LTh4A.15.pdf
Latin America Optics and Photonics Conference (LAOP) © OSA 2014
Development and evaluation of a double-pulse LIBS system: Application for soil analysis Gustavo Nicolodelli1, Jader Cabral1,2, Bruno Marangoni1,3, Ivan L O Perazzoli1,3, Renan A Romano1,4, Débora M B P Milori1 1 * Embrapa Instrumentação, P.O. Box 741, 13560-970, São Carlos, SP, Brazil Instituto de Física, Universidade Federal de Uberlândia, P.O. Box 593, 38400-902, Uberlândia, MG, Brazil 3 Departamento de Física, Universidade Federal de São Carlos, P.O. Box 676, 13565-905, São Carlos, SP, Brazil 4 Instituto de Física de São Carlos, Universidade de São Paulo, P.O. Box 369, 13560-970, São Carlos, SP, Brazil *Corresponding author:
[email protected] 2
Abstract: One of the approaches to overcome sensibility limitation of conventional LIBS system is use a double pulse (DP) configuration. The use of the DP technique allowed enhancing of line emission intensity, when compared with conventional. OCIS codes: (300.6365) General; (300.0300) General
1. Introduction Laser-induced breakdown spectroscopy (LIBS) is a well-known analytical technique to analyze solid, liquid, gaseous and aerosol samples. However, it shows a lower sensitivity compared to other spectrometric methods [1]. LIBS has been successfully employed for qualitative and quantitative analysis of samples of very different nature and origin, including the determination of the C, Al, Si, Ti and Fe content, nutrients and pollutants [2,3]. Furthermore, a lot of techniques, such as dual-pulse excitation setup, it have been used in order to improve the LIBS´s sensitivity and spectral resolution [1, 4-6]. In this way, the LIBS´s sensitivity can be improved due to better laser´s energy –target and laser´s energy-ablated material couplings, leading to an atomic analytes production more efficient, in the excited states. This paper aims to compare a DP-LIBS apparatus, which was assembled and characterize in our research laboratory, to a conventional single pulse LIBS system. Our system will be used for soils characterization in order to develop rapid, accurate and reproducible methods. The samples used in this project are soils from the Amazon rainforest. The results showed that the use of DP setup enhanced the line emission intensity at least twice when compared with conventional single pulse LIBS. 2. Materials and Methods The single pulse LIBS spectra (SP-LIBS) of soil pellets were acquired using a Q-switched Nd:YAG laser at 532 nm and a StellarNet Inc spectrometer (EPP2000-HR) with spectral range from 188 to 400 nm and resolution 0.4 nm (FWHM). The double pulse LIBS system (DP-LIBS) was built with two Q-switched Nd:YAG lasers: an Ultra 50 from Quantel manufacturer at 1064 nm (width pulse of 6 ns and repetition rate fixed at 1 Hz) and a Brillian from Quantel manufacturer at 532 nm (second harmonic, width pulse of 6 ns and repetition rate fixed at 1 Hz). The comparative study was performed between SP-LIBS and DP-LIBS systems. The total energy pulse was fixed in 90 mJ. Both laser pulses were directed toward the target (soil sample) through dichroic mirrors properly chosen for the laser wavelengths. For a better optical efficiency of the laser energy, one coated lens (532/1064 nm anti-reflection) with focal distance of 100 mm was used in order to focus the laser pulse in the sample surface. Furthermore, to a fast and simple sample scan, the sample holder is free to move in a micro-controlled xy board. The delay time between the laser´s pulses and the spectrometer acquisition was produced by an eight channel delay generator, model 9618 from Quantum Composers manufacturer. A simplified diagram of our DP-LIBS setup, assembled at Embrapa Instrumentation is showed in Figure 1. A comparative study using the DP-LIBS system in three collinear configurations was performed: a) only the 532 nm laser with energy of 90mJ; b) First laser pulse at 532 nm and second laser pulse at 1064 nm, both with energy of 45 mJ; c) First laser pulse at 1064 nm and second laser pulse at 532 nm, both with energy of 45 mJ. For each pellet sample 45 measurements (15 for each setup above) were performed on each face. The offset of the LIBS spectra was corrected by subtracting the average of the random noise region close to the element emission line. After the offset correction, the relation signal/noise was improved by averaging 15 spectra. The same procedure was performed for all pallets.
LTh4A.15.pdf
Latin America Optics and Photonics Conference (LAOP) © OSA 2014
Figure 1 - Diagram of a DP-LIBS system developed in Embrapa Instrumentation.
3. Results and discussion In Figure 2 is showed a comparison between LIBS systems at SP (conventional) and DP setups with same total energy.
Figure 2 – Comparison between LIBS setups (SP setup - solid line and DP LIBS - dot line). From the Figure 2 is possible to observe that despite the same energy placed in sample surface in each setup, the DP-LIBS shown a better sensibility and the LIBS signal was improved. Additionally, even that not appears emission lines in single pulse, it were present in DP-LIBS emission spectra (around 226 nm on the spectra – Co II lines). Figure 3 show a comparative study of the Si I line (212.41 nm) using the three setups proposed in this study. Preliminary results showed an increase of twice in the line intensity for Si in DP-LIBS in comparison with SP-LIBS, using the configuration "b", where the laser (532 nm) reaching the first target and the IR (1064 nm) is used for re-heating of the plasma. The same does not occur when we change the sequence of laser. Furthermore, other elements was analyzed, such as C, Si, Fe and Ti, their emission lines increase at least twice with the setups (b) and (c) (DP-LIBS) in comparison with the single pulse LIBS. These analysis were performed with line intensity as well as peak area, both results show the same behavior. The importance of longer wavelength reheating in dualpulse laser-induced breakdown spectroscopy was reported in literature [7], due to efficient inverse Bremsstrahlung absorption that is proportional to λ3.
LTh4A.15.pdf
Latin America Optics and Photonics Conference (LAOP) © OSA 2014
Figure 3 – SI I 212.41 nm line intensity for all the three systems setups: SP LIBS , double pulse first visible (532nm) than infra-red laser and double pulse first IR laser than visible laser. 4. Conclusion This study aimed the development of a versatile LIBS system double pulse, able to be applied to different kind of soils. In these preliminary results, we worked with a collinear DP-LIBS system. This configuration showed a signal improvement in soil samples for all the explored lines (C, Al, Fe, Si, Ti and Co). The use of the IR laser for reheating the plasma plume showed more gain in the signal. The tools used in this study have low cost of analyses and requires little sample preparation, so it is considered a technique environment clean. LIBS spectrum with emission signal enhanced, spectrum repeatability and decrease of matrix effect in the samples are expected with double pulse system. Next step of this project is to replace the current detection system for ICCD and system with higher spatial resolution.
Acknowledgements The authors thank FAPESP (Process: 2012/24349-0), Cepof, and EMBRAPA for their financial support of this study.
4. References [1] D.W. Hahn and N.Omenetto, “Laser-Induced Breakdown Spectroscopy (LIBS), Part I: Review of Basic Diagnostics and Plasma–Particle Interactions: Still-Challenging Issues Within the Analytical Plasma Community”, Appl. Spectrosc 64 , 335A-366A (2010) [2] V.S. Burakov, S.N. Raikov, N.V. Tarasenko, M.V. Belkov and V.V. Kiris, “Development of a Laser-Induced Breakdown Spectroscopy Method for Soil and Ecological Analysis (Review),” J. Appl. Spectrosc 77, 595-608 (2010). [3] G. Nicolodelli, B.S. Marangoni, J.S. Cabral, P.R. Villas-Boas, G.S. Senesi, C.H. Santos, R.A. Romano, A. Segnini,Y. Lucas, C.R. Montes, and D.M.B.P. Milori,“Quantification of Total Carbon in Soil Using Laser-Induced Breakdown Spectroscopy (LIBS): a Method to Correct Interference Lines,” AO 53, 2170-2176 (2014). [4] B. Rashid, R. Ahmed, R. Ali, and M. A. Baig, “A comparative study of single and double pulse of laser induced breakdown spectroscopy of silver,” Phys. Plasmas 18, 073301 (2011). [5] C. Gautier, P. Fichet, D. Menut, J.-L. Lacour, D. L'Hermite, J. Dubessy, “Study of the double-pulse setup with an orthogonal beam geometry for laser induced breakdown spectroscopy,” Spectrochim. Acta Part B 59, 975–986 (2004) [6] St-Onge, V. Detalle, M. Sabsabi, “Enhanced laser-induced breakdown spectroscopy using the combination of fourth-harmonic and fundamental Nd:YAG laser pulses,” Spectrochim. Acta Part B 57, 121–135 (2002). [7] R.W. Coons, S.S. Harilal, S.M. Hassan, A. Hassanein. “The importance of longer wavelength reheating in dual-pulse laser-induced breakdown spectroscopy,” Appl Phys B 107, 873–880 (2012)
