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 PLM has been cast in a quantitative measurement role by the EPA requirement of determining whether a school building material meets the 1% asbestos level defining ACM. Many variables including particle size, density and shape are not adequately controlled or measured in the analysis and contribute to errors in the percent mass estimate. Thus, PLM analysis is at best a semi-quantitative technique. Chatfield indicated that the accuracy of PLM for low concentrations of asbestos was poor and described a set of procedures that concentrated the asbestos into a weighable fraction [63]. An EPA report describes, in addition to the PLM and Chatfield’s gravimetry methods, a TEM and an X-ray diffraction method for bulk analysis of asbestos [63]. NIOSH Method 9000 describes an X-ray diffraction method for chrysotile. 5.

ELECTRON MICROSCOPY Scanning electron microscopy (SEM) has not been the focus of as much method development as either light microscopy or transmission electron microscopy (TEM). PCM found favor because of the low equipment cost and lower training level required for analysis. TEM is preferred for environmental and research studies because it offers the highest resolution and the most positive identification capabilities. TEM allows visibility of all asbestos fibers down to the individual fibrils, electron diffraction for crystal structure identification, and energy dispersive x-ray analysis for elemental measurement. SEM has intermediate resolution, with many instruments of this type not able to see all asbestos fibers. However, many modern SEMs have the capability of detecting asbestos fibrils, though contrast with background may be poor for some fiber types, especially if a high contrast substrate is not used. Energy dispersive x-ray analysis is also available for many SEMs, providing some qualitative information of fiber type. However, since electron diffraction typically cannot be performed by SEM, this often leaves open the question of positive identification of fibers.

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SCANNING ELECTRON MICROSCOPY (SEM) Particles are observed in the SEM when a beam of electrons is focused onto the sample surface and scanned over an area. The electrons are scattered from the surface and detected above the surface synchronously with the beam scan rate and an image of the scanned surface is created. Thus, the SEM measures the surface of particles on a substrate. The best image can be obtained on conducting objects deposited on a smooth, conducting substrate. Particles are often deposited on aluminum or carbon planchets that fit directly into the SEM or onto polycarbonate membrane (track-etched, Nuclepore®) filters. The samples are usually coated with gold or carbon to increase conductivity. There have been some SEM methods developed for fiber counting [65-68]. These methods are primarily used for inorganic man-made fibers that have larger diameter fibers than can occur with asbestos. Thus, all the fibers are potentially visible using the SEM.

7.

TRANSMISSION ELECTRON MICROSCOPY (TEM) The transmission electron microscope (TEM) allows detection of particle shape and structure down to the smallest asbestos fibers (Figure 2) and can be used to determine crystal structure from electron diffraction as well as determining elemental composition from energy

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