Dispersion Staining - Historical Development

Historical Development

Isaac Newton demonstrated that “white” light was actually composed of many different “simple” colors and that materials had different optical properties depending on which of the simple colors was being used to measure them. He demonstrated these facts with a series of experiments using one or more prisms. The difference in the optical properties of materials as a function of the “simple” or monochromatic colors of light is called dispersion. He was also the first person to note that different materials had different dispersion properties. “Sulfurous” liquids (organic liquids) had a higher refractive index than was expected based on their specific gravity and had a steeper dispersion curve than most solids. These well documented observations would take just over two centuries to become an analytical technique.

The first paper documenting dispersion effects seen through the microscope was written in 1872 by O. Maschke in Germany. This paper discussed the occurrence of colored Becke` lines when a particle was in a liquid of matching refractive index. Prior to this paper these colors were thought to be the result of the microscope lenses (chromatic aberration) and not the result of the slide mounted subject and the medium in which it was mounted. In 1884 and 1895 Christian Christiansen published his data on the first analytical application of dispersion colors, the Christiansen filter. He found that by placing a colorless transparent powder into a vial of a colorless organic liquid he could create monochromatic light from white light if the liquid and powder had the same refractive index for just that wavelength. Only that wavelength would see an optically homogeneous media and pass directly through the vial. The other wavelengths would be scattered in all directions by the particles in the liquid. Monochromatic light could be viewed by looking through the vial along the path of the direct beam of light. At any other angle the complementary color of that wavelength would be observed. If he chose a liquid that matched the refractive index of the powder in the far red, 700 nanometers wavelength, he could create any other wavelength by heating the vial, thereby changing the wavelength at which the powder and liquid’s refractive index matched. This technique did not work for any powder or liquid. For optimal effects the powder and the liquid had to be carefully selected so that the intersection of their dispersion curves created as large an angle as possible over the full range of visible wavelengths. Christiansen’s interest was in the creation of monochromatic filters and not the development of an analytical technique. It wasn’t until 1911 that the analytical potential of dispersion effects was reported by F. E. Wright. He observed that the colored Becke` lines noted by Maschke could be used to distinguish between two materials with the same refractive index but different dispersion curves. The colors could also indicate the region of the visible light spectrum for which a particle and liquid it was mounted in had a refractive index match. Wright also noted that by using oblique transmitted illumination the particle would show these colors without having to inspect the Becke` line.

The technical literature had little additional discussion of dispersion effects until 1948. That year S. C. Crossmon, N. B. Dodge, and co-authors R. C. Emmons and R. N. Gates all wrote papers on the use of dispersion effects through the microscope to characterize particles. Crossmon seems to have coined the term “Dispersion Staining” as any optical technique that used the “Christiansen Effect” to produce color in the image of colorless particles. He demonstrated the use of Becke` Line, Oblique Illumination, Darkfield, and Phase Contrast Dispersion Staining methods. S. C. Crossmon and W. C. McCrone have published numerous papers on the use of objective back focal plane stop dispersion staining techniques since that time. Yu. A. Cherkasov published an excellent paper on this topic in 1958 and it was translated into English in 1960. Well over 100 papers have been written on the various methods of dispersion staining and their application since about 1950 and most of these since 1960.

In spite of the early work done on this technique it was not until the 1950s that it became generally known among microscopists. It is now recognized as a powerful tool in the characterization of materials and the detection of low level contaminants. It has demonstrated sensitivity for particle contaminants in powders down to the parts per million.

The dispersion of the refractive index is a fundamental property of matter. It may be thought of as the result of the relative proximity of the harmonic frequencies of the outer shell electrons in a compound to the frequencies of visible light. The harmonic frequency of the bonding electron is the result of the energy of that bond. If the bond is very strong the frequency will be very high. The higher the frequency the less effect the difference in frequencies from blue to red will have on the refractive index. For the relatively high energy bonds in most inorganic solids this means their refractive indices change very little over the visible range of frequencies. The refractive indices of organic compounds on the other hand, with their lower bonding energies, change significantly over the visible range. This difference in dispersion is the basis of the Christiansen effect and the dispersion staining methods.