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Laser-induced breakdown spectroscopy of solid aerosols produced by optical catapulting
Spectrochimica Acta Part B 64 (2009) 642–648
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Laser-induced breakdown spectroscopy of solid aerosols produced by optical catapulting F.J. Fortes, L.M. Cabalín, J.J. Laserna ⁎ Department of Analytical Chemistry, Faculty of Sciences, University of Malaga, Campus de Teatinos s/n, 29071 Malaga, Spain
a r t i c l e
i n f o
Article history: Received 25 November 2008 Accepted 9 May 2009 Available online 18 May 2009 Keywords: Laser-induced breakdown spectroscopy LIBS Optical catapulting Solid aerosol
a b s t r a c t Laser-induced breakdown spectroscopy of particles ejected by optical catapulting is discussed for the first time. For this purpose, materials deposited on a substrate were ejected and transported from the surface in the form of a solid aerosol by optical catapulting using a neodymium-doped yttrium aluminum garnet (Nd: YAG) laser at 1064 nm. A Q-switched Nd:YAG laser at 532 nm was used for chemical characterization of the particles by laser-induced breakdown spectroscopy. Both lasers were synchronized in order to perform suitable spectral detection. The optical catapulting was optimized and evaluated using aluminum silicate particles, nickel spheres, and quartz and stainless steel particles. Experimental parameters such as the interpulse delay time, the sampling distance, the laser fluence, the sampling rate and the particle size have been studied. A correlation between these parameters and the particle size is reported and discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Laser–matter interaction involves a widespread range of mechanisms including scattering, absorption, desorption of the irradiated area, boiling vaporization, explosive decomposition of the overheated surface as well as the ejection of solid fragments due to photomechanical processes [1–4]. The prevalence of some processes depends on external aspects such as the irradiance, the laser wavelength and the sample morphology among others. During laser–matter interaction acoustic waves due to heating and thermal expansion of the medium can be observed [1]. These waves lead to thermoelastic stress which may result in fracture and ejection of material. Moreover, formation of optical breakdown plasma onto the sample surface is an efficient mechanism for stress wave generation which is almost independent of linear optical absorption. Details on the mechanism of shock generation, the applied pressure and the evolution of the shock wave through the target have been discussed [5]. If the expansion of the plasma generated on the sample surface is delayed by confining it in water or glass, the pressure generated is enhanced. This mode of laser–matter interaction is called confined ablation and it can be used to investigate the shock wave generated by the laser pulse. Fabbro et al. [6] studied the physics of the confinement of laser generated plasmas on a transparent overlay. Experimental parameters such as the amplitude, the time duration and the impulse momentum of the shock wave were
⁎ Corresponding author. Fax: +34 952132376. E-mail address: [email protected] (J.J. Laserna). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.05.006
experimentally and theoretically evaluated. Shock wave generation at irradiances below the plasma formation threshold can be used as an effective method for material ejection when a laser pulse come into contact with the sample surface on which the material has been deposited. This process is termed optical catapulting (OC). This method has been used for some time for single cells isolation in combination with laser microdissection of biological tissues [7–13]. Some advantages of optical catapulting as a sampling method include fast response, no direct contact with the sample and freedom of specimen contamination. Vogel et al. [8,10] studied and evaluated several parameters including the mechanism of laser dissection, the dynamics of the initial phase of laser-induced transport and the driving forces for catapulting with focused and defocused pulses, which could affect the laser catapulting of biological materials. In this work, optical catapulting is used for the first time as a sampling method for particle analysis using laser-induced breakdown spectroscopy (LIBS). As known, LIBS is recognized as a useful analytical method [14–17] in a broad front of applications, including cultural heritage [18–21], environmental monitoring [22–24], underwater analysis [25,26], materials analysis [27–30], aerosols [31–34] and detection of energetic materials [35,36]. The goal of OC–LIBS is to combine the advantages of optical catapulting with the fast analytical response and the multielemental capability of LIBS. Advantages of optical catapulting include the absence of contamination of the specimen analyzed and the freedom from spectral contribution of the substrate where the sample is placed. Also, as demonstrated here, each particle behaves independently in the catapulted material. Thus it is possible to discriminate between different particulate material in the aerosol based on particle size and physical properties of the
F.J. Fortes et al. / Spectrochimica Acta Part B 64 (2009) 642–648 Table 1 Samples used in the chemical detection of optical catapulted material. Material
Component
Density (g/cm3)
Formula
Diametera (μm)
Minerals Metals Metals Quartz
Aluminum silicate Nickel spheres Stainless steel spheres Quartz
2.6 8.9 8.0 2.6
Al2O3·2SiO3 Ni Various SiO2
0.2–6 3–24 64–76 0.35–35
a
The term diameter refers to the size range of the particles analyzed.
sample. LIBS analysis offers the added possibility to identify the catapulted material. OC–LIBS may find applications when chemical analysis of residues and fingerprints is needed, for instance in forensic analysis. In general, the analysis of particulate matter would benefit from OC–LIBS. In this report, several experimental parameters including the laser fluence for optical catapulting, the interpulse delay time, the sampling distance, the sampling rate and the particle size are discussed. Analysis of catapulted particles of various compositions and size are presented. 2. Experimental set-up 2.1. Instrumentation Briefly, the instrument consists of two lasers for catapulting and analysis purposes, respectively. A pulsed Q-switch Nd:YAG laser (Big Sky Laser, Ultra CFR model, MO, USA) operating at its fundamental wavelength was directed via a reflective 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 by a fused silica lens, 100 mm focal length. The laser beam was 20 mm defocused so the lens to sample distance was set at 120 mm. The deposited material was ejected or catapulted out of the glass slide plane without any mechanical contact. The output energy was established at 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. The catapulted sample was analyzed with a second laser, a pulsed Q-switched Nd:YAG laser (Spectron, model SL 284, 5 ns pulse width and 4 mm of diameter) operating at the second harmonic. The laser beam was directed, perpendicular to the first laser beam and parallel to the glass slide surface, through a reflective mirror of 25.4 mm in diameter and focused by a 75 mm focal length achromatic lens on the particles catapulted by the first laser pulse. Operation of the lasers was externally controlled by a pulse delay generator (Stanford Research Systems model DG535) which allows the synchronization of both lasers pulses and the control of the interpulse delay and data acquisition parameters. The plasma emission was collected by an optical fiber (length = 5 m, diameter = 600 μm, NA = 0.22) and guided to the entrance slit (width = 50 μm) of a 0.5 m focal length Czerny–Turner imaging spectrograph (Chromex, model 500 IS, f#8) where it was dispersed by a 300 grooves mm− 1 grating. A 120 nm wavelength range from the spectrally resolved light was detected by an intensified charge-coupled device (ICCD) detector (Stanford Computer Optics, model 4Quick 05, 768 × 512 pixels, 7.8 × 8.7 μm pixel size). The total active area was 6 × 4.5 mm2 and the spectral resolution of the system was 0.15 nm pixel− 1. Operation of the detector is controlled by 4Spec 1.20 software and the values of acquisition delay time, integration time and MCP gain are modified according to the experimental requirements. The laser pulse energies can be varied independently for the two beams. The flashlamp sync of second laser pulse was used to trigger the ICCD detector. The emission signal was corrected by subtraction of the detector dark current which was separately measured for the same experimental conditions. Temporal conditions
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of ICCD detector used during the LIBS analysis were delay (d): 500 ns, integration time (t): 1 μs, gain (g): 700v. The entrance slit was 50 μm and a 300 grooves mm− 1 grating was used. 2.2. Samples Materials employed in this study were standard materials from Duke Scientific Corporation. Polydisperse samples were selected according to different criteria such as the type of material, the size and the density, among others. This series of particles comprises different materials: aluminum silicate, nickel spheres, quartz and stainless steel. Table 1 summarizes the most important features of the samples used. In order to maintain the homogeneity of the sample and the reproducibility of the results, samples were placed on a glass slide holder of 1 mm in depth. 3. Results and discussion 3.1. Optical catapulting For chemical detection by LIBS of particles ejected by optical catapulting without mechanical contact, the first laser pulse works below the plasma formation threshold irradiance of the glass support. The laser radiation impacts (at low irradiance) on one side of a glass slide in order to start a shock wave on the material. After crossing the glass body, the shock wave arrives at the surface of the slide where the sample is located. Ejection of particles follows due to the accumulated stress by the shock wave along the substrate. The material ejected by optical catapulting seems to be similar to a solid aerosol which is then analyzed by LIBS. The spreading of the aerosol particles depends on the particle size distribution (poly-dispersity) and other parameters, including the catapulting beam pulse width, the distance from the surface considered and the time at which the aerosol is interrogated, among others. Fig. 1 depicts a schematic diagram of the particle distribution of the catapulted particles at different times t1 b t2 b t3 after the arrival of the shock wave to the support surface. Particles are ejected from the surface with a hemispherical geometry, but most
Fig. 1. Schematic detail of the optical catapulting fundamentals.
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Fig. 2. Standard deviation spectra in the selected spectral range for A) aluminum silicate, B) nickel spheres, C) quartz and D) stainless steel spheres particles. The energy of laser analysis was fixed at 100 mJ/pulse.
particles follow the preferential direction of the catapulting beam. The speed and distance traveled by the particles are also correlated with the particle size and the irradiance. A higher specimen velocity is expected when the particle size is small and the irradiance is just below the plasma formation threshold. Contrarily, with larger particles at lower irradiances the velocity would be considerably decreased. The specimen velocity and the particle size will also contribute to the sampling rate and the distance from the substrate at which the particles have been catapulted.
been proven well suited for the analysis of largely varying signals and has been applied here to the analysis of LIBS signals from solid particles aerosolized by optical catapulting.
3.2. LIBS analysis of catapulted particles It is well known that the LIBS sampling rate for aerosol analysis is very low, resulting in a significant shot-to-shot signal fluctuation and a large number of acquired spectra with no spectral information regarding the interrogated specimen. Although the ensemble average of LIBS spectral data has been commonly used in aerosol analysis by several authors [31,32], signal-to-noise ratio (SNR) and limit of detection (LOD) calculated using this method is far from being satisfactory. Recently, a new approach based on the standard deviation of the acquired spectra has been proposed for data analysis of aerosols [33,34]. The standard deviation (SD) method represents the variability of the signal and provides information of an unknown sample, avoiding the lost of spectral information due to the low sampling rate when applying the averaging spectra procedure. This SD method has
Fig. 3. Variation of the signal-to-noise ratio of aluminum calculated for each single shot spectrum. The emission line at 396.26 nm was used for SNR estimation.
F.J. Fortes et al. / Spectrochimica Acta Part B 64 (2009) 642–648
Fig. 2A–D shows the standard deviation LIBS spectra of catapulted aluminum silicate, nickel, quartz and stainless steel particles in a kinetical series of 250 spectra in the spectral range from 230 nm to 470 nm. The emission lines of Al, Ni, Si and Fe in the aforementioned spectral range are labeled in the spectra. As observed, spectra obtained by the SD procedure provide spectral information with satisfactory SNR values. In order to discriminate the LIBS spectra corresponding to the catapulted specimens from those data with no spectral information, conditional data analysis [31–34] has been used. In this procedure, the LIBS signal (in a focal volume of 9.5 mm3) is calculated as the integrated area of the emission line normalized to the baseline of the spectra and only those spectra presenting a SNR higher than 3, named real hit, will be considered for data processing. The SNR value in each conventional single shot (intensity-wavelength) spectrum was calculated in order to evaluate the LIBS sampling rate. An example of the number of real hits of Al particles detected by LIBS is shown in Fig. 3. As shown, the emission line of aluminum at 396.26 nm was detected 126 times from the total of 250 single shot acquired spectra, which indicates a sampling rate of 50.4%. The particle sampling rate calculated for nickel, silicon and iron was 39%, 41% and 23%, respectively. With the objective of evaluating the particle distribution along the preferential aerosol growing axis, the interpulse delay time has been studied. Interpulse delay time (Δt) is defined as the time from the
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catapulting shot at which the particle cloud is examined by LIBS. The LIBS sampling rate as a function of Δt (up to 100 μs) for the different materials is shown in Fig. 4. The emission lines of Al (I) at 396.26 nm, Ni (I) at 352.55 nm, Si (I) at 288.16 nm and Fe (I) at 275.09 nm were used for SNR calculations. Series of 250 single shot spectra were acquired at 4 mm from the surface. The error bars represent the standard deviation calculated for 5 replicate experiments. As shown, the particle distribution depends on the interpulse time and the particle size. Small specimens such as aluminum silicate (Fig. 4A) present a particle distribution nearly constant as function of Δt, which indicates that the solid aerosol persist a long time after the catapulting laser shot. On the other hand, aerosols particles with a larger particle diameter (Fig. 4D, stainless steel spheres) survive a lesser time and could only be detected at short interpulse delays. With intermediate particle sizes, the particle distribution exhibits two relative maxima at Δt 20 μs and 70 μs, respectively. It is thus clear that for a given interrogation distance, each particle set is to be sampled according to its size at a given interpulse delay. For the present particles, Δt is 25 μs for aluminum silicate and nickel, 15 μs for quartz and 20 μs for stainless steel. Fig. 5 shows the distribution of particles along the propagation axis. Series of 250 single shots at different distances from the surface were evaluated. This analysis was accomplished at the optimum interpulse delay time previously established for each particle. As
Fig. 4. LIBS sampling rate in the focal volume as a function of the interpulse delay time (up to 100 μs) for the different materials: A) aluminum silicate, B) nickel spheres, C) quartz and D) stainless steel spheres. The emission lines of Al (I) at 396.26 nm, Ni (I) at 352.55 nm, Si (I) at 288.16 nm and Fe (I) at 275.09 nm were used for SNR calculations.
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Fig. 5. LIBS sampling rate in the focal volume as a function of the distance from surface from 1 mm to 11 mm for the materials: A) aluminum silicate, B) nickel spheres, C) quartz and D) stainless steel spheres. The emission lines of Al (I) at 396.26 nm, Ni (I) at 352.55 nm, Si (I) at 288.16 nm and Fe (I) at 275.09 nm were used for SNR calculations.
shown, aluminum silicate particles have a homogeneous distribution along the y axis due to their small size, while nickel and quartz particles present a higher density at 4–5 mm from the surface. In contrast, iron particles are more located near the sample holder due to their higher particle size (64–76 μm). It should be noted that at distances beyond 11 mm, no LIBS signal was detected in all the cases studied probably due to the dispersal of the particle cloud in the open atmosphere and the consequent low particle density in the laser focal volume. Taken into account the particle distribution and the time (previously calculated) at which the aerosol was detected, the specimen velocity could be estimated. In this sense, both aluminum and nickel present the higher specimen velocity, 330 m/s, while a velocity of 260 m/s and 100 m/s was estimated for quartz and stainless steel particles, respectively. The effect of the catapulting pulse energy on the particle distribution has been also studied. Several LIBS spectra at different fluences were acquired at distances from the surface ranging from 1 to 11 mm. The results are plotted in Fig. 6. As shown, for all the materials, a threshold fluence close to 7 J cm–2 should be reached before catapulting starts to occur. In addition, the distance travelled strongly increased with the laser fluence until reaching a maximum value (11 J cm–2). From this irradiance value, the behaviour tends to saturation and only the glass slide support is ablated. For the present glass support, the plasma formation threshold is close to 9.5 J cm–2. The results in Fig. 6 also suggest that the catapulting thresholds depend on particle size, with smaller threshold for lightweight particles.
The final step in this work is the possibility to discriminate particles by LIBS in a heterogeneous mixture of materials. For this purpose, a mixture from 50% aluminum silicate and 50% stainless steel
Fig. 6. Effect of the catapulting laser's fluence respect to the sampling distance at which the maximum number of real hits is detected. LIB spectra in the range 1–11 mm were acquired at different fluence.
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Fig. 7. LIBS discrimination between Al and Fe particles in a heterogeneous mixture 50% aluminum silicate and 50% stainless steel spheres at different distance from surface: A) 2 mm, B) 4 mm and C) 10 mm. LIBS spectrum in the spectral range 230–350 nm showing the emission lines of Al and Fe at different sampling distance.
spheres was prepared. Fig. 7 shows a comparison of the particles hits for Al and Fe as a function of the distance at which the particle distribution is examined. As demonstrated, heavy particles (stainless steel) are distributed closer to the glass support, while lightweight particles (aluminum) are detected at longer distances. Thus, in a mixture of particles each material behaves in respect to optical catapulting according to its physical properties. 4. Conclusions The capability of laser induced plasmas for the analysis of catapulted materials has been studied for the first time by OC–LIBS. LIBS detection of ejected particles by optical catapulting has been demonstrated in the present report. Optical catapulting was used to transport small amounts of different polydisperse samples in the form of a solid aerosol. Data conditional analysis was employed in order to
evaluate the sampling rate of the elements by LIBS. Optimum interpulse delay time of 25 μs for aluminum silicate and nickel, 15 μs for quartz and 20 μs for stainless steel particles were accomplished. A correlation between the distance from surface at which the particle distribution was detected and the particle size have been established. Materials presenting a lower particle size will be catapulted with a higher specimen velocity and consequently could be detected at relatively long distances from the substrate surface. In contrast, weighty particles will be detected very close to the substrate surface due to the lower specimen velocity. In this sense, diameter of the particle could influence in the sampling rate and the maximum distance of particle detection. The capability of LIBS to discriminate a mixture of catapulted materials based on different size and composition has been also demonstrated. Thus, in a mixture of particles each material behaves in respect to optical catapulting according to its physical properties.
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Acknowledgements This work was supported by project CTQ2007-60348/BQU of the Ministerio de Ciencia e Innovación, Madrid, Spain. Authors are grateful to N. Omenetto for introducing us the topic of optical catapulting. References [1] G. Paltauf, P.E. Dyer, Photomechanical processes and effects in ablation, Chem. Rev. 103 (2003) 487–518. [2] S. Georgiou, A. Koubenakis, Laser-induced material ejection from model molecular solids and liquids: mechanisms, implications, and applications, Chem. Rev. 103 (2003) 349–393. [3] I. Apitz, A. Vogel, Material ejection in nanosecond Er:YAG laser ablation of water, liver, and skin, Appl. Phys. A 81 (2005) 329–338. [4] L.V. Zhigilei, E. Leveugle, B.J. Garrison, Y.G. Yingling, M.I. Zeifman, Computer simulations of laser ablation of molecular substrates, Chem. Rev. 103 (2003) 321–347. [5] J.P. Romain, P. Darquey, Shock waves and acceleration of thin films by laser pulses in confined plasma interaction, J. Appl. Phys. 68 (1990) 1926–1928. [6] R. Fabbro, J. Fournier, P. Bailard, D. Devaux, J. Virmont, Physical study of laserproduced plasma in confined geometry, J. Appl. Phys. 68 (1990) 775–784. [7] S. Thalhammer, G. Lahr, A. Clement-Sengewald, W.M. Heckl, R. Burgemeister, K. Schütze, Laser microtools in cell biology and molecular medicine, Laser Phys. 13 (2003) 681–691. [8] A. Vogel, K. Lorenz, V. Horneffer, G. Hüttmann, D. von Smolinski, A. Geberty, Mechanisms of laser-induced dissection and transport of histologic specimens, Biophys. J. 93 (2007) 4481–4500. [9] K. Schutze, Y. Niyaz, M. Stich, A. Buchstaller, Noncontact laser microdissection and catapulting for pure sample capture, Laser Manip. Cells Tissues 82 (2007) 649–673. [10] A. Vogel, V. Horneffer, K. Lorenz, N. Linz, G. Huttmann, A. Gebert, Principles of laser microdissection and catapulting of histologic specimens and live cells, Laser Manip. Cells Tissues 82 (2007) 153–205. [11] A. Clement-Sengewald, T. Buchholz, K. Schutze, U. Berg, F.D. Berg, Noncontact, laser-mediated extraction of polar bodies for prefertilization genetic diagnosis, J. Assist. Reprod. Genet. 19 (2002) 183–194. [12] V. Horneffer, N. Linz, A. Vogel, Principles of laser-induced separation and transport of living cells, J. Biomed. Opt. 12 (2007) 054016. [13] A. Vogel, J. Noack, G. Huttmann, G. Paltauf, Mechanisms of femtosecond laser nanosurgery of cells and tissues, Appl. Phys. B. 81 (2005) 1015–1047. [14] A.W. Miziolek, V. Palleschi, I. Schechter, Laser induced breakdown spectroscopy (LIBS) Fundamentals and applications, Cambridge University Press, UK, 2006. [15] D.A. Rusak, B.C. Castle, B.W. Smith, J.D. Winefordner, Fundamentals and applications of laser-induced breakdown spectroscopy, Crit. Rev. Anal. Chem. 27 (1997) 257–290. [16] J.D. Winefordner, I.B. Gornushkin, D. Pappas, O.I. Matveev, B.W. Smith, Novel uses of lasers in atomic spectroscopy, J. Anal. At. Spectrosc. 15 (2000) 1161–1189. [17] J.M. Vadillo, J.J. Laserna, Laser-induced plasma spectrometry: truly a surface analytical tool, Spectrochim. Acta Part B 59 (2004) 147–161. [18] F.J. Fortes, M. Cortés, M.D. Simón, L.M. Cabalín, J.J. Laserna, Chronocultural sorting of archaeological bronze objects using laser-induced breakdown spectrometry, Anal. Chim. Acta 554 (2005) 136–143. [19] F.J. Fortes, J. Cuñat, L.M. Cabalín, J.J. Laserna, In situ analytical assessment and chemical imaging of historical buildings using a man-portable laser system, Appl. Spectrosc. 61 (2007) 558–564.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Laser-induced breakdown spectroscopy of solid aerosols produced by optical catapulting F.J. Fortes, L.M. Cabalín, J.J. Laserna ⁎ Department of Analytical Chemistry, Faculty of Sciences, University of Malaga, Campus de Teatinos s/n, 29071 Malaga, Spain
a r t i c l e
i n f o
Article history: Received 25 November 2008 Accepted 9 May 2009 Available online 18 May 2009 Keywords: Laser-induced breakdown spectroscopy LIBS Optical catapulting Solid aerosol
a b s t r a c t Laser-induced breakdown spectroscopy of particles ejected by optical catapulting is discussed for the first time. For this purpose, materials deposited on a substrate were ejected and transported from the surface in the form of a solid aerosol by optical catapulting using a neodymium-doped yttrium aluminum garnet (Nd: YAG) laser at 1064 nm. A Q-switched Nd:YAG laser at 532 nm was used for chemical characterization of the particles by laser-induced breakdown spectroscopy. Both lasers were synchronized in order to perform suitable spectral detection. The optical catapulting was optimized and evaluated using aluminum silicate particles, nickel spheres, and quartz and stainless steel particles. Experimental parameters such as the interpulse delay time, the sampling distance, the laser fluence, the sampling rate and the particle size have been studied. A correlation between these parameters and the particle size is reported and discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Laser–matter interaction involves a widespread range of mechanisms including scattering, absorption, desorption of the irradiated area, boiling vaporization, explosive decomposition of the overheated surface as well as the ejection of solid fragments due to photomechanical processes [1–4]. The prevalence of some processes depends on external aspects such as the irradiance, the laser wavelength and the sample morphology among others. During laser–matter interaction acoustic waves due to heating and thermal expansion of the medium can be observed [1]. These waves lead to thermoelastic stress which may result in fracture and ejection of material. Moreover, formation of optical breakdown plasma onto the sample surface is an efficient mechanism for stress wave generation which is almost independent of linear optical absorption. Details on the mechanism of shock generation, the applied pressure and the evolution of the shock wave through the target have been discussed [5]. If the expansion of the plasma generated on the sample surface is delayed by confining it in water or glass, the pressure generated is enhanced. This mode of laser–matter interaction is called confined ablation and it can be used to investigate the shock wave generated by the laser pulse. Fabbro et al. [6] studied the physics of the confinement of laser generated plasmas on a transparent overlay. Experimental parameters such as the amplitude, the time duration and the impulse momentum of the shock wave were
⁎ Corresponding author. Fax: +34 952132376. E-mail address: [email protected] (J.J. Laserna). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.05.006
experimentally and theoretically evaluated. Shock wave generation at irradiances below the plasma formation threshold can be used as an effective method for material ejection when a laser pulse come into contact with the sample surface on which the material has been deposited. This process is termed optical catapulting (OC). This method has been used for some time for single cells isolation in combination with laser microdissection of biological tissues [7–13]. Some advantages of optical catapulting as a sampling method include fast response, no direct contact with the sample and freedom of specimen contamination. Vogel et al. [8,10] studied and evaluated several parameters including the mechanism of laser dissection, the dynamics of the initial phase of laser-induced transport and the driving forces for catapulting with focused and defocused pulses, which could affect the laser catapulting of biological materials. In this work, optical catapulting is used for the first time as a sampling method for particle analysis using laser-induced breakdown spectroscopy (LIBS). As known, LIBS is recognized as a useful analytical method [14–17] in a broad front of applications, including cultural heritage [18–21], environmental monitoring [22–24], underwater analysis [25,26], materials analysis [27–30], aerosols [31–34] and detection of energetic materials [35,36]. The goal of OC–LIBS is to combine the advantages of optical catapulting with the fast analytical response and the multielemental capability of LIBS. Advantages of optical catapulting include the absence of contamination of the specimen analyzed and the freedom from spectral contribution of the substrate where the sample is placed. Also, as demonstrated here, each particle behaves independently in the catapulted material. Thus it is possible to discriminate between different particulate material in the aerosol based on particle size and physical properties of the
F.J. Fortes et al. / Spectrochimica Acta Part B 64 (2009) 642–648 Table 1 Samples used in the chemical detection of optical catapulted material. Material
Component
Density (g/cm3)
Formula
Diametera (μm)
Minerals Metals Metals Quartz
Aluminum silicate Nickel spheres Stainless steel spheres Quartz
2.6 8.9 8.0 2.6
Al2O3·2SiO3 Ni Various SiO2
0.2–6 3–24 64–76 0.35–35
a
The term diameter refers to the size range of the particles analyzed.
sample. LIBS analysis offers the added possibility to identify the catapulted material. OC–LIBS may find applications when chemical analysis of residues and fingerprints is needed, for instance in forensic analysis. In general, the analysis of particulate matter would benefit from OC–LIBS. In this report, several experimental parameters including the laser fluence for optical catapulting, the interpulse delay time, the sampling distance, the sampling rate and the particle size are discussed. Analysis of catapulted particles of various compositions and size are presented. 2. Experimental set-up 2.1. Instrumentation Briefly, the instrument consists of two lasers for catapulting and analysis purposes, respectively. A pulsed Q-switch Nd:YAG laser (Big Sky Laser, Ultra CFR model, MO, USA) operating at its fundamental wavelength was directed via a reflective 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 by a fused silica lens, 100 mm focal length. The laser beam was 20 mm defocused so the lens to sample distance was set at 120 mm. The deposited material was ejected or catapulted out of the glass slide plane without any mechanical contact. The output energy was established at 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. The catapulted sample was analyzed with a second laser, a pulsed Q-switched Nd:YAG laser (Spectron, model SL 284, 5 ns pulse width and 4 mm of diameter) operating at the second harmonic. The laser beam was directed, perpendicular to the first laser beam and parallel to the glass slide surface, through a reflective mirror of 25.4 mm in diameter and focused by a 75 mm focal length achromatic lens on the particles catapulted by the first laser pulse. Operation of the lasers was externally controlled by a pulse delay generator (Stanford Research Systems model DG535) which allows the synchronization of both lasers pulses and the control of the interpulse delay and data acquisition parameters. The plasma emission was collected by an optical fiber (length = 5 m, diameter = 600 μm, NA = 0.22) and guided to the entrance slit (width = 50 μm) of a 0.5 m focal length Czerny–Turner imaging spectrograph (Chromex, model 500 IS, f#8) where it was dispersed by a 300 grooves mm− 1 grating. A 120 nm wavelength range from the spectrally resolved light was detected by an intensified charge-coupled device (ICCD) detector (Stanford Computer Optics, model 4Quick 05, 768 × 512 pixels, 7.8 × 8.7 μm pixel size). The total active area was 6 × 4.5 mm2 and the spectral resolution of the system was 0.15 nm pixel− 1. Operation of the detector is controlled by 4Spec 1.20 software and the values of acquisition delay time, integration time and MCP gain are modified according to the experimental requirements. The laser pulse energies can be varied independently for the two beams. The flashlamp sync of second laser pulse was used to trigger the ICCD detector. The emission signal was corrected by subtraction of the detector dark current which was separately measured for the same experimental conditions. Temporal conditions
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of ICCD detector used during the LIBS analysis were delay (d): 500 ns, integration time (t): 1 μs, gain (g): 700v. The entrance slit was 50 μm and a 300 grooves mm− 1 grating was used. 2.2. Samples Materials employed in this study were standard materials from Duke Scientific Corporation. Polydisperse samples were selected according to different criteria such as the type of material, the size and the density, among others. This series of particles comprises different materials: aluminum silicate, nickel spheres, quartz and stainless steel. Table 1 summarizes the most important features of the samples used. In order to maintain the homogeneity of the sample and the reproducibility of the results, samples were placed on a glass slide holder of 1 mm in depth. 3. Results and discussion 3.1. Optical catapulting For chemical detection by LIBS of particles ejected by optical catapulting without mechanical contact, the first laser pulse works below the plasma formation threshold irradiance of the glass support. The laser radiation impacts (at low irradiance) on one side of a glass slide in order to start a shock wave on the material. After crossing the glass body, the shock wave arrives at the surface of the slide where the sample is located. Ejection of particles follows due to the accumulated stress by the shock wave along the substrate. The material ejected by optical catapulting seems to be similar to a solid aerosol which is then analyzed by LIBS. The spreading of the aerosol particles depends on the particle size distribution (poly-dispersity) and other parameters, including the catapulting beam pulse width, the distance from the surface considered and the time at which the aerosol is interrogated, among others. Fig. 1 depicts a schematic diagram of the particle distribution of the catapulted particles at different times t1 b t2 b t3 after the arrival of the shock wave to the support surface. Particles are ejected from the surface with a hemispherical geometry, but most
Fig. 1. Schematic detail of the optical catapulting fundamentals.
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Fig. 2. Standard deviation spectra in the selected spectral range for A) aluminum silicate, B) nickel spheres, C) quartz and D) stainless steel spheres particles. The energy of laser analysis was fixed at 100 mJ/pulse.
particles follow the preferential direction of the catapulting beam. The speed and distance traveled by the particles are also correlated with the particle size and the irradiance. A higher specimen velocity is expected when the particle size is small and the irradiance is just below the plasma formation threshold. Contrarily, with larger particles at lower irradiances the velocity would be considerably decreased. The specimen velocity and the particle size will also contribute to the sampling rate and the distance from the substrate at which the particles have been catapulted.
been proven well suited for the analysis of largely varying signals and has been applied here to the analysis of LIBS signals from solid particles aerosolized by optical catapulting.
3.2. LIBS analysis of catapulted particles It is well known that the LIBS sampling rate for aerosol analysis is very low, resulting in a significant shot-to-shot signal fluctuation and a large number of acquired spectra with no spectral information regarding the interrogated specimen. Although the ensemble average of LIBS spectral data has been commonly used in aerosol analysis by several authors [31,32], signal-to-noise ratio (SNR) and limit of detection (LOD) calculated using this method is far from being satisfactory. Recently, a new approach based on the standard deviation of the acquired spectra has been proposed for data analysis of aerosols [33,34]. The standard deviation (SD) method represents the variability of the signal and provides information of an unknown sample, avoiding the lost of spectral information due to the low sampling rate when applying the averaging spectra procedure. This SD method has
Fig. 3. Variation of the signal-to-noise ratio of aluminum calculated for each single shot spectrum. The emission line at 396.26 nm was used for SNR estimation.
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Fig. 2A–D shows the standard deviation LIBS spectra of catapulted aluminum silicate, nickel, quartz and stainless steel particles in a kinetical series of 250 spectra in the spectral range from 230 nm to 470 nm. The emission lines of Al, Ni, Si and Fe in the aforementioned spectral range are labeled in the spectra. As observed, spectra obtained by the SD procedure provide spectral information with satisfactory SNR values. In order to discriminate the LIBS spectra corresponding to the catapulted specimens from those data with no spectral information, conditional data analysis [31–34] has been used. In this procedure, the LIBS signal (in a focal volume of 9.5 mm3) is calculated as the integrated area of the emission line normalized to the baseline of the spectra and only those spectra presenting a SNR higher than 3, named real hit, will be considered for data processing. The SNR value in each conventional single shot (intensity-wavelength) spectrum was calculated in order to evaluate the LIBS sampling rate. An example of the number of real hits of Al particles detected by LIBS is shown in Fig. 3. As shown, the emission line of aluminum at 396.26 nm was detected 126 times from the total of 250 single shot acquired spectra, which indicates a sampling rate of 50.4%. The particle sampling rate calculated for nickel, silicon and iron was 39%, 41% and 23%, respectively. With the objective of evaluating the particle distribution along the preferential aerosol growing axis, the interpulse delay time has been studied. Interpulse delay time (Δt) is defined as the time from the
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catapulting shot at which the particle cloud is examined by LIBS. The LIBS sampling rate as a function of Δt (up to 100 μs) for the different materials is shown in Fig. 4. The emission lines of Al (I) at 396.26 nm, Ni (I) at 352.55 nm, Si (I) at 288.16 nm and Fe (I) at 275.09 nm were used for SNR calculations. Series of 250 single shot spectra were acquired at 4 mm from the surface. The error bars represent the standard deviation calculated for 5 replicate experiments. As shown, the particle distribution depends on the interpulse time and the particle size. Small specimens such as aluminum silicate (Fig. 4A) present a particle distribution nearly constant as function of Δt, which indicates that the solid aerosol persist a long time after the catapulting laser shot. On the other hand, aerosols particles with a larger particle diameter (Fig. 4D, stainless steel spheres) survive a lesser time and could only be detected at short interpulse delays. With intermediate particle sizes, the particle distribution exhibits two relative maxima at Δt 20 μs and 70 μs, respectively. It is thus clear that for a given interrogation distance, each particle set is to be sampled according to its size at a given interpulse delay. For the present particles, Δt is 25 μs for aluminum silicate and nickel, 15 μs for quartz and 20 μs for stainless steel. Fig. 5 shows the distribution of particles along the propagation axis. Series of 250 single shots at different distances from the surface were evaluated. This analysis was accomplished at the optimum interpulse delay time previously established for each particle. As
Fig. 4. LIBS sampling rate in the focal volume as a function of the interpulse delay time (up to 100 μs) for the different materials: A) aluminum silicate, B) nickel spheres, C) quartz and D) stainless steel spheres. The emission lines of Al (I) at 396.26 nm, Ni (I) at 352.55 nm, Si (I) at 288.16 nm and Fe (I) at 275.09 nm were used for SNR calculations.
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Fig. 5. LIBS sampling rate in the focal volume as a function of the distance from surface from 1 mm to 11 mm for the materials: A) aluminum silicate, B) nickel spheres, C) quartz and D) stainless steel spheres. The emission lines of Al (I) at 396.26 nm, Ni (I) at 352.55 nm, Si (I) at 288.16 nm and Fe (I) at 275.09 nm were used for SNR calculations.
shown, aluminum silicate particles have a homogeneous distribution along the y axis due to their small size, while nickel and quartz particles present a higher density at 4–5 mm from the surface. In contrast, iron particles are more located near the sample holder due to their higher particle size (64–76 μm). It should be noted that at distances beyond 11 mm, no LIBS signal was detected in all the cases studied probably due to the dispersal of the particle cloud in the open atmosphere and the consequent low particle density in the laser focal volume. Taken into account the particle distribution and the time (previously calculated) at which the aerosol was detected, the specimen velocity could be estimated. In this sense, both aluminum and nickel present the higher specimen velocity, 330 m/s, while a velocity of 260 m/s and 100 m/s was estimated for quartz and stainless steel particles, respectively. The effect of the catapulting pulse energy on the particle distribution has been also studied. Several LIBS spectra at different fluences were acquired at distances from the surface ranging from 1 to 11 mm. The results are plotted in Fig. 6. As shown, for all the materials, a threshold fluence close to 7 J cm–2 should be reached before catapulting starts to occur. In addition, the distance travelled strongly increased with the laser fluence until reaching a maximum value (11 J cm–2). From this irradiance value, the behaviour tends to saturation and only the glass slide support is ablated. For the present glass support, the plasma formation threshold is close to 9.5 J cm–2. The results in Fig. 6 also suggest that the catapulting thresholds depend on particle size, with smaller threshold for lightweight particles.
The final step in this work is the possibility to discriminate particles by LIBS in a heterogeneous mixture of materials. For this purpose, a mixture from 50% aluminum silicate and 50% stainless steel
Fig. 6. Effect of the catapulting laser's fluence respect to the sampling distance at which the maximum number of real hits is detected. LIB spectra in the range 1–11 mm were acquired at different fluence.
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Fig. 7. LIBS discrimination between Al and Fe particles in a heterogeneous mixture 50% aluminum silicate and 50% stainless steel spheres at different distance from surface: A) 2 mm, B) 4 mm and C) 10 mm. LIBS spectrum in the spectral range 230–350 nm showing the emission lines of Al and Fe at different sampling distance.
spheres was prepared. Fig. 7 shows a comparison of the particles hits for Al and Fe as a function of the distance at which the particle distribution is examined. As demonstrated, heavy particles (stainless steel) are distributed closer to the glass support, while lightweight particles (aluminum) are detected at longer distances. Thus, in a mixture of particles each material behaves in respect to optical catapulting according to its physical properties. 4. Conclusions The capability of laser induced plasmas for the analysis of catapulted materials has been studied for the first time by OC–LIBS. LIBS detection of ejected particles by optical catapulting has been demonstrated in the present report. Optical catapulting was used to transport small amounts of different polydisperse samples in the form of a solid aerosol. Data conditional analysis was employed in order to
evaluate the sampling rate of the elements by LIBS. Optimum interpulse delay time of 25 μs for aluminum silicate and nickel, 15 μs for quartz and 20 μs for stainless steel particles were accomplished. A correlation between the distance from surface at which the particle distribution was detected and the particle size have been established. Materials presenting a lower particle size will be catapulted with a higher specimen velocity and consequently could be detected at relatively long distances from the substrate surface. In contrast, weighty particles will be detected very close to the substrate surface due to the lower specimen velocity. In this sense, diameter of the particle could influence in the sampling rate and the maximum distance of particle detection. The capability of LIBS to discriminate a mixture of catapulted materials based on different size and composition has been also demonstrated. Thus, in a mixture of particles each material behaves in respect to optical catapulting according to its physical properties.
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