User`s guide

Sampling Techniques
327
Protocol for Assessing the Performance
of a Diffusive Sampler
A joint US National Institute of Occupational Safety and Health
(NIOSH)/UK TDE protocol for evaluating diffusive monitors has
been agreed upon by research workers in these two organizations
and was presented at the American Industrial Hygiene Conference in
May 1988.
The protocol is almost identical to that published by the UK TDE,
Method for the Determination of Hazardous Substances
, Number 27,
PerkinElmer Thermal Desorption Data Sheet GCA-77. It has also
been accepted as the basis of a draft European Committee for
Normalization (CEN) standard currently being published as a
discussion document.
Theory of Pumped Air Monitoring
Diffusive sampling is not applicable to all monitoring tasks. In
certain circumstances, for example, when relevant uptake-rate data is
not easily obtained or when the expected air concentration is very
low, pumped air monitoring provides a useful alternative.
During pumped sampling, the storage caps are removed and air is
drawn through the front, grooved end of the tube.
Figure 98 Pumped Air Monitoring
NOTE: Diffusion caps (without the silicone membrane) can be
Glass Wool
o
r Frit
Stainless Steel Tube
Pumped
Flow
Retaining
Spring
Stainless Steel
Gauze
Adsorbent
Glass Tube
Pumped
Flow
Adsorbent