Emission spectroscopy of long cylindrical laser spark with additional coaxial excitation

Spectrochimica Acta Part B 157 (2019) 22–26

Contents lists available at ScienceDirect

Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab

Technical note

Emission spectroscopy of long cylindrical laser spark with additional coaxial excitation Aleksandr S. Zakuskin, Andrey M. Popov, Timur A. Labutin

T

⁎

Department of Chemistry, Lomonosov Moscow State University, 119234, Leninskie Gory, Moscow, Russia

A R T I C LE I N FO

A B S T R A C T

Keywords: Long laser spark Laser-induced breakdown spectroscopy Double-pulse Electron density Temperature

Low intensity lines of elements with high excitation potentials, especially non-metals, is one of the main problems of laser-induced breakdown spectroscopy (LIBS). To our knowledge, we report the first combination of long spark produced by a cylindrical lens with an additional orthogonal excitation along the long axis. We studied temporal evolution of the plasma parameters at delays in the range of 100 to 500 ns and inter-pulse delays of 600 to 1800 ns. This variant of double pulse LIBS leads to the formation of a rather large (approx. 14 mm length) and hot laser-induced plasma, (2.0–3.8) × 104 K, with a relatively low electron density, (2–8) × 1017 cm−3. Our analysis reveal that the proposed optical configuration provides the optimal conditions for observing ionic lines with high excitation potentials in comparison with the conventional LIBS arrangement.

1. Introduction The interaction of powerful laser pulse with solids results in plasma formation, and this effect is widely used in technology and science, e.g. in a micromachining of materials [1], in production of nanostructured surfaces [2], as a nonlinear medium for generating ultrashort attosecond laser pulses [3], as well as in remote spectral analysis of various substances [4] etc. Laser-induced breakdown spectroscopy (LIBS) is a simple and versatile method for direct determination of almost all elements in solids, liquids and gases/aerosols [5,6]. However, the relatively low sensitivity and reproducibility due to pulse-to-pulse variations and plasma inhomogeneity resulting in self-absorption [7] are still the most important problems for analytical applications of LIBS that are similar to other direct analytical techniques as XRF, arc, spark etc. Plasma homogeneity can be improved by focusing laser radiation with a cylindrical lens in a line (or long spark) with length up to 20 mm and diameter of ~150–200 μm. Such a radiation source is used for accurate determination of Stark broadening [8] and for lasing [9]. The considerably increased area of detector covered with plasma spectral image due to the long length of plasma improves a signal-to-noise ratio. The long spark regime was also used for rapid chemical mapping of heterogeneous samples [10,11]. At the same time, the plasma temperature of this source is distinctly lower than for common focusing into spot that, as expected, hamper determination of elements with high excitation potential (primarily, non-metals). Several approaches were proposed to improve LIBS sensitivity; among them are double-pulse

⁎

LIBS [12,13], measurements at low pressure in a helium atmosphere [14], a combination of LIBS with fluorescence [15,16], spatial confinement of plasma [17,18], etc. The most popular and promising technique of additional plasma excitation is double pulse one [19]. Generally, there are two configurations of double-pulse LIBS [20]: two laser beams (or subsequent pulses) going to the focal point along the same path (collinear scheme), and an ablative beam combined with a perpendicular one passing above the ablation spot (orthogonal scheme). The collinear scheme is simple for arranging an experimental set-up, while enhancement factor for the orthogonal one depends linearly on the excitation potential of element that makes it more suitable for determination of non-metals [21]. Orthogonal scheme seems to be the most reasonable for the long spark accounting its shape. A second beam can pass along the plasma axis transferring most of energy for additional plasma excitation. There are no any mentions, in the best of our knowledge, about such a combination. Therefore, we focused this study on experimental arrangement of orthogonal double-pulse LIBS with long spark, as well as on characterization of such a plasma source. 2. Experimental We used the new optical configuration of our laboratory LIBS set-up [22] that produced a long spark followed by an orthogonal laser pulse as shown in Fig. 1. Radiation of the second harmonics (532 nm, beam diameter = 6 mm, 8 ns, 82 mJ/pulse) of the first Q-switched Nd:YAG laser (model LS-2134UTF, LOTIS TII, Belarus) was focused on the

Corresponding author. E-mail address: [email protected] (T.A. Labutin).

https://doi.org/10.1016/j.sab.2019.05.007 Received 12 April 2019; Received in revised form 8 May 2019; Accepted 10 May 2019 Available online 15 May 2019 0584-8547/ © 2019 Elsevier B.V. All rights reserved.

Spectrochimica Acta Part B 157 (2019) 22–26

A.S. Zakuskin, et al.

Fig. 2. Experimental spectra for delay of 100 ns at interpulse delay 1200 ns.

made software previously described elsewhere [24]. We used a sample of magnesium alloy MA21–323 with the following content of major elements: 3.09 ± 0.06 wt% Al, 550 ± 30 ppm Mn, for all measurements. A typical spectrum for the sample and the identified lines in accordance with NIST database [25] are presented in Fig. 2. 3. Results and discussion Since estimated impact of Doppler broadening on the lines was less than 7 pm, while the observed FWHM varied between 120 and 450 pm, we neglected the influence of the Doppler broadening on the line profile. For all the considered calculations we fitted the lines by Lorentzian profile taking into account the instrumental width of 66.8 ± 0.4 pm in the range near to 395 nm [23]. We subtracted the instrumental width from the experimentally measured line width, and then estimated electron number density (Ne) based on resulting value. We calculated Ne of plasma by both Stark widths and shifts of two Al I lines at 394.40 and 396.15 nm, two N II lines at 399.50 and 661.06 nm, and Hα 656.28 nm. Their Stark parameters are listed in Table 2. We used formulas for the quadratic Stark effect [30] for calculations based on aluminium and nitrogen lines and the linear Stark effect [31,32] for the Balmer alpha line. Fig. 3 shows electron density calculated by these lines (uncertainties include both the approximation error of the line profile and the uncertainty of Stark parameters) for the interpulse delay of 1200 ns and the delay after pulse of 100 ns. Since Balmer alpha line [33] may be self-absorbed at the shortest delay that was observed by Swafford et al. [34], the calculated full-widths at half maximum (FWHM) resulted in the overestimated values of Ne. Note that Hα and N II 661.06 nm have slightly asymmetric profiles, and, therefore, their apparent shifts are preferable for electron density calculation. Shifts of Balmer alpha line and N II 661.06 nm are consistent with those of Al I lines. The close values of electron density calculated by the lines of atmospheric elements and by the lines of aluminium originating from the solid target, indicate that the plasma source is relatively homogeneous. The aluminium lines were free from self-absorption, since the observed intensity ratio in the doublet was close to the theoretical value of 1:2. Moreover, the atomic fraction of Al was very small (about 0.01%) following NIST Saha-Boltzmann plot under our conditions (T = 2.8 eV, Ne = 5 × 1017), and the optical path along the observation

Fig. 1. Scheme of experimental arrangement of optical elements to produce long plasma with the second orthogonal laser beam. Table 1 Temporal parameters for spectra registration.

Delay, ns Gate, ns

Double-pulse

Single-pulse

390–410 nm

278–286 nm

100 50

500 50

500 250

surface of magnesium alloy by a glass cylindrical lens (f = 500 mm). It produced 14 mm long plasma on the surface. The beam (532 nm, diameter = 8 mm, 4 ns, 240 mJ/pulse) of the second laser (model LS2137/2UTF, LOTIS TII, Belarus) was directed along the axis of the long spark at a height of 0.5 mm above the target surface and focused by an achromatic doublet (f = 150 mm) in the middle of the long plasma. Aspheric quartz lens with short focal length (f = 30 mm) was placed ~6 cm above the sample surface to provide maximal collection of plasma radiation with minimal aberrations on the end of an optical fiber. Plasma radiation was projected onto the optical fiber with reduction ratio of 2:1. The fiber projected an image of the long plasma onto the slit (25 μm) of Czerny Turner spectrometer (HR-320, ISA, USA) that provides medium resolution of ≈5900 at 400 nm and ≈12,800 at 650 nm with grating 1800 lines/mm. Two lasers were synchronized by a digital pulse generator (Sapphire 9214-BT, Quantum Composers, USA) which made it possible to set delay between laser pulses. Since the position and width of the emission lines may vary due to electron density variations within gate time, we chose delay and gate times as presented in Table 1 according to recommendations of Aragon and Aguilera [23]. Plasma emission spectra were recorded by an ICCD camera (Nanogeit-2 V, NANOSCAN, Russia) with the use of laboratory-

Table 2 Stark broadening and shift parameters of the lines used for Ne calculation. Line, nm Al I 394.40 Al I 396.15 Hα 656.28 N II 661.06

23

ws = 46.5 ds = +14.0 wS = 269 dS = +40 ws = 81.3 ds = −6.20

T, 104 K

Ne, 1017 cm−3

Ref.

1.0 1.0 2.0 – 2.8 2.8

1.42 1.42 0.1 6.3 1.0 1.0

[26] [27] [28] [29]

Spectrochimica Acta Part B 157 (2019) 22–26

A.S. Zakuskin, et al.

Fig. 3. Comparison of values for electron density calculated by Stark parameters of various lines (delay– 100 ns, interpulse delay – 1200 ns).

Fig. 4. Evolution of emission spectra at different interpulse delays.

Therefore, this plasma is produced both from atmospheric gases and from major components of the Mg target and allows one to observe atomic and ionic transitions with very high energy of the upper levels. The recorded spectra in the range of 396 nm, obtained for different interpulse delays and different delays after the second pulse, are shown in Fig. 4. We chose interpulse delays of 600, 1200 and 1800 ns and delays after the second pulse of 100, 300 and 500 ns, since their combinations allow us to observe plasma with significantly different parameters. Table 4 contains the measured values of the plasma temperature and electron number density. As one can see, temperature decreases faster with the longer interpulse delay. The value of T for the shortest delay after the second pulse (100 ns) is around 30,000 K, which is significantly higher than the temperature of the commonly used laser-induced plasmas. On the contrary, we observe a strong dependency of electron number density on the interpulse delay. We observed the highest value, (8.9 ± 0.9) × 1017 cm−3, for the shortest delay between pulses and significantly lower values, 4.1 × 1017 and 1.8 × 1017 cm−3, for interpulse delays of 1200 and 1800 ns, respectively. Conditions in this plasma can be described as very hot with relatively low electron density. The advantages of the combination are well illustrated by the pair of lines: O II 391.19 nm and N II 391.90 nm. The upper level of the oxygen line is 5.5 eV higher. We did not observe these lines in case of the simple long spark at any delay, which is not surprising given the low fluence. In case of spherical single pulse plasma, these lines were completely overlapped at the early delays, and the O II line presented as a shoulder or poorly resolved peak on the wing of the N II line at delay of 200–300 ns. The highest intensity ratio obtained after deconvolution was 4 times smaller than for the long spark with additional orthogonal excitation, while the uncertainty of intensities was 4 times higher. Therefore, the suggested combination is optimal for registration of emission lines with large excitation potentials. Earlier, for a short delay time, Parigger et al. [29] determined the electron density in laser

was very small too. To obtain a more reliable estimation of electron number density, we averaged values from Al I lines and shifts of nitrogen line (See Fig. 3). Plasma temperature was calculated by the Boltzmann plot [24] in approximation of existence of Local Thermodynamic Equilibrium (LTE). In this case, the measured electron number density should at least fulfill the McWhirter criterion necessary for LTE. Taking the largest gap between the energy levels of 3.163 eV (N II 391.90 nm), electron number density should exceed 9 × 1015 cm−3 and 7 × 1015 cm−3 for the shortest (100 ns) and longest (500 ns) delays, respectively. All our values (Table 4) are well above the given limits, so we assumed the existence of LTE. The lines used for Boltzmann plot and their transitions' parameters are listed in Table 3. These lines were selected as visible on different observation delays and almost isolated, that allowed fitting them by Lorentzian profile and calculation of integral intensities. The components of N II multiplet 3F°– 2[9/2] were not resolved due to Stark broadening, but they are free from spectral interferences. Since the differences in the energy of the upper level and the wavelength are small, the intensity of an individual component can be presented as

Ii = Imult ∗ gi Ai / ∑ gn An

(1)

n

We studied temporal evolution of long laser spark with auxiliary longitudinal excitation by the second pulse at earlier stages of plasma expansion (delays from 100 to 500 ns). Most of the observed lines (Fig. 2) within the range from 382 to 413 nm are ionic lines of nitrogen and oxygen with the energy of the upper level from 23 to 29 eV. Also, we observed two strong resonance transitions of aluminium atom. Table 3 Parameters of transitions for N II lines. Configurations and terms

Wavelength, nm

gkAki, 107 s−1

ΔA/ A, %

Elower, eV

Eupper, eV

[2s22p3p] 1P – [2s22p3d] 1P° [2s22p3s] 1P° [2s22p3p] 1D [2s22p3d] 3F° – [2s22p(2P°3/2) 4f] 2[9/2]

391.90

22.7

7

20.409

23.572

399.50

61.0

3

18.497

21.599

402.61 403.51 404.13 404.35 404.48 405.69

80.7 172 291 226 15.9 14.9

– – – – – –

23.132 23.124 23.142 23.132 23.132 23.142

26.210 26.196 26.209 26.197 26.196 26.197

Table 4 Temperature and electron number densities.

Interpulse delay 600 ns Interpulse delay 1200 ns Interpulse delay 1800 ns Single-pulse

24

Delay, ns

T, 103K

Ne, 1017 cm−3

100 500 100 500 100 500 500

29 ± 5 18 ± 2 33 ± 8 12.7 ± 0.2 30 ± 5 12.3 ± 0.5 11.4 ± 0.5

8.9 ± 0.9 8.2 ± 0.6 4.1 ± 0.4 4.2 ± 0.3 1.8 ± 0.4 1.9 ± 0.5 4.32 ± 0.05

Spectrochimica Acta Part B 157 (2019) 22–26

A.S. Zakuskin, et al.

induced plasma in air of 5 × 1018 cm−3 and temperature of 22,500 K. The similar conditions at short delays were recently observed in a spherical laser-induced plasma for both single- and double-pulse configuration [35]. Measurements of air plasma by an independent technique (Thomson scattering) showed the electron density of 1 × 1018 cm−3 at 100 ns delay for very similar excitation conditions (532 nm, 312 mJ/pulse) [36]. Generally, the electron density was higher by an order of magnitude (2–5 × 1018) at such a delay after the laser pulse for single-pulse configuration with the fluence in focus of 10 J/cm2. Thus, the suggested double pulse configuration allows the significant reduction of electron density (at least one order of magnitude), while maintaining the high temperature in laser-induced plasma. In conjunction with the fact that the long spark provides more uniformly distributed plasma with lower variations of parameters along its axis, it can be a good emission source both for analytical purposes and for determination of lines parameters, such as parameters of Stark broadening and shift. For example, one of the poorly determined by LIBS nonmetals is chlorine. The commonly used [37] non-resonant line Cl I 837.59 nm limits detection capabilities due to low sensitivity of either intensified or non-intensified cameras in near-IR. It should be noted that the observed conditions may allow the utilization of the ionic line in visible range (Cl II 479.46 nm, the most intense in hot and low density helium microwave plasma [38]). The suggested configuration can provide favourable conditions for its observation: the intensity ratio of the ionic to the atomic chlorine lines would be 100 (calculated by NIST Saha-LTE Spectrum [25], 2.6 eV, 4 × 1017 cm−3).

[3] X.F. Zhang, X.S. Zhu, X. Liu, D. Wang, Q.B. Zhang, P.F. Lan, P.X. Lu, Ellipticitytunable attosecond XUV pulse generation with a rotating bichromatic circularly polarized laser field, Opt. Lett. 42 (2017) 1027–1030. [4] A.K. Shaik, Ajmathulla, and V. R. Soma, Discrimination of bimetallic alloy targets using femtosecond filament-induced breakdown spectroscopy in standoff mode, Opt. Lett. 43 (2018) 3465–3468. [5] D.W. Hahn, N. Omenetto, Laser-induced breakdown spectroscopy (LIBS), part II: review of instrumental and methodological approaches to material analysis and applications to different fields, Appl. Spectrosc. 66 (2012) 347–419. [6] N.B. Zorov, A.M. Popov, S.M. Zaytsev, T.A. Labutin, Qualitative and quantitative analysis of environmental samples by laser-induced breakdown spectrometry, Russ. Chem. Rev. 84 (2015) 1021–1050. [7] J. Li, L. Guo, C. Li, N. Zhao, X. Yang, Z. Hao, X. Li, X. Zeng, Y. Lu, Self-absorption reduction in laser-induced breakdown spectroscopy using laser-stimulated absorption, Opt. Lett. 40 (2015) 5224–5226. [8] A.M. Popov, T.A. Labutin, S.M. Zaytsev, N.B. Zorov, Experimental Stark parameters of Mn I lines in the y 6P° → a 6S multiplet under conditions of "long" laser plasma, Opt. Spectrosc. 123 (2017) 521–525. [9] L. Nagli, M. Gaft, Y. Raichlin, I. Gornushkin, Cascade generation in Al laser induced plasma, Opt. Commun. 415 (2018) 127–129. [10] K.T. Rodolfa, D.A. Cremers, Capabilities of surface composition analysis using a long laser-induced breakdown spectroscopy spark, Appl. Spectrosc. 58 (2004) 367–375. [11] M.P. Mateo, S. Palanco, J.M. Vadillo, J.J. Laserna, Fast atomic mapping of heterogeneous surfaces using microline-imaging laser-induced breakdown spectrometry, Appl. Spectrosc. 54 (2000) 1429–1434. [12] E. Tognoni, G. Cristoforetti, Basic mechanisms of signal enhancement in ns doublepulse laser-induced breakdown spectroscopy in a gas environment, J. Anal. At. Spectrom. 29 (2014) 1318–1338. [13] V.I. Babushok, F.C. DeLucia Jr., J.L. Gottfried, C.A. Munson, A.W. Miziolek, Double pulse laser ablation and plasma: laser induced breakdown spectroscopy signal enhancement, Spectrochim. Acta B 61 (2006) 999–1014. [14] P. Veis, A. Marín-Roldán, J. Krištof, Simultaneous vacuum UV and broadband UV–NIR plasma spectroscopy to improve the LIBS analysis of light elements, Plasma Sources Sci. Technol. 27 (2018) 095001. [15] L. Nagli, M. Gaft, Combining laser-induced breakdown spectroscopy with molecular laser-induced fluorescence, Appl. Spectrosc. 70 (2016) 585–592. [16] R. Yi, J. Li, X. Yang, R. Zhou, H. Yu, Z. Hao, L. Guo, X. Li, X. Zeng, Y. Lu, Spectral interference elimination in soil analysis using laser-induced breakdown spectroscopy assisted by laser-induced fluorescence, Anal. Chem. 89 (2017) 2334–2337. [17] A.M. Popov, F. Colao, R. Fantoni, Spatial confinement of laser-induced plasma to enhance LIBS sensitivity for trace elements determination in soils, J. Anal. At. Spectrom. 25 (2010) 837–848. [18] A.S. Zakuskin, A.M. Popov, N.B. Zorov, T.A. Labutin, Confinement of laser plasma by shock waves for increasing signal intensity in spectrochemical determination of trace elements in ores, Tech. Phys. Lett. 44 (2018) 73–76. [19] W. Li, X. Li, X. Li, Z. Hao, Y. Lu, X. Zeng, A review of remote laser-induced breakdown spectroscopy, Appl. Spectrosc. Rev. 53 (2018) 1–25. [20] J. Scaffidi, S.M. Angel, D.A. Cremers, Emission enhancement mechanisms in dualpulse LIBS, Anal. Chem. 78 (2006) 24–32. [21] A.S. Zakuskin, A.M. Popov, S.M. Zaytsev, N.B. Zorov, M.V. Belkov, T.A. Labutin, Orthogonal and collinear configurations in double-pulse laser-induced breakdown spectrometry to improve sensitivity in chlorine determination, J. Appl. Spectrosc. 84 (2017) 319–323. [22] A.M. Popov, T.F. Akhmetzhanov, T.A. Labutin, S.M. Zaytsev, N.B. Zorov, N.V. Chekalin, Experimental measurements of Stark widths for Mn I lines in long laser spark, Spectrochim. Acta B 125 (2016) 43–51. [23] C. Aragón, J.A. Aguilera, Characterization of laser induced plasmas by optical emission spectroscopy: a review of experiments and methods, Spectrochim. Acta B 63 (2008) 893–916. [24] S.M. Zaytsev, A.M. Popov, N.B. Zorov, T.A. Labutin, Measurement system for highsensitivity LIBS analysis using ICCD camera in LabVIEW environment, J. Instrum. 9 (2014) Article number P06010. [25] A. Kramida, Yu. Ralchenko, J. Reader, NIST ASD Team, NIST Atomic Spectra Database (ver. 5.6.1), [Online]. Available: https://physics.nist.gov/asd 2019, April 10 (2018), https://doi.org/10.18434/T4W30F. [26] T. Bach, Stark broadening of the Al resonance lines, J. Quant. Spectrosc. Radiat. Transf. 19 (1978) 483–491. [27] M.A. Gigosos, V. Cardenoso, New plasma diagnosis tables of hydrogen Stark broadening including ion dynamics, J. Phys. B 29 (1996) 4795–4838. [28] C.G. Parigger, D.H. Plemmons, E. Oks, Balmer series Hβ measurements in a laserinduced hydrogen plasma, Appl. Opt. 42 (2003) 5992–6000. [29] S. Mar, J.A. Aparicio, M.I. de la Rosa, J.A. del Val, M.A. Gigosos, V.R. Gonzalez, C. Perez, Measurement of Stark broadening and shift of visible N II lines, J. Phys. 33 (1169) (2000). [30] H.R. Griem, Spectral Line Broadening by Plasmas, (1974). [31] C.G. Parriger, J.O. Hornkohl, L. Nemes, Measurements of aluminum and hydrogen microplasma, Appl. Opt. 46 (2007) 4026–4031. [32] C.G. Parriger, K.A. Drake, C.M. Helstern, G. Gauntam, Laboratory hydrogen-beta emission spectroscopy for analysis of astrophysical white dwarf spectra, Atoms 6 (2018). [33] C.G. Parigger, D.M. Surmick, G. Gautam, A.M. El Sherbini, Hydrogen alpha laser ablation plasma diagnostics, Opt. Lett. 40 (2015) 3436–3439. [34] L.D. Swafford, D.M. Surmick, M.J. Witte, A.C. Woods, G. Gautam, C.G. Parriger, Hydrogen Balmer series measurements in laser-induced air plasma, J. Phys. Conf. Ser. 548 (2014).

4. Conclusions Combination of both techniques of long spark with double pulse excitation was demonstrated to provide optimal conditions for observation of ionic lines of non-metals in laser plasma due to reheating of the whole volume of the initial plasma by high-power laser radiation passing along the axis. The use of the combination resulted in formation of a rather large (approx. 14 mm length) and hot laser-induced plasma (29,000–33,000 K) with relatively low electron density (2 × 1017–9 × 1017 cm−3). The latter effect seems to be related with underpressure after the first pulse. Obvious strong Stark broadening of emission lines, leading to the formation of intense pseudo-continuous background, was observed at early times of laser-induced plasma in air, but not in orthogonal double-pulse long spark. The results confirmed the significant decrease of the electron density in comparison with the known data for similar conditions of plasma ignition. Thus, the developed double pulse technique has a great potential for studying of broadening parameters of ionic lines of non-metals, as well as improving their LIBS determination. Also, we would make some recommendations for application of such a plasma source. To diagnose plasma via Stark broadening, the interpulse delay should be as short as possible, e.g. several hundred nanoseconds. Another possibility is to detect ionic lines with high energy of the upper level, e.g. Cl II or S II lines, for determination of element content at long interpulse delay (~2 μs) combined with short (~100 ns) delay after the second pulse. Acknowledgements This work was supported by the Russian Science Foundation (grant No 18-13-00269). References [1] J. Skruibis, O. Balachninaite, S. Butkus, V. Vaicaitis, V. Sirutkaitis, Multiple-pulse laser-induced breakdown spectroscopy for monitoring the femtosecond laser micromachining process of glass, Opt. Laser Technol. 111 (2019) 295–302. [2] L. Xu, J.W. Zhang, H. Zhao, H.B. Sun, C.X. Xu, Enhanced photoluminescence intensity by modifying the surface nanostructure of Nd3+−doped (Pb, La) (Zr, Ti)O3 ceramics, Opt. Lett. 42 (2017) 3303–3306.

25

Spectrochimica Acta Part B 157 (2019) 22–26

A.S. Zakuskin, et al.

[37] S. Millar, C. Gottlieb, T. Günther, N. Sankat, G. Wilsch, S. Kruschwitz, Chlorine determination in cement-bound materials with laser-induced breakdown spectroscopy (LIBS) – a review and validation, Spectrochim. Acta B 147 (2018) 1–8. [38] J.F. Alder, Q. Jin, R.D. Snook, The determination of chloride by microwave helium plasma emission spectrometry using a hydrogen chloride generation technique, Anal. Chim. Acta 123 (1981) 329–333.

[35] L. Wermer, J.K. Lefkowitz, T. Ombrello, M.S. Bak, S.K. Im, Spatiotemporal evolution of the plasma from dual-pulsed laser-induced breakdown in an atmospheric air, Plasma Sources Sci. Technol. 27 (2018) 015012. [36] P.K. Diwakar, D.W. Hahn, Study of early laser-induced plasma dynamics: transient electron density gradients via Thomson scattering and Stark broadening, and the implications on laser-induced breakdown spectroscopy measurements, Spectrochim. Acta B 63 (2008) 1038–1046.

26