Some laboratories certify a canister for VOC stability by introducing a low con-
centration test mixture into the canister and measuring degradation over a
specified time period. If the canister meets the specification, it is certified for
use. We recommend using such studies to ensure the effectiveness of a canister
or group of canisters for a proposed application.
Once the canister is certified clean, evacuate the canister to a final vacuum of
10-50mtorr, using either the canister cleaning system or a clean final vacuum
system. This vacuum is critical to ensure the correct amount of sample is col-
lected. Use an accurate test gauge or digital pressure tester to ensure final vac-
uum has been reached and document the final vacuum reading for the chain
of custody. Install a brass plug nut into the canister valve to ensure no con-
tamination can enter the sample pathway during shipment to the field.
Allot an individual identity to the canister, either with a label and serial num-
ber or with a bar code.
Some analysts prefer to introduce surrogate standards into the canister prior
to sampling. Debate on this practice revolves around theories that there are
potential loss issues due to low humidity and inadequate surface passivation by
water. Neither Restek chemists nor our consulting experts recommend adding
surrogates to the canisters. If you choose to introduce surrogates into your
canisters prior to sampling be sure to recheck the vacuum reading for each
canister after adding the surrogates, and record the reading.
V. Field Sampling, Using a Passive Sampling Train and Canister
It is important to mention again that the sampling train and canister must be
leak tested and certified clean prior to use. To properly begin field sampling,
we recommend bringing a "practice" evacuated canister and a flow measuring
device with you to the field. Use this canister to verify the flow rate through the
passive sampling train prior to using the train to obtain samples of record. To
verify the flow rate, connect the passive sampling train to the "practice" canis-
ter. Attach a flow meter to the inlet of the sampling train. Open the canister,
and measure the flow rate through the sampling train. If the flow rate is with-
in +/- 10% of the flow rate set in the lab, the train is ready to be used on the
formal sampling canister. If the flow rate is not within these limits, adjust the
flow rate by adjusting the piston gap screw.
When the flow rate is confirmed, record the rate as the canister flow rate for
the chain of custody form.
To begin sampling, using the formal sampling canister, follow these steps:
1. Remove the brass plug nut from the canister valve.
2. If you are using a test gauge, attach the gauge to the canister and record
the vacuum reading. If you choose not to use a test gauge under field con-
ditions, record the reading on the vacuum gauge that is part of the passive
sampling train.
3. Attach the verified passive sampling train to the canister.
4. Record the begin sampling time and necessary meteorological data.
5. Open the canister valve and begin sampling.
6. Periodically check the canister throughout the sampling period to ensure
the partial pressure reading is accurate and sampling is proceeding as
planned.
7. Once the sampling period is complete, close the valve and remove the
sampling train. Check the final partial pressure within the canister, using
the test gauge or the vacuum gauge in the sampling train.
Phone: 800-356-1688 or 814-353-1300
8
did you
know
?
Restek canisters are shipped in boxes with handles for ease of
carrying and transporting, eliminating costly carrying cases.
