- Email: [email protected]
Advancing the experimental design for simultaneous acquisition of laser induced plasma and Raman signals using a single pulse
Spectrochimica Acta Part B 123 (2016) 1–5
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Advancing the experimental design for simultaneous acquisition of laser induced plasma and Raman signals using a single pulse☆ Soo-Jin Choi, Jae-Jun Choi, Jack J. Yoh ⁎ Department of Mechanical and Aerospace Engineering, Seoul National University, 1 Gwanakro, Gwanakgu, Seoul 151-742, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 November 2015 4 July 2016 Accepted 17 July 2016 Available online 18 July 2016 Keywords: Laser-Induced Breakdown Spectroscopy Raman spectroscopy Simultaneous detection Combined LIBS-Raman
a b s t r a c t Simultaneous acquisition was performed of combined signals that show highly resolved and identifiable peaks of both LIBS and Raman signals. A LIBS-Raman combination using a single light source is a daunting task, because the energy required for Raman shift is relatively low, compared to the energy required for laser ablation. Here, we utilize an expanded-focused beam that allows simultaneous detection of the signals of laser induced plasma and Raman shift. A beam expander obtains the Raman signal with minimized interference from the plasma, and a focusing lens of small diameter generates strong laser induced plasma for LIBS. The position of the focusing lens can be adjusted to control the area of Raman scattering to ensure a strong Raman signal. In the proposed design, the key to minimized interference is to generate the Raman scattering apart from the plasma, which allows for sufficiently long gate width and wide area for Raman detection. Furthermore, axial relocation of the end of the optical fiber can easily optimize the Raman, LIBS, or combined Raman-LIBS signal. © 2016 Published by Elsevier B.V.
1. Introduction Laser-Induced Breakdown Spectroscopy (LIBS) is a technique to analyze atomic composition by detecting plasma generated by laser ablation. A breakdown of the chemical bonds occurs when a high powered pulsed laser focuses on the sample surface. Then plasma is generated that contains electrons, atoms, and ions. Raman spectroscopy, on the other hand, uses the shifted emission light, which is unique for each molecule, for its composition analysis. When a monochromatic light source such as laser irradiates a sample surface, a very small amount of the scattered light shifts, which involves the energies of molecular vibrations. Studies on a combined LIBS-Raman system have been performed for the noticeable advantages when the two are combined into a single unit. The two techniques are complementary to each other, in that the LIBS signal is sensitive to elemental composition, while Raman gives information on the mineral structure and polymorphs. Also, LIBS and Raman spectroscopy share various optical components for laser irradiation and detection, and thus the development of a compact system in theory is straightforward. The combined system is capable of depth profiling by laser ablation, which is not possible in Raman spectroscopy alone. Combined LIBS and Raman spectroscopy has the potential to unveil geological structures in planetary science and geology [1–10]. In
☆ Selected paper from the 8th Euro-Mediterranean Symposium on Laser Induced Breakdown Spectroscopy (EMSLIBS 2015), Linz, Austria, 14–18 September 2015. ⁎ Corresponding author. E-mail address: [email protected] (J.J. Yoh).
http://dx.doi.org/10.1016/j.sab.2016.07.006 0584-8547/© 2016 Published by Elsevier B.V.
particular, its stand-off capability has been exploited to detect and categorize explosive materials [11,12]. Depth profiling analysis of multi-layered materials, such as metal alloy, polymer, fresco and terra-cotta, has also been carried out [13,14]. Furthermore, it has been used to classify pigments, inks, etc., by providing complementary information on both the molecular and elemental composition of samples [15,16]. However, most studies have used two laser pulses, even though a single laser has been used, as the energy required for Raman shift is relatively low compared to that required for laser ablation. Sharma et al. adjusted the energy and spot size for LIBS and Raman detection of calcite, gypsum, and barite minerals [4], and simultaneously acquired both signals. Giakoumaki et al. used the energy range of 0.01 to 10 mJ/pulse for LIBS [5], and employed a variable angle attenuator to reduce the laser energy for Raman detection. Dreyer et al. obtained LIBS and Raman signals using a diode pumped Nd:YLF laser operating at 523 nm [6], by adjusting the proper energy for LIBS or Raman detection by a change in the Qswitch repetition rate. Hoehse et al. employed a diode pumped solid state laser for LIBS-Raman study [8], and used output wavelengths of 1064 nm and 532 nm to obtain LIBS and Raman signals, respectively. Glaus et al. applied a fiber-coupled LIBS-Raman system to perform a depth profiling study [13], irradiating laser energy of 0.02 to 0.15 mJ, and 1.0 mJ, on the samples for Raman and LIBS detection, respectively. Three experimental setups have been suggested for simultaneous detection of laser induced plasma and Raman shift. Moros et al. employed time-resolved study to carry out simultaneous detection of explosive at stand-off distance [11]. The Raman shift signal occurred immediately after laser irradiation. Therefore, they detected the Raman signal early with a very short integration time, and then obtained the LIBS signal. They used a holographic imaging spectrograph with a
2
S.-J. Choi et al. / Spectrochimica Acta Part B 123 (2016) 1–5
Fig. 1. (a) Schematic of the expanded-focused laser beam for simultaneous LIBS-Raman signals, and direction of the collection part for (b) LIBS, (c) Raman, and (d) simultaneous detection.
detector-intensified CCD for Raman detection, and employed a Czerny– Turner spectrograph with a detector-intensified CCD for LIBS data acquisition. Sharma et al. used a pulsed laser operating at dual wavelength of 1064 nm and 532 nm for simultaneous excitation of both LIBS and Raman spectra at stand-off distance [1]. When the laser beam passes the distance of 8.6 m, the 532 nm laser beam focuses in front of the 1064 nm beam, due to chromatic aberration in the beam expander. Consequently, the diameters of the concentric laser spots were 600 μm and 900 μm for 1064 nm and 532 nm beams, respectively so that simultaneous acquisition was possible. Matroodi et al. used a Glan-Taylor prism to divide a laser pulse into two beams [7]. A focused beam irradiated the sample surface to generate micro plasma for emission of the LIBS signal. The other beam directly irradiated the same point on the sample without focusing, for the Raman signal. The average laser energy
for Raman and LIBS excitation was 35 and 5 mJ, with diameters of 5 mm and 0.1 mm, respectively. The Raman shift signal mixed with the plasma emission, and was detected in a single spectrum. The present study suggests a combined LIBS-Raman system, and employs a beam expander to expand a laser beam to obtain the Raman signal without interference with the plasma. Also, we use a focusing lens of small diameter to generate the laser induced plasma for LIBS. The position of the focusing lens controls the area of Raman scattering. The proposed experimental design enables the simultaneous acquisition of laser induced plasma and Raman signals, with minimum interference between plasma and Raman scattering. Additionally, the LIBS, Raman, or combined LIBS-Raman signals can be independently obtained, by adjusting the axial location of the end of the optical fiber.
Fig. 2. LIBS spectra of aragonite sample versus energy change.
Fig. 3. Raman spectra of aragonite sample versus energy change.
S.-J. Choi et al. / Spectrochimica Acta Part B 123 (2016) 1–5
Fig. 4. Simultaneous LIBS-Raman spectra of an aragonite sample versus energy change.
2. Experimental setup A Q-switched Nd:YAG laser (Surelite I, Continuum) that operates at 532 nm with a pulse duration of 5–7 ns at 10 Hz repetition rate is focused onto the sample surface. The output pulse energy was adjusted 9 to 21 mJ for an aragonite sample to find out the optimized energy for simultaneous detection, and then the energy was fixed at 15 mJ for the other samples. Fig. 1 (a) shows that two irradiated areas of high and low irradiances form, because the energy required to generate a laser induced plasma and Raman scattering differs. A laser beam passes through the beam expander with an expansion ratio of four and a clear entrance aperture of 8.6 mm, to expand the beam diameter. A small diameter (12.7 mm) BK7 lens of 50 mm focal length focuses the expanded beam onto a sample surface. The position of the focusing lens can be adjusted to control the area of Raman scattering, to secure sufficient area to obtain Raman signals. The collection part is mounted on an optical rail to adjust the direction for LIBS, Raman, or simultaneous detection. Plasma was detected at the focused area (Fig. 1(b)), while Raman shift was detected at the expanded area (Fig. 1(c)). Also, for simultaneous detection, both LIBS and Raman signals are collected in the detecting lens (Fig. 1(d)). These signals do not provide information of the same region. Nevertheless, this setup allows the possibility of miniaturization of the combined
3
LIBS-Raman system by using a single pulse. Besides, it is possible to compare the distribution of LIBS and Raman information of sample by separate 2D measurements. 50 pulses were accumulated in the experiment, and it was repeated 5 times in each case. A Czerny-Turner spectrometer (MonoRa320i, Andor) coupled with intensified CCD (iStar, Andor) is employed for the acquisition of both laser induced plasma and Raman shift. The spectral resolution of the spectrometer is 0.09 nm at 10 μm slit with 1200 g/mm. The grating was adjusted to 150, 600, and 600 grooves for LIBS, Raman, and simultaneous detection, respectively. As an exception, 1200 groove was used for dolomite, to obtain a high resolution LIBS spectrum. Time delays of 0.5 μs were used for LIBS, and 0 s for both Raman and simultaneous detection. The gate width (integration time) was fixed at 0.1 ms. A fused silica lens of 100 mm focal length focused the emitted light, while 600 μm optical fiber was used to collect the signals. The axial location of the end of the optical fiber was adjustable, to permit detection of the LIBS, Raman, or combined LIBS-Raman signals. In order to cut off the strong Rayleigh scattered light, a long-pass filter (LP03532RU-25) was used for Raman spectroscopy. Various geological samples were analyzed (Hansol Education Co., hs-97-2001), namely Aragonite (CaCO3), Dolomite (CaMg(CO3)2), and Gypsum (CaSO4 ·2H2 O). Samples were analyzed without any processing.
3. Results and discussion To obtain LIBS signals, the end of the optical fiber was oriented to the focused area (high irradiance) where the plasma is generated. Fig. 2 shows the LIBS spectra of an aragonite (CaCO3) sample according to energy change. Most of the peaks are neutral and ionized calcium atoms, and the broad shape of CaO molecular bands also appear in the 547– 560 nm and 580–650 nm ranges. The energy range that is suitable for the LIBS signal is 15 to 21 mJ. Note that in Figs. 2 to 4, the Origin program automatically offset the spectra with respect to each other. To obtain Raman signals, the end of the optical fiber was oriented to the expanded area (low irradiance), where the Raman scattering occurs. Fig. 3 shows that the Raman spectra of aragonite according to energy displayed both internal and lattice (external) modes. The internal mode involves vibration of the molecule itself, while the lattice mode involves the interaction between the molecule and other species, such as calcium. The ν1 symmetric stretch of carbonate ions is at 1086 cm−1, and the ν4 symmetric deformation of carbonate ions is at 704 cm−1. Two peaks at 153 and 206 cm−1 are characteristics of aragonite lattice modes. Carbonate minerals can be identified based on the
Fig. 5. LIBS spectra of a dolomite sample of (a) low resolution, and (b) high resolution.
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Fig. 6. (a) Raman, and (b) simultaneous spectra, of a dolomite sample.
internal and lattice modes of Raman vibrations. Although LIBS signals appear at around 900 cm−1 and 2500 cm−1, Raman signals of both internal and lattice modes are successfully obtained without noticeable interference of the LIBS signal, even when using the high energy of 21 mJ. Here, the energy that is suitable for obtaining the Raman signal is 12–21 mJ. For simultaneous detection, the end of the optical fiber was oriented toward the area between the focused area and expanded area. Fig. 4 shows simultaneous LIBS-Raman detection by single pulse and single acquisition. Most of the Raman signals of both the internal and lattice modes are detected, except that the ν4 symmetric deformation of carbonate ions is at 704 cm−1. Fig. 3 shows that the signal of ν4 vibration mode easily interferes with the LIBS signal, essentially because it is weak. The LIBS and Raman signals were obtained with nearly equivalent intensity, but the LIBS signal became more intense when the laser energy increased above 21 mJ. Thus, 15 mJ/pulse energy was applied to all the other samples. The results that Figs. 2 to 4 show indicate that the suggested combined LIBS-Raman system is adequate for simultaneous detection of laser induced plasma and Raman shift. Furthermore, the LIBS, Raman, or combined signals can be easily optimized by simple movement of the end of the optical fiber. The LIBS signal of dolomite (CaMg(CO3)2) in Fig. 5 shows two major cations of calcium and magnesium, and CaO molecular band. Grating for the LIBS measurement was 150 groove to obtain a broad range of spectra. Hence Fig. 5(a) shows that a low resolution signal was obtained, so that the signal of magnesium appears as a broad peak. In order to enhance the resolution, the grating was adjusted to 1200 groove. Consequently, Fig. 5(b) clearly shows magnesium peaks of 516, 517, and 518 nm. Thus spectra with high resolution can be obtained, simply by adjusting the grating in the spectrometer program. Fig. 6 (a) shows the Raman signal of dolomite. The ν1 symmetric stretch of carbonate ions is at 1087 cm−1, and the ν4 symmetric deformation of carbonate ions is at 714 cm− 1. Two peaks at 187 and 285 cm−1 are characteristic of dolomite lattice modes. The LIBS signals of calcium and CaO also appear, where the Raman signals are still detectable. Fig. 6 (b) shows the simultaneous result. Most of the Raman signals of both internal and lattice modes and LIBS signals are detected, except that the Raman ν4 symmetric deformation of carbonate ions is at 714 cm−1, which has a weak signal intensity. Fig. 7 shows the LIBS spectrum of Gypsum (CaSO4·2H2O) sample. The major cation of gypsum is calcium; thus, calcium atom and CaO molecular bands dominate the spectrum. The signals of oxygen at 777,
844 nm and hydrogen at 656 nm are detected with low intensity. However the existence of water is not clear, because the bonding structure between oxygen and hydrogen cannot be discovered by LIBS analysis. Therefore, Raman spectroscopy should support the LIBS results. Sulfur that is contained in gypsum does not appear in the LIBS spectra. Fig. 8 (a) shows the Raman signal of gypsum. The ν1 symmetric stretch of SO4 is at 1015 cm−1, and a doublet for ν2 symmetric bending at 416 cm−1 and 495 cm−1 appear. The peak at 1143 cm−1 was assigned to the ν3 asymmetric stretch vibration mode. This experiment did not detect peaks at 622 cm−1 and 674 cm−1 for the ν4 asymmetric bending vibration modes. The detection of SO4 is used to identify sulfurcontaining minerals. The characteristic band for the stretching vibration modes of water in gypsum is detected around 3400 to 3500 cm− 1. The detection of O\\H stretching region enables the discrimination between hydrous and anhydrous minerals. Fig. 8 (b) shows that all of the Raman signals, including SO4 internal and lattice modes and water molecules, are detected by simultaneous acquisition. The LIBS signal was also successfully obtained by simultaneous detection with sufficiently strong intensities. 4. Conclusion We have demonstrated the simultaneous acquisition of combined signals that show highly resolved and identifiable peaks of both LIBS and Raman signals. Both LIBS and Raman signals can be obtained separately, by adjusting the axial location of the end of the optical fiber. The
Fig. 7. LIBS spectrum of a gypsum sample.
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Fig. 8. (a) Raman and (b) simultaneous spectra, of a gypsum sample.
expanded-focused beam concept enables the detection of signals of Raman-only, LIBS-only, or combined Raman-LIBS with definite and identifiable signals. Acknowledgements Authors wish to acknowledge the financial support from the National Research Foundation of Korea under the National Space Laboratory Program 2014 (NRF-2014M1A3A3A02034903) through the IAAT at Seoul National University and the Brain Korea 21 Plus Project in 2016. References [1] S.K. Sharma, A.K. Misra, P.G. Lucey, R.C. Wiens, S.M. Clegg, Combined remote LIBS and Raman spectroscopy at 8.6 m of sulfur-containing minerals, and minerals coated with hematite or covered with basaltic dust, Spectrochim. Acta A 68 (4) (2007) 1036–1045. [2] S.M. Clegg, R. Wiens, A.K. Misra, S.K. Sharma, J. Lambert, S. Bender, R. Newell, K. Nowak-Lovato, S. Smrekar, M.D. Dyar, S. Maurice, Planetary geochemical investigations using Raman and laser-induced breakdown spectroscopy, Appl. Spectrosc. 68 (9) (2014) 925–936. [3] R.C. Wiens, S.K. Sharma, J. Thompson, A. Misra, P.G. Lucey, Joint analyses by laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy at stand-off distances, Spectrochim. Acta A 61 (10) (2005) 2324–2334. [4] S.K. Sharma, A.K. Misra, P.G. Lucey, R.C.F. Lentz, A combined remote Raman and LIBS instrument for characterizing minerals with 532 nm laser excitation, Spectrochim. Acta A 73 (3) (2009) 468–476. [5] A. Giakoumaki, I. Osticioli, D. Anglos, Spectroscopic analysis using a hybrid LIBSRaman system, Appl. Phys. A Mater. Sci. Process. 83 (4) (2006) 537–541.
[6] C.B. Dreyer, G.S. Mungas, P. Thanh, J.G. Radziszewski, Study of sub-mJ-excited laserinduced plasma combined with Raman spectroscopy under Mars atmosphere-simulated conditions, Spectrochim. Acta B 62 (12) (2007) 1448–1459. [7] F. Matroodi, S.H. Tavassoli, Simultaneous Raman and laser-induced breakdown spectroscopy by a single setup, Appl. Phys. B Lasers Opt. 117 (4) (2014) 1081–1089. [8] M. Hoehse, I. Gornushkin, S. Merk, U. Panne, Assessment of suitability of diode pumped solid state lasers for laser induced breakdown and Raman spectroscopy, J. Anal. At. Spectrom. 26 (2011) 414–424. [9] G.B. Courrèges-Lacoste, B. Ahlers, F.R. Pérez, Combined Raman spectrometer/laserinduced breakdown spectrometer for the next ESA mission to Mars, Spectrochim. Acta A 68 (4) (2007) 1023–1028. [10] I. Escudero-Sanz, B. Ahlers, G.B. Courrèges-Lacoste, Optical design of a combined Raman–laser-induced-breakdown-spectroscopy instrument for the European Space Agency ExoMars Mission, Opt. Eng. 47 (3) (2008) 033001. [11] J. Moros, J.A. Lorenzo, J.J. Laserna, Standoff detection of explosives: critical comparison for ensuing options on Raman spectroscopy–LIBS sensor fusion, Anal. Bioanal. Chem. 400 (10) (2011) 3353–3365. [12] J. Moros, J.A. Lorenzo, P. Lucena, L.M. Tobaria, J.J. Laserna, Simultaneous Raman spectroscopy−laser-induced breakdown spectroscopy for instant standoff analysis of explosives using a mobile integrated sensor platform, Anal. Chem. 82 (4) (2010) 1389–1400. [13] R. Glaus, D.W. Hahn, Fiber-coupled laser-induced breakdown and Raman spectroscopy for flexible sample characterization with depth profiling capabilities, Spectrochim. Acta B 100 (2014) 116–122. [14] I. Osticioli, N.F.C. Mendes, S. Porcinai, A. Cagnini, E. Castellucci, Spectroscopic analysis of works of art using a single LIBS and pulsed Raman setup, Anal. Bioanal. Chem. 394 (4) (2009) 1033–1041. [15] M. Hoehse, A. Paul, I. Gornushkin, U. Panne, Multivariate classification of pigments and inks using combined Raman spectroscopy and LIBS, Anal. Bioanal. Chem. 402 (4) (2012) 1443–1450. [16] L. Burgio, K. Melessanaki, M. Doulgeridis, R.J.H. Clark, D. Anglos, Pigment identification in paintings employing laser induced breakdown spectroscopy and Raman microscopy, Spectrochim. Acta B 56 (6) (2001) 905–913.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Advancing the experimental design for simultaneous acquisition of laser induced plasma and Raman signals using a single pulse☆ Soo-Jin Choi, Jae-Jun Choi, Jack J. Yoh ⁎ Department of Mechanical and Aerospace Engineering, Seoul National University, 1 Gwanakro, Gwanakgu, Seoul 151-742, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 November 2015 4 July 2016 Accepted 17 July 2016 Available online 18 July 2016 Keywords: Laser-Induced Breakdown Spectroscopy Raman spectroscopy Simultaneous detection Combined LIBS-Raman
a b s t r a c t Simultaneous acquisition was performed of combined signals that show highly resolved and identifiable peaks of both LIBS and Raman signals. A LIBS-Raman combination using a single light source is a daunting task, because the energy required for Raman shift is relatively low, compared to the energy required for laser ablation. Here, we utilize an expanded-focused beam that allows simultaneous detection of the signals of laser induced plasma and Raman shift. A beam expander obtains the Raman signal with minimized interference from the plasma, and a focusing lens of small diameter generates strong laser induced plasma for LIBS. The position of the focusing lens can be adjusted to control the area of Raman scattering to ensure a strong Raman signal. In the proposed design, the key to minimized interference is to generate the Raman scattering apart from the plasma, which allows for sufficiently long gate width and wide area for Raman detection. Furthermore, axial relocation of the end of the optical fiber can easily optimize the Raman, LIBS, or combined Raman-LIBS signal. © 2016 Published by Elsevier B.V.
1. Introduction Laser-Induced Breakdown Spectroscopy (LIBS) is a technique to analyze atomic composition by detecting plasma generated by laser ablation. A breakdown of the chemical bonds occurs when a high powered pulsed laser focuses on the sample surface. Then plasma is generated that contains electrons, atoms, and ions. Raman spectroscopy, on the other hand, uses the shifted emission light, which is unique for each molecule, for its composition analysis. When a monochromatic light source such as laser irradiates a sample surface, a very small amount of the scattered light shifts, which involves the energies of molecular vibrations. Studies on a combined LIBS-Raman system have been performed for the noticeable advantages when the two are combined into a single unit. The two techniques are complementary to each other, in that the LIBS signal is sensitive to elemental composition, while Raman gives information on the mineral structure and polymorphs. Also, LIBS and Raman spectroscopy share various optical components for laser irradiation and detection, and thus the development of a compact system in theory is straightforward. The combined system is capable of depth profiling by laser ablation, which is not possible in Raman spectroscopy alone. Combined LIBS and Raman spectroscopy has the potential to unveil geological structures in planetary science and geology [1–10]. In
☆ Selected paper from the 8th Euro-Mediterranean Symposium on Laser Induced Breakdown Spectroscopy (EMSLIBS 2015), Linz, Austria, 14–18 September 2015. ⁎ Corresponding author. E-mail address: [email protected] (J.J. Yoh).
http://dx.doi.org/10.1016/j.sab.2016.07.006 0584-8547/© 2016 Published by Elsevier B.V.
particular, its stand-off capability has been exploited to detect and categorize explosive materials [11,12]. Depth profiling analysis of multi-layered materials, such as metal alloy, polymer, fresco and terra-cotta, has also been carried out [13,14]. Furthermore, it has been used to classify pigments, inks, etc., by providing complementary information on both the molecular and elemental composition of samples [15,16]. However, most studies have used two laser pulses, even though a single laser has been used, as the energy required for Raman shift is relatively low compared to that required for laser ablation. Sharma et al. adjusted the energy and spot size for LIBS and Raman detection of calcite, gypsum, and barite minerals [4], and simultaneously acquired both signals. Giakoumaki et al. used the energy range of 0.01 to 10 mJ/pulse for LIBS [5], and employed a variable angle attenuator to reduce the laser energy for Raman detection. Dreyer et al. obtained LIBS and Raman signals using a diode pumped Nd:YLF laser operating at 523 nm [6], by adjusting the proper energy for LIBS or Raman detection by a change in the Qswitch repetition rate. Hoehse et al. employed a diode pumped solid state laser for LIBS-Raman study [8], and used output wavelengths of 1064 nm and 532 nm to obtain LIBS and Raman signals, respectively. Glaus et al. applied a fiber-coupled LIBS-Raman system to perform a depth profiling study [13], irradiating laser energy of 0.02 to 0.15 mJ, and 1.0 mJ, on the samples for Raman and LIBS detection, respectively. Three experimental setups have been suggested for simultaneous detection of laser induced plasma and Raman shift. Moros et al. employed time-resolved study to carry out simultaneous detection of explosive at stand-off distance [11]. The Raman shift signal occurred immediately after laser irradiation. Therefore, they detected the Raman signal early with a very short integration time, and then obtained the LIBS signal. They used a holographic imaging spectrograph with a
2
S.-J. Choi et al. / Spectrochimica Acta Part B 123 (2016) 1–5
Fig. 1. (a) Schematic of the expanded-focused laser beam for simultaneous LIBS-Raman signals, and direction of the collection part for (b) LIBS, (c) Raman, and (d) simultaneous detection.
detector-intensified CCD for Raman detection, and employed a Czerny– Turner spectrograph with a detector-intensified CCD for LIBS data acquisition. Sharma et al. used a pulsed laser operating at dual wavelength of 1064 nm and 532 nm for simultaneous excitation of both LIBS and Raman spectra at stand-off distance [1]. When the laser beam passes the distance of 8.6 m, the 532 nm laser beam focuses in front of the 1064 nm beam, due to chromatic aberration in the beam expander. Consequently, the diameters of the concentric laser spots were 600 μm and 900 μm for 1064 nm and 532 nm beams, respectively so that simultaneous acquisition was possible. Matroodi et al. used a Glan-Taylor prism to divide a laser pulse into two beams [7]. A focused beam irradiated the sample surface to generate micro plasma for emission of the LIBS signal. The other beam directly irradiated the same point on the sample without focusing, for the Raman signal. The average laser energy
for Raman and LIBS excitation was 35 and 5 mJ, with diameters of 5 mm and 0.1 mm, respectively. The Raman shift signal mixed with the plasma emission, and was detected in a single spectrum. The present study suggests a combined LIBS-Raman system, and employs a beam expander to expand a laser beam to obtain the Raman signal without interference with the plasma. Also, we use a focusing lens of small diameter to generate the laser induced plasma for LIBS. The position of the focusing lens controls the area of Raman scattering. The proposed experimental design enables the simultaneous acquisition of laser induced plasma and Raman signals, with minimum interference between plasma and Raman scattering. Additionally, the LIBS, Raman, or combined LIBS-Raman signals can be independently obtained, by adjusting the axial location of the end of the optical fiber.
Fig. 2. LIBS spectra of aragonite sample versus energy change.
Fig. 3. Raman spectra of aragonite sample versus energy change.
S.-J. Choi et al. / Spectrochimica Acta Part B 123 (2016) 1–5
Fig. 4. Simultaneous LIBS-Raman spectra of an aragonite sample versus energy change.
2. Experimental setup A Q-switched Nd:YAG laser (Surelite I, Continuum) that operates at 532 nm with a pulse duration of 5–7 ns at 10 Hz repetition rate is focused onto the sample surface. The output pulse energy was adjusted 9 to 21 mJ for an aragonite sample to find out the optimized energy for simultaneous detection, and then the energy was fixed at 15 mJ for the other samples. Fig. 1 (a) shows that two irradiated areas of high and low irradiances form, because the energy required to generate a laser induced plasma and Raman scattering differs. A laser beam passes through the beam expander with an expansion ratio of four and a clear entrance aperture of 8.6 mm, to expand the beam diameter. A small diameter (12.7 mm) BK7 lens of 50 mm focal length focuses the expanded beam onto a sample surface. The position of the focusing lens can be adjusted to control the area of Raman scattering, to secure sufficient area to obtain Raman signals. The collection part is mounted on an optical rail to adjust the direction for LIBS, Raman, or simultaneous detection. Plasma was detected at the focused area (Fig. 1(b)), while Raman shift was detected at the expanded area (Fig. 1(c)). Also, for simultaneous detection, both LIBS and Raman signals are collected in the detecting lens (Fig. 1(d)). These signals do not provide information of the same region. Nevertheless, this setup allows the possibility of miniaturization of the combined
3
LIBS-Raman system by using a single pulse. Besides, it is possible to compare the distribution of LIBS and Raman information of sample by separate 2D measurements. 50 pulses were accumulated in the experiment, and it was repeated 5 times in each case. A Czerny-Turner spectrometer (MonoRa320i, Andor) coupled with intensified CCD (iStar, Andor) is employed for the acquisition of both laser induced plasma and Raman shift. The spectral resolution of the spectrometer is 0.09 nm at 10 μm slit with 1200 g/mm. The grating was adjusted to 150, 600, and 600 grooves for LIBS, Raman, and simultaneous detection, respectively. As an exception, 1200 groove was used for dolomite, to obtain a high resolution LIBS spectrum. Time delays of 0.5 μs were used for LIBS, and 0 s for both Raman and simultaneous detection. The gate width (integration time) was fixed at 0.1 ms. A fused silica lens of 100 mm focal length focused the emitted light, while 600 μm optical fiber was used to collect the signals. The axial location of the end of the optical fiber was adjustable, to permit detection of the LIBS, Raman, or combined LIBS-Raman signals. In order to cut off the strong Rayleigh scattered light, a long-pass filter (LP03532RU-25) was used for Raman spectroscopy. Various geological samples were analyzed (Hansol Education Co., hs-97-2001), namely Aragonite (CaCO3), Dolomite (CaMg(CO3)2), and Gypsum (CaSO4 ·2H2 O). Samples were analyzed without any processing.
3. Results and discussion To obtain LIBS signals, the end of the optical fiber was oriented to the focused area (high irradiance) where the plasma is generated. Fig. 2 shows the LIBS spectra of an aragonite (CaCO3) sample according to energy change. Most of the peaks are neutral and ionized calcium atoms, and the broad shape of CaO molecular bands also appear in the 547– 560 nm and 580–650 nm ranges. The energy range that is suitable for the LIBS signal is 15 to 21 mJ. Note that in Figs. 2 to 4, the Origin program automatically offset the spectra with respect to each other. To obtain Raman signals, the end of the optical fiber was oriented to the expanded area (low irradiance), where the Raman scattering occurs. Fig. 3 shows that the Raman spectra of aragonite according to energy displayed both internal and lattice (external) modes. The internal mode involves vibration of the molecule itself, while the lattice mode involves the interaction between the molecule and other species, such as calcium. The ν1 symmetric stretch of carbonate ions is at 1086 cm−1, and the ν4 symmetric deformation of carbonate ions is at 704 cm−1. Two peaks at 153 and 206 cm−1 are characteristics of aragonite lattice modes. Carbonate minerals can be identified based on the
Fig. 5. LIBS spectra of a dolomite sample of (a) low resolution, and (b) high resolution.
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Fig. 6. (a) Raman, and (b) simultaneous spectra, of a dolomite sample.
internal and lattice modes of Raman vibrations. Although LIBS signals appear at around 900 cm−1 and 2500 cm−1, Raman signals of both internal and lattice modes are successfully obtained without noticeable interference of the LIBS signal, even when using the high energy of 21 mJ. Here, the energy that is suitable for obtaining the Raman signal is 12–21 mJ. For simultaneous detection, the end of the optical fiber was oriented toward the area between the focused area and expanded area. Fig. 4 shows simultaneous LIBS-Raman detection by single pulse and single acquisition. Most of the Raman signals of both the internal and lattice modes are detected, except that the ν4 symmetric deformation of carbonate ions is at 704 cm−1. Fig. 3 shows that the signal of ν4 vibration mode easily interferes with the LIBS signal, essentially because it is weak. The LIBS and Raman signals were obtained with nearly equivalent intensity, but the LIBS signal became more intense when the laser energy increased above 21 mJ. Thus, 15 mJ/pulse energy was applied to all the other samples. The results that Figs. 2 to 4 show indicate that the suggested combined LIBS-Raman system is adequate for simultaneous detection of laser induced plasma and Raman shift. Furthermore, the LIBS, Raman, or combined signals can be easily optimized by simple movement of the end of the optical fiber. The LIBS signal of dolomite (CaMg(CO3)2) in Fig. 5 shows two major cations of calcium and magnesium, and CaO molecular band. Grating for the LIBS measurement was 150 groove to obtain a broad range of spectra. Hence Fig. 5(a) shows that a low resolution signal was obtained, so that the signal of magnesium appears as a broad peak. In order to enhance the resolution, the grating was adjusted to 1200 groove. Consequently, Fig. 5(b) clearly shows magnesium peaks of 516, 517, and 518 nm. Thus spectra with high resolution can be obtained, simply by adjusting the grating in the spectrometer program. Fig. 6 (a) shows the Raman signal of dolomite. The ν1 symmetric stretch of carbonate ions is at 1087 cm−1, and the ν4 symmetric deformation of carbonate ions is at 714 cm− 1. Two peaks at 187 and 285 cm−1 are characteristic of dolomite lattice modes. The LIBS signals of calcium and CaO also appear, where the Raman signals are still detectable. Fig. 6 (b) shows the simultaneous result. Most of the Raman signals of both internal and lattice modes and LIBS signals are detected, except that the Raman ν4 symmetric deformation of carbonate ions is at 714 cm−1, which has a weak signal intensity. Fig. 7 shows the LIBS spectrum of Gypsum (CaSO4·2H2O) sample. The major cation of gypsum is calcium; thus, calcium atom and CaO molecular bands dominate the spectrum. The signals of oxygen at 777,
844 nm and hydrogen at 656 nm are detected with low intensity. However the existence of water is not clear, because the bonding structure between oxygen and hydrogen cannot be discovered by LIBS analysis. Therefore, Raman spectroscopy should support the LIBS results. Sulfur that is contained in gypsum does not appear in the LIBS spectra. Fig. 8 (a) shows the Raman signal of gypsum. The ν1 symmetric stretch of SO4 is at 1015 cm−1, and a doublet for ν2 symmetric bending at 416 cm−1 and 495 cm−1 appear. The peak at 1143 cm−1 was assigned to the ν3 asymmetric stretch vibration mode. This experiment did not detect peaks at 622 cm−1 and 674 cm−1 for the ν4 asymmetric bending vibration modes. The detection of SO4 is used to identify sulfurcontaining minerals. The characteristic band for the stretching vibration modes of water in gypsum is detected around 3400 to 3500 cm− 1. The detection of O\\H stretching region enables the discrimination between hydrous and anhydrous minerals. Fig. 8 (b) shows that all of the Raman signals, including SO4 internal and lattice modes and water molecules, are detected by simultaneous acquisition. The LIBS signal was also successfully obtained by simultaneous detection with sufficiently strong intensities. 4. Conclusion We have demonstrated the simultaneous acquisition of combined signals that show highly resolved and identifiable peaks of both LIBS and Raman signals. Both LIBS and Raman signals can be obtained separately, by adjusting the axial location of the end of the optical fiber. The
Fig. 7. LIBS spectrum of a gypsum sample.
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Fig. 8. (a) Raman and (b) simultaneous spectra, of a gypsum sample.
expanded-focused beam concept enables the detection of signals of Raman-only, LIBS-only, or combined Raman-LIBS with definite and identifiable signals. Acknowledgements Authors wish to acknowledge the financial support from the National Research Foundation of Korea under the National Space Laboratory Program 2014 (NRF-2014M1A3A3A02034903) through the IAAT at Seoul National University and the Brain Korea 21 Plus Project in 2016. References [1] S.K. Sharma, A.K. Misra, P.G. Lucey, R.C. Wiens, S.M. Clegg, Combined remote LIBS and Raman spectroscopy at 8.6 m of sulfur-containing minerals, and minerals coated with hematite or covered with basaltic dust, Spectrochim. Acta A 68 (4) (2007) 1036–1045. [2] S.M. Clegg, R. Wiens, A.K. Misra, S.K. Sharma, J. Lambert, S. Bender, R. Newell, K. Nowak-Lovato, S. Smrekar, M.D. Dyar, S. Maurice, Planetary geochemical investigations using Raman and laser-induced breakdown spectroscopy, Appl. Spectrosc. 68 (9) (2014) 925–936. [3] R.C. Wiens, S.K. Sharma, J. Thompson, A. Misra, P.G. Lucey, Joint analyses by laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy at stand-off distances, Spectrochim. Acta A 61 (10) (2005) 2324–2334. [4] S.K. Sharma, A.K. Misra, P.G. Lucey, R.C.F. Lentz, A combined remote Raman and LIBS instrument for characterizing minerals with 532 nm laser excitation, Spectrochim. Acta A 73 (3) (2009) 468–476. [5] A. Giakoumaki, I. Osticioli, D. Anglos, Spectroscopic analysis using a hybrid LIBSRaman system, Appl. Phys. A Mater. Sci. Process. 83 (4) (2006) 537–541.
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