v°/EPA
United States
Environmental Protection
Agency
Environmental Monitoring Systems?
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-82-059 Dec. 1982
Project Summary
Sampling Air for Gaseous
Organic Chemicals Using
Solid Adsorbents.
Application to Tenax
J. F. Walling, R. E. Berkley, D. H. Swanson, and F. J. Toth
A simple input-output model for a
two-phase flow system with fixed bed
was applied to Tenax adsorption of
atmospheric contaminants. The model
fit laboratory data acceptably and
relationships with previous approaches
were stated. Chromatographic reten-
tion volumes are used in this model
which provides a means of scaling
adsorbent bed dimensions and flow
rates to preserve desired output
characteristics.
Suggestions are made about how to
use the model for rational sampling
design when quantitative information
is needed. Practical limitations of field
operations and fundamental know-
ledge and the heed to evaluate every
datum lead to practical suggestions
for sampling. At least two samples
collected simultaneously but at very
different flow rates are suggested as
necessary for quantitative work.
This Project Summary was devel-
oped by EPA's Environmental Monitor-
ing Systems Laboratory Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Solid adsorbents are very convenient
to use in field sampling and therefore
have been employed extensively to
concentrate trace organic pollutants in
sampling ambient air. It was usual to
assume that standard Chromatographic
theory could be applied. Since the main
objectives of chromatography are not
the same as those of field sampling with
solid adsorbents, that theory was not
developed with the goal of solving the
practical problems of establishing field
sampling procedures. Variables that are
critical in the field situation are not
explicitly handled in a convenient way
and a huge fundamental data base is
needed for application.
The objectives of this work were:
1. To obtain a simple alternative
model which incorporates a minimum
number of adjustable parameters and
establishes a relationship among im-
portant sampling variables:
(a) weight of adsorbent
(b) adsorbent bed (sampling tube)
geometry
(c) flow rate
(d) input concentration to the
adsorbent bed
(e) adsorbent bed output concen-
tration
(f) elapsed sampling time
2. To test the accuracy of the function
obtained in describing a few simplified
controlled laboratory experiments; and
3. To design a simple practical
scheme for sampling and data interpre-
tation which also can provide evidence of
consistency in the results.
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Procedure
The model investigated can be written
as:
1n
K L
C _o
C0 VL
C is the bed's exit concentration at
elapsed time t, g/cm3
C0 is the bed inlet concentration,
g/cm3
Ko is a pseudo first-order rate con-
stant, min~1
L is the adsorbent bed length, cm
VL is a superficial flow velocity,
cm/min
t is the elapsed time after beginning of
sampling, min.
ts is the elapsed time required to reach
the adsorbing capacity of the bed. Un-
der restricted conditions it is equal to
ratio of the familiar chromatographic
retention volume to the volumetric
flow rate.
The model is an approximate relation
applicable to any adsorbent. However,
its validity is limited to a range of
elapsed times from perhaps O.K t_ <
0.8. ts
The utility of the model was.tested by
simulated sampling using a permeation
tube system to supply known concentra-
tion of cleaned pollutant doped air at
various flow rates. Flame ionization
detection of the output of the Tenax
^cartridge was utilized to obtain a
relationship between elapsed time and
concentration of the pollutant in the
effluent from the cartridge. The effects
of changes in concentration tempera-
ture, flow rate, and bed size were
investigated. Chloroform, trichloroethy-
lene, benzene, tetrachloroethylene,
toluene, 1,2-dichloroethane and 1,3-
dichloropropane were used in the
experiments.
Computation of the retention volume
and Ko from the constants of the
empirical fits was done by simple
rearrangement of the basic equation to
match the empirical form and identifying
the appropriate collection of variables
with the empirical constants.
were observed on the same chemical
systems at 10°C. K0 was less well
behaved and characterized but exhibited
a typical value of about 200 min"1.
Conclusions
The model is capable of giving a
reasonable and compact description of
the sampling behavior of the Tenax
cartridges over a range of variables
likely to be met in the field:
35 cmVmm < flow rate < 500
cmVmin
1.25 cm < bed radius < 0.6 cm
when
2<
Bed Length
Bed Radius
and
30 cm/min < Flow Rate _ 400 cm/min
n (Bed Radius)2
Recommendations
A suggested simplified approach for
routine sampling is as follows:
1. Use a fixed cartridge size. (At
present GC/MS sample desorbers limit
severely the options with respect to
cartridge size.)
2. Allow the single substance, whose
determination is required and which
has the smallest published retention
volume at 38°C (100°F), control the
sampling calculations. If necessary,
divide the list into ranges of retention
volumes and sample independently for
each range.
3. The desired sample averaging time
is usually specified by the situation.
4. Compute a flow rate conservative-
ly using the model and an acceptable
value of C/C0. Or, more simply, use the
published retention volume divided by
the desired sampling time. If ambient
temperatures are expected to be cool,
use the value computed. If they are near
29°C (85°F) or higher, use one-half the
computed value.
5. Present evidence in every situation
of the data credibility. Complete every
sampling in at least duplicate (parallel)
at different flow rates, e.g., the flow rate
computed in 4 above and another at
perhaps half of that value. "Agreement"
of the analyses on duplicate samples
would be an indication of adequate
retention.
The EPA authors J. F. Walling, R, E. Berkley. D. H. Swanson. and F. J. Toth
are with the Environmental Monitoring Systems Laboratory, Research Triangle
Park, NC 27711.
The complete report, entitled "Sampling Air for Gaseous Organic Chemicals
Using Solid Adsorbents. Application to Tenax," (Order No. PB 82-262 189;
Cost: $8.50, subject to change} will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA authors can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC27711
Results
Agreement of computed retention
volumes with published values was
generally within ±25 percent. No
concentration dependences or changes
due tothepresence of another substance
were observed at 38°C. Both effects
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•& U.S. GOVERNMENT PRINTING OFFICE: 1982—• 659-O17/O875
United States
Environmental Protection
Agency
Center for Environmental Research
Information
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Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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