Both air analysis methods use the same field collection; only the resolution of the microscope is different.
Asbestos TEM (Transmission Electron Microscopy) – Air Sample Testing
Asbestos TEM air sampling uses electrons to create images of fibers or structures encountered in the air sample.
The machinery required is not mobile and is sophisticated.
It magnifies objects at least 20,000 times their actual size. This method is asbestos-specific; so specific that it can indicate the type of asbestos fibers in the air sample. This method is often used when there are interfering fibers which cause the PCM results to be very high but asbestos fibers are not suspected as the cause of their high-fiber count.
This method is most appropriate for final clearance samples either due to regulation (AHERA) or liability issues. By law, schools, etc., must use this method. This is also most appropriate for litigation and real estate transactions.
Asbestos TEM samples are considered the gold standard for asbestos air testing.
Asbestos PCM (Phase Contrast Microscopy) – Air Sampling
Asbestos PCM (Phase Contrast Microscopy) NIOSH 7400 Method samples use a light microscope to see all fibers. The microscope magnifies objects approximately 400 times their actual size. All bulk samples collected are sent to an accredited asbestos environmental laboratory.
The main disadvantage of the Phase Contrast Microscopy methodology is that it is not asbestos specific.
The analyst counts any fiber that falls into a field of view that is greater than or equal to 5 μm in length, with a 3:1 aspect ratio (three times longer than its width).
You need to be aware of this when requesting that we use this type of sample analysis there are limitations. Items such as carpets, wood, fiberglass, and drywall can produce fibers that fall in this size range and will be counted along with asbestos fibers in determining exposure.
A dust free environment with negative air machines running is recommended, per industry standards.
Asbestos PLM (Polarized Light Microscopy) – Bulk Sampling
The polarized light microscopy technique utilizes the unique features of polarized light to observe mineral specific optical properties. In this manner, PLM can differentiate asbestos from non-asbestos fibers and further classify the various species that compose the asbestos mineral family. Moreover, the technique records the identity of the non-asbestos fibrous component of each bulk building material sample.
The PLM procedure provides an economical technique for screening large numbers of samples. However, as with PCM, there are limitations to light microscopy testing due to the magnification (100-400X) employed and due to other interferences present in the building material matrix (ex: tar and petroleum binding components, sub-micron particulate adhering to the surface of an asbestos mineral, etc.).
PLM results are reported as a percentage of the total sample. PLM utilizes a few protocols for the quantification process. These include visual estimation and point counting. Depending on the sample matrix, PLM analytical sensitivity can be a fraction of a percent. Gravimetric reduction protocols (ELAP 198.1, EPA 600) further enhance this technique’s ability to accurately quantify and qualify asbestos.
The current method employed for these analyses is found in EPA 600/R-93/116. Other procedures are also utilized to supplement this method such as NIOSH 9002, and OSHA ID 191. Accreditation is primarily provided by the National Institute of Standards and Technology (NIST) through the National Voluntary Laboratory Accreditation Program (NVLAP).