VI.AnalysisofCollectedSamples
Once receivedby the lab,eachcanister is identified from the information in the
chainof custody report.The final partial pressure is checked toensureno leaks
appearedduring transport. Itmight benecessary topressurize a canisterprior
to the analysis; do this by adding humidifiednitrogenor air to the canister to
a pressure greater than 5 psig or higher, depending on the sample volume
needed for analysisor for suitablydiluting the sample (e.g.,Table4).Theneed
to dilute is determined by the preconcentrator instrument. Some air precon-
centrators can be operated while the canister is under slight vacuum. Check
with your instrumentmanuals, orwith themanufacturer, todetermine if you
must dilute your samples prior to analysis.
Equation3:
dilution factor= (P
after dilution
+P
lab atmosphere
) / (P
lab atmosphere
- P
before dilution
)
Thedilution factor is calculated from thepost-samplingpressure (beforedilu-
tion), the final pressure (after dilution), and the atmospheric pressure in the
laboratory.The factor for converting "Hg topsi =0.491.
Example:At the endof a samplingperiod the gaugepressure in a canisterwas
7"Hg.The canisterwas pressurizedwithnitrogen to14.7psig (1Atm.).
The dilution factor is (14.7+14.7) / (14.7 - (7 x 0.491))=2.61
To analyze the sample, withdraw an aliquot of the sample from the canister.
For low level ambient air analysis, withdraw 250-500mL of sample from the
canister and concentrate the analytes by using a mass flow controller and a
cryogenically cooled trap (e.g., glass beads and/or a solid sorbent).Desorb the
concentratedanalytes from the trapanddeliver them toacryofocuser, to focus
the sample bandwidth prior to introduction onto the GC column. A 60m x
0.32mm IDx1.0µmRtx®-1column typically isused forEPAMethodTO-14A
orMethodTO-15 ambient air analysis; anMSD is a commondetector. Figure
7 shows a typical TIC spectrum for aTO-14A/TO15 ambient air analysis.
Procedures used in these chromatographic analyses generally include amulti-
point calibration, using gas standards. Therefore calculations of organic com-
pounds in collected samples are straightforward - only volumes analyzed and
dilution rates are needed to determine sample concentrations. High concen-
tration calibration gas standards are commercially available (e.g., 1ppmv or
100ppbv); introduceanaliquotof stockmaterial intoacanisteranddilutewith
humidified air or nitrogen. After analyzing the calibration standards, deter-
mine the response factor for each analyte, using thepeak area counts per con-
centration.
After analyzing the multipoint calibration standards and calculating peak
area/concentration response factors, analyze the “real world” samples. If an
"unknown" samplehasnotbeendilutedapply thecorresponding response fac-
Phone: 800-356-1688 or 814-353-1300
10
frequently
asked
question
Where can I find EPAAir ToxicMethods?
pdf files of US EPAAirToxicMethods are available at thisweb
address:
Table4
Dilution factors toadjust final sampling
pressure to
14.7psig
for a6-liter canister.
Final Vacuum
("Hg)
29"
27"
25"
23"
20"
17"
15"
12"
10"
7"
5"
3"
0"
Sample Volume
(liters)
0 0.414L 0.83L 1.24L 1.86L 2.48L 2.90L 3.52L 3.93L 4.55L 5.0L 5.38L 6L
Dilution Factor
63.71 20.37 10.12 8.63 6.02 4.63 4.01 3.34 3.00 2.61 2.40 2.22 2.00
Table3
Final vacuumandvolumeof
sample collected in6-liter canister.
Final Vacuum
("Hg)
29"
27"
25"
23"
20"
17"
15"
12"
10"
7"
5"
3"
0"
Sample Volume
(liters)
0 0.414L 0.83L 1.24L 1.86L 2.48L 2.90L 3.52L 3.93L 4.55L 5.0L 5.38L 6L
