6
steel wetted surfaces, and a packless design (a completely enclosed system, to
ensure no contamination from lubricants or packing material). Various valves
are used on various models of canisters; the most commonly used valves are
the Nupro 4H4 series metal bellows valve and the Parker Hannafin diaphragm
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 a maximum temperature limit (250°C) that is much
higher than that for a bellows valve. Further, a Parker Hannafin diaphragm
valve can be rebuilt if it is damaged; it does not have to be replaced.
The connection of the valve to the canister is critical. The connection must be
leak tight, to ensure a correct sampling flow rate, but extreme caution must be
taken to prevent overtightening the tube compression fittings.
Siltek® and Sulfinert® Treatment
Siltek® treatment is a proprietary process, developed by Restek Corporation,
through which an inert layer is chemically bonded to a metal surface. The sur-
face produced by this treatment is virtually inert to active compounds. The
stainless steel pathway described in this guide is sufficient for sampling atmos-
pheres containing only non-reactive compounds, but for reactive compounds
the entire sampling pathway should be Siltek® treated to eliminate contact
between the reactive analytes and the metal surfaces. Siltek® treatment can be
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 the most effective means of deactivating the sample pathway and can-
ister.
III. Preparing the Sampling Train for Use
The sampling train must be prepared in the laboratory before it can be used in
the field. The train must be assembled and leak tested, the flow rate must be
set, and the train must be certified clean. All of the following information
should be documented for the chain of custody for the passive sampling train
and the sample collected with it.
Assemble, Leak Test, and Set the Flow Rate of the Passive Sampling Train
Choose the critical orifice (Table 1, page 3) according to the sampling period
and flow rate you anticipate using (Table 2). This will ensure an accurate and
valid sample. There should be a marking 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 in Figure 1 (page 3). 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 leaks within the sampling train. There are two ways to leak test the
train:
1. Pass helium gas through the flow controller and use a sensitive helium
leak detector to test for leaks (e.g., Restek Leak Detector).
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 and monitor any pressure change in the static sampling
train. Leaks of less than 1 mL/min. can be detected in 1-2 minutes.
This is a good practical test - the small internal volume of the passive sampling
train, combined with even a small leak, will produce a large change in moni-
tored pressure.
After you are certain the sampling train is leak-free, set the desired sampling
flow rate.
Fundamentals of Air Sampling
This book explains the fundamentals of air sam-
pling, develops the theory of gas measurement, and
presents several how-to examples of calibration
and use of air and gas sampling devices. Other top-
ics include the basics of pressure measurement and
units conversion, and specific discussions regard-
ing the use of a Volatile Organic Sampling Train or
a SUMMA®-polished canister sampling system.
G. D. Wight, CRC Press LLC, 1994, 272pp.,
cat.# 20492
