United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S7-88/011 Nov. 1 988
v°/EPA Project Summary
Development of a Breadboard
CO2 Laser Photoacoustic Toxic
Vapor Monitor
G.L. Loper, R.C. Corbin, M.L. Takayama, R.A. Clifton, J.A. Gelbwachs, and
S.M. Beck
This report describes the development
of a breadboard version of CO2 laser
photoacoustic detector. The CO2 laser
photoacoustic technique has been
demonstrated to be capable of detec-
ting, with high specificity, a variety of
toxic compounds a low parts-per-billion
(ppb) levels in multicomponent air
samples. The technique can be used for
monitoring trace levels of various hazar-
dous compounds in ambient air
samples.
Key achievements during the program
included: (1) The determination of CO2
laser absorption cross-section data or
seven compounds of EPA concern that
are volatile constituents of hazardous
chemical waste. These data show that
the laser photoacoustic method could
detect these compounds in the ambient
air at levels well below their most
stringent threshold limit values. (2) The
development of an acoustic-frequency-
tracking device that allows the use of a
resonant photoacoustic cell for the first
time under field conditions of changing
temperature and humidity. (3) The iden-
tification of nonadsorptive materials for
use in air sampliong lines and for coating
the cell's interior to minimize sample ad-
sorption and reaction losses. (4) The
development of a pyro-electronic device
that automatically determines the wave-
length of a CO2 laser transition. This
device allows laser-wavelength selection
to be under closed-loop control. (5) The
placement of the breadboard detector
under microcomputer control such that
all wavelength selection and the cal-
culation and display of the unknown
concentrations of the gases in the air
sample are automatic. (6) The analysis
of laboratory-prepared air mixtures to
determine the performance capability of
the instrument.
Experiments on the laboratory-
prepared mixtures showed that the fully
automatic breadboard instrument can
detect hydrazine, a toxic rocket fuel used
by the Air Force, at concentrations as
low as 5 ppb in the presence of three in-
terfering gases at concentrations as
much as 600 times greater.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research 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
A breadboard version of a CO2 laser
photoacoustic detector has been developed
for the U.S. Air Force Space Division
(AFSD) and the Environmental Protection
Agency (EPA), Air and Energy Engineering
Research Laboratory. This detector can
automatically and simultaneously monitor
low parts-per-billion (ppb) level concentra-
tions of several gases in laboratory-
prepared air samples with high specificity.
When fully developed, this detector will
be able to monitor areas that have poten-
tial hazards from toxic compounds will pro-
vide personnel protection not possible at
this time. The Air Force is interested in us-
ing a fully developed version of this instru-
ment for the rapid detection, at their
threshold-limit-values (TLVs), of various tox-
ic or carcinogenic compounds that may in-
advertently be present in air samples at
satellite launch sites.
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Conclusions/Recommendations
for Future Work
Several key results were achieved in the
program. (1) Resonant photoacoustic cell
designs were shown to offer advantages
over nonresonant cells in terms of
discriminating against window absorption
and sample flow noise as well as minimiz-
ing the adsorption of polar compounds. (2)
Signal risetime experiments carried out
with the photoacoustic detector using 1
ppm hydrazine flows indicated that TFE
Teflon is suitable as a cell-coating- and
sampling-line-material to minimize adsorp-
tion losses of sampled gases. (3) An elec-
tronic device was developed that automati-
cally maintains the modulation frequency
of the laser at the acoustic resonance fre-
quency of a resonant photoacoustic cell.
This allows resonant cell designs to be
used for the first time in field environments
where the sample gas temperature and
humidity may change often. (4) A pyroelec-
tronic device was developed that can be us-
ed to determine the CO2 laser wavelength.
(5) This wavelength verification device
allowed the laser's grating and PZT drive
to be placed under computer control such
that sample adsorbance measurements
can be automatic at any desired set of laser
tines. The computer can then use the ab-
sorbence data at m different laser lines to
calculate the concentrations of n < m com-
ponents in a mixture using a matrix inver-
sion program. The program employs a
library of data containing absorption cross
sections for the n compounds of interest at
the m laser lines used for monitoring.
It should also be possible to use the
detector in multipoint monitoring.
Several areas require resolution before
a prototype version of the instrument can
be developed for use in multipoint
detection:
(1) The demonstrated feasibility of using
CO2 laser photoacoustic detection for
multipoint detection.
(2) Improvement of the response time of
the instrument so that it can provide
a 90% full scale response in less
than 2 minutes for two compounds of
EPA concern at concentrations of 200
ppb in the presence of two
interferences.
(3) The effects of humidity on the instru-
ment's detection limits.
(4) The effects of aerosols that may be
present in the air at monitoring loca-
tions on the instrument's detection
limits.
(5) CO2 laser absorption cross-section
data, if not previously determined, for
new compounds of EPA concern and
for unexpected interfering com-
pounds that may be present at
monitoring locations.
(6) Verification that the water continuum,
aerosol, and unexpected interferent
effects described in items (3) through
(5), above, are adequately addressed.
(7) Results of analyses of mixtures
prepared with air from a site selected
by EPA.
(8) Field evaluations of the instrument at
the EPA site.
G. Loper, R, Corbin, M, Takayama, R. Clifton, J. Gelbwachs, and S. Beck are
with The Aerospace Corporation, £1 Segundo, CA 90245.
Merrill D. Jackson is the EPA Project Officer (see below).
The complete report, entitled "Development of a Breadboard COz Laser
Photoacoustic Toxic Vapor Monitor," (Order No. PB 88-224 332/AS; Cost:
$19.95. 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:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
f? r
Official Business
Penalty for Private Use $300
EPA/600/S7-88/011
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