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Because it resists acids, is noncombustible, and can he woven into fabrics, asbestos was commonly used as fireproofing and insulation in many huildings built before the mid-19708 as well as in brake linings, heat-proof gloves, and other commercial products. Concern about adverse effects on health from exposure to asbestos originally centered on miners, insulation workers, and others who were exposed to large amounts of asbestos in their jobs. But recent studies indicate that even low levels of airborne asbestos may cause cancer, and concern has mounted over the effects on the general public of exposure to asbestos in the environment. Because most of the methods that were developed for the analysis of bulk asbestos samples are not appropriate for the analysis of air samples, new methods capable of detecting small amounts of asbestos in ambient air have been developed. Asbestos is a generic name used to
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describe a variety of hydrated silicate materials that can, under certain conditions, crystallize in bundles of fibrils that look like organic fibers. Although there are several kinds of asbestos, most of the asbestos used in the United States is chrysotile (or white) asbestos. The average asbestos fiber is -100 pm long and 100 nm wide when mined. It is then manufactured into fabrics and boards and used in a variety of commercial applications. Asbestos can get into the air from many sources, including natural rock, mining and manufacturing operations, brake linings of cars, and aging or damaged insulation. Its widespread use over the past 100 years has caused asbestos to be generally present in the atmosphere. Microscopic methods, including phase-contrast microscopy (PCM), po-
larized light microscopy (PLM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and analytical electron microscopy (AEM),are the most common means of determining asbestos in air samples. Other methods that can be used for the analysis of bulk asbestos samples, such as infrared spectrometry (IR) and X-ray diffraction (XRD), cannot he used for air samples because they do not have sufficient sensitivity to see the low levels of asbestos present in air samples. Optbl mkroscapy The first two microscopic methods, PCM and PLM, are optical or light microscopic methods. In the PCM metbod, differences in refractive index between an object and its surrounding medium are made visible in the form of a black-and-white image as a result of differences in phase of some of the light rays passing through the system. For many years PCM was the standard method for determining airborne asbestos. But PCM has two limitations. First, it cannot distinguish asbestos from other types of fibers (such as glass, cellulose, or carbon)-all fibers are counted as asbestos. In addition, PCM does not have sufficient resolution to see asbestos fibers that are less than -0.25 pm in diameter, and many asbestos fibrils found in air samples are only 0.03-0.04 pm in diameter. Thus for chrysotile asbestos, only those asbestos fibers found in bundles can he seen by PCM. Despite these limitations, PCM can he a useful technique for an asbestos mill or plant, where it is reasonable to assume that most of the fibers in the sample are asbestos and bundles of asbestos fibers will he common. For example, PCM is the method currently recommended by OSHA and NIOSH for monitoring occupational exposure to asbestos and for evaluating asbestos abatement efforts. PLM also uses refractive index dif-
can be used to determine
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ambient air ferences to visualize the asbestos fihers, hut it differs from PCM in that these refractive index differences are used to distinguish between different types of asbestos rather than between the fibers and the background, Optical properties, including refractive index, birefringence, morphology, extinction angle, and sign of elongation, are measured to distinguish the fiber types. Like PCM, PLM can only detect fairly large asbestos fibers and thus ,is not particularly useful for the analysis of ambient air samples. It is used, however, for determination of asbestos in bulk samples.
Electron microscopy The greater magnification and resolution of electron microscopes compared with optical microscopes allows smaller asbestos fibers to be detected. Three types of electron microscopy are used to determine asbestos fibers in ambient air samples: TEM, SEM. and a comhination of the two, known as AEM. In TEM, a beam of electrons is passed through the sample, where some of the electrons are diffracted, to a detector. The pattern of electron scattering generates an image in much the same way that the image in a light microscope is formed as a result of the reduction of light intensity by the ohject. Because TEM normally includes electron diffraction capability, which
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6. MARCH 15, 1988
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FOCUS provide the crysi igraphic structure of a fiber, it can distinguish asbestos from other types of fibers. I t can also distinguish chrysotile asbestos from other forms of asbestos. In SEM, the electron beam is collimated into a small (100 A) spot, and the spot is swept across the sample surface, which has been coated with a thin layer of gold or other conductive suhstance. As the beam impinges on the surface and penetrates into it a short distance, electrons are emitted either as secondary emissions or as directly backscattered electrons. The image is then formed from the collected electrons. SEM is usually used in combination with an energy-dispersive X-ray spectroscopic detection system (EDXS), which can qualitatively and semiquantitatively identify the elemental content of a fiber. An AEM has both the high-resolution and high-contrast imaging capabilities of a TEM and the chemical analysis capabilities of an SEM/ EDXS. The TEM imaging mode of the AEM is used to find the asbestos fibers, and the TEM electron diffraction capabilities and the SEM X-ray spectrometry capabilities are used to identify the fibers. Chrysotile asbestos can be identified using electron diffraction, whereas X-ray analysis generally is necessary to identify other types of asbestos. Asbestos analysis with TEM or AEM is performed a t 20,000X magnification; a PCM analysis is performed a t a magnification of 500X. Although it takes longer to analyze an entire sample by TEM or AEM, these methods may often see -10 times the numbers of chrysotile asbestos fibers in a given sample. Put another way, PCM will only see -10-20% of the asbestos fibers seen with a TEM or AEM. SEM lies somewhere between PCM and AEM in capability hut is not recommended for ashestos determination in air samples because it has not been characterized as completely as PCM or AEM and researchers do not know its limitations. Sampling and analysk Although all the methods use essentially the same sample collection techniques, sample preparation after collection varies according to the method used. For sample collection, a known volume of air is drawn through a cellulose acetate or perforated polycarbonate membrane filter, which is then analyzed by either optical or electron microscopy. Because it isn't feasible to evaluate the complete sampling area of the filter, a random selection of fields of view is evaluated and the rest of the 9S6A
sample is assumed to have an even distribution. The measured concentration of the original air sample is calculated from the total number of fibers estimated to be on the filter and the amount of air drawn through the filter. For light microscopy, a section of the filter is cut and put in an acetone vapor bath, which causes the filter to become transparent. It can then be observed under the microscope. The procedure is a bit more complicated for electron microscopy. For SEM, the surface of the filter normally is coated with an electrical conducting material such as gold, silver, carbon, or silicon monoxide and then viewed under the microscope. Because the electron beam actually passes through the
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airborne asbestos monitoring methodology is primarily toward "EM or A E M . .
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sample, TEM analysis requires that the matrix be nearly transparent to electrons. This involves making a carbon film replica of the filter sample, which is done by coating the filter with carbon and then gently washing the filter away, leaving a replica of the filter on a special grid. Although this procedure is not particularly labor intensive, it does require quite a bit of timeanywhere from 4 to 48 h. Future trends Although asbestos analysis is an area in which existing methodology is primarily being used and refined, some work is being done on new methodology. Fred Adams and his colleagues a t the University of Antwerp are investigating the use of LAMMA (laser microprobe mass analysis) to detect chemical impurities on the asbestos fiber surface. I t has been postulated that the toxicity of asbestos fibers may result not only from the fibers acting as an irritant in the lungs but also from the asbestos carrying carcinogenic compounds to the lungs. Carcinogenic organic pollutants or their precursors may adsorb onto the surface of the asbestos fibers; they could then be transported to the lung and deposited in the tissue. Adams's group bas used LAMMA, in which laser desorption is used to obtain mass spectra of the sample, to detect trace quantities of henzi-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
dine, N&-dimethylaniline, and henzo[a]pyrene on doped asbestos fibers as well as to determine phthalate contamination resulting from polyethylene packing material. Detection limits are better than 500 pg/g for the phthalate experiments with reproducibility of -20%.
Adams and his co-workers admit that more work needs to be done before LAMMA can be considered a routine method for the determination of asbestos in air samples. And although the use of LAMMA for detection of impurities is intriguing, it will probably not gain wide acceptance for routine ashestos analysis for two reasons. First, there are very few LAMMA instruments, and second, like PCM, LAMMA has an optical resolution limit of -0.25 pm, so it cannot see all of the asbestos fibers present in a particular sample. Nevertheless, it may prove useful in correlatine carcinoeenic - impurities with asbestos toxicity. The trend in airborne asbestos monitoring methodology is primarily toward TEM or AEM, in part because they provide superior resolution and in part because these are the methods recommended by the U.S. Environmental Protection Agency. New regulations require accreditation of asbestos-testing laboratories, and the National Bureau of Standards (NBS) will add two new accreditation programs within the next two years. In October 1988, NBS will start an accreditation program for the use of PLM for bulk sample analysis in response to passage of the Asbestos Hazard Emergency Response Act last October. In October 1989, NBS will begin a lab accreditation program for the use of TEM in the determination of airborne asbestos. These programs will join one that is already in operation by the American Industrial Hygiene Association for the use of PCM in asbestos analysis. Mary Warner I
sussested -ding Beckett. S. T. In Asbestos: Properties, Applications, ond Hazards; Michaels, L.; Chissick, S. S., Eds.; John Wiley & Sons: New York, 1980. De Wade, J. K.; Vansant, E. F.; Van Epsen, P.: Adams. F. C. Anal. Chem. 1983. 55, 61'-7.
DeWaele,J.K.;Gybels.J.J.;Vansant,E.F.; Adams, F. C. Anal. Chem. 1983,55.225560. Proceedings of Workshopon Asbestos: Definitions and Measurement Methods; Gravett, C. C.; LaFleur, P. D.; Heinrich, K.F.J., Eds.; National Bureau of Stan-
dards: Gaithersbure. Md.. 1978.
Small, J. A,; Steel, E: B.; Sheridan, P. J. Anal. Chem. 1985.57,204-8. Steel, E. B.; Small, J. A. Anal. Chem. 1985, 57,209-13.
Zurer, P. S. Chem. Eng. News, March 4, 1985, pp. 28-41,
