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Enhancement effects of different elements by argon shield in laser induced breakdown spectroscopy
Optik - International Journal for Light and Electron Optics 179 (2019) 1134–1139
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Enhancement effects of different elements by argon shield in laser induced breakdown spectroscopy Jingjun Lina, Xiaomei Lina, , Lianbo Guob, ⁎
a b
T
⁎
School of Electrical &Engineering, Changchun University of Technology, 130012 Changchun, Jilin, China Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology, 430074 Wuhan, Hubei, China
ARTICLE INFO
ABSTRACT
OCIS codes: (100.7410) Wavelets (300.6365) Spectroscopy Laser induced breakdownKeywords: Laser induced breakdown spectroscopy Limits of detection Detection accuracy
Laser induced breakdown spectroscopy (LIBS) has been demonstrated to be a promising element analysis technique due to its unique advantages such as rapid in-situ and on-line monitoring of materials in various forms. However, using LIBS to analyze materials with low concentrations of different elements remains a challenge. To improve the detection accuracy and limits of detection (LOD) of low concentration elements, the plasma produced by steel alloys with an argon shield to reduce the influence of the matrix effect in the detection process was investigated. A number of lowconcentration elements, such as manganese (Mn), chromium (Cr), silicon (Si) and carbon (C) in steel samples were studied. After optimization of the argon shield environment, significant enhancement factors of 1.89, 1.654, 2.44, and 5.689 in the emission intensity of the Mn, Cr, Si, and C lines were achieved. More importantly, after data processing, the correlation coefficient of IMnII294.38nm/ IFeII294.76nm in an argon shield increased from 0.997 to 1.149; and similar results for ICrII284.29nm/ IFeII295.34nm, ISiII212.41nm/IFeII200.04nm, and ICI193.09nm/IFeII200.04nm were from 0.448 to 0.664, from 0.601 to 0.941, and from0.672 to 1.439. The correlation coefficients of the ratio (R2) also increased to 98%. Therefore, we can create an argon environment to eliminate the interference of the experimental environment and realize the rapid detection of multiple-elements in low concentrations.
1. Introduction Laser-induced breakdown spectroscopy (LIBS) is a spectrometry technology for material element analysis based on measuring the atomic emission of plasma induced by a high power laser pulse. The basic principle is the focusing of a high power laser beam on the surface of a sample to produce plasma and collect emitting radiation [1–3]. The information obtained from the plasma can be used to obtain qualitative and quantitative information about the target’s elemental composition. LIBS can be used to detect samples of any physical form (liquid, solid, powder, gas) at a high speed [4–6]. Therefore, during the last decade, LIBS technology has been applied to various fields, including iron and steel analysis, ocean detection, soil composition analysis, coal quality testing and so on, for realtime, online chemical assays or monitoring [7]. However, the sensitivity and precision of LIBS remains a challenge due to the fluctuations of various factors, such as the laser energy and environmental interference. More and more researchers spent their efforts on system improvement and spectral processing algorithms to solve the problem. They have proposed various methods to enhance the signal stability, such as the multiple pulse approach [8–11], spatial confinement, and so on [12–14]. Aside from the methods mentioned above, a widely used approach is introduction of inert gas, for instance, argon, nitrogen, and helium, which can easily improve the detection sensitivity of LIBS. These gases can be adopted to protect the plasma from the
⁎
Corresponding authors. E-mail addresses: [email protected] (X. Lin), [email protected] (L. Guo).
https://doi.org/10.1016/j.ijleo.2018.10.144 Received 2 September 2018; Accepted 23 October 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 179 (2019) 1134–1139
J. Lin et al.
Fig. 1. Schematic diagram of the experimental setup.
reaction between the air and the surface of the sample. By considering the spectral line intensity and the signal-to-background ratio of the elements in different inert gases, argon was selected as the protective gas for this experiment [15]. At the same time, during the shock wave expansion, argon can provide a transparent medium for the light in the ultraviolet region [16–18].The activation of the sample in argon replaces the oxygen and nitrogen in air, preventing the selective oxidation of the sample during excitation. In recent years, the gas protection in LIBS has been studied by several groups for main matrix elements. A previous work on aluminum (Al) plasmas by gas protection, which confined the temporal and spatial dynamics of laser-induced aluminum plasma in an argon background at atmospheric pressure, was conducted by Q. Ma [19]. Q. L. Ma and V. Motto-Ros also studied the effects of ablation laser wavelength on the expansion of laser-induced plasma in argon gas [20]. Many other researchers studied qualitative analysis, such as spectral enhancement and plasma distribution [21–32]. In the study, our main aim was to investgate enhancement effects with different elements for quantitative analysis by argon shield in LIBS. 2. Experimental methods 2.1. Experiment setup The schematic diagram of the experimental setup used in this work is shown in Fig. 1. The experiments were conducted in a vacuum tank. First, the experiments were carried out in the atmospheric environment. Then the air in the vacuum tank was pumped out by a vacuum pump. At the same time, we added argon to the vacuum tank. The pressure in the tank was controlled to the atmospheric pressure. Finally, the experiments were carried out with the argon shield. A Q-switched Nd: YAG laser (Beam-tech, Vlite-200, pulse duration 8 ns) operating at 1,064 nm with a repetition rate of 2 Hz was used for plasma generation. We set 580 V as the working voltage. The laser emits a laser beam which was reflected by a dichroic mirror and focused by an ultraviolet (UV)-grade quartz lens (focal length, 25 cm), through the quartz window at the top of the vacuum tank onto the surface of the sample. The reflective spectrum range of this 45°dichroic mirror is 1064 nm. The dichroic mirror can reflect a 1,064 nm laser beam and transmit all of the other wavelengths. The focal point was 2 mm below the sample surface where plasma plumes the size of several millimeters at the center of the hemisphere were generated. The emissions from the laser-induced plasmas were collected through the window located on the upper side which was clamped at 30 ° with the laser beam, and coupled to an optical fiber by the UV-grade quartz lens and a light collector (AvaSpace-2048, wavelength range: 180–300 nm). An intensified charge-coupled device (ICCD) was used to detect the spectrally resolved line that was operated in the gated mode. The gate delay and width of the ICCD were adjusted to each other. Thus, we were able to control the gate delay to obtain the spectra at different time delays after the laser pulse. At the same time, the plasma image was collected by a highspeed camera at different time delays. All of the data acquisition and analysis were performed with a personal computer. 2.2. Sample preparation Seven steel samples (CSBS11071-2012, Research Institute of certified reference materials, Twelfth Research Institute of China Shipbuilding Industry Corporation) with manganese (Mn), chromium (Cr), silicon (Si) and carbon (C) concentrations lower than 1.34%, 2.33%, 1.38%, and 0.881%, respectively, as well as the matrix element iron (Fe) were used in this study. The concentrations with their standard deviations of main elements in the study are shown in Table 1. The experimental conditions were kept constant during the experiment. 3. Results and discussion 3.1. Enhancement ratio of low-concentration elements with argon shield in the steel samples To demonstrate the intensity enhancement of the low-concentration metallic elements, the time-integrated LIBS spectra of laserinduced Mn and Cr elements with argon shield in the steel samples were measured. The matrix element Fe was used as a contrast element. As shown in Figs. 2 and 3, the emission spectra of Mn, Cr, and Fe from sample No. 2 were obtained in the deep ultraviolet 1135
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J. Lin et al.
Table 1 Composition of C, Si, Mn, Cr and Fe elements from the CrFe samples.
Sample
Fe
Standard Standard Standard Standard Standard Standard Standard Standard Standard Standard
Mn Cr Si C
values (%) uncertainty values (%) uncertainty values (%) uncertainty values (%) uncertainty values (%) uncertainty
(%) (%) (%) (%) (%)
No.1
No.2
No.3
No.4
No.5
No.6
No.7
96.492 0.02 0.352 0.006 1.010 0.008 1.380 0.009 0.881 0.008
96.489 0.02 1.340 0.009 0.736 0.007 0.439 0.003 0.580 0.005
97.238 0.02 0.624 0.005 1.280 0.009 0.298 0.003 0.418 0.004
94.166 0.02 0.579 0.009 1.190 0.009 0.249 0.003 0.358 0.004
97.236 0.02 1.040 0.01 2.330 0.01 0.601 0.004 0.222 0.003
96.914 0.02 0.589 0.009 1.050 0.009 0.336 0.003 0.122 0.002
98.773 0.02 0.470 0.007 0.071 0.007 0.520 0.004 0.081 0.001
Fig. 2. Metallic elements emission spectra obtained in the deep ultraviolet region (180–300 nm) for CrFe sample No.2.
region (180–300 nm).The Q-switched Nd-YAG laser beam was focused to 2 mm below the surface of the sample. The energy density of the laser was 200 mJ. The emission intensities for the Mn (294.38 nm), Cr (284.29 nm), Si (212.41 nm) and C (193.09 nm) atomic lines were all obviously enhanced with argon shield. The enhancement ratio of the nonmetallic elements in Fig. 3 was much higher than that of the metallic elements in Fig. 2. Argon, as a protective gas in the reaction, not only isolates the reaction of air from the surface of the sample, but also provides a transparent medium for the light of the ultraviolet region. Moreover, in argon, the oxygen and nitrogen in the air were isolated, which prevented the selective oxidation of the sample during the excitation process and stabilized the discharge state. Since oxygen has a strong absorption of the spectrum in 140–195 nm, it will have an impact on the detection of 193.09 nm C lines. Argon can eliminate the interference of oxygen. Because argon reduces the background noise, it provides a good signal-to-back ratio for experiments. Therefore, the experimental data in argon is relatively stable. The presence of argon enhances the spectral lines of neutral atoms and reduces the interference of the background. Therefore, the detection intensity of trace elements has been greatly improved. The increases in the signal are caused by the lower transfer rate of energy between the plasma and the argon shield [33]. The emission intensities for the Mn (294.38 nm), Cr (284.29 nm), Si (212.41 nm), and C (193.09 nm) atomic lines were all obviously enhanced with argon shield. The corresponding elemental enhancement ratios were 1.654, 1.896, 2.440, and 5.689, as shown in Table 2. The enhancement ratios of Mn and Cr are obviously lower than those of Si and C. Manganese and Cr are shown in the form of ions. Silicon and C exist in the form of atoms. It can be found that the enhancement effect of the atomic spectrum is obviously higher than that of the ion spectrum. This is mainly because the lifetime of the atoms in the plasma is longer than that of the ions. It was found that the content of C was higher than that of Si, and the enhancement ratio was 2.5 times higher than that of Si. This was mainly because oxygen in the atmosphere can absorb the spectrum of C elements, and argon blocks the contact between oxygen and the C element [34]. 3.2. Influence of delay times with argon shield To visually understand the mechanism of enhancement effects at the argon shield, the lateral images of the plasmas at different delay times were investigated. The time-resolved plasma plumes of the sample No. 2 were photographed by a high speed ICCD camera. The exposure time was set to 2 ns, and the gain was set to 30. The aperture starts from the minimum value of the corresponding value 22 mm. Since the distance between the lens and the sample was about 50 mm, the focus of the lens is adjusted to about 50 mm and then fine-tuned. Fig. 4 presents the time-resolved plasma images acquired at different delay times. As shown in Fig. 4, the plasma plumes enhanced more strongly than in the air condition. Their plasma morphology increased with the time delay at the very beginning and then slowly decreased to ablation. The plasmas reached the maximum at 1.5 s. This phenomenon coincided with the maximum enhancement time of the iron ion. This phenomenon verified the principal position of iron in the alloy, and the influence of nonmetal and trace metals on the plasma pattern can be neglected in the process of plasma evolution. 1136
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Fig. 3. Non-metallic emission spectra obtained in the deep ultraviolet region (180–300 nm) for CrFe sample No.2. Table 2 Enhancement ratio of different elements with argon shield. Element Wavelength
Mn II 294.38 nm
Cr II 284.29 nm
Si I 212.41 nm
C I 193.09 nm
With Ar shield No confinement Enhancement ratio
8269.66 4998.88 1.654
8807 4645.4 1.896
391.29 160.31 2.440
1454.72 255.72 5.689
Fig. 4. Lateral images of the plasma plumes under different delay times with argon shield.
3.3. Calibration curves for Mn, Cr, Si and C The internal standardization method has been widely used in the analysis of LIBS to avoid the influence of a number of external experimental parameters. The ratio of the line intensity of a low element to the emission line of the internal standard was measured, and plotted as a function of the known concentration ratios in the reference samples. The calibration curves defined by these plots allowed quantitative analysis for unknown samples. The intrinsic delay of the spectrometer was 1.28 μs. The emission spectra of the plasma were measured with a delay time of 2.5 μs and a gate width of 0.5 μs. To make the data more stable, the integration time was set to 20 μs; and each measurement was carried out by accumulating LIBS spectra with three laser shots. Based on the results previously shown, all of the lines of the low-concentration elements were enhanced in the argon shield; and non-metallic elements had a better confinement effect than metallic elements. So the low-concentration elements, Mn, Cr, Si, and C, were used for quantitative analysis. Fig. 5(a)–(d) show the calibration curves for Mn (0.352–1.340%), Cr (0.071–2.33%), Si (0.298–1.380%), and C (0.081–0.881%). According to the standard concentrations in the experimental samples, the relative intensity ratios of line IMnII294.38nm/IFeII294.76nm, ICrII284.29nm/IFeII295.34nm, ISiII212.41nm/IFeII200.04nm, and ICI193.09nm/IFeII200.04nm were plotted as a function of the relative concentration in a linear scale. From Fig. 5, we can obviously see that the calibration curve is nearly a straight line, which indicates a nearly linear relationship between the concentration and the emission intensity. From the above results, it was calculated that the correlation coefficients of the ratio (R2) of the IMnII294.38nm/IFeII294.76nm with the internal calibration method changed from 0.959 to 0.988, as shown in Fig. 5(a). Similar results for ICrII284.29nm/IFeII295.34nm, ISiII212.41nm/IFeII200.04nm, and ICI193.09nm/IFeII200.04nm were 0.957 to 0.985, 0.944 to 0.989 and 0.953 to 0.988, were also shown in Fig. 5(b)–(d). Therefore, the accuracy of quantitative analysis of low concentration elements, such as Mn, Cr, Si, and C, in steel samples was improved with argon shield. In the experiment, argon provides a transparent medium for the light in the ultraviolet region. The presence of argon enhances the spectral lines of neutral atoms in the emitting particles, prevents selective oxidation of the sample during excitation, leads to a steady state of discharge, and improves the precision of spectral analysis. 1137
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Fig. 5. Calibration curves vs. concentration in CrFe alloys: (a)Mn (I) 294.38 nm/ Fe(II) 294.78 nm vs. Mn concentration; (b) Cr(I) 284.29 nm/Fe(II) 286.34 nmvs. Cr concentration; (c) Si(I) 212.41 nm/Fe(II) 200.04 nmvs. Si concentration; (d) C (I) 193.09 nm/ Fe(II) 200.04 nm vs. C concentration.
Table 3 LODs of the detected elements in CrFe alloys. Element name
Slope Standard deviation LOD
MnII:294.38 nm
CrII:284.29 nm
SiI:212.41 nm
CI:193.09 nm
Ar
Air
Ar
Air
Ar
Air
Ar
Air
1.149 0.049 128
0.997 0.104 314
0.664 0.030 135
0.448 0.051 339
0.941 0.025 78
0.601 0.054 272
1.449 0.026 55
0.672 0.024 106
Limit of detection refers to the minimum concentration of the substance to be measured from the sample. In the daily inspection process, LOD is the specific measurement index. The analysis error of low-concentration elements and the content of the sample have a direct influence on the calculation of LOD, especially in the analysis of low-concentration elements. The calculated LODs of Mn, Cr, Si, and C elements in Sample No. 2 are given in Table 3. The LODs of low-concentration elements with argon shield increased more than two times than in air. The strengthening factor of nonmetallic elements in argon can reach to 55 ppm, which is lower by two times than metallic elements in the air. We suggest that the main reason of high sensitivity for low-concentration in argon is that the interference of other elements in the air has been eliminated. So the demonstration of improving LIBS sensitivity by argon shield is clear. 4. Conclusions In summary, the enhancement effect of different elements by argon shield in LIBS was studied. Significant enhancements in LIBS of alloy samples with different concentrations were observed. After the optimization of the argon shield environment, the whole spectral line strength was improved; but the enhancement effect of an atom is larger than that of an ion. The correlation coefficient of IMnII294.38nm/IFeII294.76nm in argon shield increased from 0.997 to 1.149; and similar results for ICrII284.29nm/IFeII295.34nm, ISiII212.41nm/ IFeII200.04nm, and ICI193.09nm/IFeII200.04nm were from 0.448 to 0.664, from 0.601 to 0.941, and from 0.672 to 1.439. The experimental results are consistent with the spectrum line intensity. The correlation coefficients of the ratio (R2) of the low-concentration elements have also increased to 98%. Therefore, the presence of argon enhances the spectral lines of neutral atoms in the emitting particles, prevents selective oxidation of the sample during excitation, leads to a steady state of discharge, and improves the precision of spectral analysis. 1138
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Acknowledgments This paper was supported by National Natural Science Foundation of China (NSFC, No. 51374040) and National Key Scientific Instrument and Equipment Development Project (No. 2014YQ120351). References [1] H. Zhang, F.Y. Yueh, J.P. Singh, Laser-induced breakdown spectrometry as a multi-metal continuouse-mission monitor, Appl. Opt. 38 (9) (1999) 1459–1466. [2] D.W. Hahn, M.M. Lunden, Detection and analysis of aerosol particles by laser-induced breakdownspectroscopy, Aerosol Sci. Technol. 33 (1-2) (2000) 30–48. [3] 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 spatialconfinement and dual-pulse irradiation, Opt. Express 20 (2) (2012) 1436–1443. [4] R. Hedwig, K. Lahna, Z.S. Lie, M. Pardede, K.H. Kurniawan, M.O. Tjia, K. 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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Enhancement effects of different elements by argon shield in laser induced breakdown spectroscopy Jingjun Lina, Xiaomei Lina, , Lianbo Guob, ⁎
a b
T
⁎
School of Electrical &Engineering, Changchun University of Technology, 130012 Changchun, Jilin, China Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology, 430074 Wuhan, Hubei, China
ARTICLE INFO
ABSTRACT
OCIS codes: (100.7410) Wavelets (300.6365) Spectroscopy Laser induced breakdownKeywords: Laser induced breakdown spectroscopy Limits of detection Detection accuracy
Laser induced breakdown spectroscopy (LIBS) has been demonstrated to be a promising element analysis technique due to its unique advantages such as rapid in-situ and on-line monitoring of materials in various forms. However, using LIBS to analyze materials with low concentrations of different elements remains a challenge. To improve the detection accuracy and limits of detection (LOD) of low concentration elements, the plasma produced by steel alloys with an argon shield to reduce the influence of the matrix effect in the detection process was investigated. A number of lowconcentration elements, such as manganese (Mn), chromium (Cr), silicon (Si) and carbon (C) in steel samples were studied. After optimization of the argon shield environment, significant enhancement factors of 1.89, 1.654, 2.44, and 5.689 in the emission intensity of the Mn, Cr, Si, and C lines were achieved. More importantly, after data processing, the correlation coefficient of IMnII294.38nm/ IFeII294.76nm in an argon shield increased from 0.997 to 1.149; and similar results for ICrII284.29nm/ IFeII295.34nm, ISiII212.41nm/IFeII200.04nm, and ICI193.09nm/IFeII200.04nm were from 0.448 to 0.664, from 0.601 to 0.941, and from0.672 to 1.439. The correlation coefficients of the ratio (R2) also increased to 98%. Therefore, we can create an argon environment to eliminate the interference of the experimental environment and realize the rapid detection of multiple-elements in low concentrations.
1. Introduction Laser-induced breakdown spectroscopy (LIBS) is a spectrometry technology for material element analysis based on measuring the atomic emission of plasma induced by a high power laser pulse. The basic principle is the focusing of a high power laser beam on the surface of a sample to produce plasma and collect emitting radiation [1–3]. The information obtained from the plasma can be used to obtain qualitative and quantitative information about the target’s elemental composition. LIBS can be used to detect samples of any physical form (liquid, solid, powder, gas) at a high speed [4–6]. Therefore, during the last decade, LIBS technology has been applied to various fields, including iron and steel analysis, ocean detection, soil composition analysis, coal quality testing and so on, for realtime, online chemical assays or monitoring [7]. However, the sensitivity and precision of LIBS remains a challenge due to the fluctuations of various factors, such as the laser energy and environmental interference. More and more researchers spent their efforts on system improvement and spectral processing algorithms to solve the problem. They have proposed various methods to enhance the signal stability, such as the multiple pulse approach [8–11], spatial confinement, and so on [12–14]. Aside from the methods mentioned above, a widely used approach is introduction of inert gas, for instance, argon, nitrogen, and helium, which can easily improve the detection sensitivity of LIBS. These gases can be adopted to protect the plasma from the
⁎
Corresponding authors. E-mail addresses: [email protected] (X. Lin), [email protected] (L. Guo).
https://doi.org/10.1016/j.ijleo.2018.10.144 Received 2 September 2018; Accepted 23 October 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 179 (2019) 1134–1139
J. Lin et al.
Fig. 1. Schematic diagram of the experimental setup.
reaction between the air and the surface of the sample. By considering the spectral line intensity and the signal-to-background ratio of the elements in different inert gases, argon was selected as the protective gas for this experiment [15]. At the same time, during the shock wave expansion, argon can provide a transparent medium for the light in the ultraviolet region [16–18].The activation of the sample in argon replaces the oxygen and nitrogen in air, preventing the selective oxidation of the sample during excitation. In recent years, the gas protection in LIBS has been studied by several groups for main matrix elements. A previous work on aluminum (Al) plasmas by gas protection, which confined the temporal and spatial dynamics of laser-induced aluminum plasma in an argon background at atmospheric pressure, was conducted by Q. Ma [19]. Q. L. Ma and V. Motto-Ros also studied the effects of ablation laser wavelength on the expansion of laser-induced plasma in argon gas [20]. Many other researchers studied qualitative analysis, such as spectral enhancement and plasma distribution [21–32]. In the study, our main aim was to investgate enhancement effects with different elements for quantitative analysis by argon shield in LIBS. 2. Experimental methods 2.1. Experiment setup The schematic diagram of the experimental setup used in this work is shown in Fig. 1. The experiments were conducted in a vacuum tank. First, the experiments were carried out in the atmospheric environment. Then the air in the vacuum tank was pumped out by a vacuum pump. At the same time, we added argon to the vacuum tank. The pressure in the tank was controlled to the atmospheric pressure. Finally, the experiments were carried out with the argon shield. A Q-switched Nd: YAG laser (Beam-tech, Vlite-200, pulse duration 8 ns) operating at 1,064 nm with a repetition rate of 2 Hz was used for plasma generation. We set 580 V as the working voltage. The laser emits a laser beam which was reflected by a dichroic mirror and focused by an ultraviolet (UV)-grade quartz lens (focal length, 25 cm), through the quartz window at the top of the vacuum tank onto the surface of the sample. The reflective spectrum range of this 45°dichroic mirror is 1064 nm. The dichroic mirror can reflect a 1,064 nm laser beam and transmit all of the other wavelengths. The focal point was 2 mm below the sample surface where plasma plumes the size of several millimeters at the center of the hemisphere were generated. The emissions from the laser-induced plasmas were collected through the window located on the upper side which was clamped at 30 ° with the laser beam, and coupled to an optical fiber by the UV-grade quartz lens and a light collector (AvaSpace-2048, wavelength range: 180–300 nm). An intensified charge-coupled device (ICCD) was used to detect the spectrally resolved line that was operated in the gated mode. The gate delay and width of the ICCD were adjusted to each other. Thus, we were able to control the gate delay to obtain the spectra at different time delays after the laser pulse. At the same time, the plasma image was collected by a highspeed camera at different time delays. All of the data acquisition and analysis were performed with a personal computer. 2.2. Sample preparation Seven steel samples (CSBS11071-2012, Research Institute of certified reference materials, Twelfth Research Institute of China Shipbuilding Industry Corporation) with manganese (Mn), chromium (Cr), silicon (Si) and carbon (C) concentrations lower than 1.34%, 2.33%, 1.38%, and 0.881%, respectively, as well as the matrix element iron (Fe) were used in this study. The concentrations with their standard deviations of main elements in the study are shown in Table 1. The experimental conditions were kept constant during the experiment. 3. Results and discussion 3.1. Enhancement ratio of low-concentration elements with argon shield in the steel samples To demonstrate the intensity enhancement of the low-concentration metallic elements, the time-integrated LIBS spectra of laserinduced Mn and Cr elements with argon shield in the steel samples were measured. The matrix element Fe was used as a contrast element. As shown in Figs. 2 and 3, the emission spectra of Mn, Cr, and Fe from sample No. 2 were obtained in the deep ultraviolet 1135
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Table 1 Composition of C, Si, Mn, Cr and Fe elements from the CrFe samples.
Sample
Fe
Standard Standard Standard Standard Standard Standard Standard Standard Standard Standard
Mn Cr Si C
values (%) uncertainty values (%) uncertainty values (%) uncertainty values (%) uncertainty values (%) uncertainty
(%) (%) (%) (%) (%)
No.1
No.2
No.3
No.4
No.5
No.6
No.7
96.492 0.02 0.352 0.006 1.010 0.008 1.380 0.009 0.881 0.008
96.489 0.02 1.340 0.009 0.736 0.007 0.439 0.003 0.580 0.005
97.238 0.02 0.624 0.005 1.280 0.009 0.298 0.003 0.418 0.004
94.166 0.02 0.579 0.009 1.190 0.009 0.249 0.003 0.358 0.004
97.236 0.02 1.040 0.01 2.330 0.01 0.601 0.004 0.222 0.003
96.914 0.02 0.589 0.009 1.050 0.009 0.336 0.003 0.122 0.002
98.773 0.02 0.470 0.007 0.071 0.007 0.520 0.004 0.081 0.001
Fig. 2. Metallic elements emission spectra obtained in the deep ultraviolet region (180–300 nm) for CrFe sample No.2.
region (180–300 nm).The Q-switched Nd-YAG laser beam was focused to 2 mm below the surface of the sample. The energy density of the laser was 200 mJ. The emission intensities for the Mn (294.38 nm), Cr (284.29 nm), Si (212.41 nm) and C (193.09 nm) atomic lines were all obviously enhanced with argon shield. The enhancement ratio of the nonmetallic elements in Fig. 3 was much higher than that of the metallic elements in Fig. 2. Argon, as a protective gas in the reaction, not only isolates the reaction of air from the surface of the sample, but also provides a transparent medium for the light of the ultraviolet region. Moreover, in argon, the oxygen and nitrogen in the air were isolated, which prevented the selective oxidation of the sample during the excitation process and stabilized the discharge state. Since oxygen has a strong absorption of the spectrum in 140–195 nm, it will have an impact on the detection of 193.09 nm C lines. Argon can eliminate the interference of oxygen. Because argon reduces the background noise, it provides a good signal-to-back ratio for experiments. Therefore, the experimental data in argon is relatively stable. The presence of argon enhances the spectral lines of neutral atoms and reduces the interference of the background. Therefore, the detection intensity of trace elements has been greatly improved. The increases in the signal are caused by the lower transfer rate of energy between the plasma and the argon shield [33]. The emission intensities for the Mn (294.38 nm), Cr (284.29 nm), Si (212.41 nm), and C (193.09 nm) atomic lines were all obviously enhanced with argon shield. The corresponding elemental enhancement ratios were 1.654, 1.896, 2.440, and 5.689, as shown in Table 2. The enhancement ratios of Mn and Cr are obviously lower than those of Si and C. Manganese and Cr are shown in the form of ions. Silicon and C exist in the form of atoms. It can be found that the enhancement effect of the atomic spectrum is obviously higher than that of the ion spectrum. This is mainly because the lifetime of the atoms in the plasma is longer than that of the ions. It was found that the content of C was higher than that of Si, and the enhancement ratio was 2.5 times higher than that of Si. This was mainly because oxygen in the atmosphere can absorb the spectrum of C elements, and argon blocks the contact between oxygen and the C element [34]. 3.2. Influence of delay times with argon shield To visually understand the mechanism of enhancement effects at the argon shield, the lateral images of the plasmas at different delay times were investigated. The time-resolved plasma plumes of the sample No. 2 were photographed by a high speed ICCD camera. The exposure time was set to 2 ns, and the gain was set to 30. The aperture starts from the minimum value of the corresponding value 22 mm. Since the distance between the lens and the sample was about 50 mm, the focus of the lens is adjusted to about 50 mm and then fine-tuned. Fig. 4 presents the time-resolved plasma images acquired at different delay times. As shown in Fig. 4, the plasma plumes enhanced more strongly than in the air condition. Their plasma morphology increased with the time delay at the very beginning and then slowly decreased to ablation. The plasmas reached the maximum at 1.5 s. This phenomenon coincided with the maximum enhancement time of the iron ion. This phenomenon verified the principal position of iron in the alloy, and the influence of nonmetal and trace metals on the plasma pattern can be neglected in the process of plasma evolution. 1136
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Fig. 3. Non-metallic emission spectra obtained in the deep ultraviolet region (180–300 nm) for CrFe sample No.2. Table 2 Enhancement ratio of different elements with argon shield. Element Wavelength
Mn II 294.38 nm
Cr II 284.29 nm
Si I 212.41 nm
C I 193.09 nm
With Ar shield No confinement Enhancement ratio
8269.66 4998.88 1.654
8807 4645.4 1.896
391.29 160.31 2.440
1454.72 255.72 5.689
Fig. 4. Lateral images of the plasma plumes under different delay times with argon shield.
3.3. Calibration curves for Mn, Cr, Si and C The internal standardization method has been widely used in the analysis of LIBS to avoid the influence of a number of external experimental parameters. The ratio of the line intensity of a low element to the emission line of the internal standard was measured, and plotted as a function of the known concentration ratios in the reference samples. The calibration curves defined by these plots allowed quantitative analysis for unknown samples. The intrinsic delay of the spectrometer was 1.28 μs. The emission spectra of the plasma were measured with a delay time of 2.5 μs and a gate width of 0.5 μs. To make the data more stable, the integration time was set to 20 μs; and each measurement was carried out by accumulating LIBS spectra with three laser shots. Based on the results previously shown, all of the lines of the low-concentration elements were enhanced in the argon shield; and non-metallic elements had a better confinement effect than metallic elements. So the low-concentration elements, Mn, Cr, Si, and C, were used for quantitative analysis. Fig. 5(a)–(d) show the calibration curves for Mn (0.352–1.340%), Cr (0.071–2.33%), Si (0.298–1.380%), and C (0.081–0.881%). According to the standard concentrations in the experimental samples, the relative intensity ratios of line IMnII294.38nm/IFeII294.76nm, ICrII284.29nm/IFeII295.34nm, ISiII212.41nm/IFeII200.04nm, and ICI193.09nm/IFeII200.04nm were plotted as a function of the relative concentration in a linear scale. From Fig. 5, we can obviously see that the calibration curve is nearly a straight line, which indicates a nearly linear relationship between the concentration and the emission intensity. From the above results, it was calculated that the correlation coefficients of the ratio (R2) of the IMnII294.38nm/IFeII294.76nm with the internal calibration method changed from 0.959 to 0.988, as shown in Fig. 5(a). Similar results for ICrII284.29nm/IFeII295.34nm, ISiII212.41nm/IFeII200.04nm, and ICI193.09nm/IFeII200.04nm were 0.957 to 0.985, 0.944 to 0.989 and 0.953 to 0.988, were also shown in Fig. 5(b)–(d). Therefore, the accuracy of quantitative analysis of low concentration elements, such as Mn, Cr, Si, and C, in steel samples was improved with argon shield. In the experiment, argon provides a transparent medium for the light in the ultraviolet region. The presence of argon enhances the spectral lines of neutral atoms in the emitting particles, prevents selective oxidation of the sample during excitation, leads to a steady state of discharge, and improves the precision of spectral analysis. 1137
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Fig. 5. Calibration curves vs. concentration in CrFe alloys: (a)Mn (I) 294.38 nm/ Fe(II) 294.78 nm vs. Mn concentration; (b) Cr(I) 284.29 nm/Fe(II) 286.34 nmvs. Cr concentration; (c) Si(I) 212.41 nm/Fe(II) 200.04 nmvs. Si concentration; (d) C (I) 193.09 nm/ Fe(II) 200.04 nm vs. C concentration.
Table 3 LODs of the detected elements in CrFe alloys. Element name
Slope Standard deviation LOD
MnII:294.38 nm
CrII:284.29 nm
SiI:212.41 nm
CI:193.09 nm
Ar
Air
Ar
Air
Ar
Air
Ar
Air
1.149 0.049 128
0.997 0.104 314
0.664 0.030 135
0.448 0.051 339
0.941 0.025 78
0.601 0.054 272
1.449 0.026 55
0.672 0.024 106
Limit of detection refers to the minimum concentration of the substance to be measured from the sample. In the daily inspection process, LOD is the specific measurement index. The analysis error of low-concentration elements and the content of the sample have a direct influence on the calculation of LOD, especially in the analysis of low-concentration elements. The calculated LODs of Mn, Cr, Si, and C elements in Sample No. 2 are given in Table 3. The LODs of low-concentration elements with argon shield increased more than two times than in air. The strengthening factor of nonmetallic elements in argon can reach to 55 ppm, which is lower by two times than metallic elements in the air. We suggest that the main reason of high sensitivity for low-concentration in argon is that the interference of other elements in the air has been eliminated. So the demonstration of improving LIBS sensitivity by argon shield is clear. 4. Conclusions In summary, the enhancement effect of different elements by argon shield in LIBS was studied. Significant enhancements in LIBS of alloy samples with different concentrations were observed. After the optimization of the argon shield environment, the whole spectral line strength was improved; but the enhancement effect of an atom is larger than that of an ion. The correlation coefficient of IMnII294.38nm/IFeII294.76nm in argon shield increased from 0.997 to 1.149; and similar results for ICrII284.29nm/IFeII295.34nm, ISiII212.41nm/ IFeII200.04nm, and ICI193.09nm/IFeII200.04nm were from 0.448 to 0.664, from 0.601 to 0.941, and from 0.672 to 1.439. The experimental results are consistent with the spectrum line intensity. The correlation coefficients of the ratio (R2) of the low-concentration elements have also increased to 98%. Therefore, the presence of argon enhances the spectral lines of neutral atoms in the emitting particles, prevents selective oxidation of the sample during excitation, leads to a steady state of discharge, and improves the precision of spectral analysis. 1138
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