United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-85-002 Jan. 1985
Project Summary
Evaluation of Cryogenic
Trapping as a Means for
Collecting Organic Compounds
in Ambient Air
M. Holdren, S. Rust, R. Smith, and J. Koetz
The methodology used in reduced
temperature preconcentration of vola-
tile organic compounds has been tested
using a prototype automated gas chro-
matographic system. Mixtures of vola-
tile organic compounds in humidified
zero air were passed through a Nafion
tube dryer and the organic compounds
were collected on a reduced-tempera-
ture trap. The dryer reduced the water
concentration without significantly
affecting the integrity of the trace
organic species. The selective reduction
of water vapor improves the chroma-
tography of the trace organics and
likewise permits processing larger
sample volumes.
Collection and recovery efficiencies
of the volatile organic compounds at
low ppbv levels (.3 to 3) were 100 ±5
percent with this preconcentration
technique. The integrity of sample
components was unaffected by
co-collection of ozone and nitrogen
dioxide at typical ambient concentra-
tions. Two nominally identical automa-
ted gas chromatographic instruments
were used for simultaneous monitoring
of calibration mixtures and laboratory
air. For calibration mixtures, percent
relative errors were less than 10 percent
over the concentration range of 0 to 50
ppbv for fourteen of the sixteen com-
pounds; benzyl chloride and hexachloro-
butadiene gave errors of 20 percent.
For laboratory air analysis, deviation
from the mean concentration for
twenty-four comparisons ranged from
0.0 to 20.5 percent. Sixteen target
compounds were stored at low ppb
concentrations (2-5 ppbv) in passivated
stainless steel containers and examined
over a seven-day period. Statistical
treatment of the data indicated that
after four days of storage, measured
concentrations for all sixteen com-
pounds were within ±8 percent of the
initial values. After seven days the
average benzyl chloride concentration
decreased by ~19 percent while hexa-
chlorobutadiene increased by ~11
percent.
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
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The atmosphere contains a complex
mixture of organic compounds. Many of
these emitted chemjcals have been found
to be highly toxic substances. Data on the
identity and concentration levels of these
compounds in urban and rural environ-
ments are continually being gathered by
researchers who are attempting to better
understand the chemistry and fate of
these chemicals as well as the extent of
human exposure to these compounds.
From an analytical standpoint, -this data
gathering task has been formidable.
However, these efforts have been
dramatically improved in recent years
through the use of capillary column gas
chromatography coupled to compound-
specific detection systems. Furthermore,
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the interfacing of the gas chromatograph
to a mass spectrometer detector has
permitted positive identification of many
of these atmospheric constituents.
Most of these toxic chemicals are
present at ppb levels or less and
preconcentration is often necessary for
accurate chemical analysis. Presentlythe
two primary ambient air preconcentra-
tion techniques employed in delineating
atmospheric organic burdens are
cryogenic trapping and the use of solid
adsorbents.
Cryogenic trapping can involve either
of two collection procedures. With the
first procedure, whole air samples are
entirely condensed within suitable col-
lection devices by cryogens such as liquid
-_niix>gen or liquid helium. Both cryogens
are sufficiently cool to serve as the
cryogenic pump in this collection process.
This approach is normally used in studies
where remote sampling is being con-
ducted. Once the sample is collected, the
coolant is removed and the container
returned to the laboratory for subsequent
analysis.
The other cryogenic trapping procedure
involves passing air through a reduced-
temperature trap. At the appropriate
temperature, trace organic species will
condense onto the trap surfaces while
oxygen and nitrogen pass through the
system. Reduced-temperature trapping
has several limitations which must be
considered when designing a sampling
and analytical system. A limiting factor of
major importance is the co-collection of
water in the sampling trap. One liter of air
at 50 percent relative humidity and 25 C
will contain approximately 10 mg of
water, which appears as ice in the
collection trap. The possibility of the ice
plugging the trap and stopping sample
flow is of concern, and water transferred
to the gas chromatographic capillary
column may also cause plugging and
deleterious column effects. Furthermore,
during sample preconcentration,
chemical reactions may also occur in the
collection trap. Possible reactants could
include ammonia/acids, ozone/olefins,
etc.
The Advanced Analysis Techniques
Branch of the Environmental Monitoring
Systems Laboratory (EMSL) is
responsible for the development and
evaluation of state-of-the-art and emer-
ging analytical techniques for the
determination of organic compounds in
ambient air. Recently a priority listing of
volatile organics has been established
and the EMSL is focusing on further
development of analytical methodology
associated with the detection of these
compounds. Primary emphasis has been
placed on developing field-compatible
analytical systems.
Recently a prototype automated
analytical system incorporating cryogenic
trapping for sample preconcentration has
been developed jointly by EPA and the
Battelle Columbus Laboratory. System
hardware utilizes capillary column gas
chromatographic separation techniques
along with flame ionization and electron
capture detection. Software development
using the basic programming capability of
the GC system permits calibration and
ambient sampling to be achieved with
minimal operator interfacing. In this
report, we shall describe instrumenta-
tion hardware and software and shall
discuss laboratory experiments designed
to test the suitability of the prototype
system for preconcentration of volatile
organic compounds in ambient air. Table
1 lists the volatile organic compounds
that were examined. The following
laboratory studies were carried out
during this program.
(1) A Perma-Pure dryer was tested to
determine if water vapor could be
selectively removed from the gas
stream also containing the 16
target compounds without
affecting the integrity of the organ-
ics in the gas phase.
(2) Collection and recovery efficiencies
of the organic compounds were
determined.
(3) Studies were also conducted with
the target compounds to examine
potential interference or artifact
effects from co-collected ozone and
nitrogen dioxide gases.
(4) A prototype calibration device was
interfaced to the overall system and
tested. Software was developed to
permit automatic operation and
calibration of the GC system.
(5) Side-by-side comparison of two
prototype automated sampling and
analysis units was carried out at the
EPA facility. Calibration mixtures
and ambient air samples were
analyzed.
(6) In a joint effort with EPA, a compar-
ison was made between analytical
results for samples collected and
temporarily stored in small metal
cylinders, with data collected
during real-time sampling.
Conclusions
During the laboratory experiments, two
Nafion tube dryers were evaluated and
found to reduce water vapor selectively in
the gas phase without affectfng the integ-
rity of trace organic species also present.
The selective reduction of water vapor
improves the chromatographic resolution
of the trace organics and likewise per-
mits larger sample volumes to be
processed. Experiments showed that
collection and recovery efficiencies of the
volatile organic compounds at low ppbv
levels were 100 ± 5 percent with th is pre-
concentration technique. Ozone and
nitrogen dioxide interference studies
indicated that none of the target
compounds was affected by the
additional presence of these reactive
species. Furthermore, no artifact peaks or
deleterious column effects were ob-
served during or after these tests.
In a cooperative effort with EPA, two
nominally identical automated GC
systems were intercompared with
calibration mixtures and ambient air
samples drawn from a common manifold.
Statistical analysis of the calibration data
involved regressing concentration on raw
area and determining the least squares
regression line. A percent relative error of
less than 10 was obtained for thirteen of
the fifteen tested compounds when
comparing the estimated concentration
to the actual concentration. The remain-
ing two compounds, benzyl chloride and
hexachlorobutadiene, exhibited relative
Table 1.
Volatile Organic Compounds Examined During Laboratory Studies
Propane
Vinyl Chloride
Vinyl/dene Chloride
Trichlorotrifluoroethane
Chloroform
1,2-Dichloroethane
Methyl Chloroform
Benzene
Carbon Tetrachloride
Trichloroethylene
1,3-Dichloropropene fcis and trans)
Toluene
1,2-Dibromoethane
Tetrachloroethylene
Chlorobenzene
o-Xylene
Benzyl Chloride
Hexachlorobutadiene
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error values of —20 percent at the lowest
non-zero concentration level (4 ppbv).
Simultaneous sampling of laboratory air
from a common manifold resulted in
reasonable agreement between the two
instruments. Nine target compounds
were identified and quantified. The
results indicated an overall precision of
±10 percent.
A test of the storage characteristics of
passivated stainless steel canisters
(seven) for a set of sixteen volatile organ-
ic compounds (2 to 5 ppbv) showed less
than ±8 percent average change for all of
the compounds over a four-day period.
After seven days of storage, the average
benzyl chloride concentration decreased
by —19 percent while hexachlorobuta-
diene increased by —11 percent.
During the program, software was
developed to more easily facilitate the
monitoring and calibration needs of the
automated GC sampling and analysis
system. In addition to controlling zone
heating and cooling, sample collection
and injection, chromatographic run con-
ditions and data processing, the devel-
oped software provided single point or
multipoint calibration subroutines to be
nested into the overall sampling and
analysis strategy.
Recommendations
A gas chromatographic system
employing reduced temperature precon-
centration for the collection of VOC's has
been laboratory tested with respect to
sample drying procedures, co-collection
of reactive ambient air species, and
collection and release efficiency. An
automated GC system has been used to
more easily facilitate the laboratory tests.
The following recommendations are
suggested:
i Many of the target compounds tested
in this study will co-elute with other
ambient air species. Although the
combination of capillary column and
flame ionization and electron
capture detectors used in the current
program help alleviate some of these
concerns, other more selective
detectors are needed. Integrating a
mass-selective detector into the
automated gas chromatographic
system is recommended. The mass-
selective detector offers both total
ion and selected ion monitoring
capability and will thus provide both
qualitative and quantitative informa-
tion. The increased specificity of this
detector over other detection
systems will allow better
differentiation of co-eluting GC
peaks and thus improve present
quantitative capability.
The automated GC system should be
field tested. The two automated
systems should be evaluated side-
by-side during the first phase of the
field program. Subsequently, the
sample collection cycle would be
offset so as to permit complete time
coverage with both instruments. The
cryogenic sampling system should
also be compared with other
preconcentration techniques such
as solid adsorbents (Tenax, Carbo-
sieve) and passive dosimeters. These
tests would provide much needed
information regarding the
advantages and disadvantages of
each technique.
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M. Holdren. S. Rust/R. Smith, and J. Koetz are with Battelle Columbus
Laboratories, Cdfumbus, OH 43201.
W. M. McClenrty is.'the EPA Project Officer (see below).
The completgjrefbrt, entitled "Evaluation of Cryogenic Trapping as a Means for
ColtectingSfganic Compounds in Ambient Air," (Order No. PB 85-144 046;
Cost: $ IQJjv. subject to change) will be available only from:
tidnal Technical Information Service
36 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
fifte £ PA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
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