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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 11 — Jun. 2, 2014
  • pp: 12909–12914
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Combination of cylindrical confinement and spark discharge for signal improvement using laser induced breakdown spectroscopy

Zongyu Hou, Zhe Wang, Jianmin Liu, Weidou Ni, and Zheng Li  »View Author Affiliations


Optics Express, Vol. 22, Issue 11, pp. 12909-12914 (2014)
http://dx.doi.org/10.1364/OE.22.012909


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Abstract

Spark discharge has been proved to be an effective way to enhance the LIBS signal while moderate cylindrical confinement is able to increase the signal repeatability with limited signal enhancement effects. In the present work, these two methods were combined together not only to improve the pulse-to-pulse signal repeatability but also to simultaneously and significantly enhance the signal as well as SNR. Plasma images showed that the confinement stabilized the morphology of the plasma, especially for the discharge assisted process, which explained the improvement of the signal repeatability.

© 2014 Optical Society of America

1. Introduction

Relatively low sensitivity and pulse-to-pulse repeatability has always been regarded as the obstacle for wide commercialization of laser induced breakdown spectroscopy (LIBS). For applications such as trace element analysis, a very low limit of detection (LOD) is desired, while for on-line measurement and inhomogeneous sample analysis, high repeatability is necessary. Therefore, a strong and highly repeatable signal is of great importance for LIBS applications. Researchers have proposed various methods to enhance the signal such as dual-pulse excitation technique [1

1. V. I. Babushok, F. C. DeLucia Jr, J. L. Gottfried, C. A. Munson, and A. W. Miziolek, “Double pulse laser ablation and plasma: Laser induced breakdown spectroscopy signal enhancement,” Spectrochim. Acta, B At. Spectrosc. 61(9), 999–1014 (2006). [CrossRef]

4

4. M. Weidman, M. Baudelet, S. Palanco, M. Sigman, P. J. Dagdigian, and M. Richardson, “Nd:YAG-CO2 double-pulse laser induced breakdown spectroscopy of organic films,” Opt. Express 18(1), 259–266 (2010). [CrossRef] [PubMed]

], spark discharge [5

5. W. D. Zhou, K. X. Li, Q. M. Shen, Q. L. Chen, and J. M. Long, “Optical emission enhancement using laser ablation combined with fast pulse discharge,” Opt. Express 18(3), 2573–2578 (2010). [CrossRef] [PubMed]

9

9. Z. Wang, T.-B. Yuan, Z.-Y. Hou, W.-D. Zhou, J.-D. Lu, H.-B. Ding, and X.-Y. Zeng, “Laser-induced breakdown spectroscopy in China,” Frontiers Phys. 8, 1–19 (2013).

], and spatial confinement [10

10. R. Hedwig, “Confinement effect in enhancing shock wave plasma generation at low pressure by TEA CO2 laser bombardment on quartz sample,” Spectrochim. Acta, B At. Spectrosc. 58(3), 531–542 (2003). [CrossRef]

17

17. Z. Hou, Z. Wang, J. Liu, W. Ni, and Z. Li, “Signal quality improvement using cylindrical confinement for laser induced breakdown spectroscopy,” Opt. Express 21(13), 15974–15979 (2013). [CrossRef] [PubMed]

]. The commonly applied methods to improve the signal repeatability is normalization [18

18. N. B. Zorov, A. A. Gorbatenko, T. A. Labutin, and A. M. Popov, “A review of normalization techniques in analytical atomic spectrometry with laser sampling: From single to multivariate correction,” Spectrochim. Acta, B At. Spectrosc. 65(8), 642–657 (2010). [CrossRef]

] and other data processing methods [19

19. L. Li, Z. Wang, T. Yuan, Z. Hou, Z. Li, and W. Ni, “A simplified spectrum standardization method for laser-induced breakdown spectroscopy measurements,” J. Anal. At. Spectrom. 26(11), 2274–2280 (2011). [CrossRef]

, 20

20. Z. Hou, Z. Wang, S.- Lui, T. Yuan, L. Li, Z. Li, and W. Ni, “Improving data stability and prediction accuracy in laser-induced breakdown spectroscopy by utilizing a combined atomic and ionic line algorithm,” J. Anal. At. Spectrom. 28(1), 107–113 (2013). [CrossRef]

].

In the present work, the effects of combining spatial confinement and spark discharge were investigated comparing with spark discharge or spatial confinement alone. It was demonstrated that the spark discharge was more effective in signal enhancement while moderate cylindrical cavity confinement was more effective in improving pulse-to-pulse signal repeatability. Combining spark discharge and moderate cylindrical cavity confinement, the signal was enhanced significantly and the pulse-to-pulse signal repeatability was also improved. That is, the combination of these two methods is able to provide a better signal than either method alone and conventional LIBS. The fluctuation of the plasma images was quantitatively analyzed and clearly demonstrated the repeatability improvement of plasma morphology using cavity confinement.

2. Experimental setup

The sample used in this work was powdery bituminous coal with grain diameter less than 0.2mm. The main components of the sample are C (78.98%wt), H (4.95%wt), N (1.38%wt), Si (1.7%wt), S (1.70%wt), which were certified by the China Coal Research Institute (CCRI). About 3 gram of coal powder was pressed with pressure of 20 tons to form a pellet with 30mm in diameter. In addition, the coal pellet sample surface was made very smooth by using smooth pressing mould.

The cylindrical cavity used in this work was 1.5mm high and 3mm diametric, which was drilled in a 1.5mm thick polytetrafluoroethylene (PTFE) plate. For cases with cavity, the PTFE plate was placed closely on the sample surface, and the laser was shot through the center of the cylindrical hole. For cases without cavity, nothing was placed on the sample surface. Two cylindrical electrodes (3mm in diameter) with hemispherical endpoint were paced 15 degree with the horizontal and the lowest point of the electrodes was 2.5mm above the sample surface. An electric capacitor with 20nF and a high-voltage direct current power supply were parallel connected with the electrodes. In this work, the high-voltage was set to be 7.5kV. The discharge process was triggered passively, which means the laser induced plasma increases the conductivity of air near the electrode so as to automatically trigger the discharge process.

For each sample, 40 spectra and 40 plasma images were collected from 40 different locations on the sample surface, and these 40 spectra were used to calculate the average parameters and RSD of the signal. Each spectrum and image was from a single laser shot. After each pulse, an air flow was used to clear the laser ablated aerosols above the sample. The spectra were background subtracted before data processing. Carbon is a major element in coal and is of most interest in coal analysis so the carbon line C(I) 193.09 nm was selected for analyses in this work. Some other spectral lines such as C(I)247.856 nm, Si(I)288.158 nm were also analyzed and the results were found to be similar with that of C(I) 193.09nm, so they had not been displayed.

3. Results and discussion

As described above, the main objective of the present work is combining moderate cylindrical cavity confinement and spark discharge to improve signal repeatability and enhance the signal. In this section, the plasma character and optical signals of four configurations were compared, namely the conventional LIBS (without confinement and spark discharge), cylindrical cavity confined LIBS, spark discharge assisted LIBS, and the combination of cylindrical cavity confinement and spark discharge LIBS.

3.1 Plasma character

Plasma temperature and electron number density are two key parameters to characterize the plasma. Five Si I spectral lines (243.515nm, 250.69nm, 251.432nm, 251.61nm, 288.158nm) were used to build Boltzmann plot for temperature calculation. The full width at half maximum (FWHM) of Hα (656.28nm) line was used to calculate the electron number density according to [21

21. H. R. Griem, ed., Plasma Spectroscopy (McGraw-Hill, 1964).

, 22

22. C. Aragón and J. A. Aguilera, “Characterization of laser induced plasmas by optical emission spectroscopy: A review of experiments and methods,” Spectrochim. Acta, B At. Spectrosc. 63(9), 893–916 (2008). [CrossRef]

].

Figure 2
Fig. 2 Plasma temperature and electron density under different laser energy.
shows that the spatial confinement increased the plasma temperature and electron number density distinctly for all laser energies, mainly because the reflected shockwave added extra energy to the plasma [16

16. Z. Wang, Z. Hou, S. L. Lui, D. Jiang, J. Liu, and Z. Li, “Utilization of moderate cylindrical confinement for precision improvement of laser-induced breakdown spectroscopy signal,” Opt. Express 20(S6), A1011–A1018 (2012). [CrossRef]

, 17

17. Z. Hou, Z. Wang, J. Liu, W. Ni, and Z. Li, “Signal quality improvement using cylindrical confinement for laser induced breakdown spectroscopy,” Opt. Express 21(13), 15974–15979 (2013). [CrossRef] [PubMed]

]. While for spark discharge, the plasma temperature and electron density were lower than conventional LIBS. Our results is mainly agreed with the results obtained by other researchers [8

8. O. A. Nassef and H. E. Elsayed-Ali, “Spark discharge assisted laser induced breakdown spectroscopy,” Spectrochim. Acta, B At. Spectrosc. 60(12), 1564–1572 (2005). [CrossRef]

], where the spark discharge has no obvious effect on plasma temperature and electron density. Though the spark discharge adds additional electric energy into the laser induced plasma, it can increase the plasma volume (seen in Fig. 3
Fig. 3 Plasma images of different configurations, a: conventional LIBS, b: cavity, c: discharge, d: cavity and discharge. Laser energy = 65mJ, delay time = 1μs, gate width = 1ms.
). Therefore, the plasma temperature and electron density may be higher or lower than that of conventional LIBS for different samples, laser energy, delay time and gate width. For example, Zhou et al. [7

7. W. Zhou, K. Li, X. Li, H. Qian, J. Shao, X. Fang, P. Xie, and W. Liu, “Development of a nanosecond discharge-enhanced laser plasma spectroscopy,” Opt. Lett. 36(15), 2961–2963 (2011). [CrossRef] [PubMed]

] indicated that the spark discharge can obviously increase the plasma temperature and electron density for soil samples, with the voltage of 8kV and capacitor of 4nF. The difference between Zhou et al. and the present work may come from the samples’ matrix and electric circuit property such as the voltage and capacitor. More detailed researches are needed to investigate the effect of spark discharge on plasma temperature and electron density.

3.2 Plasma morphology stabilization

For each configuration, 40 plasma images from 40 laser pulses were captured. 10 typical plasma images for each configuration were shown in Fig. 3. As Fig. 3(c) shows, when only using spark discharge assistance, the plasma morphology can be more unstable than that of conventional LIBS, which may increase signal uncertainty. The probable reason is that even a slight plasma morphology difference at the initial expansion stage may be enlarged by the discharge process. As shown, the cavity can stabilize the expansion and morphology of the plasma. Especially for Figs. 3(c) and 3(d), the discharge process and the plasma were much more stable when the cavity confinement was used. The probable reason is the cavity confinement reduced the pulse-to-pulse fluctuation of the air conductivity between the plasma and the electrodes so to stabilize the discharge process.

3.3 Signal enhancement, SNR improvement and uncertainty reduction

The line intensity (peak area) and signal-to-noise ratio (SNR) of the C(I) 193.09 nm for different configurations under different laser energy were shown in Fig. 5
Fig. 5 Intensity and SNR of C(I) 193.09nm under different laser energy.
. The noise was defined as the standard deviation of the continuum emission intensity near the characteristic line. Both the cavity confinement and spark discharge are able to increase the line intensity, so the combination of cylindrical cavity confinement and spark discharge generated the strongest line intensity in four configurations. For larger laser energy, such as 80mJ, the cavity confinement enhanced the line intensity more than spark discharge, probably because the higher laser energy generates a stronger shockwave.

Comparing with the signal intensity, SNR was more important for the sensitivity and limit of detection (LOD) of LIBS analysis. Figure 5 shows that when combining cylindrical cavity confinement and spark discharge, the SNR was the highest in four configurations, which was helpful to improve the LOD and sensitivity of LIBS analysis.

For the signal repeatability, we find that the spark discharge increase the pulse-to-pulse RSD of carbon line, as shown in Fig. 6
Fig. 6 RSD of C(I) 193.09nm under different laser energy.
. When combining cylindrical cavity confinement and spark discharge, the pulse-to-pulse RSD can be greatly reduced, similar with that of cylindrical cavity confinement alone. This result was accordance with the fluctuation of the plasma morphology in section 3.2. As shown in Fig. 6, the effect of cavity confinement on uncertainty reduction was more significant for spark discharge assisted LIBS. As discussed in section 3.2, when using discharge alone, the plasma morphology was more unstable than that of conventional LIBS. Therefore, the plasma stabilization effect of cavity confinement will become more prominent and necessary for spark discharge assisted LIBS. The signal repeatability can be influenced by voltage, capacitance, electrode distance, cavity shape, etc., more detailed experiments and optimization are necessary in the future research.

4. Conclusions

Using coal sample and C(I)193.09 nm for analysis, the spark discharge had been found to be an effective way to enhance the LIBS signal, while it may decrease the pulse-to-pulse signal repeatability due to the instability of the discharge process. Spatial confinement can regulate the expansion the plasma so as to stabilize the discharge process and finally stabilize the plasma morphology. Combining the moderate cylindrical cavity confinement and spark discharge, the pulse-to-pulse signal repeatability, signal intensity, and SNR were all improved comparing with only using confinement or discharge. The fluctuation of the plasma images was quantitatively analyzed to demonstrate the effect of cavity confinement on plasma morphology stabilization.

Acknowledgments

The authors are grateful for the financial support from National Natural Science Foundation of China (NO. 51276100) and National Basic Research Program (973 Program) of China (NO. 2013CB228501).

References and links

1.

V. I. Babushok, F. C. DeLucia Jr, J. L. Gottfried, C. A. Munson, and A. W. Miziolek, “Double pulse laser ablation and plasma: Laser induced breakdown spectroscopy signal enhancement,” Spectrochim. Acta, B At. Spectrosc. 61(9), 999–1014 (2006). [CrossRef]

2.

L. B. Guo, B. Y. Zhang, X. N. He, C. M. Li, Y. S. Zhou, T. Wu, J. B. Park, X. Y. Zeng, and Y. F. Lu, “Optimally enhanced optical emission in laser-induced breakdown spectroscopy by combining spatial confinement and dual-pulse irradiation,” Opt. Express 20(2), 1436–1443 (2012). [CrossRef] [PubMed]

3.

D. K. Killinger, S. D. Allen, R. D. Waterbury, C. Stefano, and E. L. Dottery, “Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse,” Opt. Express 15(20), 12905–12915 (2007). [CrossRef] [PubMed]

4.

M. Weidman, M. Baudelet, S. Palanco, M. Sigman, P. J. Dagdigian, and M. Richardson, “Nd:YAG-CO2 double-pulse laser induced breakdown spectroscopy of organic films,” Opt. Express 18(1), 259–266 (2010). [CrossRef] [PubMed]

5.

W. D. Zhou, K. X. Li, Q. M. Shen, Q. L. Chen, and J. M. Long, “Optical emission enhancement using laser ablation combined with fast pulse discharge,” Opt. Express 18(3), 2573–2578 (2010). [CrossRef] [PubMed]

6.

L. I. Kexue, W. D. Zhou, Q. M. Shen, J. Shao, and H. G. Qian, “Signal enhancement of lead and arsenic in soil using laser ablation combined with fast electric discharge,” Spectrochim. Acta, B At. Spectrosc. 65(5), 420–424 (2010). [CrossRef]

7.

W. Zhou, K. Li, X. Li, H. Qian, J. Shao, X. Fang, P. Xie, and W. Liu, “Development of a nanosecond discharge-enhanced laser plasma spectroscopy,” Opt. Lett. 36(15), 2961–2963 (2011). [CrossRef] [PubMed]

8.

O. A. Nassef and H. E. Elsayed-Ali, “Spark discharge assisted laser induced breakdown spectroscopy,” Spectrochim. Acta, B At. Spectrosc. 60(12), 1564–1572 (2005). [CrossRef]

9.

Z. Wang, T.-B. Yuan, Z.-Y. Hou, W.-D. Zhou, J.-D. Lu, H.-B. Ding, and X.-Y. Zeng, “Laser-induced breakdown spectroscopy in China,” Frontiers Phys. 8, 1–19 (2013).

10.

R. Hedwig, “Confinement effect in enhancing shock wave plasma generation at low pressure by TEA CO2 laser bombardment on quartz sample,” Spectrochim. Acta, B At. Spectrosc. 58(3), 531–542 (2003). [CrossRef]

11.

A. M. Popov, F. Colao, and R. Fantoni, “Enhancement of LIBS signal by spatially confining the laser-induced plasma,” J. Anal. At. Spectrom. 24(5), 602–604 (2009). [CrossRef]

12.

A. M. Popov, F. Colao, and R. Fantoni, “Spatial confinement of laser-induced plasma to enhance LIBS sensitivity for trace elements determination in soils,” J. Anal. At. Spectrom. 25(6), 837–848 (2010). [CrossRef]

13.

P. Yeates and E. T. Kennedy, “Spectroscopic, imaging, and probe diagnostics of laser plasma plumes expanding between confining surfaces,” J. Appl. Phys. 108(9), 093306 (2010). [CrossRef]

14.

M. Corsi, G. Cristoforetti, M. Hidalgo, D. Iriarte, S. Legnaioli, V. Palleschi, A. Salvetti, and E. Tognoni, “Effect of laser-induced crater depth in laser-induced breakdown spectroscopy emission features,” Appl. Spectrosc. 59(7), 853–860 (2005). [CrossRef] [PubMed]

15.

L. B. Guo, W. Hu, B. Y. Zhang, X. N. He, C. M. Li, Y. S. Zhou, Z. X. Cai, X. Y. Zeng, and Y. F. Lu, “Enhancement of optical emission from laser-induced plasmas by combined spatial and magnetic confinement,” Opt. Express 19(15), 14067–14075 (2011). [CrossRef] [PubMed]

16.

Z. Wang, Z. Hou, S. L. Lui, D. Jiang, J. Liu, and Z. Li, “Utilization of moderate cylindrical confinement for precision improvement of laser-induced breakdown spectroscopy signal,” Opt. Express 20(S6), A1011–A1018 (2012). [CrossRef]

17.

Z. Hou, Z. Wang, J. Liu, W. Ni, and Z. Li, “Signal quality improvement using cylindrical confinement for laser induced breakdown spectroscopy,” Opt. Express 21(13), 15974–15979 (2013). [CrossRef] [PubMed]

18.

N. B. Zorov, A. A. Gorbatenko, T. A. Labutin, and A. M. Popov, “A review of normalization techniques in analytical atomic spectrometry with laser sampling: From single to multivariate correction,” Spectrochim. Acta, B At. Spectrosc. 65(8), 642–657 (2010). [CrossRef]

19.

L. Li, Z. Wang, T. Yuan, Z. Hou, Z. Li, and W. Ni, “A simplified spectrum standardization method for laser-induced breakdown spectroscopy measurements,” J. Anal. At. Spectrom. 26(11), 2274–2280 (2011). [CrossRef]

20.

Z. Hou, Z. Wang, S.- Lui, T. Yuan, L. Li, Z. Li, and W. Ni, “Improving data stability and prediction accuracy in laser-induced breakdown spectroscopy by utilizing a combined atomic and ionic line algorithm,” J. Anal. At. Spectrom. 28(1), 107–113 (2013). [CrossRef]

21.

H. R. Griem, ed., Plasma Spectroscopy (McGraw-Hill, 1964).

22.

C. Aragón and J. A. Aguilera, “Characterization of laser induced plasmas by optical emission spectroscopy: A review of experiments and methods,” Spectrochim. Acta, B At. Spectrosc. 63(9), 893–916 (2008). [CrossRef]

OCIS Codes
(300.6210) Spectroscopy : Spectroscopy, atomic
(300.6365) Spectroscopy : Spectroscopy, laser induced breakdown

ToC Category:
Spectroscopy

History
Original Manuscript: April 7, 2014
Revised Manuscript: May 8, 2014
Manuscript Accepted: May 14, 2014
Published: May 20, 2014

Citation
Zongyu Hou, Zhe Wang, Jianmin Liu, Weidou Ni, and Zheng Li, "Combination of cylindrical confinement and spark discharge for signal improvement using laser induced breakdown spectroscopy," Opt. Express 22, 12909-12914 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-11-12909


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References

  1. V. I. Babushok, F. C. DeLucia, J. L. Gottfried, C. A. Munson, A. W. Miziolek, “Double pulse laser ablation and plasma: Laser induced breakdown spectroscopy signal enhancement,” Spectrochim. Acta, B At. Spectrosc. 61(9), 999–1014 (2006). [CrossRef]
  2. L. B. Guo, B. Y. Zhang, X. N. He, C. M. Li, Y. S. Zhou, T. Wu, J. B. Park, X. Y. Zeng, Y. F. Lu, “Optimally enhanced optical emission in laser-induced breakdown spectroscopy by combining spatial confinement and dual-pulse irradiation,” Opt. Express 20(2), 1436–1443 (2012). [CrossRef] [PubMed]
  3. D. K. Killinger, S. D. Allen, R. D. Waterbury, C. Stefano, E. L. Dottery, “Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse,” Opt. Express 15(20), 12905–12915 (2007). [CrossRef] [PubMed]
  4. M. Weidman, M. Baudelet, S. Palanco, M. Sigman, P. J. Dagdigian, M. Richardson, “Nd:YAG-CO2 double-pulse laser induced breakdown spectroscopy of organic films,” Opt. Express 18(1), 259–266 (2010). [CrossRef] [PubMed]
  5. W. D. Zhou, K. X. Li, Q. M. Shen, Q. L. Chen, J. M. Long, “Optical emission enhancement using laser ablation combined with fast pulse discharge,” Opt. Express 18(3), 2573–2578 (2010). [CrossRef] [PubMed]
  6. L. I. Kexue, W. D. Zhou, Q. M. Shen, J. Shao, H. G. Qian, “Signal enhancement of lead and arsenic in soil using laser ablation combined with fast electric discharge,” Spectrochim. Acta, B At. Spectrosc. 65(5), 420–424 (2010). [CrossRef]
  7. W. Zhou, K. Li, X. Li, H. Qian, J. Shao, X. Fang, P. Xie, W. Liu, “Development of a nanosecond discharge-enhanced laser plasma spectroscopy,” Opt. Lett. 36(15), 2961–2963 (2011). [CrossRef] [PubMed]
  8. O. A. Nassef, H. E. Elsayed-Ali, “Spark discharge assisted laser induced breakdown spectroscopy,” Spectrochim. Acta, B At. Spectrosc. 60(12), 1564–1572 (2005). [CrossRef]
  9. Z. Wang, T.-B. Yuan, Z.-Y. Hou, W.-D. Zhou, J.-D. Lu, H.-B. Ding, X.-Y. Zeng, “Laser-induced breakdown spectroscopy in China,” Frontiers Phys. 8, 1–19 (2013).
  10. R. Hedwig, “Confinement effect in enhancing shock wave plasma generation at low pressure by TEA CO2 laser bombardment on quartz sample,” Spectrochim. Acta, B At. Spectrosc. 58(3), 531–542 (2003). [CrossRef]
  11. A. M. Popov, F. Colao, R. Fantoni, “Enhancement of LIBS signal by spatially confining the laser-induced plasma,” J. Anal. At. Spectrom. 24(5), 602–604 (2009). [CrossRef]
  12. 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(6), 837–848 (2010). [CrossRef]
  13. P. Yeates, E. T. Kennedy, “Spectroscopic, imaging, and probe diagnostics of laser plasma plumes expanding between confining surfaces,” J. Appl. Phys. 108(9), 093306 (2010). [CrossRef]
  14. M. Corsi, G. Cristoforetti, M. Hidalgo, D. Iriarte, S. Legnaioli, V. Palleschi, A. Salvetti, E. Tognoni, “Effect of laser-induced crater depth in laser-induced breakdown spectroscopy emission features,” Appl. Spectrosc. 59(7), 853–860 (2005). [CrossRef] [PubMed]
  15. L. B. Guo, W. Hu, B. Y. Zhang, X. N. He, C. M. Li, Y. S. Zhou, Z. X. Cai, X. Y. Zeng, Y. F. Lu, “Enhancement of optical emission from laser-induced plasmas by combined spatial and magnetic confinement,” Opt. Express 19(15), 14067–14075 (2011). [CrossRef] [PubMed]
  16. Z. Wang, Z. Hou, S. L. Lui, D. Jiang, J. Liu, Z. Li, “Utilization of moderate cylindrical confinement for precision improvement of laser-induced breakdown spectroscopy signal,” Opt. Express 20(S6), A1011–A1018 (2012). [CrossRef]
  17. Z. Hou, Z. Wang, J. Liu, W. Ni, Z. Li, “Signal quality improvement using cylindrical confinement for laser induced breakdown spectroscopy,” Opt. Express 21(13), 15974–15979 (2013). [CrossRef] [PubMed]
  18. N. B. Zorov, A. A. Gorbatenko, T. A. Labutin, A. M. Popov, “A review of normalization techniques in analytical atomic spectrometry with laser sampling: From single to multivariate correction,” Spectrochim. Acta, B At. Spectrosc. 65(8), 642–657 (2010). [CrossRef]
  19. L. Li, Z. Wang, T. Yuan, Z. Hou, Z. Li, W. Ni, “A simplified spectrum standardization method for laser-induced breakdown spectroscopy measurements,” J. Anal. At. Spectrom. 26(11), 2274–2280 (2011). [CrossRef]
  20. Z. Hou, Z. Wang, S.- Lui, T. Yuan, L. Li, Z. Li, W. Ni, “Improving data stability and prediction accuracy in laser-induced breakdown spectroscopy by utilizing a combined atomic and ionic line algorithm,” J. Anal. At. Spectrom. 28(1), 107–113 (2013). [CrossRef]
  21. H. R. Griem, ed., Plasma Spectroscopy (McGraw-Hill, 1964).
  22. C. Aragón, J. A. Aguilera, “Characterization of laser induced plasmas by optical emission spectroscopy: A review of experiments and methods,” Spectrochim. Acta, B At. Spectrosc. 63(9), 893–916 (2008). [CrossRef]

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