[A comparison between phase-contrast optical microscopy and scanning electron microscopy for the analysis of...

There is no general agreement on the relationship between results obtained by optical microscopy (PCOM) and electron scanning microscopy (SEM) or electron transmission microscopy (TEM) for airborne asbestos fibres. However, a considerable amount of data has been produced in latter years indicating that a general correlation factor, not differing greatly from unity, can be established between the two analytical techniques if counts are limited to fibres longer than 5 microns. A study was made of the relationship between SEM and PCOM when used simultaneously for determination of background concentrations of asbestos in office buildings. Twenty-four couples of samples were collected in parallel for PCOM and SEM in different locations distributed over the various floors of two buildings containing amosite sprayed on structural parts. Asbestos and total fibre counts by SEM were performed following the Verein Deutscher Ingenieure (VDI) method 3492. For total fibre counts the European PCOM method was mainly used. Discrimination of asbestos from non-asbestos fibres was performed using criteria based on birefringence and morphological characteristics of fibres. Counts included only those fibres longer than 5 microns, diameter less than 3 microns and aspect ratio greater than 3. Both total fibres and asbestos fibres were recorded separately by PCOM and SEM. the arithmetic and geometric means of the ratio between PCOM and SEM were 1.3 and 1.1 respectively for total fibres and 1.2 and 1.1 for asbestos fibres (excluding one outlying value). The regression study showed a significant linear correlation (P = 0.01) with correlation coefficients of 0.84 for asbestos fibres and 0.52 for total fibres.