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Production of aerosols by optical catapulting: Imaging, performance parameters and laser-induced plasma sampling rate
Spectrochimica Acta Part B 89 (2013) 1–6
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Production of aerosols by optical catapulting: Imaging, performance parameters 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 b
National Institute of Laser Enhanced Science, NILES, Cairo University, Giza, Egypt Department of Analytical Chemistry, Faculty of Sciences, University of Malaga, 29071 Malaga, Spain
a r t i c l e
i n f o
Article history: Received 12 April 2013 Accepted 15 August 2013 Available online 20 August 2013 Keywords: Optical catapulting Laser-induced breakdown spectroscopy LIBS Shadowgraphy Solid aerosol
a b s t r a c t Optical catapulting (OC) is a sampling and manipulation method that has been extensively studied in applications ranging from single cells in heterogeneous tissue samples to analysis of explosive residues in human fingerprints. Specifically, analysis of the catapulted material by means of laser-induced breakdown spectroscopy (LIBS) offers a promising approach for the inspection of solid particulate matter. In this work, we focus our attention in the experimental parameters to be optimized for a proper aerosol generation while increasing the particle density in the focal region sampled by LIBS. For this purpose we use shadowgraphy visualization as a diagnostic tool. Shadowgraphic images were acquired for studying the evolution and dynamics of solid aerosols produced by OC. Aluminum silicate particles (0.2–8 μm) were ejected from the substrate using a Q-switched Nd:YAG laser at 1064 nm, while time-resolved images recorded the propagation of the generated aerosol. For LIBS analysis and shadowgraphy visualization, a Q-switched Nd:YAG laser at 1064 nm and 532 nm was employed, respectively. Several parameters such as the time delay between pulses and the effect of laser fluence on the aerosol production have been also investigated. After optimization, the particle density in the sampling focal volume increases 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 extensively tested for aerosol analysis [1,2] for applications in process control, environmental monitoring, biomedicine, security and forensics. Furthermore, the interaction between discrete particles with laser-induced plasma presents some complexities that directly affects the analysis of bulk aerosols by LIBS. Hence, fundamental studies are still needed for a better understanding of the involved processes. In this sense, problems arise from the use of the same laser pulse to induce all the physical processes, including vaporization, dissociation and atomic excitation. In the last few years, two overviews of plasma–particle interaction and its implications for LIBS [3] and LA-ICP [4] analysis have been published. In general, sampling representativeness, quantitative analysis, analysis of nano-materials and the sampling efficiency for LIBS analysis are some of the common issues covered in the literature. Among the aforementioned inconveniences, those concerning matrix effects and fragmentation processes are of upmost importance for quantitative LIBS purposes [5]. For instance, Strauss et al. [6] observed a non-linear correlation between the analyte signal and mass concentration when analyzing large particles during a size-resolved analysis (20–800 nm) of CaCl2 particles. Also, Diwakar and co-workers found that the analyte emission derived from aerosol particles is ⁎ Corresponding author. Fax: +34 952132376. E-mail address: [email protected] (J.J. Laserna). 0584-8547/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sab.2013.08.003
affected by the presence of concomitant elemental fractions [7]. In an attempt to overcome matrix effects, Windom and Hahn [8] proposed a novel configuration, named laser-ablation-LIBS (LA-LIBS), based on a 2-step scheme for i) laser-assisted sampling (LA) of the solid target and transportation of particles for the ii) excitation of analyte species by LIBS. With this configuration, authors maximize the aerosol analyte response and mitigate matrix effects while avoiding the inconveniences appearing during laser–bulk sample interaction. In the last few years, research is mostly focused in a critical understanding of the sampling probability and the selection of appropriate processing algorithms. Although the ensemble average of LIBS spectral data has been commonly used in aerosol analysis [9], signal-to-noise ratios (SNR) and limits of detection (LOD) calculated using this method are far from being satisfactory. Due to this constrain, conditional data analysis emerged as a solution to discard the acquired spectral with no analytical information while improving the SNR and LOD's [10,11]. As commented above, sampling rate (defined as the percentage of LIBS spectra presenting a SNR higher than 3 over a total of 100 analyzed spectra) remains a significant issue for a proper scheme of aerosol analysis based on LIBS. When systems containing a low particle density in the sampling volume must be analyzed, individual particle analysis turns even more critical and single-shot LIBS analysis gains greater significance [12]. Nowadays, new set-ups or methodologies have addressed by several authors in order to overcome this problem [13]. Recently, the combination of optical catapulting with LIBS has been demonstrated as a promising method for aerosol analysis [14,15].
2
M. Abdelhamid et al. / Spectrochimica Acta Part B 89 (2013) 1–6
Optical catapulting produces the aerosolization of particles deposited on a solid substrate by means of the pressure wave generated when a laser beam is transmitted along the solid. The catapulted particles seem to be similar to a solid aerosol which is then analyzed by LIBS. The advantages of OC-LIBS result from the absence of contamination of the material analyzed and the freedom from spectral contribution of the substrate where the specimen is placed that gives optical catapulting, with the fast analytical response of LIBS. Although the use of multiple laser sources may increase the complexity and cost of the experiment, it is a highly effective method for the chemical analysis of residues and fingerprints, especially useful in forensic analysis. Fortes et al. [14] performed a complete study based on this methodology and demonstrated the potential of the technique for the detection of several materials (aluminum silicate, nickel, quartz and stainless-steel). After optimization of experimental parameters (interpulse delay time, sampling distance, laser fluence and the particle size) the sampling rate calculated for aluminum, nickel, silicon and iron was 50.4%, 39%, 41% and 23%, respectively. In addition, fundamental studies on OC and its influence on the aerosol particle formation and the velocity at which the particles are ejected have been also reported [15]. Most recently, OC-LIBS was proposed for the analysis of explosive residues in human fingerprints [16,17]. MNT, DNT and TNT were successfully identified with a sampling rated calculated in 20–25% for real human fingerprints of mass quantity in the range of 0.8–1.7 μg mm−2. Although previous results are quite satisfactory, fundamental studies were focused on the parameters affecting LIBS analysis while the information concerning the dynamics and evolution of the generated aerosol by OC remains unclear. Thus, OC-LIBS methodology requires further research for improving the LIBS sampling rate. In this work, we focus our attention on those experimental parameters that must be optimized for a proper aerosol generation while increasing the particle density in the focal volume region, using shadowgraphy visualization as a diagnostic tool [18–22]. In addition, LIBS analysis would benefit from this optimization. 2. Experimental set-up 2.1. Instrumentation The experimental set-up used in the present work has been described elsewhere [14–16]. Briefly, the instrument consists of two lasers.
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 the focusing lens, beneath the sample platform where the material was deposited over a glass slide. The laser beam is focused into the back of the glass slide via a fused silica biconvex lens of 100 mm focal length. The deposited material was ejected or catapulted out of the upper surface of the glass slide without any mechanical contact. The laser energy was 40 mJ per pulse at a repetition rate of 2 Hz. With this configuration, the ablation formation threshold of the glass slide was calculated at 9.5 J/cm2. The energy density during the experiments was below the plasma formation threshold of glass. Then, catapulted material was analyzed with a second pulsed Qswitched Nd:YAG laser (@1064 nm, 5 ns pulse width and 4 mm beam diameter). The laser beam was directed perpendicular to the catapulting laser beam and parallel to the glass slide surface, and focused by a 75 mm focal length achromatic lens on the particles catapulted by the first laser pulse. In addition, shadowgraphy images were acquired to study the propagation and dynamics of the solid aerosol produces by OC. A pulsed Q-switched Nd:YAG laser (@532 nm) was used for shadowgraphy visualization. Fig. 1 shows a schematic diagram of the experimental set-up used in this report. In the case of shadowgraphy, the laser pulse energy was fixed at 1.4 mJ. The propagating light is collected by a fused silica planoconvex lens, 93 mm focal length and directed to a high resolution spectrometer coupled to an ICCD (AndorIstar DH740-25F-03, 512 × 1024 pixels). Images were taken using an integration time of 50 ns. Operation of the lasers was externally controlled by a pulse delay generator (Stanford Research, model DG535) which allows the synchronization of both lasers and the control of the interpulse delay and data acquisition parameters. 2.2. Samples Material employed in this study was standard aluminum silicate from Duke Scientific Corporation. It is a polydisperse material with particle size ranging from 0.2 to 8 μm. With the objective of maintaining the sample homogeneity and the reproducibility of LIBS results, the same quantity of material was weighed and placed in a glass slide holder of 1 mm depth. The protocol for sample handling was carefully controlled to get a homogeneous and reproducible particle distribution on the holder. The glass was completely free of contamination since its manipulation was made with gloves and the glass slide was cleaned (following the
Fig. 1. Schematic diagram for experimental set up of shadowgraphy experiment.
M. Abdelhamid et al. / Spectrochimica Acta Part B 89 (2013) 1–6
normal procedures of glass surface cleaning) from one experiment to another. In addition, in order to avoid any air turbulence, the whole setup was protected and isolated in a glass housing. 3. Results and discussion OC-LIBS is a new sampling method that has been satisfactory tested in the analysis of solid aerosols. OC-LIBS combines the attributes of LIBS with the advantages of OC in terms of the absence of contamination of the specimen analyzed. OC-LIBS operates at atmospheric pressure and room temperature. Briefly, this approach produces the ejection of particles deposited on a substrate by means of the (almost) instantaneous pressure wave generated when a pulsed laser beam is transmitted along the substrate. The catapulted particles (without any mechanical contact and free from interferences from the substrate) are subsequently analyzed by LIBS. Most particles are ejected from the surface following the preferential direction of the catapulting beam. However, as the time elapses from the laser pulse, the particle cloud spreads, the extent of this process depends on the particle size distribution (poly-dispersity of the original material) and other parameters, including the time at which the aerosol is interrogated (Δt), the laser fluence and the working distance of the catapulted beam, among others.
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height with the maximum density in the aerosol is proportional to the catapulting laser fluence, until plasma formation occurs in the glass support. Fig. 3 shows the influence of the working distance (WD) of the catapulting laser on the aerosol propagation. As noted, the spot area has an effect on the number of catapulted particles and consequently, on the aerosol density. For a better understanding, the fluence of the catapulting 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) the aerosol production is driven by plasma formation and subsequently, the glass substrate is ablated. This fact reduces the maximum distance traveled by the aerosol front and the particle density in the focal volume. On the other hand, fluence values below 9.5 J/cm2 (WD = 120, 130 and 140 mm) favor the transmission of the shock wave through the substrate. In this case, the optimum results were obtained when using 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 the shock wave is less effective. As a conclusion, both parameters (laser fluence and working distance) affect not only the propagation and the aerosol density, but also the spreading of the particles along the propagation axis.
3.1. Effect of laser fluence on the elevation of the aerosol cloud
3.2. Dynamics and evolution of the aerosol produced by optical catapulting
The effect of laser fluence on the production and dynamics of the aerosol will be discussed in this section. In this study, the fluence has been modified to change the catapulting pulse energy. This effect is of great importance since it affects the distance traveled by the aerosol front and the dispersion of the aerosol cloud. For comparative purposes, a fixed Δt of 1 μs, on which the aerosol can be clearly monitored, was selected in order to investigate the effect of laser fluence in the earlier stages of aerosol formation. Fig. 2 shows the maximum distance traveled by the front of the aerosol cloud as a function of the catapulting laser fluence (data were acquired at a working distance of 120 mm). As shown, a clear correspondence between the aerosol propagation distance and the fluence of the catapulting laser can be established. The propagation distance by the aerosol cloud increases until it reaches its maximum value at a fluence of 9.1 J/cm2 as a result of the increased pressure wave experienced by the glass slide. For fluences above the plasma formation threshold of the glass slide, the distance traveled by the aerosol front starts to decrease due to the ablation of the substrate (9.5 J/cm2). In other words, transmission of the pressure wave is less effective when the laser beam energy is converted into plasma state. In fact, this graph corroborates that the
In order to evaluate the spreading of the aerosol, the distribution of particles along the propagation axis at variable times (t) following the catapulting laser event has been studied. For this purpose, shadowgraphic images were acquired. Fig. 4 shows the results obtained from 0 μs to 3 ms in temporal windows of 50 ns. As shown, the propagation of the particle cloud follows the preferential axis of the catapulting beam due to the effect of the pressures waves generated when the laser beam interacts with the rear side of the support. The distance traveled by the aerosol front increases with time until reaching a maximum height at about 50 μs (free-expansion and aggregation phases). From this point, re-deposition of the particles can be observed in the following pictures. After 2 ms, the shape of the aerosol cloud disappears and only clusters of particles remain suspended, thus leading to the collapse of the aerosol (coagulation phase). Consequently, the spreading of the aerosol increases with time and the aerosol cloud becomes widely dispersed at longer t. Also, Fig. 4 shows that due to the effect of gravity and collisions with air molecules (air friction) in the surrounding atmosphere, the aerosol cloud decelerates with time.
Fig. 2. The effect of laser fluence on the maximum distance traveled by the aerosol front.
Fig. 3. The effect of defocusing pulses on the maximum distance traveled by the aerosol front.
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Fig. 4. Shadowgraphy images of the aerosol propagation until 3 ms.
Several mechanisms may take part in aerosol spreading, while at prompt delay times after catapulting the main force contributing to decelerate the aerosol should be the air friction (that highly depends on the density media), at late delay times after the catapulting pulse the main force should be the gravity, which is the same for all the particles. Hence the uniformity in the decay of particles in long delay times while 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 aerosol particles are not in line with the optical axis of the catapulting beam. The maximum distance dcloud traveled by the aerosol cloud was measured at each time t. Fig. 5A presents the data obtained in the range 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 in the first few microseconds) due to the almost instantaneous force exerted by the pressure wave arriving at the particles after crossing the glass slide. Immediately after, the aerosol cloud starts to decelerate by the force of gravity and the friction of the particles against the surrounding atmosphere (air friction). The time range from 0 μs to 5 μs is characterized by the expansion of the particle cloud as shown in Fig. 4, and this period is followed by clustering of the aerosol. Although the aerosol front continues traveling (at reduced speed) beyond 5 μs, the width of the cloud is progressively decreasing due to aggregation. This aggregation phase lasts until approximately 40–60 μs during which the front travels ca. 6.5 mm. Afterwards, the particle cloud tends to stop and only a residual aerosol lasts, which slowly is deposited on the support in the form of large particles as depicted in Fig. 4 for observation times beyond 1 ms. We assume that part of the particles remains suspended in the atmosphere above the support. Unfortunately, these particles 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 velocity. For nanometric sized particles, the initial instantaneous speed may reach 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 difficulties that lie in the nature of the LIBS plasma and also the consequent complexities involved in plasma–particle interaction. This fact results in a decreased sampling rate, non-equilibrium conditions and matrix and fractionation effects. LIBS is a sampling method that has been satisfactory tested for particle detection of optically catapulted particles. An average LIBS spectrum on catapulted aluminum silicate particles is shown in Fig. 6. The emission lines of Al in the spectral range from 350 nm to 460 nm are labeled in the spectra. As observed, spectra obtained provide analytical information with satisfactory SNR values. On the basis of improving the LIBS sampling rate, LIB spectra of Al2O3·2SiO3 particles were acquired. Hence, the influence of analyzing laser energy (ranging from 30 to 180 mJ/pulse) as function of sampling rate was evaluated. Results are plotted in Fig. 7. As shown, sampling rate increased with the laser energy until behavior tends to saturate at 140 mJ/pulse. Thus, it exists a linear correlation between factors that increased the sampling rate from 7% to 85%. Nevertheless, observed saturation could be attributed to plasma shielding, selfabsorption, or even to a completely ablation of particles in the focal volume. These effects may be overcoming or minimized, while working in the range 100–140 mJ/pulse, in order to improve LIBS analysis in terms of signal emission and/or sampling rate.
M. Abdelhamid et al. / Spectrochimica Acta Part B 89 (2013) 1–6
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Fig. 7. % LIBS sampling rate as a function of the analyzing laser energy.
Fig. 5. A) Maximum distance traveled by the aerosol front and B) velocity of the aerosol front as function of time. The free-expansion, aggregation and coagulation phases are labeled in the graph.
4. Conclusion In this work, aerosol generation by optical catapulting has been successfully optimized. Shadowgraphy images were acquired for studying the evolution and dynamics of solid aerosols produced by OC. Effects
of temporal conditions and laser fluence on the elevation of the aerosol cloud are some of the experimental parameters investigated here. At the earliest stages, free-expansion phase (0–2 μs), the catapulted particles are greatly accelerated thanks to the force exerted by the pressure wave arriving the particles. In the range 2–50 μs, the aerosol comprises the aggregation phases in which the particles are slowed down by the air friction and in competition with the pressure wave generated here. The velocity decreases from 15,000 m/s to 2000 m/s in the first 2 μs. At long time, the shape of the aerosol cloud disappears and only re-deposited particles remain suspended, approaching the collapse of the aerosol (coagulation phase). In addition, the dynamics of the aerosol also depends on the mechanisms performed during the catapulting process, driven by plasma formation (catapulting with tightly focused pulses) or driven by the pressure wave transmission along the support (catapulting with defocused pulses, as reported here). Also, the poly-dispersity of particles may also contribute to a differential particle speed inside the aerosol (the smaller the particle sizes the faster the velocities). Then, after optimization, particle density in the focal volume region increases, which improves LIBS performance for aerosol detection in terms of signal emission and/or sampling rate. The observed LIBS sampling rate increased from 50% reported before to approximately 90%, an outstanding result for aerosol analysis. Acknowledgment Research supported by Project CTQ2011-24433 of the Spanish Ministerio de EconomiayCompetitividad. Financial support from Project P07-FQM-03308 of Consejeria de Innovacion, Ciencia y Empresa de la Junta de Andalucia is also gratefully acknowledged. M. Abdelhamid would like to acknowledge the support and hospitality of Malaga University during his stay in Malaga. References
Fig. 6. LIB spectra of catapulted aluminum silicate particle in the spectral range 350– 460 nm.
[1] A.W. Miziolek, V. Palleschi, I. Schechter, Laser Induced Breakdown Spectroscopy (LIBS) Fundamentals and Applications, Cambridge University Press, UK, 2006. [2] F.J. Fortes, J. Moros, P. Lucena, L.M. Cabalin, J.J. Laserna, Laser-induced breakdown spectroscopy, Anal. Chem. 85 (2013) 640–669. [3] V. Hohreiter, D.W. Hahn, Plasma–particle interactions in a laser-induced plasma: implications for laser-induced breakdown spectroscopy, Anal. Chem. 78 (2006) 1509–1514. [4] H. Lindner, K.H. Loper, D.W. Hahn, K. Niemax, The influence of laser–particle interaction in laser induced breakdown spectroscopy and laser ablation inductively coupled plasma spectrometry, Spectrochim. Acta Part B 66 (2011) 179–185. [5] T. Amodeo, C. Dutouquet, F. Tenegal, B. Guizard, H. Maskrot, O. Le Bihan, E. Fréjafon, On-line monitoring of composite nanoparticles synthesized in a pre-industrial laser
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[14] F.J. Fortes, L.M. Cabalin, J.J. Laserna, Laser-induced breakdown spectroscopy of solid aerosols produced by optical catapulting, Spectrochim. Acta Part B 64 (2009) 642–648. [15] F.J. Fortes, J.J. Laserna, Characteristics of solid aerosols produced by optical catapulting studied by laser-induced breakdown spectroscopy, Appl. Surf. Sci. 256 (2010) 5924–5928. [16] M. Abdelhamid, F.J. Fortes, M.A. Harith, J.J. Laserna, Analysis of explosive residues in human fingerprints using optical catapulting-laser-induced breakdown spectroscopy, J. Anal. At. Spectrom. 26 (2011) 1445–1450. [17] M. Abdelhamid, F.J. Fortes, J.J. Laserna, M.A. Harith, Optical catapulting laser induced breakdown spectroscopy (OC-LIBS) and conventional LIBS: a comparative study, AIP Conf. Proc. 1380 (2011) 55–59. [18] J. Koch, S. Schlamp, T. Rösgen, D. Fliegel, D. Günther, Visualization of aerosol particles generated by near infrared nano- and femtosecond laser ablation, Spectrochim. Acta Part B 62 (2007) 20–29. [19] C. Porneala, D.A. Willis, Time-resolved dynamics of nanosecond laser-induced phase explosion, J. Phys. D: Appl. Phys. 42 (2009) 155503(7 pp.). [20] J. Koch, S. Heiroth, T. Lippert, D. Günther, Femtosecond laser ablation: visualization of the aerosol formation process by light scattering and shadowgraphic imaging, Spectrochim. Acta Part B 65 (2010) 943–949. [21] T.A. Schmitz, J. Koch, D. Günther, R. Zenobi, Characterization of aerosol plumes in nanosecond laser ablation of molecular solids at atmospheric pressure, Appl. Phys. B 100 (2010) 521–533. [22] A. Vogel, V. Venugopalan, Mechanisms of pulsed laser ablation of biological tissues, Chem. Rev. 103 (2003) 577–644.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Production of aerosols by optical catapulting: Imaging, performance parameters 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 b
National Institute of Laser Enhanced Science, NILES, Cairo University, Giza, Egypt Department of Analytical Chemistry, Faculty of Sciences, University of Malaga, 29071 Malaga, Spain
a r t i c l e
i n f o
Article history: Received 12 April 2013 Accepted 15 August 2013 Available online 20 August 2013 Keywords: Optical catapulting Laser-induced breakdown spectroscopy LIBS Shadowgraphy Solid aerosol
a b s t r a c t Optical catapulting (OC) is a sampling and manipulation method that has been extensively studied in applications ranging from single cells in heterogeneous tissue samples to analysis of explosive residues in human fingerprints. Specifically, analysis of the catapulted material by means of laser-induced breakdown spectroscopy (LIBS) offers a promising approach for the inspection of solid particulate matter. In this work, we focus our attention in the experimental parameters to be optimized for a proper aerosol generation while increasing the particle density in the focal region sampled by LIBS. For this purpose we use shadowgraphy visualization as a diagnostic tool. Shadowgraphic images were acquired for studying the evolution and dynamics of solid aerosols produced by OC. Aluminum silicate particles (0.2–8 μm) were ejected from the substrate using a Q-switched Nd:YAG laser at 1064 nm, while time-resolved images recorded the propagation of the generated aerosol. For LIBS analysis and shadowgraphy visualization, a Q-switched Nd:YAG laser at 1064 nm and 532 nm was employed, respectively. Several parameters such as the time delay between pulses and the effect of laser fluence on the aerosol production have been also investigated. After optimization, the particle density in the sampling focal volume increases 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 extensively tested for aerosol analysis [1,2] for applications in process control, environmental monitoring, biomedicine, security and forensics. Furthermore, the interaction between discrete particles with laser-induced plasma presents some complexities that directly affects the analysis of bulk aerosols by LIBS. Hence, fundamental studies are still needed for a better understanding of the involved processes. In this sense, problems arise from the use of the same laser pulse to induce all the physical processes, including vaporization, dissociation and atomic excitation. In the last few years, two overviews of plasma–particle interaction and its implications for LIBS [3] and LA-ICP [4] analysis have been published. In general, sampling representativeness, quantitative analysis, analysis of nano-materials and the sampling efficiency for LIBS analysis are some of the common issues covered in the literature. Among the aforementioned inconveniences, those concerning matrix effects and fragmentation processes are of upmost importance for quantitative LIBS purposes [5]. For instance, Strauss et al. [6] observed a non-linear correlation between the analyte signal and mass concentration when analyzing large particles during a size-resolved analysis (20–800 nm) of CaCl2 particles. Also, Diwakar and co-workers found that the analyte emission derived from aerosol particles is ⁎ Corresponding author. Fax: +34 952132376. E-mail address: [email protected] (J.J. Laserna). 0584-8547/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sab.2013.08.003
affected by the presence of concomitant elemental fractions [7]. In an attempt to overcome matrix effects, Windom and Hahn [8] proposed a novel configuration, named laser-ablation-LIBS (LA-LIBS), based on a 2-step scheme for i) laser-assisted sampling (LA) of the solid target and transportation of particles for the ii) excitation of analyte species by LIBS. With this configuration, authors maximize the aerosol analyte response and mitigate matrix effects while avoiding the inconveniences appearing during laser–bulk sample interaction. In the last few years, research is mostly focused in a critical understanding of the sampling probability and the selection of appropriate processing algorithms. Although the ensemble average of LIBS spectral data has been commonly used in aerosol analysis [9], signal-to-noise ratios (SNR) and limits of detection (LOD) calculated using this method are far from being satisfactory. Due to this constrain, conditional data analysis emerged as a solution to discard the acquired spectral with no analytical information while improving the SNR and LOD's [10,11]. As commented above, sampling rate (defined as the percentage of LIBS spectra presenting a SNR higher than 3 over a total of 100 analyzed spectra) remains a significant issue for a proper scheme of aerosol analysis based on LIBS. When systems containing a low particle density in the sampling volume must be analyzed, individual particle analysis turns even more critical and single-shot LIBS analysis gains greater significance [12]. Nowadays, new set-ups or methodologies have addressed by several authors in order to overcome this problem [13]. Recently, the combination of optical catapulting with LIBS has been demonstrated as a promising method for aerosol analysis [14,15].
2
M. Abdelhamid et al. / Spectrochimica Acta Part B 89 (2013) 1–6
Optical catapulting produces the aerosolization of particles deposited on a solid substrate by means of the pressure wave generated when a laser beam is transmitted along the solid. The catapulted particles seem to be similar to a solid aerosol which is then analyzed by LIBS. The advantages of OC-LIBS result from the absence of contamination of the material analyzed and the freedom from spectral contribution of the substrate where the specimen is placed that gives optical catapulting, with the fast analytical response of LIBS. Although the use of multiple laser sources may increase the complexity and cost of the experiment, it is a highly effective method for the chemical analysis of residues and fingerprints, especially useful in forensic analysis. Fortes et al. [14] performed a complete study based on this methodology and demonstrated the potential of the technique for the detection of several materials (aluminum silicate, nickel, quartz and stainless-steel). After optimization of experimental parameters (interpulse delay time, sampling distance, laser fluence and the particle size) the sampling rate calculated for aluminum, nickel, silicon and iron was 50.4%, 39%, 41% and 23%, respectively. In addition, fundamental studies on OC and its influence on the aerosol particle formation and the velocity at which the particles are ejected have been also reported [15]. Most recently, OC-LIBS was proposed for the analysis of explosive residues in human fingerprints [16,17]. MNT, DNT and TNT were successfully identified with a sampling rated calculated in 20–25% for real human fingerprints of mass quantity in the range of 0.8–1.7 μg mm−2. Although previous results are quite satisfactory, fundamental studies were focused on the parameters affecting LIBS analysis while the information concerning the dynamics and evolution of the generated aerosol by OC remains unclear. Thus, OC-LIBS methodology requires further research for improving the LIBS sampling rate. In this work, we focus our attention on those experimental parameters that must be optimized for a proper aerosol generation while increasing the particle density in the focal volume region, using shadowgraphy visualization as a diagnostic tool [18–22]. In addition, LIBS analysis would benefit from this optimization. 2. Experimental set-up 2.1. Instrumentation The experimental set-up used in the present work has been described elsewhere [14–16]. Briefly, the instrument consists of two lasers.
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 the focusing lens, beneath the sample platform where the material was deposited over a glass slide. The laser beam is focused into the back of the glass slide via a fused silica biconvex lens of 100 mm focal length. The deposited material was ejected or catapulted out of the upper surface of the glass slide without any mechanical contact. The laser energy was 40 mJ per pulse at a repetition rate of 2 Hz. With this configuration, the ablation formation threshold of the glass slide was calculated at 9.5 J/cm2. The energy density during the experiments was below the plasma formation threshold of glass. Then, catapulted material was analyzed with a second pulsed Qswitched Nd:YAG laser (@1064 nm, 5 ns pulse width and 4 mm beam diameter). The laser beam was directed perpendicular to the catapulting laser beam and parallel to the glass slide surface, and focused by a 75 mm focal length achromatic lens on the particles catapulted by the first laser pulse. In addition, shadowgraphy images were acquired to study the propagation and dynamics of the solid aerosol produces by OC. A pulsed Q-switched Nd:YAG laser (@532 nm) was used for shadowgraphy visualization. Fig. 1 shows a schematic diagram of the experimental set-up used in this report. In the case of shadowgraphy, the laser pulse energy was fixed at 1.4 mJ. The propagating light is collected by a fused silica planoconvex lens, 93 mm focal length and directed to a high resolution spectrometer coupled to an ICCD (AndorIstar DH740-25F-03, 512 × 1024 pixels). Images were taken using an integration time of 50 ns. Operation of the lasers was externally controlled by a pulse delay generator (Stanford Research, model DG535) which allows the synchronization of both lasers and the control of the interpulse delay and data acquisition parameters. 2.2. Samples Material employed in this study was standard aluminum silicate from Duke Scientific Corporation. It is a polydisperse material with particle size ranging from 0.2 to 8 μm. With the objective of maintaining the sample homogeneity and the reproducibility of LIBS results, the same quantity of material was weighed and placed in a glass slide holder of 1 mm depth. The protocol for sample handling was carefully controlled to get a homogeneous and reproducible particle distribution on the holder. The glass was completely free of contamination since its manipulation was made with gloves and the glass slide was cleaned (following the
Fig. 1. Schematic diagram for experimental set up of shadowgraphy experiment.
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normal procedures of glass surface cleaning) from one experiment to another. In addition, in order to avoid any air turbulence, the whole setup was protected and isolated in a glass housing. 3. Results and discussion OC-LIBS is a new sampling method that has been satisfactory tested in the analysis of solid aerosols. OC-LIBS combines the attributes of LIBS with the advantages of OC in terms of the absence of contamination of the specimen analyzed. OC-LIBS operates at atmospheric pressure and room temperature. Briefly, this approach produces the ejection of particles deposited on a substrate by means of the (almost) instantaneous pressure wave generated when a pulsed laser beam is transmitted along the substrate. The catapulted particles (without any mechanical contact and free from interferences from the substrate) are subsequently analyzed by LIBS. Most particles are ejected from the surface following the preferential direction of the catapulting beam. However, as the time elapses from the laser pulse, the particle cloud spreads, the extent of this process depends on the particle size distribution (poly-dispersity of the original material) and other parameters, including the time at which the aerosol is interrogated (Δt), the laser fluence and the working distance of the catapulted beam, among others.
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height with the maximum density in the aerosol is proportional to the catapulting laser fluence, until plasma formation occurs in the glass support. Fig. 3 shows the influence of the working distance (WD) of the catapulting laser on the aerosol propagation. As noted, the spot area has an effect on the number of catapulted particles and consequently, on the aerosol density. For a better understanding, the fluence of the catapulting 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) the aerosol production is driven by plasma formation and subsequently, the glass substrate is ablated. This fact reduces the maximum distance traveled by the aerosol front and the particle density in the focal volume. On the other hand, fluence values below 9.5 J/cm2 (WD = 120, 130 and 140 mm) favor the transmission of the shock wave through the substrate. In this case, the optimum results were obtained when using 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 the shock wave is less effective. As a conclusion, both parameters (laser fluence and working distance) affect not only the propagation and the aerosol density, but also the spreading of the particles along the propagation axis.
3.1. Effect of laser fluence on the elevation of the aerosol cloud
3.2. Dynamics and evolution of the aerosol produced by optical catapulting
The effect of laser fluence on the production and dynamics of the aerosol will be discussed in this section. In this study, the fluence has been modified to change the catapulting pulse energy. This effect is of great importance since it affects the distance traveled by the aerosol front and the dispersion of the aerosol cloud. For comparative purposes, a fixed Δt of 1 μs, on which the aerosol can be clearly monitored, was selected in order to investigate the effect of laser fluence in the earlier stages of aerosol formation. Fig. 2 shows the maximum distance traveled by the front of the aerosol cloud as a function of the catapulting laser fluence (data were acquired at a working distance of 120 mm). As shown, a clear correspondence between the aerosol propagation distance and the fluence of the catapulting laser can be established. The propagation distance by the aerosol cloud increases until it reaches its maximum value at a fluence of 9.1 J/cm2 as a result of the increased pressure wave experienced by the glass slide. For fluences above the plasma formation threshold of the glass slide, the distance traveled by the aerosol front starts to decrease due to the ablation of the substrate (9.5 J/cm2). In other words, transmission of the pressure wave is less effective when the laser beam energy is converted into plasma state. In fact, this graph corroborates that the
In order to evaluate the spreading of the aerosol, the distribution of particles along the propagation axis at variable times (t) following the catapulting laser event has been studied. For this purpose, shadowgraphic images were acquired. Fig. 4 shows the results obtained from 0 μs to 3 ms in temporal windows of 50 ns. As shown, the propagation of the particle cloud follows the preferential axis of the catapulting beam due to the effect of the pressures waves generated when the laser beam interacts with the rear side of the support. The distance traveled by the aerosol front increases with time until reaching a maximum height at about 50 μs (free-expansion and aggregation phases). From this point, re-deposition of the particles can be observed in the following pictures. After 2 ms, the shape of the aerosol cloud disappears and only clusters of particles remain suspended, thus leading to the collapse of the aerosol (coagulation phase). Consequently, the spreading of the aerosol increases with time and the aerosol cloud becomes widely dispersed at longer t. Also, Fig. 4 shows that due to the effect of gravity and collisions with air molecules (air friction) in the surrounding atmosphere, the aerosol cloud decelerates with time.
Fig. 2. The effect of laser fluence on the maximum distance traveled by the aerosol front.
Fig. 3. The effect of defocusing pulses on the maximum distance traveled by the aerosol front.
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Fig. 4. Shadowgraphy images of the aerosol propagation until 3 ms.
Several mechanisms may take part in aerosol spreading, while at prompt delay times after catapulting the main force contributing to decelerate the aerosol should be the air friction (that highly depends on the density media), at late delay times after the catapulting pulse the main force should be the gravity, which is the same for all the particles. Hence the uniformity in the decay of particles in long delay times while 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 aerosol particles are not in line with the optical axis of the catapulting beam. The maximum distance dcloud traveled by the aerosol cloud was measured at each time t. Fig. 5A presents the data obtained in the range 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 in the first few microseconds) due to the almost instantaneous force exerted by the pressure wave arriving at the particles after crossing the glass slide. Immediately after, the aerosol cloud starts to decelerate by the force of gravity and the friction of the particles against the surrounding atmosphere (air friction). The time range from 0 μs to 5 μs is characterized by the expansion of the particle cloud as shown in Fig. 4, and this period is followed by clustering of the aerosol. Although the aerosol front continues traveling (at reduced speed) beyond 5 μs, the width of the cloud is progressively decreasing due to aggregation. This aggregation phase lasts until approximately 40–60 μs during which the front travels ca. 6.5 mm. Afterwards, the particle cloud tends to stop and only a residual aerosol lasts, which slowly is deposited on the support in the form of large particles as depicted in Fig. 4 for observation times beyond 1 ms. We assume that part of the particles remains suspended in the atmosphere above the support. Unfortunately, these particles 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 velocity. For nanometric sized particles, the initial instantaneous speed may reach 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 difficulties that lie in the nature of the LIBS plasma and also the consequent complexities involved in plasma–particle interaction. This fact results in a decreased sampling rate, non-equilibrium conditions and matrix and fractionation effects. LIBS is a sampling method that has been satisfactory tested for particle detection of optically catapulted particles. An average LIBS spectrum on catapulted aluminum silicate particles is shown in Fig. 6. The emission lines of Al in the spectral range from 350 nm to 460 nm are labeled in the spectra. As observed, spectra obtained provide analytical information with satisfactory SNR values. On the basis of improving the LIBS sampling rate, LIB spectra of Al2O3·2SiO3 particles were acquired. Hence, the influence of analyzing laser energy (ranging from 30 to 180 mJ/pulse) as function of sampling rate was evaluated. Results are plotted in Fig. 7. As shown, sampling rate increased with the laser energy until behavior tends to saturate at 140 mJ/pulse. Thus, it exists a linear correlation between factors that increased the sampling rate from 7% to 85%. Nevertheless, observed saturation could be attributed to plasma shielding, selfabsorption, or even to a completely ablation of particles in the focal volume. These effects may be overcoming or minimized, while working in the range 100–140 mJ/pulse, in order to improve LIBS analysis in terms of signal emission and/or sampling rate.
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Fig. 7. % LIBS sampling rate as a function of the analyzing laser energy.
Fig. 5. A) Maximum distance traveled by the aerosol front and B) velocity of the aerosol front as function of time. The free-expansion, aggregation and coagulation phases are labeled in the graph.
4. Conclusion In this work, aerosol generation by optical catapulting has been successfully optimized. Shadowgraphy images were acquired for studying the evolution and dynamics of solid aerosols produced by OC. Effects
of temporal conditions and laser fluence on the elevation of the aerosol cloud are some of the experimental parameters investigated here. At the earliest stages, free-expansion phase (0–2 μs), the catapulted particles are greatly accelerated thanks to the force exerted by the pressure wave arriving the particles. In the range 2–50 μs, the aerosol comprises the aggregation phases in which the particles are slowed down by the air friction and in competition with the pressure wave generated here. The velocity decreases from 15,000 m/s to 2000 m/s in the first 2 μs. At long time, the shape of the aerosol cloud disappears and only re-deposited particles remain suspended, approaching the collapse of the aerosol (coagulation phase). In addition, the dynamics of the aerosol also depends on the mechanisms performed during the catapulting process, driven by plasma formation (catapulting with tightly focused pulses) or driven by the pressure wave transmission along the support (catapulting with defocused pulses, as reported here). Also, the poly-dispersity of particles may also contribute to a differential particle speed inside the aerosol (the smaller the particle sizes the faster the velocities). Then, after optimization, particle density in the focal volume region increases, which improves LIBS performance for aerosol detection in terms of signal emission and/or sampling rate. The observed LIBS sampling rate increased from 50% reported before to approximately 90%, an outstanding result for aerosol analysis. Acknowledgment Research supported by Project CTQ2011-24433 of the Spanish Ministerio de EconomiayCompetitividad. Financial support from Project P07-FQM-03308 of Consejeria de Innovacion, Ciencia y Empresa de la Junta de Andalucia is also gratefully acknowledged. M. Abdelhamid would like to acknowledge the support and hospitality of Malaga University during his stay in Malaga. References
Fig. 6. LIB spectra of catapulted aluminum silicate particle in the spectral range 350– 460 nm.
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