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. 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 conformation 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:
- Colored Becke` Line Dispersion Staining (Maschke, 1872; Wright, 1911)
- Oblique Illumination Dispersion Staining (Wright, 1911)
- Darkfield Dispersion Staining (Crossmon, 1948)
- Phase Contrast Dispersion Staining (Crossmon, 1949)
- Objective Stop Dispersion Staining (Cherkasov, 1958)
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. 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.
Read more about Dispersion Staining: Historical Development
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