JP7760944B2 - Analysis method and analysis device using optical microscope - Google Patents

Description

The present invention relates to an analytical method and analytical device using an optical microscope, and in particular to an analytical method and analytical device that can easily and inexpensively determine the content ratios of multiple types of compounds in an analytical sample using an optical microscope.

There is a need in various fields to accurately understand the proportions of the components that make up a substance. For example, in the field of non-ferrous metal smelting, there is a need for technology that can accurately quantitatively analyze the composition of useful minerals and less valuable gangue contained in mined ores in order to determine the types of flotation agents, such as inhibitors, to be used during flotation, as well as their dosage. Chemical analysis methods such as ICP emission spectroscopy and X-ray fluorescence analysis are known as methods for analyzing the composition of substances. While these chemical analysis methods can quantitatively analyze the overall composition of the sample being analyzed, when the sample being analyzed contains multiple types of components, such as the ore mentioned above, it is difficult to accurately quantitatively analyze the proportions of these components.

In response, a method has been proposed that combines imaging with an optical microscope with image analysis technology to analyze the content ratios of multiple types of components in a sample to be analyzed. For example, Patent Document 1 discloses a technique in which the sample to be analyzed is embedded in resin and the analysis surface is mirror-finished, and then this analysis surface is imaged with a reflected light microscope, and the resulting image is binarized using hue, brightness, and saturation, thereby quantitatively analyzing the sample to be analyzed.

In recent years, the use of mineral particle analyzers (MLA) for quantitative analysis of materials has been increasing. This technology, also known as SEM-EDS (Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy), involves scanning an electron beam across the surface of a sample containing multiple compositions to detect the secondary and backscattered electrons generated, thereby identifying the compositional morphology of the sample surface. It also performs elemental analysis by detecting the characteristic X-rays generated by the electron beam irradiation, allowing the content of each of the multiple compositions contained in the sample to be determined.

Japanese Patent Application Publication No. 01-307664

By using the technology described in Patent Document 1, it is believed that it will be possible to easily perform quantitative analysis of samples containing multiple compositions without requiring any particular expertise. However, when a sample contains multiple compositions with very small differences in the intensity of reflected light, it is difficult to distinguish between these multiple compositions when imaging the analysis surface with an optical microscope and processing the image. On the other hand, by using an MLA, accurate quantitative analysis can be performed even for samples containing multiple compositions with very small differences in the intensity of reflected light. However, because MLA is an extremely expensive analytical device, it may be overkill for some applications, such as when analyzing mined ore. Therefore, it is not suitable for applications requiring simple, low-cost analysis.

The present invention was made in consideration of the above circumstances, and aims to provide an analytical method and analytical device that can easily and inexpensively determine the content ratios of multiple types of compositions in an analytical sample containing these compositions.

As a result of extensive research into achieving the above-mentioned objectives, the inventors discovered that for an analysis sample containing multiple types of compositions, by understanding the unique optical properties of each of those compositions in advance, it is possible to easily and inexpensively determine the content ratios of those compositions, even when the sample contains multiple types of compositions with only small differences in the intensity of reflected light, and this led to the completion of the present invention.

In other words, the analytical method using an optical microscope according to the present invention is a method for quantitatively analyzing the content ratio of each of multiple types of compositions contained in a sample to be analyzed using an optical microscope, and is characterized by comprising the following steps: a first step of performing spectroscopic analysis by irradiating light onto each region of the multiple types of compositions present in an image obtained by imaging the sample to be analyzed with the optical microscope; a second step of characterizing each of the multiple types of compositions with a specific wavelength that exhibits unique optical properties based on the results of the spectroscopic analysis; a third step of imaging the sample to be analyzed while irradiating the sample with light of the characterized specific wavelength; a fourth step of measuring the area of the region occupied by the composition characterized by the specific wavelength from the image obtained by imaging; and a fifth step of calculating the content ratio by calculating the ratio of the area of the measured region to the area of all regions of the compositions in the image.

The analytical device of the present invention is an apparatus for quantitatively analyzing the content ratio of each of multiple types of compositions contained in a sample to be analyzed, and is composed of an optical microscope that images the analysis surface of the sample to be analyzed, a spectroscopic analyzer that performs spectroscopic analysis by irradiating light onto each region of the multiple types of compositions present in the captured image, and an image analyzer that performs image analysis based on the image captured by the optical microscope and the data obtained by the spectroscopic analyzer.The image analyzer is characterized by comprising: means for characterizing each type of composition with a specific wavelength that exhibits unique optical properties through the spectroscopic analysis; means for measuring the area of the region occupied by the composition characterized by the specific wavelength from an image captured while irradiating the sample to be analyzed with light of the characterizing specific wavelength; and means for calculating the content ratio by calculating the ratio of the area of the measured region to the area of all regions of the compositions in the image.

This invention makes it possible to easily and inexpensively determine the content ratios of multiple types of compositions in an analytical sample containing those compositions, making it of great industrial value.

FIG. 1 is a block flow diagram showing an embodiment of an analysis method using an optical microscope of the present invention. 1 is a configuration diagram of a specific example of an analytical device using an optical microscope of the present invention. 1 is a graph showing the spectral profiles of compositions A to D obtained in examples of the present invention. 1A and 1B are schematic diagrams of images acquired by an analysis method according to an embodiment of the present invention.

An embodiment of an analytical method using an optical microscope according to the present invention is described in detail below. This analytical method, which is a method for quantitatively analyzing the content ratio of each of multiple types of compositions contained in a sample to be analyzed using an optical microscope, comprises the following steps: a first step of performing spectroscopic analysis by irradiating light onto each region of the multiple types of compositions present in an image obtained by imaging the sample to be analyzed with the optical microscope; a second step of characterizing each of the multiple types of compositions with a specific wavelength that exhibits unique optical properties based on the results of the spectroscopic analysis; a third step of imaging the sample to be analyzed while irradiating the sample with light of the characterized specific wavelength; a fourth step of measuring the area of the region occupied by the composition characterized by the specific wavelength from the image obtained by the imaging; and a fifth step of calculating the content ratio by calculating the ratio of the area of the measured region to the area of all regions of the compositions in the image.

As described above, samples targeted by the analytical methods of embodiments of the present invention contain multiple types of compositions. Examples of such samples include ore raw materials for dry copper smelting, which contain useful metals such as sulfide ore and oxide ore, gangue with little utility value, and non-metallic oxides; concentrates recovered by flotation of the ore raw materials and the remaining tailings; and intermediates containing useful metals, slag composed of slag components, and other oxides, which are produced by smelting the concentrates in dry copper smelting using the concentrates as intermediate raw materials.

As mentioned above, when a sample to be analyzed contains multiple types of compositions, a known method for determining the content ratio of each of those multiple types of compositions is to first embed the sample to be analyzed in resin, and then mirror-polish the embedded sample to expose the analytical surface in a thin slice form. An image is then taken using an optical microscope, the image obtained is analyzed to determine the area of the region occupied by each composition in the image, and the content ratio of the multiple types of compositions is determined based on the area ratio obtained.

When using the optical microscope described above, so-called white light, which contains a wide range of wavelengths, is used as the light source, such as a halogen lamp. However, depending on the sample being analyzed, the image of reflected light captured when irradiated with white light may contain multiple types of compositions, making it difficult to distinguish one type from another. In such cases, it is difficult to accurately determine the proportions of the multiple compositions contained in the sample being analyzed.

In contrast, the analytical method of an embodiment of the present invention first performs spectroscopic analysis on the sample to be analyzed using a spectroscopic analyzer incorporated into an optical microscope, as described above, to characterize each of the multiple types of compositions contained in the sample to be analyzed by a specific wavelength that exhibits unique optical properties.The sample to be analyzed is then imaged while being irradiated with light of this specific wavelength, making it possible to clearly distinguish areas in the resulting image occupied by compositions characterized by this specific wavelength from areas occupied by other compositions.

To explain in more detail, as shown in Figure 1, an embedded sample is prepared by embedding a powdered or granular sample to be analyzed in resin and then polishing it to a mirror finish. In the first step, the embedded sample is placed on the stage of an optical microscope, and measurement points are determined for each region of multiple types of composition that appear on the analysis surface of the embedded sample through visual observation using an optical microscope with a normal light source. Light is then irradiated onto these measurement points for spectroscopic analysis. This results in a spectrum for each composition, providing a result of spectroscopic analysis. In this first step, spectroscopic analysis may be performed by determining only one measurement point for each type of composition that can be visually distinguished by its shape or brightness, or by determining measurement points for all composition regions that appear within the field of view, regardless of the type of composition that can be visually distinguished.

Next, in the second step, the spectral profiles of the multiple types of compositions obtained in the first step above are compared, and a portion of each spectrum whose relative intensity significantly differs from the relative intensity of the other spectra is selected, and each composition is characterized by the wavelength of that portion. For example, if a certain composition contained in the sample to be analyzed has the optical property of absorbing light of a specific wavelength, its spectrum will have a characteristic profile that drops sharply downward in a convex shape at that specific wavelength. On the other hand, if another composition has the optical property of not absorbing light at all within a specific range of wavelengths, its spectrum will have a characteristic profile that rises gently upward in a convex shape within that wavelength range. By comparing the profiles in this way, each composition can be characterized by a specific wavelength.

Even if the relative intensity of a certain spectrum is difficult to clearly distinguish from other spectra, as long as this difficult-to-distinguish spectrum is only one type of composition among multiple compositions, this does not pose a particular problem, as will be described below. Furthermore, if the profile of one spectrum is nearly identical to the profile of another spectrum across the entire wavelength range, it can be determined that both spectra belong to the same type of composition. Therefore, even if spectroscopic analysis is performed on the areas of all compositions that appear within the field of view in the previous first step, as described above, the specific wavelengths for each type of composition can be determined in this second step.

Next, in the third step, the embedded sample on the stage is irradiated with light of a specific wavelength, which was determined for each composition in the previous step, and then imaged using an optical microscope. The specific wavelength of light may be obtained by using a single-wavelength light source that emits that specific wavelength, or by passing light emitted from a general light source through a single-wavelength filter that transmits only that specific wavelength. Furthermore, the specific wavelength of light may be irradiated either by an epi-illumination method, in which the embedded sample is irradiated from the objective lens side, or by a transmission method, in which the embedded sample is irradiated from the opposite side of the objective lens. Generally, the former method is preferred for samples that are difficult for light to transmit or for thick samples.

Next, in the fourth step, the area of the region occupied by the composition characterized by the specific wavelength is measured from the image obtained by imaging while irradiating with light of the specific wavelength in the third step. For example, when comparing four spectral profiles obtained by spectroscopic analysis of an embedded sample obtained in the second step before the previous step by embedding a powdered analysis target sample composed of three types of compositions B to D in a resin composed of composition A, the profile of composition A shows a sharp downward convex decrease in relative intensity at wavelength _λA that is significantly different from those of the other compositions B, C, and D, the profile of composition B shows a gradual upward convex increase in relative intensity at wavelength _λB that is significantly different from those of the other compositions A, C, and D, the profile of composition D shows a significant upward deviation in relative intensity from those of the other compositions A, B, and C over almost the entire wavelength range, and the relative intensity of composition C is not significantly different from those of the other compositions A and B, then composition A can be characterized by wavelength _λA , composition B can be characterized by wavelength _λB , and composition D can be characterized over the entire wavelength range.

Therefore, in an image obtained by first irradiating the embedded sample with light of wavelength _λA and photographing it with an optical microscope, the region where composition A is present appears darker than other regions because the light of wavelength _λA is absorbed. Therefore, by measuring the area of this dark region, the area _SA of the region occupied by composition A in the image can be easily determined. On the other hand, in an image obtained by similarly irradiating the embedded sample with light of wavelength _λB and photographing it with an optical microscope, the region where composition B is present appears brighter than other regions because almost all of the light of wavelength _λB is reflected. Therefore, by measuring the area of this bright region, the area _SB of the region occupied by composition B in the image can be easily determined.

Since composition D has a different relative intensity from the other compositions A to C across almost the entire wavelength range, the region where it is present can be easily identified, and therefore the area S _D of the region occupied by composition D in the image can be easily calculated. The area S _C of the region where composition C is present can be calculated by subtracting the area S A of the region of composition A measured in the image captured with light of wavelength λ _A , the area S _B of the region of composition B measured in the image captured with light of wavelength λ _B , and the area S _D of the region of composition _D from the area of the image where compositions A to D are present.

Next, in step 5, the analysis surface of the embedded sample, in which a powdery sample to be analyzed composed of compositions B to D is embedded in a resin made of composition A as described above, is imaged multiple times using an optical microscope while irradiating the sample with light of different wavelengths, and the areas S _A to S _D of the regions occupied by compositions A to D are measured for each of the multiple images obtained, and the content ratio of each composition in the sample to be analyzed can be determined from the ratio of these values to the total value. For example, the volumetric content V _B of composition B can be determined from the following formula 1.

[Formula 1]
If the density of each composition is known, the mass-based content ratio M _B of composition B can be calculated using the following formula 2: where ρ _B , ρ _C , and ρ _D are the densities of composition B, composition C, and composition D, respectively.

[Formula 2]

In the analytical method according to an embodiment of the present invention, it is preferable to repeat steps 1 to 5 described above using an optical microscope for multiple locations on the analysis surface of the embedded sample, and then calculate the arithmetic mean of the composition content determined for each composition in the group of images obtained by capturing images of these multiple locations. This makes it possible to average out the variation in the composition content depending on the location where the images are captured, thereby improving the accuracy of the analytical results.

When using an optical microscope to image multiple locations on the analysis surface of an embedded sample as described above, it is preferable to use an optical microscope with a tiling function, as this allows for efficient quantitative analysis. The tiling function fully automatically controls the XY movement and imaging of the stage on which the embedded sample is placed, thereby continuously imaging the analysis surface of the embedded sample, and then stitches together the multiple images thus acquired to ultimately obtain a single image covering a wide area (combining multiple fields of view) in the optical microscope field of view.

The analytical method of the embodiment of the present invention described above can be suitably carried out using an analytical device such as the one shown in Figure 2. That is, this analytical device is composed of at least an optical microscope 1 that captures an image of the analysis surface of the sample to be analyzed; a spectroscopic analyzer 2 that performs spectroscopic analysis by irradiating light onto each region of the multiple types of compositions present in the captured image; an image analyzer 3 that performs image analysis based on the image captured by the optical microscope 1 and the data obtained by the spectroscopic analyzer 2; and a control device 4 such as a personal computer that controls these components.

The image analysis device 3 includes a means for characterizing each type of composition with a specific wavelength that exhibits unique optical properties based on the spectroscopic analysis performed by the spectroscopic analysis device 2, a means for measuring the area of the region occupied by the composition characterized by the specific wavelength from an image captured while irradiating the sample to be analyzed with light of the specific wavelength, and a means for calculating the content ratio by calculating the proportion of the area of the measured region to the area of all regions of the composition in the image.

The above-described analytical device may be equipped with a mechanism capable of irradiating the embedded sample with single-wavelength light by replacing the light source 5 of the optical microscope 1 with a single-wavelength light source. It may also be equipped with a mechanism capable of irradiating the sample with single-wavelength light by placing a single-wavelength filter 6 on the optical axis between the light source 5 of the optical microscope 1 and the sample to be observed. The analytical method of the above-described embodiment of the present invention will now be described in more detail using the following examples and comparative examples.

[Example]
The sample to be analyzed was a matte, an intermediate product produced in a flash furnace of a dry copper smelting process, which was cooled and then crushed to a particle size of a few millimeters or less. Analysis of this sample by ICP atomic emission spectroscopy revealed that it was a mixture of three compositions: composition B, which consisted of oxides derived from the copper concentrate raw material that did not react during the smelting process in the flash furnace; composition C, which served as slag consisting of slag components produced during the smelting process in the flash furnace; and composition D, which served as metal consisting of metal components produced during the smelting process in the flash furnace. Separate analysis of the compositions revealed that the majority of the sample to be analyzed, accounting for more than 80%, was metal from composition D, with the remaining components accounting for less than 10% of each.

Because the sample to be analyzed is in granular form, it is difficult to observe it as is using an optical microscope. Therefore, the granular sample to be analyzed was mixed with powdered resin and heated to harden, embedding the sample in the solidified resin. Specifically, a thermosetting phenolic resin was used as the embedding resin, and the sample was placed in a vial in a ratio of 1 part sample to 10 parts resin by volume, and mixed using a rocking mill.

The resulting mixture was heated to 90°C using a pressure heating device (CitoPress-20) manufactured by Marumoto Struers K.K., held there for 4 minutes, then heated to 180°C and held there for 5 minutes under a pressure of 75 bar to harden. To expose the observation surface (analysis surface) of the resulting embedded sample, it was polished with abrasive paper and buffed, and the exposed surface of the cross section of the embedded sample was then polished to a mirror finish to obtain the embedded sample.

The resulting embedded sample was placed on the stage of a Hirox Corporation optical microscope (Microscope RH-2000) with a tiling function, and the content of each composition was determined by performing spectroscopic and image analysis on the analysis surface according to the procedures (1) to (3) below. Image analysis was performed using a personal computer equipped with Mitani Corporation's image analysis software (WinROOF2018), and spectroscopic analysis was performed using a JASCO Corporation microscopic ultraviolet-visible-near-infrared spectrophotometer (MSV-5200). The light source for the optical microscope was a 300W xenon light source (MAX303) manufactured by Asahi Spectroscopy Co., Ltd.

(1) Determination of the specific wavelength of each composition based on spectroscopic analysis. Using a halogen lamp or xenon lamp as the light source for an optical microscope, measurement targets were determined for each of the four compositions A to D that appeared on the analysis surface of the embedded sample by visual observation, and light was irradiated onto the target targets for spectroscopic analysis. This spectroscopic analysis yielded four spectra, as shown in Figures 3(a) and 3(b). Comparing the profiles of these four spectra, it was found that the spectrum of composition A had a distinctive optical characteristic of a downwardly convex slope at a wavelength of 270 nm, distinct from the other spectra, while the spectrum of composition B had a distinctive optical characteristic of an upwardly convex slope at a wavelength of 440 nm, distinct from the other spectra. Therefore, compositions A and B were characterized at these wavelengths, 270 nm and 440 nm, respectively. Furthermore, the spectrum of composition D had a higher relative intensity than the other spectra across almost the entire wavelength range, and composition D was characterized across this entire wavelength range.

(2) Area measurement of each composition by irradiation with light of a specific wavelength As described above, composition D could be characterized over the entire wavelength range, so the analysis surface of the embedded sample was first irradiated with light from the halogen lamp used as the light source for the optical microscope to obtain the first image shown in Figure 4(a). This first image was taken while irradiating with light of a wavelength that characterizes composition D, so the area with the highest brightness occupied by composition D could be easily distinguished from the areas occupied by the other compositions. This made it possible to measure the area of the region occupied by composition D in the image.

Next, since composition A could be characterized at a wavelength of 270 nm, the light from a xenon lamp, the light source for the optical microscope, was passed through a 270 nm wavelength filter (LX0270) manufactured by Asahi Spectroscopy Co., Ltd., and the light obtained was irradiated onto the analysis surface of the embedded sample, yielding the second image shown in Figure 4(b). Because this second image was captured while irradiating it with light of a wavelength that characterizes composition A, the dark areas occupied by composition A, which have the lowest brightness, can be easily distinguished from areas occupied by other compositions, making it possible to measure the area of the composition A region.

Next, since composition B could be characterized at a wavelength of 440 nm, the analysis surface of the embedded sample was irradiated with light obtained by passing light from a halogen lamp (the light source of the optical microscope) through a 440 nm filter (#86-350) manufactured by TECHSPEC Corporation, resulting in the third image shown in Figure 4(c). Because this third image was captured while irradiating the analysis surface with light at a wavelength characteristic of composition B, it was possible to distinguish the brighter regions occupied by composition B and the brighter regions occupied by composition D from the regions of compositions A and C. Among the regions of compositions B and D, the area of the region of composition D was already determined when processing the first image described above, so by excluding this area, it became possible to measure the area of the region occupied by composition B. The areas of the regions occupied by compositions A, B, and D could be determined by processing the first through third images described above, and by subtracting these from the area of the entire image, the area of the region occupied by composition C within the image could be determined.

(3) Calculation of the Content of Each Composition When capturing the first to third images, the optical microscope was set at a magnification of 300x, and a tiling function was used to continuously capture a matrix of 10 fields of view vertically and horizontally (i.e., a total of 100 fields of view). The resulting images were then stitched together to form a single image covering a wide area (combining multiple fields of view) in the optical microscope field. The area measurements were performed by importing image data into an image analyzer and determining a threshold value using a conventional method to calculate the area occupied by each composition. From the resulting area ratios of compositions A to D, the content of each composition could be calculated as shown in Table 1 below.

[Comparative Example]
Instead of characterizing each composition by a specific wavelength based on spectroscopic analysis as in the above-described Examples, an attempt was made to calculate the content ratio of each composition by analyzing images taken with an optical microscope of an embedded sample similar to that of the Examples while irradiating it with light from a halogen lamp used as a light source. As a result, it was difficult to distinguish the areas occupied by compositions A to C, which had weak relative intensities, on the image, and therefore it was not possible to measure the area ratio except for 85% of composition D. The results of this Comparative Example, together with those of the above-described Examples, are shown in Table 2 below.

Separate analysis of each composition using MLA revealed that composition A was a resin. Therefore, to determine the content ratios of compositions B, C, and D that made up the sample being analyzed, the value for composition A was subtracted from the values in Table 1 above, and the total of the values for the remaining three compositions, B, C, and D, was set to 100%, and the content ratios of each composition were converted. The results are shown in Table 3 below.

The results of the above examples and comparative examples demonstrate that by adopting the analytical method of the examples of the present invention, it is possible to easily and inexpensively analyze the content ratios of multiple types of compositions, even in samples containing only small differences in the intensity of reflected light.

REFERENCE SIGNS LIST 1 Optical microscope 2 Spectroscopic analyzer 3 Image analyzer 4 Control means 5 Light source 6 Single wavelength filter

Claims (8)

A method for quantitatively analyzing the content ratio of each of multiple types of compositions contained in a sample to be analyzed using an optical microscope, comprising: a first step of performing spectroscopic analysis by irradiating light onto each region of the multiple types of compositions present in an image obtained by imaging the sample to be analyzed with the optical microscope; a second step of characterizing each of the multiple types of compositions with a specific wavelength that exhibits unique optical properties based on the results of the spectroscopic analysis; a third step of imaging the sample to be analyzed while irradiating the sample with light of the characterized specific wavelength; a fourth step of measuring the area of the region occupied by the composition characterized by the specific wavelength from the image obtained by the imaging; and a fifth step of calculating the content ratio by calculating the ratio of the area of the measured region to the area of all regions of the composition in the image. The analysis method using an optical microscope described in claim 1, characterized in that a single-wavelength light source is used for the light having the specific wavelength. The analysis method using an optical microscope described in claim 1, characterized in that light having the specific wavelength is passed through a single-wavelength filter. The analytical method using an optical microscope according to any one of claims 1 to 3, wherein the imaging of the sample to be analyzed using the optical microscope involves taking multiple images using a tiling function possessed by the optical microscope, repeating steps 1 to 5 for each of the group of images thus acquired, and arithmetically averaging the multiple content ratios obtained. An apparatus for quantitatively analyzing the content ratio of each of multiple types of compositions contained in a sample to be analyzed, comprising an optical microscope that images the analysis surface of the sample to be analyzed, a spectroscopic analyzer that performs spectroscopic analysis by irradiating light onto each region of the multiple types of compositions present in the captured image, and an image analyzer that performs image analysis based on the image captured by the optical microscope and the data obtained by the spectroscopic analyzer. The image analyzer comprises: means for characterizing each type of composition with a specific wavelength that exhibits unique optical properties through the spectroscopic analysis; means for measuring the area of the region occupied by the composition characterized by the specific wavelength from an image captured while irradiating the sample to be analyzed with light of the characterizing specific wavelength; and means for calculating the content ratio by calculating the ratio of the area of the measured region to the area of all regions of the composition in the image. The analytical device described in claim 5, wherein the optical microscope's light source is replaceable with a single-wavelength light source. The analytical device described in claim 5, characterized in that the optical microscope is capable of mounting a single-wavelength filter on the optical axis between its light source and the mounting portion for the sample to be analyzed. The analytical device according to any one of claims 5 to 7, wherein the optical microscope has a tiling function.