Asbestos analysis methods include Transmission Electron Microscopy

Asbestos analysis methods include Transmission Electron Microscopy

The National Institute for Occupational Safety and Health’s (NIOSH) method 7400 uses Phase Contrast Microscopy (PCM) to determine airborne fibers in the workplace. This method cannot distinguish asbestos from fiberglass, cellulose or other fiber types.

Therefore, it is useful only when the main source of dust is expected to be asbestos, and not in situations where asbestos fibers are mixed with other fiber types. In addition, PCM may not detect very thin fibers. As a result of these limitations, a PCM count can be biased high if fibers other than asbestos are present, or low if thin asbestos fibers are present.

Polarized Light Microscopy (PLM) is the EPA accepted screening method for asbestos in bulk samples. The main purpose of the PLM analysis is to determine if asbestos exists in a medium. The limit of detection for this method is typically 0.25-1% asbestos.

PLM uses polarized light to compare refractive indices of minerals and can distinguish between asbestos and non-asbestos fibers and between different types of asbestos. The method fails, however, in samples where asbestos fibers are fine or obscured by a tightly binding matrix.

Transmission Electron Microscopy (TEM) can also analyze for asbestos in air and bulk samples. Light microscopy can detect particles only down to a diameter of about 0.3 µm. Many asbestos fibers have smaller diameters than this and are thus invisible to a light microscope. TEM uses electron diffraction and energy-dispersive x-ray methods, which give information on crystal structure and elemental composition, respectively. This information can help determine the elemental composition of the visualized fibers.

One disadvantage of TEM is that it has difficulty determining asbestos concentration in soils and other bulk materials. Advantages of using TEM are that it can detect smaller fibers than PCM or PLM and can also determine the fiber type.

When soil asbestos concentrations are low, the number of grids that are examined for a given sample can limit the accuracy of the TEM method. If the laboratory analyst does not count enough grids, the sample results will likely be inaccurate. In general the TEM approach is more tedious and time consuming – and therefore more costly – than PLM, but it provides the best approach for fiber identification and quantification at low sample concentrations.

Courtesy of the Washington Department of Health