VISIBILITY OF FINE FIBRES OF ASBESTOS DURING ROUTINE ELECTRON MICROSCOPICAL ANALYSIS

Abstract

Most analyses for asbestos in samples from the non-occupational environment are carried out using techniques based upon either scanning (SEM) or transmission (TEM) electron microscopy. The mode of image formation and resolution obtainable differ between SEM and TEM and the present study was undertaken in order to provide some indication of the comparability of observations made by the two techniques, with particular reference to the visibility of fine, sub-micrometre fibres. The results indicate that, in general, at a given magnification finer fibres are visible when using TEM than when using SEM. When using SEM, fibre visibility is improved by the use of higher magnifications, by the use of slower raster speeds and by the use of gold rather than carbon for specimen coating. The use of photomicrographs leads to improved fibre visibility in both SEM and TEM images. The study suggests that comparable results can probably be obtained from the two techniques, although fibres S≥O. 1 μn dia may be close to the limit of visibility in the SEM. In order to facilitate inter-laboratory comparison of results it would seem appropriate for standard samples to be available so that optimum conditions can be determined for the observation of fine fibres.