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blanks, when preparing samples for fiber counting and analysis. These blanks confirm a clean preparation environment or bear witness to laboratory contamination. Another preparation technique which has been used off and on is the so-called "rub- out" technique. This was used early in the electron microscopy analysis of microfibers and has been applied by the Mount Sinai group [22]. High particle losses and the destruction of the true particle size distribution to produce only a mass concentration are cited as disadvantages with this technique. Other techniques have also been cited as viable, including that in a recent EPA report [5]. However, most have been discarded in favor of the direct transfer method alone or preceded by ashing only when necessary. The added specimen handling necessary for transmission electron analysis has often been cited as a serious disadvantage to TEM, STEM and TSEM analysis. However, experienced laboratories have developed preparation routines and techniques which make particle losses, contamination and labor time negligible. The usual amount of time lag in preparation of a transmitted electron sample is about four hours. Analysis Cost The amount of electron microscope time necessary for an analysis is the major consideration affecting cost, and is dependent upon many factors, not the least of which is the sample from which the specimen was produced. The size distribution, particle loading and uniformity of distribution are just three of these. If a very limited amount of microscope time requires that the analyst use only a low magnification, e.g., 4000X, then the small microfibers may be missed. Computer image analysis has been used by a few laboratories [9] and can be applied directly on an electronic image as produced in the SEM, STEM and TSEM or on photomicrographs produced by an electron microscope. Direct computer image analysis is also possible with suitably modified imaging devices mounted into TEM's. This technique can greatly reduce the amount of microscope time requireil for microfiber searches but is prone to certain errors, especially where high concentrations of microfibers and other particles are present. The application of microfiber identification techniques affects the microscope time as well as imaging. The TEM image is essentially instantaneous, whereas an SEM image must be acquired with time and takes several seconds to form. Furthermore, on a typical SEM the time for one EDXS analysis is 100 or more seconds. As a conse- quence most analysts working with SEM and STEM only obtain analyses from selected microfibers, not all of those found. SAED usually requires 10 to 30 seconds to form an image suitable for recognition by the microscopist and is usually performed on all microfibers found. Recording of this image is done selectively on a few microfibers and usually requires 100 to 200 seconds. The beam focusing feature available on all STEM and some TEM can reduce the recording time by producing a brighter SAED pattern. Technique Development A number of laboratories are evaluating the various electron microscopy techniques used in the analysis of microfibers. That this is necessary is evident from the wide discrepancy in results produced on similar samples by different laboratories and/or microscopists (23]. No two laboratories perform sample preparation or microfiber analysis exactly the same and some are markedly different. However, over the past three years a number of laboratories have markedly improved their analytical reliability in spite of the overwhelming statistical uncertainties. This author is aware of some new approaches to the identification, counting and measurement of microfibers. United States Steel Research Laboratories are applying a specially equipped TSEM with an EDXS detector located for a very high x-ray take-off angle, higher than possible in a standard unit. This sytem is computer controlled using criteria from the transmitted electron image data at 1g,000X magnification processed through an image analyzer to locate microfibers. The geometry of the system and the sample and x-ray detector distance (less than 1 cm) are such that a very adequate EOXS spectrum can be accumulated an order of magnitude faster than with standard electron microscopes, SEM or STEM. After a statistically significant number 231 2063105026

of microfibers are found and EDXS data obtained from each, they are classified with respect to aspect ratio and EOXS spectrum. The specimen is then transferred to a TEM, a 1200 KeV instrument in this case, where some microfibers in each EDXS clas- sification are selected and an SAED pattern obtained for identification. It is recog- nized that a 1200 KeV TEM is not' readily available; however, the SAED could be performed on most TEN instruments with 80 to 100 KeV. The advantage in the transmitted image over that usually produced in an SEM is greater visibility of particles, as has previously been stated. Moreover, the tech- nique has a great advantage over those presently applied from the standpoint that a large number of microfibers are analyzed at least through classification and this is a tremendous statistical advantage. Conclusion In conclusion there are a few points that should be made. 1. The transmitted electron image is generally accepted as being superior for counting and measuring microfibers as compared with a secondary or backscatter electron image. 2. Selected area electron diffraction is generally accepted as the best criterion for the identification of asbestos mineral microfibers, although a few non-asbestos minerals may be mistaken for asbestos. 3. The statistical consideration affecting electron microscopy of microfibers is a source of considerable error and new techniques are being and must be developed to relieve these problems. 4. There are a few specific situations where the SEM can be applied to the counting of microfibers, especially where the source and species mixture are well characterized. 5. Although the TEN-SAED method of asbestos mineral microfiber counting and identifica- tioh is not absolute, it is the best compromise of accuracy and cost available. The author would like to thank C. S. Barrett and J. M. Dement for their contribution to this paper. References [1] National Academy of Sciences, "Drinking Water and Health," Part I, 1977. [2] Flinckinger, J. and Standridge, J., Identification of fibrous material in two public water supplies, Env. aci. and Tech. 10, No. 10, 1028-1032 (Oct. 1976). [3] Ruud, C. 0., Barrett, C. S., Russell, P. A., and Clark, R. L., Selected area electron diffraction and energy dispersed x-ray analysis for the identification of asbestos fibers, a comparison, Micron 7, 115-132 (1976). [4] Mumpton, F. A., Characterization of chrysotile asbestos and other members of the serpentine group .inerals, Siemens Review XLI, 7th Special Issue, 75-84 (1974). [5] Gerber, R. M. and Rossi, R. C., Evaluation of electron microscopy for process control in the asbestos industry, EPA-600/2-77-059 (Feb. 1977). [6] Rubin, I. B. and Maggiore, C. J., Elemental analysis of asbestos fibers by means of electron probe techniques, Env. Health Perspect. 9, 81-84 (1974). 232

[7] Ferrell, Jr., R. E., Paulson, C. G., and Walker, C. W. , Evaluation of an SEM-ES method for identification of chrysotile, In Proc. 8th Annual SEM Symposium, Johari, 0. and Corvin, E. (eds.). IIT Research InstStute, Chicago, I11., 537-546 (1975). [8] Langer, A. M. , Rubin, I., and Selikoff, I. J., Electron microprobe analysis of asbestos bodies, Histochem. Cytochem. J. 20, 735-740 (1975). [9] Pattnaik, A. and Maakin, J. D., Development of scanning electron microscopy for measurement of airborne asbestos concentration, U.S.E.P.A., Office of R and 0 Pub. No. EPA 650/2-75-029. Research Triangle Park, N.C., (1975). [10] Speil, S. and Leineweber, J. P., Asbestos minerals in modern technology, Env. Res. 2, 166-208 (1969). [11] Pooley, F. D., The identification of asbestos dust with an electron microscope microprobe analyzer, Norelco Reporter, 23, No_2, 5-9 (Oct. 1976). [12] Dement, J. M. , Zumwalde, R. D., and Wallingford, K. M. , Asbestos fiber exposures in a hard rock gold mine, In Proc. N.Y. Acad. of Scci. Conf. Occup. Carcinogenesis. (1975). [13] Clark, R. L. and Ruud, C. 0., Transmission electron microscopy standards for asbestos, Micron 5, 83-88 (1974). [14] Barrett, C. S. and Massalski, T. R., Structure of Metals, (McGraw-Hill, New York, 654, 1966). [15] Zussman, J., Brindley, G. W., and Comer, J. J., Electron diffraction studies of serpentine minerals, Amer. Mineralogist 42, 133-153 (1957). [16] Whittaker, E. J. W. , The structure of chrysotile, Acta Cryst. 27, 659-664 (1956). r [17] Yada, K., A study of microstructures of chrysotile asbestos by high resolution microscopy, Acta Cr st. 9, 855-857 (1971). [18] Deer, W. A., Howie, R. A., and Zussman, J., Rock Forming Minerals, (Wiley, New York, Vol. 3, p. 270, 1963). [19] Cook, P. M., Rubin, J. B., Maggiore, C. J., and Nicholson, W. J., X-ray diffraction and electron beam analysis of asbestiform minerals in Lake Superior waters. In Trans. Inst. Electrical Electronic Eng. (in press) (1974). [20] Beaman, D. R. and File, D. M., The quantitative determination of asbestos fiber concentrations. The Dow Chemical Company, unpublished report (1975). [21] Oriz, L. W. and Loom, B. L., Transfer technique for electron microscopy of membrane filter samples, Am. Ind. ~jg. Assoc. J., 423-425 (1974). [22] Rohl, A. N., Langer, A. M., and Selikoff, I. J., Environmental asbestos pollution related to use of quarried serpentine rock, Science, 196, 1319-1322 (17 June 1977). [23] Brown, A. L., Taylor, W. F., and Carter, R. E., The reliability of measures of amphibole fiber concentrations in water, Env. Res. 12, 150-160 (1976). N ~ Y O 233 0 ~

Discussion J. LEINEWEBER: I would like to make one comment with regard to Clay Ruud's remark about using the central channel of the chrysotile fiber for identification. This is good a reasonable percentage of the time, but you can run into chrysotile fibers such that this channel is not very visible and may be pretty well filled up with the non-cystalline material. So, it cannot be used as positive identification. C. RUUD: I know. R. FISHER: I want to get clarification whether you advocate visual identification from the diffraction patterns and visual counts in contrast to recording micrographs. It seems to me desirable to have your data in a form that others can confirm, look at your diffraction patterns, look at your counts, and not rely on visual observations that are just recorded in a pad or notebook. RUUD: I hear what you say, and I would like to record every pattern or every micro- graph that is projected on the screen, but I can't afford to do this; my sponsor won't stand for it. So, what we do is, we record typical SAD patterns we see in particular samples or sets of samples. When we see something different than that, something unusual, strange, we record it. I agree that it would be nice to have everything recorded for posterity, but it takes too much time. FISHER: Well, at this stage it is essential to have records that can be accepted by others, I am afraid. I agree the costs are high, and people will have to pay them, but I think that any data that are not recorded for confirmation and detailed examination are going to be challenged in all kinds of situations. RUUD: As I say, we record typical ones; we save the samples and since the samples are on finder grids any grid can be found and the data confirmed. J. ZUSSMAN: I'd like to make three comments concerning Dr. Ruud's paper. One concerning electron diffraction patterns. I think he has very much underplayed the varia- tions and variability one can get in electron diffraction patterns, depending very much upon the orientation of the grain, the way it lays on the stage. If you look at these patterns carefully, you see enormous numbers of different effects; I would have made this comment anyway; I make it still much more strongly now, having heard a lot of judgments are made perhaps without even taking photographs. From looking down on the screen you can certainly not see the subtle variations which are nevertheless important, produced by orientation effects. Secondly, you mention the scanning electron microscope as being best for chemical analytical purposes. I don't think it is capable of an accuracy that can be obtained by the transmission electron microscope with suitable attachments, or STEM, which brings me to the third point. You showed that lizardite and anthophyllite were not distinguishable from their x-ray fluorescence spectra, and this is surprising. The magnesium-to-silicon ratio for lizardite is 1.5 to I in atomic ratio, the other ratio is 0.9 to 1, and I think there is a detectable difference. The reason why we may not pick up this difference is that your crystal has the wroog kind of thickness so that the crude ratios of peak height are not indicative of concentration. The crystal has to be of a suitable thickness for this to be so. RUUD: Regarding the last comment, we can rotate the fiber or change the position in the microscope, and get different ratios, and, as someone pointed out yesterday, just be going along the fiber you may get different ratios. So, that's one reason why I do not have too much confidence in energy-dispersive x-ray spectroscopy. The first comment had to do with selected-area diffraction and the variability of patterns. We do not study them that carefully. We do not try to distinguish between the various amphiboles, amphibole asbestos materials. We do not have the time to study individual patterns that carefully. We looked at the possibilities of trying to get good d-spacings from them; it seems like it is a good possibility if we could connect the computer into a vidicon or a camera tube in the bottom of a TEM or STEM and put it directly into a computer; I think that would be great. But, so far I know of only one microscope equipped that way, and it is not used for asbestos analysis. 234