6
steel wetted surfaces, and a packless design (a completely enclosed system, to
ensureno contamination from lubricants or packingmaterial).Various valves
are used on variousmodels of canisters; themost commonly used valves are
theNupro4H4 seriesmetal bellows valveand theParkerHannafindiaphragm
valve with metal seat. At Restek we incorporate the Parker Hannafin
diaphragm valve on canisters because of its ability to hold vacuum, its dura-
bility and longevity, and amaximum temperature limit (250°C) that ismuch
higher than that for a bellows valve. Further, a Parker Hannafin diaphragm
valve canbe rebuilt if it is damaged; it does not have tobe replaced.
The connectionof the valve to the canister is critical.The connectionmust be
leak tight, to ensure a correct sampling flow rate, but extreme cautionmust be
taken toprevent overtightening the tube compression fittings.
Siltek®andSulfinert®Treatment
Siltek® treatment is a proprietary process, developed by Restek Corporation,
throughwhichan inert layer is chemicallybonded toametal surface.The sur-
face produced by this treatment is virtually inert to active compounds. The
stainless steel pathwaydescribed in this guide is sufficient for samplingatmos-
pheres containing onlynon-reactive compounds, but for reactive compounds
the entire sampling pathway should be Siltek® treated to eliminate contact
between the reactive analytes and themetal surfaces. Siltek® treatment canbe
applied to the interior surfaces of the canister and valve, to ensure an inert
sample pathway. If the samples will contain reduced sulfur-containing ana-
lytes, an alternative proprietary Restek deactivation process, Sulfinert® treat-
ment, is themost effectivemeansof deactivating the samplepathwayand can-
ister.
III.Preparing theSamplingTrain forUse
The sampling trainmust beprepared in the laboratorybefore it canbeused in
the field. The trainmust be assembled and leak tested, the flow ratemust be
set, and the train must be certified clean. All of the following information
shouldbe documented for the chainof custody for the passive sampling train
and the sample collectedwith it.
Assemble, LeakTest, andSet theFlowRateof thePassiveSamplingTrain
Choose the critical orifice (Table 1, page 3) according to the sampling period
and flow rate you anticipate using (Table 2). Thiswill ensure an accurate and
valid sample. There should be amarking on the outside of the critical orifice
fitting indicating the size of the orifice. In a clean environment, assemble the
sampling train components as shown inFigure1 (page3). It is imperative that
you leak test the assembled train. If the sampling train leaks during sampling,
the final partial pressure in the canister will not be the desired final partial
pressure, making the sample invalid. The most common reason for invalid
samples is leakswithin the sampling train. There are twoways to leak test the
train:
1. Pass helium gas through the flow controller anduse a sensitivehelium
leakdetector to test for leaks (e.g.,RestekLeakDetector).
2. Cap the inlet, attach the sampling train to an evacuated canister, open the
valve on the canister and evacuate the sampling train.
Close the valve andmonitor anypressure change in the static sampling
train. Leaks of less than1mL/min. canbe detected in1-2minutes.
This is agoodpractical test - the small internal volumeof thepassive sampling
train, combinedwith even a small leak, will produce a large change inmoni-
toredpressure.
After you are certain the sampling train is leak-free, set the desired sampling
flow rate.
FundamentalsofAirSampling
This book explains the fundamentals of air sam-
pling,develops the theoryof gasmeasurement,and
presents several how-to examples of calibration
anduseof airandgas samplingdevices. Other top-
ics include thebasicsof pressuremeasurement and
units conversion, and specific discussions regard-
ing theuseof aVolatileOrganic SamplingTrainor
a SUMMA®-polished canister sampling system.
G. D.Wight, CRCPress LLC, 1994, 272pp.,
cat.#20492
