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Analytical methods
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The identification of asbestos minerals is based on:
the appearance of the minerals: long fibers (length:thickness > 3) forming bundles; fibers split into fine fibrils
the chemical composition of the minerals
the fact that the material is crystalline
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The asbestos minerals can be identified using several methods. Commonly, phase contrast polarized light microscopy (PLM) and transmission electron microscopy (TEM) are used. Other potential techniques include X-ray diffraction (XRD) and possibly infra-red spectroscopy (IRS). |
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Phase contrast polarizied light microscopy (PLM)
Asbestos fibers can be identified using
appearance
optical properties
refractive index
The presence of asbestos in a material sample is normally confirmed by using PLM. The asbestos minerals can be identified by their appearance in a microscope. They form bundles, and the individual fibers split at the ends. The fibers have to be crystalline, and the appearance has to be in accordance with the optical properties of the asbestos minerals.
The asbestos minerals have characteristic refractive indices. By embedding the fibers in well-defined index-matching liquids, the different asbestos types can be distinguished. Phase contrast microscopy is also used for fiber counting when different mineral types have to be separated.
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Crocodolite through a polarized light microscope. The fibers are usually very fine, but may also form bundles that split.
(Photo: Mary Ekström)
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Scanning electron microscopy (SEM)
Scanning electron microscopy is primarily used for asbestos analyses in air and dust samples. The asbestos fibers are identified using their
chemical composition
appearance
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With SEM, asbestos fibers too fine for detection with light microscopy can be detected.
Analyses for asbestos in dust are usually performed in order to assess if asbestos has been dispersed during asbestos removal work.
Likewise, the asbestos concentration in air is tested after removal work to ascertain that these hazardous fibers have not spread to the surroundings.
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Amosite in dust (tape sample). Magnification 1000x, the dashed line equals 10 µm. Amosite asbestos, synthetic inorganic fibers (MMMF) and an organic fiber (ORG) can also be seen. The picture shows that amosite fibers split in the ends, in contrast to MMMf fibers.
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Chrysotile in dust (tape sample). Magnification 2000x, the dashed line equals 10 µm. Chrysotile fibers are usually bent and split into very fine fibrils. |
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Transmission electron microscopy (TEM)
Asbestos fibers can be identified using TEM by means of their
appearance
chemical composition
crystal structure
An advantage of using TEM is that very fine fibers can be analyzed and, simultaneously, their chemical composition and crystal structure can be determined. Fibers to be analyzed using TEM need to have a diameter <0,1 µm. TEM uses an energy dispersive X-ray detector for the determination of the chemical composition of the fibers. The spectra resemble the EDX diagrams discussed earlier. Using another detector, a refraction spectrum, reflecting the crystal structure, can be obtained. In this way, it is easy to separate amorphous fibers from crystalline fibers, since the former do not exhibit refraction spectra.
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Amphibole structure (CrystalMaker™): The atoms are structured in a 3D lattice (SiO4 tetrahedra in dark blue, Mg and Fe octahedral sites in light blue and green, yellow balls represent sites for larger ions, such as Na, grey balls indicate OH groups. In X-ray diffraction, the refraction of the X-ray at a crystal lattice, and the resulting characteristic interference pattern, is used to identify the mineral. |
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X-Ray Diffraction (XRD) and IR
With X-ray diffraction, only crystalline material can be detected. The minerals are identified using their crystal structures. The XRD technique can not be used at lower asbestos concentrations than 1% (wt). Furthermore, XRD can not be used to establish whether the amphibole minerals are asbestiform or only prismatic.
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The IR technique is not used for the identification of asbestos minerals. Although the IR spectra of asbestos minerals are well-known, there is no methodology for using these spectra on a routine basis.
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