Airborne inorganic fibre level monitoring by transmission electron microscope (TEM): comparison of direct and indirect sample transfer methods.

The direct and indirect sample transfer techniques for measuring airborne inorganic fibres concentrations were studied by TEM for airborne rockwool fibres, chrysotile, amosite and tungsten oxide whiskers. The number and mass concentrations of fibres with aspect ratios > or = 5, prepared by these two techniques for fibres longer than 0.5 mu m and 5 mu m, and the aspect ratios were evaluated and compared. It was found that the indirect sample transfer technique affects the fibre size distribution of different materials differently, and that the mass concentrations did not relate to fibre number concentrations of ambient air, irrespective of the sample transfer method used or the materials studied. It is concluded that the direct sample transfer method incorporating the etching stage should be preferred over the indirect method. Moreover, a need for an extra stage in-between the etching and carbon coating stages for the direct sample transfer method is suggested; this would be a stage where soluble materials could be removed.