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Plasma Sci. Technol.  
  Plasma Sci. Technol.--2015, 17 (8)   Published: 24 July 2015
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A Rising Force for the World-Wide Development of Laser-Induced Breakdown Spectroscopy

WANG Zhe (王哲)1, DONG Fengzhong (董凤忠)2, ZHOU Weidong (周卫东)3
Plasma Sci. Technol. 2015, 17 (8): 617-620 ;  doi: 10.1088/1009-0630/17/8/01
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Wavelength Dependence in the Analysis of Carbon Content in Coal by Nanosecond 266 nm and 1064 nm Laser Induced Breakdown Spectroscopy

LI Xiongwei (李雄威), WANG Zhe (王哲), FU Yangting (傅杨挺), et al.
Plasma Sci. Technol. 2015, 17 (8): 621-624 ;  doi: 10.1088/1009-0630/17/8/02
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The wavelength dependence of laser induced breakdown spectroscopy (LIBS) in the analysis of the carbon contents of coal was studied using 266 nm and 1064 nm laser radiations. Compared with the 1064 nm wavelength laser ablation, the 266 nm wavelength laser ablation has less thermal effects, resulting in a better crater morphology on the coal pellets. Besides, the 266 nm wavelength laser ablation also provides better laser-sample coupling and less plasma shielding, resulting in a higher carbon line intensity and better signal reproducibility. The carbon contents in the bituminous coal samples have better linearity with the line intensities of atomic carbon measured by the 266 nm wavelength than those measured by the 1064 nm wavelength. The partial least square (PLS) model was established for the quantitative analysis of the carbon content in coal samples by LIBS. The results show that both of the 266 nm and 1064 nm wavelengths are capable of achieving good performance for the quantitative analysis of carbon content in coal using the PLS method.

A Study on the Characteristics of Carbon-Related Spectral Lines from a Laser-Induced Fly Ash Plasma

PAN Gang (潘刚), LU Jidong (陆继东), DONG Meirong (董美蓉), YAO Shunchun (姚顺春),et al.
Plasma Sci. Technol. 2015, 17 (8): 625-631 ;  doi: 10.1088/1009-0630/17/8/03
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A 1064 nm Nd:YAG laser was used to ablate fly ash samples. The characteristics of the spectral lines measured from the laserablated fly ash plasmas are presented with special attention to atomic and molecular carbon emission. It is shown that the intensity of the atomic line C I 192.9 nm is weak and the shot-to-shot intensity is fluctuant. The carbon atomic line C I 247.7 nm is relatively intensive and stable, however it is seriously interfered with by Fe I 247.8 nm. The intensity of the CN molecular line is close to that of C I 247.7 nm and the CN line is stable and less interfered with. The comparison of molecular CN emission under different conditions (air, Ar and N2) shows that the CN lines detected from the plasmas formed in an atmospheric environment are correlated with the reaction of carbon atoms in the plasma with the nitrogen in air, which indicates that the CN line is also important in pulsed laser ablation fly ash plasmas and this information can be incorporated in the detection of unburned carbon content in fly ash. Finally, a calibration curve is established with a correlation coefficient R2 of 0.999, using C I 247.7 nm and the CN molecular line as associated variables. In addition, accuracy is improved to a certain extent.

Heavy Metal Detection in Soils by Laser Induced Breakdown Spectroscopy Using Hemispherical Spatial Confinement

MENG Deshuo (孟德硕), ZHAO Nanjing (赵南京), MA Mingjun (马明俊),et al.
Plasma Sci. Technol. 2015, 17 (8): 632-637 ;  doi: 10.1088/1009-0630/17/8/04
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Spatial confinement has great potential for Laser Induced Breakdown Spectroscopy (LIBS) instruments after it has been proven that it has the ability to enhance the LIBS signal strength and repeatability. In order to achieve in-situ measurement of heavy metals in farmland soils by LIBS, a hemispherical spatial confinement device is designed and used to collect plasma spectra, in which the optical fibers directly collect the breakdown spectroscopy of the soil samples. This device could effectively increase the stability of the spectrum intensity of soil. It also has other advantages, such as ease of installation, and its small and compact size. The relationship between the spectrum intensity and the laser pulse energy is studied for this device. It is found that the breakdown threshold is 160 cm?2, and when the laser fluence increases to 250 J/cm2, the spectrum intensity reaches its maximum. Four different kinds of laser pulse energy were set up and in each case the limits of detection of Cd, Cu, Ni, Pb and Zn were calculated. The results show that when the laser pulse fluence was 2.12 GW/cm2, we obtained the smallest limits of detection of these heavy metals, which are all under 10 mg/kg. This device can satisfy the needs of heavy metal in-situ detection, and in the next step it will be integrated into a portable LIBS instrument.

Spatial Resolution Measurements of C, Si and Mo Using LIBS for Diagnostics of Plasma Facing Materials in a Fusion Device

LI Cong (李聪), ZHAO Dongye (赵栋烨), WU Xingwei (吴兴伟), et al.
Plasma Sci. Technol. 2015, 17 (8): 638-643 ;  doi: 10.1088/1009-0630/17/8/05
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Recently, a laser-induced breakdown spectroscopic (LIBS) system has been devel?oped for in situ measurements of the chemical compositions of plasma facing materials (PFMs) in the Experimental Advanced Superconducting Tokamak (EAST). In this study, a LIBS system, which was used in a similar optical configuration to the in situ LIBS system in EAST, has been developed to investigate the spatial distribution of PFM elements at 10?4 Pa. The aim of this study was to understand the nature of the spatial distribution of atoms or ions of different elements in the plasma plume and optimize the signal to background ratio for the in situ LIBS diagnosis in EAST. The spatial profiles of the LIBS signals of C, Si, Mo and the continuous background were measured. Moreover, the influence of laser spot size and laser energy density on the LIBS signals of C, Si, Mo and H was also investigated. The results show that the distribution of the C, Si and Mo peaks’intensities first increased and then decreased from the center to the edge of the plasma plume. There was a maximum value at R ≈ 1.5 mm from the center of the plasma plume. This work aims to improve the understanding of ablating plasma dynamics in very low pressure environments and give guidance to optimize the LIBS system in the EAST device.

Determination of the Insoluble Aluminum Content in Steel Samples by Using Laser-Induced Breakdown Spectroscopy

LIU Jia (刘佳), JIA Yunhai (贾云海), ZHANG Yong (张勇), SUN Nian (孙念)
Plasma Sci. Technol. 2015, 17 (8): 644-648 ;  doi: 10.1088/1009-0630/17/8/06
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The insoluble aluminum content in steel samples has a significant influence on the quality of the steel. In this paper, laser-induced breakdown spectroscopy (LIBS) is used to analyze the insoluble aluminum content in steel samples using a scanning mode. The average intensity plus 2.5 standard deviations was iterated and the final iteration value was taken as the threshold that distinguishes soluble and insoluble aluminum, and thus total and soluble aluminum content calibration curves were generated. Using the relevant total and soluble aluminum content calibra?tion curves, the total and soluble aluminum contents in steel samples could be determined. The insoluble aluminum content could be determined by subtracting the soluble aluminum content from the total aluminum content. The insoluble aluminum content of standard samples and pro?cess product samples were determined using the present mathematical model; the results agreed well with the certified reference values. This method could be used to rapidly characterize the insoluble aluminum content in steel samples.

Temporal and Spatial Evolution of Laser-Induced Plasma from a Slag Sample

WANG Jingge (王静鸽)1, FU Hongbo (付洪波)1, NI Zhibo (倪志波)1,et al.
Plasma Sci. Technol. 2015, 17 (8): 649-655 ;  doi: 10.1088/1009-0630/17/8/07
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Laser-Induced Breakdown Spectroscopy (LIBS) has been demonstrated to be an effective method for slag analysis. In order to better clarify the nature of the plasma generated from a slag sample, an Nd:YAG pulse laser at 1064 nm wavelength was used to ablate the slag sample in air. The temporal and spatial evolutions of plasma parameters, including emission intensity, electronic density and plasma temperature, have been studied. It is shown that the electron density and plasma temperature drop off rapidly with the delay time as a result of plasma expansion and cooling. It has been found that the electron density of the whole plasma is close to that of the center regions in the plasma. The results of the spatial distributions on the two-dimensional plane have shown that there is a big region with lower electron density values caused by the recombination process in the center of the plasma. The maximum of the plasma temperature takes place at the regions close to the target, and the border of the plasma front-head has higher plasma temperatures than that of the center part.

Rock and Soil Classification Using PLS-DA and SVM Combined with a Laser-Induced Breakdown Spectroscopy Library

YANG Guang (杨光), QIAO Shujun (乔淑君), CHEN Pengfei (陈鹏飞), et al.
Plasma Sci. Technol. 2015, 17 (8): 656-663 ;  doi: 10.1088/1009-0630/17/8/08
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Laser-induced breakdown spectroscopy (LIBS) has become a powerful technology in geological applications. The correct identification of rocks and soils is critical to many geological projects. In this study, LIBS database software with a user-friendly and intuitive interface is developed based on Windows, consisting of a database module and a sample identification module. The database module includes a basic database containing LIBS persistent lines for elements and a dedicated geological database containing LIBS emission lines for several rock and soil reference standards. The module allows easy use of the data. A sample identification module based on partial least squares discriminant analysis (PLS-DA) or support vector machine (SVM) algorithms enables users to classify groups of unknown spectra. The developed system was used to classify rock and soil data sets in a dedicated database and the results demonstrate that the system is capable of fast and accurate classi?cation of rocks and soils, and is thus useful for the detection of geological materials.

The Spectral Emission Characteristics of Laser Induced Plasma on Tea Samples

ZHENG Peichao (郑培超)1,2, SHI Minjie (石珉杰)1, WANG Jinmei (王金梅)1,et al.
Plasma Sci. Technol. 2015, 17 (8): 664-670 ;  doi: 10.1088/1009-0630/17/8/09
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Laser induced breakdown spectroscopy (LIBS) provides a useful technique for food security as well as determining nutrition contents. In this paper, optical emission studies of laser induced plasma on commercial tea samples were carried out. The spectral intensities of Mg, Mn, Ca, Al, C and CN vibration bands varying with laser energy and the detection delay time of an intensified charge coupled device were studied. In addition, the relative concentrations of six microelements, i.e., Mg, Mn, Ca, Al, Na and K, were analyzed semi-quantitatively as well as H, for four kinds of tea samples. Moreover, the plasma parameters were explored, including electron temperature and electron number density. The electron temperature and electron number density were around 11000 K and 1017 cm?3 , respectively. The results show that it is reasonable to consider the LIBS technique as a new method for analyzing the compositions of tea leaf samples.

Characterization of the Delamination Defects in Marine Steel Using Laser-Induced Breakdown Spectroscopy

YANG Chun (杨春)1,2, JIA Yunhai (贾云海)1, ZHANG Yong (张勇)1,et al.
Plasma Sci. Technol. 2015, 17 (8): 671-675 ;  doi: 10.1088/1009-0630/17/8/10
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In this paper, two types of comparison analyses, bulk analysis and defect analysis, were carried out for marine steel. The results of laser-induced breakdown spectroscopy (LIBS) were compared with those of spark optical emission spectrometry (Spark-OES) and scanning electron microscopy/energy dispersion spectroscopy (SEM/EDS) in the bulk and defect analyses. The comparison of the bulk analyses shows that the chemical contents of C, Si, Mn, P, S and Cr obtained from LIBS agree well with those determined using Spark-OES. The LIBS is slightly less precise than Spark-OES. Defects were characterized in the two-dimensional distribution analysis mode for Al, Mg, Ca, Si and other elements. Both the LIBS and SEM/EDS results show the enrichment of Al, Mg, Ca and Si at the defect position and the two methods agree well with each other. SEM/EDS cannot provide information about the difference in the chemical constituents when the differences between the defect position and the normal position are not signi?cant. However, LIBS can provide this information, meaning that the sensitivity of LIBS is higher than that of SEM/EDS. LIBS can be used to rapidly characterize marine steel defects and provide guidance for improving metallurgical processes.

Application of Stand-off Double-Pulse Laser-Induced Breakdown Spectroscopy in Elemental Analysis of Magnesium Alloy

QI Lifeng (齐立峰), SUN Lanxiang (孙兰香), XIN Yong (辛勇),et al.
Plasma Sci. Technol. 2015, 17 (8): 676-681 ;  doi: 10.1088/1009-0630/17/8/11
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In this study, a stand-off and collinear double pulse laser-induced breakdown spectroscopy (DP LIBS) system was designed, and the magnesium alloy samples at a distance of 2.5 m away from the LIBS system were measured. The effect of inter-pulse delay on spectra was studied, and the signal enhancement was observed compared to the single pulse LIBS (SP LIBS). The morphology of the ablated crater on the sample indicated a higher efficiency of surface pretreatment in DP LIBS. The calibration curves of Ytterbium (Y) and Zirconium (Zr) were investigated. The square of the correlation coefficient of the calibration curve of element Y reached up to 0.9998.

A Study of the Laser-Induced Breakdown Spectroscopy of Carbon in the Ultraviolet Wavelength Range Under Vacuum Conditions

PAN Congyuan (潘从元) 1,DU Xuewei (杜学维) 1,ZENG Qiang (曾强) 1,et al.
Plasma Sci. Technol. 2015, 17 (8): 682-686 ;  doi: 10.1088/1009-0630/17/8/12
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The influence of a vacuum on the laser-induced breakdown spectroscopy (LIBS) of carbon in the ultraviolet wavelength range is studied. Experiments are performed with graphite using a LIBS system, which consists of a 1064 nm Nd:YAG laser, a vacuum pump, a spectrometer and a vacuum chamber. The vacuum varies from 10 Pa to 1 atm. Atomic lines as well as singly and doubly charged ions are confirmed under the vacuums. A temporal evolution analysis of intensity is performed for the atomic lines of C I 193.09 nm and C I 247.86 nm under different vacuum conditions. Both time-integrated and time-resolved intensity evolutions under vacuums are achieved. The lifetimes of the two atomic lines have similar trends, which supports the point of view of a ‘soft spot’. Variations of plasma temperature and electron density under different vacuums are measured. This study is helpful for research on carbon detection using LIBS under vacuum conditions.

The Enhanced Effect of Optical Emission from Laser Induced Breakdown Spectroscopy of an Al-Li Alloy in the Presence of Magnetic Field Confinement

LIU Ping (刘平), HAI Ran (海然), WU Ding (吴鼎), XIAO Qingmei (肖青梅), et al.
Plasma Sci. Technol. 2015, 17 (8): 687-692 ;  doi: 10.1088/1009-0630/17/8/13
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In this paper, the influence of magnetic field strength on laser-induced breakdown spectroscopy (LIBS) has been investigated for various pressures. The plasma plume was produced by employing Q-switch Nd:YAG laser ablation of an Al-Li alloy operating at a 1064 nm wavelength. The results indicated that the LIBS intensity of the Al and Li emission lines is boosted with an increase of magnetic strength. Typically, the intensity of the Al I and Li I spectral emissions can be magnified by 1.5-3 times in a steady magnetic field of 1.1 T compared with the field-free case. Also, in this investigation we recorded time-resolved images of the laser-produced plume by employing a fast ICCD camera. The results show that the luminance of the plasma is enhanced and the time of persistence is increased signi?cantly, and the plasma plume splits into two lobes in the presence of a magnetic field. The probable reason for the enhancement is the magnetic con?nement effect which increases the number density of excited atoms and the population of species in a high energy state. In addition, the electron temperature and density are also augmented by the magnetic field compared to the field-free case.

The Effect of an External Magnetic Field on the Plume Expansion Dynamics of Laser-Induced Aluminum Plasma

Atif HUSSAIN, LI Qi (李奇), HAO Zuoqiang (郝作强), et al.
Plasma Sci. Technol. 2015, 17 (8): 693-698 ;  doi: 10.1088/1009-0630/17/8/14
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In this work, we investigated the plasma morphology induced by a Nd:YAG laser with the aim of improving the understanding of the formation and dynamics of the plasma in two cases, with and without a magnetic field. Single laser pulse production of a plasma in the absence and presence of a magnetic field was performed with an aluminum target in air. A fast photography technique was employed to obtain information about the expansion dynamics and confinement of the aluminum plasma in each case. The generation of the laser plasma was allowed to expand at two locations with different magnetic field strengths, which correspond to the strength 0.58 T in the center of two magnetic poles and 0.83 T at a distance of 4 mm from the upper pole (N). The plume showed lateral con?nement at longer delays when the target was placed at the center of the two poles. When the target was placed at a distance of 4 mm from the upper pole it was observed that the plume was divided into two lobes at the initial stage and traveled towards the center of the magnetic field with further elapse of time.

Quantitative Analysis of Heavy Metals in Water Based on LIBS with an Automatic Device for Sample Preparation

HU Li (胡丽), ZHAO Nanjing (赵南京), LIU Wenqing (刘文清), MENG Deshuo (孟德硕),et al.
Plasma Sci. Technol. 2015, 17 (8): 699-703 ;  doi: 10.1088/1009-0630/17/8/15
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Heavy metals in water can be deposited on graphite flakes, which can be used as an enrichment method for laser-induced breakdown spectroscopy (LIBS) and is studied in this paper. The graphite samples were prepared with an automatic device, which was composed of a loading and unloading module, a quantitatively adding solution module, a rapid heating and drying module and a precise rotating module. The experimental results showed that the sample preparation methods had no significant effect on sample distribution and the LIBS signal accumulated in 20 pulses was stable and repeatable. With an increasing amount of the sample solution on the graphite flake, the peak intensity at Cu I 324.75 nm accorded with the exponential function with a correlation coe?cient of 0.9963 and the background intensity remained unchanged. The limit of detection (LOD) was calculated through linear fitting of the peak intensity versus the concentration. The LOD decreased rapidly with an increasing amount of sample solution until the amount exceeded 20 mL and the correlation coefficient of exponential function fitting was 0.991. The LOD of Pb, Ni, Cd, Cr and Zn after evaporating different amounts of sample solution on the graphite flakes was measured and the variation tendency of their LOD with sample solution amounts was similar to the tendency for Cu. The experimental data and conclusions could provide a reference for automatic sample preparation and heavy metal in situ detection.

Development of an Automated LIBS Analytical Test System Integrated with Component Control and Spectrum Analysis Capabilities

DING Yu (丁宇), TIAN Di (田地), CHEN Feipeng (陈飞鹏), CHEN Pengfei (陈鹏飞), et al.
Plasma Sci. Technol. 2015, 17 (8): 704-710 ;  doi: 10.1088/1009-0630/17/8/16
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The present paper proposes an automated Laser-Induced Breakdown Spectroscopy (LIBS) analytical test system, which consists of a LIBS measurement and control platform based on a modular design concept, and a LIBS qualitative spectrum analysis software and is developed in C#. The platform provides flexible interfacing and automated control; it is compatible with different manufacturer component models and is constructed in modularized form for easy ex?pandability. During peak identification, a more robust peak identification method with improved stability in peak identification has been achieved by applying additional smoothing on the slope obtained by calculation before peak identification. For the purpose of element identification, an improved main lines analysis method, which detects all elements on the spectral peak to avoid omission of certain elements without strong spectral lines, is applied to element identification in the tested LIBS samples. This method also increases the identification speed. In this paper, actual applications have been carried out. According to tests, the analytical test system is compatible with components of various models made by different manufacturers. It can automatically control components to get experimental data and conduct filtering, peak identification and qualitative analysis, etc. on spectral data.

Nondestructive Determination of Cu Residue in Orange Peel by Laser Induced Breakdown Spectroscopy

HU Huiqin (胡慧琴), HUANG Lin (黄林), LIU Muhua (刘木华), CHEN Tianbing (陈添兵),et al.
Plasma Sci. Technol. 2015, 17 (8): 711-715 ;  doi: 10.1088/1009-0630/17/8/17
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Laser induced breakdown spectroscopy (LIBS) is an emerging tool with rapid, nonde?structive, green characteristics in qualitative or quantitative analyses of composition in materials. But LIBS has its shortcomings in detect limit and sensitivity. In this work, heavy metal Cu in Gannan Navel Orange, which is one of famous fruits from Jiangxi of China, was analyzed. In view of LIBS’s limit, it is difficult to determinate heavy metals in natural fruits. In this work, nine orange samples were pretreated in 50-500 μg/mL Cu solution, respectively. Another one orange sample was chosen as a control group without any pollution treatment. Previous researchers ob-served that the content of heavy metals is much higher in peel than in pulp. So, the content in pulp can be reflected by detecting peel. The real concentrations of Cu in peels were acquired by atomic absorption spectrophotometer (AAS). A calibration model of Cu I 324.7 and Cu I 327.4 was constructed between LIBS intensity and AAS concentration by six samples. The correlation coefficient of the two models is also 0.95. All of the samples were used to verify the accuracy of the model. The results show that the relative error (RE) between predicted and real concentration is less than 6.5%, and Cu I 324.7 line has smaller RE than Cu I 327.4. The analysis demonstrated that different characteristic lines decided different accuracy. The results prove the feasibility of detecting heavy metals in fruits by LIBS. But the results are limited in treated samples. The next work will focus on direct analysis of heavy metals in natural fruits without any pretreatment. This work is helpful to explore the distribution of heavy metals between pulp and peel.

Quantitative Analysis of Mg in Pipeline Dirt Based on Laser-Induced Breakdown Spectroscopy

WANG Shaolong (王绍龙)1, WANG Yangen (王阳恩)1, CHEN Shanjun (陈善俊)1,et al.
Plasma Sci. Technol. 2015, 17 (8): 716-720 ;  doi: 10.1088/1009-0630/17/8/18
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In order to maintain the pipeline better and remove the dirt more effectively, it was necessary to analyze the contents of elements in dirt. Mg in soil outside of the pipe and the dirt inside of the pipe was quantitatively analyzed and compared by using the laser-induced breakdown spectroscopy (LIBS). Firstly, Mg was quantitatively analyzed on the basis of Mg I 285.213 nm by calibration curve for integrated intensity and peak intensity of the spectrum before and after subtracting noise, respectively. Then calibration curves on the basis of Mg II 279.553 nm and Mg II 280.270 nm were analyzed. The results indicated that it is better to use integrated intensity after subtracting noise of the spectrum line with high relative intensity to make the calibration curve.
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