Direct-on-Filter Disadvantages
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The main disadvantages to DOF: 1) difficulty in adapting the €nylon cyclone€ to
a 25mm sampling cassette. x-ray instruments are normally setup to accept 25mm
diameter filters; thus a cyclone normally designed to accept a 37mm cassette is
adapted to fit the 25mm system with varying degrees of success. 2) Some samples
do not present themselves as homogeneous. Employees sampled may be involved in
many different operations during the workday and samples taken on these
employees will tend to have different layers of particulate which may not be
fully penetratable by x-rays if they are not redeposited in a thin-layer
deposition. 3) the biggest drawback is the need to prepare standards using
aerosol generation systems to assure standard/sample matching. Most small labs
are unable to generate aerosols to produce similar standards. The possibility of
a private enterprise (i.e. NIST) generating standards as a commercial product
would enable this technique to enjoy more popularity; however, at present, there
are no takers. In addition if the filter deposit is distributed in transit, have
to resort to the old technique of redeposition, and prepare similar standards.
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