United States
Environmental Protection
Agency
Environmental Monitoring Systems
Laboratory
Research Triangle Park NC 2771
Research and Development
EPA/600/S4-85/034 June 19851
S€PA Project Summary
Evaluation of Passive
Sampling Devices (PSDs)
Robert W. Coutant
The basic objectives of this study
were to evaluate the performance of the
EPA passive sampling device (PSD) for
sampling of ambient level volatile or-
ganic compounds (VOCs); to develop
an understanding of the mechanics of
passive sampling using reversible ad-
sorption; and to apply this understand-
ing to development of an improved PSD
that is usable for sampling of VOCs over
periods of 8 to 24 hours. Laboratory and
limited field evaluations of the standard
and modified PSDs were conducted
and a model relating sorbent properties
and device design to sampling rates was
developed. The results show the stand-
ard PSDs to be useful for sampling of
VOCs having large retention volumes.
Modified PSDs having greatly reduced
sampling rates show promise for sam-
pling compounds having retention vol-
umes as low as 5 to 10 L/g over 8- to
24-hour sampling periods. The use of
Spherocarb as an alternative sorbent to
Tenax® GC also was investigated as a
means for improving the performance
of the PSD. This sorbent was found to
be unsuitable because of the high tem-
peratures required for desorption. It is
recommended that the model which
was developed be used for developing
sampling plans for specific applications,
and that more extensive field evaluation
of the reduced-rate PSDs be conducted.
This report was submitted in fulfill-
ment of Contract No. 68-02-3487 by
Battelle Columbus Laboratories under
the sponsorship of the U.S. Environ-
mental Protection Agency. It covers a
period from April 15.1982 to October
31.1984, and work was completed as
of October 31,1984.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Research Triangle
Park. NC. to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
In recent years, there has been an
increased awareness of the need for
monitoring individual or personal expo-
sures to pollutants and toxic chemicals.
This awareness has prompted the devel-
opment of a variety of personal sampling
devices including battery-driven pump
systems, passive systems having high
specificity for individual compounds, and
generalized passive systems intended for
the collection of volatile organic com-
pounds. Within this latter category, the
primary commercial emphasis has been
on the use of carbon-based sorbents for
monitoring of the relatively high concen-
trations of contaminants found in indus-
trial workplaces. In two earlier programs
for the Environmental Monitoring Sys-
tems Laboratory, U.S. Environmental
Protection Agency, Research Triangle
Park, North Carolina (RTP), Battelle's
Columbus Laboratories (BCL) explored
the problems and limitations of using
commercially available passive devices
for monitoring ambient level organic
chemicals. The performance of one of
these devices under simulated ambient
conditions also was subjected to a de-
tailed evaluation.
Results of these earlier studies have
shown that the commerciail^available
carbon-based devices are satisfactory for
ambient monitoring o> selected volatile
organic compounds/under some condi-
tions, but they are by no means complete-
ly general in their applicability under
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realistic ambient conditions. For example,
their performance is impaired under
conditions of high relative humidity. The
Environmental Monitoring Systems Lab-
oratory therefore has undertaken, under
separate contract with the Monsanto
Research Corporation (MRC), the devel-
opment of a passive sampler that is not
subject to the same restrictions as the
commercially available devices. The basic
concept invovled in the development of
this new device has been to employ
relatively hydrophobic porous polymer
sorbents in order to evolve a system that
is readily subject to thermal desorption
for analysis. While much of the initial
work with this device has been conducted
using Tenax® GC as the sorbent, the
fundamental applications concept is flex-
ible to permit the use of other porous
polymer sorbents, or even activated car-
bon, as may be required for specific
applications.
This report addresses the findings of
three Work Assignments (13,21, and 33)
conducted consecutively at BCL. The
primary objective was an independent
evaluation of the applicability of the MRC
passive sampling device (PSD) with re-
spect to monitoring of volatile organic
compounds (VOCs) under ambient condi-
tions. In the first Work Assignment,
sampling rates were determined for chlor-
oform, 1,1,1-trichloroethane, carbon tet-
rachloride, trichloroethylene, tetrachloro-
ethylene, benzene, and chlorobenzene. A
general model of passive sampling using
thermally reversible adsorption also was
developed. The second Work Assignment
then was conducted to extend the list of
chemicals to include acrylonitrile, 1,1-
dichloroethylene, trichlorotrifluoroeth-
ane, 1,2-dichloroethane, cr-chlorotoluene,
and hexachlorobutadiene, and to test the
general applicability of the performance
model. In the third Work Assignment, the
general precepts of the performance
model were applied for the purpose of
modifying the PSD to enable long-term
(8-24 h) sampling of VOCs. Results of the
first two work assignments and some of
the developmental work performed by
MRC are summarized in two papers
accepted for publication in the January
1985 issue of Analytical Chemistry.
Procedures
Exposure Studies
The U.S. EPA passive sampling devices
(PSDs) were exposed in triplicate to
controlled compositions of the test chem-
icals at ppbv concentration levels in the
Battelle dosimeter test facility. This fac-
ility consists of a 200 L chamber in which
air velocity, gas composition, relative
humidity, and temperature can be con-
trolled. Associated with this chamber is
appropriate monitoring equipment includ-
ing a capillary GC facility having a triple
detector (in series electron capture and
photoionization with parallel flame ioniza-
tion) arrangement to provide both high
sensitivity and selectivity for component
analysis. The devices were handled under
a protective atmosphere of zero-nitrogen
at all times except during the actual
exposures to the test atmospheres. Test
compositions in the chamber were mon-
itored both by periodic direct analysis
with the GC and by a pair of Tenax® GC
traps that were used to sample the
chamber gases in an active mode. The
PSDs and Tenax traps were analyzed by
thermal desorption into the GC. Expo-
sures were conducted at different relative
humidities, different concentrations (in
the low ppbv range), different air veloc-
ities, and for various lengths of time (up to
24 hr).
Results
Inasmuch as sampling rates for most of
the compounds studies had not previously
been determined for the EPA PSD, this
parameter was used as the basis for
comparison of the device performance. A
generalized model relating sampling rate
to the device geometry and chemical
retention volumes was developed. This
model takes into account the reversible
nature of the adsorption of volatile organ-
ics on Tenax® GC, and permits the
prediction of the time weighted average
sampling rate as a function of time.
Conversely, it has been demonstrated
that the model can be used to derive
Tenax retention volumes from measure-
ments of the apparent sampling rates
with the MRC device. More importantly,
the model provides guidelines for design
and application of passive dosimeters
employing reversible adsorption.
Apparent sampling rates measured for
the 18 test chemicals agreed well with
those predicted by the model (generally
within about 6 percent), except in the
cases of carbon tetrachloride and tetra-
chloroethylene. With the latter chemicals,
the apparent rates were only about one-
half those expected. Battelle researchers
previously have noted difficulties with
passive sampling for carbon tetrachloride
and other authors have noted similar
problems with active sampling for this
chemical. No appreciable effect of relative
humidity was found at humidities as high
as 90 percent. The precision of measured
sampling rates was generally of the order
of 20-25 percent, with approximately
one-half of this variation being traceable
to differences between individual devices.
The remainder of this variation can be
associated with analytical error.
Two approaches to modification of the
PSDs were pursued for the purpose of
improving their performance for long-
term (8 to 24 hour) sampling. These
consisted of (1) replacement of theTenax®
GC with Spherocarb in order to increase
the retention times of VOCs and (2)
alteration of the diffusion barrier to yield
sampling rates reduced by a factor of 30
in comparison with the standard PSDs.
The use of Spherocarb proved unsatis-
factory because of the high temperatures
required for desorption of most of the
target compounds. Results obtained in
the laboratory tests of the reduced-rate
PSDs showed promise for the use of
these devices in sampling of chemicals
having retention volumes as low as 5 to
lOL/g.
Limited field studies of the perform-
ances of both the standard and reduced-
rate PSDs yielded overall precisions com-
parable to those determined in the lab-
oratory tests. These field studies illus-
trated problems associated with the in-
appropriate use of the standard PSDs for
sampling of very volatile chemicals, and
also indicated potential problems with
using the reduced-rate devices for very
short time periods. In the latter case,
uncertainties in blank levels impact se-
verely on the relatively small amounts of
sample collected by the reduced-rate
devices when they are used for times that
are too short. In any case, these results
emphasize the need to use the model as
an aid to design of a sampling plan for a
particular application.
Conclusions and
Recommendations
Based on the results of this work, it
concluded that the EPA PSDs offer some
distinct advantages over other available
designs of passive sampling devices in
application to sampling of ambient level
volatile organic compounds. The ability to
thermally desorb collected samples pro-
vides more than adequate amounts of
sample for use with conventional GC and
GC/MS analytical procedures, and their
independence from high relative humid-
ities yields more flexible field applicability
than the commercially available devices
using activated carbon. Finally, the
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Tenax® GC can easily be replaced with
other sorbents for customized sampling
applications. The PSDs should not, how-
ever, be utilized without strict attention to
the mechanics of reversible adsorption
and their implications with the respect to
particular sampling requirements. Specif-
ically:
1. The standard EPA PSDs can be
used for sampling of ambient levels
of VOCs, but careful attention must
be paid to the retention volumes of
target compounds and the appro-
priate sampling period. In general,
the standard PSDs are useful for
chemicals having large retention
volumes (>100 L/g), but can be
used only for short sampling periods
a few hours or less) for compounds
having small retention volumes.
2. Reduced rate PSDs having nominal
sampling rates of the order of 2.5
cc/min show promise for applica-
tions requiring the sampling of
VOCs over extended periods of 8 to
24 hours. These reduced rate de-
vices, however, should not be used
for compounds having retention
volumes less than about 5-10 L/g.
Also, the current results show
potential blank problems with the
use of the reduced rate devices over
short sampling periods.
3. In any case, the time-weighted
average sampling rate for a partic-
ular sampling requirement should
be estimated using the model pre-
sented in this report and should be
used as a guide in designing the
sampling plan for a particular appli-
cation. In general, rates significant-
ly less than about 70-80 percent of
the initial rate (Ro) may indicate
potential sampling error.
4. The model of PSD performance
presented in this report has been
shown to represent correctly the
effects of retention volume, sam-
pling time, and air velocity on the
effective time-weighted average
sampling rates of the EPA PSDs.
5. The use of Spherocarb in the EPA
PSDs may offer some advantages
for sampling of a few selected
VOCs, but general use of this
sorbent is not recommended be-
cause of problems generally assoc-
iated with the high temperatures
needed for desorption of this sor-
bent. Contamination of the sorbent
by unpyrolyzed polymer presents
some special difficulties with clean-
up and preparation of this sorbent
for use in the PSDs.
6. The EPA PSDs are not affected by
high humidities, and therefore are
not subject to the same limitations
as the commercially available de-
vices based on activated carbon.
7. Protective shields developed for the
EPA PSDs on this program provided
protection against contamination
during handling of the devices in
the field without significantly af-
fecting their sampling rates.
8. Blank contamination of the EPA
PSDs has generally not been a
problem, but a few instances of
such have been observed. In recog-
nition of the fact that the PSDs may
not always be handled by trained
laboratory personnel in the field, it
is recommended that a formalized
procedure and containment system
be developed for cleanup and pro-
tection of the PSDs.
9. It is recommended that further field
testing of the reduced rate PSDs be
considered.
10. Investigation of the application of
the EPA PSD to sampling of pol-
lutants other than VOCs (e.g., N02,
volatile polar organics, etc.) is rec-
ommended to take advantage of the
capacity for use of different sor-
bents with this device.
R. W. Coutant is with Battelle's Columbus Laboratories, Columbus, OH 43201.
James D. Mulik is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Passive Sampling Devices (PSD's),"
(Order No. PB 85-196 418/AS; Cost: $10.00, 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 Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
i, U.S. GOVERNMENT PRINTING OFFICE: 1985-559416/27082
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Environmental Protection Information
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