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Abstract
Heavy metal pollution is one of the main problems in water pollution, which is harmful to humans. Surface-enhanced laser-induced breakdown spectroscopy (SENLIBS) has been applied to detect trace amounts of heavy metal elements in aqueous solution; however, it is still a big challenge to explore the relationship between the LIBS detection sensitivity and the substrate’s physical properties. In this work, four typical substrates, zinc (Zn), magnesium alloy (Mg), nickel (Ni), and silicon (Si), were compared; and the mechanism of spectral enhancement by different substrates in SENLIBS was investigated. The results indicated that the limit of detection (LoD) of heavy metal elements on different substrates is positively proportional to the boiling of the substrate. That is mainly because a higher plasma excitation temperature and electron density are obtained, leading to more intense collision between particles. The signal enhancement is associated with the lower boiling point of the substrate (corresponding to a lower ablation threshold and higher ablation quantity from the substrate). As a result, the best LoD was 0.0011 mg/L for chromium (Cr) and 0.004 mg/L for lead (Pb) on an optimal Zn substrate, respectively. The LoDs were sufficiently low to meet the drinking water sanitation standard. These results showed that the detection sensitivity of heavy metal elements in aqueous solution can be improved by choosing a substrate with a lower boiling point in SENLIBS.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The pollution of water, especially heavy metal pollution, has recently been attracting more attention. Heavy metal elements in an aqueous solution are highly toxic, even at trace levels, and can cause lasting damage to organisms due to the high enrichment and difficult degradation [1]. For example, chromium (Cr) may be carcinogenic to humans, and lead (Pb) can wreak havoc on human body systems, including the nervous system [2]. China has issued a series of standards for heavy metal detection to protect people’s health. Therefore, the detection of heavy metals in water is of great significance.
Laser-induced breakdown spectroscopy (LIBS) is an atomic spectrum analysis method. With the advantages of minimal sample preparation, rapidity, and simple operation [3,4], LIBS has shown great potential for detecting elemental composition in many fields, such as industry [5–9], agriculture [10,11], water [12–14], food [15], and explosives [16]. So far, researchers have done extensive work on element detection in an aqueous solution [17–23]. For example, Kuwako et al. [17] detected sodium (Na) in water using dual-pulse LIBS with the liquid flow; and the limit of detection (LoD) of Na was achieved in the range of 0.1 parts per billion (ppb). Koch et al. [18] detected indium (In) in water using resonance fluorescence spectroscopy in laser-induced cavitation bubbles, and an LoD of 10 mg/L was obtained. Lui et al. [13] detected Pb in water using LIBS and laser-induced fluorescence (LIBS-LIF) with a liquid jet, and the LoD was found to be 35 ppb. However, the above liquid predisposal measures not only increase the complexity of the instrument but also have poor reproducibility due to water splashing and quenching of intensity. For the above defects, the transfer of liquid samples into solid samples before analysis is an effective and simple method for effectively improving stability and sensitivity. For example, Wang et al. [24] used LIBS based on metal precipitation and membrane separation to analyze copper (Cu), silver (Ag), manganese (Mn), and Cr in liquid samples; and LoDs within the range of 0.957-2.59 ug/L were achieved. Alamelu et al. [25] detected samarium (Sm), europium (Eu), and gadolinium (Gd) in aqueous samples using LIBS with the samples supported by a membrane-based filter paper. The LoDs were found to be 1.3 ppm (Sm), 1.9 ppm (Eu), and 2.3 ppm (Gd). In addition, other solid-phase extraction adsorbents have been developed, such as wood slice [26], bamboo charcoal [27], plant fiber spunlace nonwovens [28], paper substrate [29], porous electrospun ultrafine polymer fibers [30], electrical deposition on an aluminum sheet [31], nonabsorbent solid surface [32–35], and so on. It is worth mentioning that surface-enhanced LIBS (SENLIBS), based on a nonabsorbent solid surface, exhibits superior spectral enhancement and repeatability. For example, Aguirre et al. [34] used aluminum substrate to dry microdroplets; and a LoD of 6 ug/g for Mn was achieved. Bae et al. [33] analyzed potassium (K) ions by using a laser-patterned silicon wafer (LPSW) substrate, and a LoD of 0.23 × 10−9 mol and an analysis precision of ~4% relative standard deviation were obtained. Vander Wal et al. [36] found that the substrates of different materials have different enhancement effects on the spectrum, and SENLIBS possesses sufficient reproducibility. All of these studies achieved the spectral enhancement with different substrates. However, the LoDs of elements such as Pb failed to meet the drinking water sanitation standard of China (0.01 mg/L for Pb, GB5749-2006); and the relationship between the LIBS detection sensitivity and the substrate physical properties, including the boiling point, has not been reported thus far. Therefore, it is very significant and interesting to study the mechanism of spectral enhancement by different substrates in SENLIBS.
In this work, the effect and mechanism of different substrates on the spectral intensity was investigated. The Cr and Pb elements in an aqueous solution were analyzed as two examples; and four typical substrates (zinc (Zn), magnesium alloy (Mg), nickel (Ni), and silicon (Si)), which covered a wide range of thermal properties, were selected to study the mechanism of spectral enhancement. There was no spectral line interference between the element to be tested and the substrate element. Quantitative analyses of Cr and Pb were conducted. The relationship between the LoD and the boiling point of the substrate was studied. The effect of the substrate’s boiling point on the plasma excitation temperature and electron density during the ablation process were discussed.
2. Experimental setup and sample preparation
2.1 Experimental setup
Figure 1 is a schematic of the experimental setup. The sample was ablated using a Q-switch Nd: YAG laser (Quantel Ultra 100; pulse width: 7 ns; repetition rates: 10 Hz; wavelength: 532 nm). The laser beams were focused onto a sample surface by a 25 mm plano-convex quartz lens. The pulse energy was set to 3 mJ in our experiments. The diameter of the ablation spot was approximately 20 μm. The sample was located on a motorized translational stage that exposed a fresh region of the sample for analysis, and the platform worked at a speed of 0.5 mm/s. A Czerny-Turner (C-T) spectrometer (Andor Tech. Shamrock 500i; grating: 2400 l/mm; slit width: 200 μm) was employed to detect the plasma emission which was collected by a light collector. The spectrometer was equipped with an intensified charge-coupled device (ICCD) camera (Andor Tech., iStar 320T: DH320T-18F-E3-26mm). In particular, the C-T spectrometer was replaced by an echelle spectrometer (Andor Tech., Mechelle 5000, 200-950 nm, λ/Δλ = 5000) when the plasma excitation temperature was evaluated. The Nd: YAG laser and the ICCD camera were sequentially triggered by a digital delay generator (Wuhan N&D Laser Engineering, LDG3.0). The delay time of 2 μs and the gate width of 2 μs were optimized in our experiments to obtain optimal spectral intensity and signal-to-noise ratio (SNR). Each spectrum was performed by accumulating the signal from 360 ablation shots to scanning the whole area of heavy metal dry microdroplets.
2.2 Sample preparation
Four typical targets, including Zn (99.993-99.995%); Mg (AZ31BMg: 95.56 wt. %; aluminum (Al): 3.1 wt. %; Zn: 0.82 wt. %), Ni (99.995%), and Si (above 99.9999%) were used as substrates to investigate the mechanism of spectral enhancement by different substrates. The boiling points of the substrates are given in Table 1.
Table 1. Boiling points of the substrates in this work [37].
The stock solutions (10 mg/L) were prepared by dissolving analytical reagents of chromium (III) chloride (CrCl3) and lead nitrate (Pb(NO3)2) in deionized water, respectively. The analytical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd; and no further treatment was carried out. The concentrations of Cr and Pb in the standard solutions ranged from 0.02 to 0.8 mg/L and 0.04-1 mg/L, respectively. The concentration details are given in Table 2. The sample pretreatment process is shown in Fig. 2. The sample preparation method consisted of two steps. First, 20 μL microdroplets were deposited on a substrate, in an average of four drops, using a micropipette. Then the microdroplets on the substrate were heated to 70 °C on a hot plate, and a heavy metal layer was prepared about a circular area of 3 mm in diameter on the surface of the substrate.
Table 2. Concentrations of Cr and Pb in an aqueous solution.
3. Results and discussion
3.1 Comparison of spectra intensity obtained on different substrates
A representative example of spectra was obtained on different substrates, as presented in Fig. 3. The atomic emission lines of Cr I 425.43 nm and Pb I 405.78 nm were chosen as analytical lines. Figure 3 illustrates that the spectral intensities were different on different substrates, and the spectra of the Zn substrate was stronger than those of the other three.
Fig. 3 Emission spectral intensity of Cr I 425.43 nm (a, 0.6 mg/L Cr of CrCl3) and 405.78 nm (b, 0.6 mg/L Pb of Pb(NO3)2) prepared on Zn, Mg, Ni, and Si substrates.
3.2 Calibration curves and limits of detection
To further study the effects of different substrates on the analysis of heavy metal elements in SENLIBS, quantitative analyses of the trace amounts of Cr and Pb on different substrates were carried out. The calibration curves of Cr I 425.43 nm on different substrates are shown in Fig. 4(a), and the calibration curves of Pb I 405.78 nm are shown in Fig. 4(b). The R2 of Cr were 0.996, 0.984, 0.979, and 0.991; and the R2 of Pb were 0.992, 0.998, 0.919, and 0.980 for the Zn, Mg, Ni, and Si substrates, respectively. The LoDs of Cr calculated by the 3σ criterion were 0.0011, 0.0022, 0.0038, and 0.0041mg/L; and the LoDs of Pb were 0.004, 0.012, 0.021, and 0.024 mg/L for Zn, Mg, Ni, and Si substrates, respectively. Moreover, the root mean square error of cross-validation (RMSECV) of Cr was in the range of 0.026-0.040 mg/L; and the RMSECV of Pb was in the range of 0.0095-0.1869 mg/L. More detailed results of the quantitative analyses are listed in Table 3. The results showed that the LoD of Cr obtained on a Zn substrate was greater than other substrates and gradually deteriorated in the order of Mg, Ni, and Si. The same result was obtained for Pb.
Fig. 4 Calibration curves of Cr I 425.43 nm (a) and Pb I 405.78 nm (b) for Zn, Mg, Ni, and Si substrates.
Table 3. Comparison of LoDs, RSDs, and RMSECVs for the Cr and Pb elements on different substrates.
A comparison of the LoDs of Cr and Pb acquired in this work with other works in the literature is provided in Table 4. There was a lower LoD in this work than in the reports by others. What is more noteworthy is that the LoD of Pb and Cr in this work met the drinking water sanitation standard of China (0.01 mg/L for Pb and 0.05 mg/L for Cr, GB5749-2006), and the LoD of Cr in this work was one order of magnitude lower than that in previous reports. Unlike the fibers and foil, the Zn substrate can be reused. These investigations show that SENLIBS is a more sensitive method for detecting heavy metal elements in an aqueous solution.
Table 4. A comparison of the LoD values of the present work and other reported works.
3.3 Mechanisms of spectral enhancement on different substrates
As depicted in Tables 3 and 4, the different LoDs were obtained on different substrates; and a better LoD was acquired using a Zn substrate compared to other works. To clearly investigate the effect and mechanism of SENLIBS in different substrates, the physical properties of a substrate, especially the thermal features, such as the boiling point, were considered. The relationship between the LoDs and the boiling point of substrates is shown in Fig. 5. There was a positive correlation between the boiling points of the substrates and the LoDs.
Fig. 5 The relationship between the LoDs of Cr I 425.43 nm (a) and Pb I 405.78 nm (b) obtained on different substrates and the boiling point.
To further demonstrate the effect of the boiling point on the LoDs in SENLIBS, using the Cr element as an example, the excitation temperature and electron density were calculated. In laser-induced plasma, the excitation temperature and electron density are the main factors that determine spectral intensity in local thermodynamic equilibrium (LTE) [38]. The excitation temperature can be calculated by the following formula [39]:
where is the spectral intensity, and are the upper and lower levels of the transition process of the corresponding wavelength, is the degeneracy of the upper level, is the transition coefficient, is the Planck constant, is the energy of the upper level, is the instrument receiving efficiency, is the content of particle in plasma, is the plasma excitation temperature, and is the partition function of particle at temperature . The excitation temperature on different substrates was calculated by atomic lines of the main elements of the substrates. The correlation between the boiling points and the excitation temperatures is shown in Fig. 6. There is a negative correlation between the excitation temperatures and the boiling points. A higher plasma excitation temperature was obtained on the Zn substrate, which has a lower boiling point.Fig. 6 The relationship between the boiling point and plasma excitation temperature (0.6 mg/L Cr of CrCl3).
The electron density is given by [39]:
where is the full width at half maximum (FWHM), is the electron collision coefficient, and is the electron density. As shown in Eq. (2), is only related to the FWHM when the same spectrum line is used, so the electron density can be expressed in terms of . The relationship between the boiling point and electron density (∝) is shown in Fig. 7. As the boiling point increases, the decreases from the Zn substrate to the Si substrate, i.e., the electron density decreases.In general, as the number of heavy metals deposited was the same and the heavy metal layers were thin enough, that the ablation of layers was completed; and then the substrates were further ablated. Taking Cr as an example, the total number of Cr particles was approximately the same in the plasma plume during ablation of different substrates. Therefore, the spectral intensity was mainly affected by the plasma temperature and electron density. The difference in matrix elements of the substrate was the primary cause of the difference between the plasma temperature and the electron density; and the different content of the matrix in the plasma plume was mainly caused by the difference in the ablation mass of the substrate [40–42]. When the laser interacted with matter, the ablation amount of the target increased with the decrease in the ablation threshold of the target. The ablation threshold of the target was positively correlated with the boiling point of the target when the ablation laser wavelength was 532 nm [37]. Therefore, the ablation amount of the target was negatively correlated with the boiling point. As a result, the content of the matrix in the plasma plume increased as the boiling point decreased. Correspondingly, the collision between particles was more intense, resulting in an increase in plasma temperature and electron density. Eventually, higher spectral intensities and lower detection limits were obtained.
4. Conclusions
In summary, the relationship between the detection sensitivity and the substrate’s physical properties was investigated in SENLIBS. In this work, the Cr and Pb elements were detected. Four different substrates, including Zn, Mg, Ni, and Si targets, were compared to investigate the mechanism of spectral enhancement by different substrates. The results showed that the LoD proved to be positively correlated with the boiling point of the substrate in SENLIBS. This is mainly because the plasma excitation temperature and electron density improved, and the boiling point of the substrate decreased. Correspondingly, the ablation threshold decreased and the ablation quantity of the substrate increased, resulting in a more intense interparticle collision in the plasma plume. The results showed that the best LoD was 0.0011 mg/L for Cr and 0.004 mg/L for Pb under the optimal Zn substrate. The RESECV was 0.026 mg/L and 0.0095 mg/L for Cr and Pb, respectively. The LoD of Pb and Cr in this work meets the drinking water sanitation standard of China (0.01 mg/L for Pb and 0.05 mg/L for Cr, GB5749-2006); and the LoD of Cr was one order of magnitude lower than previous reports. The results showed that higher detection sensitivity can be obtained by selecting the low boiling point substrate in SENLIBS.
Funding
National Natural Science Foundation of China (No.61575073).
References
1. J. O. Duruibe, M. Ogwuegbu, and J. Egwurugwu, “Heavy metal pollution and human biotoxic effects,” Int. J. Phys. Sci. 2, 112–118 (2007).
2. A. H. Smith and C. M. Steinmaus, “Health effects of arsenic and chromium in drinking water: recent human findings,” Annu. Rev. Public Health 30(1), 107–122 (2009). [CrossRef] [PubMed]
3. H. Ebrahimzadeh, E. Moazzen, M. M. Amini, and O. Sadeghi, “Pyridine-2,6-diamine-functionalized Fe₃O₄ nanoparticles as a novel sorbent for determination of lead and cadmium ions in cosmetic samples,” Int. J. Cosmet. Sci. 35(2), 176–182 (2013). [CrossRef] [PubMed]
4. J. D. Winefordner, I. B. Gornushkin, T. Correll, E. Gibb, B. W. Smith, and N. Omenetto, “Comparing several atomic spectrometric methods to the super stars: special emphasis on laser induced breakdown spectrometry, LIBS, a future super star,” J. Anal. At. Spectrom. 19(9), 1061–1083 (2004). [CrossRef]
5. R. Noll, H. Bette, A. Brysch, M. Kraushaar, I. Mönch, L. Peter, and V. Sturm, “Laser-induced breakdown spectrometry—applications for production control and quality assurance in the steel industry,” Spectrochim. Acta B At. Spectrosc. 56(6), 637–649 (2001). [CrossRef]
6. Z. Hao, L. Guo, C. Li, M. Shen, X. Zou, X. Li, Y. Lu, and X. Zeng, “Sensitivity improvement in the detection of V and Mn elements in steel using laser-induced breakdown spectroscopy with ring-magnet confinement,” J. Anal. At. Spectrom. 29(12), 2309–2314 (2014). [CrossRef]
7. J. Li, Z. Zhu, R. Zhou, N. Zhao, R. Yi, X. Yang, X. Li, L. Guo, X. Zeng, and Y. Lu, “Determination of carbon content in steels using laser-induced breakdown spectroscopy assisted with laser-induced radical fluorescence,” Anal. Chem. 89(15), 8134–8139 (2017). [CrossRef] [PubMed]
8. Z. Zhu, J. Li, Y. Guo, X. Cheng, Y. Tang, L. Guo, X. Li, Y. Lu, and X. Zeng, “Accuracy improvement of boron by molecular emission with a genetic algorithm and partial least squares regression model in laser-induced breakdown spectroscopy,” J. Anal. At. Spectrom. 33(2), 205–209 (2018). [CrossRef]
9. R. Yuan, Y. Tang, Z. Zhu, Z. Hao, J. Li, H. Yu, Y. Yu, L. Guo, X. Zeng, and Y. Lu, “Accuracy improvement of quantitative analysis for major elements in laser-induced breakdown spectroscopy using single-sample calibration,” Anal. Chim. Acta 1064, 11–16 (2019). [CrossRef] [PubMed]
10. R. Yi, X. Yang, R. Zhou, J. Li, H. Yu, Z. Hao, L. Guo, X. Li, Y. Lu, and X. Zeng, “Determination of Trace Available Heavy Metals in Soil Using Laser-Induced Breakdown Spectroscopy Assisted with Phase Transformation Method,” Anal. Chem. 90(11), 7080–7085 (2018). [CrossRef] [PubMed]
11. M. Knadel, Y. Peng, R. Gislum, and M. H. Greve, “Comparing predictive ability of Laser-Induced Breakdown Spectroscopy to Near Infrared Spectroscopy for soil texture and organic carbon determination,” in 4th Global Workshop on Proximal Soil Sensing(2015).
12. D. H. Lee, S. C. Han, T. H. Kim, and J. I. Yun, “Highly sensitive analysis of boron and lithium in aqueous solution using dual-pulse laser-induced breakdown spectroscopy,” Anal. Chem. 83(24), 9456–9461 (2011). [CrossRef] [PubMed]
13. S. L. Lui, Y. Godwal, M. T. Taschuk, Y. Y. Tsui, and R. Fedosejevs, “Detection of lead in water using laser-induced breakdown spectroscopy and laser-induced fluorescence,” Anal. Chem. 80(6), 1995–2000 (2008). [CrossRef] [PubMed]
14. X. Wang, Y. Wei, Q. Lin, J. Zhang, and Y. Duan, “Simple, fast matrix conversion and membrane separation method for ultrasensitive metal detection in aqueous samples by laser-induced breakdown spectroscopy,” Anal. Chem. 87(11), 5577–5583 (2015). [CrossRef] [PubMed]
15. P. Yang, R. Zhou, W. Zhang, R. Yi, S. Tang, L. Guo, Z. Hao, X. Li, Y. Lu, and X. Zeng, “High-sensitivity determination of cadmium and lead in rice using laser-induced breakdown spectroscopy,” Food Chem. 272, 323–328 (2019). [CrossRef] [PubMed]
16. T. Delgado, J. M. Vadillo, and J. J. Laserna, “Pressure effects in laser-induced plasmas of trinitrotoluene and pyrene by laser-induced breakdown spectroscopy (LIBS),” Appl. Spectrosc. 68(1), 33–38 (2014). [CrossRef] [PubMed]
17. A. Kuwako, Y. Uchida, and K. Maeda, “Supersensitive detection of sodium in water with use of dual-pulse laser-induced breakdown spectroscopy,” Appl. Opt. 42(30), 6052–6056 (2003). [CrossRef] [PubMed]
18. S. Koch, W. Garen, W. Neu, and R. Reuter, “Resonance fluorescence spectroscopy in laser-induced cavitation bubbles,” Anal. Bioanal. Chem. 385(2), 312–315 (2006). [CrossRef] [PubMed]
19. J. I. Yun, T. Bundschuh, V. Neck, and J. I. Kim,, “Selective Determination of Europium(III) Oxide and Hydroxide Colloids in Aqueous Solution by Laser-Induced Breakdown Spectroscopy,” Appl. Spectrosc. 55(3), 273–278 (2001). [CrossRef]
20. Y. Godwal, S. L. Lui, M. T. Taschuk, Y. Y. Tsui, and R. Fedosejevs, “Determination of lead in water using laser ablation–laser induced fluorescence ☆,” Spectrochim. Acta B At. Spectrosc. 62(12), 1443–1447 (2007). [CrossRef]
21. H. Loudyi, K. Rifaï, S. Laville, F. Vidal, M. Chaker, and M. Sabsabi, “Improving laser-induced breakdown spectroscopy (LIBS) performance for iron and lead determination in aqueous solutions with laser-induced fluorescence (LIF),” J. Anal. At. Spectrom. 24(10), 1421–1428 (2009). [CrossRef]
22. X. Wang, L. Shi, Q. Lin, X. Zhu, and Y. Duan, “Simultaneous and sensitive analysis of Ag (i), Mn (ii), and Cr (iii) in aqueous solution by LIBS combined with dispersive solid phase micro-extraction using nano-graphite as an adsorbent,” J. Anal. At. Spectrom. 29(6), 1098–1104 (2014). [CrossRef]
23. J. Cortez and C. Pasquini, “Ring-oven based preconcentration technique for microanalysis: simultaneous determination of Na, Fe, and Cu in fuel ethanol by laser induced breakdown spectroscopy,” Anal. Chem. 85(3), 1547–1554 (2013). [CrossRef] [PubMed]
24. A. Matsumoto, A. Tamura, R. Koda, K. Fukami, Y. H. Ogata, N. Nishi, B. Thornton, and T. Sakka, “On-site quantitative elemental analysis of metal ions in aqueous solutions by underwater laser-induced breakdown spectroscopy combined with electrodeposition under controlled potential,” Anal. Chem. 87(3), 1655–1661 (2015). [CrossRef] [PubMed]
25. D. Alamelu, A. Sarkar, and S. K. Aggarwal, “Laser-induced breakdown spectroscopy for simultaneous determination of Sm, Eu and Gd in aqueous solution,” Talanta 77(1), 256–261 (2008). [CrossRef] [PubMed]
26. Z. Chen, H. Li, M. Liu, and R. Li, “Fast and sensitive trace metal analysis in aqueous solutions by laser-induced breakdown spectroscopy using wood slice substrates,” Spectrochim. Acta B At. Spectrosc. 63(1), 64–68 (2008). [CrossRef]
27. D. Zhu, J. Chen, J. Lu, and X. Ni, “Laser-induced breakdown spectroscopy for determination of trace metals in aqueous solution using bamboo charcoal as a solid-phase extraction adsorbent,” Anal. Methods 4(3), 819–823 (2012). [CrossRef]
28. C. Chen, G. Niu, Q. Shi, Q. Lin, and Y. Duan, “Laser-induced breakdown spectroscopy technique for quantitative analysis of aqueous solution using matrix conversion based on plant fiber spunlaced nonwovens,” Appl. Opt. 54(28), 8318–8325 (2015). [CrossRef] [PubMed]
29. Y. Yu, W. Zhou, H. Qian, X. Su, and K. Ren, “Simultaneous Determination of Trace Lead and Chromium in Water Using Laser-Induced Breakdown Spectroscopy and Paper Substrate,” Plasma Sci. Technol. 16(7), 683–687 (2014). [CrossRef]
30. X. Wen, Q. Lin, G. Niu, Q. Shi, and Y. Duan, “Emission enhancement of laser-induced breakdown spectroscopy for aqueous sample analysis based on Au nanoparticles and solid-phase substrate,” Appl. Opt. 55(24), 6706–6712 (2016). [CrossRef] [PubMed]
31. Z. Chen, H. Li, F. Zhao, and R. Li, “Ultra-sensitive trace metal analysis of water by laser-induced breakdown spectroscopy after electrical-deposition of the analytes on an aluminium surface,” J. Anal. At. Spectrom. 23(6), 871–875 (2008). [CrossRef]
32. X. Y. Yang, Z. Q. Hao, C. M. Li, J. M. Li, R. X. Yi, M. Shen, K. H. Li, L. B. Guo, X. Y. Li, Y. F. Lu, and X. Y. Zeng, “Sensitive determinations of Cu, Pb, Cd, and Cr elements in aqueous solutions using chemical replacement combined with surface-enhanced laser-induced breakdown spectroscopy,” Opt. Express 24(12), 13410–13417 (2016). [CrossRef] [PubMed]
33. D. Bae, S. H. Nam, S. H. Han, J. Yoo, and Y. Lee, “Spreading a water droplet on the laser-patterned silicon wafer substrate for surface-enhanced laser-induced breakdown spectroscopy,” Spectrochim. Acta B At. Spectrosc. 113, 70–78 (2015). [CrossRef]
34. M. A. Aguirre, S. Legnaioli, F. Almodóvar, M. Hidalgo, V. Palleschi, and A. Canals, “Elemental analysis by surface-enhanced Laser-Induced Breakdown Spectroscopy combined with liquid–liquid microextraction,” Spectrochim. Acta B At. Spectrosc. 79–80, 88–93 (2013). [CrossRef]
35. M. A. Aguirre, E. J. Selva, M. Hidalgo, and A. Canals, “Dispersive liquid-liquid microextraction for metals enrichment: a useful strategy for improving sensitivity of laser-induced breakdown spectroscopy in liquid samples analysis,” Talanta 131, 348–353 (2015). [CrossRef] [PubMed]
36. R. L. Vander Wal, T. M. Ticich, J. R. West Jr., and P. A. Householder, “Trace metal detection by laser-induced breakdown spectroscopy,” Appl. Spectrosc. 53(10), 1226–1236 (1999). [CrossRef]
37. J. J. L. L. M. Cabalin and J. J. Laserna, “Experimental determination of laser induced breakdown thresholds of metals under nanosecond Q-switched laser operation,” Spectrochim. Acta B At. Spectrosc. 53(5), 723–730 (1998). [CrossRef]
38. E. C. Benck, “Plasma diagnostic techniques,” Phys. Today 19(9), 94–95 (1966). [CrossRef]
39. N. S. Tan Halid, R. Zainal, and Y. Mat Daud, “Estimation of Temperature and Electron Density in Stainless Steel Plasma Using Laser Induced Breakdown Spectroscopy,” Jurnal Teknologi 781–5 (2016).
40. L. Caneve, F. Colao, R. Fantoni, and V. Spizzichino, “Laser ablation of copper based alloys by single and double pulse laser induced breakdown spectroscopy,” Appl. Phys., A Mater. Sci. Process. 85(2), 151–157 (2006). [CrossRef]
41. D. N. Stratis, K. L. Eland, and S. M. Angel, “Effect of pulse delay time on a pre-ablation dual-pulse LIBS plasma,” Appl. Spectrosc. 55(10), 1297–1303 (2001). [CrossRef]
42. G. Cristoforetti, S. Legnaioli, V. Palleschi, A. Salvetti, and E. Tognoni, “Effect of target composition on the emission enhancement observed in Double-Pulse Laser-Induced Breakdown Spectroscopy,” Spectrochim. Acta B At. Spectrosc. 63(2), 312–323 (2008). [CrossRef]
