User`s guide

Sampling Techniques
326
Alternatively, the uptake rate can be determined experimentally.
One quick experiment that can be used either to determine
approximate uptake rates or to confirm published data is as follows:
1. Monitor an atmosphere containing the component of
interest, using at least 6 identical diffusion tubes.
2. Monitor the same atmosphere, in parallel, using an
independent method, three pumped sample tubes, for
example.
3. If the atmospheric concentration is determined using the
independent method, an approximate value for uptake
rate can then be calculated from the masses of the
component retained by the diffusive monitors.
Diffusion Monitoring Applications
Diffusive monitoring with PerkinElmer tubes typically occurs at
rates equivalent to approximately 1 mL/min pumped sampling flow.
This relatively slow sampling rate restricts the application of
diffusive monitoring to ambient air concentrations greater than 5 ppb
when used with 8-hour exposure times and flame ionization
detection. However, diffusive sampling onto PerkinElmer tubes is
suitable for considerably lower atmospheric concentrations,
provided the appropriate selective detector or a mass spectrometer is
used.
The range of components that can be monitored diffusively at any
one time is restricted to those that are quantitatively retained by the
single adsorbent selected, unless two or more tubes are used in
parallel.
Tubes packed with more than one adsorbent in series, cannot be used
for diffusive monitoring, as the diffusion gap for the rear adsorbent
would exceed the standard 1.5 cm, thus reducing the uptake rate.
Within these limits, diffusive sampling contributes greatly to the
routine monitoring of vapor phase pollutants, particularly in
workplace environments. When applicable, it avoids the use of
expensive and variable sample pumps and thus facilitates collection
of the large amounts of data required for accurate occupational
hygienic assessments.