An evaluation of the precision and accuracy of the direct transfer method for the analysis of asbestos fibers with comparison to the NIOSH method.

Transmission electron microscopy (TEM) of asbestos on polycarbonate membrane filters prepared using the direct transfer (DT) method is used as a reference for determining the precision and accuracy of phase contrast microscopy (PCM) of the same filters and for determining the correlation between DT-PCM and the NIOSH method of asbestos analysis. Replicate aliquots of an aqueous suspension of chrysotile were filtered through either polycarbonate or cellulosic membrane filters. The polycarbonate filters were prepared using the DT method and analyzed using TEM and PCM; the cellulosic filters were treated and analyzed using the NIOSH method. Compared to the results of the TEM examination of the polycarbonate filters, about 50% of the fibers longer than 5 micron were detected using DT-PCM, which was more than 3 times the result obtained using the NIOSH method. The improvement in accuracy using DT-PCM is attributed to the increased visibility of thin fibers (diameters less than 0.1 micron). The largest variation in results was found to be due to the fiber distribution on the filters. This factor may be eliminated from interlaboratory comparisons, etc. by the use of pre-designated and well-defined fields-of-view.