Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5066 

ARL-TR-2387 February 2001 

Detection of Energetic Materials by Laser 

Photofragmentation/Fragment Detection 

and Pyrolysis/Laser-Induced Fluorescence 

Rosario C. Sausa and Vaidhianat Swayambunathan 

Weapons and Materials Research Directorate, ARL 

Gurbax Singh 

University of Maryland Eastern Shore, Department of Natural Sciences 

. 

Approved for public release; distribution is uniimited. 
Abstract 

Trace concentrations of energetic materials such as 2,4,6-trinitrotoluene (TNT), 

pentaerythritol tetranitrate (PETN), and hexahydro-1;3,5-trinitro-s-triazine (RDX) are 

detected by laser photofragmentationlfragment detection (PF/FD) spectrometry. In this 

technique, a single laser operating near 227 mn photofragtnents the parent molecule and 

facilitates the detection of the characteristic NO fragment by means of its A2C+-X2 II 

(0,O) transitions near 227 nm. Fragment detection is accomplished by 

resonance-enhanced multiphoton ionization (REMPI) with miniature electrodes and by 

laser-induced fluorescence (LlF) with a photodetector. Experiments are also conducted in 

the visible region using 453.85-11111 radiation for photofragmentation and fragment 

detection. Sand samples contaminated with PETN and RDX are analyzed by a 

pyrolysis/LIP technique, which involves pyrolysis of the energetic material with 

subsequent detection of the pyrolysis products NO and NO2 by LIF and PF/LIF, 

respectively, near 227 nm. The application of these techniques to the trace analysis of 

TNT, PETN, and RDX at ambient pressure in room air is demonstrated with limits of 

detection (S/N = 3) in the range of low parts-per-billion to parts-per-million for a 20-s 

integration time with 10-120 N of laser energy at 226.8 nm and - 5 mJ at 453.85 nm. 

An increase in detection sensitivity is projected with an increase in laser energy and an 

improved system design. The analytical merits of these techniques are discussed and 

compared to other laser-based techniques. 
I ’ 

Acknowledgments 

We thank R. Pesce-Rodriquez and J. Morris of the U.S. Army Research Laboratory (ARL) and 

E. Cespedes of the U.S. Army Corps of Engineers, Waterway Experiment Station, for providing us 

with the samples. We also thank J. Simeonsson of the University of Iowa, R. Pesce-Rodriquez, 

E. Cespedes, and B. Homan of ARL for many helpful discussions. Support from the National 

Research Council/ARL Senior Research Associateship Program (V. Swayambunathan) and Summer 

Faculty Program (G. Singh) is gratefully acknowledged. 
INTENTIONALLY LEFT BLANK. 

iv 
I ’ 

1. 

2. 

3. Results and Discussion . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....t.._.....- . . . . . . . . .._.._... *._ _. . .._ .. . . . . . 6 

3.1 

3.2 

3.3 

4. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +. . . . . . . . . . . . . . . . . . . . . . . . . . . _ 18 

5. References .. ...-....-*.. 11 --...* * -.* + .. . . . . . . . . ..*.................................................... 21 

Table of Contents 

Acknowledgments.... ,....,........................_.......*.. I.*..__.. _. . .._...... _ ..+__..._... I ..,.,........... iii 

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -.. . . . . .-.... vii 

List of Tables .. . ..*..**..~...~..~..*...~...........~.~~....~.........~....~~....~~.~......~~...~~......*.*.~.~..~..~* Vii 

Ultraviolet PF/REMPI ...................................................................................... 

Ultraviolet PF/LJF and Pyrolysis/LIF ................................................................ 

Visible PF/REMPI and PyrolysisAJF.............................................................. 

6 

10 

16 

Report Documentation Page ~.....-..*1~,..1..~.......*...~.~..~..,...,~...........~.*........~.~.~...~.~~. 27 

V 
i 

. 

bJTENTIONALLYLEFTBLANK. 

vi 
List of Figures 

Figure 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

Schematic of the Experimental Apparatus: Second Harmonic Generator (SHG), 

Computer (PC), Photomultiplier Tube @‘MT), and Digital (DIG) . .._.......__............... 

NO Spectra: (a) Computer Simulation, (b) PyrolysisUF of PETN, and 

(c) PFREMPI of PETN ____.~~................__..._._________....~~..........~.....____..._.._____________~___ 

PF/REMPI Response Plots of PETN With (a) -100 fl of 227-nm Laser Radiation, 

and (b) -5 rnJ of 454-nm Laser Radiation . . . . . . . . . ..f..............._._.___.f___............f............. 

227-nm LIF Emission Spectra of 0.1% NO in Nz at Room Temperature and NO 

From PETN _.........................________........................ + .. . . . ..________....... *..* . . . ..f.......f....f..ff..._ 

Intensity vs. Time Plot of NO From 20 ppm of PETN in Sand .__...._____....._..__._...._f._f 

PyrolysisUF Response Curves of NO From RDX (m) and PETN (0) _.__..___......._.... 

Two-Photon LIF Excitation Spectrum of NO From (a) PETN and (b) 0.1% NO 

in Nz ._. . . . . . . . . . . ..~~.~..............~..~....*............*.........~..~~~.~~*..~~....***~...................~....~..~.~.~..~.. 

List of Tables 

& 

3 

7 

9 

14 

15 

15 

17 

vii 

Table & 

1. Limits of Detection of PETN, RDX, and TNT _____..................f....f..f......_.._.___._._.__.ff.. 10 
hTENTIONALLY LEFT BLANK. 

. . . 

Vlll 
1. Introduction 

. The application of laser-based methods for detecting trace levels of energetic materials has been 

an active area of research in recent years [l-6]. The trace detection of these materials finds 

important military and civilian applications in areas such as mine sweeping operations, aviation 

security, forensic science, and site characterization for cleanup and remediation measures. The 

detection sensitivities needed range from low parts-per-trillion (ppt) for mine sweeping operations to 

hundreds of parts-per-million @pm) for cleanup of contaminated sites. Although several methods, 

including pyrolysis with subsequent fragment detection spectroscopy, are available for the analysis 

of these molecules [2], research that is focused on the development of laser-based methods has 

become increasingly popular because of the high selectivity and sensitivity and their real-time and in 

situ capabilities [1,3]. 

I 

A laser-based technique for energetic material detection that has received considerable attention 

in recent years is laser photofragrnenation&agment detection (PF/FD) spectrometry [1,3] m In this 

technique, an ultraviolet laser both photofragments the energetic molecule and facilitates the 

detection of the characteristic NO photofragment by resonance-enhanced multiphoton ionization 

(REMPI) or laser-induced fluorescence &IF). The NO fragment is characteristic of the NO2 

functional group, which is present in these parent molecules and contributes to the selectivity of the 

analytical method. The energetic material concentration is inferred from the intensity of the NO 

FCIMPI or LIF signal. 

i 

Hexahydro- 1,3,5-trinitro-s-bkzine (RDX), 2,4,6-knitrotohiene (TNT), and pentaerythritol 

tetranitrate (PETN) are three of the most powerful and most brisant energetic materials. RDX has 

been detected by time-of-flight mass spectrometry for analytical applications by the 227-run [7,8] or 

193~nm [9] laser photofragmenation of the vapor with subsequent REMPI detection of NO by means 

of its A2c’- X2n (0,O) transitions near 227 run or its A2x+-X2n (3,0), B2n-X2n (7,0), and 

D”C”- X2n (0,l) transitions at 193 nm. TNT has also been detected by the 227”nm laser 

photofragmentation of the vapor, with subsequent REMPI [7] detection of NO using mass 

spectrometry or by LIF [ 10,l l] detection of NO using photometry. Limits of detection range from 

1 
ppt to ppm, depending on the energetic material, laser wavelength, laser energy, and sampling 

conditions. 

PETN has not been detected by PF/FD; although RDX and TNT have been detected by PF/FD, 

this was done under vacuum conditions with ultraviolet lasers and bulky time-of-flight mass 

spectrometers- PF/FD detection of these materials at atmospheric pressure in room air is a 

prerequisite if such a technique is to be used in the field. Also, visible lasers could be potentially 

more useful than ultraviolet lasers for field applications, particularly those applications that require 

the laser radiation to be transmitted through optical fibers at long distances, such as in the analysis of 

contaminated underground soil at depths up to 30 m using laser-based, cone penetrometer systems 

[ 121. Although time-of-flight mass spectrometers offer a high degree of selectivity, they are bulky 

and cannot be incorporated in many small devices (such as hand-held sensors and cone penetrometer 

heads), as can miniature electrodes. 

In this-report, we present PF/FD studies on TNT, RDX, and PETN for real-time and in situ 

detection applications. The detection of the energetic materials is accomplished by both PFREMPI 

and PF/UF at atmospheric pressure in room air. A single laser, operating at 227 run or 454 nm, 

photofragments the parent molecule and also facilitates the detection of the characteristic NO 

photofragment by REl’@?I with miniature electrodes or by LIF with a photodetector. Sand samples 

doped with PETN and RDX are analyzed by a pyrolysis/LIF technique, in which the pyrolysis 

products NO and NO2 are detected by LIF and PF/LIF, respectively, near 227 nm and 454 m-n. The 

analytical utility of these techniques is demonstrated on these energetic compounds and their limits 

of .detection (LODs) are determined. 

2. Experimental 

A schematic of the experimental apparatus is presented in Figure 1, Briefly, a compact 1 O-Hz 

Neodynium-Yttrium-Aluminum-Garnet (Nd-YAG) pumped dye laser (Continuum Surelite Ill and 

Lumonics HyperDye 300), with a frequency doubling beta barium borate crystal, provideslO- mJ 

2 
w 

Oven (25-200 ‘C) 0 Lens 

Monochromator 

Energy 

Meter 

BOXCAR 

DIG. SCOPE 

Figure 1. Schematic of the Experimental Apparatus: Second Harmonic Generator (SHG), Computer (PC), 

Photomultiplier Tube (PMT), and DigitaI (DIG). 
of tunable ultraviolet (W) radiation in the 225-228-nm wavelength range with a pulse width of 

- 6 ns. The REMPI and LIF experiments were performed at ambient pressure in room air on 

samples of pure PETN, RDX, and TNT, or on samples of sand doped with known concentrations of 

the energetic material. The sample cell consists of a miniature, electrically-heated oven, which was 

designed and built at the US. Army Research Laboratory. It housed a removable small glass cup of 

c 5-mm diameter and - lo-mm length that contained milligram quantities of the samples. The 

temperature of the oven was precisely controlled (51 K) with a thermostat and ranged from room 

temperature to 473 K. The output vapors, photolysis products, or both were directed to the entrance 

of a pipe located approximately 20 cm from the top of the oven. The pipe led to an exhaust hood 

having a linear air velocity of - 50 cm/s (Kurtz Air Velocity, Model 441s). The laser beam was 

steered with quartz prisms and focused with a 2%cm focal-length lens at a distance of 2-3 mm above 

the tip of the sample tube. At the sampling point, the air-flow velocity was less than ~0.05 cm/s, and 

the LlF and REMPI signal intensities were the same with and without the pipe. 

REMPI detection was accomplished with a pair of miniature electrodes (1.25 cm/edge) 

positioned H 2-3 mm above the center of the sample cup. We focused the laser beam at the center of 

the electrodes to optimize ion collection and minimize the background signal from photoelectron 

emission. The ion signal from the electrodes was amplified with a current arnplifier’(Keithley Model 

427), which was set at a gain and time constant of lo6 V/A and 0.01 ms, respectively. The ion signal 

increased linearly with increasing voltage across the electrodes (O-l 20 V) for all three compounds. 

A nominal voltage of 50 V was used for the REMPI spectral recordings and LOD measurements. 

For the LIF experiments, the fluorescence was collected with a &-cm focal-length lens at an angle 

normal to the laser excitation beam and focused onto the slits of a O-125- or 0.5-m monochromator 

(Jarrell Ash or Spex-500) equipped with a photomultiplier tube (Hamamatsu R955). A 250-nm 

interference filter (FWHM, 30 nm) was also used for signal discrimination for minimizing the 

intensity of the scattered light. We recorded the excitation spectra by setting the monochromator at 

237 nm, corresponding to the NO A2c”- X2n (0,l) band, and scanning the laser at a rate of 0.003 or 

0.005 rim/s; we recorded the emission spectra by setting the excitation laser near 227 nm and 

4 
(1) 

(2) 

scanning the monochromator at a rate of 0.1 run/s. The alignment of the detection optics was 

optimized using the strong LlF signal from PETN. 

We recorded the REMPI and LIF excitation spectra with lo-shot averaging, using a boxcar 

integrator (Stanford Research Systems, SRS-250) interfaced to a personal computer. A 125-MHz 

digital oscilloscope (LeCroy, Model 9400) was employed for both signal display and LOD 

measurements. The LODs were obtained by 200-shot averaging of the REMPI or LIF signal at its 

maximum with three times the standard deviation of the background noise, which was determined 

from twenty measurements under the same experimental conditions with no sample in the glass cup. 

Our colleagues at the U.S. Army Research Laboratory provided us with the PETN, TNT, and 

RDX samples. The PETN and TNT samples were obtained from Ensign and Bickford, and Eastman 

Chemical Company, respectively, whereas the RDX sample (>99% pure) was obtained from 

recrystallizing an RDX composite gun propellant (M43) twice with an American Chemical 

Society-specified reagent acetone. Our colleagues at the U.S. Army Corps of Engineers, Waterway 

Experiment Station, provided us with the soil samples contaminated with known concentrations of 

RDX. Analysis for energetic material content in these soils was performed according to 

U.S. Environmental Agency (SW-846) method 8330. We prepared sand samples containing PETN 

and RDX in the laboratory, and then analyzed them by high-pressure liquid chromatography 

(ISCCO, Model 2350, Spectra Physics, Model Focus-SMSOOO). 

Concentrations of the energetic materials as a function of temperatures were determined from the 

global vapor-pressure expressions reported by Dionne et al. [ 131. These expressions were derived 

from both a least-squares fit of their vapor-pressure data, and of data reported by previous workers, 

which they critically reviewed. The Clausius-Clapeyron expressions for PETN, RDX, and TNT in 

the temperature range of 278-417 K are as follows: 

Log P[PETN@pt)] = -7243/T(K) + 25.56, 

Log P[RDX@pt)] = -6473/T(K) + 22.50, and 

5 
Log P[TNT(ppb)J = -5481/T(K) -t 19.37. (3) 

Stimac [Z, 141 reveals, however, that the RDX vapor pressure may be approximately five times 

greater than that calculated from equation 2. 

3. Results and Discussion 

3.1 Ultraviolet PFLREMPI. The RJZMPI excitation spectrum of PETN in the region of 

225.8-227 nm is shown in part C of Figure 2. The prominent features are attributed to the P21+Qi, 

Pi, P+Qiz, and P12 branches of the NO A2C n+ - X2 (0,O) band that result from one-photon 

selection rules (AJ = 0, +l) that govern the REMPI process. Evident from part C in Figure 2 are the 

spectral features due to the 2n1,2 and 2n 312 spin-orbit components of the NO ground electronic state. 

The NO spectral features clearly arise through a photofiagmentation/fragrnent ionization mechanism 

of the PETN vapor that involves the following steps: 

R-N0,2h-&+NO(XzII)+Oand (4) 

NO (X217)hz NO (A2 x+)h2 NO’ (X’ c’)+ e , (5) 

where step (4) represents fragmentation of the parent molecule yielding the electronic ground state 

NO, and step (5) represents the (l+l) REMPI of the NO fragment by means of its A2C+ intermediate 

state. An enhancement of the NO ion signal is observed because the energy of the A2C* state is 

\: 

resonant with the energy of the 227~nm photon. The processes represented by steps (4) and (5) are 

very fast; they occur within the duration of the laser pulse, - 6 ns for our experiment. Also, f 

hvi = hv2 because a single laser operating near 227 nm is used for both fragmentation and fragment 

ionization. This mechanism is proposed based on our earlier studies of the 227~nm excitation of 

dimethylnitramine (DMNA), a simple analogue of RDX and PETN, with time-of-flight mass 

6 
Q, + p,, 

PI 

P2 + Ql2 

226.0 

WAVELENGTH (NM) 

227.0 

Figure 2. NO Spectra: (a) Computer Simulation, (b) PyrolysisLIF of PETN, and 

(c) PF/ftEMPI of PETN. 

7 
analysis of the fragments [7]. The time-of-flight mass spectrum of DMNA revealed a strong ion 

signal at a mass-to-charge ratio of 30 that corresponds to the NO’ ion. 

It is important to realize that because the REMPI experiments conducted here are nonselective 

(i.e., no mass selection is involved), the analytical selectivity of the PF/FD technique depends on 

laser wavelength excitation for NO detection. As shown in Figure 2 (c)i the rotational lines of NO 

are well-resolved, which confirms that this technique does indeed exhibit sufficient spectral 

resolution to permit the NO fragment generated from PETN to be identified unequivocally, even at 

atmospheric pressure and in room air. 

The NO’ signal dependence on the 227-m-n laser intensity is measured, and the data are fitted 

with the equation S = CI”, where S is the ion signal intensity, I is the laser beam intensity, C is a 

system-dependent constant, and n is the number of photons. The slope of the log-log plot of ion 

signal vs. intensity yields a value of 1.64 for n. This is somewhat surprising in light of the 

mechanism already presented which suggests a value of four (i.e., two photons to generate NO from 

PETN and two photons to produce NO+). The reason for the lower than expected “n” value is not 

well understood and is under investigation. The dependence of ion signal on laser intensity suggests 

that a significant enhancement in sensitivity can be achieved at higher laser intensities. A similar 

trend is also observed for RDX and TNT. 

In Figure 3 (a), a response curve for PETN shows the variation of ion signal with concentration 

in the 298-393 K temperature range on excitation with 227-nm laser radiation. The PETN sample is 

heated to different temperatures, and at each temperature the ion signal from the vapor is measured. 

The concentration at each temperatnre is calculated using equation 1. We use a similar procedure to 

obtain the response curves for TNT and RDX in the ranges of 326-393.5 K and 32H16.5 R, 

respectively. 

+ 

8 
0 

0 50 100 150 200 250 

CONCENTRATION (ppb) 

7 

100 200 300 

CONCENTRATION (ppm) 

Figure 3. PF/REMPI Response Plots of PETN With (a) -100 PJ of 227-lam Laser Radiation, 

and (b) -5 mJ of 454”nm Laser Radiation. 

9 
The LOD in this study is defined as the concentration that produces a signal equal to three times 

the standard deviation of the background noise divided by the slope of the response curve for a 

particular compound. Table 1 shows the PFREMPI LODs for the compounds studied. They are 2 

ppb for PETN, 7 ppb for RDX, and 70 ppb for TNT. We calculate the LODs for RDX and TNT by 

using the vapor-pressure expressions presented in equations 2 and 3, respectively. The high 

detection sensitivity achieved for PETN indicates that both the photofragmentation and subsequent 

ionization processes are highly efficient. The lower sensitivity of TNT suggests that its absorption-.-- 

cross-section at 227 m-n, the efficacy of NO production, or both, are compared less to PETN and 

WX because the photoionization process for NO is the same for all three compounds. It should be 

noted that the LODs are determined at ambient pressure in room air, suggesting that collisional 

quenching of the NO (A2Z’) by N2 or 02, and reactions of NO (A217 with 02, do not significantly 

decrease the REMPI signal. The fact that these energetic materials can be detected by PF/REMPI 

with high sensitivity in room air at ambient pressure without the samples being heated to high 

temperatures enhances the analytical utility of this technique for field applications. 

Table 1. Limits of Detection of PETN, RDX, and TNT 

Compound 

PETN 

Wavelength (227 nrn) 

PFREMPI PFiLlF 

2 nnb NDa 

RDX 7 ppb ND 

TNT 70 PPb 37 PPm 

“ND = Not determined because the signal is weak, of the order of the background noise. 

3.2 Ultraviolet PF/LIF and Pyrolysis/LIF. LIF experiments were also conducted on the same 

three energetic materials so we could understand their photofiagmentation pathways and determine 

and compare the LOD values with those obtained from REMPI experiments. We determined the 

PF/LIF LOD of TNT by measuring the LIF signal as a function of its concentration in the 

354-397 K temperature range and calculating the LOD by using the same procedures followed for 

the PFREMPI experiments. The temperatures used were below 423 K to comply with the 

vapor-pressure expression presented in equation 3, and to ensure that TNT did not decompose. 

10 
t 

Eiceman et al. [ 141, using a diffusion generator with gravimetry and ion mobility slkctrometry, 

determined that TNT does not decompose at 423 K. The PF/LIF LOD value obtained for TNT is 

37 ppm and is listed in Table 1. This value is - 500 times greater than that obtained by PFREMPI. 

This difference, and the reduced PF/Lll? sensitivity for PETN and RDX at lower temperatures, are 

attributed both to fundamental limitations of the PF/LlF technique and to our system design. 

Collisional quenching of NO (A2C’) by Nz and 02, and reactions with 02 are more pronounced in 

the LIF experiments than in the R.EMPI experiments because the radiative lifetime of the A2Cf 

intermediate state is relatively long, - 2 15 ns [ 151. In the REMPI experiments, ionization from this 

state is instantaneous; it occurs within the 6-ns laser pulse and allows little time for quenching and 

other reactions to occur. A higher sensitivity for REMPI than LIF is also achieved because 

practically all of the ions produced byREMP1 are collected by the electrodes. In contrast, the signal 

in the LIF experiments is isotropic, and only part of it is collected because it is viewed through a 

small cone in a direction normal to the excitation laser beam. Collecting more of the signal using 

parabolic mirrors should yield higher sensitivities. 

Wu et al. [lo] recently reported their studies of the detection of TNT by PF/LlF. As in our study, 

a 227”nm laser photofragmented the precursor molecule and subsequently induced fluorescence from 

the resulting NO fragment. Concentrations of TNT were measured at various temperatures in a lowpressure, static environment. Their research reveals that when the TNT sample was heated from 

room temperature to 373 K, the LIF intensity did not increase much until the sample temperature 

reached about 343 K, close to its melting point. At temperatwes above 343 K, the LIF intensity 

increased rapidly with increasing temperature. These results are consistent with our observations on 

how sample temperature effects the LIF intensity of TNT. The detection limit of TNT was estimated 

at - 0.8 ppm, which was much lower than our LIF results. Possible reasons for the higher sensitivity 

~ 

;. reported in Wu et al. [lo] include the following: (1) the use of higher laser energies, although they 

are not reported, (2) less collisional quenching of NO (A2C”) by Nz or 02 and reactions with 02 in 

. 

their low-pressure environment, and (3) possible product accumulation of NO, contributing to an 

increase in the NO LIF signal because of the static environment used in their study. 

11 
There is one important difference between the REMPI and LIE experiments of PETN and RDX. 

Uulike the REMPI experiments, which show a measurable ion signal of NO from PETN even at 

room temperature, the LlF experiment shows a very weak emission signal below the melting point of 

PETN, - 413 K. The emission intensity increases sharply above the melting point, and at 

temperatures above 438 K, the PETN liquid boils so vigorously that rapid sample loss occurs 

because of evaporation. Light brown vapors from PETN at temperatures above the melting point 

can be clearly seen, indicating thermal decomposition, possibly to form NOz. The extent of 

decomposition at each temperature is not known, however. A similar trend is observed for RDX. 

The decomposition of RDX is indicated by the distinct but slow color change from white to brown to 

dark brown at temperatures close to its melting point, 476-477 R; this is followed by vigorous 

boiling of the liquid and rapid sample loss because of evaporation. TNT, on the other hand, melts at 

-353 K, but the light yellow liquid starts to turn brown only around 473 K. The color change 

becomes more intense and rapid at higher temperatures. Also, unlike RDX and PETN, TNT liquid 

boils less vigorously and remains in the sample tube for a long time. 

In view of the above observations, we used a different approach from PF/REMFI to determine 

the LODs for PETN and RDX. This approach involved pyrolysis of the energetic material with 

subsequent detection of the characteristic NO and NO2 products by LIF and PF/LIF, respectively, 

near 227 run. Sand samples contaminated with known amounts of PETN (20.0 ppm) and RDX 

(24.7 ppm) were placed in a miniature glass cup and then dropped into an oven maintained at 

predetermined temperatures of 416.5 K for PETN and 47 1.5 K for RDX. Figure 2 (b) shows the LIF 

excitation spectrum of PETN in the 225%227-nm region, obtained by heating PETN to 413 K. A 

simulation of the spectrum, shown in Figure 2 (a), is also included for comparison- The simulation 

was performed using a multiparameter computer program based on a Boltzmarm rotational 

distribution analysis [ 16,171. The program utilizes one-photon line strengths and rotational energy 

levels calculated using ground and excited electronic spectroscopic constants for NO, which were 

reported by Henry et al. [ 181 and Engleman and Rouse [ 193, respectively and a radiative lifetime was 

reported by McDennid and Laudenslager [ 151. 

12 
13 

A comparison of Figures 2 (b) and 2 (a) reveals that the observed spectral features from PETN in 

the region near the electronic origin are identical to those calculated for NO; this suggests that there 

exists an efficient pathway of PETN that leads to the formation of NO. Figures 2 (a) and 2 (b) also 

suggest that there is no spectral interference from other fragments that may be produced from PETN 

because all of the observed spectral features are attributed to NO. This supposition is confirmed by 

the virtually identical emission spectra for PETN and NO shown in Figure 4. The spectra clearly 

show a progression in the N-O stretch mode with a frequency of -1900 cm-’ for both NO and NO 

produced from PETN. Excitation and emission spectral features identical to those of NO were also 

recorded for RDX and TNT. 

Figure 5 shows a signal vs. time curve that we recorded by measuring the LIF signal intensity 

resulting from 227-run excitation of PETN as a function of 25-s intervals after the sand sample was 

dropped into the oven. The LIF intensity rose from zero, went through a maximum, and fmally fell 

to zero as the energetic material, which was initially at room temperature, first melted and then 

evaporated or decomposed from the sand. The total area under the curve, which we determined 

using a linear combination of Gaussian and Lorentian functions, represents the amount of PETN 

evaporated, decomposed, or both, and is assumed to be equal to the initial quantity of PETN. This 

assumption was verified from high-pressure liquid chromatography analysis of both neat and 

PETN-doped sand samples before and after the pyrolysis&IF experiments, which showed that >95% 

of the PETN decomposed, evaporated, or both. The amount of PETN lost from the mixture during 

the 25-s interval near the emission maximum was calculated from the ratio of the area under the 25-s 

segment of the curve at the maximum height to the total area of the curve. The results of a number 

of these experiments with different amounts of sand yielded the response curves for PETN and RDX 

that are presented in Figure 6. The LODs were calculated l?om the slope of the respective response 

curve using the same procedure previously described, and their values are 2.2 ppm (ng/mg) for 

PETN and 1.6 ppm (ng/mg) for RDX. 

The 227-nm LODs for PETN and RDX, determined by pyrolysis/I-IF, are -3 orders of 

magnitude greater than those obtained by PFREMPI. This difference resulted from the fact that the 

two methods are mechanistically different. The REMPI measurements involve ‘laser 
I I I I I I 1 

to,@ (&I) (0,2) (0,3) (0,4) (OS) ((46) 

(---) NO 

(-) NO from PETN 

320 

WAVELENGTH (NM) 

Figure 4. 227-nm LIF Emission Spectra of 0.1% NO in Nz at Room Temperature and NO 

From PETN. 

14 
10 

6 

h 

i$ 

-6 

z 

z 

34 

z 

2 

C 

0 1 2 3 4 5 6 7 

TIME (min) 

Figure 5. Intensity vs. Time Plot of NO From 20 ppm of PETN in Sand. 

b[l]= 0.329 

r==0.929 

b[l]= 0.204 

r 1 = 0.974 

20 40 60 80 100 120 

Weight of Contaminated Sand (mg) 

Figure 6. PyrolysWLIF Response Curves of NO From RDX (m) and PETN (0). 

15 
photofragmentation of the precursor molecule to NO with subsequent NO ionization. In contrast, the 

LIF measurements involve pyrolysis of the precursor molecule to NO with subsequent NO LlF at 

227 urn, or pyrolysis of the precursor molecule to NO2 with subsequent NO* laser 

photofragmentation to NO, followed by laser excitation of NO. This mechanism is consistent with 

open literature reports on pyrolysis of RDX [2&23] and PETN [ 14,24,25 1. 

3.3 Visible Pl?/FtEMPI and Pyrolysis/LIF. PF/FD experiments on the energetic materials 

were also performed with a visible laser because of the potential advantages offered by a visible 

laser over an ultraviolet laser. A PF/REMPI response curve for PETN at 453.85 nm is presented in 

Figure 3 (b). The slope of the response curve, together with the noise, yields a LOD of 16 ppm. 

This value is several orders of magnitude higher than that obtained at 227 run and is attributed to 

(1) the lower photofragmentation efficiency of the precursor molecule to generate NO, and (2) the 

lower NO ionization efficiency when visible laser radiation rather than ultraviolet laser radiation is 

used. Although the PFREMPI technique at 454 run is less sensitive than that at 227 run, it is still 

very useful for many applications, including those involving the analysis of underground 

contaminated soil with laser-based, cone-penetrometer systems. The PFREMPI technique would be 

useful for such applications because of the modest sensitivities needed, typically hundreds of ppm, 

and because the laser beam can easily be transmitted distances over l&30 m through optical fibers. 

Figure 7 shows the LIF excitation spectra of NO and NO generated from the pyrolysis of PETN 

in the 452454 nm region. The spectral features of NO from PETN are identical to those that result 

from the two-photon excitation of NO by means of its A2c’-X2n (0,O) band. The emission 

spectrum recorded following 454-nm laser excitation is also identical to the emission spectrum of 

NO shown in Figure 4. These observations reconfirm the fact that NO is a major stable product of 

the decomposition of PETN. The LOD for PETN is 140 ppm (ng/mg), approximately nine times 

greater than that produced by PFREMPI at the same wavelength. The LODs for RDX and TNT are 

not reported because the NO LIF signals were of the order of the background noise. 

16 
A 

B 

011 022 +b2 

I 

I 

P22 + Q12 

I I I I I I I I I I I I I I I I I I 1 

452.5 

WAVELENGTH, nm 

Figure 7. Two-Photon LIF Excitation Spectrum of NO From (a) PETN and (b) 0.1% NO 

in Nz. 

17 
4. Conclusion 

The analytical utility of PF/FD spectroscopy and pyrolysis&IF spectroscopy has been 

demonstrated for the trace detection of PETN, FCDX, and TNT in air (at atmospheric pressure) using 

both ultraviolet and visible laser radiation. In the PF/FD technique, a laser operating at 227 or 

454 nm photofragments the energetic material and facilitates detection of the characteristic NO 

fragment byREh4PI or LIF, using the NO A2c’-X2n (0,O) transitions near 227 nm. In contrast, the 

pyrolysis/LIF technique involves pyrolysis of the energetic material with subsequent detection of the 

NO and NO* products by LIF and PF/‘LIF, respectively. The PFREMPI, PF/LIF, and pyrolysisiLIF 

excitation spectra unequivocally reveal the presence of NO without any spectral interference from 

other photolysis products and that the NO rotational lines are well-resolved at atmospheric pressure. 

This result indicates that these techniques can be highly selective on the basis of wavelength. The 

LIF emission spectra also reveal several NO spectral features in the 22&305-nm range, which can 

be used as an additional fingerprint for NO detection, thus enhancing the selectivity of the method. 

. 

0 

We could measure all three energetic compounds by 227-nm PF/REMPI without heating the 

sample. The LODs ranged from 2 to 70 ppb, with PETN having the highest sensitivity. The 

differences in the sensitivities are attributed to the differences in NO generation because the NO 

detection efficiency is the same for all three compounds. The PFREMPI LOD of PETN at 454 nm 

is 16 ppm. This value is much greater than that obtained using 227~nm radiation and is attributed to 

the lower NO formation and higher ionization efficiencies in the visible laser radiation than in the 

ultraviolet laser radiation. The PF/LIF LOD value of TNT at 227 nm is 37 ppm- This value is 

greater than that obtained by PFREMPI at the same wavelength and is attributed to a lower NO 

detection efficiency. The pyrolysis/LIF LODs of PETN ranged from low ppm for PETN and RDX 

at 227 nm, to 140 ppm for PETN at 454 nm. The lower sensitivity at 454 nm than at 227 nm results 

from a lower NO excitation efficiency in the visible than in the ultraviolet. 

The PF/REMPI approach exhibits great potential for detecting trace levels of energetic materials 

in real-time and in situ because of its high sensitivity, selectivity, and instrumentation simplicity. An 

18 
increase in sensitivity is projected for both PF/FD and pyrolysis/I3 techniques with the use of 

higher pulse energies and for LIP-based techniques by improving the system design. 

Nute Added in Proo$ We have recently performed PF/LIF experiments on neat RDX and TNT 

at 1 atm and in room air with 227-nm radiation and a photomultiplier tube/interference filter 

configuration; we determined their LODs to be in the low ppm. 

19 
INTENTIONALLY LEFT BLANK. 

20 
5. References 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

Simeonsson, 5. B., and R. C. Sausa. “Laser Photofragtnentation/Fragment Detection 

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Huang, S. D., L. Kolaitis, and D. M. Lubman. “Detection Of Explosives Using Laser 

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Riris, H., C. B. Cat-isle, D. F. McMillen, and D. E. Cooper. “Explosives Detection With a 

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Capellos, C., P. Papagiannakopoulos, and Y. Liang. “The 24%nm Photodecomposition of 

Hexahydro-1,3,5-Trinitro-1,3,5-Triazine.” Chemical Physics Letters, vol. 164, no. 5, 

pp. 533-538,1989. 

Lemire, G. W., J. B. Simeonsson, and R. C. Sausa. “‘Monitoring of Vapor-Phase 

Nitrocompounds Using 226-run Radiation: Fragmentation With Subsequent NO 

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pp. 529-533,1994. 

Clark, A., K. W. D. Ledingham, A. Marshall, J. Sander, and R. P. Singhal. “‘Attomole 

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Mass-Spectrometry.” Analyst, vol. 118, no. 6, pp. 601-607, 1993. 

Simeonsson, J. B., G. W. Lemire, and R. C. Sausa. “Trace Detection of Nitrocompounds by 

ArF Laser Photofragmentationnization Spectrometry.” AppEied Spectroscopy, vol. 47, 

no. 11, pp. 1907-1912,1993. 

10. Wu, D., J. Singh, F. Yueh, and D. Monts. “2,4,6-Trinitrotoluene Detection by 

Laser-Photofragmentatiofiaser-Induced Fluorescence.” Applied Optics, vol. 35, no. 2 1, 

pp. 3998-4003,1996. 

21 
8 

11. Boudreaux, G. M., T. S. Miller, A. J. Kunefke, J. P. Singh, F. Yueh, and D. Monts. 

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1999. 

12. Davis, W. M., E. R. Cespedes, L. T. Lee, J. F. Powell, and R. A. Goodson. “Rapid Delineation 

of Subsurface Petrolium Contamination Using the Site Characterization and Analysis 

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Explosives.” Journal of Energetic Materials, vol. 4, pp. 447-472, 1986. 

14. Eiceman, G. A., D. Preston, G. Tiano, J. Rodriguez, and J. E. Parmeter. “Quantitative 

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17. Vanderhoff, J. A., M. W. Teague, and A. J. Kotlar. “Detection of Temperature and NO 

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pp. 1915-1922,1992. 

18. Henry, A., M. F. Le Moal, P. Cardinet, and A. Valentin. “Overtone Bands of 14N160 and 

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19. Engleman, Jr., R., and P. E. Rouse. “The j3 and y Bands of Nitric Oxide Observed During Flash 

Photolysis of Nitrosyl Chloride.” JournaZMoZecuZar Spectroscopy, vol. 37, pp. 24&25 1,1971_ 

20. Errnolin, N. E., and V. E. Zarko. “Mechanism and Kinetics of the Thermal Decomposition of 

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1997. 

21_ Behrens, R., and S. Bulusu. “Thermal Decomposition of Energetic Materials 3: Tempera1 

Behaviors of the Rates of Formation of the Gaseous Pyrolysis Products From Condensed-Phase 

Decomposition of 1,3,5-Trinitrohexahydro-s-triazine.” Journal ofphysical Chemistry, vol. 96, 

no. 22, pp. 8877-8891,1992. 
22. Behrens, R., and S. Bulusu. “Thermal Decomposition of Energetic Materials 4: Deuteriurn 

Isotope Effects and Isotopic Scrambling (H/D, ‘3C/‘g0, ‘%/‘5N) in Condensed-Phase 

Decomposition of 1,3,5-Trinitrohexahydro-s-triazine.” Journal ofPhysical Chemisty, vol. 96, 

no. 22, pp- 8891-X897,1992. 

23. Oyumi, Y., and T. Brill. “Thermal Decomposition of Energetic Materials 3: A High-Rate 

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Variables.” Combustion and Flame, vol. 62, pp. 213-224, 1985. 

24. Oyumi, Y., and T. Brill. “Thermal Decomposition of Energetic Materials 4: Selective Product 

Distributions Evidenced in Rapid, Real-Time Thermolysis of Nitrate Esters at Various 

Pressures.” Combustion and Flame, vol. 66, no. 9, pp. 9-16, 1986. 

25. Ng, W. L., J. E. Field, and H. M. Hauser. “Study of Thermal Decomposition of Pentaerythritol 

Tetranitrate.” Journal Chemistry Social Perkin Translation, vol. 2, no. 6, pp- 637-639,1976. 

23 
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26 
Rosario C. Sausa, Vaidhianat Swayambunathan, and Gurbax Singh* 

J.S. Army Research Laboratory 

41TN: AMSRL-WM-ED 

4berdeen Proving Ground, MD 2 1005-5066 

F University of Maryland Eastern Shore, Department of Natural Sciences 

12a. DISTRlBUTION/AVAlLAE4LllY STATEMENT 12b. DISTRIBUTION CODE 

4pproved for public release; distribution is unlimited. 

13. ABSTFtACT(Maximum 200 wwds) 

Trace concentrations of energetic materials such as 2,4,6-trinitrotoluene (TNT), pentaerythritol teuanitrate 

(PETN), and hexahydro-1,3,5-triniiro-s-triazme (RDX) are detected by laser photofiagmentation/fragment detection 

(PF/FD) spectrometry. In this technique, a single laser operating near 227 nm photofiagments the parent molecule 

and facilitates the detection of the characteristic NO fragment by means of its A2c-X2 II (0,O) transitions near 

227 run. Fragment detection is accomplished by resonance-enhanced multiphoton ionization (REMPI) with 

miniature electrodes and by laser-induced fluorescence (LlF) with a photodetector. Experiments are also conducted 

in the visible region using 453.8%mn radiation for photofragmentation and fragment detection. Sand samples 

contaminated with PETN and RDX are analyzed by a pyrolysis/LIF technique, which involves pyrolysis of the 

energetic material with subsequent detection of the pyrolysis products NO and NO2 by LlF and PF/LIF, 

respectively, near 227 nrn. Applying these techniques to the trace analysis of TNT, PETN, and RDX at ambient 

pressure in room air is demonstrated with limits of detection (S/N = 3) in the range of low parts-per-billion to 

parts-per-million for a 20-s integration time with l&120 uJ of laser energy at 226.8 nm and - 5 mJ at 453.85 nrn+ 

An increase in detection sensitivity is projected with an increase in laser energy and an irnproved system design 

The analytical merits of these techniques are discussed and compared to other laser-based techniques. 

4. SUBJECT TERMS 15. NUMBER OF PAGES 

aser photofragmentationlfragment detection, pyrolysis/laser-induced fluorescence, 30 

:nergetic material detection, TNT, PETN 16. PRICE CODE 

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