Nearly all current methods for asbestos analysis rely on the use of a filter to separate particulate from the aerosol, or in the case of indirect preparations, particulate from a liquid. The filter that is most commonly used is formed from cellulose acetate or cellulose nitrate or a mixture of both. The resulting membrane has a sponge — like structure forming a series of tortuous pores which have dimensions larger than the nominal pore size of the filter. This allows particulates to penetrate into the filter before capture so that for many types of microscopy analysis [e.g. phase contrast microscopy (PCM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM)], it is important how far the fibre penetrates into (or through) the membrane, as it may not be recoverable for analysis.
Confocal microscopy has the advantage over conventional light microscopy of being able, through means of an adjustable slit, to discard light from above and below the focal plane, to makes it possible to observe a precise point within a transparent film. Computer controlled quantitative scanning confocal microscopy may be used to carry out precise measurements of depth profiles of entrapped particulates by scanning a vertical series of 2D planes into the membrane. The use of pixel connectivity routines also allow the 3D orientation of fibres to be determined so foreshortening effects on fibre length can be measured. The technique has been applied to 0.8μm nominal pore size filters, which had been loaded with crocidolite asbestos, to determine the fibre penetration and orientation.