to the optical axis. And a beam shaping means for shaping into a shape.

  As a more specific aspect, the beam shaping means can use a polarizing optical element provided with a plurality of half-wave plates so that the polarization main axis sequentially rotates in a circumferential direction centered on the optical axis. This is known, for example, as a circumferential polarizer, a Z polarizer or the like. As another aspect, the beam shaping means may use a polarizing optical element made of a refracting plate whose thickness changes continuously or stepwise in the circumferential direction around the optical axis. This is known, for example, as a spiral phase plate.

  In any of the aspects, energy is canceled out by interference at the central portion along the optical axis at a position further away from the insertion position of the polarizing optical element to the rear (beam exit side) at a predetermined distance. A beam showing a light intensity distribution is stably formed. Although high accuracy is required for the manufacturing of the polarizing optical element itself and the accuracy of the insertion position (optical axis alignment) of the element, the structure is basically simple and the part is mechanically driven. Therefore, the cost can be easily reduced, and it is suitable for reducing the size and weight of the apparatus.

  The laser desorption / ionization apparatus according to the second invention is naturally applicable to general MALDI-TOFMS, but usually in such MALDI-TOFMS, the beam focusing diameter of the laser beam on the sample is not much. There is no need to squeeze, and detection sensitivity is not a problem. On the other hand, in the microscopic mass spectrometer, a high spatial resolution is required, that is, there is a demand for reducing the beam focusing diameter of the laser beam on the sample as small as possible. Therefore, the laser desorption ionization apparatus according to the second invention Is particularly beneficial.

That is, the third invention is a mass spectrometer using the laser desorption ionization apparatus according to the second invention,
Position scanning means for scanning the irradiation position of the laser beam on the sample one-dimensionally or two-dimensionally;
A mass analyzing means for separating and detecting ions generated from the sample by irradiation with the laser beam according to the mass;
A distribution investigation means for examining a mass distribution state in the predetermined range by repeating mass analysis of the micro area by the mass analysis means while performing position scanning by the position scanning means for the predetermined range on the sample;
It is characterized by having.

  In the laser desorption / ionization apparatus according to the second invention, for example, even if a laser beam having a beam diameter of about 3 to 20 mm is condensed to near the diffraction limit (approximately wavelength / numerical aperture), the energy distribution of the light spot on the sample Is maintained in a donut shape. Thereby, for example, mass analysis of a very small region of 5 μm or less can be performed with sufficient detection sensitivity, and a two-dimensional substance distribution image with high spatial resolution can be created.

  In the mass spectrometer according to the third aspect of the present invention, it is preferable that the area of the minute region is changed by changing the outer diameter of the laser light applied to the sample. According to this configuration, when it is sufficient that a two-dimensional substance distribution image of a predetermined range on the sample can be acquired with low spatial resolution, by increasing the condensing diameter of the laser beam on the sample and reducing the number of analysis points, Results can be obtained in a short time. On the other hand, when it is desired to acquire a two-dimensional substance distribution image of a predetermined range on the sample with high spatial resolution, it takes time by performing mass analysis densely by reducing the condensing diameter of the laser beam on the sample. A precise two-dimensional material distribution image can be obtained.

  According to the laser desorption / ionization method according to the first invention and the laser desorption / ionization apparatus according to the second invention, high ion generation efficiency can be achieved even when the beam focusing diameter of the laser beam is reduced to execute mass analysis of a minute region. Achieved, thereby obtaining sufficient detection sensitivity. Further, according to the mass spectrometer according to the third invention using the laser desorption ionization apparatus according to the second invention, a two-dimensional substance distribution image of a predetermined range on the sample can be obtained with high spatial resolution. For example, it becomes possible to obtain knowledge about samples that have been difficult to obtain in the past.

  Conventionally, beam shaping that changes the cross-sectional shape of the beam includes beam splitting using a mask, correction using an attenuation (ND) filter, wavefront by a diffractive optical element (DOE = Diffractive Optical Elements) such as a computer generated hologram. There are several methods such as generation, but when a single optical element is used, the required light energy distribution can be obtained only at spatially limited positions even on the optical axis. That is, even if it is on the optical axis, if it deviates from the position, the light energy distribution does not have a desired shape. Therefore, when it is desired to make the light energy distribution in a desired shape at an arbitrary position on the optical axis, it is necessary to switch between a plurality of optical elements having different characteristics, which is inevitably large in terms of mechanism. .

  On the other hand, when a donut-shaped beam such as a Laguerre-Gaussian beam is used, the shape of the hollow energy distribution is maintained in a wide range along the optical axis. It is possible to set an ionization region having a size of. Thereby, when realizing a microscopic mass spectrometer, the stability and reproducibility of the results obtained by changing the spatial resolution are increased, and high performance can be realized.

  First, an embodiment of a laser desorption / ionization apparatus for carrying out the laser desorption / ionization method according to the present invention will be described with reference to FIGS. FIG. 1 is a schematic configuration diagram of a laser desorption / ionization apparatus according to this embodiment.

  A laser light source (laser light source in the present invention) 1 is an Nd: YAG / THG laser having a wavelength of 355 nm (ultraviolet wavelength region), and the intensity distribution of the beam waist can be regarded as a Gaussian distribution by single mode oscillation. The laser beam is expanded to a beam diameter of about 10 mm by the beam expander 2. When this beam is condensed by a lens 4 (condensing means in the present invention) 4 having a focal length of 80 mm, a spot having a diffraction limit of 3.5 μm is obtained on the surface of the sample 5 at the minimum. Immediately after the beam expander 2 along the optical axis C of the laser beam, a circumferential polarizer (also called a Z polarizer) 3 as a beam shaping means in the present invention is inserted. Due to the optical interference action, the beam is shaped into an annular cross section in which no optical energy exists in the vicinity of the optical axis C and energy exists in a donut shape around the optical energy.

  FIG. 2 is a plan view (a) and a side view (b) of an example of the circumferential polarizer 3 in a plane orthogonal to the optical axis C. A plurality of circular polarizers 3 (four in this example) are arranged so that the angle of the polarization main axis (arrow in the figure) is rotated 180 ° on the circumference around the optical axis C. ) Half-wave (λ / 2) plates 3a, 3b, 3c, 3d are arranged and bonded together. The half-wave plates 3a to 3d are generally made of an optically anisotropic crystal such as calcite, but may be made artificially of a photonic crystal. When linearly polarized light is incident in a direction substantially orthogonal to the half-wave plates 3a to 3d, the direction of polarized light changes at an angle twice the rotation angle of the polarization main axis. The polarized light of the light emitted with respect to the incident polarized light is aligned in the radial direction or the azimuth direction, and the light energy on the optical axis C at a position more than a certain distance from there is zero or the vertical direction (that is, the optical axis C direction). Only a weak electric field is obtained, and a donut-shaped beam is obtained.

  In particular, in the case of radial polarization, a light spot having an electric field in the vertical direction (direction of the optical axis C) can be obtained by narrowing the beam focusing diameter with a lens having a large numerical aperture (NA). Utilizing this property, circumferential polarizers are used in near-field Raman microspectrophotometers and the like.

  FIG. 3 is a simulation result of beam shaping by the circumferential polarizer 3 having the configuration shown in FIG. 2, (a) is a three-dimensional display of the X-axis direction position, the Y-axis direction position, and the energy intensity, (b). Is a plane graph showing energy intensity in shades with respect to the X-axis direction position and Y-axis direction position on the vertical axis and on the horizontal axis. FIG. 4 is a diagram showing a calculation result simulating the condensing of a Gaussian beam obtained when a conventional, that is, a circular polarizer is not inserted.

  As apparent from FIG. 3, it can be seen that a clean donut-shaped beam is formed using the circumferential polarizer of FIG. That is, the energy distribution becomes zero at and near the optical axis C, the energy becomes maximum at a position away from the center by a predetermined distance (radius) in a plane orthogonal to the optical axis C, and the energy is further attenuated toward the outer periphery. The energy becomes zero when the distance is more than a predetermined distance. Therefore, the conditions for increasing the ion generation efficiency as described above are satisfied in that the light energy is within a finite range on the sample and a region having a small light energy density exists in the vicinity of the optical axis C. . Moreover, since the shape of such an energy distribution is maintained even when the light is condensed by the lens 4, high ion generation efficiency can be ensured even if the focusing diameter is reduced to near the diffraction limit.

  In order to obtain a laser beam having the same energy distribution as in the above embodiment, a spiral phase plate 6 may be inserted in place of the circumferential polarizer 3 in the configuration of FIG. FIG. 5 is a plan view (a) and a side view (b) of an example of the spiral phase plate 6 on a plane orthogonal to the optical axis C. The thickness of the spiral phase plate 6 was changed by cutting the surface so that there was a change in the optical path length for one wavelength (λ) on the circumference around the optical axis C. It consists of a refracting plate. The change in thickness may be continuous in the circumferential direction, but this is not always necessary. In this example, the optical path length changes in order of λ / 4 in the clockwise direction in FIG. In this way, four-step spiral stair-shaped refracting plates 6a, 6b, 6c, and 6d having thicknesses adjusted as described above are used. The surface shape thus formed is a reason called a spiral phase plate.

  The material of the refracting plate does not need to be special, and generally optically isotropic glass is used. However, since the required surface step size is inversely proportional to the number (n-1) obtained by subtracting 1 from the refractive index n of the material, a material with an appropriate refractive index is selected according to the realizable processing accuracy. There is a need to.

  Since the laser beam that has passed through the spiral phase plate has the property that the diameter of the hollow region where the energy generated around the center of the beam, that is, the optical axis C is zero, is less than the diffraction limit, it has been generally two wavelengths. Used for erase light for fluorescence dip spectroscopy.

  FIG. 6 is a simulation result of beam shaping by the spiral phase plate 6 having the configuration shown in FIG. 5, where (a) is a three-dimensional display of the X-axis direction position, the Y-axis direction position, and the energy intensity, and (b) is X It is the plane graph which showed the energy intensity with the light / dark on the vertical axis | shaft and the horizontal axis | shaft in the axial direction position and the Y-axis direction position. This energy distribution is almost the same as that of the circumferential polarizer 3 described above, and has the same effect.

  In addition to the use of the above-described circumferential polarizer and spiral phase plate, there is a method for obtaining a beam having a circular cross section in a plane perpendicular to the optical axis. For example, a combination of a multimode laser and a polarizer can be considered. The multimode laser can be generated from a single mode laser or directly by generating multimode oscillation in a laser resonator.

  The laser desorption ionization apparatus described above can be used for ordinary MALDI-TOFMS, but is particularly useful for a microscopic mass spectrometer for performing mass imaging of a biological sample or the like. An embodiment of such a micromass spectrometer will be described with reference to FIGS. FIG. 8 is an overall configuration diagram of the micro mass spectrometer according to the present embodiment.

  A sample stage 12 that is movable in at least two directions of the x-axis and y-axis by a stage drive unit 14 is disposed in an airtight chamber 11 that is maintained in a substantially atmospheric pressure atmosphere, and the sample 13 is placed on the sample stage 12. Placed. When the sample stage 12 is at a position indicated by a solid line in the figure, the laser beam emitted from the laser irradiation unit 10 and converged by the lens 4 strikes the upper surface of the sample 13. Upon receiving this laser light irradiation, ions derived from the sample are generated from the vicinity of the laser light irradiation position in the sample 13.

  Ions generated from the sample 13 by laser light irradiation in the hermetic chamber 11 are transported through the ion transport tube 15 into the vacuum chamber 20 evacuated by a vacuum pump (not shown). In the vacuum chamber 20, the ions are converged by the ion lens 21 and sent to the ion trap 22 at the subsequent stage. The ion trap 22 has a three-dimensional quadrupole configuration composed of a ring electrode and a pair of end cap electrodes, and temporarily accumulates and holds ions by an electric field formed by a high-frequency voltage applied to these electrodes. These ions are discharged almost simultaneously at a predetermined timing and introduced into a time-of-flight mass analyzer (TOF) 23. The time-of-flight mass analyzer 23 is a reflectron type equipped with a reflective electrode 24, and ions are turned back by a DC electric field formed by the reflective electrode 24. Various ions introduced almost simultaneously fly in different flight times according to their mass and reach the ion detector 25, and the ion detector 25 outputs a detection signal corresponding to the amount of incident ions.

  In the ion trap 22, ions having a specific mass among various ions are selected as precursor ions, then cleaved by CID (collision-induced dissociation), and the product ions generated thereby are converted into time-of-flight masses. It may be introduced into the analyzer 23 for mass analysis, that is, MS / MS analysis or mass analysis with multi-stage cleavage may be performed.

  Within the hermetic chamber 11, the sample stage 12 can be moved along the guide 16 to a position (observation position) 12B indicated by a dotted line in the drawing, and a CCD camera 17 is positioned outside the hermetic chamber 11 above the observation position 12B. Is disposed, and a transmission illumination unit 18 is installed below the observation position 12B. In a state where the sample stage 12 is moved so as to come to the observation position 12B, the light emitted from the transmission illumination unit 18 hits the lower surface of the sample 13 through the opening formed in the sample stage 12, and the sample image by the transmitted light is converted into a lens. 19 can be photographed by the CCD camera 17. A microscopic image captured by the CCD camera 17 can be displayed on the screen of the display unit 34 via the control unit 32. Of course, in addition to such transmission observation, an illumination system for reflection observation and fluorescence observation may be provided separately. Further, instead of taking an image with the CCD camera 17, an optical microscope may be provided so that the operator can directly observe the microscopic observation image.

  A detection signal obtained by the ion detector 25 by mass analysis is converted into a digital value by the A / D converter 30 and input to the data processing unit 31, and the data processing unit 31 represents the relationship between the mass and the signal intensity. Mass spectral data is determined. Further, the data processing unit 31 executes various types of data processing using the mass spectrum data, and creates, for example, a two-dimensional substance distribution image representing the distribution of predetermined molecules. The control unit 32 controls each unit including the stage driving unit 14 in order to execute a mass analysis operation on the sample, and displays a microscopic observation image, an analysis result (for example, the above two-dimensional substance distribution image), and the like on the display unit 34. To do. The operation unit 33 is a keyboard, a pointing device, or the like, and is used for input setting of various parameters for analysis and various instructions.

  The control unit 32 and the data processing unit 31 can be constituted by a general-purpose personal computer, for example, and various control and data processing functions are achieved by executing dedicated control / processing software installed in the computer. can do.

The basic operation for obtaining a two-dimensional substance distribution image of a target substance using the mass spectrometer is as follows.
First, the operator determines which part of the sample 13 is to be analyzed. For this purpose, the sample stage 12 is moved to the position 12B, and in this state, the CCD camera 17 acquires a microscopic observation image of the sample 13 and displays it on the screen of the display unit 34. The operator determines the range in which mass analysis is performed, sets the range by the operation unit 33, and instructs the start of analysis after setting one or more masses of the target substance, for example.

  When the analysis is started, the laser light that is the above-described donut-shaped beam emitted from the laser irradiation unit 10 is condensed by the lens 4 and irradiated onto the sample 13. The lens 4 is movable by the lens driving unit 40 so as to approach or move away from the sample 13. As shown in FIG. 7, when the position of the lens 4 is changed by the lens driving unit 40, the beam converging diameter on the sample 13 changes. Thereby, the area irradiated with the laser beam is changed by one laser irradiation, and the spatial resolution of the mass spectrometry is also changed. Therefore, when the spatial resolution is designated by an operator before the analysis is executed, the position of the lens 4 is automatically set so as to obtain a beam condensing diameter corresponding to the spatial resolution.

  When the laser beam is irradiated, various substances existing in the vicinity of the irradiation range on the sample 13 are ionized, and ions are mainly emitted in a direction substantially orthogonal to the surface of the sample 13, that is, directly above. Since the air in the hermetic chamber 11 is sucked into the ion transport pipe 15 due to the differential pressure across the ion transport pipe 15, the ions are mainly sucked into the ion transport pipe 15 by this air flow and transported into the vacuum chamber 20. Then, for example, ions generated by a plurality of times of laser light irradiation on the same position on the sample 13 are once collected in the ion trap 22 and then introduced into the time-of-flight mass analyzer (TOF) 23 all at once. Mass analyzed.

  The control unit 32 controls the stage driving unit 14 so that the irradiation position of the laser beam sequentially moves within the analysis range set on the sample 13, and moves the sample stage 12 in a step shape. The movement step width at this time is determined in accordance with the spatial resolution. That is, the beam condensing diameter of the laser beam on the sample 13 and the step width of the movement of the sample 13 in the xy plane are determined according to the spatial resolution. Then, each time the sample stage 12 is moved by a minute distance and stopped, the sample 13 is irradiated with the laser beam as described above, and mass spectrometry is executed. After mass analysis is performed over the entire set analysis range, for example, a two-dimensional material distribution image showing a two-dimensional distribution of a specific mass is created.

  When the set spatial resolution is large, that is, coarse, the beam focusing diameter of the laser beam on the sample 13 is relatively increased, and the step width of the sample movement is also increased. Therefore, if the analysis area has the same area, the number of analysis points is reduced and the time required for the analysis can be shortened. On the contrary, when the spatial resolution is small, the beam focusing diameter of the laser beam on the sample 13 is relatively small, and the step width of the sample movement is also small. Therefore, if the analysis range has the same area, the number of analysis points increases, and the number of analysis points is large and requires time for analysis. However, the obtained two-dimensional substance distribution image becomes precise. Thus, even when the position of the lens 4 is changed so as to change the beam focusing diameter of the laser light on the sample 13, the energy distribution of the light spot on the sample 13 becomes a donut shape. Therefore, regardless of the spatial resolution, ionization can be performed under substantially the same conditions and with high ion generation efficiency, and an accurate two-dimensional substance distribution image can be created with high detection sensitivity.

  The above-described embodiment is an example of the present invention, and it is a matter of course that changes, modifications, and additions within the scope of the present invention are included in the scope of the claims of the present application. For example, in the above embodiment, the case where the sample component is ionized by irradiating the sample with laser light has been described. However, the sample in this case may be a mixture of the matrix and the target sample. When a sample having a certain large area is ionized by MALDI, the sample can be prepared by various methods such as coating or spraying a matrix on the sample.

1 is a schematic configuration diagram of a laser desorption / ionization apparatus according to an embodiment of the present invention. The top view (a) in the surface orthogonal to an optical axis of an example of a circumferential polarizer, and its side view (b). The figure which shows the simulation result of the beam shaping by the circumferential polarizer of the structure shown in FIG. The figure which shows the simulation result of the condensing of the former (Gaussian beam). The top view in the surface orthogonal to an optical axis of an example of a spiral phase plate, and its side view (b). The figure which shows the simulation result of the beam shaping by the spiral phase plate of the structure shown in FIG. FIG. 9 is an explanatory diagram of a relationship between a change in lens position and a beam focusing diameter on a sample in the micro mass spectrometer of FIG. The whole block diagram of the micro mass spectrometer by a present Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Laser light source 2 ... Beam expander 3 ... Circumferential polarizer 3a, 3b, 3c, 3d ... Half-wave plate 4 ... Lens 5, 13 ... Sample 6 ... Spiral phase plate C ... Optical axis 10 ... Laser irradiation part 11 Airtight chamber 12 Sample stage 14 Stage drive unit 15 Ion transport tube 20 Vacuum chamber 21 Ion lens 22 Ion trap 23 Time-of-flight mass analyzer 24 Reflecting electrode 25 Ion detector 30 A / D converter 31... Data processing unit 32... Control unit 40.

Claims (7)

  A laser desorption / ionization method that ionizes components contained in a sample by irradiating the sample with a laser beam, the laser beam having a circular beam cross-sectional shape perpendicular to the optical axis being focused and irradiated on the sample And a laser desorption ionization method.   A laser desorption / ionization apparatus that ionizes components contained in the sample by irradiating the sample with laser light, the laser irradiation unit emitting laser light having a circular beam cross-sectional shape perpendicular to the optical axis; A laser desorption / ionization apparatus comprising: a condensing unit that condenses the laser light and irradiates the sample.   3. The laser desorption / ionization apparatus according to claim 2, wherein the laser irradiation means includes a laser light source and a laser beam emitted from the laser light source so that a beam cross-sectional shape perpendicular to the optical axis is annular. And a beam shaping means for shaping into a laser desorption ionization apparatus.   4. The laser desorption / ionization apparatus according to claim 3, wherein the beam shaping means is provided with a plurality of half-wave plates so that a polarization main axis is sequentially rotated in a circumferential direction around the optical axis. A laser desorption / ionization apparatus characterized by being an element.   4. The laser desorption / ionization apparatus according to claim 3, wherein the beam shaping means is a polarizing optical element comprising a refracting plate whose thickness changes continuously or stepwise in a circumferential direction centering on the optical axis. There is provided a laser desorption ionization apparatus. A mass spectrometer using the laser desorption / ionization apparatus according to any one of claims 2 to 5,
Position scanning means for scanning the irradiation position of the laser beam on the sample one-dimensionally or two-dimensionally;
A mass analyzing means for separating and detecting ions generated from the sample by irradiation with the laser beam according to the mass;
A distribution investigation means for examining a mass distribution state in the predetermined range by repeating mass analysis of the micro area by the mass analysis means while performing position scanning by the position scanning means for the predetermined range on the sample;
A mass spectrometer comprising:   The mass spectrometer according to claim 6, wherein the area of the minute region is changed by changing an outer diameter of laser light applied to the sample.