9
Thereare fourpossiblescenarios:
A. Ideally therewill be a vacuumof 7"-4"Hg in the canister (see, e.g.,
Table 3onpage 10).
B. Ifmore than7"Hg vacuum remains, less samplewas collected than
initially anticipated.The samplewill be valid, but the detection limit
maybehigher than expected.Youmight have topressurize the canister
prior to the analysis,whichwill dilute the sample, then youwill have to
use adilution factor todetermine final concentrations of target com-
pounds.
C. A vacuumof less than4"Hg indicates the samplemight be skewed
toward the initial part of the samplingperiod.This assumptionusually
is validbecause the flow rate through the flow controllerwill fall once
the vacuum falls below5"Hg (Figure 5, page 5),when the change in
pressure across the flow controller diaphragmbecomes too small and
the flow controller is unable tomaintain a constant flow.Although
flowwas not constant over the entire samplingperiod, the sample
might beusable because samplewas collectedover the entire interval.
D. If the ending vacuum is less than1"Hg the sample shouldbe consid-
ered invalidbecause itwill be impossible to tell when the sample flow
stopped.
8. Record the final partial pressure in the canister and replace the plugnut.
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 oftenprove veryuseful 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
Equation1:
Equation1:
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
readingwhen the canister left the labwas 29.92"Hg vacuum; the final gauge
pressure readingwhen the canisterwas returned to the labwas 7"Hgvacuum.
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 samplevolume (Equation2).Laboratories typicallyallowamax-
imum deviation of +/-10% to +/-25% between the initial flow rate and the
post-sampling flow rate.
Equation2:
samplevolume=[(initial flowrate+post-sampling flowrate)/2]xsampling time
Example: A flow controller was set at 3.3mL/min. After obtaining a 24 hour
sample the flow ratewas 3.0mL/min.
sample volume= [(3.3mL/min.+3.0mL/min.) / 2] x 1440min.=4536mL.
MethodsofAirSamplingandAnalysis,3
rd
Edition
This book includes precisedirections for analyzing
a long list of air contaminants. All contaminants
one can analyze ormonitor using a givenmethod
areconsolidated to facilitateuse.Anexcellent refer-
encemanual forall analytical laboratoriesconduct-
ing air analyses.
J. P. Lodge, CRCPress LLC, 1988, 784pp.,
cat.#20493
