VI. Analysis of Collected Samples
Once received by the lab, each canister is identified from the information in the
chain of custody report. The final partial pressure is checked to ensure no leaks
appeared during transport. It might be necessary to pressurize a canister prior
to the analysis; do this by adding humidified nitrogen or air to the canister to
a pressure greater than 5 psig or higher, depending on the sample volume
needed for analysis or for suitably diluting the sample (e.g., Table 4). The need
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 instrument manuals, or with the manufacturer, to determine if you
must dilute your samples prior to analysis.
Equation 3:
dilution factor = (P
after dilution
+ P
lab atmosphere
) / (P
lab atmosphere
- P
before dilution
)
The dilution factor is calculated from the post-sampling pressure (before dilu-
tion), the final pressure (after dilution), and the atmospheric pressure in the
laboratory. The factor for converting "Hg to psi = 0.491.
Example: At the end of a sampling period the gauge pressure in a canister was
7"Hg. The canister was pressurized with nitrogen to 14.7psig (1 Atm.).
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 250-500 mL 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
concentrated analytes from the trap and deliver them to a cryofocuser, to focus
the sample bandwidth prior to introduction onto the GC column. A 60m x
0.32mm ID x 1.0µm Rtx®-1 column typically is used for EPA Method TO-14A
or Method TO-15 ambient air analysis; an MSD is a common detector. Figure
7 shows a typical TIC spectrum for a TO-14A/TO15 ambient air analysis.
Procedures used in these chromatographic analyses generally include a multi-
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); introduce an aliquot of stock material into a canister and dilute with
humidified air or nitrogen. After analyzing the calibration standards, deter-
mine the response factor for each analyte, using the peak 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" sample has not been diluted apply the corresponding response fac-
Phone: 800-356-1688 or 814-353-1300
10
frequently
asked
question
Where can I find EPA Air Toxic Methods?
pdf files of US EPA Air Toxic Methods are available at this web
address:
Table 4
Dilution factors to adjust final sampling
pressure to
14.7psig
for a 6-liter canister.
Final Vacuum
("Hg)
29"
27"
25"
23"
20"
17"
15"
12"
10"
7"
5"
3"
0"
Sample Volume
(liters)
0 0.58L 0.99L 1.39L 1.99L 2.59L 2.99L 3.59L 3.99L 4.60L 5.0L 5.40L 6L
Dilution Factor
63.77 20.37 12.12 8.63 6.02 4.63 4.01 3.34 3.00 2.61 2.40 2.22 2.00
Table 3
Final vacuum and volume of
sample collected in 6-liter canister.
Final Vacuum
("Hg)
29"
27"
25"
23"
20"
17"
15"
12"
10"
7"
5"
3"
0"
Sample Volume
(liters)
0 0.58L 0.99L 1.39L 1.99L 2.59L 2.99L 3.59L 3.99L 4.60L 5.0L 5.40L 6L
