United States
Environmental Protection
Agency
Environmental Monitoring Systems -
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-85/043 Aug. 1985
Project Summary
Development of an Optical
Monitor for Toxic Organic
Compounds in Air
T. Hadeishi, M. Pollard, R. Mclaughlin, and M. Koga
The objectives of this study were: (a)
to design, construct, and deliver a
prototype atomic line molecular spec-
trometer (ALMS) benzene monitor and
(b) to locate matches of atomic lines
and sharp molecular absorption features
in other toxic organic compounds for
possible use in ALMS or TALMS tech-
niques. ALMS and TALMS are newly
developed, high resolution molecular
absorption techniques which are used
in the vacuum-ultraviolet and ultraviolet
regions of the optical spectrum to detect
organic molecules in the gas phase. The
dual beam prototype ALMS instrument
was designed, constructed, tested and
delivered to the Environmental Moni-
toring Systems Laboratory, USEPA,
Research Triangle Park, NC, in Decem-
ber 1984. It was designed for monitor-
ing benzene with the 184.9 and 253.7
nm mercury lines using the ALMS
technique. The instrument consisted of
three units: the optical unit (weight: 28
Ibs, dimensions: 28 x 10 x 12"); the
electronics unit (weight: 6 Ibs, dimen-
sions: 19 x 7 x 5.25"); and a lamp driver
(weight: 24 Ibs, dimensions: 14.5 x 14
x 6.5"). The total weight was 58 Ibs.
which is less than that of the TALMS
benzene monitor previously developed
(82 Ibs). Tests of the performance of the
benzene monitor showed an approxi-
mate detection limit of 250 ppbv at
184.9 nm.
The process of searching for TALMS
signals in organic compounds was sim-
plified by the development of a com-
puter accessible data base of atomic
line locations and relative intensities.
This data base was used to select lines
for ALMS detection of o- and m-xylenes.
Line matches and TALMS signals were
found for three new compounds: p-
difluorobenzene (Pt: 266.9 nm); m-
dichlorobenzene (Ge: 269.1 nm) and p-
chlorofluorobenzene (Fe: 275.6 nm).
The high resolution absorption spectrum
of p-difluorobenzene was determined
near the platinum 265.9 nm line.
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 Protect Report ordering in-
formation at back).
Introduction
There is a great need for instruments to
detect and monitor specific organic pollu-
tants and classes of pollutants in ambient
air and near sources such as waste
disposal and industrial production sites.
Direct monitors are needed for specific
chlorinated hydrocarbons and various
aromatic hydrocarbons including ben-
zene, toluene and other substituted
benzenes. Since substituted benzenes
and other organic compounds absorb light
in the vacuum ultraviolet and ultraviolet
regions, one possible detection method is
the use of optical absorption techniques
in the gas phase. The use of high resolu-
tion absorption methods should increase
the selectivity of the technique by ex-
ploiting the rotational-vibrational fine
structure in the absorption spectra of the
compounds.
Tunable atomic line molecular spectros-
copy (TALMS) is a newly developed high
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resolution, molecular absorption approach
to monitoring organic vapors that is
different from most present analytical
techniques. Whereas present methods
depend on some form of chromatographic
separation for compound identification,
the TALMS technique depends only on
ultraviolet-visible absorption properties.
There is no separation procedure in-
volved. It is highly specific because it
responds only to very sharp rotational-
vibrational molecular absorption features.
However, TALMS has not been shown to
be very sensitive due to difficulties in
locating atomic probe lines near high
intensity molecular absorption maxima.
The lowest detection limit found for
benzene with the TALMS technique is 10
ppmv, which is too high for direct ambient
air measurements.
A related technique, atomic line mo-
lecular spectroscopy (ALMS), resulted
from extending the ideas behind the
TALMS technique in an attempt to im-
prove the detection limit. Differential
absorption at two different wavelengths
is the basis of both methods. In the
TALMS case the two wavelength posi-
tions are determined by the very small
Zeeman splitting of the atomic line chosen
for measurement. The small splitting
ensures that the background correction
will eliminate most interferences. In the
ALMS case the wavelength positions are
determined by choosing two different
atomic lines that are necessarily separ-
ated by a much greater distance than in
the TALMS technique. The inherent
detection limits of both techniques de-
pend upon the difference in intensities of
the molecular absorption spectrum at the
two wavelength positions. Because the
wavelength separation is much greater
with ALMS, the absorption difference can
be much larger yielding a great improve-
ment in detection limits. For example, a
TALMS detection limit of 10 ppmv has
been found for benzene at 253.6 nm
where the molecular extinction coeffic-
ient is approximately 100. Judging from
the much higher extinction coefficients in
the vacuum ultraviolet region for ben-
zene, detection limits should be at least
one thousand times better with the ALMS
technique. This would result in detection
at 10 ppbv levels, a useful limit for
ambient air monitoring.
The major disadvantage of the ALMS
technique is the possibility of interfer-
ences from compounds other than these
sought. The use of multiple analysis lines
will reduce this problem. However, for
accurate background correction with ad-
ditional analysis lines, the absorption by
interfering compounds must be constant
over the wavelength interval used. If
several different lines are used to monitor
the concentration of the same molecule,
the presence of an interference can be
detected because different apparent con-
centration values will be obtained at
different wavelengths. Although this
measurement may not provide an accu-
rate background correction, it will alert
the analyst to the presence of a problem.
With the ALMS technique the use of
multiple analysis lines at properly chosen
wavelengths in the vacuum ultraviolet
and ultraviolet region may also allow the
determination of classes of organic com-
pounds.
Previous studies with the TALMS tech-
niques have resulted in construction and
evaluation of prototype instruments for
general laboratory use and for monitoring
benzene. The goals of the present study
are to: (a) design, construct, and deliver to
the Environmental Monitoring Systems
Laboratory, Research Triangle Park, NC,
an ALMS monitor and (b) to determine the
spectral location of absorption features
and matching atomic lines in toxic organic
compounds other than benzene. An
ALMS instrument would be useful in
laboratory detection and field monitoring
for benzene and other toxic compounds.
Determination of spectral locations of
molecular absorption maxima is neces-
sary to optimize instrument performance
and to extend the technique to other
organic compounds.
Conclusions and
Recommendations
Design and construction of a prototype
ALMS instrument for the detection of
benzene and other compounds was com-
pleted on schedule; and the prototype
was delivered to the Environmental Mon-
itoring Systems Laboratory, Research
Triangle Park, NC, in December 1984. It
consisted of three modules: an optical
module; an electronics module; and a
lamp driver. The sensing module weighs
28 Ibs, the electronics module, 6 Ibs, and
the lamp driver, 24 Ibs. An instruction
manual and schematics of the electronics
were also supplied. It was constructed to
operate in the vacuum ultraviolet and
ultraviolet spectral regions and was
equipped with a mercury lamp. Benzene
and other organic compounds can be
detected with the mercury 184.9 and
253.7 nm lines. Tests with benzene at
184.9 nm gave approximate detection
limits of 250 ppbv. This is an improvement
by a factor of 40 over the best TALMS
detection limits. Other atomic lamps car
be used to obtain different analysis
wavelengths as required.
Searches for TALMS signals in othei
organic molecules were continued. This
tedious process was greatly improved by
the development of a computer search
technique using National Bureau o1
Standards atomic line information. Aftei
considerable experimentation with a vari-
ety of lamps and medium resolution
absorption spectra, TALMS signals were
found for p-difluorobenzene; (Pt: 265.9
nm); m-dichlorobenzene; (Ge: 269.1 nm);
and p-chlorofluorobenzene; (Fe: 275.6
nm). TALMS signals have now been found
for benzene, bromobenzene, chloroben-
zene, toluene, p-xylene, aniline, phenol,
pyridine, formaldehyde, m-dichloroben-
zene, p-chlorofluorobenzene, and p-di-
fluorobenzene. Wavelengths for the
ALMS detection of ortho- and meta-xy-
lenes were selected. The high resolution
absorption spectrum of p-difluorobenzene
was determined near the platinum 265.9
nm line.
It was recommended that several modi-
fications be made to the prototype to
improve ALMS instrument performance.
An arrangement should be devised to
alternately send two different lines from
the same light source through the sample.
Electronic subtraction of the signals from
the two lines will greatly reduce light
source noise and electronic noise with a
corresponding increase in sensitivity. For
simultaneous detection of several com-
pounds, a device that automatically posi-
tions different wavelength regions on the
exit slit of the monochromator should be
constructed. If it is necessary to make
measurements in the vacuum ultraviolet
region or use weak emission lines, an
arrangement for purging the instrument
and optical path will be important in order
to increase transmission. Addition of
commercially available intense lamps of
other elements is also important. The
goal of these modifications is to improve
sensitivity, reliability and portability.
Since major decreases in detection
limits and extension of this technique to
other compounds are dependent upon
the location of proper analysis lines, it is
recommended that more studies be
carried out to locate new analysis lines
for compounds of interest.
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T. Hadeishi, M. Pollard, R. McLaughlin, and M. Koga are with the University of
California, Berkeley, CA 94720.
D. R. Scott is the EPA Project Officer fsee below).
The complete report, entitled "Development of an Optical Monitor for Toxic
Organic Compounds in Air." (Order No. PB 85-225 027/AS; Cost: $8.50,
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 Protect/on Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-85/043
OC00329 PS
U S ENVIR PROTECTION AGiNCY
REGION 5 LIBRARY
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