Dispersion staining

Dispersion staining is useful for rapid deterrnination of refractive index and dispersion. It is appHed most often, however, for needle-in-a-haystack detection of any particular substance in a mixture such as chrysotile in insulation, cocaine in dust samples, quartz in mine samples, or any particular mineral, eg, tourmaline, in a forensic soil sample. 

Applications of visible light such as the use of polarized light, birefringence, retardation, angles of extinction, dispersion staining, and phase contrast will be explained, discussed and related primarily to asbestos with some discussion of quartz. 

If a mineral is known to be either anthophyllite or tremolite by dispersion staining tests, the angle of extinction can then be used to distinguish between the two. Caution It is possible for a mineral which usually has inclined extinction to have a few fibers with parallel or close to parallel extinction, depending upon orientation. These fibers can be rolled into a position of maximum extinction. (See section on rolling fibers.)... 

Plant fibers will show cells of color with the first order red compensator. The complicated color pattern reveals the complicated internal structure of the plant fiber and shows it to be a non mineral fiber. Occasionally, plant fibers show a single color when examined with crossed polars and first order red plate. In such cases, close observation of morphology or dispersion staining can be used to make a distinction. 

If fibers remain unidentified after examination by phase contrast, polarized light, compensators, or measurement of angle of extinction, the fibers can be removed from the membrane for dispersion stain analysis by ashing, particle picking, or dissolving the membrane. 

Dispersion staining is a convenient technique for determining the identity of fibers and particles. Dispersion staining is taught by McCrone Research Institute (33). 

Identification of asbestos by microscopical dispersion staining has been described by Julian (34) and the technique is described in "The Particle Atlas" (35JT... 

Zeiss and many other phase contrast microscopes produce the equivalent of a central stop dispersion stain by using a phase 2 16X phase contrast objective with the phase 3 condenser ring in place. Leitz manufactures a phase contrast microscope which produces central stop dispersion colors at 400X by using the number 5 condenser ring position. If the microscopes in use at other labs do not produce a central stop dispersion stain in this way, a "dispersion stainer" is available (36). The color resolution (not the particle resolution) of a McCrone dispersion stainer is a little better than that obtained from mismatching phase rings. 

For dispersion staining analysis, it is necessary to have quality high dispersion liquids. One source is R. P. Cargille Laboratories, Inc. (37). 

Wollastonite. Wollastonite is a fibrous calcium silicate sometimes used in industry as a substitute for asbestos. Further testing will be necessary if wollastonite is present. See the discussion on fiber rolling, and the table on identification of fibers by dispersion stain. 

Due to its blocky appearance, gypsum is unlikely to be confused with asbestos by an experienced analyst. Gypsum will often show particles with aspect ratio of 3 1 or greater. However, gypsum can be identified by dispersion staining using cargille HD medium 1.530. 

These are organic fibers, usually plant fibers. Occasionally, plant fibers show a single color with a retardation plate which changes in color in a manner similar to asbestos. If morphology suggests the possibility of plant fibers, do a dispersion staining test. 

Especially in a talc sample, suspect the presence of talc fiber bundles. This can be verified by dispersion staining if the analyst is uncertain. 

Suspect the fibers to be asbestos. The variety of asbestos can be determined by dispersion staining. Chrysotile will be recognized by its morphology. 

Step III. If the identity of the fibers remains doubtful after phase contrast and polarized light study on the membrane, remove fibers from the membrane and examine by dispersion staining. All of the optical tests described herein can be performed while the fibers are on the membrane except fiber rolling, dispersion staining, and determination of n. 

Tremolite and anthophylUte can be identified in 1.620 medium. In a talc sample where talc fibers may also be present, 1.605 medium is prefered. This will distinguish between talc fibers and asbesestos fibers by the dispersion staining colors. 1.605 medium will also distinguish between wollastonite and asbestos. 

If dispersion staining shows that a fiber is either tremolite or anthophyllite, one can distinguish between the two by carefully determining the slope of the dispersion staining curve or by measuring the angle of extinction. 

The sTope of the dispersion staining curve is characteristic for each mineral. Dispersion staining curves of various minerals have been plotted in the Particle Atlas (43) and dispersion data are given by Winchell (44). 

A 1.547 medium is often used for examination of quartz by dispersion staining. This medium will show the F wave length color golden magenta (486.0 nanometers) for epsilon and the C wave length color blue green (656.0 nanometers) for omega as viewed with central stop. 

The dispersion staining colors described are for measurements at room temperature (25° C). It is possible to vary the index of the liquid by using a hot stage (46) and obtain a dispersion slope with a single particle without necessity of changing the mounting medium. This technique is also useful in analysis of other minerals including asbestos. 

Asbestos, quartz or other minerals can be analyzed by consideration of mineralogical principles and crystal systems. Polarized light, compensation plates, measurement of angles of extinction and dispersion staining are useful techniques. Optical behavior of a mineral is related to the internal crystal structure of the mineral. Tables of optical constants are useful for mineral identification. The microscope is a powerful tool for analysis that should not be overlooked by the industrial hygiene chemist. 

Julian and McCrone, "Identification of Asbestos Fibers by Microscopical Dispersion Staining". The Microscope 1J3 (l)... 

Note "The Microscope" has a series of articles on dispersion staining. 

Various means of particle identification are possible with optical microscopy. These include dispersion staining for identification of asbestos particles [44] and the use of various mounting media [45], Proctor et. al. [46,47] dispersed particles in a solidifying medium of Perspex monomer and hardener. This was poured into a plastic mold that was slowly rotated to ensure good mixing. Microscope analyses were carried out on thick sections a lower size limit of 5 p,m was due to contamination. 

KEY WORDS gypsum, microscopes, polarization, birefringence, limestone, silicon dioxide. anhydrite, refractivity. extinction angle, dispersion staining... 

There are five major clues to help us identify a particle. They are morphology, refractive index, birefringence, angle of extinction, and dispersion staining. These clues will help us identify impurities in gypsum, mainly, limestone, silica, and natural anhydrite. They will also help distinguish the different phases of gypsum dihydrate, hemihydrate, alpha and beta, and anhydrite. 

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