at the same location. Sampling from multiple locations and compositing samples helps improve the likelihood of obtaining a representative sample. The material should be wetted or sealed during sample removal. A small coring device, such as a cork borer, can be used to obtain a sample from the full depth of the material. At least 2 three samples per 1000 ft of ACM should be taken [45]. The sample should be placed in a well sealed, rugged container. Finally, the sampled area should be repaired or sealed to minimize further fiber release. Surface sampling has been proposed by several groups, but there is no relationship between airborne fibers and those found on surfaces [57]. Therefore, surface sampling for fibers is not recommended. b. Sample preparation and analysis Sample preparation for a PLM analysis involves grinding the material to the optimum particle size range (1-15-:m diameter) and dispersing the particles in a liquid of known refractive index on a glass slide [51]. Particle size uniformity in the prepared sample is extremely important. A few large chunks of material may contain more asbestos than hundreds of much smaller particles. Friable material, i.e., that which is crumbly or can be crushed by hand, may readily release fibers and is considered more hazardous. Friable materials are generally easier to prepare for analysis than some other ACMs, such as vinyl asbestos floor tiles, which may require dissolution or ashing of the matrix material so that the fibers are separated and visible in the microscope. Before and after preparation, the sample is observed with a stereomicroscope at 10-100X magnification to evaluate sample uniformity and observe whether fibrous material is present. Some materials that interfere with accurate fiber identification either by their similarity or by covering up the fibers can be removed by physical treatment of the sample. For instance, organic materials, such as cellulose fibers or diesel soot can be removed by low temperature, oxygen-plasma ashing [58]. Leather fibers and chrysotile have a similar appearance and refractive index. The leather can be removed by ashing at 400/ C [59]. Fiber morphology, i.e. the structure and shape of the fiber, can be used to assist in its identification. Morphology of fibers can give some indication of fiber type. For instance, chrysotile fibers tend to be curly, while amphibole fibers are straight, especially when they are shorter than 50 :m. Asbestos fibers often have frayed or split ends, while glass or mineral wool fibers are typically straight or slightly curved with fractured or bulbous ends. Many plant fibers are flattened and twisted, with diameters between 5-20 :m. Note that it is not recommended to base identification solely on morphology. Fiber refractive index and other crystalline properties can be used to identify fiber type with reasonable certainty. Several techniques for determining these properties can be used in a polarizing light microscope. When viewed in the microscope with crossed polarizing filters, isotropic (isometric or amorphous) fibers appear consistently bright when rotated, while anisotropic (uni- or biaxial crystal structure) fibers appear bright, but disappear when rotated to their extinction angle, which is a function of crystal structure. Thus, amorphous materials such as glass or mineral wool fibers can easily be discriminated from asbestos.
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