WHAT DO POSITIVE NOA LAB RESULTS MEAN (PART II): CONSIDERATIONS WHEN ORDERING AND INTERPRETING TRANSMISSION ELECTRON MICROSCOPY (TEM) ANALYSES OF ROCK/SOIL AND AIR SAMPLES

Accurate identification and quantitation of naturally occurring asbestos (NOA) in rock/soil and air samples is fraught with difficulties, any of which can lead to serious error. When small asbestos fibers and/or low concentrations of asbestos must be detected, transmission electron microscopy (TEM), which can obtain magnifications of 250,000X or greater, is the method of choice. For rock/soil samples the TEM Quantitative Bulk method is used, and for air samples a modified TEM AHERA method is used.

The TEM Quantitative bulk technique can identify asbestos fiber concentrations below 0.0001 weight (wt.) %. TEM is applied to rock/soil samples following a modified EPA 600/R-93/116 method. Samples are prepared using the CARB 435 crushing and grinding prep method with resultant dust suspended in liquid, and emplaced onto a filter. TEM is applied to air filters from air sample cassettes using a modified AHERA method. In both cases, a carbon replica of a filter is created, placed onto a TEM sample grid and analyzed. Selected Area Electron Diffraction analysis measuring crystal layer spacing is performed on all fibers. Suspect amphibole fibers are subjected to Energy Dispersive X-Ray chemical element analysis. For bulk samples, asbestos structure lengths and widths are measured to allow calculation of the wt. % asbestos. For air samples, an asbestos structure count per unit volume of air is determined.

Geologists must exercise considerable caution in relying upon TEM analytical results for NOA samples produced by asbestos testing laboratories. TEM Quantitative Bulk methods are being pushed to their limits with the recent lowering of accepted regulatory amounts of NOA to 0.001 wt. % for rock/soil in school yards. Unfortunately, the methods do not address statistical uncertainties present at these low concentrations. It also has not been established that commercial asbestos testing laboratories as a group, are up to the task of providing accurate, reproducible NOA analytical results. These labs are geared toward analyzing high quality, ore grade asbestos used in building materials, not weathered dirt. No certification, proficiency testing, or well defined test methods currently exist for NOA analysis. As a result, it is important to rigorously assess the credentials and capabilities of labs used for NOA analysis.