9
There are four possible scenarios:
A. Ideally there will be a vacuum of 7"-4" Hg in the canister (see, e.g.,
Table 3 on page 10).
B. If more than 7" Hg vacuum remains, less sample was collected than
initially anticipated. The sample will be valid, but the detection limit
may be higher than expected. You might have to pressurize the canister
prior to the analysis, which will dilute the sample, then you will have to
use a dilution factor to determine final concentrations of target com-
pounds.
C. A vacuum of less than 4" Hg indicates the sample might be skewed
toward the initial part of the sampling period. This assumption usually
is valid because the flow rate through the flow controller will fall once
the vacuum falls below 5" Hg (Figure 5, page 5), when the change in
pressure across the flow controller diaphragm becomes too small and
the flow controller is unable to maintain a constant flow. Although
flow was not constant over the entire sampling period, the sample
might be usable because sample was collected over the entire interval.
D. If the ending vacuum is less than 1" Hg the sample should be consid-
ered invalid because it will be impossible to tell when the sample flow
stopped.
8. Record the final partial pressure in the canister and replace the plug nut.
Information that should be acquired at the sampling site includes the start
time and interval time, the stop time, atmospheric pressure and temperature
and, for ambient sampling, wind direction. Include elevation if it is a factor.
These parameters often prove very useful toward interpreting results.
After sampling, the canisters are sent back to the laboratory, where the final
vacuum is measured once again, with a test gauge. Using the initial vacuum
and final vacuum, the sample volume collected can be determined from
Equation 1:
Equation 1:
pressure change*
sample volume = ________________________ x canister volume
atmospheric reference pressure
*initial pressure - final pressure
Example: A sample is collected in a 6-liter canister. The initial gauge pressure
reading when the canister left the lab was 29.92" Hg vacuum; the final gauge
pressure reading when the canister was returned to the lab was 7" Hg vacuum.
sample volume = [(29.92"Hg - 7"Hg) / 29.92"Hg] x 6L = 4.59 liters collected.
It is also a good practice to recheck the flow rate after sampling, because this
will affect the sample volume (Equation 2). Laboratories typically allow a max-
imum deviation of +/-10% to +/-25% between the initial flow rate and the
post-sampling flow rate.
Equation 2:
sample volume = [(initial flow rate + post-sampling flow rate)/2] x sampling time
Example: A flow controller was set at 3.3mL/min. After obtaining a 24 hour
sample the flow rate was 3.0mL/min.
sample volume = [(3.3mL/min. + 3.0mL/min.) / 2] x 1440 min. = 4536mL.
Methods of Air Sampling and Analysis, 3
rd
Edition
This book includes precise directions for analyzing
a long list of air contaminants. All contaminants
one can analyze or monitor using a given method
are consolidated to facilitate use. An excellent refer-
ence manual for all analytical laboratories conduct-
ing air analyses.
J. P. Lodge, CRC Press LLC, 1988, 784pp.,
cat.# 20493
