differential absorption lidar co_2 laser system for remote sensing of tatp related gases

Differential absorption lidar CO2 laser system forremote sensing of TATP related gases

Avishekh Pal,1 C. Douglas Clark,2 Michael Sigman,2 and Dennis K. Killinger1,*1University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, USA

2University of Central Florida, 4000 Central Florida Boulevard, Orlando, Florida 32816, USA

*Corresponding author: [email protected]

Received 19 August 2008; accepted 23 October 2008;posted 5 December 2008 (Doc. ID 100259); published 1 January 2009

A CW tunable 10:6 μm CO2 laser differential absorption lidar (DIAL) system has been developed, for thefirst time to our knowledge, for the remote sensing of triacetone triperoxide (TATP) gas vapors, whichhave strong absorption lines at several wavelengths, including 3.3, 8.3, and 10:6 μm. The DIAL laserbeam was transmitted through an enclosed absorption cell containing TATP or SF6, and backscatteredreturns were measured from a retroreflector array target at ranges of 5–100m. DIAL sensitivity forthe detection of TATP was about 0:5ng=μl [52parts in106ðppmÞ] for a 0:3m path. © 2009 OpticalSociety of America

OCIS codes: 010.280, 010.3640, 280.1910.

1. Introduction

The differential absorption lidar (DIAL) techniquehas proved to be a sensitive method for the longrange remote sensing of molecular constituents inthe ambient atmosphere [1]. DIAL systems operat-ing in the infrared region have been used formonitoring the presence of major atmospheric consti-tuents such as CO as well as trace constituents suchas C2H2 and various types of hazardous chemicalagents and land mines [1–9]. We report on the devel-opment and use of a CO2 DIAL system for the poten-tial remote sensing of triacetone triperoxide (TATP)vapors. Our studies are similar to recent laser detec-tion studies of TATP but are directed more towardremote sensing applications than point detection[10,11]. TATP is notable as an explosive that doesnot contain nitrogen but does have a significant va-por pressure [12–16].Our experiments were carried out using a CW CO2

laser DIAL system in conjunction with a large opticalcell that contained the target gas. Lidar returns wereobtained from laser beams that passed through the

cell and were reflected from a retroreflector arrayplaced at a range of 5–100m. The experimentalresults established initial experimental capabilityof a CW CO2 DIAL lidar system to detect TATP.DIAL measurements of SF6 were also made for sys-tem calibration purposes. Our results indicate thatTATP can be detected by a remote CO2 DIAL systemat concentrations near the vapor pressure of TATPbut that the TATP concentration measured may bevariable due to chemical instabilities, surface-relatedabsorption, and recrystallization of the TATP vaporson the windows of our optical absorption cell. Our in-itial results are positive but also point toward theneed for better control of the TATP concentrationto better quantify our DIAL remote sensing sensitiv-ity measurements.

2. Introduction to TATP

Triacetone triperoxide (TATP or TCAP), also knownas acetone peroxide, is an organic peroxide explosivewith a molecular weight of 222:24 g=mole and wasdiscovered in 1895 by Richard Wolffenstein [12–16].The vapor pressure of TATP at 25 °C is 7Pa [i.e.,0:052Torr, 0:62ng=μl, or 65parts in 106ðppmÞ in anatmospheric background at STP], which is about14,000 times that of other explosives such as TNT

0003-6935/09/04B145-06$15.00/0© 2009 Optical Society of America

1 February 2009 / Vol. 48, No. 4 / APPLIED OPTICS B145

[15]. TATP also has a very high sublimation rate andloses about 6.5% of its volume into the vapor phase ina 24 h period and 68.6% during 14 days [16]. It is thishigh vapor pressure that may produce a gas plume ofTATP surrounding a large sample of solid TATP andis thus susceptible for detection using a DIAL lidarsystem.The absorption spectrum of solid state and vapor

phase TATP was measured using Fourier transforminfrared spectrometers (Perkin Elmer, Spectrum 1)at a resolution of 4 cm−1. For the vapor phase mea-surements, a 5 cm long optical absorption cell wasused at a temperature of 28 °C. The concentrationof TATP in the vapor phase was estimated to be8:28Pa (0:062Torr) under these conditions. Figure 1shows the measured vapor phase FTIR spectrum ofTATP. As can be seen, the absorption lines of TATPare fairly strong with lines occurring near 8.36, 7.29,3.38, and near 10:59 μm. These results are similarto spectra obtained recently by Bauer et al. [10].For example, the absorbance, A, of TATP in Fig. 2at 10:59 μm is about 0.0024. Relating the absor-bance, A, to the transmission, T, as A ¼ −log10ðTÞ,and noting the Beers–Lambert relation of T ¼expð−PgαLÞ, one calculates that the attenuation coef-ficient, α, is 0:014=Pa-m (or 1:75=Torr-m), where Pg isthe partial pressure of the absorbing gas in Pa and Lis the path length in meters. For the solid phase ab-sorption measurements, a FTIR-attenuated total re-flectance (a microscope was used. The solid statespectral signature for the TATP was consistent withthose obtained for the vapor phase TATP.It is informative to predict the differential absorp-

tion due to a remote 1m diameter cloud of TATP; it isanticipated that a large sample of TATP may besurrounded by a saturated vapor cloud of TATP ap-proximately 1m in diameter, although direct experi-mental measurements of the cloud size still need tobe conducted. For a 1m diameter cloud of TATP hav-ing a saturated vapor phase concentration of about7Pa at 25 °C, the absorption for a two-way DIALpath would be 31% for 3:32 μm, 73% for 8:36 μm,and 17% for the 10:59 μm line. As such, a cloud of

TATP could potentially be detected using a DIALsystem at wavelengths near 7.29, 8.4, 3.3, 10.6, or11:2 μm. We chose to use the lines near 10:6 μm sincethey coincide with the laser lines of a tunable CO2laser, which was available in our laboratory andmore importantly this wavelength region has lessspectral interferences from other TATP mixing sol-vents such as acetone as reported by other groups[10,11]. It should be added that other tunable lasersources such as a pulsed optical parametric oscillatoror a CW quantum cascade laser could be used forDIAL measurements of TATP at these other wave-lengths; however, a preference would be for use ofa pulsed high-power laser as opposed to a CW laserfor longer DIAL and lidar ranges.

3. Experimental Setup

We constructed a laboratory DIAL lidar system usinga grating tuned CW CO2 laser. A schematic of theDIAL lidar system is given in Fig. 2. The gratingtuned CW CO2 laser (Edinburgh Instruments ModelWL-86T) was used for producing the line-tunableemission near 10:6 μm. The laser had a CW power le-vel of about 1W, TEMoo beam mode, a linewidth ofabout 0:03 cm−1, and could be tuned over about 40different lines from 9:7 μm to about 11:2 μm. The out-put from the laser was sent through an optical chop-per (SRS Model SR540), and directed via beamsplitters toward a CO2 laser line spectrum analyzer(Optical Engineering Model LSA 16-A), a pyroelec-tric detector (Eltec Model 420-0-1491) for power mon-itoring, and through either an absorption cellcontaining SF6 or through a test absorption cell con-taining the TATP gas sample; the SF6 cell was 5 cmlong with ZnSe windows, while the test absoprtioncell was 1:75m long with Mylar windows and wasconstructed using PVC pipe. The laser beam wassampled and detected after passage through thecells, but the major portion of the beams was directedvia mirrors to a large beam steering mirror towardtargets outside our laboratory window or toward aretroreflector array target; the retroreflector arraytarget consisted of a grouping of 30 3 in: (8 cm) dia-meter gold-coated retroreflectors.

The backscattered lidar returns were collected by a16 in: (41 cm) diameter telescope (Meade Model DS16) and detected by a liquid nitrogen cooled merucrycadmium telluride (MCT) detector (Electro OpticalSystems Inc., Model MCT10-040). The chopped sig-nal was detected using a lock-in amplifier (SRS, Mod-el SR810 DSP) and interfaced to a computer with aLabVIEW software program; a chopper frequency of330Hz was usually used in our experiments.

A photograph of our setup is shown in Fig. 3. DIALexperiments were conducted by tuning the CO2 laserwavelength to an “offline” wavelength and then to an“online” wavelength and deducing the concentrationof the target gas from the differential absorption ordifferent intensities of the online and offline lidar re-turned signals.

Fig. 1. (Color online) Absorbance spectrum for vapor phase TATP(cell path length of 5 cm; TATP concentration/partial pressure8:28Pa; 28 °C).

B146 APPLIED OPTICS / Vol. 48, No. 4 / 1 February 2009

4. DIAL Calibration with SF6

SF6 was used as the calibration gas for our DIAL sys-tem because of its strong absorption lines near10:6 μm and its past use by various groups [17–20]. A5 cm long aluminum cell with ZnSe windows wasused and was filled with 0.2 or 0:5Torr of SF6. Fig-ure 4 shows the qualitative transmission spectra ofSF6 gas as a function of wavelength between 10and 11 μm obtained from the NIST database [21].As can be seen the Pð24Þ line at 10:632 μm can beused as the online wavelength, and the Rð24Þ lineat 10:220 μm can be used as the offline wavelengthfor the DIAL measurements.The DIAL beam was directed through the SF6 cell

and then toward the retroreflector array targetplaced at a range of 100m outside our lab window.

Lidar returns as a function of time for each of theon/off resonance CO2 wavelengths were recorded andare shown in Figs. 5 and 6 for two different SF6 con-centrations. As can be seen in Fig. 5, there was about80% transmission (i.e., 20% absorption) for the caseof SF6 gas at 0:2Torr and a 5 cm path length. Simi-larly, Fig. 6 shows about 50% transmission (i.e., 50%absorption) for the case of 0:5Torr of SF6 gas and a5 cm path length. The theoretical or expected absorp-tion values are shown as dotted lines in the figures,using an attenuation coefficient value of 26=Torr-mfor the 10Pð24Þ line at 10:632 μm obtained from otherstudies [20]. As can be seen, our DIALmeasurementsare consistent with these previously measured va-lues. Using the Beer–Lambert equation and Figs. 5and 6 the attenuation coefficient α was calculated

Fig. 2. (Color online) Schematic of laboratory DIAL/lidar setup.

Fig. 3. (Color online) Photograph of laboratory CO2 laser DIALsystem.

Fig. 4. (Color online) Qualitative transmission spectra of gaseousSF6 as a function of wavelength near the Rð24Þ and Pð24Þ CO2 la-ser lines.


and found to be 27:7=Torr-m and 22:3=Torr-m for 0.5and 0:2Torr concentrations of SF6, respectively.

5. DIAL Lidar Detection of TATP Gas

The CO2 DIAL system was used with TATP gas inthe test absorption cell. In this case, the test absorp-tion cell in Fig. 2 was a 175 cm long PVC plastic pipecell with Mylar windows to transmit the CO2 wave-lengths. The cell had injection ports on the side forthe delivery of TATP into the cell. The expectedTATP transmission spectrum for a 1:75m path ofTATP vapor at a concentration or partial pressureof 4:3Pa was calculated from the FTIR spectra ofTATP of Fig. 1 and is shown in Fig. 7. As can be seenfrom Fig. 7 the absorption between the offline andthe online resonance DIAL wavelength should beabout 10%.Initial DIAL experiments were conducted by pas-

sing the CO2 laser beam through the 1:75m test ab-soprtion cell, toward the retroreflector array target,and detecting the backscatter using the telescopeand liquid nitrogen cooledMCT detector. In this case,the target was placed at a range of 5m to increase thesignal-to-noise ratio (SNR) due to losses experienced

directing the beam through the test absorption celldue to increased scatter of the Mylar windows andpossible alignment effects on the light collection effi-ciency of the system. A small sample of TATP (about1mg) was prepared and left in a chloroform (CHCl3)solvent. The sample was then injected via a syringeinto the absorption cell and allowed tomix for severalminutes. During this time, the CO2 laser was operat-ing on the “on-resonance” Pð24Þ line so that absorp-tion due to TATP could be observed. However, in thiscase negligible absorption was observed. It was de-termined that the TATP was difficult to disperseevenly throughout the entire cell and that possiblystratification could have occurred even though large(>200 cc) syringe pumps were used to circulate andmix the gas inside the cell. Calculations indicate that1mg of TATP within the cell (volume of 17; 500 cm3)would produce a concentration of about 0:05ng=μland produce only about 2% absorption of the single-pass online beam. Our measurements are consistentwith these limitations. It was determined that use ofa larger sample size was not prudent because largesizes are often unstable. It should be added that theconversion between partial pressure Pg and concen-tration is 1Pa ¼ 0:088ng=μl at 25 °C and 1Pa ¼0:077ng=μl at 70 °C using the ideal gas law.

To increase the TATP vapor pressure and the op-tical path length inside the absorption cell a second(different) glass absorption cell was used that couldbe heated so that the vapor pressure and the concen-tration of TATP were increased; the cell was 30 cmlong and had Mylar windows. The cell was heatedto a temperature near 70 °C by using heating tapewrapped around the 20 cm central portion of the cell.A 200 μl sample of TATP solution (concentration of1 μg of TATP in 1 μl of CHCl3 solution) was injectedinto the cell, and the “on-resonance” DIAL laserbeam transmission through the cell was recorded.Figure 8 shows our measured transmission signalas a function of time over the period when the TATPwas injected. As can be seen, there was about 10%

Fig. 5. (Color online) DIAL transmission measurements as afunction of time for 0:2Torr of SF6 (5 cm cell) and lidar target rangeof 100m. The predicted absorption for the online Pð24Þ line isshown as a dotted line.

Fig. 6. (Color online) DIAL transmission measurements as afunction of time for 0:5Torr of SF6 (5 cm cell) and lidar target rangeof 100m. The predicted absorption for the online Pð24Þ line isshown as a dotted line.

Fig. 7. (Color online) Expected transmission spectra of TATP fora 175 cm path length and concentration of 4:3Pa near the Rð24Þand Pð24Þ CO2 laser lines.


absorption when the gas was injected; our measure-ment uncertainty was about 3% in the transmissionvalues most probably due to mechanical changes inthe optical alignment of the laser beam and cell, andetalon effects of the windows and beam splitters.Such measurement errors can be reduced by furtherrefinement of the DIAL optical system.Using the attenuation coefficient of TATP near

10:63 μm and our DIAL data in Figs. 8 and 9 showsthe measured concentration of TATP as the TATPwas injected. The measurements indicated a TATPconcentration of about 1:8ng=μl (or 26:4Pa) when thesample was first injected, with sensitivity (noise leveluncertainty) of about 0:5ng=μl , or about 52ppm. Inorder to better quantify the concentration inside thecell, a small sample of the gas inside the cell was ob-tained using a precession volumetric gas-tight syr-inge, and the sample was injected into a calibratedion mobility spectrometer (IMS, Smith Detection400B) operating in the positive ion mode. Two differ-ent IMS readings were obtained over a period ofabout 20 min and yielded values of 0.72 and0:99ng=μl. Our DIAL measured value is consistentwith that measured by the IMS taking into accountsome variability after the time of introduction of thesample into the cell. These values can be compared tothe theoretical concentrations within the cell of200 μg injected into the cell volume of 135 cm3, whichyields a maximum concentration of about 1:5ng=μl.Since our DIAL related measurement is slightlyhigher than this value, it is possible that some stra-tification or layering of the TATP vapor could haveoccurred within the cell, thus leading to an increasedconcentration being measured by the relatively smalllaser beam as it traversed the cell.It was noticed that there were often long term (few

minutes) temporal changes in the DIAL signal. Theobserved variation in transmission may be due tochemical induced changes in TATP with tempera-ture and time as reported by other groups [12–16].Upon further examination of the cell, it was foundthat TATP crystals were being formed upon the

unheated Mylar windows. Attempts to heat the win-dows slightly were not successful. We are now work-ing on a new absorption cell with ZnSe windows thatcan be heated uniformly, including the windows. Weanticipate that such a cell will be able to better sta-bilize the concentration of TATP within the absorp-tion cell. Finally, field tests are being planned to testour CO2 DIAL system for detection of a potentialTATP plume surrounding a large sample [approxi-mately 1 lb (0:45kg)] of TATP.

6. Conclusion

A tunable CO2 DIAL system has been developed forthe first time to our knowledge for the potential de-tection of TATP gas clouds. The system has beenused to measure gas samples of SF6 and has showninitial absorption measurements of samples of TATPcontained within an enclosed optical test absorptioncell. DIAL/lidar returns from a remote retroreflectortarget array were used for the DIAL measurementsafter passage through the laboratory cell containingthe TATP gas. DIAL measured concentrationsagreed well with those obtained using a calibratedionmobility spectrometer. DIAL detection sensitivityof the TATP gas concentration in the cell was about0:5ng=μl. However, the concentration of TATP wasfound to be unstable over long periods of time duepossibly to reabsorption and crystallization of theTATP vapors on the absorption cell windows. Aheated cell partially mitigated these effects, butfurther detailed studies to control the TATP chemis-try are required to better quantify our results. Weplan to extend these preliminary one-way DIALmeasurements to a two-way DIAL measurementby placing the absorption cell outside the laboratorywindow, if the TATP concentration within an exter-nal cell can be better controlled and known. Inaddition, a more optimized high-power pulsed CO2laser DIAL system could be used for greater detec-tion ranges, and pulsed DIAL systems near 3.3,7.3, and 8:4 μm could also be used for TATP detec-tion. It is anticipated that quantum cascade lasers

Fig. 8. (Color online) Online DIAL transmission as a function oftime showing injection of TATP into a heated 30 cm long absorp-tion cell.

Fig. 9. (Color online) DIAL measured TATP concentration insidea heated 30 cm long absorption cell as a function of time.


and optical parametric oscillator lasers could also beused at these other wavelengths, with even greatersensitivity than our work shown here since the ab-sorption of TATP is stronger at 3.3, 7.3, and 8:4 μmthan at 10:6 μm, and could potentially have greaterdetection sensitivity and lower measurement errors.However, the ease of use, linewidths required, andwavelength stability would need to be optimized.We plan to study the use of these other lasers forTATP DIAL detection.

This work was partially supported by U.S. AirForce Office of Scientific Research (AFOSR) grantFA96550-06-1-0363.

References1. E. R. Murray and J. E. Laan, “Remote measurement of ethy-

lene using a CO2 differential-absorption lidar,” Appl. Opt. 17,814–817 (1978).

2. J. R. Quagliano, P. O. Stoutland, and R. R. Petrin, “Quantita-tive chemical identification of four gases in remote infrared(9–11 μm) differential absorption lidar experiments,” Appl.Opt. 36, 1915–1927 (1997).

3. D. K. Killinger and N. Menyuk, “Laser remote sensing of theatmosphere,” Science 235, 37–45 (1987).

4. Markus W. Sigrist, Air Monitoring by Spectroscopic Techni-ques (Wiley, 1994).

5. W. B. Grant, “Lidar for atmospheric and hydrospheric stu-dies,” in Tunable Laser Applications, F. J. Duarte, ed. (MarcelDekker, 1995), pp. 213–305.

6. M. J. T. Milton, T. D. Gardiner, F. Molero, and J. Galech,“Injection-seeded optical parametric oscillator for range-resolved DIAL measurements of atmospheric methane,”Opt. Commun. 142, 153–160 (1997).

7. J. A. Shaw, N. L. Seldomridge, D. L. Dunkle, P. W. Nugent,L. H. Spangler, J. J. Bromenshenk, C. B. Henderson,J. H. Churnside, and J. J. Wilson, “Polarization lidar measure-ments of honey bees in flight for locating land mines,” Opt.Express 13, 5853–5863 (2005).

8. V. V. Vaicikauskas, V. Kabelka, Z. Kuprionis, V. Svedas,M. Kaucikas, and E. K. Maldutis, “Infrared DIAL system

for remote sensing of hazardous chemical agents,” Proc. SPIE5613, 21–28 (2004).

9. V. Vaicikauskas, M. Kaucikas, V. Svedas, and Z. Kuprionis,“Mobile spectroscopic system for trace gas detection usinga tunable mid-IR laser,” Rev. Sci. Instrum. 78, 023106(2007).

10. C. Bauer, U. Willer, A. Sharma, and W. Schade, “A new photo-nic sensor device for TATP detection,” in Laser Applicationsfor Chemical, Security and Environmental Analysis (OpticalSociety of America, 2008), paper LThB3.

11. I. Dunayevskiy, A. Tsekoun, M. Prasanna, Rowel Go,C. Kumar, and N. Patel, “High-sensitivity detection of triace-tone triperoxide (TATP) and its precursor acetone,” Appl. Opt.46, 6397–6406 (2007).

12. R. Matyas, J. Pachman, and H.-G. Ang, “Study of TATP: spon-taneous transformation of TATP to DADP,” Propellants Ex-plos. Pyrotech. 33, 89–91 (2008).

13. D. Armitt, P. Zimmermann, and S. Ellis-Steinborner, “Gaschromatography/mass spectrometry analysis of triacetone tri-peroxide (TATP) degradation products,” Rapid Commun.Mass Spectrum 22, 950–958 (2008).

14. R. Matyas and J. Pachman, “Thermal stability of triacetonetriperoxide,” Sci. Tech. Energetic Materials 68, 111–116(2007).

15. J. C. Oxyley, L. James, J. L.Smith, K. Shinde, andJ. Morgan, “Determination of the vapor density of triacetonetriperoxide (TATP) using a gas chromatography headspacetechnique,” Propellants Explos., Pyrotech. 30, 127–130 (2005).

16. A. J. Bellamy, “Triacetone triperoxide: its chemicaldestruc-tion,” J. Forensic Sci. 44, 603–608 (1999).

17. C. K. N. Patel and R. E. Slusher, “Self- induced transpar-ency in gases,” Phys. Rev. Lett. 19, 1019–1022 (1967).

18. L. Lyman, G. R. Anderson, A. Fisher, and B. J. Feldman, “Ab-sorption of pulsed CO2—laser radiation by SF6 at 140K,”Opt.Lett. 3, 238–240 (1978).

19. H. R. Carlon, “Infrared absorption coefficients (3–15 μm) forsulphur hexafluoride (SF6) and freon (CCl2F2),” Appl. Opt.18, 1474–1475 (1979).

20. E. E. Uthe, “Airborne CO2DIAL measurement of atmospherictracer gas concentration distributions,” Appl. Opt. 25, 2492–2498 (1986).

21. NIST Standard Reference Database 79: Infrared Database.


differential absorption lidar co_2 laser system for remote sensing of tatp related gases

Documents