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 be missed because they are difficult to see in the background clutter of the sample [72]. Taylor et al. found that TEM counting gave poorer precision than counting the same sample by PCM and recommended that the fraction of asbestos fibers counted by TEM be applied to the PCM count as indicated in Method 7402 [73]. This combined PCM/TEM approach gave better precision than counting by TEM alone. In addition to recognizing fibrous shape and structure of the several asbestos minerals, qualitative analysis of fibers by TEM primarily involves two techniques, energy dispersive xray analysis and electron diffraction. X-ray analysis produces responses for each of the elements (typically atomic number > 6, but is instrument dependent) present in a particle; the responses occur as peaks in an energy spectrum. Specific asbestos minerals can be identified using peak intensity ratios observed in standard samples and as specified in the method. The crystal structure of individual fibers is evaluated using electron diffraction. Focusing the TEM electron beam on a single fiber produces a diffraction pattern consisting of a number of spots. The spot locations depend not only on the particle crystal structure, but also on the geometry of the electron beam optics and other instrumental parameters. The diffraction spot locations relative to one another give a very specific identification of crystal structure. For easily recognized minerals, such as chrysotile, the visual identification of the diffraction pattern is often sufficient. However, to identify fibers not fitting the x-ray analysis pattern for standard asbestos minerals, careful measurement, or indexing, of the diffraction spots is important. The combination of x-ray analysis and electron diffraction gives a highly definitive identification of specific minerals. However, as with any analytical methods, there are exceptions that require greater expertise to recognize potential interferences. Some minerals that are difficult to differentiate from regulated asbestos minerals include non-regulated amphiboles and fibrous talcs. There are several established methods for analyzing fibers, especially asbestos fibers, by TEM [45, 74-77]. 8.

OPTICAL DETECTION (LIGHT SCATTERING) Two types of light scattering detectors are commonly used for measuring airborne dust concentrations: the optical particle counter (OPC), which detects and counts individual particles, and the photometer (sometimes called a nephelometer), which detects the scattering from all particles in a defined detection volume. A standard OPC was used to detect asbestos concentrations in a workplace where the aerosol was primarily fibrous and good correlation with fiber counts was obtained [78]. A nephelometer may also be used, but may have an even greater interference from non-fibrous dusts. The fibrous aerosol monitor (Model FM-7400, MIE, Inc. Bedford MA) used an electrostatic alignment technique by applying a field that aligns and rotates individual fibers in a laser beam. The light scattered from the fibers uniquely identified the presence of individual fibers. This allowed specific detection of fibers [79] and was even used to measure fiber length [80].

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