SERA
United States
Environmental Protection
Agency
•Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-81-110 Sept. 1981
Project Summary
Response of Portable VOC
Analyzers to Chemical
Mixtures
D. A. DuBose, G. E. Brown, and G. E. Harris
The report gives the responses of
two types of portable VOC analyzers
(Century Systems OVA-108 and
Bacharach TLV Sniffer), calibrated
with methane and used to measure a
variety of chemical vapor mixtures.
Instrument response data for both
binary and ternary mixtures of selected
chemicals are presented. Various
empirical models were evaluated to
determine an appropriate method of
estimating mixture concentration
based on instrument response. The
evaluation concluded that the instru-
ment response for a mixture falls
between the responses expected for
the pure compounds in the mixture.
Thus, an interpolation or weighted
average model can be used to predict
the response for mixtures based on
known responses for individual chem-
icals. Both linear and logarithmic
weighted average models are applied
to the data and presented with esti-
mates of accuracy. In general, these
models predicted the instrument
response to within 30 percent of the
observed value.
This Project Summary was developed
by EPA's Industrial Environmental
Research 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
Portable VOC (Volatile Organic Com-
pound) analyzers have been widely used
in the identification of fugitive emission
sources, a process commonly called
"screening." Screening is important
both in research to develop emission
factors and in proposed regulatory
monitoring of fugitive emissions. While
these VOC analyzers will respond to
mo.st organic vapors, they respond with
varying sensitivity to different chemical
species.
Previous to the present study. Radian
Corporation performed a laboratory
evaluation of the response character-
istics of two common VOC analyzers
(the Century Systems OVA-108 and the
J.W. Bacharach TLV Sniffer) to a large
variety of individual organic compounds
in air. The results of this previous EPA-
funded study are available in two
reports EPA-600/2-81-002 (NTIS PB81
136194) and EPA-600/2-81 -051.
This summary presents the results of
testing in the area of chemical mixtures.
The data base reported here includes
response data on nine binary chemical
combinations of acetic acid, acetone,
chloroform, cyclohexane, benzene,
methanol, and methyl ethyl ketone plus
one ternary combination.
The results of these analyses show
that the response to a binary chemical
mixture falls between the responses of
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the two individual chemicals at the total
concentration of the mixture. This
indicates that synergistic effects are
weak and suggests that some form of
interpolation or weighted average can
be used to predict the response of the
mixtures based on individual chemical
response characteristics.
Computation Methods
This section describes and discusses
methods for estimating concentrations
when VOC instrument responses are
known. The fractional composition of
the detected vapors must also be
known. Computed and graphical esti-
mates are discussed.
In general, the VOC instrument
response to a mixture of compounds is
intermediate to the individual responses
to each one of them at the same total
concentration. This effect may be
approximated by a weighted average of
the responses (or linear interpolation in
the case of two chemicals). An estimate
of the weighted average response is:
P,aiCTbie1/2Si
(1)
where
RA = the estimated weighted average
response,
P, = the fraction of the mixture total
concentration accounted for by
compound i (P, = Ci/CT),
a, = exp(A) with "A" from Brown, et
al. (1980)* for component i,
b, = coefficient "B"from Brown, etal.
(1980) for component i,
s, = parameters "SE" from Brown
et al. (1980) for component i,
CT = £- C,, the total concentration,
and
d = the concentration in the mixture
of compound i.
The coefficients A, B, and SE can be
found in Tables 5-169 and 5-170 of
Brown, et al. (1980) for selected com-
pounds.
The above discussion involves the
prediction of an instrument response
when the actual concentration of
mixture components is known. In a
practical situation, the reverse is the
case: the response is observed and it is
desired to estimate the concentration of
the constituents. Basically, this cannot
be done without some additional
information. The compound identifica-
tion of the constituents must be known.
If the constituent proportions are also
known, the total concentration can be
computed assuming the above model is
correct. The total concentration (Cr) is
estimated by solving Equation (1).
Equation (1) cannot be solved explicitly
for total concentration. An iterative
solution is required. This can be done
using the Newton-Rhapson method.
Letting
p,a,CTi e1/2Si - R (2)
where R is the observed instrument
response, and
,-1 1/2Sl
2. pib,a,CT
f(CT)= 2L Pib,a
,-1 1/2S2
e i
(3)
then the iteration formula is:
Cl+i = C, - f(C,)/f (C,). (4)
A reasonable starting value C0 is R, the
observed instrument response.
Alternatively, a weighted logarithmic
average may be used. In this case:
log (RL) = Z. p, [log a, + V4s? + b, log CT]
i (5)
where RL is the estimated instrument
response using a weighted logarithmic
average and logarithms are to base e.
In contrast to the previously given
weighted arithmetic average model,
Equation (1), this weighted logarithmic
average model. Equation (5) has an
explicit solution for actual total con-
centration:
log CT =
log R -
IP,
(log a, + 1/2 s,2)
p,b,
(6)
For binary mixtures, the solutions are as
follows. The arithmetic weighted aver-
age iteration formula is:
•In report EPA-600/2-81-002 (NTIS PB81-
136194)
C, + 1 = C, - f(C,)/f'(Ci)
where
bi 1/2S?
f(CT) = piai CT e +
bz 1/2 si
p2a2 CT e -R,
(7)
and
C0 = R or another suitable starting value.
The logarithmic weighted average solu-
tion is:
log CT =
(8)
Results
The models discussed above predict
the mixture response by weighted
average or linear interpolation between
the response of two pure chemicals on
either an arithmetic or a logarithmic
scale. This "weighted average" model is
readily extendable to three or more
compounds. Asummary of the response
data and the precision of these two
models is given in Table 1. Neither the
arithmetic nor logarithmic scale model
shows any clear advantage
applied over the entire data set.
ever, the logarithmic model gives much
poorer estimates than the arithmetic
model in some cases. Both models are
able to predict mixture response to
within about ±30 percent of the
observed value, which is about as good
as the single chemical data on which
the models are based. There is some
advantage to the logarithmic scale
model in that it has a discrete solution,
while the arithmetic scale model must
be solved iteratively or graphically.
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Table 1. Comparison of Percent Error for Arithmetic and Logarithmic Weighted A verage Models for Mixtures
Average Percent Error of Model Prediction
OVA
Compound Mixtures
Arithmetic
Logarithmic
Compound
Response Factor*
Compound
OVA
1.64
0.80
0.29
9.28
TLV
15.60
1.22
1.07
Cyclohexane
Methanol
Methyl Ethyl Ketone
TLV
Arithmetic
Logarithmic
Acetic Acid and Chloroform
Acetic Acid and Cyclohexane
Acetic Acid and Acetone
Acetone and Chloroform
Acetone and Cyclohexane
Benzene and Methyl Ethyl Ketone
Benzene and Methanol
Chloroform and Cyclohexane
Methanol and Methyl Ethyl Ketone
Benzene, Methanol, and
Methyl Ethyl Ketone
25
31
33
39
31
-9
-1
29
-16
-9
20
11
27
17
28
19
-121
-5
-87
-50
-83
4
-3
-21
7
9
10
-13
12
12
-88
-37
-28
-80
3
9
8
-90
9
9
Response Factor*
OVA
0.47
4.39
.0.64
Acetic Acid
Acetone
Benzene
Chloroform
* Ratio of actual concentration to instrument response at 10,000 ppmv response (EPA -600/2 -81 -051).
**A 10,000 ppmv response to chloroform with TLV is not achievable.
TLV
0.70
2.01
1.12
D. A. DuBose, G. E. Brown, andG. E. Harris are with Radian Corp., P.O. Box 9948,
Austin, TX 78766.
Bruce A. Tichenor is the EPA Project Officer (see below).
The complete report, entitled "Response of Portable VOC Analyzers to Chemical
Mixtures," (Order No. PB 81 -234 262; Cost: $8.00, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
it US GOVERNMENT PRINTING OFFICE, 1981—757-012/7336
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