libs como detección de explosivos

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Analysis of explosive and other organic residues by laser induced breakdown spectroscopy V. Lazic a, , A. Palucci a , S. Jovicevic b , C. Poggi a , E. Buono a a ENEA, FIS-LAS, Via. E. Fermi 45, 00044 Frascati (RM), Italy b Institute of Physics, 11080 Belgrade, Pregrevica 118, Serbia abstract article info Article history: Received 30 November 2008 Accepted 31 July 2009 Available online 12 August 2009 Keywords: LIBS Explosives Organic Residues Laser With the aim of realizing a compact instrument for detection of energetic materials at trace levels, laser induced breakdown spectroscopy was applied on residues from nine explosives in air surroundings. Different potentially interfering organic materials were also analyzed. The residues were not uniformly distributed on an aluminum support and single-shot discrimination was attempted. For a single residue type, large shot-to-shot uctuations of the line intensity ratios characteristic for organic samples were observed, which made material classication difcult. It was found that both atomic and molecular emission intensities, as well as their ratios, are strongly affected by an amount of the ablated support material, which mainly determines the plasma temperature. With respect to the spectra from the clean support, emission intensities of atomic oxygen and nitrogen are always reduced in the presence of an organic material, even if its molecules contain these elements. This was attributed to chemical reactions in a plasma containing carbon or its fragments. Hydrogen atomic emission depends strongly on the local humidity above the sampled point and its line intensity shows shot to shot variations up to 50%, also on a homogeneous sample. It is argued that shock waves generated by previous spatially and/or temporally close laser pulses blow away a relatively heavy water aerosol, which later diffuses slowly back towards the sampled point. C 2 and CN exhibit a peak emission behavior with atomic Al emission, and their variable ratio indicates an existence of different formation or removal mechanisms from the plasma, depending on the plasma parameters and on the composition of the organic residue. On the basis of these observations, an attempt is made to establish a suitable procedure for data analysis and to determine the optimal experimental conditions, which would allow for discrimination of explosives from other, potentially interfering, residues. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Explosives are notoriously difcult to detect and methods for their detection can address vapors or particles, dissolved or suspended solids in solutions, or direct probing of solid materials [1,2]. One of the techniques recently proposed for identication of solid explosives and their residues is LIBS, which allows for multi- elemental, in-situ measurements in different surroundings [25]. The measurements are without contact, making stand-off detection possible up to 130 m [69]. LIBS is based on plasma generation by an intense laser pulse, which duration is in nanosecond range or shorter. In the case of solid samples, plasma is produced through laser-induced ablation of the surface layer [3]. An intense laser pulse is also responsible for atomization and ionization of the material. The plasma growth and decay leads to different processes such as: expansion, shock waves formation, continuum (bremsstrahlung) emission and light absorption by free electrons (inverse bremsstrahlung), collisions in the gas with excitation and relaxation of atoms/ions, chemical recombination and, as important for LIBS, de-excitation of the species (atoms, ions and molecules) through optical emission. In LIBS, this radiation is usually detected in the spectral range covering near UV, visible and near IR. The capability to perform non-contact, long-distance, real time multi-element detection makes LIBS very interesting for different applications [35], including detection of hazardous materials [813] and classication of biological agents [1315]. However, LIBS sensitivity and accuracy are generally lower than for some other techniques, such as ICP-OES, mass spectrometry and gas chromatography. In addition, the laser characteristics and the acquisition parameters, the surrounding atmosphere (gas composition and pressure), interaction geometry, surface conditions and collecting optics geometry strongly affect the analytical capabilities of the method [1621]. Under xed experi- mental conditions, the signal intensities and analytical accuracy are Spectrochimica Acta Part B 64 (2009) 10281039 "This paper was presented at the 5th International Conference on Laser-Induced Breakdown Spectroscopy (LIBS 2008), held in Berlin, Adlershof, Germany, 2226 September 2008, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference". Corresponding author. Tel.: +39 06 94005885; fax: +39 06 94005400. E-mail address: [email protected] (V. Lazic). 0584-8547/$ see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.07.035 Contents lists available at ScienceDirect Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab

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Page 1: Libs como detección de explosivos

Spectrochimica Acta Part B 64 (2009) 1028–1039

Contents lists available at ScienceDirect

Spectrochimica Acta Part B

j ourna l homepage: www.e lsev ie r.com/ locate /sab

Analysis of explosive and other organic residues by laser inducedbreakdown spectroscopy☆

V. Lazic a,⁎, A. Palucci a, S. Jovicevic b, C. Poggi a, E. Buono a

a ENEA, FIS-LAS, Via. E. Fermi 45, 00044 Frascati (RM), Italyb Institute of Physics, 11080 Belgrade, Pregrevica 118, Serbia

☆ "This paper was presented at the 5th InternationaBreakdown Spectroscopy (LIBS 2008), held in BerlinSeptember 2008, and is published in the Special Issuededicated to that conference".⁎ Corresponding author. Tel.: +39 06 94005885; fax:

E-mail address: [email protected] (V. Lazic).

0584-8547/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.sab.2009.07.035

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 November 2008Accepted 31 July 2009Available online 12 August 2009

Keywords:LIBSExplosivesOrganicResiduesLaser

With the aim of realizing a compact instrument for detection of energetic materials at trace levels, laserinduced breakdown spectroscopy was applied on residues from nine explosives in air surroundings.Different potentially interfering organic materials were also analyzed. The residues were not uniformlydistributed on an aluminum support and single-shot discrimination was attempted. For a single residue type,large shot-to-shot fluctuations of the line intensity ratios characteristic for organic samples were observed,which made material classification difficult. It was found that both atomic and molecular emissionintensities, as well as their ratios, are strongly affected by an amount of the ablated support material, whichmainly determines the plasma temperature. With respect to the spectra from the clean support, emissionintensities of atomic oxygen and nitrogen are always reduced in the presence of an organic material, even ifits molecules contain these elements. This was attributed to chemical reactions in a plasma containingcarbon or its fragments. Hydrogen atomic emission depends strongly on the local humidity above thesampled point and its line intensity shows shot to shot variations up to 50%, also on a homogeneous sample.It is argued that shock waves generated by previous spatially and/or temporally close laser pulses blow awaya relatively heavy water aerosol, which later diffuses slowly back towards the sampled point. C2 and CNexhibit a peak emission behavior with atomic Al emission, and their variable ratio indicates an existence ofdifferent formation or removal mechanisms from the plasma, depending on the plasma parameters and onthe composition of the organic residue. On the basis of these observations, an attempt is made to establish asuitable procedure for data analysis and to determine the optimal experimental conditions, which wouldallow for discrimination of explosives from other, potentially interfering, residues.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Explosives are notoriously difficult to detect and methods for theirdetection can address vapors or particles, dissolved or suspendedsolids in solutions, or direct probing of solid materials [1,2].

One of the techniques recently proposed for identification of solidexplosives and their residues is LIBS, which allows for multi-elemental, in-situ measurements in different surroundings [2–5].The measurements are without contact, making stand-off detectionpossible up to 130 m [6–9].

LIBS is based on plasma generation by an intense laser pulse, whichduration is in nanosecond range or shorter. In the case of solid samples,plasma is produced through laser-induced ablation of the surface layer

l Conference on Laser-Induced, Adlershof, Germany, 22–26of Spectrochimica Acta Part B,

+39 06 94005400.

ll rights reserved.

[3]. An intense laser pulse is also responsible for atomization andionization of the material. The plasma growth and decay leads todifferent processes such as: expansion, shock waves formation,continuum (bremsstrahlung) emission and light absorption by freeelectrons (inverse bremsstrahlung), collisions in the gas with excitationand relaxation of atoms/ions, chemical recombination and, as importantfor LIBS, de-excitation of the species (atoms, ions and molecules)through optical emission. In LIBS, this radiation is usually detected in thespectral range covering near UV, visible and near IR. The capability toperform non-contact, long-distance, real time multi-element detectionmakes LIBS very interesting for different applications [3–5], includingdetection of hazardous materials [8–13] and classification ofbiological agents [13–15]. However, LIBS sensitivity and accuracyare generally lower than for some other techniques, such as ICP-OES,mass spectrometry and gas chromatography. In addition, the lasercharacteristics and the acquisition parameters, the surroundingatmosphere (gas composition and pressure), interaction geometry,surface conditions and collecting optics geometry strongly affect theanalytical capabilities of the method [16–21]. Under fixed experi-mental conditions, the signal intensities and analytical accuracy are

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1029V. Lazic et al. / Spectrochimica Acta Part B 64 (2009) 1028–1039

further compromised by the so-called matrix effect [21–29], Thematrix effect can be partially overcome by applying calibration withmatrix-matched reference samples. However, the latter is difficult toimplement in multi-elemental in-situ analyses of complex sampleswith unknown or variable matrices. In such applications, it ispreferable to include into calibration different types of the samplematrices [28] and to take into account also the plasma parameters[28,29].

Explosives are organic compounds, containing carbon, hydrogen andoxygen, while nitrogen is present in almost all the high explosives.Commonly, explosives are rich in N and O, and poor in H and C withrespect to other organic substances. LIBS spectra from energeticmaterials normally contain atomic lines from these four elements [8–12], andmolecular bands of CN and C2. On the organic compounds, ioniclines from oxygen and nitrogen are also sometimes observed [30].Intensity of C2 emission can be correlated with number of double CfCbonds in the sample [31,32]. Differently, CN spectra produced fromablation of organic compounds in air surrounding are mainly result ofrecombination through reaction C2+N2=2CN [31–33]. LIBS detectionof native CN bonds in organic materials was obtained only at lowfluences of a UV exciting laser [33].

Rapid LIBS detection of energetic materials with compact instru-ments or at distance is normally performed in air surroundings, sointerference from air components on the LIBS spectra must be takeninto account. The influence of air components can be reduced, but noteliminated, by applying a double-pulse laser excitation [9]. In suchcase, expanding shock-waves following breakdown produced by thefirst laser pulse, reduces locally the gas (air) pressure, so thesecondary, analytical plasma expands into more rarefied atmosphere[34].

Recognition of bulk organic compounds by LIBS can be performedby comparison between the sample spectra and previously estab-lished library, for example through linear correlation [8,11,12,30].Discrimination of organic materials was obtained by comparing lineintensity ratios from H, C, N and O, eventually including some otherelements present as impurities [8]. Molecular emission was some-times also considered in order to improve the discrimination [8]. Aprocedure for recognition of energetic materials is then based onproperly constructed algorithms [7,8], Principal Component Analysis(PCA) [8–10,14,15], Soft Independent Modeling of Class Analogy(SIMCA) [8,13], Neural Networks [10,35] or Partial Least SquareDiscriminant Analysis (PLS-DA) [8].

Analysis of residue materials placed on a support, even if the latterhas a well known composition, is difficult due to the variable amountof sample from one point to another. In such case, the previouslydiscussed matrix effect must be taken into account and differentspectral libraries/models should be constructed for different supportmaterials. When probing residues by LIBS, even if placed on a fixedtype of support, variations in the line intensities and their ratios areexpected to be large, due to other causes such as variable amount ofablated sample, variable quantity of the ablated support material,possible differences in the plasma shape, temperature and electrondensity, and finally possible differences in the chemical reactionsoccurring in the plasma. Chemical reactions depend on the plasmacomposition and its parameters, where higher reached temperaturesalso lead to a more complete sample fragmentation and atomization.Chemistry of LIBS plasma from organic samples is very complex, andits simulation for pure RDX explosive includes 137 species and 577reactions [36]. All the mentioned reasons might contribute to a failureof residue explosive recognition from the LIBS spectra.

Given this background, we studied variations of the characteristicatomic (C, H, N and O) and molecular (C2 and CN) lines in the LIBSspectra obtained on different organic residues, not uniformlydistributed over the substrate. It has been reported [8] that 100%correct residue classification (considering 3 explosives and 3 inter-ferents) andwith an improved class separation, can be obtainedwhen

including into model (PCA or PLS-DA) also impurities (Na, Ca and K)beside C, C2, CN, H, N, and O line intensities. However, we intentionallydid not consider impurities here as their concentrations change withthe origin of the organic material.

We analyzed nine types of residues from pure explosives andresidues of six other organic materials as possible interferents. In allthe cases, residues were placed on a clean aluminum support and LIBSspectra were saved after each laser shot. We identified and explainedsome sources of changes of line intensity ratios, then proposed anddiscussed some procedures for recognition of explosives, based on theline intensities and PCA analysis.

2. Experimental

2.1. Laboratory set-up

The system described here is intended only for laboratorymeasurements and does not represent a prototype system fordetection of explosive residues, which is presently in the testingphase. The laser source is an Nd:YAG (Quantel — Ultra 200) operatedat 1064 nm, characterized by a super Gaussian beam profile. The laserpulse width is about 6 ns, the maximum energy is 300 mJ and themaximum repetition rate is 10 Hz. The laser was triggered manuallyafter repositioning of the sample. Timing between two successivepulses was not fixed and always longer than 10 s. The laser beam wasdeflected towards a horizontally placed sample and focused by aquartz lens with 150 mm focal length. In the optimal position for theLIBS signal intensities, the measured laser spot diameter was of about0.94 mm at an energy of 250 mJ. This energy was used for all themeasurements on organic residues.

Plasma emission was collected at an angle by a wide-angle quartzobjective, and directed to a spectrometer by 0.1 mm diameter quartzfiber. The fiber end was mounted on an entrance slit of a Mechellespectrograph (Andor, Mod. 5000), which allows for simultaneousspectral detection in the range 200–950 nm with resolving power(λ/Δλ)=5000. The spectra were recorded by a gated ICCD (iStarDH734), whose gate aperture was synchronized with the laser burstthrough an optical trigger. After preliminary optimization of thegating parameters, all the LIBS measurements on residues or onclean support were performed with gate delay of 800 ns and gatewidth of 30 µs. After each laser shot, a spectrum was saved forfurther analysis. This approach also allows monitoring of the shot-to-shot variations of the spectral intensities.

The sample holder was placed on an X–Y micrometric table. Aftereach laser shot, the sample was displaced by 1.5 mm, which wassufficient to avoid sampling in areas affected by material redepositedfrom previous shots.

2.2. Samples

As a support material for residues, we used aluminum (Antic-orodal) discs, whose surface was intentionally machined roughly(Fig. 1) in order to increase the sticking of the residues and to avoidhigh surface reflectivity. Before sampling, the discs were cleaned inultrasound bath, first with pure acetone and then with distilled water.

All the explosives considered, except DNT (see Table 1), arecommercial solutions with concentration of 1.0 mg/ml or 0.1 mg/ml(TATP) in methyl or ethyl alcohol, or acetonitrile. DNT was purchasedin form of crystal grains and dissolved in pure acetone before placingit onto support. Small droplets of solution containing explosivesspread over aluminum surface were allowed to evaporate thermally,leaving unevenly distributed residues (Fig. 1). Other analyzed, notenergetic materials were directly distributed over the support in thinlayers of uncontrolled thickness.

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Fig. 1. Photo of the HMX residue (white) on the aluminum support; the width of thearea is 1.5 mm.

Table 2Wavelengths of the emission lines used for peak analyses.

Species Peak (nm)

C I 247.8CN 388.3C2 516.3H I 656.2O I 777.4 (triplet)N I 746.8Al I 309.3

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3. Results

LIBS spectrawere initially acquired on the coal sample pressed intoa pellet, by accumulating over 20 laser shots with gate delay andwidth of 800 ns and 50 µs, respectively. The aim of thesemeasurements was to optimize the spectral intervals for calculationof the line peaks/integrals and corresponding background levels. Thecoal pellet was chosen due to its good homogeneity, presence of allthe four elements of interest (C, H, N and O) and because it containedalso other elements (Al, Fe, K, Mg, Ca, Na), which are often present asimpurities in other organic materials. Some emission lines from theseimpurities are close to the to atomic/molecular features characteristicfor the LIBS spectra of explosives and might lead to an erroneousevaluation of the characteristic line intensities and/or their back-ground level.

Temporal behaviors of the atomic and molecular emissions werealso studied on the same coal sample, by changing the gate delay andusing short gate widths (250 ns, 500 ns or 1000 ns). Hydrogenemission disappears after about 7 µs, a time about twice longer thanthat observed for nitrogen lines. Emission from CN and C2 wasdetectable up to 30 µs, when enlarging the gate width to 5 µs. All theconsidered lines (Table 2) have smooth, approximately exponentialdecay with time constants of 1–1.5 µs for atomic lines (the longest isfor H) and 12–13 µs for CN and C2. The duration of the emission linesin the plasma was further confirmed by measurements at a fixed gatedelay (800 ns), while increasing progressively the gate width.

Table 1Residues analyzed.

Name Composition Notes

EGDN C2H4N2O6 ExplosiveNG C3H5N3O9 ExplosiveRDX C3H6N6O6 ExplosiveTNT C7H5N3O6 Explosive, contains aromatic ringDNT C7H6N2O4 Explosive, contains aromatic ringPETN C6H8N4O12 ExplosiveHMX C4H8N8O8 ExplosiveTETRYL C7H5N5O8 Explosive, contains aromatic ringTATP C9H18O6 Explosive, not containing NDiesel oil C10H22–C15H32 Might contain aromatic rings up to 25%Paraffin wax C20H42–C40H82 High purity, for dental useGrease lubricant Unknown Contains hydrocarbonsCoal NIST 1632b C 78%, H 5.1%, N 1.6% Volatile matter 35.4 %wtGlue LOCTITE C5H5NO2 Main componentHand cream – Containing glycerol

Molecular line intensities reach the maximum Signal-to-Noise ratio(SNR) for gate width of about 30 µs, while for the atomic lines thisoccurs for gate widths below 10 µs. In particular, C emission reachesthe maximum SNR already for gate width of 1 µs. Increasing the signalintegration time up to 3 ms does not deteriorate SNR of the lines,which is important in a view of employingmuch cheaper and compactspectrometers without gating option.

In the following, the gate width was fixed to 30 µs in order tocapture the whole molecular emission. The optimal acquisition delaywas then searched for the spectra produced on clean support and forthe same in presence of diesel residues. In both cases, the optimaldelay was between 800 and 1000 ns. In all the successive measure-ments, the gate delay was fixed at 800 ns.

On all the residues considered, in addition to the emission from H,N and O, it was possible to obtain single-shot spectra containingsufficiently intense C and CN lines. Inmany cases, C2 emissionwas alsodetectable. When analyzing the lines, we initially considered theirpeak values (Table 2) after subtraction of an average, nearbybackground. Before processing, the spectra were corrected for thewavelength dependent instrumental response. All the files inside asingle directory, corresponding to one measurement run per sample,were processed automatically by custom routines written underLabView. Intensities of the lines with calculated SNR below 3.0 wereautomatically settled to zero.

The measured correlations between different line intensities ortheir ratios were rather poor and one of the reasons is a frequently lowSNR for C, CN and C2 peaks. Some slight improvements were obtainedby calculating the line integrals instead of their peak values, but thescattering of the data points remained large. As a consequence, byapplying an algorithm or PCA analyses on the single shot spectra, inmany cases it was not possible to discriminate the explosives frominterfering materials. In the following, the plots relative to the lineemission intensities and their ratios were examined in order todetermine the reasons for such large fluctuations.

3.1. Clean support

First, the signal behavior on clean support was investigated. In thiscase, emission from H, N and O in plasma can be attributed to the airsurroundings. A potential small contribution to O emission comingfrom surface oxidation can be considered as a characteristic of thesupport. However, this contribution should not be significant, as thematerial is Anticorodal, chosen for its low oxidation. In order toextend a comparison between plasma on clean support and the samein presence of residue, the laser energy was varied from 160 to 250mJ.

We observed that O and N emission intensities slightly fluctuatefrom one laser shot to another, around a mean value which was lowerat the minimum energy used. The O/N ratio, measured over 30 shotsat 250 mJ, remains stable within RSDb0.05. The corresponding RSDfor O/H and H/N ratios was as high as 0.26 (Table 3). The last tworatios might significantly change from one day to another, dependingon the environmental humidity. However, this fact does not explainthe high RSD of the line ratios that include H, particularly for thespectra acquired in the same hour of the same day. Successively, weobserved that the highest H/N and the lowest O/H ratio always

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Table 3Average peak ratios O/H, H/N and O/N and corresponding RSD over 30 laser shots,measured on the clean aluminum support in three different days.

Energy (mJ) O/H RSD H/N RSD O/N RSD

Day 1 190 4.74 0.26 0.59 0.26 2.642 0.047Day 2 190 6.68 0.15 0.40 0.12 2.633 0.044Day 3 190–250 4.35 0.22 0.65 0.23 2.689 0.037

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appeared after the first laser shot of the measurement run. Later, H/Nand O/H fluctuate strongly, whereas the H line intensity seems todepend on the timing interval between the successive shots. Sucheffect could be explained by a reduction of local humidity above thesampled point, caused by the shock waves generated after ablation bythe previous laser shot. Considering that air humidity at roomtemperature is not related to single H2O molecules but rather tomuch heavier water aerosol droplets, a relatively slow aerosoldiffusion back to the proximity of the sampling spot seems realistic.A confirmation of this supposition was obtained by using another LIBSset up, not well comparable with the one described in this paper.Although the laser energy at 1064 nm was the same (250 mJ), thepulse duration, beam profile, spot size and collecting optics weredifferent in the two cases. These LIBS measurements were performedat a fixed laser repetition rate (1 Hz), while constantly moving thesample at different linear velocities. The spectra registered after thefirst laser shot within the measurement run, always exhibit thestrongest H emission for spot-to-spot distances up to about 10 mm(Fig. 2). The closer are the successive spots, the more laser shots areneeded to reach a stable H emission intensity, which might be as lowas 50% of the value measured by the first laser shot. If the distancebetween the laser spots exceeds a certain value, in our case estimatedto about 10 mm, the H line intensity does not vary evidently from oneshot to another but fluctuates around some mean value. On ahomogeneous sample (as the clean Al support) these fluctuationscould be minimized by keeping constant the laser repetition rate andscanning speed, and by discarding the spectra acquired after the firstlaser shots. However, in the case of non-uniformly distributedresidues, the ablation rate and shock wave intensity vary from shotto shot. Consequently, changes in H emission intensity caused by thelaser-induced shock waves must be considered during LIBS measure-ments in air surroundings. This fact makes more difficult to identifyresidues through the emission intensity of this element.

In the experiment described in this paper, the timing between thelaser pulses was not constant (manual triggering) and alwaysexceeding 10 s. Although this timing is very large, we assume thatthe observed significant variations of H intensity were indeed caused

Fig. 2. Hydrogen peak intensity measured on clean aluminum support as a function ofthe shot number: d is the center-to-center distance between the successive laser spots.The laser repetition rate is 1 Hz, and the pulse energy is 250 mJ.

by the shockwaves as the distance between the laser spots (measuredfrom center-to-center) was small (1.5 mm).

3.2. Effects of chemical reactions in plasma

We compared emission intensities of H, N and O obtained on thesupport with and without organic residues. In Fig. 3 the results areshown for diesel, TNT and HMX. Themeasured H emission is higher inpresence of the residues, as expected from their molecular formula.Differently, the lines from O and N are always less intense than in thecase of clean support. These two elements are not present in diesel,containing only carbon, hydrogen and some impurities, and theobserved O and N lines in the spectra are also coming exclusively fromdissociation of air molecules. TNT and HMX contain both O and N;however, O and N line intensities are again much lower than in thecase of clean support. This is a clear indication that atomic O and N arepartially lost due to the occurrence of some chemical reactions duringthe detection window. Considering the molecular composition ofdiesel (hydrocarbon), it could be deduced that a presence of carbon(atoms, molecules or fragments) or an excess of hydrogen in theplasma, is responsible for the depletion of N and O atoms (Fig. 3). Ifthis depletion was caused by hydrogen, we would observe it from thespectra obtained on clean support in the conditions corresponding tovery different atmospheric humidity. From one day to another, duringablation of the support we detected changes in the average Hemission intensity (line integrals) up to 40%. However, such largechanges did not induce any visible variations in O and N lineintensities. Consequently, we might deduce that a presence of atomiccarbon or carbon containing molecules/fragments in the plasma isresponsible for a partial removal of atomic O and N through somechemical reactions. Placing an organic residue on the support,reduction of O and N emission intensities are evident with respectto the clean support. This offers an alternative possibility to detectcarbon in residues andwith a higher sensitivity than through its muchweaker atomic and/or molecular emissions. Emission from atomiccarbon, after discarding the spectra not containing it, has a growingtendency with the support ablation, although corresponding to asmaller amount of residue inside the laser spot. An increase of Cemission with reduced quantity of residue shall be discussed later.

The considered atomic lines have similar trends with the Alintensity (Fig. 3) once the latter appears in the spectra (thinnerresidue), but their ratios are not constant. In Fig. 4 some of the atomicratios proposed for identification of explosives by LIBS [8] are shownas a function of Al emission intensity. In the case of clean support,increase of the material ablation, obtained by changing the laserenergy from 160 mJ to 250 mJ, leads to O/H and H/N changes of about24%, with decreasing and growing trends, respectively. Concurrently,O/N ratio is slightly reduced by about 7%. On different residues, H/Nand O/N ratios are constant or decreasing with Al emission intensity,except in absence of support ablation, as well observable on dieselresidue. The measured ratios of C/N and O/H have different trends(increasing, decreasing or almost constant) with support ablation,depending on the residue type. A special case is represented bynitroglycerin, in which the atomic line ratios are almost independenton Al emission in the plasma (not shown). Comparing the behavior ofthese four ratios with respect to Al emission, it was not possible todistinguish explosives from other organic residues. Increase of theatomic emission from residues with an amount of support ablation(Figs. 3 and 5) is caused by a much more efficient ionization of the Alcompared to that of the organicmolecules. This leads to higher plasmatemperatures, as shown in Fig. 5 (diesel residue). Here, the plasmatemperatures were calculated as average values obtained from thefrom the Boltzmann plot and relative to the spectra with similar Alintensities (cases A–C). The Boltzmann plot was applied on Al atomiclines with the upper level energies above 4.5 eV (cases B, C) or above4 eV (case A). This limitation was set in order to reduce self-

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Fig. 3. Comparison of the integrated line intensities from O, N, H and C observed for the clean support and in the presence of organic residues: diesel, TNT and HMX.

Fig. 4. Integrated line intensity ratios H/N, O/CN, C/N and O/H as a function of Al intensity, measured on the support and on diesel, RDX and TETRYL residues; the data were smoothedover 3–5 adjacent points.

1032 V. Lazic et al. / Spectrochimica Acta Part B 64 (2009) 1028–1039

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Fig. 5. Line integrated intensity of C, C2, N, and CN, smoothed over 3 adjacent points, as a function of Al emission intensity; the sample is diesel residue. Plasma temperatures averagedover different spectra i.e. points (encircled) are shown.

1033V. Lazic et al. / Spectrochimica Acta Part B 64 (2009) 1028–1039

absorption effects, expected at high Al concentrations. The plasmabecomes hotter for increased support ablation, leading not only to amore efficient line excitation, but also to higher fragmentation andatomization rates of residue material, as evident from Fig. 5. Changingfrom the spectra not containing Al lines to those where they start to

Fig. 6. Integrated intensity ratios of CN/N, C2/CN, C2/N and C2/C as a function of Al inten

appear, the emission intensity of C2 rises abruptly, thus indicating thetransition to a more efficient fragmentation of the organic material. Cand N emissions intensities show a similar initial steep increase. In thecase of diesel, the growth of N intensity is also related to an increaseddissociation rate of N2 present in air. Unlike the case of the atomic

sity, measured on diesel residues; the data were smoothed over 3 adjacent points.

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Fig. 7. Behavior of the C2/CN ratio and of the N and O emission intensities as a functionof parameter B for diesel residue; the arrows at lower figure indicate a growth of Alemission in plasma.

1034 V. Lazic et al. / Spectrochimica Acta Part B 64 (2009) 1028–1039

lines, C2 emission shows a peak behavior with Al intensity. Once themaximum fragmentation is reached, bond breaking of C2 molecules inhotter plasma supplies further C atoms, also contributing to itsintensity growth. This dissociation leaves fewer molecules availablefor formation of CN fragments through the reaction:

C2 þ N2→2CN ð1ÞEmission from CN molecules (Fig. 5) also reaches the maximum at

certain Al intensity, i.e., at a certain plasma temperature. For most ofthe residues analyzed, the maximum in the CN emission appears forapproximately the same intensity of aluminum atomic lines. However,C2/CN ratio also exhibits a peak behavior with the support ablation, i.e.plasma temperature (Fig. 6). This peak occurs in correspondence of theappearance of Al emission. The concentration of N2 molecules comingfrom air is higher at low plasma temperatures and decreases in thehotter plasma due to dissociation, as observable from the trend of Natomic emission. Given the availability of N2 molecules, if reaction (1)was the only pathway for CN formation, its peak emission should occurconcurrentlywith theC2 emissionmaximum(Fig. 5) and theC2/CN ratio(Fig. 6) should be a smooth function of the Al line intensity. On thecontrary, the peak behavior of the C2/CN ratio indicates that also otherchemical reactions leading to CN formation [36] are present in theplasma during the chosen observation window. Due to the differentbehavior ofmolecular and atomic emission intensities in the plasmaas afunction of the support ablation, the ratios of their lines are not constant(Fig. 6), their variations being particularly pronounced for very low orabsent support ablation. Evendiscarding these low intensity spectra, it isclear that the considered ratios CN/N, C2/CN, C2/N and C2/C have a non-linear dependence on Al intensity.

The changes of the line intensity ratios with the amount of residueinside the sampling point could be as large as one order of magnitude.This fact excludes the possibility to identify residues by a simplecomparison of the line intensity ratios relative to the sampleconstituents. Also, it is evident that performing signal averagingover the spectra corresponding to very different support ablationrates is not adequate. When applying chemometric tools, which relyon the trends within the data set, an abrupt change of the lineintensities and their ratios when passing from the bulk similaranalysis (thick residue) to a concurrent support ablation (thinresidue) might compromise the correct sample classification. Fur-thermore, the line intensity ratios do not exhibit linear trendswith thesupport ablation (Fig. 4), even after discarding the spectracorresponding to the absence or to a low emission from the supportmaterial. As the molecular emissions themselves or rationed to otherlines (atomic or molecular) exhibit a peak dependence on Al ablation(see Figs. 5 and 6), their simplistic inclusion into themodel might leadto a reduced data correlation, as we checked on our data set byapplying PCA. In order to improve classification of the organicresidues, it would be important to introduce some other parameters,related to the plasma temperature and chemical reactions, and toobtain a more linear behavior of the line intensities and/or their ratiosinside the data set.

Considering again chemical reaction (1), we compared the CNemission intensity with a parameter B, defined as:

B ¼ IðC2Þ−2⁎K1IðNÞ ð2ÞHere, the first term represents the C2 intensity measured from the

spectra, while the second term takes into account that the N emissionintensity detected from the spectra corresponds to a reduced N2

availability in the plasma. As explained before, both line intensities, I(C2) and I(N), are temperature dependent. In the case of thecompounds containing nitrogen, I(N) increases with the plasmatemperature more rapidly than in the case of pure dissociation of N2

molecules from air. Numerical value of the constant K1 was chosen tobe 0.6, which gives similar weights to both right hand terms in Eq. (2)

for diesel residue when considering the spectra with intermediate Alintensities. For diesel, where we sampled a number of points withthick residue coverage, I(C2) / I(CN) as a function of parameter Bshows better linearity (Fig. 7) with respect to the previous plot(Fig. 6). However, after plotting the atomic emission lines as afunction of B (Fig. 7, bottom), which to a certain extent includes theplasma parameters, we observe again an abrupt change in theirtendencies when support ablation occurs. In this figure, the arrowsindicate the direction of increasing Al emission in the plasma. Afterreaching a certain Al threshold emission, corresponding here to about5·105 counts, the atomic line intensities start to behave linearly withparameter B. This was also confirmed for other residue types afterelimination of the points with low Al intensity, and the examples arereported in Fig. 8. Here, we can observe that the trends of I(N)= f(B)can be opposite from one explosive to another. In particular, I(N) isdecreasing with parameter B in the cases of TNT and EGDN (notshown). H and O emissions decay with parameter B, but, contrary totheir behavior as a function of Al intensity (Fig. 3), it is now possible todiscern TNT and HMX from diesel residue. Carbon emission as afunction of B shows large fluctuations and is not adequate for thesingle shot sample classification. From these results, it seems that H, Nand O intensities as a function of parameter B are other importantcharacteristics to consider in residue classification by LIBS.

3.3. Recognition of explosives residues

As discussed in the previous section, the line intensity ratios of theelements forming organic compounds depend strongly on the amountof the ablated support material. In [8], PCA analysis was performed onresidues of three explosives, namely RDX, TNT and composition-B(36% TNT, 63% RDX and 1% wax), and on three different interferents(dust, fingerprint and lubricant oil). In that work, the residues werealso placed on aluminum support, whose ablation was observed as

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Fig. 8. Line intensities of N, C, H and N as a function of parameter B, for diesel, DNT, HMX and TNT residues.

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well. In PCA analysis, six line intensity ratios, based on their peakvalues, were considered: O/N, O/C, H/C, N/C and O/H. In the resultingthree-dimensional PCA plot, there was a good separation between theconsidered explosives, interferents and clean support. We performedthe same type of analysis on our LIBS data set, which includes residuesof nine explosives, 6 interferents (Table 1) and aluminum support.Except for the latter, the spectra not containing lines of C, CN or C2(SNRb3) were excluded from the analysis. Instead of the peakintensities, we used here the line integrals and obtained the followingscores in PCA analysis: the first PCA components describe 68.9% (PCA1),19.8% (PCA2) and 8.1% (PCA3) of the total variance within the data set.The results for PCA2 and PCA3 as a function of PCA1 are shown in Fig. 9.We do not show here PCA3with respect of PCA2 as it does not improvethe classification for the given data set. On the both plots from Fig. 9, thedata points belonging to the clean support arewell clustered on the left,residues from the interferents are on the extreme right side, while in amiddle majority of the points from the explosive residues are scattered.For a single residue type, increasing value of PCA1 corresponds to lowerAl intensity in spectra. Knowing this, it becomes clear that betterseparation of explosive residues is obtained for high support ablation,where the plasma is hotter and atomization ismore efficient. Also, fromthe graphs reported in the previous section (see Figs. 3 and 4), one cansee thatwith an increased support ablation, different atomic ratios startto have more linear dependence on Al intensity. The worst cases arerepresented by the spectra with very low or absent Al intensity,corresponding to an abrupt change in the line intensities and theirratios. On the plots in Fig. 9, all the points belonging to NG and TATP fallinside the area that encloses the interferents. Unlike the otherexplosives considered here, TATP does not contain nitrogen, so it ismore difficult to distinguish. Some other explosives, such as EGDN,DNT, RDX and HMX are separated from the interferents only for spectrawith high Al emission, while the spectra with low Al emissioncontribute to false negatives (Table 4).

In order to improve the separation between the explosives andinterferents, we tested different line ratios and their sums, butwithout significant improvement in PCA analysis. In particular,including molecular to atomic line ratios into the model deterioratesthe results of analysis, while including the Al intensity improves thembut only in the case of spectra where the emission is high. One of thevariables that seems to improve point clustering is the previouslydefined parameter B, used in addition to the six atomic ratios. Inalternative, we used a new variable A, defined as:

A ¼ B−IðCNÞ þ K2IðCÞ ð3Þ

When defining the parameter B, we considered a loss of N2

molecules due to dissociation. Now, in the new variable A, we also takeinto account the emission from atomic carbon in plasma, whichincreases with plasma temperature because of the more efficient lineexcitation and the dissociation of C2 molecules. To the constant K2 weattributed value of 2.0, chosen to have similarweights of both terms fordiesel residue and considering its spectra with intermediate Alintensities. For all the non explosives (except RDX), the calculatedparameter A has a smaller range of values, grouped close to zero(Fig. 10). It could be observed that the H/N ratio is always above 3.7 forthe tested non explosives. Inside the data set for a single residue, theH/N ratio is lower for higher support ablation. In Fig. 10, the straight linesencompass the area containing all the interferents, and the pointsoutside and not overlapping with well clustered points from thesupport, were considered to belong to explosives. In this case, somepoints of TATP and NG were also correctly identified (Table 4).

Repeating the PCA analysis (Fig. 9) with the six atomic line ratiosbut including the parameter A into model, we obtained the resultsshown in Fig. 11. Now, the points relative to a single residue are morelinearly distributed for high Al intensities in the spectra. Here, weconsidered that the points belonging to the explosives are between

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Fig. 9. PCA analysis for residues and clean support: the straight line of the left delimits the interferents, while the regions on the left of the lines delimit points belonging to the cleansupport.

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two straight lines. For some explosives, the percentage of correctclassification was lower than in the previous PCA analysis, anerroneous classification being always related to the spectra charac-terized by low Al lines intensity. However, some points belonging toTATP and NG, previously resulting as false negatives, now arecorrectly classified (Table 4).

Table 4Percentage of correct sample classification (explosive or not explosive) accordingFigs. 9–11.

Name Fig. 9 Fig. 10 Fig. 11

EGDN 65 65 68NG 0 39 39RDX 95 94 59TNT 100 100 100DNT 79 56 64PETN 100 100 100HMX 95 90 95TETRYL 100 100 100TATP 4 31 54Diesel oil 100 100 100Paraffin wax 100 100 100Grease lubricant 100 100 100Glue LOCTITE 100 100 100Hand cream 100 100 100Support 100 100 100

4. Conclusions

In LIBS analysis of organic residues, here performed on cleanaluminum support in air surroundings, there are different sources ofvariations in the characteristic line intensities and their ratios. For themeasurements in air, large changes of H emission intensity mightoccur also on homogeneous samples (aluminum), as here reported forthe first time. We demonstrated that this effect is due to local changesof air humidity, induced by the shock waves following laser ablationby previous laser pulses. Such changes, leading to H intensityreduction up to 50%, were detected up to a large distance betweenthe laser spots — in our case 10 mm at a laser repetition rate of 1 Hz.The same effect was also observable in a large time scale— in order of10 s, for close sampling spots (1.5 mm). A slow diffusion of airhumidity back to the focal point was explained by the presence ofwater droplet aerosols at room temperature, much heavier and lessdiffusive than air molecules. At the beginning of LIBS sampling in air,themuch higher presence of H in the plasma after the first laser shot isalso reflected in the plasma parameters. This is one of the reasonsexplaining the sometimes reported differences in the LIBS signalbetween the first and successive shots, even on homogeneoussamples.

With the successive increase of the support ablation due to asmaller quantity of residue (thickness) inside the sampled spot, theatomic line intensities (C, H, N and O) initially increase abruptly, laterfollowing an almost linear trend. Their observed rate of growth withsupport ablation, corresponding to a hotter plasma, is not the same: as

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Fig. 10. Behavior of the H/N ratio as a function of parameter A for different residues and for the substrate.

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a consequence, their ratios are subject to changeswhich could be evenas large as one order of magnitude. Furthermore, the line intensityratios show a non-linear behavior with support ablation. The latterfact makes more difficult to classify the samples through observationof their trends inside the data set (chemometrics).

In the presence of any organic residue, a part of atomic O and Nformed by dissociation of molecules from air is lost from the plasmathrough some chemical reactions. We identify carbon or some carboncontaining molecule/fragment as being responsible for this evident Oand N signal depletion. This finding suggests an alternative way ofdetecting carbon in residues through reduction of O and N emissionfrom air, instead of using much weaker carbon atomic or molecularlines.

Emissions from C2 and CN show a peak behavior with the Al lineintensity, with different respective peak positions. Their line intensityratio shows an abrupt change from the spectra not containing Alemission to the spectra where it appears. With a further increase of Al

Fig. 11. PCA analysis using six ato

spectral intensity, C2/CN trend can strongly differ from one type ofresidue to another, thus indicating an occurrence of various chemicalreactions in the plasma. Due to the very different behavior of atomicandmolecular line intensities as a function of the amount of residue, asimplistic inclusion of molecular emission features into a procedurefor residue identificationmight lead to a larger percentage of incorrectclassifications.

Considering that all the line intensities and their ratios are stronglydependent on the plasma temperature, here correlated with thesupport ablation, and that the plasma parameters affect both thefragmentation/atomization and the chemical reactions, the averaging/accumulating procedure of processing LIBS spectra is meaningful onlyif these spectra correspond to similar support ablation rates.

We examined nine types of explosive and six types of other organicresidues, which were not uniformly distributed on the support surface.By the previously proposed PCA analysis, which considers 6 atomicratios [8], enclosing properly all the interferents (identification 100%),

mic ratios and parameter A.

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we classified correctly 100% only three types of the explosives. For TATPandnitroglycerin, the identification completely failed. After including anadditional parameter into PCA analysis, which partially takes intoaccount differences in chemical reactions in plasma, these twoexplosives were also correctly identified for the spectra with strong Alemission. The percentage of incorrect identifications increased for DNTand RDXwith respect to the previous PCA analysis, but occurred alwaysin the case of spectra corresponding to low support ablation. We pointout that the emission lines from impurities were not taken into accounthere, although thismight improve the correct classification for the givenset of samples. This choice is justified by the variations in impurityconcentrations of the organic samples (also explosives) according totheir origin.

From the result presented in this work, we conclude that anaffordable recognition by LIBS of an unknown amount of organicresidues inside the probed spot, must consider both the plasmaparameters and the chemical reactions in the plasma. We also suggestthat some other parameters, in addition to the line intensity ratios,should be introduced into chemometric models in order to improvethe linearity of the trends inside the data set. In the present paper, wedo not explain or model really a complex chemistry occurring in theplasma produced on organic residues, but rather we only show that itaffects the sample classification and that it should be taken somehowinto account. We believe that further studies are necessary tounderstand better the processes in the plasma, and that this requiresmore extended and complex studies to be added to those performedonly by LIBS. Also, it requires a much more extensive data setmodeling and testing than that considered up to now.

The best conditions for LIBS analysis of organic residues corre-spond to an efficient atomization of the organic molecules in the hightemperature plasma, obtained for higher ablation rates of the supportmaterial, equivalent to a smaller amount of residue inside the sampledpoint. In absence of the support ablation (thick residue layer), LIBSprobing is equivalent to analysis of bulk explosives, which was not thesubject of our presented studies. During rapid, in-situ analysis ofresidues, the sample preparationmust beminimized, so it is necessaryto consider that the samplematerial shall not be uniformly distributedon the support. This is true also for initially uniformly distributedpowder residues, because the laser induced shock waves blow up thepowder, leaving a non controllable quantity of residue on the support.One possible way to speed the correct classification, which was foundto fail for some explosives when present in a large amount inside thefocal spot, is to insert a warning into the on-line implemented, dataanalysis software — alerting that the residue is too thick! A simple,partial removal of the residue could lead to the correct identificationalready by the successive laser shot.

The role of the acquisition gate delay and width on theclassification of organic residues should be also examined, as gatewidths shorter than those used in this work (30 µs), might reduce theinfluence of chemical reactions and recombination on the acquiredLIBS spectra.

Acknowledgments

This project was funded by EU commission, research project PASR-2006-6thFP-SEC6-PR-203600.

Authors are grateful to colleagues from ENEA: Roberta Fantoni forsuggesting some of the organic interferents to examine, RobertoCiardi and Dino Del Bulgaro for preparing the support materials.

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