User`s guide

Sampling Techniques
324
If the concentration is required in ppm, then the formula is modified
to include the molecular weight of the compound of interest and the
molar gas volume, as follows:
Where U
P
= Uptake rate in ng ppm
-1
min
-1
NOTE: 24.45 l/Mole = Molar gas volume at 760 mm Hg and 25°C
(298 K)
The atmospheric concentration of a compound in ppm can then be
found from:
The PerkinElmer stainless steel sample tube (P/N L427-0123) is
suitable for diffusive sampling, since the distance from the surface of
the adsorbent to ambient air is clearly defined. Glass and glass-lined
stainless steel tubes cannot be used.
Example: Determine the mass of styrene deposited on the
sample tube using the following parameters:
• Expected concentration is 1 ppm
• Typical sampling time is 8 hours (480 minutes)
• Uptake rate for styrene on a PerkinElmer tube packed with
Tenax is 2 ng ppm
-1
min
-1
Therefore the mass adsorbed on the tube = 480 x 1 x 2 ng
= 960 ng
≈ 1 µg
Equation 14
Equation 15
Zx 24.45
x MW Ax x D 60
=U
1
P
(min) timesamplingx U
(ng) tubeon found compound ofWeight
ppm
p
=
timeexposurex ionConcentrat catmospheri
(ng) tube theonto adsorbed mass
RateUptake
=