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 can 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 elemental properties can be gathered and graphed in their appropriate ratios. Knowing the exact ratios of the elements in the fiber, along with other analytical information, allows the microscopist to distinguish one type of asbestos fiber from another and asbestos fibers from non-asbestos fibers. Selected Area Electron Diffraction (SAED) is another component of the TEM that allows for a differentiation of fibers. It 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, and usually 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.
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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.
This article was originally published on December 2008.