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Spectrochimica Acta Part B 111 (2015) 23–29
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Spectrochimica Acta Part B
j ourna l homepage: www.e lsev ie r .com/ locate /sab
Signal enhancement in collinear double-pulse laser-induced breakdownspectroscopy applied to different soils☆
Gustavo Nicolodelli a,⁎, Giorgio Saverio Senesi b, Renan Arnon Romano a,c,Ivan Luiz de Oliveira Perazzoli a, Débora Marcondes Bastos Pereira Milori a
a Embrapa Instrumentation, Rua XV de Novembro, 1452, CEP 13560-970 São Carlos, SP, Brazilb Institute of Inorganic Methodologies and Plasmas, CNR, Bari, 70126 Bari, Italyc Physics Institute of São Carlos, University of São Paulo, IFSC-USP, Av. Trabalhador são-carlense, 400 Pq. Arnold Schimid, 13566-590 São Carlos, SP, Brazil
☆ Selected Paper dedicated to the 8th InternationalBreakdown Spectroscopy (LIBS 2014), Beijing (China), 8–⁎ Corresponding author at: Embrapa Instrumentation, R
13560-970, São Carlos, SP, Brazil. Tel.: +55 2107 2800.E-mail addresses: [email protected] (G. Nico
[email protected] (G.S. Senesi), renan.romano@[email protected] (I.L. de Oliveira Perazzoli), de(D.M.B.P. Milori).
http://dx.doi.org/10.1016/j.sab.2015.06.0080584-8547/© 2015 Elsevier B.V. All rights reserved.
a b s t r a c t
a r t i c l e i n f oArticle history:Received 30 January 2015Accepted 13 June 2015Available online 19 June 2015
Keywords:SoilCollinear DP-LIBSInterpulse delay
Laser-induced breakdown spectroscopy (LIBS) is a well-known consolidated analytical technique employed suc-cessfully for the qualitative and quantitative analysis of solid, liquid, gaseous and aerosol samples of very differentnature and origin. Several techniques, such as dual-pulse excitation setup, have been used in order to improveLIBS's sensitivity. The purpose of this paper was to optimize the key parameters as excitation wavelength,delay time and interpulse, that influence the double pulse (DP) LIBS technique in the collinear beam geometrywhen applied to the analysis at atmospheric air pressure of soil samples of different origin and texture fromextreme regions of Brazil. Additionally, a comparative study between conventional single pulse (SP) LIBS andDP LIBS was performed. An optimization of DP LIBS system, choosing the correct delay time between the twopulses, was performed allowing its use for different soil types and the use of different emission lines. In general,the collinear DP LIBS system improved the analytical performances of the technique by enhancing the intensity ofemission lines of some elements up to about 5 times, when compared with conventional SP-LIBS, and reducedthe continuum emission. Further, the IR laser provided the best performance in re-heating the plasma.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) is an emerging tech-nique that uses the emission generated from a laser-induced plasma toobtain the multi-element analysis of different materials in any physicalstate [1]. Due to its simple and direct nature, during the last decade LIBShas achieved awide popularity in severalfields of applications [2]. How-ever, conventional LIBS shows specific drawbacks for many elements,which are related to sensitivity and precision that may not fulfill theanalytical requirements requested. Anyway, in the last three decadesvarious approaches have been proposed aiming to improve LIBS analyt-ical capabilities [3–11]. Among these the double-pulse (DP) laser hasproven to be very effective in improving LIBS performance by offeringa high flexibility in the choice of wavelength, pulse width and pulse se-quence. This in order to better coupling laser energy to the target and
Conference on Laser Induced12 September 2014.ua XV de Novembro, 1452, CEP
lodelli),ail.com (R.A. Romano),
ablated material, thus leading to a more efficient production of analyteatoms in the excited state [3,11–15].
Various configurations have been used for the DP setup as it regardsthe geometry of laser beams and laser collection, laser wavelength andpulse duration, pulse energy, etc. [4]. In particular, two main beam ge-ometries, i.e. collinear and orthogonal, are used in DP LIBS [16–19].The collinear configuration in which both pulses have the same axis ofpropagation, i.e. are directed orthogonal to the sample surface, yieldsan increase of emission line intensities which ranges from a factor of 2[20] up to a factor of 100 [13,21], although the large variety of enhance-ments observed is not well understood. DP LIBS in collinear geometryhas been applied to analyze various types of materials in the liquidform [22–25], solid form, including metals and various environmentalsamples [13,14,25–31], and solid targets in liquids [32]. Further, DP ex-periments in collinear geometry can be performed either using a uniquelaser [14,20,22,32] or two lasers [13,15,23,28]. Although the use of twolasers has been shown to provide more flexibility than a single laser,for parameters such as laser pulse energy and delay between the twolaser pulses, the use of a single laser can overcome alignment problems.
Most researchers use two pulses at the same wavelength for the DPconfiguration, however, the use of pulses of different wavelengths wasshown to yield an increase in sensitivity. St-Onge et al. (2002) [13]have reported an improved sensitivity when using an ultraviolet (UV)pulse (266 nm) for ablation and a second IR (1064 nm) pulse to re-
Table 1Texture of the three soils studied.
Sample Sand (%) clay (%) Silt (%)
RO 33 48 19HS 8 27 65YO 61 26 13
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heating the plasma, if compared to the use of two infrared (IR) pulses.Laser emissions in the UV are more effective for ablation because theyreduce the shielding plasma. Rashid et al. (2011) [33] also combinedtwo lasers at different wavelengths in their LIBS DP experiment, thefirst pulse at 523 nm (for plasma generation) and the second at1064 nm (for re-heating), obtaining a 12-fold increase in intensitywhen compared to the SP LIBS signal. Piscitelli et al. (2009) [34] andDiwakar et al. (2013) [10] used three different DP combinations andfound in all cases an improvement of the LIBS broadcast signal.
The LIBS technique has been applied successfully to soils forassessing their elemental composition, classification, presence of con-taminants, etc [27,31,34–39]. LIBS is a quick and environmentallyclean technique, and does not require the use of reagents for samplepreparation, thus has great potential for application to soils.
The general purpose of this work was to optimize the key parame-ters that influence theDPLIBS technique in the collinear beamgeometrywhen applied to elemental analysis at atmospheric air pressure of soilsamples of different origin and texture. The use of a DP LIBS systemwith different laser wavelengths allows greater flexibility in research.In particular, the influence was evaluated of the delay time betweenthe two laser pulses on the background emission for different emissionlines detected in soil samples examined. Further, the whole analyticalperformance of LIBS in the SP and DP configurations was compared.
Fig. 1. Scheme of the coll
2. Materials and methods
2.1. Samples
Tree soils of different texture from two extreme regions (São Pauloand Amazonas) in Brasil were studied in this work (Table 1) [40,41].Two soil samples, a Humiluvic Spodosol (HS) and a Yellow Oxisol(YO), were collected from the Amazon forest near the city São Gabrielda Cachoeira along a depth profile of 4 m. The third soil sample, a RedOxisol (RO) was collected close a remnant Atlantic forest near SãoCarlos city, São Paulo State, at a depth from 0 to 1 m.
Soil samples were sieved to remove roots and then homogenizedfrom process of ground to obtain particles smaller than 0.15 mm. Soilpellets were prepared from homogenized samples using a pressure of10 t cm−2 for 30 s.
2.2. Experimental setup
The LIBS systemusedwas composed of twoNd:YAG lasers operatingat different wavelengths, the one at 1064 nm (IR) and the other at532 nm (VIS). The IR pulse had a maximum energy of 75 mJ, a widthof 6 ns and was generated by a Nd:YAG Q-switch Ultra (Quantel). TheVIS pulse had a maximum energy of 180 mJ, a width of 4 ns, and wasgenerated by a Nd:YAG Q-switch Brillant (Quantel) coupled with a sec-ond harmonic generator module. A 400-Butterfly Aryelle system wasused to detect and select the wavelengths. The spectrometer operatesin two spectral bands, 175–330 nm and 275–750 nm, with a spectralresolution of 13–24 pm and 29–80 pm, respectively, and has an intensi-fied charge-coupled device (ICCD) camerawith 1024× 1024 pixels. Thebeams from the two lasers were directed to the target sample bydichroic mirrors at appropriate wavelengths (532 nm and 1064 nm).
inear DP LIBS setup.
Fig. 2. SEM images of the craters generated on soil RO using four different LIBS configurations: (a) SP at 45mJ (laser, 532 nm), (b) SP at 90 mJ (laser, 532 nm), (c) DP at 1064 nm (IR) and532 nm (VIS) at 90 mJ and (d) DP at 532 nm (VIS) and 1064 nm (IR) at 90 mJ.
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Lenswith a focal length of 100mmwith anti-reflecting coating (532 nmand 1064 nm) used at the best of laser optical energy were placed be-tween the sample and the tip of the fiber for efficient collection of theemitted plasma. The sample support was placed in a micro-controlledxy stage for an easy and fast scanning of the laser beam impinging onit. A pulse generator with eight channels (Quantum Composers Manu-facturer, model 9618) was used to synchronize the delay time betweenpulses and the delay detection during the experiments.
The images of the craters generated by different settings of the lasersystem on soil pellets were analyzed by scanning electron microscopy(SEM) (JSM-6510/JEOL, Thermo Scientific). For the acquisition of DPLIBS spectra the collinear geometry assembly using two laser beamseach with energy of 45 mJ was chosen. The beams were focused andaligned to hit the sample in the overlappingmodewith a delay betweenthem. The integration timewas set at 1ms and the gate time at 5 μs. Thescheme shown in Fig. 1 represents the experimental system used.
The variation of delay time and of interpulse delay was investigatedby performing ten measurements for each parameter at different posi-tion of the sample. Further, a comparative SP LIBS and DP LIBS studywas performed by using a 532-nm laser of pulse energy fixed at90 mJ. Total of 30 measurements, 15 with the SP LIBS and 15 with DPLIBS were performed on each face of each pellet. The background ofthe LIBS spectra were corrected by subtracting the average noise regionnear the element emission line. Results were based on the areasobtained from the Lorentzian fit for one peak in each spectrum andthen by averaging the areas.
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Fig. 3. Line emission intensity of Fe II 238.84 nm line (a) and C I 247.86 nm line (b) of soilRO as a function of the delay time at four different interpulses.
3. Results and discussion
In the first part of this work, the influence of the wavelengths usedfor laser ablation and plasma re-heating was investigated on soilsamples. Fig. 2 shows the SEM images of the craters formed on RO soilpellets when four different configurations of the system were used:a) SP at 45 mJ, b) SP at 90 mJ, c) DP at 1064 nm (IR) and 532 nm (VIS)at 90 mJ, and d) DP at 532 nm (VIS) and 1064 nm (IR) at 90 mJ. Themass removal rate values according to Fabbro et al. [42] depend on irra-diance (W/cm2) and, consequently, on energy per pulse (mJ), so highermass removal is expected when 90 mJ is used rather than 45 mJ for SP
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modes. Further, images obtained show a small increase of surface dam-age when the energy of 90 mJ was used in the SP configuration (Fig. 2aand b). In the DP configuration (Fig. 2c and d) the process of ablationwas more marked, and a greater amount of mass was removed, as ex-pected. Due to the deeper crater formed, an increase of themass remov-al ≥3.5 times for DP mode was found by other authors [9,43]. Inparticular, Benedetti et al. [43] found a volume of the crater in DP LIBSlarger by a factor of 4–6with respect to that obtained in SP LIBS. Furtherthe ablation obtained for theDPmode at 90mJ (Fig. 2d) appeared clean-er than that obtained at 45mJ (Fig. 2c), where melting effects appeareddominant. This result was expected as the wavelength has a depen-dence on the Inverse Bremsstrahlung (IB) (λ3 ~ IB), and more thermaleffects are induced on the sample surface when using IR laser ablation(Fig. 2c). The background emission increased using the sequence 1064and 532 nm, which, however, hindered partially data analysis. Thisphenomenon may be associated to a process of ablation at 1064 nmhotter than that obtained at 532 nm, although no significant differenceswere measured for the relative peak intensities. Thus, the betterperformance of IR laser for plasma re-heating confirms previous litera-ture results [10,44].
The main mechanisms suggested for laser energy absorption byplasma include electron-ion IB, electron-neutral IB and photoionization.The electron-ion IB is higher at longer excitation wavelength (IR) be-cause of its λ3 dependence, while photoionization is predominant atVIS–UV wavelengths. Both processes associated with laser ablationand re-heating are influenced by collinear arrangement depending onthe interpulse delays, as will be discussed below [10].
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Fig. 4. Temperature (a) and electron density (b) as a function of the delay time for soil RO.The value of interpulse delay used was 1500 ns.
In the second part of this work the DP LIBS parameters were opti-mized for the different soils. Fig. 3 shows the intensity of the ionic Fe II238.84 nm line (a) and atomic C I 247.86 nm line (b) of the RO sampleas a function of the delay time at four different values of the interpulsedelay. The trends observed were similar for other emission lines inves-tigated. In general, the line intensity decreasedwhen the delay timewasincreased. The experiment repeated on the other two soil samples in theUV and VIS regions of the spectrometer yielded similar trends.
To better understand this behavior, the excitation temperature andelectron density were estimated for each delay time. The excitation tem-perature was determined using the Boltzmann plot for Fe emission lines,whereas the electron density was measured referring to the Fe line at426.05 nm and the calculationwere done based on the Stark broadening[45]. Fig. 4 shows the decreasing trends of excitation temperature(a) and electron density (b) as a function of the delay time for the ROsample at an interpulse delay of 1500 ns. The decrease of lines intensities(Fig. 3) appeared thus associated to the decrease of plasma excitationtemperature and of electron density with increasing the delay time. Forall emission lines the maximum intensity was obtained when the set-up was fixed at a delay time of 250 ns and an interpulse of 1500 ns.
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Fig. 5. Line emission intensity of Ti II and Ca II (a), electron excitation temperature (b), andelectron density (c) as a function of the interpulse delay for soil RO. The value of delay timeused was 250 ns.
27G. Nicolodelli et al. / Spectrochimica Acta Part B 111 (2015) 23–29
After testing the delay time and fixing it at 250 ns, the trends of theline intensities of Ti II and Ca II, of plasma temperature and of electrondensity were studied as a function of the interpulse delay varied from300 ns to 10,000 ns for the RO sample (Fig. 5). Fig. 5a shows that theline intensity of Ca II and Ti II increased from 600 ns to about 1600and 2000 ns, respectively, followed by a soft decrease to 10,000 ns. Asimilar trend was found for other lines appearing in the LIBS spectrumof RO sample. Further, a line intensity “valley” was present betweenabout 300 and 600 ns for these lines (Fig. 5) and other spectral lines ofRO. These results may be explained by the mechanisms underlyinglaser energy absorption by the plasma expansion dynamics. At veryshort interpulse delays the re-heating ismore efficient due to the higherdensity of the pre-plume, while at larger interpulse delays the laser–target coupling is higher due to the reduced density of the pre-plumewhich leads to an increased mass ablation as shown in Fig. 6. The signalenhancement may thus be attributed to both IB and photoionization
Fig. 6. SEM images of the crater generated on soil RO at interpulse delays of: (a) 500 ns,(b) 800 ns and (c) 1500 ns.
processes. However, at larger interpulse delays, most of the enhance-ment may be attributed to the increased mass ablation. Some shadow-graphy studies have shown that after the laser strikes the target, ashockwave is generated and most of the mass of the ambient gas iscompressed in a thin layer at the shockwave front, while inside theplume a sudden pressure drop occurs determining rarified ambientconditions [10,46]. However, as time progresses and the shockwavefront loses energy and pressure inside the plume, the core reaches itsoriginal ambient conditions [10].
The signal intensity dip appearing between about 300 ns and 600 nsinterpulse delay suggests a reduced coupling between the pre-pulseplasma and the re-heating beamand/or the reduction of laser–target in-teraction by the second beam. According to previous studies [10,27,43],the interaction of the second laser pulsewith the extremely rarified am-bient conditions behind the shockwave front of the pre-plasma can re-duce the emission intensity. The first laser pulse would result in theformation of a shockwave front that leads to a decrease of the buffergas density behind the shockwave front. The decrease of ambient gasdensity close to the target surface would lead to a decrease of lasershielding by the pre-plasma thereby determining an increase of thelaser ablation and signal enhancement [10]. The shockwave front de-taches from theplume front after fewhundreds of ns andmoves further,while the plume core gets stagnant at fewmm from the plasma surfacedepending on the laser pulse energy [10,47]. At higher interpulse delays(N500 ns), optimal rarified regions are generated which lead to a signalenhancementmostly due to increasedmass ablation. Overall, the signalenhancement may be attributed to a combination of increased laser
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Fe 238 Co 223 Mn 248 Mg 280 -- Fe 316 Fe 368 Na 351 Si 220 Al 2260
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Fig. 9. LIBS signal area of atomic and ionic emission lines obtained by SP and DP configu-rations on soil RO and YO. The delay time for SP mode used was 1000 ns. The delay timeand interpulse delay in DP mode were fixed at 250 ns and 1500 ns, respectively.
28 G. Nicolodelli et al. / Spectrochimica Acta Part B 111 (2015) 23–29
ablation and plume re-heating. The plasma temperature increased atinterpulse delays higher than 500 ns (Fig. 5b), thus the plasma formedby the second laser pulse expands at a faster speed in the rarified envi-ronment [48]. Above 600 ns a temperature decrease occurred and massablation became predominant again (Fig. 6). Similar to plasma temper-ature, electron density also showed a valley at about 500 ns and then in-creases at 600 ns (Fig. 5c).
The delay time and interpulse delay were optimized for soil RO, andthen confirmed by repeating the experiment with soil HS. Fig. 7 showsthe variations of line intensity (C I, 247.86 and Fe II, 238.84) as a functionof the delay time (a) and the interpulse delay (b) for soil HS. Similar tosoil RO (Fig. 3), at a fixed interpulse delay, a decrease of the intensity ofboth lines was observed (Fig. 7a). The behavior of additional atomicand ionic emission lines investigated, such as Mg II, 279.553 nm and280.270 nm, and Si I, 212.412 nm, was similar. Fig. 7b shows that, at afixed delay time, the behavior of C and Fe emission lines was similar tothose of the RO sample (Fig. 4). In particular, the line intensity “valley”shown for soil RO is also observed for soil HS near the interpulse valueof 500 ns.
After configuration optimization of DP LIBS, the SP system also wasoptimized. The laser energy was fixed at 90 mJ and the gate time at5 ns, whereas the delay time was tested from 250 ns to 3000 ns. A sim-ilar trend was measured for all ionic and atomic lines tested, i.e. Al II198.99, Fe II 238.84, C I 247,86, Mg II 279.553 and 280.270, Si I212.412 and Co II 258.22 and the maximum intensity was found at adelay time around 500–1000 ns.
Based on the best parameters obtained for SP and DP LIBS a compar-ative study was performed on soil YO. Fig. 8 shows the emission spec-trum obtained by SP LIBS and DP LIBS in two different spectral regions
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Fig. 8. SP andDP LIBS emission spectra of soil YO in twodifferent spectral regions: (a) 273/290 nmand (b) 367/378 nm. The delay time in SPmodewasfixed at 1000 ns. ForDPmodethe delay time and interpulse delay were fixed at 250 ns and 1500 ns, respectively.
of soil YO. Data in Fig. 8 indicate that, despite the same energy impingingthe sample surface in both configurations, DP LIBS showed a bettersensitivity with improvement of LIBS signals. Further, some emissionlines that did not appear in SP spectra were present in DP spectra, andan intensity increase of almost 8 times was obtained for some low-intensity emission lines.
Finally, some ionic and atomic emission lines of interest of soil YOand RO, i.e. Fe II 238.84, Mg II 280.270, Co II 223.107, Mn II 248.616, SiI 220.80 Al I 226.90, Fe I 316.88, Ti I 399.86, Fe I 368.60, and Na I351.10, were tested comparatively using the SP and DP LIBS configura-tions (Fig. 9). In all cases the signal areas measured using DP LIBSwere much higher compared to those obtained by SP LIBS at the sametotal energy. This result may be explained by the larger ablated massresulting when using DP (Fig. 1) and/or by the increase of populationdensities of upper levels of the spectral lines due to the higher plasmatemperature [49]. These results confirm those of Corsi et al. [27] whofound an increase from about 1.3 to 9.4 times of the emission lines ob-tained using DP on polluted soils.
4. Conclusions
The key parameters that influence the DP LIBS technique in the col-linear beamgeometrywhen applied to the analysis of soil sampleswereoptimized. The correct choice of the delay time between the two pulsesis crucial for the DP system, and this depends on the laser ablation andplasma formation dynamics. An optimization of DP LIBS system wasperformed, which allowed LIBS analysis of different soil types usingdifferent emission lines. The emission line intensities using the DP con-figuration were about of 5 times higher than those obtained by SP LIBS.The signal enhancement may be attributed to a combination of in-creased laser ablation and plume re-heating. The combination of differ-ent wavelengths and the selection of appropriate temporal and spatialparameters are expected to yield acceptable LOD values for LIBSspectroscopic analysis in the near future. Although soil texture is animportant factor that may affect soil LIBS analysis in various ways, anddespite the different texture of soils examined, similar results wereobtained in all cases.
Acknowledgments
The authors thank grant 2012/24349-0, São Paulo Research Founda-tion (FAPESP) and grants 403405/2013-0 and 479994/2013-7, CNPq fortheir financial support of this study and Dr Célia ReginaMontes for pro-viding the Amazon samples tested.
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