Spectrochimica Acta Part B 89 (2013) 1–6

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Spectrochimica Acta Part B

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Production of aerosols by optical catapulting: Imaging, performanceparameters and laser-induced plasma sampling rate

M. Abdelhamid a, F.J. Fortes b, A. Fernández-Bravo b, M.A. Harith a, J.J. Laserna b,⁎a National Institute of Laser Enhanced Science, NILES, Cairo University, Giza, Egyptb Department of Analytical Chemistry, Faculty of Sciences, University of Malaga, 29071 Malaga, Spain

⁎ Corresponding author. Fax: +34 952132376.E-mail address: laserna@uma.es (J.J. Laserna).

0584-8547/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.sab.2013.08.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 April 2013Accepted 15 August 2013Available online 20 August 2013

Keywords:Optical catapultingLaser-induced breakdown spectroscopyLIBSShadowgraphySolid aerosol

Optical catapulting (OC) is a sampling and manipulation method that has been extensively studied in applica-tions ranging from single cells in heterogeneous tissue samples to analysis of explosive residues in human finger-prints. Specifically, analysis of the catapultedmaterial bymeans of laser-induced breakdown spectroscopy (LIBS)offers a promising approach for the inspection of solid particulate matter. In this work, we focus our attention inthe experimental parameters to be optimized for a proper aerosol generation while increasing the particledensity in the focal region sampled by LIBS. For this purpose we use shadowgraphy visualization as a diagnostictool. Shadowgraphic images were acquired for studying the evolution and dynamics of solid aerosols producedby OC. Aluminum silicate particles (0.2–8 μm) were ejected from the substrate using a Q-switched Nd:YAGlaser at 1064 nm, while time-resolved images recorded the propagation of the generated aerosol. For LIBS anal-ysis and shadowgraphy visualization, a Q-switchedNd:YAG laser at 1064 nmand532 nmwas employed, respec-tively. Several parameters such as the time delay between pulses and the effect of laser fluence on the aerosolproduction have been also investigated. After optimization, the particle density in the sampling focal volumeincreases while improving the aerosol sampling rate till ca. 90%.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Laser-induced breakdown spectroscopy (LIBS) has been extensivelytested for aerosol analysis [1,2] for applications in process control, envi-ronmental monitoring, biomedicine, security and forensics. Further-more, the interaction between discrete particles with laser-inducedplasma presents some complexities that directly affects the analysis ofbulk aerosols by LIBS. Hence, fundamental studies are still needed fora better understanding of the involved processes. In this sense, prob-lems arise from the use of the same laser pulse to induce all the physicalprocesses, including vaporization, dissociation and atomic excitation. Inthe last few years, two overviews of plasma–particle interaction and itsimplications for LIBS [3] and LA-ICP [4] analysis have been published. Ingeneral, sampling representativeness, quantitative analysis, analysis ofnano-materials and the sampling efficiency for LIBS analysis are someof the common issues covered in the literature.

Among the aforementioned inconveniences, those concerningmatrix effects and fragmentation processes are of upmost importancefor quantitative LIBS purposes [5]. For instance, Strauss et al. [6] ob-served a non-linear correlation between the analyte signal and massconcentration when analyzing large particles during a size-resolvedanalysis (20–800 nm) of CaCl2 particles. Also, Diwakar and co-workersfound that the analyte emission derived from aerosol particles is

ghts reserved.

affected by the presence of concomitant elemental fractions [7]. In an at-tempt to overcome matrix effects, Windom and Hahn [8] proposed anovel configuration, named laser-ablation-LIBS (LA-LIBS), based on a2-step scheme for i) laser-assisted sampling (LA) of the solid targetand transportation of particles for the ii) excitation of analyte speciesby LIBS. With this configuration, authors maximize the aerosol analyteresponse andmitigatematrix effects while avoiding the inconveniencesappearing during laser–bulk sample interaction.

In the last few years, research is mostly focused in a critical under-standing of the sampling probability and the selection of appropriateprocessing algorithms. Although the ensemble average of LIBS spectraldata has been commonly used in aerosol analysis [9], signal-to-noise ra-tios (SNR) and limits of detection (LOD) calculated using this methodare far from being satisfactory. Due to this constrain, conditional dataanalysis emerged as a solution to discard the acquired spectral withno analytical information while improving the SNR and LOD's [10,11].

As commented above, sampling rate (defined as the percentageof LIBS spectra presenting a SNR higher than 3 over a total of 100 ana-lyzed spectra) remains a significant issue for a proper scheme of aerosolanalysis based on LIBS. When systems containing a low particle densityin the sampling volume must be analyzed, individual particle analysisturns evenmore critical and single-shot LIBS analysis gains greater signif-icance [12]. Nowadays, new set-ups ormethodologies have addressed byseveral authors in order to overcome this problem [13].

Recently, the combination of optical catapulting with LIBS has beendemonstrated as a promising method for aerosol analysis [14,15].

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Optical catapulting produces the aerosolization of particles depositedon a solid substrate by means of the pressure wave generated when alaser beam is transmitted along the solid. The catapulted particlesseem to be similar to a solid aerosol which is then analyzed by LIBS.The advantages of OC-LIBS result from the absence of contaminationof the material analyzed and the freedom from spectral contributionof the substrate where the specimen is placed that gives opticalcatapulting, with the fast analytical response of LIBS. Although the useof multiple laser sources may increase the complexity and cost of theexperiment, it is a highly effective method for the chemical analysis ofresidues and fingerprints, especially useful in forensic analysis. Forteset al. [14] performed a complete study based on this methodology anddemonstrated the potential of the technique for the detection of severalmaterials (aluminum silicate, nickel, quartz and stainless-steel). Afteroptimization of experimental parameters (interpulse delay time, sam-pling distance, laser fluence and the particle size) the sampling ratecalculated for aluminum, nickel, silicon and iron was 50.4%, 39%, 41%and 23%, respectively. In addition, fundamental studies on OC and itsinfluence on the aerosol particle formation and the velocity at whichthe particles are ejected have been also reported [15]. Most recently,OC-LIBS was proposed for the analysis of explosive residues in humanfingerprints [16,17]. MNT, DNT and TNT were successfully identifiedwith a sampling rated calculated in 20–25% for real human fingerprintsof mass quantity in the range of 0.8–1.7 μg mm−2.

Although previous results are quite satisfactory, fundamental stud-ies were focused on the parameters affecting LIBS analysis while the in-formation concerning the dynamics and evolution of the generatedaerosol by OC remains unclear. Thus, OC-LIBS methodology requiresfurther research for improving the LIBS sampling rate. In this work, wefocus our attention on those experimental parameters that must be op-timized for a proper aerosol generation while increasing the particledensity in the focal volume region, using shadowgraphy visualizationas a diagnostic tool [18–22]. In addition, LIBS analysis would benefitfrom this optimization.

2. Experimental set-up

2.1. Instrumentation

The experimental set-up used in the present work has been de-scribed elsewhere [14–16]. Briefly, the instrument consists of two lasers.

Fig. 1. Schematic diagram for experimenta

For catapulting purposes, a Q-switch Nd:YAG laser (@1064 nm, 2 Hz,6.5 ns pulse width) was directed via a mirror placed just before thefocusing lens, beneath the sample platform where the material wasdeposited over a glass slide. The laser beam is focused into the back ofthe glass slide via a fused silica biconvex lens of 100 mm focal length.The deposited material was ejected or catapulted out of the uppersurface of the glass slide without any mechanical contact. The laser en-ergy was 40 mJ per pulse at a repetition rate of 2 Hz. With this configu-ration, the ablation formation threshold of the glass slide was calculatedat 9.5 J/cm2. The energy density during the experiments was below theplasma formation threshold of glass.

Then, catapulted material was analyzed with a second pulsed Q-switchedNd:YAG laser (@1064 nm, 5 ns pulsewidth and4 mmbeamdi-ameter). The laser beam was directed perpendicular to the catapultinglaser beam and parallel to the glass slide surface, and focused by a75 mm focal length achromatic lens on the particles catapulted by thefirst laser pulse. In addition, shadowgraphy images were acquired tostudy the propagation and dynamics of the solid aerosol produces byOC. A pulsed Q-switchedNd:YAG laser (@532 nm)was used for shadow-graphy visualization. Fig. 1 shows a schematic diagramof the experimen-tal set-up used in this report. In the case of shadowgraphy, the laser pulseenergy was fixed at 1.4 mJ. The propagating light is collected by a fusedsilica planoconvex lens, 93 mm focal length and directed to a high res-olution spectrometer coupled to an ICCD (AndorIstar DH740-25F-03,512 × 1024 pixels). Images were taken using an integration time of50 ns. Operation of the lasers was externally controlled by a pulsedelay generator (Stanford Research, model DG535) which allows thesynchronization of both lasers and the control of the interpulse delayand data acquisition parameters.

2.2. Samples

Material employed in this study was standard aluminum silicatefrom Duke Scientific Corporation. It is a polydisperse material withparticle size ranging from 0.2 to 8 μm.With the objective of maintainingthe sample homogeneity and the reproducibility of LIBS results, the samequantity of material was weighed and placed in a glass slide holder of1 mm depth. The protocol for sample handling was carefully controlledto get a homogeneous and reproducible particle distribution on the hold-er. The glass was completely free of contamination since itsmanipulationwas made with gloves and the glass slide was cleaned (following the

l set up of shadowgraphy experiment.

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normal procedures of glass surface cleaning) from one experiment to an-other. In addition, in order to avoid any air turbulence, the whole setupwas protected and isolated in a glass housing.

3. Results and discussion

OC-LIBS is a new sampling method that has been satisfactory testedin the analysis of solid aerosols. OC-LIBS combines the attributes of LIBSwith the advantages of OC in terms of the absence of contamination ofthe specimen analyzed. OC-LIBS operates at atmospheric pressure androom temperature. Briefly, this approach produces the ejection of parti-cles deposited on a substrate by means of the (almost) instantaneouspressure wave generated when a pulsed laser beam is transmittedalong the substrate. The catapulted particles (without any mechanicalcontact and free from interferences from the substrate) are subsequentlyanalyzed by LIBS. Most particles are ejected from the surface followingthe preferential direction of the catapulting beam. However, as the timeelapses from the laser pulse, the particle cloud spreads, the extent ofthis process depends on the particle size distribution (poly-dispersity ofthe original material) and other parameters, including the time atwhich the aerosol is interrogated (Δt), the laser fluence and the workingdistance of the catapulted beam, among others.

3.1. Effect of laser fluence on the elevation of the aerosol cloud

The effect of laserfluence on the production anddynamics of the aero-sol will be discussed in this section. In this study, the fluence has beenmodified to change the catapulting pulse energy. This effect is of great im-portance since it affects the distance traveled by the aerosol front and thedispersion of the aerosol cloud. For comparative purposes, a fixed Δt of1 μs, on which the aerosol can be clearly monitored, was selected inorder to investigate the effect of laser fluence in the earlier stages of aero-sol formation. Fig. 2 shows themaximumdistance traveled by the front ofthe aerosol cloud as a function of the catapulting laser fluence (data wereacquired at a working distance of 120 mm). As shown, a clear correspon-dence between the aerosol propagation distance and the fluence of thecatapulting laser can be established. The propagation distance by theaerosol cloud increases until it reaches its maximum value at a fluenceof 9.1 J/cm2 as a result of the increased pressure wave experienced bythe glass slide. For fluences above the plasma formation threshold of theglass slide, the distance traveled by the aerosol front starts to decreasedue to the ablation of the substrate (9.5 J/cm2). In other words, transmis-sion of the pressure wave is less effective when the laser beam energyis converted into plasma state. In fact, this graph corroborates that the

Fig. 2. The effect of laser fluence on the maximum distance traveled by the aerosol front.

height with the maximum density in the aerosol is proportional to thecatapulting laser fluence, until plasma formation occurs in the glasssupport.

Fig. 3 shows the influence of the working distance (WD) of thecatapulting laser on the aerosol propagation. As noted, the spot areahas an effect on the number of catapulted particles and consequently,on the aerosol density. For a better understanding, the fluence of thecatapulting laser at each working distance is also plotted in the graph.As shown, at WD = 100 mm (spot area: 400 μm; 36 J cm−2) theaerosol production is driven by plasma formation and subsequently,the glass substrate is ablated. This fact reduces the maximum distancetraveled by the aerosol front and the particle density in the focal vol-ume. On the other hand, fluence values below 9.5 J/cm2 (WD = 120,130 and 140 mm) favor the transmission of the shock wave throughthe substrate. In this case, the optimum results were obtained whenusing defocused pulses at a WD of 120 mm (spot area: 790 μm;8.96 J cm−2). From this point, the efficiency in the transmission of theshock wave is less effective. As a conclusion, both parameters (laserfluence and working distance) affect not only the propagation and theaerosol density, but also the spreading of the particles along the propa-gation axis.

3.2. Dynamics and evolution of the aerosol produced by optical catapulting

In order to evaluate the spreading of the aerosol, the distributionof particles along the propagation axis at variable times (t) followingthe catapulting laser event has been studied. For this purpose,shadowgraphic images were acquired. Fig. 4 shows the results obtainedfrom 0 μs to 3 ms in temporal windows of 50 ns. As shown, the propa-gation of the particle cloud follows the preferential axis of thecatapulting beam due to the effect of the pressures waves generatedwhen the laser beam interacts with the rear side of the support. The dis-tance traveled by the aerosol front increases with time until reaching amaximum height at about 50 μs (free-expansion and aggregationphases). From this point, re-deposition of the particles can be observedin the following pictures. After 2 ms, the shape of the aerosol clouddisappears and only clusters of particles remain suspended, thus leadingto the collapse of the aerosol (coagulation phase). Consequently, thespreading of the aerosol increases with time and the aerosol cloudbecomes widely dispersed at longer t. Also, Fig. 4 shows that due tothe effect of gravity and collisions with air molecules (air friction) inthe surrounding atmosphere, the aerosol cloud decelerates with time.

Fig. 3. The effect of defocusing pulses on the maximum distance traveled by the aerosolfront.

Fig. 4. Shadowgraphy images of the aerosol propagation until 3 ms.

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Several mechanisms may take part in aerosol spreading, while atprompt delay times after catapulting the main force contributing todecelerate the aerosol should be the air friction (that highly dependson the density media), at late delay times after the catapulting pulsethemain force should be the gravity, which is the same for all the parti-cles. Hence the uniformity in the decay of particles in long delay timeswhile the propagation of the aerosol is non-uniform at prompt ones.These results are in good agreement with those published elsewhere[14]. Fig. 4 also shows that parts of the flight trajectories of the aerosolparticles are not in line with the optical axis of the catapulting beam.

The maximum distance dcloud traveled by the aerosol cloud wasmeasured at each time t. Fig. 5A presents the data obtained in therange 0–50 μs, whereas Fig. 5B shows the speed of aerosol propagation.Initially, the aerosol is impulsed at a very high speed (above 10 km/s inthe first few microseconds) due to the almost instantaneous forceexerted by the pressure wave arriving at the particles after crossingthe glass slide. Immediately after, the aerosol cloud starts to decelerateby the force of gravity and the friction of the particles against the sur-rounding atmosphere (air friction). The time range from 0 μs to 5 μs ischaracterized by the expansion of the particle cloud as shown in Fig. 4,and this period is followed by clustering of the aerosol. Although theaerosol front continues traveling (at reduced speed) beyond 5 μs, thewidth of the cloud is progressively decreasing due to aggregation. Thisaggregation phase lasts until approximately 40–60 μs during whichthe front travels ca. 6.5 mm. Afterwards, the particle cloud tends tostop and only a residual aerosol lasts, which slowly is deposited on thesupport in the formof large particles as depicted in Fig. 4 for observationtimes beyond 1 ms. We assume that part of the particles remainssuspended in the atmosphere above the support. Unfortunately, theseparticles are too small to be imaged by our shadowgraphic system.The behavior described here corresponds to a polydisperse aluminum

silicate powder with particle sizes in the range from 0.2 μm to 8 μm.Presumably, the smaller the particle, the faster is the propagation veloc-ity. For nanometric sized particles, the initial instantaneous speed mayreach several tens of km/s.

3.3. Improvement of the aerosol sampling rate by OC

LIBS analysis of aerosols is not a trivial task, presenting some difficul-ties that lie in the nature of the LIBS plasma and also the consequent com-plexities involved in plasma–particle interaction. This fact results in adecreased sampling rate, non-equilibrium conditions and matrix andfractionation effects.

LIBS is a sampling method that has been satisfactory tested forparticle detection of optically catapulted particles. An average LIBSspectrum on catapulted aluminum silicate particles is shown inFig. 6. The emission lines of Al in the spectral range from 350 nm to460 nm are labeled in the spectra. As observed, spectra obtained pro-vide analytical information with satisfactory SNR values. On the basisof improving the LIBS sampling rate, LIB spectra of Al2O3·2SiO3 par-ticles were acquired. Hence, the influence of analyzing laser energy(ranging from 30 to 180 mJ/pulse) as function of sampling rate wasevaluated. Results are plotted in Fig. 7. As shown, sampling rateincreased with the laser energy until behavior tends to saturate at140 mJ/pulse. Thus, it exists a linear correlation between factorsthat increased the sampling rate from 7% to 85%. Nevertheless,observed saturation could be attributed to plasma shielding, self-absorption, or even to a completely ablation of particles in the focalvolume. These effects may be overcoming orminimized, while work-ing in the range 100–140 mJ/pulse, in order to improve LIBS analysisin terms of signal emission and/or sampling rate.

Fig. 5. A) Maximum distance traveled by the aerosol front and B) velocity of the aerosolfront as function of time. The free-expansion, aggregation and coagulation phases arelabeled in the graph.

Fig. 7. % LIBS sampling rate as a function of the analyzing laser energy.

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4. Conclusion

In this work, aerosol generation by optical catapulting has been suc-cessfully optimized. Shadowgraphy images were acquired for studyingthe evolution and dynamics of solid aerosols produced by OC. Effects

Fig. 6. LIB spectra of catapulted aluminum silicate particle in the spectral range 350–460 nm.

of temporal conditions and laser fluence on the elevation of the aerosolcloud are some of the experimental parameters investigated here.

At the earliest stages, free-expansion phase (0–2 μs), the catapultedparticles are greatly accelerated thanks to the force exerted by thepressure wave arriving the particles. In the range 2–50 μs, the aerosolcomprises the aggregation phases in which the particles are sloweddown by the air friction and in competition with the pressure wavegenerated here. The velocity decreases from 15,000 m/s to 2000 m/sin the first 2 μs. At long time, the shape of the aerosol cloud disappearsand only re-deposited particles remain suspended, approaching thecollapse of the aerosol (coagulation phase). In addition, the dynamicsof the aerosol also depends on the mechanisms performed duringthe catapulting process, driven by plasma formation (catapulting withtightly focused pulses) or driven by the pressure wave transmissionalong the support (catapulting with defocused pulses, as reportedhere). Also, the poly-dispersity of particles may also contribute to a dif-ferential particle speed inside the aerosol (the smaller the particle sizesthe faster the velocities).

Then, after optimization, particle density in the focal volume regionincreases, which improves LIBS performance for aerosol detection interms of signal emission and/or sampling rate. The observed LIBS sam-pling rate increased from 50% reported before to approximately 90%, anoutstanding result for aerosol analysis.

Acknowledgment

Research supported by Project CTQ2011-24433 of the SpanishMinisterio de EconomiayCompetitividad. Financial support from ProjectP07-FQM-03308 of Consejeria de Innovacion, Ciencia y Empresa de laJunta de Andalucia is also gratefully acknowledged. M. Abdelhamidwould like to acknowledge the support and hospitality of MalagaUniversity during his stay in Malaga.

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