United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-85/077 Jan. 1986
v°/EPA Project Summary
Comparison of Solid
Adsorbent Sampling
Techniques for Volatile
Organic Compounds in
Ambient Air
R. M. Riggin and R. A. Markle
The objective of this study was to
compare the performance of three solid
adsorbents (Tenax®, an experimental
polyimide resin, and Spherocarb®) as
well as whole air collection in canisters
followed by cryogenic trapping/gas
chromatography for sampling and anal-
ysis of a target list of volatile organic
compounds in ambient air.
A series of 14 experimental sampling
runs, wherein parallel samples were
collected using each of the techniques,
were conducted over a one-month pe-
riod. Several of the runs used audit or
other reference standards as a check on
method performance for known ana-
lyte concentrations.
Compared to the three adsorbent
methods, whole air collection in canis-
ters followed by cryogenic trapping/gas
chromatography offered better preci-
sion and accuracy for the compounds of
interest, especially when a mass selec-
tive detection system was employed.
None of the three adsorbents gave opti-
mal performance for the entire list of
compounds, although in general
Tenax® gave the best results. Sphero-
carb® was the best adsorbent for
chloroethene (vinyl chloride),
dichloromethane, and 1,1,2-trichloro-
1,2,2-trif luoroethane.
The polyimide material suffered from
a number of operational problems
which weigh heavily against its use in
ambient air sampling.
This Project Summary was devel-
oped by EPA's Environmental Monitor-
ing Systems Laboratory, Research Tri-
angle 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 Methods Development and Anal-
ysis Division of the Environmental Mon-
itoring Systems Laboratory (EMSL) of
the U.S. Environmental Protection
Agency (EPA) develops and evaluates
state-of-the-art and emerging analytical
techniques for determining organic
compounds in ambient air. Recently a
priority listing of volatile organics has
been established, and EMSL is focusing
on further development of analytical
methodology for the determination of
these compounds.
In general one of two approaches,
cryogenic trapping or gas/solid adsorp-
tion, is used to preconcentrate volatile
organics in ambient air. Each approach
has advantages and limitations. Cryo-
genic trapping has been demonstrated
to provide excellent recovery and preci-
sion for a number of volatile organic
compounds but is somewhat inconve-
nient when used in the field because of
the complexity and size of the appara-
tus. Solid adsorbents can be conve-
niently transported to the field for sam-
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pling and returned to the laboratory for
analysis. Unfortunately none of the ex-
isting solid adsorbents show recoveries
comparable to cryogenic trapping tech-
niques for a wide range of components.
By far the most widely employed ad-
sorbent for volatile organic compounds
is Tenax® GC. Tenax® has the advan-
tage of good thermal stability which al-
lows for efficient desorption of higher
boiling compounds (e.g., C-12 hydro-
carbons) during the analysis step. A pri-
mary limitation of Tenax® is the low
retention volume of highly volatile
compounds (e.g., vinyl chloride, 1,2-
dichloroethane, etc.). In order to extend
the applicability of solid adsorbent col-
lection to more volatile compounds,
EMSL has conducted development and
evaluation studies for various adsor-
bents which could be used in place of or
in combination with Tenax®. The two
most promising materials for this pro-
gram are a polyimide material formed
from pyromellitic anhydride and 4,4'-
diaminodiphenylsulfone, and carbon
molecular sieves (CMS) sold under the
tradenames Spherocarb®, and Car-
bosieve®, or Carbosphere®.
The objective of the work described in
the full report was to compare the per-
formance of the three adsorbents
(Tenax®, polyimide, and Spherocarb®)
as well as whole air collection in canis-
ters followed by cryogenic trapping for
sampling and analysis of representative
volatile organic compounds at realistic
concentrations in ambient air. The
target compounds of concern are listed
in Table 1.
Procedure
Three adsorbents—Tenax®, poly-
imide, and Spherocarb®—as well as
cryogenic trapping/gas chromatogra-
phy were operated in parallel to sample
ambient air, using the sampling mani-
fold shown in Figure 1. The air stream
was spiked with 1-10 ng/liter levels of
each target compound in order to en-
sure the presence of a detectable con-
centration.
Cryogenic sampling was accom-
plished by collecting integrated sam-
ples, over the entire two-hour sampling
period, using specially treated stainless
steel canisters. The canister samples
were then analyzed by cryogenic trap-
ping/gas chromatography using flame
ionization (FID), electron capture, and
mass selective detectors.
Duplicate ten-liter samples were col-
lected using each of the three adsor-
bents. In addition five and twenty-liter
samples were collected for analysis by
EPA.
All adsorbent samples were analyzed
by gas chromatography/mass spec-
trometry. The analytes were thermally
desorbed from the cartridges onto a liq-
uid nitrogen-cooled trap and subse-
quently transferred onto a wide-bore
SE-30 capillary column. The individual
compounds were eluted using a tem-
perature program of -70°C to 150°C at
8°/minute. Components were quantified
by comparing the integrated ion inten-
sity for a characteristic ion of each com-
pound to that of standard injected on
the same day.
Results and Discussion
A series of experimental runs, listed
in Table 2, were conducted during July
and August of 1984. As indicated in
Table 2, Runs 4 and 11 were clean air
experiments using an audit cylinder
supplied by EPA and Run 5 was a simi-
lar experiment using the Battelle cali-
bration cylinder.
In order to compare the performance
of the various methods for ambient air,
the apparent recovery of the adsorbent
samples for each run, relative to the
cryogenic trapping value, was calcu-
lated. Use of the cryogenic trapping
value was considered to be most appro-
priate, since this technique generally
agreed best with the expected concen-
tration, and gave better precision than
did any of the adsorbent techniques.
Most of the values used for the cryo-
genic trapping were obtained using the
mass selective detector, because this
detection system was less subject to po-
Table 1. List of Target Compounds
Compound
tential interferences. However, the
toluene and 1,2-dimethylbenzene val-
ues were obtained using FID due to lim-
itations on the number of ions which
could be monitored using the mass se-
lective detector.
Comparative data for the adsorbents
is summarized in Table 3. For poorly re-
tained compounds the low volume
Tenax® sample (nominally 5 liters) was
used, whereas for better retained com-
pounds the duplicate 10 liter Tenax®
samples were used for these calcula-
tions.
As expected, none of the adsorbents
performed best for all of the target com-
pounds. Tenax® performed best for
3-chloropropene (allyl chloride),
trichloromethane (chloroform), 1,2-
dichloroethane, 1,1,1-trichloroethane,
benzene, tetrachloromethane (carbon
tetrachloride), trichloroethene, tetra-
chloroethene (perchloroethylene), and
1,2-dimethylbenzene (o-xylene).
Tenax® and polyimide gave essentially
identical results for toluene. As ex-
pected, on the basis of breakthrough
volume data, Tenax® gave essentially
no recovery for chloroethene (vinyl
chloride) and 1,1-dichloroethene (vinyli-
dene chloride).
Polyimide performed better than both
Spherocarb® and Tenax® only for 1,1-
dichloroethene, although only 55 per-
cent recovery and 28 percent standard
deviation were obtained. Polyimide
gave essentially no recovery for chloro-
ethene, despite the good recovery ob-
tained for this compound in high purity
air sampling. Polyimide also gave dis-
appointingly low and variable recovery
for 1,1,2-trichloro-1,2,2-trif luoroethane
Concentration
in Calibration
Cylinder, \>.g/La
Chloroethene (Vinyl chloride)
Acrylonitrile
1,1-Dichloroethene (Vinylidene chloride)
Dichloromethane (Methylene chloride)
3-Chloropropene (Allyl chloride)
Trichlorotrifluoroethane (Freon 113)
Trichloromethane (Chloroform)
1,2-Dichloroethane
1,1,1-Trichloroethane (Methyl chloroform)
Benzene
Tetrachloromethane (Carbon tetrachloride)
Trichloroethene
Toluene
Tetrachloroethene (Perchloroethylene)
Chlorobenzene
1,2-Dimethylbenzene (o-xylene)
2.58
1.30
1.70
1.33
1.94
3.41
2.67
2.42
2.97
1.47
3.62
2.69
1.77
3.39
2.20
2.04
aAt 25°C and 1 atmosphere.
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Exhaust
Solid
Adsorbent
Sampling
System
Calibration Cyl.
Audit Cyl.
Figure 1. Sampling manifold.
(Freon 113), 1,1,1-trichloroethane, and
tetrachloromethane. An operational
problem that weighs heavily against the
use of this material for ambient air sam-
pling is the adsorption of a significant
amount of moisture onto the adsorbent.
The presence of water in the matrix led
to the chromatographic column plug-
ging during virtually all of the ambient
air sampling runs (plugging of the
column was not observed for any dry air
runs using the audit or calibration cylin-
ders). A similar phenomenon is ob-
served for Spherocarb®. However, in
that case the cartridge is prepurged with
dry air, at room temperature, prior to
analysis to remove adsorbed moisture.
A similar approach was not successful
in eliminating the problem for the poly-
imide material, indicating that the ad-
sorbed moisture is difficult to desorb
due to kirietic or thermodynamic fac-
tors.
Spherocarb® gave the best results of
the three resins only for chloroethene,
dichloromethane, and 1,1,2-trichloro-
1,2,2-trifluoroethane and extremely
poor results for 1,1-dichloroethene,
3-chloropropene, 1,2-dimethylbenzene,
and tetrachloromethane. In the case of
1,1-dichloroethene artifactually high re-
covery, possibly due to dehydrohalo-
genation of 1,1,1-trichloroethane, was a
major problem. Although low recover-
ies were anticipated for the other three
compounds, on the basis of earlier
work, the low recovery of 1,1-
dichloroethene was not observed previ-
ously.
Despite the problems discussed
above, the data set provides important
information for ambient air sampling.
Inspection of the raw data reveals that,
except for the major problem areas dis-
cussed above, the individual values
generally agree with the values ob-
tained by cryogenic trapping within a
factor of two or better (i.e., 50 to 150%
relative recovery). In view of the large
temporal and spatial variability of or-
ganic pollutant concentrations ob-
served in ambient air, this extent of
agreement between methods is quite
good. Furthermore, the health effects
information and mathematical models
used to assess the significance of the
data are subject to much greater uncer-
tainties.
Another encouraging aspect of the
data set is the very low blank levels ob-
served. Our previous work with the
open-style Tenax® tubes commonly
employed in other laboratories resulted
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Table 2. List of Sampling Runs
Run #
1
2
3
4
5
6
7
8
9
10
n
12
13
14
Sampling
Date
07/12/84
07/17/84
07/19/84
07/20/84
07/24/84
07/26/84
07/27/84
07/30/84
07/31/84
08/02/84
08/03/84
08/06/84
08/07/84
08/09/84
Description
Trial Run, Ambient Air
Ambient Air
Ambient Air
Audit Cylinder
Calibration Cylinder
Ambient Air
Ambient Air
Ambient Air
Ambient Air
Ambient Air
Audit Cylinder
Ambient Air
Ambient Air
Ambient Air
Table 3. Performance Data for Solid Adsorbents Relative to Cryogenic Trapping
Compound
Chloroethene
1, 1-Dichloroethene
Dichloromethane
3-Chloropropenec
1,1,2-Trichloro-1,2,2-
Trifluoroethane
Trichloromethanec
1,2-Dichloroethane°
1, 1, 1-Trichloroethanec
Benzene
Tetrachloromethanec
Trichloroethene
Toluene
Tetrachloroethene
Chlorobenzene
1,2-Dimethylbenzene
Tenax
Breakthrough
Volume3,
Liters/Cartridge
0.8
Not Given
4
6
Not Given
13
18
9
27
13
28
122
106
249
334e
Average Recovery Relative
To Cryogenic Trapping, %
Tenax
t>
b
83(21)
87(35)
39(25)
100(36)
100(15)
130(42)
100(18)
1 10(37)
1 12(26)
70(19)
88(27)
78(35)
55(21)
Polyimide
b
52(28)
86(31)
140(68)
17(14)
75(25)
65(15)
51(14)
130(34)
53(19)
100(33)
70(17)
78(30)
57(21)
40(15)
Spherocarb
73(23)"
410(260)
85(12)
29(14)
69(30)
65(17)
75(14)
46(10)
140(63)
29(9)
90(33)
43(8)
72(30)
53(19)
20(7.9)
aData from Reference 2 at 90°F.
bNo meaningful data obtained.
°Low volume (nominally 5 liters) Tenax valued used for these compounds. Medium volume
(nominally 10 liters) Tenax value used for all other compounds.
dValue in parentheses is standard deviation for all sampling runs, except for audit and calibra-
tion cylinder sampling.
eValue for ethylbenzene.
in much larger blank levels being ob-
tained. In our hands the VOST-style
traps, which are positively sealed dur-
ing storage and analysis (desorption),
provide much lower bank levels.
One must recognize that the composi-
tion of the atmosphere being sampled
may have a significant impact on the
performance of any of the adsorbents,
due to chemical and physical effects
which are not fully understood. There-
fore, the results obtained in this study
may not be observed in all ambient air
sampling situations. Ideally one should
check the method performance using
the air sample of interest spiked with
known quantities of the compounds to
be determined. Since such experiments
are expensive to conduct, it is usually
not possible to obtain such data. For
this reason any ambient air sampling
data should be viewed as qualitative, or
at least semiquantitative, unless such
data are provided. The use of directly
spiked cartridges or spiked clean air
studies cannot substitute for such data.
Conclusions and Recommenda-
tions
The significant conclusions to be
drawn from this study are presented
below:
1) Compared to the three adsorbent
methods tested, canister sampling
followed by cryogenic trapping/
gas chromatography offers better
precision and accuracy for the
compounds of interest in this
study, especially when a mass se-
lective detection system is em-
ployed to overcome most poten-
tial interferences.
2) None of the three adsorbents eval-
uated gave the best performance
for all of the target compounds.
Tenax® gave the best performance
for 3-chloropropene, trichloro-
methane (chloroform), 1,2-
dichloroethane, 1,1,1-
t rich I oroetha ne, benzene,
tetrachloromethane (carbon tetra-
chloride), trichloroethene, tetra-
chloroethene, and 1,2-dimethyl-
benzene. Polyimide performed
best only for 1,1-dichloroethene
(vinylidene chloride). Spherocarb®
performed best only for
chloroethene (vinyl chloride),
dichloromethane, and 1,1,2-
trichloro-1,2,2-trifluoroethane
(Freon 113). Toluene gave com-
parable results when Tenax® and
polyimide were used, but lower re-
coveries on Spherocarb®.
3) The polyimide adsorbent has sev-
eral operational difficulties which
diminish its usefulness in ambient
air sampling. A major artifact peak,
acetonitrile, is generated when
clean cartridges are stored longer
than 24-48 hours before analysis.
In addition, a significant amount of
water is adsorbed during sam-
pling, which leads to plugging of
the chromatographic column dur-
ing analysis.
4) Spherocarb® should be used only
if highly volatile compounds such
as chloroethene (vinyl chloride) or
dichloromethane are of interest,
since several of the target com-
pounds appear to be degraded in
the sampling and analysis process
for this adsorbent. High levels (ar-
tifacts) of 1,1-dichloroethene
(vinylidene chloride) are formed
on Spherocarb® in many in-
stances.
5) With a few exceptions, the recov-
ery relative to cryogenic trapping/
GC and precision obtained for the
target compounds on solid adsor-
bents is considered to be accept-
able for most ambient air studies.
Generally, individual data points
for the solid adsorbents agreed
within a factor of two with the
cryogenic sampling data.
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6) The Volatile Organic Sampling
Train (VOST) style cartridges gave
low blank levels and are suggested
as an alternative to the open style
cartridge.
On the basis of the results obtained in
this study, further evaluation of the
polyimide material for ambient air sam-
pling appears to be unwarranted, since
a number of operational difficulties
were encountered for which no obvious
solution exists. Furthermore, no signifi-
cant advantages in terms of recovery of
highly volatile compounds were ob-
served, in comparison to Tenax®, when
sampling ambient air.
The results of this study indicate that
the use of the VOST style cartridge of-
fers substantially better blank levels for
Tenax® than the conventional open-
style cartridges. If this finding is inde-
pendently verified, all further studies
should use the VOST style cartridge.
Ralph M. Riggin and Richard A. Markleare with Battelle's Columbus Laboratories,
Columbus, OH 43201.
James D. Mulik is the EPA Project Officer (see below).
The complete report, entitled "Comparison of Solid Adsorbent Sampling
Techniques for Volatile Organic Compounds in Ambient Air," (Order No. PB
86-127 651 /AS; Cost: $11.95. 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, NC27711
U. S. GOVERNMENT PRINTING OFFICE-. 1986/646-116/20760
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United States
Environmental Protection
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