A discussion was provided on the problems which have arisen with Method 7402 for the removal of asbestos (1332214). An example of the problems was that some of the procedural steps, such as ashing, were not compatible with all the options, such as the asbestos fraction determination. In addition, some confusion has arisen as to the best option to be followed in Method 7402 for analysis of occupational exposure samples for asbestos. As a result of experiments involving scanning electron microscopy analysis of low temperature ashed filters, Method 7402 was revised to clarify these issues. Because low temperature ashing removes organic fibers such as cellulose when preparing samples for transmission electron microscopy (TEM) analysis, the step now has been removed from Method 7402, Revision Number 1, to more accurately determine the asbestos fiber fraction. While air filtration of the fibers usually resulted in most of the fibers being collected on the surface of the filter, some may deposit in the interior of the filter. Some fibers, particularly the shorter, thinner ones, may be washed down into the interior of the filter when acetone is condensed onto the surface during the clearing step. If ashing is not performed, the asbestos fibers in the interior of the fiber is not able to be detected during TEM.
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