Bulk Sample
Analysis
If a material is suspected to contain asbestos, then a representative sample
is taken for analysis. Once a sample is received by the laboratory
it is analysed under a stereo microscope to identify and isolate any
visible bundles of fibres. The sample may be prepared mechanically
and/or chemically to aid the release of bundles. These bundles are
characterised and an appropriate refractive index (RI) liquid is selected,
the fibres are mounted on a slide in the RI liquid before being examined
under a polarised light microscope (PLM) with a dispersion staining
objective.
Asbestos is a birefringant material: Birefringance can be observed
with a PLM microscope by looking at the sample with crossed polars,
allowing only certain wavelengths of light pass through the sample.
When the fibre is at the N-S and E-W position the fibre appears
to disappear, between these angles the fibre appears bright.
Asbestos fibres are also anisotropic; this is recognised by observing
the fibre under crossed polars with a red tint plate to make to
colours easier to see. Anisotropic materials show a yellow colour
in one diagonal direction and a blue colour in the other, this is
called the sign of elongation. Depending on which direction the
yellow and blue lines go distinguishes whether the fibre is length
fast or length slow, Crocidolite is length fast whereas Chrysotile,
Amosite, Tremolite, Anthophyllite and Actinolite are all length
slow.
The final way that PLM can identify asbestos is by a process called
dispersion staining, this involves part of the colour fan being
blocked out. If the refractive index of the fibres matches that
of the liquid then the analyst will see specific fibre colours.
This test is used as the final conformation as to the identity of
the fibres in the sample.
References taken from HSG 248; The Analysts Guide
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