SEPARATION OF ASBESTOS FROM ENVIRONMENTAL MEDIA
BY ZONAL CENTRIFUGATION
William P. Bonner, Professor
Rafael B. Bustamante, Chairman
Department of Civil Engineering
Tennessee Technological University
Cookeville, Tennessee 38501
C. W. Isham, Engineer
J.I. Serene Company
Greenville, South Carolina 29615
BACKGROUND
The Problem
Asbestos determination by the NIOSH procedure has found widespread application in the field of
industrial hygiene for the determination of fibrous bodies in air [1]. After collection of a sample on a
0.45 p.m membrane filter, it is placed on a glass slide and made transparent to light with a few drops of
mounting solution. When the filter has cleared sufficiently, the slide is examined using a polarized
light microscope. Fibrous particles which rotate the plane of polarized light are counted as asbestos
fibers. This procedure accomplishes its intended purpose, i.e., control of asbestos fibers in areas
where asbestos is handled. The application of this technique to samples from unknown sources and to
media other than air poses several problems. The limit of optical resolution is satisfactory for industrial exposure where exposure limits are based upon particles having a length greater than 5 /um. In
addition, large quantities of debris reduce light transmission and mask the presence of fibrous particles.
Electron microscopy has many advantages over optical methods particularly for viewing and identifying crystalline particles in the sub-turn size range. Sample preparation and treatment prior to the
application of one of the several electron microscopic techniques varies with the type of microscopy
and the type of information desired.
Water samples to be analyzed for asbestos using electron microscopy are first filtered through a
membrane filter. The use of filters having 0.22 to 0.45 (im pore diameters has been reported in the
literature, the larger pore diameter being generally recommended [2]. A portion of this filter is then
placed upside down on a specimen grid and the filter paper dissolved by washing in a condenser
washer containing an appropriate solvent [3,4]. Other preparation techniques include ashing in a
plasma at an oxygen pressure of abut 10 Torr at a temperature of less than 100-120 C for 6-72 hours
with subsequent resuspension (including low energy ultrasonics), Filtration and extraction [4,5]. The
ashing step is included to remove the mass of organics which are frequently present in surface waters.
The grid is then examined by electron microscopy. If transmission electron microscopy is used, the
sample is examined without any further treatment. When an asbestiform particle is observed, a
selected area electron diffraction pattern is obtained from that fiber. This pattern depends uniquely
on the crystal structure of the material and provides positive identification of the various forms of
asbestos. The entire grid is examined and the number of asbestos fibers is determined. Since the
volume of the original sample is known, the number of fibers per unit volume can be calculated.
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