Asbestos PLM (Polarized Light Microscopy) bulk sampling for asbestos content in building materials is primarily conducted by the method.
The 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).