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Canister
The canister is a stainless steel vessel designed toholdvacuum to less than10mTorrorpressure to40psig.
Canistersareavailable ina rangeof volumes: 400mL,1.0 liter,3.0 liter,6.0 liter,and15 liter.The sizeof can-
ister usedusually depends on the concentrationof the analytes in the sample, the sampling time, the flow
rate, and the samplevolume required for the samplingperiod (Table II,page3).Typically, smaller canisters
are used formore concentrated samples, such as soil gas collection, 3-liter and 6-liter canisters are used to
obtain integrated (TWA) ambient air samples at sampling timesof up to24hours, and large15-liter canis-
ters areused for reference standards. Sampling timewill be limitedby the combinationof canister size and
the flow rate atwhich the sample is tobe collected.
Awell-designed canister is essential to the success of the sampling project. First, the canister should be
made of stainless steel, so the collected samplewill not permeate through the vessel wall or degrade due
to exposure to light during shipment to the analytical laboratory. Second, the interior surface of the can-
ister shouldbe inert, to reduce thepotential for interactionswith theanalytes in the sample.Third,all can-
isters involved in a particular application should be of consistent volume, to simplify calculating sample
volumes. Finally, the canister should have a high quality valve that resists abuse in the field (e.g., over-
tightening that potentially could cause leaks).An inferior valve can fail, causing sample loss and incurring
replacement costs. It canbemore expensive to sample again than to replace a valve.
Two types of canisters are available, the difference being the interior surface.The traditional canister is the
stainless steel SUMMA®orTO-Can® canister.The interior of this typeof canister is electropolished,using
apolishingprocedure (developedbyMolectrics) that enriches thenickel andchromium surfaceandmakes
it more inert than untreated stainless steel. The new generation of sampling canister is typified by the
SilcoCan® canister. Like the SUMMA® or TO-Can® canisters, the SilcoCan® canister ismade of stainless
steel, and the interior is electropolished, but in an additional step—Siltek® treatment—an inert layer is
chemically bonded to the interior surface. Siltek® treatmentmakes the surface inert not only for relatively
inactive organic compounds, but also for compounds that are highly reactivewithmetal surfaces, such as
sulfur-containing compounds. Thus, surface inertness for SilcoCan® canisters exceeds that for SUMMA®
andTO-Can® canisters.
CanisterValve
The valve on a sampling canistermust be of highquality,with the following characteristics: leak integri-
ty, ametal seat, stainless steel wetted surfaces, and apackless design.Ametal seat eliminates offgassingof
seat components into the sample andmemory effects in the seat material. A packless design provides a
completely enclosed system, to ensure no contamination from lubricants or packing material occurs.
Various valves are used, the most common being the Swagelok® SS4H bellows valve and the Parker
Hannafindiaphragm valvewithmetal seat. Several valve options are available forRestek canisters.
Theconnectionof thevalve to thecanister is critical.Theconnectionmust be leak tight, toensureacorrect
sampling flow rate, but use extreme caution to prevent overtightening the tube compression fittings. To
ensure a leak tight valve, alwaysuse apre-filter (such as an inline filter) toprevent valve seat damage.
EnsureAccurateSamplingof
ReactiveCompounds
with
Siltek®
Treatment
Siltek® treatment is a proprietary process, developed by Restek Corporation,
throughwhichan inert layer ischemicallybonded toametal surface.Thesur-
faceproducedby this treatment is virtually inert to active compounds. The
stainless steel pathway described in this guide is sufficient for sampling
atmospherescontainingonlynonreactivecompounds,but for reactivecom-
pounds the entire samplingpathway shouldbe Siltek® treated to eliminate
contact between the reactive analytes and themetal surfaces. Siltek® treat-
ment can be applied to the interior surfaces of the canister and valve, to
ensurean inert samplepathway.
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