Perkins, R., & Beard, M. (1991). Asbestos bulk sample analysis: Visual estimates and point counting. In Proceedings of the 1991 U.S. EPA/A&WMA International Symposium: Measurement of Toxic and Related Air Pollutants, Volume 1, VIP-21, EPA/600/9-91/018 (pp. 214-219)
Quantitation of asbestos content in bulk materials continues to present difficulties to laboratories. Methods of quantitation commonly used by laboratories include volume estimation using the simple stereomicroscope, visual area estimation from slide mounts, and area estimation by point counting of slide mounts. Each of these methods may present problems that preclude obtaining accurate results. If values are to be given in weight percent, results obtained by volume estimation may be in error if significant differences in specific gravities of the asbestos and binder/matrix material are not considered. Quantitation by area estimation, visually or by point counting, must take into account not only differences in the specific gravities of the asbestos and binder/matrix, but also siginificant differences in the thicknesses of the asbestos particles as compared to the binder/matrix particles if results are to be expressed as weight percent. Large variations in particle thickness may cause significant bias because area estimates are comparisons of projected areas and not volumes of the sample components. It is suggested that calibration standards comprised of a variety of asbestos types and concentrations, mixed with a variety of common binder/matrix materials, be prepared for "in-house" calibration of microscopists.