Asbestos detection techniques

There are a number of asbestos detection techniques that have been developed over the years, the most important and widely used of which are microscopy techniques, such as phase contrast microscopy (PCM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and polarized light microscopy (PLM). Having accurate techniques for measuring asbestos levels is critical in determining the extent of asbestos contamination and the health risks for humans. The techniques mentioned in the previous section vary significantly, so it is important to understand their individual strengths and weaknesses to determine when they should be used and how they can be used most effectively. In this report, all of the established techniques will be analyzed to determine their ability to detect asbestos levels in air (and not in soil). Detecting asbestos in air is a relatively easy process because obtaining a sample only requires one to filter particles out of the air. However, it is extremely important to have effective techniques and methods for measuring asbestos in the air because this is the medium in which asbestos is usually measured and in which health risks can most easily be determined.We are now going to discuss three detection techniques in more detail.

1. Phase Contrast Microscopy (PCM)
PCM is an optical microscopy analytical technique to determine indoor asbestos air levels for occupational settings to ensure a safe working environment. Several methods have been developed for PCM, but the most prevalent one was developed by the National Institute for Occupational Safety and Health (NIOSH) and is referred to as “NIOSH 7400.” This method establishes requirements for both the preparation and microscopic examination of air samples. To conduct a PCM analysis following NIOSH 7400 guidelines, users must follow a number of steps from air sample collection to the documentation of results. The first step requires the collection of an air sample done using a personal sampling pump to force air through a membrane filter to capture airborne asbestos fibers. The amount of time over which pumping occurs and the flow rate must be recorded to later calculate the number of fibers present per volume of air. Other methods of collecting air samples exist, such as using a personal passive dust sampler, but then other methods than the NIOSH 7400 have to be used. NIOSH 7400 advocates the use of a cellulose-ester membrane filter with 0.45 µm to 1.2 µm sized pores. In preparing for analysis following sample collection, the portion of the filter that is to be examined first has to be made “cleared” or “collapsed” (i.e. made transparent).Fibers are counted according to strict guidelines. Fibers from a minimum of 20 random areas on the filter are counted and fibers are only accepted if they have a length greater than 5 µm and have an aspect ratio of 3:1 or greater. Some other counting restrictions also apply.There is widespread agreement in the superiority of NIOSH 7400 over other PCM methods. Among other scientific applications, PCM is a fairly common, standard analytical technique utilized for testing air monitoring samples for airborne fiber concentrations pertaining to asbestos-related work, such as projects related to: abatement, repair, clean-up, worker exposure, ambient background, etc. Further, PCM equipment is relatively inexpensive, portable, and sturdy enough that it can be setup directly on many project sites, a particularly convenient advantage.But, there are a number of significant issues regarding over-reliance of PCM. One main cause for concern are serious limitations of PCM's optical resolution. Unknown quantities of asbestos fibers too fine for observation at 400x may not be discernible using PCM, but are better detected using more advanced and asbestos-specific imaging techniques such as transmission-electron microscopy (TEM). PCM may be considered more as a simple "screening" method and possibly inadequate for truly accurate analyses of airborne asbestos fiber concentrations; another debate that typically boils down to Health vs. Cost, as TEM sample analysis is comparatively many times more costly than PCM sample testing fees.The main disadvantage with PCM is that it cannot distinguish between asbestos and non-asbestos fibers, which causes great uncertainty about the actual asbestos fiber concentration for a given area nor can it distinguish between different types of asbestos fibers. Also, chain-like particles often appear fibrous when using PCM and may be counted as asbestos fibers (NIOSH, 1994a). A number of non-asbestos fiber-like structures can interfere if present and artificially boost the asbestos fiber count. Therefore, to have an accurate estimate of the asbestos fiber concentration one must be sure that a given site is devoid of any kind of interfering material. To ensure that interfering materials are kept out of the asbestos fiber count, “differential counting” can be used. To perform differential counting, electron microscopy (i.e., TEM), optical tests, or dispersion staining can be used in conjunction with PCM to identify the fraction of the sample representing asbestos fibers.

2. Transmission Electron Microscopy (TEM)
Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera. TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to the small de Broglie wavelength of electrons. This enables the instrument's user to examine fine detail—even as small as a single column of atoms, which is tens of thousands times smaller than the smallest resolvable object in a light microscope. TEM forms a major analysis method in a range of scientific fields, in both physical and biological sciences. TEMs find application in cancer research, virology, materials science as well as pollution and semiconductor research. The transmission electron microscope (TEM) operates on the same basic principles as the light microscope but uses electrons instead of light. What you can see with a light microscope is limited by the wavelength of light. with their much lower wavelength enable TEM to get a resolution a thousand times better than with a light microscope. The possibility for high magnifications has made the TEM a valuable tool in both medical, biological and materials research. TEM uses electromagnetic lenses to focus the electrons into a very thin beam that travels through the specimen you want to study. The image can be studied directly by the operator or photographed with a camera. TEM could be considered a superior technique to PCM and SEM for several reasons.First, transmission electron microscopes have greater resolution and thus can better detect smaller fibers and better examine a particulate’s morphology. Secondly, TEM methods for analyzing airborne asbestos use EDXA to determine the elemental makeup of a fiber, which enables this technique to be able to determine if a fiber possesses a chemical composition characteristic of asbestos or not. Despite many strengths of TEM there also exists some disadvantages. Both the TEM sample preparation and analysis are more complicated than PCM, making it more labor intensive. It is also more expensive, partly because of its lack of simplicity, but also because the equipment needed is much more expensive than PCM and because it is performed less frequently (it is not used to test airborne asbestos levels in the workplace, like PCM). Another disadvantage is that it has a high detection level because a much smaller portion of the collecting filter (or analysis filter in the case of indirect-transfer TEM) is examined. This introduces a greater uncertainty about any results obtained from TEM. PCM and SEM do not have this problem because more of the filter can be examined.

 3. Scanning Electron Microscopy (SEM)
SEM is another electron microscopy approach used to detect asbestos in ambient air by analyzing the image produced by scanning a targeted surface with electron beam.Compared to PCM and TEM, SEM is better for examining the morphology of particulates because of the greater resolution of the scanning electron microscope, making it easier to detected fibers with smaller diameters. SEM has lower detection limit due to simpler sample preparation methods and a greater proportion of a collection filter can be analyzed,and it is more likely that the results an SEM analysis will be reproducible. Also, SEM better equipped than PCM is that EDXA can be used with SEM to determine the elemental composition of a given fiber. Despite these strengths there are still some significant disadvantages to using SEM. While SEM benefits from having greater resolution for analyzing samples and the ability to use EDXA to help identify structures, TEM is still better suited for determining if a fiber is asbestos or non-asbestos and for identifying the specific type of asbestos because of its ability to use ED to determine crystal structure. But itis far more expensive than TEM and less widely available, probably partly due to the fact that its use is not required by any governmental regulations.

Although asbestos detection techniques have been used for years, new techniques and methods are always being developed, and old techniques and methods are always being improved. The techniques discussed above have been used with mixed success. All techniques (and methods) have strengths and weaknesses and there is no technique (or method) that is superior to all others. In order to effectively detect asbestos one must take into consideration all the likely advantages and disadvantages, weigh them carefully, and then choose the best technique and method for a given task.


References:

http://www.flickr.com/photos/asbestos_pix/3538567254/
http://www.clu-in.org/download/studentpapers/Asbestos_Paper_Perry.pdf
http://en.wikipedia.org/wiki/
http://nobelprize.org/educational_games/physics/microscopes/tem/index.html