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Asbestos Identification and Quantification in Bulk Samples
By Alana Graham,
EMLab P&K Asbestos Analyst
What is asbestos?
Asbestos is a fibrous mineral silicate with a crystalline structure that
comes from a mineral rock. It is a great building material because it has
high tensile strength, is very flexible, and is resistant to chemicals,
high temperatures and stress. As a result, it can be found in buildings in
many places including floor tiles, mastics, caulking, roofing material,
joint compound, plaster, pipe insulation, popcorn ceiling, stucco, some
wall textures, and vermiculite insulation. It is estimated that thirty
million tons of asbestos building materials were used in the U.S. between
1900 and 1975 with probably eighty to ninety percent of that still in
place. This is why it can be found in many commercial and residential
buildings built before 1975. Little did we know then that this amazing
addition to building material would cause serious long term health
problems. Because of its needle-like shape, an asbestos fiber can stick to
lung tissue if inhaled and cause an inflammation. This can lead to many
internal health problems such as asbestosis, mesothelioma, lung cancer or
interstitial fibrosis. There are six types of asbestos fibers. Chrysotile,
the most widely used asbestos fiber, is the only one in the serpentine
group of phyllosilicates (sheet silicates). It has a crystalline structure
that is very different from the rest. The remaining five, amosite,
crocidolite, anthophyllite, tremolite and actinolite, are all amphiboles.
 
Fig. 1. Chrysotile asbestos fiber from Swift Creek viewed in a
Transmission Electron Microscope (TEM).
Source: U.S. Environmental Protection Agency
How does a microscopist identify asbestos in bulk material?
The presence of asbestos in bulk building material can be detected by three
methods of microscopy. The most popular method is by Polarized Light
Microscopy (PLM). With this method, a small amount of the sample is crushed
and placed on a microscope slide using the appropriate refractive index
oil. Polarized light is used to observe the many optical properties of
asbestos fibers that allow the microscopist to distinguish it from
non-asbestos fibers. A polarizing filter on a light microscope is used to
force light to vibrate in one particular plane. The polarizing filter,
along with other additions to the light microscope, such as the analyzer, a
530nm waveplate, and a dispersion staining lens, allow the microscopist to
observe properties of light and crystals such as pleochroism (the
phenomenon of substances showing different absorption colors under
transmitted light when viewed in different vibration directions), the sign
of elongation (describing the relation between the principal vibration
directions and the length of the substance), the extinction angle (the
angle at which no light passes through the substance under crossed polars),
the refractive indices of the fibers (the ratio of the velocity of light in
a vacuum to the velocity of light in a substance) and birefringence (the
numerical difference between the two refractive indices of a substance).
Nonetheless, the first property observed is the morphology of the fiber.
Asbestos analysis by PLM is the least expensive and quickest method to
identify and quantify asbestos fibers in friable and non-friable material.
However, there are two other methods that will produce a more accurate
quantification.
Scanning Electron Microscopy (SEM) and Transmission Electron
Microscopy (TEM) are also used to identify and quantify asbestos in a bulk
sample. Both microscopes use a beam of electrons from a filament in a
vacuum. The SEM produces an image of the topography of the sample. The
electron beam interacts with the atoms on the surface of the sample and
information on the sample's composition can be collected. The TEM's electron
beam passes through the sample and an image is displaced onto a screen.
Using an energy dispersive X-ray (EDX) and a computer system, information
about the fiber's chemical properties can be gathered and graphed in their
appropriate ratios. Knowing the exact ratios of the chemicals in the fiber
allows the microscopist to distinguish one type of asbestos fiber from
another and asbestos fibers from non-asbestos fibers. X-ray diffraction
(XRD) is another component of the TEM that allows for a differentiation of
fibers. XRD allows the microscopist to observe the diffraction patterns of
the crystalline structures of the fiber being analyzed.
The advantage of using an SEM for asbestos analysis is it
has better resolution than the PLM. As a result, the sample can be observed
at higher magnifications and at a greater depth of focus. TEM, which is
more widely used than SEM, has the best resolution out of the three
microscopes and therefore is mandated for analysis of asbestos in water.
Using the TEM, asbestos sample composition can be observed at
magnifications of up to 19,000x, as opposed to the PLM which allows for a
maximum magnification of 400x, or the SEM which can be used at
magnifications of up to 5,000x for asbestos analysis. The higher
magnification used by the TEM allows for more accurate asbestos
concentration estimations in samples that contain very little asbestos
(less than 5%). In addition, more accurate identifications and estimations
are attributed to the gravimetric reduction of non-friable organically
bound bulk samples selected for TEM analysis. That is, TEM samples are
ashed at 480°C for 6-8 hours to eliminate the organic binder and/or tar
material, and are then treated with acid for a few minutes to eliminate the
carbonate material. This allows hidden asbestos fibers to surface and be
easily noticed and identified under the TEM.
Fig. 2. Anthophyllite asbestos, Georgia.
Source: U.S. Geological Survey Denver Microbeam Laboratory
How do microscopists quantify asbestos in a bulk sample?
Calibrated visual estimation (CVES) is the quantification method widely
used in asbestos analysis via PLM, SEM, and TEM. Initially, as they are
being trained to quantify asbestos in a sample, microscopists use reference
slides/samples of known concentrations and compare with reference documents
having photomicrographs to learn how to estimate asbestos percentages. The
microscopists continue to estimate percentages in different samples until
the lab manager/trainer deems them proficient in quantification. Then, the
microscopists view reference slides daily, analyze proficiency samples, analyze
monthly unknown samples, and perform inter- and intra-laboratory analyses
and other comparisons to test the continual accuracy of their CVES. Having
well trained analysts employing good quality control processes is the best
way to identify and quantify asbestos consistently and accurately.
Another method of quantification is the point count method.
This method is only used with PLM at a magnification of 100x. In this
method, a special reticule is used in the microscope allowing the
microscopist to count fibrous materials that land under a point of
reference in the reticule. Multiple preparations of the sample are made on
a slide, covered with a cover slip, and then point counted. A point count
allows for lower detection limits than CVES offers. A 200 point count has
a detection limit of 0.5%, a 400 point count of
0.25% and a 1000 point count of
0.1%. This method is seemingly more accurate, yet it is not highly
recommended because in some cases, asbestos-containing materials do not
have the same asbestos concentrations in every area of the sample and thus
different preparations will yield results that are inconsistent and
imprecise. Nevertheless, when this method is requested, the microscopist is
expected to thoroughly mix and homogenize the sample so as to evenly
disperse the asbestos so the point count estimation will be as accurate as
possible.
In TEM analysis, the CVES of asbestos concentration in the
gravimetrically reduced sample is multiplied by the percentage of sample
remaining after ashing and acid treatment to give the estimation of
asbestos in the entire sample. The same method applies to a PLM gravimetric point count
where the samples are gravimetrically reduced and then point counted using
PLM. The point counted percentage is multiplied by the percentage of sample
remaining after ashing and acid treatment to give the estimation of
asbestos in the entire sample.
References:
1. Oklahoma Department of Labor: Asbestos
Background and History
2. McCrone, Walter C. 1987. Asbestos Identification. Chicago: McCrone
Research Institute.
3. Purdue University: Scanning
Electron Microscope
4. De Stefano, Luca. 2002. SEM Quantitative Determination of Asbestos in
Bulk Materials. Microscopy and Analysis 16(3): 13-15.
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