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800-356-1688
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VIII.Cleaning theCanister
Every air sampling canister,whether newor used,must be cleaned and certifiedbefore it is used for sam-
pling. Some laboratories batch test and certify canisters. This is done by testing and certifying one canis-
ter out of 10 following cleaning.We recommend certifying each canister cleanprior touse—especially if
there is potential for litigation.
For years therehas beenmuchdiscussion regardingwhat constitutes aproper procedure for cleaning can-
isters.US EPAMethodTO-15 has provided guidance, and in the last 5–10 years automated commercially
available canister cleaning systems have evolved. Becausemany of these systems are quite expensive, and
somedesignshave limitations,analystsoftendesign theirown systemsandmethodologies forcleaningcan-
isters.The cleaningproceduredescribed in this section is apractical approach thatwill ensure canisters are
suitably cleaned for ambient air sampling,whether you areusing a commercially available cleaning system
or a system of your own design. There are minor differences when cleaning SilcoCan® or TO-Can®
(SUMMA®) canisters.Wewill discuss thesedifferences in this procedure.
AirVersusNitrogen
The two gases recommended for cleaning canisters are humidified ultra-high purity air and ultra-high
purity nitrogen. Thewater in the humidified gas hydrolyzes impurities in the canister and, according to
theory,will occupy the active sites on the interior surface, displacing the impurities and allowing them to
be removed.Air is recommendedwhenoxidationof the interior surface isdesired.Theoxygen content of
air, 21%, is sufficient for this surface oxidation; it is not necessary to use pure oxygen gas. Nitrogen is
equally effective for cleaning ambient air canisters, butwill not oxidize the surface of the canister.
HeatorNoHeat*
Some user-designed canister cleaning systems do not heat the canisters. Typically this does not create a
problemwhencleaningcanisters that areused inambientaircollection,but asa safeguardwe recommend
heating the canisters during the cleaning process.Compounds collected inmost ambient air samples are
in the low ppbv range, and can be removed from a canister by multiple cycles of pressurization with
humidified air or nitrogen followedby evacuation. If there are higher concentrations of contaminants in
the canister, heatmight be required to clean the canister satisfactorily. In addition, the cleaning cyclemay
be reducedwhenheat is applied.
Caution:
Addingheat andhumidifiedgas toacanistermaycreatea steampressurevessel.Somecommercial cleaning systems
incorporate apressure release valve to ensure the pressure does not exceed the pressure ratingof the canisters.
CleaningSystems
•Oven
Some canister cleaning systems are incorporatedwithin anoven.Batch size is determined
by thenumber of canisters that can fit inside the oven.The supply line for thehumidified air or
nitrogen stream and the line to the vacuum system areplumbeddirectly into theoven.A cold trap
is employed to trap impurities.Accuratemonitoringof vacuum andpressure is required. In this
arrangement, the entire canister, including the valve,will beheated.Thiswill help remove contam-
inants if both the valve and the canister aredirty.Typically,whenusingheat, it is helpful to create
steam from thehumidified air ornitrogen stream.Anoven temperatureof at least 120 °C is
required, but higher temperatures often areused.
•HeatBands
Abandheaterplacedaround theequatorof thecanister typically iscapableof heat-
ing the canister toapproximately130 °C.There is aheat gradient, and thevalvemight only receive
radiant heat (approximately 70–100 °C). In most sampling situations, this lower temperature
shouldbe sufficient for effectively removing contaminants from the valve.
• InsulatedHeat Jackets
Insulated heat jackets surround and heat each canister. These jackets
typicallyhavea siliconeorTeflon®-coated fiberglass fabricexterioranda fiberglass insulation inte-
rior. Some operate at a fixed temperature; others canprovide variable temperature.Restek’s heat-
ing jacket offers a significant advantage over alternatives because it encompasses the valve area.
• InfraredHeat
An infraredheating system includes an infraredheat source and a reflectivepanel
similar to the cylinder drying rack on a gas cylinder system. The infrared source and the reflective
panel areplacedonopposing sidesof the canisters. Infrared rays from the sourceheat the canisters;
rays that pass the canisters strike the reflectivepanel andheat the canisters from theopposing side.
•UserDesigned
Figure 10 shows an example of a“homemade”systemdesigned to clean 24 six-
liter canisters.Thisdesigndoesnot employheat, but aheater canbe added (see
HeatorNoHeat
).
It provides ahumidified air or nitrogen stream to all canisters and the roughingpumpon the bot-
tom shelf is the vacuum source.This system is computer operated to automate the cleaning cycles.
*If you are cleaning any fused
silica lined canisters, andwill
be using heat, use humidified
nitrogen, not air.
Cleaning any fused silica lined
canisterswith humidified air
and heat above 80 °Cmay
damage the fused silica sur-
face, resulting in reduced
recoveries of sulfur and other
reactive compounds.
Air Canister
Heating Jacket
TO-CleanCanister
CleaningSystem
Figure10
User-designed system
for cleaning24
six-liter canisters.
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