Real-Time Detection of Chemical Warfare Agents Using 

Multi-Wavelength Photoacoustics 

by Kristan Gurton, Melvin Felton, and Richard Tober 

ARL-TR-4782 April 2009 

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Army Research Laboratory 

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ARL-TR-4782 April 2009 

Real-Time Detection of Chemical Warfare Agents Using 

Multi-Wavelength Photoacoustics 

Kristan Gurton and Melvin Felton 

Computational and Information Sciences Directorate, ARL 

Richard Tober 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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4. TITLE AND SUBTITLE 

Real-Time Detection of Chemical Warfare Agents Using Multi-Wavelength 

Photoacoustics 

5c. PROGRAM ELEMENT NUMBER 

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6. AUTHOR(S) 

Kristan Gurton, Melvin Felton, and Richard Tober 

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U.S. Army Research Laboratory 

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2800 Powder Mill Road 

Adelphi, MD 20783-1197

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Approved for public release; distribution unlimited. 

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14. ABSTRACT 

We present a proof-of-concept study designed to investigate the utility of operating a conventional photoacoustic 

spectroscopy technique in a “multi-wavelength” mode applied to chemical vapor/aerosols for application of trace species 

detection and identification. The technique involves propagating three or more laser sources through a non-resonate, flowthrough photoacoustic cell. Each laser source is modulated at a different frequency, chosen at some convenient acoustic 

frequency. A portion of each laser’s power is absorbed by a particular test gas/aerosol that is passing through the PA cell, 

resulting in a acoustic signal that is found to be proportional to the absorption cross section of the gas/vapor at the particular 

laser wavelength. A superposition of frequency component (equal to the number of laser wavelengths used), combines with 

the ambient acoustic noise spectrum and is recorded by an electret microphone housed in the photoacoustic cell. The signal is 

deconvolved using phase sensitive detection where each component (one corresponding to a particular modulation frequency 

for a particular laser) is amplified and recorded as function of species concentration. Ratios of the resultant absorption 

information are used to produce an identifiable metric that remains constant for all concentrations. For the study presented 

here, we used 3 laser wavelengths all lying in the spectrally rich long-wave infrared (LWIR), i.e., 8.72, 9.27, and 10.35 μm. 

Test nerve agents simulants include (but not limited to), diethyl phosphonate (DEMP), dimethyl methylphosphonate (DMMP), 

and diisopropyl phosphonate (DIMP). Measured photoacoustic absorption results compare well with Fourier Transform 

Infrared (FTIR) analysis that is conducted in situ with the photoacoustic portion of the measurement. 

15. SUBJECT TERMS

photoacoustic, nerve agent, Chem/Bio detection, DEMP, DMMP, DIMP 

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19a. NAME OF RESPONSIBLE PERSON

Kristan Gurton 

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301-394-2093 

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18 
iii 

Contents 

List of Figures iv

1. Background 1

2. Method 1

3. Preliminary Result/Conclusions 4

4. References 9

List of Symbols, Abbreviations, and Acronyms 10

Distribution List 11 
iv 

List of Figures 

Figure 1. Flow-through photoacoustic cell. ....................................................................................2 

Figure 2. Multi-wavelength photoacoustic test bed........................................................................3 

Figure 3. AC component of the electret signal sampled at 2.5 kHz while isopropyl vapor is 

drawn through the photoacoustic cavity. ..................................................................................5 

Figure 4. AC component of the electret signal sampled at 2.1 kHz while isopropyl vapor is 

drawn through the photoacoustic cavity ...................................................................................5 

Figure 5. AC component of the electret signal sampled at 1.7 kHz while isopropyl vapor is 

drawn through the photoacoustic cavity ...................................................................................6 

Figure 6. Resultant isopropyl vapor absorptance spectra measured simultaneous with the 

photoacoustic portion of the study (path length 10 cm). Note that for the laser wavelength 

chosen that the absorptance are very similar in magnitude, i.e., 8.72 μm=2.39, 9.27 

μm=2.29, 10.35 μm=2.30. .........................................................................................................7 

Figure 7. New photoacoustic cell design(s). Design a is based on the multiple reflections 

inherent in integrating spheres. Design b maintains a cylindrical column of vapor (into 

the page) but now the lasers are brought into the cell perpendicular to the flow in order to 

limit unwanted laser attenuation. In addition, the right cell shows possible enhancement 

of the signal by incorporating resonate effects by spacing opposing mirrors at distances 

on the order of quarter-multiples of the wavelength of sound associated with each 

modulation frequency. ...............................................................................................................8 
1 

1. Background 

Currently, there are no reliable means for accurate “real-time” detection and identification of 

poisonous chemical and/or biological substances that are airborne, either in gaseous or aerosol 

form. As a result, early-warning systems that are capable of instantaneous detection of such 

materials are badly needed. Because rapid detection and identification is desired, an optically 

based technique should be considered. 

To this end, we propose a new approach based on the detection of the optical absorption 

measured in situ for various organophosphorus-based nerve agent simulants. The novelty of the 

proposed study is to modify a traditional ultra-sensitive spectroscopic technique, i.e., gaseous 

and aerosol photoacoustics, by incorporating multiple laser-line sources. Photoacoustic 

techniques that utilize laser sources routinely measure absorption characteristics of trace gases in 

the ppb, and are considered among one of the most sensitive spectroscopy methods available 

(1, 2). Unfortunately, conventional photoacoustic techniques have been greatly limited by the 

availability of particular laser sources (3). This has been particularly true for infrared (IR) 

photoacoustic spectroscopy, in which the researcher typically had only a hand full of 

wavelengths available that were usually produced by cumbersome carbon dioxide (CO2) and lead 

salt (PbS/PbSe) lasers. However, with the advent of the newly developed Quantum Cascade IR 

laser diode substrates being developed at the U.S. Army Research Laboratory (ARL) and 

elsewhere, practically any wavelength within 3–14 μm is now readily available. 

Our approach is simply to incorporate multiple Quantum Cascade (QC) laser sources in an ultrasensitive spectroscopic method that has traditionally only used one laser source at a time. By 

doing so, we intend to measure multiple (three or more) absorption coefficients simultaneously 

of a given nerve agent simulant in vapor form. We intend to show that a reliable metric will 

result based on the relationships between the measured absorption parameters that can be used to 

detect and identify trace species of a given chemically based toxin. 

2. Method 

Detection of optical absorption using a photoacoustic method is fairly simple. For gaseous 

photoacoustic spectroscopy, a small-diameter laser beam is modulated at some convenient 

acoustic frequency, 1 kHz, for example, and is passed through a sealed cylindrical gas cell, often 

via two or more transmitting windows. The absorbed IR energy raises the absorbing molecules 

from the ground vibrational state to an excited vibrational-rotational state. Collisional processes 

then redistribute the energy into translation and rotation, with a resulting increase in the pressure 

of the gas. The time required for vibrational relaxation to occur at standard temperature and 
2 

pressure (STP) is on the order of 1 μs. The resultant rapid change in pressure at the modulation 

frequency of the laser results in an acoustic signal that is detected by an electret microphone 

usually with the aid of a lock-in amplifier. The measured acoustic, or pressure, signal is found to 

be proportional to the average power of the optical source, the absorption cross-section of the 

species, and the sensitivity of the electret. The last two parameters are usually known, and one is 

then interested in measuring the absorption response of the gas at the laser-line. 

We have developed a modified flow-through photoacoustic cavity designed specifically to 

overcome the limited sensitivity inherent in closed-cell designs that use windows to transmit the 

laser radiation into a photoacoustic cell, as shown in figure 1* (4, 5). 

Figure 1. Flow-through photoacoustic cell. 

Not seen in the photograph is a series of acoustic dampeners mounted inside the cell at both ends 

of the tube that serve to suppress any ambient noise. Situated in the center of the cell is a small 

field-effect transistor (FET) based electret microphone. The faint acoustic signal resulting from 

the absorbing gas is amplified and fed into a 24-Bit dynamic signal acquisition device that 

digitizes the analog signal. By accurately digitizing and processing the acoustic signal using a 

PC, we avoid having to use multiple lock-ins that are usually needed to deconvolve each signal. 

Instead we conduct a fast Fourier transform (FFT) on the signal and filter only frequency 

components that correspond to the modulation frequencies of our lasers. Using this approach, 

we are able to deconvolve up to eight separate laser-induced signals simultaneously with 

just a single PC. 

Initially, we had hoped to use only QC IR laser diode sources that were to be provided by our 

corporate partner, Maxion Lasers Inc., College Park, MD, but due to production difficulties, we 

were only provided one QC source (operating at 8.72 μm with CW power 60 mW at 14 °C) at 

 *Often such system are noise limited as a result of residual absorption from the transmission windows that if often greater that

the faint acoustic signal resulting from the absorbing species. 
3 

the time of this report. In order to move forward with our proof of concept, we procured two 

other conventional CO2 laser sources operating at 10.54 and 9.27 μm. The three sources were 

made coincident and propagated axially through the cell. Each source was modulated at a 

different acoustic frequency, e.g., 10.35 μm at 1700 Hz, 9.27 μm at 1900 Hz, and the 

8.72 μm QC at 1300 Hz. 

Examples of the organophosphorus nerve agent simulants considered include diethyl 

phosphonate (DEMP), dimethyl methylphosphonate (DMMP), and diisopropyl phosphonate 

(DIMP). Simulant vapor was generated by heating the liquid (low vapor pressure materials), 

where residual particles were removed by passing the vapor through a condenser that was cooled 

with liquid nitrogen (LN2). Simulant concentrations were varied by mixing with dry air that was 

pumped through the system at differing flow-rates. 

Complete IR absorption spectral for each simulant was simultaneously measured from 3–13 μm 

by passing the gas sample through a Fourier Transform IR (FTIR) spectrometer monitored 

transmission cell. This allowed for direct comparison with the three measured absorption 

coefficients that result from the photoacoustic portion of the study. A simplified schematic of 

the experiment can be seen in figure 2. 

Figure 2. Multi-wavelength photoacoustic test bed. 

Pre-amp 

Vacuum 

PC 

Photoacoustic 

cell 

QC laser 

CO2 laser 

CO2 

laser 

flow meter 

flow meter 

flow meter 

Power meter 

FTIR 

Gas transmission 

cell

flow meter 

Pressure/vacuum 

gauge 

Simulant 

generator 

IR glow 

bar 

Air 

window 

Air 

window 
4 

3. Preliminary Result/Conclusions 

At the time of this report we had just begun testing the specified nerve agent simulants, e.g., 

DMMP, DEMP, DIMP, etc., and results from those runs must still be analyzed. However, we 

will show data based on our “practice” vapor, i.e., isopropyl alcohol, in which optimum 

operating procedures were established. The following figures show the evolution of the raw 

photoacoustic signal for 8.72 μm, 9.27 μm, and 10.35 μm laser lines, as varying concentrations 

of isopropyl alcohol are released into the cavity. Figure 3 shows the magnitude of alternating 

current (AC) component of the acoustic signal for the lock-in synced at 2.5 kHz, i.e., the 

modulation frequency of the QC laser operating at 8.72 μm. Region 1 highlighted in the figure 

shows the period just prior to the release of the vapor, and the subsequent signal increase as the 

simulant enters the active region of the photoacoustic cell. During this portion of the 

measurement, simulant is heated to about 80 °C (dependent on the vapor pressure), and a 

pressure of about 2–3 psi is allowed to build in the system. This high concentration of vapor is 

then leaked at a rate of about 100 ml/min into the FTIR transmission cell and continues to flow 

into the photoacoustic sample region. A maximum saturation is reached in region 2, at which 

point we begin to mix the simulant flow with varying concentrations of dry air which reduces the 

overall concentration, as shown in region 3. The large signal spike seen in region 2 corresponds 

to opening of valves for the fresh air mixture. We intentionally chose this particular curve to 

highlight the obvious appearance of acoustic noise seen in the trailing end of the signal in region 

3. This noise is apparent in much of the acoustic frequency spectrum above 2 kHz and results 

from the fluid flowing in the lines. We merely had to adjust our modulation frequencies to avoid 

the high frequency noise region. Figure 4 shows the corresponding raw acoustic signal for the 

10.35 μm CO2 laser line modulated at 2.1 kHz, where the noise is still present. Figure 5 shows 

the 9.27 μm signal modulated at 1.7 kHz and a much better noise profile as we move down in 

frequency. 
5 

8.72um mod freq. 2.5kHz

0

0.5

1

1.5

2

2.5

0 100 200 300 400 500 600

Time (sec)

Acoustic Signal (uV)

12 3

Figure 3. AC component of the electret signal sampled at 2.5 kHz while isopropyl vapor is drawn 

through the photoacoustic cavity. 

10.35um mod freq. 2.1kHz

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 100 200 300 400 500 600

Time (sec)

Acoustic Signal (uV)

Figure 4. AC component of the electret signal sampled at 2.1 kHz while isopropyl vapor is drawn 

through the photoacoustic cavity 
6 

9.27 um mod freq. 1.7kHz

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 100 200 300 400 500 600

Time (sec)

Acoustic Signal (uV)

Figure 5. AC component of the electret signal sampled at 1.7 kHz while isopropyl vapor is drawn 

through the photoacoustic cavity 

It is only in region 3 (see figure 3), in which uniform concentrations are acquired, where 

absorption information is most reliable. 

At first glance we were somewhat surprised to see that the relative magnitude for the three 

signals was about the same (after normalizing for the differing laser powers), i.e., peaking around 

1.2 μV. However, after reviewing the absorptance spectra measured by the FTIR, the relative 

relationships between the three photoacoustics signals appear to be justified, (figure 6). 

Currently, by using the muti-wavelength photoacoustic method as described previously, we are 

accurately measuring the absorption response for a variety of nerve agent simulants. We are 

investigating several new photoacoustic cell designs to improve optical energy density in the 

cavity by inducing multiple reflections of the laser beams (figure 7). Similarly, these new 

approaches will boost signal by reducing unwanted attenuation of the lasers by keeping the lasergas interaction region small and in close proximity to the electret. 

It is our intent to replace the two CO2 lasers with QC laser diodes chosen to operate at 10.10 μm 

and 9.62 μm during the winter of 2009. 

Additional work is needed to better quantify simulant concentrations to establish ultimate 

detectability. Similarly, we need to establish the best metric to use in the detection/identification 

of a given agent. 
7 

A possible “concentration invariant” metric would involve ratios of the integral of the electret 

signal for a predescribed period of time for each wavelength. For example, during the sampling 

of a suspect species, we measure three different acoustic signals (one for each of the three 

wavelengths chosen), and the integral over a time period ∆t results in the three independent 

quantities, S(λ1), S(λ2), and S(λ3). A three-number metric associated with the species would 

then be {S(λ1)/ S(λ2), S(λ2)/ S(λ3), S(λ1)/ S(λ3)}. 

It is our hope to expend testing to incorporate particulate matter and aerosols, and attempt to 

distinguish various biological particles based on documented absorption spectra in the mid- and 

long-wave IR (LWIR) (6, 7). 

2

2.05

2.1

2.15

2.2

2.25

2.3

2.35

2.4

2.45

2.5

6 7 8 9 10 11 12

wavelength (um)

absorbance

8.72um abs= 2.39

9.27um 10.35um abs=2.30

Figure 6. Resultant isopropyl vapor absorptance spectra measured simultaneous with the photoacoustic portion 

of the study (path length 10 cm). Note that for the laser wavelength chosen that the absorptance are 

very similar in magnitude, i.e., 8.72 μm=2.39, 9.27 μm=2.29, 10.35 μm=2.30. 
8 

Figure 7. New photoacoustic cell design(s). Design a is based on the multiple reflections inherent in integrating 

spheres. Design b maintains a cylindrical column of vapor (into the page) but now the lasers are brought 

into the cell perpendicular to the flow in order to limit unwanted laser attenuation. In addition, the right cell 

shows possible enhancement of the signal by incorporating resonate effects by spacing opposing mirrors at 

distances on the order of quarter-multiples of the wavelength of sound associated with each modulation 

frequency. 



4

n 

Spherical void 

(integrating sphere) 

Cylindrical flow-through 

a. b.

gold surface 

gold mirrors 

electret 

Top view 
9 

4. References 

1. Kerr, E. L.; Attwood, J. G. The Laser Illuminated Absorptivity Spectrophone: A Method for 

Measurement of Weak Absortivity in Gases at Laser Wavelengths. Appl. Opt. 1968, (7), 

915. 

2. Bruce, C. W.; Gurton, K. P.; Stromberg, T. F. Trans-Spectral Absorption and Scattering of 

Electromagnetic Radiation by Diesel Soot. Appl. Opt. 1991, 30 (12), 1537–1546. 

3. Kreutzer, L. B. Ultralow Gas Concentration Infrared Absorption Spectroscopy. J. Appl. 

Phys. 1971, 42, 2934. 

4. Gurton, K.; Felton, M.; Dahmani, R.; Ligon D. In Situ Aerosol Spectroscopy for a Variety of 

Nerve Agent Simulants Using Flow-through Photoacoustics. Applied Optics September 

2007, 46 (25), 6323–6329. 

5. Gurton, K.; Dahmani, R.; Ligon, D.; Bronk, B. In Situ Measurement of the optical 

Absorption and Extinction for Chemical and Biologically Derived Aerosols Using Flowthrough Photoacoustics. J. of Applied Optics July 2005, 44 (19), 4001–4006. 

6. Helm, D.; Naumann, D. Identification of Some Bacterial Cell Components by FT-IR 

Spectroscopy. FEMS Microbiol. Lett. 1995, 126, 75–80. 

7. Gurton, Kristan P.; Ligon, David; Kvavilashvili, Ramaz. Measured Infrared Spectral 

Extinction for Aerosolized Bacillus subtilis var. niger endospores from 3 to 13 m. Applied 

Optics-OT September 2001, 40 (25), 4443–4448. 
10 

List of Symbols, Abbreviations, and Acronyms 

AC alternating current 

ARL U.S. Army Research Laboratory 

CO2 carbon dioxide 

DEMP diethyl phosphonate 

DIMP diisopropyl phosphonate 

DMMP dimethyl methylphosphonate 

FET field-effect transistor 

FFT fast Fourier transform 

FTIR Fourier Transform IR 

IR infrared 

LN2 liquid nitrogen 

LWIR long-wave IR 

PbS/PbSe lead salt 

QC Quantum Cascade 

STP standard temperature and pressure 
11 

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