Document b54bgzVRbdV4VX6xe99Zpqq70

1 1 ! J -Tv-0 O^hi/4 Environmental Health Perspectives Vot. 9. pp. ST--61, 19~i Detection and Identification of Asbestos by Microscopical Dispersion Staining by Walter C. McCrone* Asb*to fibers small as 1 fin in diameter can be uniquely identified by light microscopy by employing dispersion staining methods. The technique described herein involves suspension of fibers in liquids of known refractive indices and observation ofcolor display by means ofa dispersion stain ing objective. Wavelengths or indices ofrefraction may be determined at right angles to and parallel to fiber axes. This method is rapid and sensitive for identification purposes. There is a great need for a dependable, sen sitive and rapid method for the detection and identification of asbestos. Microscopical disper sion staining satisfies all of these requirements. It is dependable because it is based on the measurement of three refractive indices as well as the dispersion of those indices. Refractive in dices are among the most valuable identifying characteristics for small particles. The method is sensitive because the refractive indices are "read" as bright dispersion staining colors against a black background. These colors can be observed on particles well below 1 pm in diameter. It is rapid because any particle in the microscopical preparation showing the optical properties of asbestos signals its presence by a unique color combination with polarized light. One particle of asbestos in a field of view con taining many thousands of other particles will be immediately apparent on scanning the eye across the field of view. Besides optical crystallographic methods like dispersion staining, only differential thermal analysis (DTA) and X-ray diffraction have this ability to tag a particular crystalline phase in a mixture. X-ray diffraction, however, is several orders of magnitude less sensitive than disper sion staining and requires much more time. McCrone Associates. Inc., Chicago, Illinois 60616. There are, however, compounds whose disper sion staining colors are, at least at first glance, confused with chrysotile colors. Here, for tunately, particle morphology is able to differentiate between these interfering sub stances. Quartz is one example, Jizardite another. The latter, however, is a tabular talclike mineral, and quartz shows conchoidal fracture into usually thin flakes. Both are easily differentiated from fibrous asbestos by morphology. Dispersion staining is, therefore, a straight forward technique easily applied by any microscopist with some knowledge of optical crystallography. We have found it extremely useful for the rapid and routine examination of any particulate samples for any of the various kinds of asbestos (1). The necessary background information for applying the method is given in Figure 1, which plot3 the matching wavelength Xo, as a function of refractive index of the Cargille refractive index liquids used in these determinations. Most of the asbestos minerals have distinctive indices without overlap. Figure 1 suggests, however, that amosite and crocidolite may overlap partially. There should be no confusion in this situation, however, since y for amosite overlaps only with a for .crocidolite. The data in Figure 1 are plotted as the averages of a considerable number of values December 1974 57 t i i r i * 1 rlt | r X i I ( 4 1 1 | }* % S I 9 XI ` PLAINTIFF'S EXHIBIT ASARCO ELP 0003797 for individual mine samples previously pub lished (1). It is interesting to look a little bit more closely at this variation in dispersion staining data from mine to mine, and Table 1 lists the matching wavelength X# for y (parallel to the fiber length) and a (perpendicular to the fiber length) for a group of more than 30 asbestos samples from different parts of the world. There is some variation from sample to sample, in dicating composition variations. However, all of the data lie in the same characteristic chrysotile region and show no overlap with any of the fibrous amphiboles. Since dispersion staining is a relatively new technique requiring a certain amount of skill, not only in reading the matching wavelengths but also in adjusting the microscope for best dis persion staining colors, it seems well to sum- ' marize a few of the common difficulties. The refractive indices given in dispersion staining tables are not dispersion data for that compound. To illustrate this, we can cite the data for the <u index of quartz (Table 2). The value 1.538 for u at 486 nm is nD for the Cargille liquid that matches quartz w at 486 nm. The actual refractive index of that liquid at 486 nm is 1.550, the same as quartz w. This op eration, which simplifies the analytical pro cedure, is used for all dispersion staining data. One can, of course, calculate the true refrac tive indices of any substance from the disper sion staining data. The necessary data to do this can be found in the table of dispersion of refrac tive index data for the Cargille liquids. True refractive index data and dispersion staining data are identical at 589 nm; hence, refractive index data at 589 nm for any sub stance become dispersion staining data for that substance. Chrysoberyl, for example, does not appear in the dispersion staining tables, but 58 Environmental Health Perspectives asarco elp 0003798 Table 1. Matching wavelength X, In H. D. 1.S50 liquid. X, nm Sample Quebec; Lake Asbestos Quebec; King Asbestos Corp, Quebec; Asbestos Corp. Quebec; Belt Mines Quebec; Johnsons Quebec; Careys, Bradford Quebec; Flintkote Quebec Normandie Ontario; Reeves Ontario; Munro Vermont; Hyde Park, GAF Vermont; Jeffrey New Foundland; Advocate New Foundland Yukon; Clinton Creek British Columbia; Cassiar California; Pacific Asbestos Corp. California; Coalings Arizona Venezuela Rhodesia Rhodesia; Shabina Rhodesia; Havelock C and G Rhodesia; Havelock HVL Rhodesia; Havelock VRA Cyprus Greece; Zandini Yugoslavia Italian; Balengera Russia Australia; Woodsreef II X sxo 510 500 510 500 480 500 570 480 560 510 500 510 590 500 500 480 590 600 610 520(460) 480 490 490 500 600 580 520 500(460) 500 610 610 610 610 600 600 590 610 610 590 610 620 580 610 620 580 580 610 630 620 680 620(550) 580 590 590 630 660 620 590 600(510) 600 680 Table 2. Refractive indices for quartz. 486 nm 589 nm 656 nm True refractive indices,w Dispersion staining data 1.550 1.538 1.544 1.544 1.542 1.547 Winchell (2) gives nD - 1.746(a), 1.748 (0), and 1.756 (y). From these data one would mount a suspected chrysoberyl in Cargilte liquid nD = 1.750 and expect to see annular stop colors rang ing from orange (fi) to greenish-yellow (y) or greenish-blue (cc) to magenta-blue (7) with the central stop. This greatly extends the usefulness of dispersion staining. We are often asked if the dispersion staining objective can be supplied with a higher magnification. This gives us the opportunity to point out that higher magnification is not desirable. One is trying to "resolve" color of the particles not the particles themselves. Central stop dispersion staining is a darkfield procedure, hence a strong light source will show points of light for particles smaller than the resolving power limit of the microscope system, i.e., 1.22 /im for NA= 0.25,10X objective. These points of light will be colored for particles show ing dispersion in that liquid. The lower limit of detection is a function of light intensity and con trast. There are a number of points of technique which greatly improve the sensitivity of the dis persion staining procedure. The particles must be well separated in the mounting liquid since nonstained particles close to or overlapping stained asbestos can mask their presence. The dispersion staining colors for chrysotile and the fiber amphiboles are more brilliant if one uses the high dispersion Cargille set of refractive in dex liquids. In spite of the above injunction con cerning high magnification, it is sometimes useful to use a 20-25X ocular with the 10X dis persion staining objective. The optics for the dis persion staining objective, the axis of stage rota tion, the substage apertures and lenses must be well aligned on the same optical axis. It is a good idea to take special pains to align the optical system and to maintain that microscope for dis persion staining examination only. The problem of glare from other particles in the field of view is solved to a great extent by having a centered and nearly closed field diaphragm in the optical system. This concentrates attention on particles in the center of the field and eliminates well over 90% of the glare which makes it difficult to see very fine asbestos fibers. It is desirable to be able to change the orientation of any particles which appear at first sight to give the distinctive dispersion staining colors characteristic of chrysotile (or other fibrous amphiboles). Rolling the particle by sliding the cover slip with a viscous liquid prep is the ideal way of doing this and helps greatly in differentiating quartz, paper fibers and mineral wool from chrysotile. The slides and cover slips used for dispersion staining preparations should be unusually clean since any optical discontinuities on any surface of the preparation cause glare and interfere with visibility of the dispersion staining colors. December 1974 59 ASARCO ELP 0003799 IJ 1 Finally, it is desirable to have standards of the substance you are looking for mounted in the same refractive index liquid. If one is looking for chrysotile, it is also useful to have standard preparations of quartz, paper fibers, and talc in order to remind oneself quickly of the essential differences in appearance and color for these substances. The color plates (Figs. 2-16) show the general nature of the dispersion staining colors and the specific appearance of the various kinds of asbestos in their specific liquids. Figure 2 shows the arrangement of annular and central stops in the objective back focal plane. These may be a centered 2-3 mm opening in any opaque film or a 3-4 mm dot of India ink on an 18 mm cover slip, respectively; although a dispersion staining objective is available commercially. Note that the substage iris is closed to allow only an axial beam of light to strike the object. The color series obtained with each stop are shown in Figure 3. Chrysotile is shown in Figures 4 and 5 mounted in two different standard Cargille refractive index media. Polarized light is used with an E-W vibration direction for Figure 4 and N-S for Figure 5. Observed Xo values are given in Table 3. Figures 6-8 show anthophyllite all with E-W polar. Figures 6 and 7 show the fibers mounted in 1.610 and 1.620 refractive index media, respectively. Figure 8 shows two differ ent anthophyllites (from Maryland and North Carolina) mounted in liquid of nD 1.627. This figure, like the others, is a double exposure with the stage rotated 90 between exposures. The two different sources result in small varia tions in Xo. Amosite is shown in Figures 9-11 mounted in Cargille liquids of nD 1.670,1.680, and 1.690, re spectively. Figure 12 shows crocidolite in Car gille liquid nD = 1.700, and Figure 3 shows one particle of chrysotile in a talc sample mounted in Cargille liquid, nD = 1.555. Conclusion , The combination of particle morphology and optics uniquely identifies any of the fibrous asbestos compounds. The most rapid method for obtaining this information is.through the use of dispersion staining. Properly carried out, the dispersion staining method is capable of sensi tivity in the ppm range. Table 3. Matching wavelength* for chrysotile. Xo, nm Refractive index of Cargille liquid n D Parallel to fiber length Crosswise to fiber 1.550 1.560 500 570 610 640 REFERENCES 1. Julian. Y.. and McCrone, \V. C., Microscope 18: 1 (19701. 2. WincheU, A. N.. and Winchell, H., Microscopical Characters of Artificial Inorganic Solid Substances, Academic Press, New York. 1954, p. 78. 60 Environmental Health Perspectives ASARCO ELP 0003800