Simultaneous Raman-LIBS for the Standoff Analysis of Explosive Materials

VOL. 22 NO. 3 (2010) ARTICLE

Simultaneous Raman-LIBS for the standoff analysis of explosive materials

Javier Moros, Juan Antonio Lorenzo, Patricia Lucena, Luciano Miguel Tobaria and José Javier Laserna* Department of Analytical Chemistry, Faculty of Sciences, University of Málaga, E-29071 Málaga, Spain

Introduction ent order, Raman signals can be over- papers have been published demon- The problem of detecting, recognising whelmed by the background continuum strating Raman and LIBS sequential data and identifying explosives at significant emission from the LIBS plasma, and laser acquisition (that is, refreshing the sample standoff distances has proved one of the power density (as a function of the area position and modifying the laser pulse most difficult—and most important—chal- size of the target where laser energy energy or the beam focal conditions) but lenges during recent years, being today, comes into contact with) imposes strict in these situations, Raman and LIBS infor- one of the most demanding applica- instrumental conditions on the detector mation may not provide any direct rela- tions of spectroscopic techniques. The as well as for the optical system used for tion as the spectral data do not belong to limited number of sophisticated avail- laser focusing. the same interrogated target area.4 able techniques potentially capable of The concept of extracting information “Sensor fusion” permits the integrated standoff detection of minimal amounts on both spectroscopic phenomena from use of the Raman and LIBS spectroscopic of explosives is based on laser spec- a sample using a single instrument has techniques for achieving either better troscopy. Of the recently developed been around for a while. Indeed, several performance or reducing the penalties techniques, Raman spectroscopy and laser-induced breakdown spectroscopy

(LIBS) are considered significant for their G H potential for homeland defence applica- tions.1,2 These two techniques both inter- rogate samples using laser beams, both use spectrographs to disperse the signal over overlapping spectral ranges and both A require approximately the same spectral resolution. Additionally, both techniques C are very complementary in the data they provide; LIBS yields detailed information on multi-elemental composition, whereas Raman spectroscopy yields information about molecular composition. Thus, LIBS and Raman spectroscopy have several I D B features that make a combined instrument for remote analysis very attractive.3 However, beyond these first quick F glances at the two complementary E approaches, there are many problems G 10 that make it difficult for both techniques to operate in a complementary, simultane- Figure 1. Experimental set-up of the standoff dual Raman-LIBS sensor. A) Nd:YAG laser ous way, because of constraints imposed (532 nm); B) beam expander; C) telescope; D) laser power sources; E) pulse and delay genera- by their signal collections. Raman scat- tors; F) spectrographs; G) bifurcated optical fibre coupled into a collimating lens; H) holographic tering and atomic emission processes SuperNotch filter; I) personal computer. The inset shows the telescope optical layout. Reprinted take place in temporal windows of differ- with permission from Reference 6 (Copyright 2010, American Chemical Society).

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occurring when each sensor is used separately. It may provide an improved DNT NH4NO3 A solution in terms of required identification confidence, since combining simultaneously collected data from both sensors then a more accurate assessment of the sensed target can be obtained.5

This article presents the first experimental fused sensor development permitting one to obtain standoff, instant, simultaneous, separate Raman spectra and LIBS data from single laser events on the same spots of the target.

Raman-LIBS hybrid sensor B system Figure 1 illustrates the mobile hybrid sensor system used for simultaneous Raman and LIBS spectroscopy based standoff analysis. The radiation source is a Quantel Brilliant Twins Q-switched –1 Nd:YAG laser (10 Hz, 400 mJ pulse , @ 532 nm, 5.5 ns pulse width); a 10× large output beam expander (Special

Optics, Wharton, NJ, USA) is employed for first expanding and then focusing the beam onto a well-defined distant spot. C

Both spectroscopic signals are collected using a home-made Cassegrain telescope (167 cm in length and 24 cm in diameter), and the return light is focused onto the tip of a bifurcated optical fibre (600 μm in diameter mounted on a precision linear stage). Fibres are coupled to a pair of identical gated Shamrock sr-303i Czerny–Turner spec- trometers, each fitted with an Andor iStar intensified charge-coupled device (CCD) Figure 2. Standoff temporal behaviour for DNT (2,4-dinitrotoluene) and NH NO Raman scat- camera as detectors. The Raman spec- 4 3 tering and LIBS emission signals using the mobile hybrid sensor system. Each Raman spectrum trometer uses a 300 grooves per mm is the result of 25 accumulated laser pulses using 0.25 GW cm–2 and 4 GW cm–2 irradiance values grating, whereas the LIBS spectrometer for out-of-focus (A) and in-focus (B) studies, respectively. Each LIBS signal (C) belongs to the uses a 150 grooves per mm grating. A ensemble-average of spectra resulting from 10 laser pulses using 4 GW cm–2. Exposure time Holographic SuperNotch filter (Kaiser values of 5.5 ns and 500 ns, for Raman and LIBS, were used, respectively. Reprinted with permis- Optical Systems) is placed in front of the sion from Reference 6 (Copyright 2010, American Chemical Society). fibre to remove the Rayleigh-scattered light at 532 nm. Two pulse and delay generators (Berkeley Nucleonics model to avoid oscillations of both the focus- incident light interaction with the mole- 565–4C) are also included to aid in the ing point on the target and on the light cule and photon scattering occur almost synchronisation of the experiment. The collection point at the optical fibre. simultaneously, the Raman phenomenon entire system is mounted on a wheeled, lifetime remaining within the laser pulse easily transportable cart equipped with Timing time regime and photons are scattered levelling feet for guaranteeing system When simultaneous molecular and during the laser pulse width (5.5 ns). stability once it is located at the desired multi-elemental analytical information Furthermore, Raman scattering competes location. Furthermore, the laser and tele- needs to be separately collected a time- with molecular fluorescence emission scope are also mounted onto a pair of resolved study for signal acquisition is the since both phenomena have similar pneumatic levelling isolation mounts first critical issue. In Raman spectroscopy origins, and they must be discriminated www.spectroscopyeurope.com SPECTROSCOPYEUROPE 19 VOL. 22 NO. 3 (2010) ARTICLE

Time-resolved, low-irradiance experi- Raman LIBS ments (Figure 2A) permit one to identify the most appropriate timing parameters to record the Raman spectrum without interference from fluorescence emission, DNT (nitroaromatic) which, as a function of its intensity, can render the acquisition of useful Raman spectra very difficult. Whenever fluorescence is excited, the Raman signature of the material “rides” on top of a broad

non-specific baseline and thus, significant

growth in the background signal can be

observed. Under the operating conditions

H15 explored, its presence slightly appears at

(nitramine) gate delays longer than a few ns.

However, for simultaneous collection

of Raman and LIBS information from the

same laser event on the same target spot,

timing parameters for Raman signals acqui-

sition must be evaluated under high-irradi-

ance experiments (Figure 2B) that cause

laser-induced breakdown to occur. Under

Goma2-ECO this condition bremsstrahlung and unspe- (plastic) cific recombination emission take place after plasma formation and the Raman signature again “rides” on top of a broad non-specific highly tilted baseline, which may totally mask the weakest Raman bands at delay times of tens of ns. Thus Raman acquisition requires a delay time to be set large enough to allow the

NaClO3 Raman signal to develop but short enough (inorganic) to avoid masking of the molecular fingerprint by the LIBS background. For optimum LIBS signal collection a gate delay of about 1 μs is required in order to avoid collecting the unspecific background signal, and the LIBS signal must be acquired during just a Figure 3. Dual behaviour plots for Raman-LIBS responses from explosive materials, as a function few μs after its occurrence, see Figure 2C. of different irradiance conditions, evaluated by checking molecular bands related to NO2 symmet- ν – ric stretching (DNT), (C–N–C) symmetric ring-breathing vibration mode (H15), NO3 symmetric – Laser power stretching (Goma2-ECO) and ClO3 symmetric stretching (NaClO3) vibrations and emission lines for C2 system (DNT), Hα (656.4 nm) (H15 and Goma2-ECO) and Na (589.8 nm) (NaClO3). For all spectroscopic information simul- Reprinted with permission from Reference 6 (Copyright 2010, American Chemical Society). taneously occurring during a single laser event, optimal combination of pulse energy and beam focal conditions by proper timing, thus permitting one to In this sense, since temporal windows resulting in effective excitation of both collect only those Raman photons scat- in all instances broadly differ, the time- phenomena must be found. The success tered during the laser pulse and rejecting resolved capability offered by gated in simultaneous information acquisition is the vast majority of fluorescence. multi-channel detectors can overcome linked to the intensity distribution profile of In contrast, atomic emission that the problem of phenomena discrimina- the laser beam. Thus, after the interaction follows laser ablation is a much slower tion. Temporal evolution of Raman and of the laser pulse on the surface of the process (heating, vaporisation, atomisa- LIBS signals (spectra acquisition at differ- target, several processes combine, includ- tion, ionisation and radiative emission ent delays—gate delay—from the output ing ablation and laser-induced breakdown must take place) with LIBS spectra being signal of the laser to the opening of the in the inner part of the beam, where the detected once the plasma continuum camera intensifier tube) are illustrated in irradiance is above the ablation threshold emission is almost extinguished. Figure 2. of the material, and molecular scattering

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occurs in the outer and less intense region x 20 4000 of the beam. 1600000 A 3500 Evaluation of the behaviour of LIBS 1400000 3000 1200000 emission lines together with observation 2500 1000000 1336 2000 of Raman bands for several materials 800000 Counts under a wide range of irradiance condi- 600000 1500

Raman intensity (a.u.) tions provides scientists with vital infor- 400000 1000 1100 1594 3070 200000 1510 2920 500 mation on optimum energy and spot 632 858 0 0 size for laser focusing. 1000 2000 3000 4000 300 400 500 600 700 800 Raman shift (cm-1) 2500 Wavelength (nm) 70000 As an example, Figure 3 shows irradi- B 1358 2000 ance effects on dual Raman-LIBS simul- 60000

1500 taneous signals illustrating the intensity of 50000 1525 1200 3098 792 2940 the most characteristic Raman feature and Counts 1000 40000

the most intense emission signal for some Raman intensity (a.u.) representative substances. As shown, a 30000 500 2 560 mJ laser pulse focused to a 2.5 mm 20000 0 1000 2000 3000 4000 300 400 500 600 700 800 Raman shift (cm-1) 2500 Wavelength (nm) area spot ensures an adequate trade-off, 120000 C since under these experimental condi- 2000 1360 tions, irradiance is well above the thresh- 100000 old level required for inducing standoff 1500 1530 80000 1614 Counts laser-induced breakdown in the differ- 1000 Raman intensity (a.u.) ent solids tested, whereas the irradiance 60000 500 is low enough to avoid totally masking of 40000 0 the Raman signature of the material. 1000 2000 3000 4000 300 400 500 600 700 800 -1 Raman shift (cm ) 2700 Wavelength (nm) 80000 D 2400 876 Signal acquisition 70000 2100

1214 1800 60000 While single-shot LIBS measurements 1272 1500 have demonstrated tremendous possibil- 50000 Counts 1200 ities for chemical analysis as well as for 40000 2940 900 2992 Raman intensity (a.u.) samples identification, including energetic 600 30000 materials located on a surface at differ- 300 20000 0 1000 2000 3000 4000 300 400 500 600 700 800 ent distances from the instrument, multi- -1 Raman shift (cm ) 1800 Wavelength (nm) 80000 ple-shots must be accumulated before a E 1600 70000 1400 useful Raman signal-to-noise ratio (SNR) 880 1262 1308 1200 builds up. 60000 1214 1000 50000 This fact is particularly relevant in simul- Counts 800 2960 40000 600

taneous spectra acquisition because the Raman intensity (a.u.) 400 number of accumulated laser pulses for 30000 200 Raman spectrum collection must be set 20000 0 1000 2000 3000 4000 300400500600700800 -1 to a minimum in order to cope with the Raman shift (cm ) 2000 Wavelength (nm) 140000 1268 1800 single-shot capability of LIBS, since the first F 1212 876 1600 120000 shot can ablate all or most of the target. 1400

2937 1200 As expected, Raman SNR grows with the 100000 1000 Counts number of accumulated laser pulses. 800 80000 600 However, a somewhat different behaviour, Raman intensity (a.u.) 60000 400 due to the distinct degree of compactness 13 200 40000 0 of each chemical, their different heteroge- 1000 2000 3000 4000 300 400 500 600 700 800 neities and the laser ablation that occurs Raman shift (cm-1) Wavelength (nm) while the Raman spectra are accumulated, can be observed as a function of Figure 4. (Continues on next page) Raw instant, simultaneous and dual standoff Raman (accu- the compound tested. mulated) and LIBS (averaged) spectra of explosive materials: A) MNT (mono-nitrotoluene), B) DNT (di-nitrotoluene), C) TNT (trinitrotoluene), D) RDX (cyclotrimethylene trinitramine), E) C4, F) While the use of accumulated laser H15, G) Goma2-ECO, H) NH4NO3 and I) NaClO3; recorded at 20 m standoff distance obtained pulses is required for a clear and useful from 10 laser pulses (560 mJ pulse–1 on a 2.5 mm2 spot). Note: the Raman spectrum of MNT is Raman fingerprint, the traditional saturated under the aforementioned experimental conditions. The MNT spectrum obtained under ensemble-average of spectra resulting lower irradiance conditions magnified 20-fold is shown in red. Reprinted with permission from from the same number of laser pulses Reference 6 (Copyright 2010, American Chemical Society). www.spectroscopyeurope.com SPECTROSCOPYEUROPE 21 VOL. 22 NO. 3 (2010) ARTICLE

140000 3000 1040 spectrum and laser-induced break- G 120000 2500 down spectrum for the instant, stand-

100000 2000 off analysis of explosive materials. This hyphenated spectroscopy approach of 80000 1500 Counts Raman-LIBS exploits the energy distribu- 60000 1000 (a.u.) intensity Raman tion profile of the laser beam to extract 40000 500 the vibrational fingerprint from the non- 20000 0 1000 2000 3000 4000 300 400 500 600 700 800 ablated section of the interrogated target Raman shift (cm-1) Wavelength (nm)

400000 within the outer part of the laser beam H 4000 350000 3500 jointly together with the atomic informa- 1040 300000 3000 tion gained from the ablated mass by the

250000 2500 inner part of the laser beam. Under well- 200000 2000 Counts defined operating conditions (timing, 150000 1500 Raman intensity (a.u.) intensity Raman laser power and acquisition) molecular 100000 1000 717 1284 1414 3185 and multi-elemental spectral informa- 50000 500

0 0 tion gathered which belongs to the same 1000 2000 3000 4000 300 400 500 600 700 800 Raman shift (cm-1) Wavelength (nm) interrogated area inspected by the same 270000 50000 laser event is demonstrated. I 935 240000 Despite this exciting achievement, 40000 210000 further evaluations through the imple- 180000 30000

150000 mentation of chemometric tools and data

Counts 20000 120000 fusion strategies for the enhancement of 618 Raman intensity (a.u.) 90000 the systematic analysis capabilities of the 10000 60000 system to be safely used in the standoff

30000 0 1000 2000 3000 4000 300 400 500 600 700 800 detection of explosives residues left (for -1 Raman shift (cm ) Wavelength (nm) example, by human fingerprints) under changing weather conditions (atmos- Figure 4. Continued pheric temperature, relative humidity, wind etc.) with a reduced spectral vari- is used for the final multi-elemental LIBS signals, it must be mentioned that a large ability must be carried out. spectrum. ablated mass per pulse does not neces- Figure 4 shows raw standoff (20 m) sarily result in an increased LIBS signal References dual Raman-LIBS spectra of several intensity since most of the material 1. J.C. Carter, S.M. Angel, M. Lawrence- substances simultaneously acquired. ablated can be ejected from the sample Snyder, J. Scaffidi, R.E. Whipple and surface in the form of non-emitting parti- J.G. Reynolds, Appl. Spectrosc. 59, Sensor features cles. On the other hand, Raman intensi- 769 (2005). In considering the operating parameters, ties do not seem to show an apparent 2. C. López-Moreno, S. Palanco, J.J. some features, including target mass abla- correlation with ablation rates, some- Laserna, F.C. De Lucia, Jr, A.W. tion rate and spectral variability derived from thing that might have been expected Miziolek, J. Rose, R.A. Walters and A. the use of the sensor, must be considered. because the Raman signals are inferred Whitehouse, J. Anal. At. Spectrom. Depending on the target, laser-created from the intact, non-ablated section of 21, 55 (2006). craters produced on solid surfaces can the target which interacts with the laser 3. G. Bazalgette Courrèges-Lacoste, B. span the range of several mm3 volumes, beam. Nevertheless, broad differences Ahlers and F. Rull Pérez, Spectrochim. evidenced by the broad differences in abla- in the properties and formulations of the Acta A 68, 1023 (2007). tion behaviour as a function of the charac- explosives compounds prevent defini- 4. S.K. Sharma, A.K. Misra, P.G. Lucey teristics and nature of the chemical. In spite tive conclusions, and it is not possible to and R.C.F. Lentz, Spectrochim. Acta of the difference in physicochemical prop- extend the aforementioned arguments to A 73, 468 (2009). erties, the degree of compactness of the all compounds. 5. S.R. Robison, The Infrared & Electro- target considered is an important param- However, in general trends, conditions optical Systems Handbook. Emerging eter in ensuring a homogeneous ablation that favour Raman scattering tend to Systems and Technologies, Volume rate (with a low variability in attained mass) reduce LIBS emission and vice versa. 8. SPIE Optical Engineering Press, and compounds with analogous compact- Washington (1993). ness exhibit similar ablation rates in the low Conclusions 6. J. Moros, J.A. Lorenzo, P. Lucena, L. hundred μg per shot. A novel mobile hybrid sensor system has Miguel Tobaria and J.J. Laserna, Anal. Regarding correlation between abla- been developed that is able to simul- Chem. 82, 1389 (2010). tion rates and Raman and LIBS spectral taneously measure both the Raman

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