United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-92/055 May 1992
EPA Project Summary
Theoretical Evaluation of
Stability of Volatile Organic
Chemicals and Polar Volatile
Organic Chemicals in Canisters
Robert W. Coutant
The potential for physical adsorption
as a mechanism for loss of volatile
organic chemicals (VOC) and polar
volatile organic chemicals (PVOC) from
the vapor phase in canister samples
was assessed using the principles em-
bodied in the Dubinin-Radushkevich
isotherm. This isotherm provides a spe-
cific relationship between the tendency
for adsorption and compound/sample
specific properties such as polarizabil-
ity, vapor concentration, temperature,
and equilibrium vapor pressure. In aid-
dition, the isotherm provides the means
for distinguishing between surfaces
having different physical and chemical
properties. A computer-based model
was developed for predicting adsorp-
tion behavior and vapor phase losses
in multicomponent systems. At present,
the data base for the model contains
relevant physicochemical data for more
than 60 compounds (42 VOC, 19 PVOlC,
and water), and provisions for inclu-
sion of additional compounds are in-
corporated in the software.
This report was submitted in partial
fulfillment of Contract No. 68-DO-0007
by Battelle, under the sponsorship of
the U.S. Environmental Protection
Agency. This report covers a period
from September, 1990 through Febru-
ary, 1991, and was completed as of
February 28,1991.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search 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
Physical adsorption of trace atmospheric
constituents on the surfaces of sampling
apparatus has long been recognized as a
contributing factor to apparent losses of
some organic compounds during sampling
and sample storage. The use of passi-
vated stainless steel canisters for sample
collection, shipping, and storage prior to
analysis has diminished this problem suf-
ficiently that these devices are widely used
for whole air sampling of non-polar vola-
tile organic compounds (VOC). Nonethe-
less, it is recognized that not all VOC are
equally stable in canisters under all pos-
sible sampling temperatures and relative
humidities. The number of compounds that
need to be accurately sampled and ana-
lyzed is expanding as the provisions of
the Clean Air Act are implemented. These
new compounds include (PVOC) and ad-
ditional VOC, some of which have lower
vapor pressures than VOC that are cur-
rently being sampled and analyzed using
canisters.
Experimental investigation of the stabil-
ity of all possible combinations of impor-
tant VOC and PVOC at all concentration
levels of interest and under all realistic
sampling conditions is not practical par-
ticularly given the dynamic nature of the
provisions of the Clean Air Act. A more
sensible approach is to develop guide-
lines for future evaluation and application
of canister sampling technology based on
the implications of fundamental processes
that govern the stability of whole air
Printed on Recycled Paper
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samples. Excluding compounds that are
inherently unstable (i.e., reactive) in the
atmosphere, such as ozone, these pro-
cesses can be limited to (1) heteroge-
neous chemical reaction (including chemi-
sorption) with surfaces within the sam-
pling system, and (2) physical adsorption
on surfaces within the sampling system.
Passivation of a surface is generally per-
formed to minimize the chemical reactivity
of the surface by either altering the chemi-
cal nature of the surface or by masking
the surface by deposition of a less reac-
tive coating. In the case of Summa™*
polished stainless steel canisters, the sur-
face area is reduced by the polishing pro-
cess, and the surface is predominantly
the relatively inert CrO2. However, exami-
nation of the Summa polished surface by
ESCA, after exposure to air, shows the
top 20A to be covered by oxygen and
carbon species. This is a common obser-
vation for most metallic surfaces that have
been exposed to air, and indicates that
the main advantage of the Summa pro-
cess may be in the reduction of the sur-
face area.
Objective
The objective of this program was to
evaluate the potential for physical adsorp-
tion for a broad range of VOC and PVOC
with the goal of developing a fundamen-
tally consistent model for assessing the
stability of such compounds in canisters.
A subsidiary goal was to collect and con-
solidate relevant experimental information
for comparison with model predictions.
Results
A computer-based model was devel-
oped to facilitate solution of the set of
simultaneous equations representing mul-
ticomponent adsorption equilibria. Code
for this model was written in Microsoft
Quick-Basic. The software package in-
cludes the main program that is used for
all calculations, a Lotus-123 file that con-
tains the listing of compounds and their
physicochemical properties, and a .prn file
that is generated from the Lotus-123 file.
The .prn file serves as a data base for the
main program, and provides a mechanism
for expansion of the list of available com-
pounds without requiring alteration of the
program.
The program is menu driven, with all
inputs necessary for a computation being
specified through use of the menu. Cer-
tain default values are specified at startup,
but these can be changed to suit the user's
needs. The program should be run on a
system capable of EGA graphics and it
Mention ol trade names or commercial products
doas not constitue endorsement or recommenda-
tion (or USB.
requires a math co-processor for comple-
tion of a calculation in a reasonable time.
The program assumes a spherical ge-
ometry for the canister, and the volume of
the canister is used to determine the avail-
able surface area. The surface roughness
also is used to determine the available
area. This factor is dependent on the ex-
tent of polishing — a perfect mirror sur-
face would have a roughness factor of 1,
(i.e., a true surface area equal to the geo-
metrical surface area). Electropolishing
typically yields roughness factors of 1.5-2,
and we have calibrated the model using a
default value of 2 for the surface rough-
ness and experimental data derived using
Summa polished canisters.
When, trying to,simulate experimental
conditions, it is necessary to know the
temperatures for sampling and analysis.
The absolute pressure of the canister
sample at the analysis condition is also
needed. The program adjusts the mea-
sured sampling humidity to the analysis
temperature and canister pressure, and
provides a warning when the sample prop-
erties are such as to lead to water con-
densation.
Three output choices are available-
screen, printer, and disk. The screen and
printer outputs yield fables of compound
names, adsorbed phase mole fractions
(X|), final gas phase pressure (PJ, initial
gas phase pressure (Pln), and the ratio of
Ph/PIn. The disk output yields a file that
can be imported into Lotus-123 for tabula-
tion and comparison with experimental
data.
Default settings are provided for all vari-
ables except sample composition which
must be supplied by the user. Listings of
compounds currently included in the data
base can be viewed either on screen by
accessing the help feature or on hard
copy by request from the main menu.
Once a particular composition is selected,
it is "remembered" by the program so that
multiple computations involving changes
in relative humidity, temperatures, etc. can
be conducted without re-entry of the com-
position data.
The program first uses the Newton-
Raphson method to determine the values
of X, at the known initial values of P,.
Then the amounts of each component
adsorbed and the corresponding residual
gas phase pressures are estimated. This
process of estimation of X, and P, is reiter-
ated until convergence is achieved to less
than 1 ppm.
The model was calibrated by compari-
son of model predictions with a single set
of experimental results for recovery of VOC
from "dry" canisters. Application of the
model to other experimental conditions
yields predictions that are qualitatively in
agreement with the experimental data. In
some cases, the agreement between pre-
dictions and experimental results is nearly
quantitative, suggesting that the use of
very well qualified data for model calibra-
tion might improve the overall quality of
the predictions.
Conclusions and
Recommendations
Based solely on the physicochemical
properties of the compounds (i.e. inde-
pendent of surface considerations), the
model predicts displacement of the more
volatile VOC and PVOC from a canister
surface by water vapor at relative humidi-
ties in the range of 1 to 20 percent. This is
generally consistent with experimental ob-
servations, but in most cases, the experi-
mental conditions are not sufficiently char-
acterized to permit detailed quantitative
comparison with the model. For example,
relative humidities less than about 5 to 10
percent are generally not measured but
rather are calculated based on the addi-
tion of a known amount of water to a "dry"
system. A different kind of uncertainty
arises when attempting to compare the
model results with field samples. In this
case, the analysis is usually conducted
for a restricted set of analytes, whereas
the model considers all components to be
in competition for the surface.
Some implications for application of can-
ister sampling for VOC and PVOC result
from this work:
(1) Measurements of relative humidity
and temperature should be made
during the sampling process. Un-
der conditions where the relative
humidity is low enough that the
model predicts loss of target com-
pounds, provision should be made
to add water vapor to the canister
prior to analysis.
(2) The sample pressure should be as
high as possible without causing
precipitation of liquid water within
the canister, but restrictions im-
posed during shipment may gov-
ern the maximum pressure allow
able.
(3) When considering the suitability of
the canister sampling method for
new compounds, the first param-
eters to be evaluated should be
chemical reactivity and the vapor
pressure of that compound. Com-
pounds with equilibrium vapor pres
sures less than about 1 torr at am-
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bient temperatures may require
heating of the canister to effect good
recovery, but heating of canisters
should be done only with recogni-
tion of all of the effects this process
may have on the recovery of the
analytes.
(4) Inasmuch as all species present
participate in the competitive ad-
. sorption process, retrospective con-
siderations of the quality of data
obtained from multiple canisters at
the same site should include at least
semi-quantitative specification (e.g.
total FID response) of non-target
species contained in the samples.
The model developed in this program
shows considerable promise for qualita-
tive and semi-quantitative explanation of
physical adsorption phenomena with mix-
tures of trace VOC and PVOC in canis-
ters. At this point, however, quantitative
data are lacking with respect to character-
ization of canister surface properties. In
particular, the surface roughness factors
for both electropolished and unpolished
canisters are unknown. At present, the
model calibration depends on an assumed
value for this parameter, and uncertainty
in the calibration could be reduced by
measurements of surface roughness. The
model calibration also is currently depen-
dent on incompletely characterized experi-
mental measurements at low relative hu-
midities. The accuracy of the calibration
could be improved by a simple set of
experiments conducted under very con-
trolled conditions. It would be desirable,
for example, to prepare a standard cylin-
der of VOC in very dry air that could be
used to charge a well-cleaned and dried
canister to various pressures. Analysis of
the residual gas concentration at each
pressure would yield a more reliable cali-
bration of the model than is currently in
place.
The parameters included in the model
suggest a complex dependence of analyte
recovery on sample temperature. This as-
pect of canister analysis should be evalu-
ated systematically.
•U.S. Government Printing Office: 1992— 648-080/60022
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Robert W. Coutantis with Battelle, Columbus, OH 43201.
William A. McClenny is the EPA Project Officer (see below).
The complete report, entitled "Theoretical Evaluation of Stability of Volatile Organic
Chemicals and Polar Volatile Organic Chemicals in Canisters," (Order No,
PB92-166941/AS; Cost: $17.00; 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 Off her can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
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