A rapid asbestos (1332214) sample filter clearing procedure was described. The procedure was based on the flash vaporization of a small amount, usually around 0.20 milliliters, of acetone injected into a heated metal block. The vapor was transported through the block and condensed on the filter. The entire procedure (injection and condensation) took less than 5 seconds. The method was tried with a large number of cellulose ester or nitrate membrane filters containing asbestos samples. Samples that were consistently flat, not wrinkled, fully cleared, and containing no air bubbles were obtained. Design specifications of the metal block and associated heating apparatus were presented. Precautions specific to the method were also presented. The authors conclude that the use of flash vaporization reduces the amount of acetone required, thereby making the procedure significantly more safe than other acetone clearing techniques. The procedure clears the filter in a more complete and consistent fashion than methods previously recommended. The authors recommend that the procedure be included in any future revision of NIOSH method 7400 for clearing asbestos sample filters.
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