Detection of Energetic Materials and Explosive Residues 

With Laser-Induced Breakdown Spectroscopy: I. 

Laboratory Measurements 

by Jennifer L. Gottfried, Frank C. De Lucia Jr., Russell S. Harmon, 

Chase A. Munson, Raymond J. Winkel, Jr., and Andrzej W. Miziolek 

ARL-TR-4240 September 2007 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5066 

ARL-TR-4240 September 2007 

Detection of Energetic Materials and Explosive Residues 

With Laser-Induced Breakdown Spectroscopy: I. 

Laboratory Measurements 

Jennifer L. Gottfried, Frank C. De Lucia Jr., Russell S. Harmon, 

Chase A. Munson, Raymond J. Winkel, Jr., and Andrzej W. Miziolek 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited. 
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Detection of Energetic Materials and Explosive Residues With Laser-Induced 

Breakdown Spectroscopy: I. Laboratory Measurements 

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6. AUTHOR(S) 

Jennifer L. Gottfried, Frank C. De Lucia Jr., Russell S. Harmon, 

Chase A. Munson, Raymond J. Winkel, Jr., and Andrzej W. Miziolek 

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14. ABSTRACT 

Laser-induced breakdown spectroscopy (LIBS) has been investigated for the detection of energetic materials. After an initial 

survey of explosives and propellants, key elemental and molecular emission lines in the LIBS spectra were identified. 

Techniques for improving the sensitivity and selectivity of LIBS for explosives detection, such as the use of an argon buffer gas 

and double pulse LIBS, have been investigated with laboratory and field-portable instruments. We present results 

demonstrating the ability of LIBS to discriminate between energetic and non-energetic materials. 

15. SUBJECT TERMS

explosive detection, LIBS 

16. SECURITY CLASSIFICATION OF: 

19a. NAME OF RESPONSIBLE PERSON

Jennifer L. Gottfried 

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iii

Contents 

List of Figures iv

List of Tables vi

1. Introduction 1

1.1 Background .....................................................................................................................1 

1.2 LIBS of Explosives .........................................................................................................2 

1.3 Complications in the LIBS Spectra Because of Air........................................................5 

2. Experimental 6

3. Results and Discussion 10

3.1 Initial Survey of Explosives and Energetic Materials...................................................10 

3.2 MP-LIBS Studies of Explosive Residues......................................................................14 

3.3 Laboratory Double Pulse LIBS Measurements of Explosives......................................16 

3.4 Discrimination of Energetic Materials With LIBS........................................................19 

4. Conclusions 23

5. References 26

Distribution List 30 
iv

List of Figures 

Figure 1. Structures or formulas of several explosives and non-energetic organic compounds.....4 

Figure 2. Basic LIBS experimental setup. (The laser is fired, passed through a turning prism 

and pierced mirror, and is focused on the sample surface to induce a microplasma. The 

resulting light emission characteristic of the sample composition is focused into a fiber 

optic and dispersed with a spectrograph to obtain the LIBS spectrum.) ...................................6 

Figure 3. LIBS sensor suite and applications. (The basic LIBS components are compactible 

and ruggedizable and have been configured by different groups for a variety of 

applications.)..............................................................................................................................7 

Figure 4. Laboratory double pulse LIBS experimental setup. (A 100-mm focal length 

convex lens is used to focus the Nd:YAG laser pulses on the sample surface. A 30-mm 

biconvex quartz lens focuses the plasma emission into the 600-μm fiber optic, which is 

sent to the spectrometer. An argon flow is positioned to eliminate the air above the 

sample surface.) .........................................................................................................................9 

Figure 5. Representative single-shot LIBS spectra of an air spark, RDX powder, and an 

RDX pellet. ..............................................................................................................................11 

Figure 6. Single-shot LIBS spectra of pure energetic compounds: NC, PETN, HMX, TNT, 

and RDX. .................................................................................................................................11 

Figure 7. Single-shot LIBS spectra of several operational explosives and propellants: A-5, 

JA2, LX-14, C-4, and M43. ....................................................................................................12 

Figure 8. Single-shot LIBS spectra of black powder and its typical components: charcoal, 

calcium sulfate, ammonium nitrate, sulfur and potassium nitrate. (Strong lines of C, Mg, 

Na, N, K, O and S are labeled.) ...............................................................................................13 

Figure 9. Single-shot LIBS spectra of RDX in air and in argon...................................................14 

Figure 10. Single-shot spectra of aluminum and RDX residue on Al in air and under argon 

acquired with the MP-LIBS system. (Approximately 400 ng/mm2

 of RDX were applied 

to the aluminum foil substrate, and 10 spectra of each sample under each condition were 

acquired. The C [247 nm] and H [656 nm] signals from the RDX are enhanced under 

argon, while the O [777 nm] and N [742 to 747 nm] signals under argon are 

representative of the sample composition rather than atmospheric contributions.).................15 

Figure 11. Relative oxygen to nitrogen peak intensities for aluminum and RDX residue on 

aluminum in air and in an argon flow (error bars represent ±σ for the 10 spectra acquired 

with the MP-LIBS system). (The RDX residue can only be discriminated from the 

aluminum substrate [with the O/N ratio] under argon.)...........................................................15 

Figure 12. Comparison of the reduction in air entrainment with a single 320-mJ pulse under 

an argon flow versus using a double laser pulse (with an interpulse separation Δt=2 μs 

and total energy=320 mJ). (Twenty spectra of the aluminum and RDX residue samples 

in air and under an argon flow were acquired with the Continuum Surelite lasers in our 

laboratory LIBS setup. The oxygen-to-nitrogen ratios for a single 320-mJ pulse in air 

[maximum air entrainment] and a double-laser pulse under an argon flow [minimal air 

entrainment] are also shown for comparison. The aluminum and RDX residue can be 
v

discriminated, based on O/N when an argon flow and/or a double pulse is used, with the 

double pulse providing smaller standard deviations [i.e., the O/N values for the two 

samples do not overlap].).........................................................................................................16 

Figure 13. LIBS spectra of bulk RDX with a double laser pulse with Δt=2 μs (top) and a 

single laser pulse with equivalent pulse energy (bottom). (The carbon [inset right top] 

and hydrogen lines increase with double pulsing, while the nitrogen [inset right middle] 

and oxygen [inset right bottom] lines decrease because of the reduction in air entrained in 

the plasma.)..............................................................................................................................17 

Figure 14. Peak atomic emission ratios from RDX residue on aluminum in air and in argon 

(top) and from bulk RDX with single pulse and double pulse (Δt=2 μs) LIBS (bottom). 

(The error bars represent one standard deviation. The same trend is observed in both 

cases, indicating that double pulse LIBS results in ratios indicative of the sample 

composition rather than the air entrained in the plasma.)........................................................18 

Figure 15. Peak atomic emission ratios of RDX residue on aluminum from various 

interpulse separation times (Δt) in argon (white square) and air (black diamond). (The 

ratios were calculated from 20 spectra collected at each Δt in both argon and air. The 

error bars represent one standard deviation.)...........................................................................19 

Figure 16. O/C peak atomic emission ration comparison of RDX and diesel fuel for single 

pulse and double pulse (Δt=2 μs) LIBS. (The error bars represent one standard deviation. 

The two samples can only be discriminated based on the O/C ratio by double pulse 

LIBS.).......................................................................................................................................20 

Figure 17. Examples of an ideal ROC curve (dashed line) and a ROC curve based on a 

random predictor (solid line). ..................................................................................................21 

Figure 18. O/C peak atomic emission ratio for each single pulse LIBS spectrum taken of 

RDX residue and diesel fuel residue. (The dotted line represents the threshold that is 

moved along the y axis in order to create the ROC curve in figure 19.) .................................22 

Figure 19. ROC curves created from O/C peak atomic emission ratios from single pulse and 

double pulse LIBS spectra of RDX and diesel fuel residues. (The circle represents a 

point on the ROC curve where 75% of the RDX samples are identified correctly as RDX 

and 7% of the diesel fuel samples are false positives [incorrectly identified as RDX]. 

Moving in the direction of the arrow increases the percentage of correct RDX 

identification but also increases the false positive rate. Moving this direction corresponds 

to lowering the threshold in figure 18. The ROC curve generated from the double pulse 

LIBS spectra is closer to an ideal predictor [figure 17].).........................................................23 

Figure 20. First principal component from atomic emission ratios O/C, O/H, N/C, N/H, and 

O/N from each single pulse LIBS spectrum of RDX and diesel fuel residue. (The dotted 

line represents the threshold that is moved along the y axis in order to create the ROC 

curve in figure 21.)...................................................................................................................24 

Figure 21. ROC curves created from the first principal component calculated from atomic 

emission ratios from single pulse and double pulse LIBS spectra of RDX and diesel fuel 

residue in figure 20. (The double pulse ROC curve represents complete separation 

between RDX and diesel fuel residue [100% correct identification].) ....................................24 
vi

List of Tables 

Table 1. Explosives and energetic materials...................................................................................8 

Table 2. Results of LIBS spectra library comparison...................................................................13 
1

1. Introduction 

1.1 Background 

The direct chemical detection of energetic materials and explosive residues in real time is a 

particularly challenging problem. Interest in overcoming the difficulties associated with the 

detection of explosives has grown over the past decade and is now shared by many Government 

organizations. The military has a vital interest in the development of field-portable sensors to 

detect land mines, improvised explosive devices, remotely detonated munitions, hidden 

armaments, and unexploded ordnance. Homeland security requires an analytical capability for 

the detection of trace amounts of explosives or their residues in a variety of different situations 

(e.g., in airplane, train, or ship passenger luggage, in vehicles, and within transport containers). 

Also, the ability to detect trace amounts of explosive residues would greatly benefit forensic 

investigations of destructive explosive events. 

Energetic and explosive materials are pure substances or mixtures that chemically react to 

rapidly liberate large amounts of heat and gas. Today, about 150 different formulations are used 

in military, commercial, and illicit manufacture of explosives (1). Energetic materials and 

explosives may be inorganic or organic in nature and can be divided into two broad categories 

(low-energy explosives and high-energy explosives) based on factors related to how readily a 

reaction is initiated and its intensity. Many explosives consist of a fuel component (usually a 

hydrocarbon) and an oxidizer (typically an oxide of nitrogen), which may be contained within 

the same molecule (1). To be of broad utility, a sensing technique would need to have the 

capability to rapidly detect and identify the wide variety of different constituents that are present 

in energetic materials and explosives. 

Although some methods for detecting bulk explosives such as x-ray analysis, neutron activation 

or scattering (which measures quantities of carbon, nitrogen, and oxygen) (2), and terahertz 

imaging are currently used or undergoing development, many of these systems are quite large 

and expensive. One of the most pressing needs for the military is the stand-off detection of 

explosives. As a result of drastically reduced sensitivity at increasing distances and the 

generation of potentially harmful ionization radiation, neither x-ray imaging nor neutron 

activation is practical at stand-off distances (3). Although terahertz imaging is a promising 

technique that employs non-ionizing radiation, absorption of water vapor and other species in the 

atmosphere could potentially limit its application to stand-off detection (4, 5). 

Most trace explosive detection techniques, such as ion mobility spectrometry and gas 

chromatography, rely on vapor detection. Unfortunately, at room temperatures, the vapor 

pressures of many common explosives are extremely small (ppbv or less), and attempts to 

conceal the explosives by sealing them in packaging materials can decrease the vapor 
2

concentrations by as many as three orders of magnitude (6). A single first generation C-4 

fingerprint, however, can contain several milligrams of hexahydro-1,3,5-trinitro-s-trazine (RDX) 

(7). Optical techniques such as cavity ringdown spectroscopy (CRDS), Raman spectroscopy, 

photoacoustic spectroscopy, and photofragmentation followed by resonance-enhanced 

multiphoton ionization (PF-REMPI) or laser-induced fluorescence (PF-LIF) have been applied to 

trace explosive detection (5). Of these techniques, only Raman has been demonstrated on solid 

explosives at stand-off distances (10 to 50 m), although long integration times (i.e. multiple laser 

shots) were required to improve the signal-to-noise ratio (8, 9). 

An alternate optical technique for the detection of explosives is laser-induced breakdown 

spectroscopy (LIBS). LIBS is a spectroscopic analysis technique that uses the light emitted from 

a laser-generated microplasma to determine the composition of the sample, based on elemental 

and molecular emission intensities. The ability of LIBS to provide remote, rapid multi-element 

micro-analysis of bulk samples (solid, liquid, gas, aerosol) in the parts-per-million range with 

little or no sample preparation has been widely demonstrated (10) and is the greatest advantage 

of LIBS compared with other analytical approaches. LIBS holds particular promise for the 

detection and identification of explosives because of its intrinsic capability for minimally 

destructive, in situ, real-time detection and analysis of chemical species. 

LIBS has the following properties: (a) it is a simple and straightforward technique; (b) no sample 

preparation is required; (c) it is very sensitive, e.g., only a very small sample is required 

(nanograms–picograms) for production of a usable LIBS spectrum; (d) it is fast, providing realtime (<1 s) response; (e) LIBS sensors can be made rugged and field portable; (f) all components 

(i.e., laser, detector, computer, etc.) can be miniaturized; and (g) LIBS offers the flexibility of 

point detection or operation in a stand-off mode. With recent advances in broadband detectors 

(multispectrometer or echelle), LIBS is now capable of detecting a wide variety of toxic and 

hazardous compounds. No other sensor is capable of detecting all classes of chemical 

compounds and all types of matter. Researchers at the U.S. Army Research Laboratory (ARL) 

have previously used LIBS for the detection of Halon alternative agents (11), tested a fieldportable LIBS system for the detection of lead in paint (12), and demonstrated the detection of 

chemical and biological warfare agent surrogates (13–16). Here, we report recent results for the 

detection of energetic materials and explosive residues under close-contact laboratory conditions. 

An ensuing report (17) will detail the extension of these results to the stand-off detection of 

explosive residues with the use of LIBS. 

1.2 LIBS of Explosives 

In the past, LIBS has been primarily used to analyze one or a few elements, mostly metals (10). 

More recently, the capability of LIBS to identify compounds has been realized with the advent of 

high-resolution broadband spectrometers. Every element on the periodic table has characteristic 

atomic emission lines that emit in the visible spectrum. A broadband spectrometer allows one to 

capture all the elements in the sample interrogated by the laser-generated plasma, provided they 
3

are present in sufficient abundance. Instead of concentrating on a small portion of a LIBS 

spectrum as with a Czerny-Turner type monochromator, all the emitting elements in a sample 

can be observed in a full broadband LIBS spectrum. LIBS can more readily be applied to a 

variety of materials beyond metals, including plastics and other organic materials, biological 

materials, and other hazardous compounds. 

The carbon, hydrogen, oxygen, and nitrogen atomic emission lines (which span the ultraviolet to 

near infrared regions) are commonly used to identify organic compounds. Anzano et al. (18) 

used LIBS to see if linear correlation techniques would allow sorting of a variety of plastics, 

including polystyrene and high-density polyethylene. Subtle differences in the carbon and 

hydrogen intensities enabled successful identification 90% to 99% of the time. Portnov et al. 

(19) used LIBS to investigate the spectral signatures of nitroaromatic and polycyclic aromatic 

hydrocarbon samples. They observed the atomic emission lines associated with C, H, N, and O, 

but they also observed emission attributable to molecular fragments associated with the CN 

(B2

Σ+–X 2Σ+) violet system and the C2 (d 3Πg–a

 3Πu) Swan system. These fragments were used 

to successfully show differences between the compounds studied. Ferioli and Buckley (20) have 

used LIBS to study hydrocarbon mixtures (C3H8, CH4, and CO2 in air). The strength of the C, N, 

and O atomic emission lines was investigated in relation to the concentration of carbon and 

hydrogen in the samples. 

In addition to the absolute intensities of the atomic emission lines, the peak intensity ratios of 

these lines have been used to analyze samples. Tran et al. (21) analyzed the atomic intensity 

ratios of several organic compounds in order to determine the empirical formula of a compound, 

based on the ratios from several elements. Calibration curves were built, based on C:H, C:O, and 

C:N atomic emission ratios, from a variety of compounds that covered a wide range of 

stoichiometries. Four compounds with known stoichiometries were tested against the calibration 

curves. The ratios determined from the calibration curves were compared with the actual 

stoichiometries and showed an accuracy of 3% on average. In the study of nitro-aromatic and 

polycyclic aromatic hydrocarbon samples described previously, the ratios between C2 and CN 

and between O and N of different samples were shown to correlate with the molecular formula 

(19). Anzano et al. also attributed the success of their correlation of plastics to differences in the 

C/H atomic emission intensity ratio of each sample (18). 

Recent efforts at ARL have focused on optimizing the sensitivity and selectivity of LIBS for the 

detection of explosive residues. The ability of LIBS to detect trace amounts of materials with a 

single laser shot is especially important for residue detection, since the first shot can ablate all or 

most of the residue. The success of LIBS for identifying organic compounds, based on atomic 

emission intensity ratios, led researchers at ARL to investigate characteristics of LIBS spectra of 

explosive compounds. Carbon, hydrogen, oxygen, and nitrogen are found in most military 

explosives. A common characteristic of most explosives is their high nitrogen and oxygen 

content relative to the amount of carbon and hydrogen (22). By tracking the amounts of oxygen 

and nitrogen in a sample relative to the other elements, it is possible to determine if a compound 
4

is energetic or non-energetic (23). Figure 1 shows the molecular formulas or structures of 

several explosives and potential interferents. The interferents have a much higher carbon and 

hydrogen content relative to the oxygen and nitrogen than the explosives. Some of the 

interferents contain no nitrogen at all; however, the oxygen content is still low relative to 

hydrogen and carbon. Furthermore, diesel fuel is a combination of chains of hydrogen and 

carbon with no oxygen or nitrogen content. These differences can be exploited with LIBS in 

order to improve discrimination between energetic and non-energetic materials. 

N

N

N

NO2

NO2 NO2

NO2

O2N NO2

CH3

O

O ON+

O

OO

N+

O

OO

N+

O

OO

N+

N

O

O

C10H22-C15H32

Methyl-2-cyanoacrylate

(adhesive)

Oils (fatty acids)

CH3(CH2)2COOH, 

CH3(CH2)18COOH, 

CH3(CH2)7CH=CH(CH2)7COOH

Diesel Fuel

N

N

N

NH2

NH2 NH2

Melamine (resins)

Nylon C12H22O2N2

Polyurethane C17H15O4N2

RDX

TNT PETN

N

N

N

NO2

NO2 NO2

N

N

N

NO2

NO2 NO2

NO2

O2N NO2

CH3

NO2

O2N NO2

CH3

O

O ON+

O

OO

N+

O

OO

N+

O

OO

N+

O

O ON+

O

OO

N+

O

OO

N+

O

OO

N+

N

O

O

C10H22-C15H32

Methyl-2-cyanoacrylate

(adhesive)

Oils (fatty acids)

CH3(CH2)2COOH, 

CH3(CH2)18COOH, 

CH3(CH2)7CH=CH(CH2)7COOH

Diesel Fuel

N

N

N

NH2

NH2 NH2

N

N

N

NH2

NH2 NH2

Melamine (resins)

Nylon C12H22O2N2

Polyurethane C17H15O4N2

RDX

TNT PETN

Figure 1. Structures or formulas of several explosives and non-energetic organic compounds. 

Although explosive residues can be identified, based on the elemental ratios of carbon, hydrogen, 

nitrogen and oxygen, entrainment of atmospheric oxygen and nitrogen into the laser-induced 

plasma complicates the discrimination of energetic and non-energetic materials. Minimizing the 

oxygen and nitrogen contribution from the atmosphere is necessary to track the true value of 

oxygen and nitrogen relative to carbon and hydrogen in a LIBS spectrum. The formulation of 
5

RDX, for example, has equal amounts of O and N. Air alone has more nitrogen (80%) relative 

to oxygen (20%). Eliminating the nitrogen and oxygen contribution from air therefore results in 

a larger O:N ratio characteristic of an explosive material (24). 

1.3 Complications in the LIBS Spectra Because of Air 

In a laboratory setting, argon can be used to displace the ambient atmosphere from the surface of 

the sample. As an added benefit, the use of argon as a buffer gas at atmospheric pressures also 

results in an increase in emission signal compared to air (25). Although we demonstrate in this 

report that using an inert gas to displace the air above the sample is effective for close-contact 

studies, it cannot practically be used for stand-off applications (17). Alternatively, multiple laser 

pulses can be used to interrogate the sample. We have used double pulse LIBS to reduce the 

amount of air entrained in the laser-induced plasma and improve the discrimination of energetic 

and non-energetic materials. 

In double pulse LIBS, two successive laser pulses are used to generate the microplasma. 

Typically, the laser pulses are separated by a few microseconds. Researchers have used a variety 

of double pulse LIBS experimental configurations ranging from collinear nanosecond laser 

pulses to orthogonal laser sparks using both a femtosecond pulse and a nanosecond pulse 

(26–31). The main advantage of double pulse LIBS for most applications is the observed 

increase in signal; consequently, studies of double pulse LIBS have increased in recent years 

(31). The extent of the reported signal increases depends on many factors, including pulse 

separation times, wavelength, pulse energies, sample, and experimental configuration. Another 

advantage of using two pulses is that the ablation pulse is separate from the analytical pulse, thus 

improving reproducibility in most cases. Since we are ultimately concerned with performing 

LIBS stand-off experiments, all our laboratory double pulse LIBS experiments were performed 

with two collinear pulses. 

A combination of several factors is thought to increase the emission signal in double pulse LIBS. 

These factors include greater mass ablation, larger plasma volume (more atoms are excited), and 

less laser shielding of the second pulse, which results from the decrease in gas density after the 

first pulse (28–31). When the first laser pulse hits, it impacts the sample and displaces the 

surrounding gas. The second pulse arrives and interacts with the material within the first plasma 

in a reduced density environment. We have attempted to exploit the decrease in gas density for 

explosives detection. The decrease of surrounding air density within the analytical (second) 

plasma should diminish the influence of oxygen and nitrogen from the atmosphere on the LIBS 

emission signals. The oxygen and nitrogen signal obtained will be more representative of the 

sample composition than the surrounding air; thus, a better determination of whether an 

unknown material is an explosive can be achieved. 
6

2. Experimental 

A standard laboratory LIBS system is shown in figure 2. The basic LIBS setup consists of a 

pulsed laser (typically the fundamental of a Nd:YAG*

 laser 1064 nm, 8-ns pulse width), optics 

for focusing the laser and light collection from the laser-induced microplasma, a spectrograph for 

dispersing the collected emission, and a computer for recording and analyzing the LIBS 

spectrum. The focused laser pulse ablates a small amount of the sample material and dissociates 

the resulting molecules and particulates within the plasma volume. For a typical setup the laser 

power at the focal point is >1 GW/cm2 in order to produce a microplasma. The subsequent 

emission can be resolved spectrally and temporally in order to generate a spectrum containing 

emission lines from the atomic, ionic, and molecular fragments created by the plasma. A single 

laser event and subsequent data analysis can take place in less than a second. 

Figure 2. Basic LIBS experimental setup. (The laser is fired, passed through a turning prism and pierced 

mirror, and is focused on the sample surface to induce a microplasma. The resulting light emission 

characteristic of the sample composition is focused into a fiber optic and dispersed with a 

spectrograph to obtain the LIBS spectrum.) 

Because of the simplicity, compactibility, and ruggedness of the instrumentation, a wide range of 

sensors based on LIBS has been developed by various groups over the years. Figure 3 illustrates 

some of the different LIBS applications and configurations that have been envisioned. 

 *

Nd:YAG = neodymium-doped yttrium aluminum garnet. 
7

Figure 3. LIBS sensor suite and applications. (The basic LIBS components are compactible and ruggedizable and 

have been configured by different groups for a variety of applications.) 

A number of LIBS systems have been developed by ARL and used to obtain spectra of energetic 

materials and residues. The following paragraphs describe the laboratory systems used in this 

research, as well as specialized instrumentation for portable applications. 

One of the earliest laboratory LIBS systems at ARL uses the pulse (~10 ns, 30 mJ) from an 

actively Q-switched Nd:YAG laser (Ultra Big Sky Laser) focused by a 50-mm convex lens onto 

a sample (24). A bundle of seven fibers (diameter, 600 μm) collects the emission from the 

plasma spark. A lens (Ocean Optics, Inc.) is placed in front of the fiber bundle so that the 

plasma spark is sufficiently defocused for each fiber to collect the same emission and to 

eliminate any spatial effects. Each of the fibers is connected to an individual high-resolution 

(0.1-nm) broadband spectrometer (Ocean Optics, Inc., LIBS2000+). Each spectrometer covers a 

different segment of the 200- to 980-nm spectral range of the LIBS system. Thus, we are able to 

observe all constituents of an energetic or explosive material. All LIBS spectra with this system 

were collected with a 1.5-μs delay to eliminate plasma continuum effects. The detectors 

remained open for 2 ms. The LIBS spectra collection software and the LIBS library software 

were provided by Ocean Optics, Inc. 

LIBS Probe for 

Humanitarian 

De-mining 

THREAT 

DETECTED 

Stand-off detection 

MP-LIBS 

Remote/FiberOptic LIBS 

Under water 

Navy Ships - 300-m standoff 

Suitcase LIBS 

“Lab-in-Field” 

Robo-LIBS 

Spectrometer Optics 

Data Processing 

0

500

1000

1500

2000

2500

3000

200 400 600 800

Wavelength (nm)

Intensity (a.u.)

Laser 
8

We surveyed a variety of energetic and explosive materials with the broadband LIBS2000+ 

system. Initially, we investigated black powder and its typical components: charcoal, potassium 

nitrate, calcium sulfate, ammonium nitrate, and sulfur, with no sample preparation. 

Subsequently, we investigated several pure organic energetics: RDX, octahydro-1,3,5,7-

tetrocine (HMX), pentaerythrite tetranitrate (PETN), nitrocellulose (NC), and 2,4,6-

trinitrotoluene (TNT), which had been prepared by dissolution into acetone and subsequent 

evaporation of the pure organic onto glass slides. We pre-treated these samples to get the purest 

possible LIBS spectrum for each explosive for use as a baseline comparison with explosives that 

had been handled or mixed into operational formulations. We also studied several operational 

explosives and propellants: C-4, M-43, LX-14, JA2, and A-5. The chemical compositions of all 

the explosive materials examined in our initial survey with the LIBS2000+ system are listed in 

table 1. The operational explosives and propellants contain a diverse suite of different binders 

and plasticizers. 

Table 1. Explosives and energetic materials. 

Sample 

(Type) 

Composition 

RDX (explosive) Hexahydro-1,3,5-trinitro-s-trazine

HMX (explosive) Octahydro-1,3,5,7-tetrocine

TNT (explosive) Trinitrotoluene

PETN (explosive) Pentaerythritol tetranitrate 

NC (propellant) Nitrocellulose

C-4 (explosive) 91% RDX, 9% plasticizer [5.3% di(2-ethylhexyl)sebacate, 2.1% polyisobutylene, 1.6% 

motor oil]

M-43 (propellant) 76% RDX, 4% nitrocellulose, 12% cellulose acetate/butyrate, 8% plasticizer, <1% 

additives

LX-14 (propellant) 95.5% HMX, 4.5% estane

JA2 (propellant) 60% nitrocellulose, 15.8% nitroglycerine, 25.2% plasticizer [diethylene glycol 

dinitrate], 0.74% Akardite II [N-methyl-N,N’-diphenyl urea (burning rate moderator and 

stabilizer)], 0.03% magnesium oxide, 0.7% graphite

A-5 (explosive) 98.5% RDX, 1.5% stearic acid

A man-portable backpack LIBS system (MP-LIBS) has been developed by ARL with Ocean 

Optics, Inc. for field use (15). A small Nd:YAG laser is contained in a hand-held wand. The 

user holds the sensing wand with the laser and control stick near the sample. Colored filters 

surrounding the sample region act as splash guards and contain the laser beam in an enclosed 

area. The focusing optics and collection optics are also in the wand. The collected light from the 

LIBS plasma is delivered to a backpack spectrometer via a fiber optic cable. The data are then 

analyzed by an on-board computer and displayed in a heads-up display. The laptop and 

spectrometer fit into a backpack or suitcase, and the entire system weighs less than 20 lb. 

A variety of LIBS spectra has previously been collected with the prototype unit shown in the 

upper left-hand corner of figure 3, ranging from plastic land mine casings to liquid chemical 

warfare simulants (15, 32–34). For this study, single-shot spectra of RDX residue on an 

aluminum (Al) substrate (~400 ng/mm2) were acquired with an MP-LIBS system with ~25 mJ 
9

per laser pulse (1064 nm, ~7 ns). An argon flow was directed across the sample region, and 

10 spectra each of the Al substrate and RDX were acquired. Air spectra were also obtained for 

comparison. 

The laboratory double pulse LIBS system at ARL is shown in figure 4. Two Surelite* Qswitched Nd:YAG lasers are used to generate the two laser pulses. The pulses from each laser 

are delayed relative to one another by a Stanford Research pulse generator. When single pulse 

operation is desired, one 320-mJ laser pulse (1064 nm, 8 ns) is generated (the flashlamps 

continuously fire at 5 Hz). In double pulse mode, each pulse is 160 mJ and separated by 1 to 

10 μs. The pulse train passes through a turning prism, a 100-mm focal length convex lens, and a 

pierced parabolic mirror. The emission from the plasma is collected by the parabolic mirror and 

focused onto the tip of a 600-μm fiber optic. The fiber is then inserted into a Catalina Scientific 

SE200† echelle spectrometer through a 25-micron pinhole. An intensified charge-coupled device 

(ICCD) serves as the detector. The intensified camera begins collecting spectra 2 μs after the 

initiation of the second plasma for 100 μs. An argon flow enables displacement of the air above 

the sample. 

Figure 4. Laboratory double pulse LIBS experimental setup. (A 100-mm focal length convex lens is used 

to focus the Nd:YAG laser pulses on the sample surface. A 30-mm biconvex quartz lens focuses 

the plasma emission into the 600-μm fiber optic, which is sent to the spectrometer. An argon 

flow is positioned to eliminate the air above the sample surface.) 

 *

Surelite is a trademark of Continuum; Continuum is a registered trademark. 

†

The SE200 is a designed and manufactured by Optomechanics Research Inc. and is protected by US patent 6,628,383. 
10

LIBS spectra from the following samples were collected with the laboratory double pulse 

system: bulk Composition-B (Comp-B) (36% TNT, 63% RDX, and 1% wax), Comp-B residue 

on Al, bulk RDX, and RDX residue on Al. We prepared the residues by dissolving the explosive 

material in acetone for a concentration of 1 mg/mL. The solution was then deposited onto an Al 

substrate and allowed to evaporate. Spectra were collected in ambient atmosphere and under 

argon. Single-pulse LIBS spectra were collected from each sample under each bath gas. 

Double-pulse spectra were also collected under each gas at each of the interpulse delays ranging 

from 1 to 10 μs. Twenty spectra were collected of each sample for each experiment. A small 

amount of diesel fuel was spread onto an Al substrate in order to study the discrimination ability 

of single pulse LIBS relative to double pulse LIBS. 

3. Results and Discussion 

3.1 Initial Survey of Explosives and Energetic Materials 

The initial survey of the explosives and energetic materials in table 1 resulted in the 

identification of the characteristic peaks observed in their LIBS spectra. Figure 5 presents a 

comparison of a pure air, RDX powder, and RDX pellet spectra. The nitrogen and oxygen 

signals in the RDX spectra are primarily from the entrainment of air into the laser-induced 

plasma, although the O/N ratio is noticeably higher in the RDX spectra. In addition to the 

nitrogen (N) (747, 821, and 869 nm) and oxygen (O) (777 and 844 nm) lines, the carbon (C) 

(247 nm) and hydrogen (H) (656 nm) lines are prevalent in the RDX spectra. The impurities 

magnesium (Mg) (279 and 285 nm), sodium (Na) (589 nm) and potassium (K) (766 and 769 nm) 

are also present in the explosives spectra. These lines can be attributed to the handling of the 

explosives or to the binders in operational explosives. The LIBS spectra of the different pure 

organic explosives are shown in figure 6. All share similar C, H, N, and O peaks. 

LIBS spectra for the suite of operational explosives included in the study are shown in figure 7. 

From table 1, it can be seen that there is a diverse mixture of exotic components in these 

explosives because of the binders and plasticizers that they contain. The peaks that are 

attributable to C, H, N, and O in the organic explosives are observed, as are peaks attributable to 

the components of these secondary constituents. For example, Mg (279 and 285 nm) is 

identified in the JA2 and A-5 spectra. Calcium (Ca) (442, 558, 643 and 854 nm), Na, and K 

lines are also observed. 

One possible way to extend LIBS as a practical approach to the detection and identification of 

explosives at an operational level is through spectral matching based on a predetermined and 

assembled spectral library of reference materials of interest. This approach would require the 

construction of a library of broadband spectra that would provide the basis for the comparison of 

the spectra for an unknown sample with those spectra contained in the library. The feasibility of 
11

Note: a.u. = arbitrary units. 

Figure 5. Representative single-shot LIBS spectra of an air spark, RDX powder, and an RDX pellet. 

Figure 6. Single-shot LIBS spectra of pure energetic compounds: NC, PETN, HMX, TNT, and RDX. 
12

Figure 7. Single-shot LIBS spectra of several operational explosives and propellants: A-5, JA2, 

LX-14, C-4, and M43. 

this approach was tested through the construction of a small library with the Ocean Optics 

software. By taking single-shot LIBS spectra of black powder and its principal components, we 

developed the library that included LIBS spectra of charcoal, a black powder pellet, sulfur, 

calcium sulfate, potassium nitrate, and ammonium nitrate. The LIBS spectrum of each of these 

component materials is shown in figure 8. We compared a series of single-shot LIBS spectra of 

a black powder pellet and powder with the library that we had constructed of black powder and 

its components. The library software identified the black powder and the black powder pellet as 

black powder in 150 of 150 tests. Charcoal, which is very similar in appearance to black 

powder, was identified as charcoal in 48 of 50 firings. Table 2 shows the results of several other 

comparisons. 

The other approach to detection and identification of explosives is through the use of the 

stoichiometry of the compound for discrimination by the taking of a ratio of the elemental peaks 

of interest. A similar approach was explored by Anzano et al. to sort types of plastic (18). The 

intensity of the ratio of the C line at 247 nm to the H line at 656 nm was found to be different for 

each plastic. Whereas the LIBS spectra of all the plastics shared similar peaks, and the overall 

appearance of the LIBS spectra for the different plastics was highly similar, the peak intensity 

ratios of the elements were different to such an extent as to make differentiation of different 

plastic types possible. By analogy, it should be possible to identify the pure organic explosives 

in figure 6, based on differences in their stoichiometry (table 1). 
13

Figure 8. Single-shot LIBS spectra of black powder and its typical components: charcoal, calcium 

sulfate, ammonium nitrate, sulfur and potassium nitrate. (Strong lines of C, Mg, Na, N, K, O 

and S are labeled.) 

Table 2. Results of LIBS spectra library comparison. 

Sample Correct Identifications 

(%) 

No. of Tests 

Black powder 100 150 

Charcoal 96 50 

KNO3 94 50 

CaSO4 100 25 

In addition to using the C:H peak intensity ratio, we might gain additional information from the 

O and N peak intensities. One concern that needs to be addressed is the interference of 

atmospheric N and O. A LIBS spectrum of air is shown in figure 5. Dry air includes ~80% N2

and ~20% O2. The ratio of the O peak at 777 nm to the N peak at 747 nm is 1.5. In figure 9, we 

compare RDX in air with RDX in argon (Ar). We have focused on the wavelength region from 

600 to 800 nm to better observe the peaks of interest. The ratio of the O peak to the N peak from 

the LIBS spectrum of RDX in air is 2.5. This ratio is attributable to the intermixing of the O and 

N in RDX with O and N from air. Isolating the O and N in RDX by blowing Ar across the 

surface produces an interesting result. The formulation of RDX has equal amounts of O and N, a 

1:1 ratio. Air alone has a 1:4 oxygen (20%) to nitrogen (80%) ratio. The LIBS peak O:N ratio in 

air is 1.5. 
14

Figure 9. Single-shot LIBS spectra of RDX in air and in argon. 

With a 1:1 O:N ratio, one would expect a LIBS peak ratio of four times a 1:4 O:N ratio. The 

LIBS peak ratio of RDX is therefore expected to be 6.0 because it has a 1:1 O:N ratio. In 

figure 9, the O:N LIBS peak intensity ratio is 5.6 for the RDX in Ar. 

3.2 MP-LIBS Studies of Explosive Residues 

The use of an argon flow to displace the air above the sample has also been tested with the 

MP-LIBS device. The single-shot spectra of RDX residue and Al acquired both in air and with 

an argon flow are shown in figure 10. The addition of argon to the LIBS plasma enhances the C 

(247 nm) and H (656 nm) lines, increasing the sensitivity of the MP-LIBS system. When argon 

is used to displace the air, the N (742, 744, and 747 nm) and O (777 nm) lines are representative 

of the sample composition rather than atmospheric contributions to the signal. 

Figure 11 compares the peak intensity ratio of the oxygen to nitrogen for the four samples (Al in 

air, RDX on Al in air, Al in argon, and RDX on Al in argon). For the Al sample, O/N is 

indicative of the amount of oxygen relative to nitrogen in the atmosphere, since the sample itself 

contains no oxygen or nitrogen (except possibly from trace surface contamination). The RDX 

residue, on the other hand, contains more oxygen relative to nitrogen than air. The two samples 

cannot be discriminated based on the O/N value in air, but in argon, the decrease in the amount 

of air entrained in the plasma results in greater separation in the O/N values. These results show 

that for low pulse powers (single pulse), the use of argon is essential for improving the 

selectivity of the MP-LIBS system for explosive residue detection. 
15

Figure 10. Single-shot spectra of Al and RDX residue on Al in air and under argon acquired with the MPLIBS system. (Approximately 400 ng/mm2

 of RDX were applied to the Al foil substrate, and 

10 spectra of each sample under each condition were acquired. The C [247 nm] and H 

[656 nm] signals from the RDX are enhanced under argon, while the O [777 nm] and N [742 to 

747 nm] signals under argon are representative of the sample composition rather than 

atmospheric contributions.) 

Figure 11. Relative oxygen to nitrogen peak intensities for Al and RDX residue on Al 

in air and in an argon flow (error bars represent ±σ for the 10 spectra 

acquired with the MP-LIBS system). (The RDX residue can only be 

discriminated from the Al substrate [with the O/N ratio] under argon.) 
16

3.3 Laboratory Double Pulse LIBS Measurements of Explosives 

We have investigated the use of argon as a buffer gas for explosive residue detection using 

higher laser powers with the laboratory double pulse LIBS system. Spectra were collected from 

samples in air and argon with double pulse (160 mJ/pulse) and single pulse (320 mJ/pulse) LIBS. 

Figure 12 compares the O/N ratios for each sample. The average O/N ratios for single pulse Al 

and RDX residue spectra in air overlap significantly as a result of the atmospheric contribution to 

the O and N signals. The use of an argon flow to displace the air results in a separation of the 

O/N values for the Al and RDX residue; this confirms the lower pulse energy MP-LIBS results 

(figure 11). 

Figure 12. Comparison of the reduction in air entrainment with a single 320-mJ pulse 

under an argon flow versus using a double laser pulse (with an interpulse 

separation Δt=2 μs and total energy=320 mJ). (Twenty spectra of the Al and 

RDX residue samples in air and under an argon flow were acquired with the 

Continuum Surelite lasers in our laboratory LIBS setup. The oxygen-tonitrogen ratios for a single 320-mJ pulse in air [maximum air entrainment] 

and a double-laser pulse under an argon flow [minimal air entrainment] are 

also shown for comparison. The Al and RDX residue can be discriminated, 

based on O/N when an argon flow and/or a double pulse is used, with the 

double pulse providing smaller standard deviations [i.e., the O/N values for 

the two samples do not overlap].) 

Although these results confirm the necessity of using argon for single pulse LIBS detection of 

explosive residues, the ideal detector would be capable of stand-off detection of explosive 

residues from tens to hundreds of meters. An argon flow cannot practically be delivered to the 

sample surface for stand-off applications. As described in section 1.3, double pulse LIBS has 

been extensively investigated because of the signal enhancement attributable to the greater mass 

ablation and increase in plasma volume caused by the reduced density environment following the 
17

first laser pulse. Figure 12 shows that separation of the O/N ratios for the Al and RDX samples 

can also be achieved with collinear double pulses. Although the separation between the samples 

is not as large as with the argon flow (since displacement of the air is not complete), the standard 

deviation of the measurements is significantly lower with double pulse. The combination of 

double pulse LIBS with an argon flow results in a large separation of the O/N ratios for the two 

samples, as well as lower standard deviations. 

The carbon, hydrogen, nitrogen, and oxygen atomic emission lines from these spectra were used 

to determine if double pulse LIBS alone (with no argon flow) will minimize atmospheric oxygen 

and nitrogen interference sufficiently to enable the discrimination of energetic and non-energetic 

materials. Figure 13 compares a single pulse bulk RDX spectrum to the double pulse bulk RDX 

spectrum. The broadband spectra look almost identical, although a closer look at the regions of 

interest shows that analogous to the MP-LIBS air vs. argon study (figure 10), the C and H lines 

are enhanced with the double pulse while the O and N lines decrease. The differences in the key 

ratios for explosives (O/C, O/H, N/C, N/H, O/N) for single vs. double pulse shown in figure 14 

follow the same trend observed with spectra obtained in air vs. argon. In other words, all the 

ratios decrease with double pulsing (and argon) except the O/N ratio, which increases. RDX 

residue, bulk Comp-B, and Comp-B residue also exhibit the same trend with double pulses. 

These results confirm that double pulse LIBS results in ratios indicative of the sample 

composition rather than the air entrained in the plasma. 

Figure 13. LIBS spectra of bulk RDX with a double laser pulse with Δt=2 μs (top) and a single laser 

pulse with equivalent pulse energy (bottom). (The carbon [inset right top] and hydrogen 

lines increase with double pulsing, while the nitrogen [inset right middle] and oxygen [inset 

right bottom] lines decrease because of the reduction in air entrained in the plasma.) 
18

Figure 14. Peak atomic emission ratios from RDX residue on Al in air and in argon (top) and 

from bulk RDX with single pulse and double pulse (Δt=2 μs) LIBS (bottom). 

(The error bars represent one standard deviation. The same trend is observed in 

both cases, indicating that double pulse LIBS results in ratios indicative of the 

sample composition rather than the air entrained in the plasma.) 

By comparing the oxygen-to-carbon atomic emission ratio of RDX residue from spectra 

collected at various delay times in air with the same ratios from spectra collected in argon, we 

can observe the effectiveness of double pulse LIBS in minimizing the influence of oxygen from 

the atmosphere on the spectra as a function of interpulse delay. Figure 15 shows the oxygen-tocarbon peak atomic emission ratios from an RDX residue on Al at a variety of interpulse delay 

times in air and argon. At Δt=0, corresponding to a single pulse with energy equal to the two 

pulses, the O/C ratio is much higher in air than argon, as observed in figure 14. As the single 

pulse is split into two pulses as an interpulse delay is introduced, the O/C ratio from the LIBS 

spectra collected in air decreases. By contrast, the O/C ratio from the spectra collected in argon 

with an interpulse delay remains fairly constant. The same trend is observed in the N/C, N/H,
19

Figure 15. Peak atomic emission ratios of RDX residue on Al from various interpulse separation times 

(Δt) in argon (white square) and air (black diamond). (The ratios were calculated from 

20 spectra collected at each Δt in both argon and air. The error bars represent one standard 

deviation.) 

and O/H ratios shown in figure 15. The behavior is similar for all the samples: bulk Comp-B, 

bulk RDX, and Comp-B residue. The trend observed in figure 15 once again demonstrates that 

double pulse LIBS diminishes the oxygen and nitrogen from atmosphere. Of course, not all of 

the oxygen and nitrogen from the atmosphere are displaced by the double pulse. This is reflected 

in the fact that the emission ratios from double pulse LIBS spectra in air are larger than those in 

argon. However, the atomic emission ratio obtained from double pulse LIBS is much closer to 

the true ratio value of the sample than the ratio obtained from single pulse LIBS. Based on the 

decrease in the emission ratios and their standard deviations, an interpulse delay of 2 μs was 

chosen as the optimal timing for the detection of explosive residues with the laboratory double 

pulse LIBS system. 

3.4 Discrimination of Energetic Materials With LIBS 

In order to determine the ultimate utility of double pulse LIBS as a tool for explosives detection, 

it is important to test the discrimination between energetic and non-energetic organic materials. 

In this case, we collected LIBS spectra of diesel fuel residue and RDX residue, both on an Al 

substrate. Single pulse LIBS spectra were collected from each sample with a 320-mJ laser pulse. 

Double pulse LIBS spectra were collected from each sample with an interpulse separation of 
20

2 μs and 160 mJ for each pulse. The peak atomic emission ratios, O/C, O/H, N/C, N/H, and 

O/N, were calculated from each spectrum collected. In order to compare the discrimination 

capability of double pulse LIBS to single pulse LIBS, we used the atomic emission ratios for 

each sample. The average O/C ratios from the single and double pulse LIBS spectra for RDX 

and diesel fuel are shown in figure 16. The RDX O/C ratio is larger than the O/C ratio from the 

diesel fuel in both cases. This result is expected since the amount of oxygen to carbon in RDX is 

higher than the amount of oxygen relative to carbon in diesel fuel (figure 1). However, the O/C 

ratio from the two samples overlaps within one standard deviation with single pulse LIBS. By 

contrast, the O/C ratio from the two samples does not overlap with double pulse LIBS. 

Figure 16. O/C peak atomic emission ration comparison of RDX and diesel fuel for single pulse 

and double pulse (Δt=2 μs) LIBS. (The error bars represent one standard deviation. 

The two samples can only be discriminated based on the O/C ratio by double pulse 

LIBS.) 

A useful way to compare the effectiveness of discrimination methods is to use receiver operating 

characteristic (ROC) curves. A ROC curve measures the sensitivity of the detection versus 

specificity, i.e., the number of false alarms, in a two-member system. A threshold is established 

in order to categorize the unknown samples as one of the two members of the system. The 

threshold sweeps through all the values of the samples in order to create a ROC curve. An ideal 

ROC curve, i.e., complete discrimination between two samples, and the worst case for a ROC 

curve, i.e., a random predictor, are shown in figure 17. 
21

Figure 17. Examples of an ideal ROC curve (dashed line) and a ROC curve based on a random predictor 

(solid line). 

In our case, the two members of our system are an energetic (RDX) and a non-energetic (diesel 

fuel). Our threshold for what is classified as an energetic is based on the value of the O/C ratio. 

Figure 18 shows the O/C ratio calculated from each individual spectrum collected with single 

pulse LIBS for the diesel fuel and the RDX residue. Everything above a certain O/C threshold is 

classified as an energetic material, while everything below it is classified as a non-energetic. 

The threshold was swept through all the O/C values calculated from spectra collected with single 

pulse LIBS in order to generate the solid ROC curve in figure 19. The circle indicates the point 

on the graph where 75% of the energetic RDX is identified with a false positive rate of 7%. In 

order to obtain >90% detection of RDX, the threshold is moved down on figure 18, and the 

position on the ROC curve follows the direction of the arrow. At this point, the false positive 

rate rises to 25%. Next, the O/C ratios from diesel fuel and RDX residue on Al were calculated 

from the spectra collected by double pulse LIBS. The threshold was swept through all the O/C 

ratios in order to generate a ROC curve based on data collected with double pulse LIBS. The 

ROC curve generated from these data is shown in figure 19. A comparison of the two ROC 

curves shows that the double pulse method improves discrimination. In order to obtain >90% 

detection of RDX in double pulse LIBS, the false positive rate is only 2.5% compared to 25% 

with single pulse LIBS. The double pulse LIBS minimizes the interference of atmospheric 

oxygen, more closely obtaining the true value of the O/C ratio of RDX and diesel fuel enhancing 

the discrimination. 
22

Figure 18. O/C peak atomic emission ratio for each single pulse LIBS spectrum taken of RDX residue 

and diesel fuel residue. (The dotted line represents the threshold that is moved along the y 

axis in order to create the ROC curve in figure 19.) 

By combining LIBS data with principal components analysis (PCA), we have previously been 

able to discriminate between similar substances (14). PCA is a technique for simplifying a data 

set in order to ease analysis. It groups variables in the data and transforms them into principal 

components that describe the variance within the data. Instead of relying on one ratio, we can 

use all the atomic emission ratios associated with the energetic material calculated from the LIBS 

spectra (O/C, O/H, N/C, N/H, and O/N) can be used. Since most of the variance occurs in the 

first principal component (PC1) (>70%), we use the PC1 score values as the threshold to create 

our ROC curves. Figure 20 shows the scores for the first principal component for each sample, 

calculated from the five atomic emission ratios for each LIBS spectrum of RDX residue and 

diesel fuel by single pulse LIBS. Everything above the threshold is considered an energetic. The 

threshold is swept through the PC1 score values in order to generate the solid ROC curve in 

figure 21. The process is repeated for the LIBS spectra collected via double pulse LIBS in order 

to generate the dashed ROC curve in figure 21. Comparing the ROC curves in figure 19 

generated from just one atomic emission ratio, O/C, with the ROC curves in figure 21 generated 

from PCA with the first principal component based on several atomic emission ratios, one can 

plainly see the improvement in discrimination. Furthermore, the data from the double pulse 

spectra in figure 21 represent the ideal ROC curve, 100% identification of RDX with 0% false 

positive rate. 
23

Figure 19. ROC curves created from O/C peak atomic emission ratios from single pulse and double 

pulse LIBS spectra of RDX and diesel fuel residues. (The circle represents a point on the 

ROC curve where 75% of the RDX samples are identified correctly as RDX and 7% of the 

diesel fuel samples are false positives [incorrectly identified as RDX]. Moving in the 

direction of the arrow increases the percentage of correct RDX identification but also 

increases the false positive rate. Moving this direction corresponds to lowering the 

threshold in figure 18. The ROC curve generated from the double pulse LIBS spectra is 

closer to an ideal predictor [figure 17].) 

4. Conclusions 

We have completed a LIBS survey of energetic materials and explosives, noting distinct 

differences between the inorganic explosive black powder and organic explosives; we also 

identified key elemental and molecular emission lines. In addition, we have tested the idea of 

using a broadband LIBS spectral library to identify an unknown explosive material (black 

powder) and its constituent components. Note that the laser-induced microplasma has never 

initiated detonation of any of the energetic materials that we have worked with because the shortlived spark provides an insufficient ignition source. However, we do expect detonation of 

materials that are especially shock sensitive as well as ignition of reactive gaseous mixtures. 

In order to better discriminate energetic material from non-energetic material, interference from 

atmospheric nitrogen and oxygen must be minimized. Two methods for diminishing the 

atmospheric interference have been studied: displacing the atmosphere with an inert gas and 
24

Figure 20. First principal component from atomic emission ratios O/C, O/H, N/C, N/H, and 

O/N from each single pulse LIBS spectrum of RDX and diesel fuel residue. (The 

dotted line represents the threshold that is moved along the y axis in order to create 

the ROC curve in figure 21.) 

Figure 21. ROC curves created from the first principal component calculated from atomic emission 

ratios from single pulse and double pulse LIBS spectra of RDX and diesel fuel residue in 

figure 20. (The double pulse ROC curve represents complete separation between RDX 

and diesel fuel residue [100% correct identification].) 
25

double pulse LIBS. Displacing the surrounding atmosphere with argon eliminates all 

atmospheric nitrogen and oxygen. The nitrogen and oxygen atomic emission lines in the LIBS 

spectra collected from samples under argon are characteristic of the material composition. We 

have also investigated the capability of double pulse LIBS to minimize the oxygen and the 

nitrogen from the surrounding atmosphere, since delivering an argon flow to the sample surface 

at stand-off distances (tens of meters) would be impractical. 

The double pulse LIBS spectra have been shown to effectively diminish the interference of 

atmospheric oxygen and nitrogen because of the decrease in gas density caused by the first laser 

pulse (28–31). Thus, the oxygen and nitrogen from the atmosphere do not interfere with the 

LIBS signal of the sample. Using ROC curves and PCA, we have shown that double pulse LIBS 

improves the discrimination between an energetic sample (RDX residue) and a non-energetic 

sample (diesel fuel). The double pulse technique, although not quite as effective as displacing 

the atmosphere with an inert gas in order to minimize atmospheric oxygen and nitrogen 

interference, has the advantage of being applicable to a stand-off LIBS system. An ensuing 

report (17) will discuss the extension of the techniques described here to stand-off distances and 

will present more advanced chemometric techniques to improve the selectivity of LIBS for 

explosives detection. 
26

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