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Dispersion staining
The optical properties of all liquid and solid materials change as a function of the wavelength of light used to measure them. This change as a function of wavelength is called the dispersion of the optical properties. The graph created by plotting the optical property of interest by the wavelength at which it is measured is called a dispersion curve.
The dispersion staining is an analytical technique used in light microscopy that takes advantage of the differences in the dispersion curve of the refractive index of an unknown material relative to a standard material with a known dispersion curve to identify or characterize that unknown material. These differences become manifest as a color when the two dispersion curves intersect for some visible wavelength. This is an optical staining technique and requires no stains or dyes to produce the color. Its primary use today is in the confirmation of the presence of asbestos in construction materials but it has many other applications.
There are five basic optical configurations of the microscope used for dispersion staining. Each configuration has its advantages and disadvantages. The first two of these, Becke` line dispersion staining and oblique dispersion staining, were first reported in the United States by F. E. Wright in 1911 based on work done by O. Maschke in Germany during the 1870s. The five dispersion staining configurations are:
All of these configurations have the same requirements for the preparation of the sample to be examined. First, the substance of interest must be in intimate contact with the known reference material. In other words, the clean solid must be mounted in a reference liquid, one mineral phase must be in intimate contact with the reference mineral phase, or the homogenous liquid must contain the reference solid. Most applications involve a solid mounted in a reference liquid (referred to as the mounting medium). Second, dispersion colors will only be present if the two materials have the same refractive index for some wavelength in the visible spectrum (referred to as λo) and they have very different dispersions curves for the refractive index. Finally, the sample must be properly mounted under a coverslip to minimize any other optical effect that could complicate the interpretation of the color seen. Once these criteria are met the sample is ready to be examined.
The starting configuration of the microscope for all of these methods is properly adjusted Köhler illumination. Some additional adjustments are required for each of the methods.
The Becke' Line method takes advantage of the fact that particles behave basically like lenses because they tend to be thinner at the edges than they are at the center. If the particle has a higher refractive index than the liquid surrounding it then it behaves as a convex lens and focuses a parallel beam of light on the side opposite the source of the light. Looking through the microscope this is seen as a bright ring of light, the Becke` Line, moving in from the edge as the particle is dropped out of focus by increasing the distance between the stage of the microscope and the objective. If the stage is moved closer to the objective then the particle behaves like a magnifying glass and the image of the Becke` Line is magnified and it appears outside the particle.
A requirement for this method is that the incoming beam of light is as parallel as possible. This requires the closing down of the sub-stage condenser iris. Closing the sub-stage condenser iris decreases the resolution of the particle and increases the depth of field over which other objects may interfere with the effect seen. For large particles this is not a significant limitation but for small particles it is a problem.
When the conditions for dispersion staining are met (the particle is mounted in a liquid with a matching refractive index in the visible range of wavelengths but with a very different refractive index) then the particle has a high refractive index in the red part of the spectrum and a lower refractive index in the blue. This is because liquids tend to have a steeper dispersion curve than colorless solids. As a result, when the particle is dropped out of focus the red wavelengths are focused inward. For the blue wavelengths the particle behaves like a concave lens and the blue Becke` Line moves out into the liquid.
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Dispersion staining AI simulator
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Dispersion staining
The optical properties of all liquid and solid materials change as a function of the wavelength of light used to measure them. This change as a function of wavelength is called the dispersion of the optical properties. The graph created by plotting the optical property of interest by the wavelength at which it is measured is called a dispersion curve.
The dispersion staining is an analytical technique used in light microscopy that takes advantage of the differences in the dispersion curve of the refractive index of an unknown material relative to a standard material with a known dispersion curve to identify or characterize that unknown material. These differences become manifest as a color when the two dispersion curves intersect for some visible wavelength. This is an optical staining technique and requires no stains or dyes to produce the color. Its primary use today is in the confirmation of the presence of asbestos in construction materials but it has many other applications.
There are five basic optical configurations of the microscope used for dispersion staining. Each configuration has its advantages and disadvantages. The first two of these, Becke` line dispersion staining and oblique dispersion staining, were first reported in the United States by F. E. Wright in 1911 based on work done by O. Maschke in Germany during the 1870s. The five dispersion staining configurations are:
All of these configurations have the same requirements for the preparation of the sample to be examined. First, the substance of interest must be in intimate contact with the known reference material. In other words, the clean solid must be mounted in a reference liquid, one mineral phase must be in intimate contact with the reference mineral phase, or the homogenous liquid must contain the reference solid. Most applications involve a solid mounted in a reference liquid (referred to as the mounting medium). Second, dispersion colors will only be present if the two materials have the same refractive index for some wavelength in the visible spectrum (referred to as λo) and they have very different dispersions curves for the refractive index. Finally, the sample must be properly mounted under a coverslip to minimize any other optical effect that could complicate the interpretation of the color seen. Once these criteria are met the sample is ready to be examined.
The starting configuration of the microscope for all of these methods is properly adjusted Köhler illumination. Some additional adjustments are required for each of the methods.
The Becke' Line method takes advantage of the fact that particles behave basically like lenses because they tend to be thinner at the edges than they are at the center. If the particle has a higher refractive index than the liquid surrounding it then it behaves as a convex lens and focuses a parallel beam of light on the side opposite the source of the light. Looking through the microscope this is seen as a bright ring of light, the Becke` Line, moving in from the edge as the particle is dropped out of focus by increasing the distance between the stage of the microscope and the objective. If the stage is moved closer to the objective then the particle behaves like a magnifying glass and the image of the Becke` Line is magnified and it appears outside the particle.
A requirement for this method is that the incoming beam of light is as parallel as possible. This requires the closing down of the sub-stage condenser iris. Closing the sub-stage condenser iris decreases the resolution of the particle and increases the depth of field over which other objects may interfere with the effect seen. For large particles this is not a significant limitation but for small particles it is a problem.
When the conditions for dispersion staining are met (the particle is mounted in a liquid with a matching refractive index in the visible range of wavelengths but with a very different refractive index) then the particle has a high refractive index in the red part of the spectrum and a lower refractive index in the blue. This is because liquids tend to have a steeper dispersion curve than colorless solids. As a result, when the particle is dropped out of focus the red wavelengths are focused inward. For the blue wavelengths the particle behaves like a concave lens and the blue Becke` Line moves out into the liquid.