Analysis of Pulverized Coal by Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy （LIBS） has been used to detect atomic species in various enviromnents. The quantitative analysis （C, H, O, N and S） of representative coal samples are being carried out with LIBS, and the effects of particle size are analyzed. A powerful pulse Nd：YAG laser is focused on the coal sample at atmosphere pressure, and the emission spectra from laser-induced plasmas are measured by time-resolved spectroscopy, and the intensity of analyzed spectral lines is obtained through observing the laser plasma with a delay time of 0.4 #s. The experimental results show that the slope of calibration curve is nearly 1 when the concentration of the analyzed element is relatively low, and the slope of curve is nearly 0.5 when the concentration of C is higher than other elements. In addition, using the calibration-free model without self-absorption effect, the results show that the decreasing of particle size leads to an increase of the plasma temperature.     

Study on Cluster Analysis Used with Laser-Induced Breakdown Spectroscopy

Supervised learning methods(eg.PLS-DA,SVM,etc.) have been widely used with laser-induced breakdown spectroscopy(LIBS) to classify materials;however,it may induce a low correct classification rate if a test sample type is not included in the training dataset.Unsupervised cluster analysis methods(hierarchical clustering analysis,K-means clustering analysis,and iterative self-organizing data analysis technique) are investigated in plastics classification based on the line intensities of LIBS emission in this paper.The results of hierarchical clustering analysis using four different similarity measuring methods(single linkage,complete linkage,unweighted pair-group average,and weighted pair-group average) are compared.In K-means clustering analysis,four kinds of choosing initial centers methods are applied in our case and their results are compared.The classification results of hierarchical clustering analysis,K-means clustering analysis,and ISODATA are analyzed.The experiment results demonstrated cluster analysis methods can be applied to plastics discrimination with LIBS.     

Influence of Ambient Gas on Laser-Induced Breakdown Spectroscopy of Uranium Metal

Laser-induced breakdown spectroscopy (LIBS) is regarded as a suitable method for the remote analysis of materials in any phase, even in an environment with high radiation levels. In the present work we used the third harmonic pulse of a Nd:YAG laser for ablation of uranium metal and measured the plasma emission with a fiber-optic spectrometer. The LIBS spectra of uranium metal and their features in different ambient gases (i.e., argon, neon, oxygen, and nitrogen) at atmospheric pressure were studied. Strong continuum spectrum and several hundreds of emission lines from UI and UII were observed. It is found that the continuum spectrum observed in uranium not only comes from bremsstrahlung emission but is also due to the complex spectrum of uranium. The influence of ambient gas and the gas flow rate for ablation of uranium metal was investigated. The experimental results indicate that the intensity of the uranium lines was enhanced in argon and nitrogen. However, the intensity of uranium lines was decreased in oxygen due to the generation of UO and other oxides. The results also showed that the highest intensity of uranium lines were obtained in argon gas with a gas flow rate above 2.5 L/min. The enhanced mechanism in ambient gas and the influence of the gas flow rate were analyzed in this work.     

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

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 calibration 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 process 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.     

Rapid Detection of Oil Pollution in Soil by Using Laser-Induced Breakdown Spectroscopy

Detection of oil pollution in soil has been carried out using laser-induced breakdown spectroscopy(LIBS). A pulsed neodymium-doped yttrium aluminum garnet(Nd:YAG) laser(1,064 nm, 8 ns, 200 mJ) was focused onto pelletized soil samples. Emission spectra were obtained from oil-contaminated soil and clean soil. The contaminated soil had almost the same spectrum profile as the clean soil and contained the same major and minor elements. However, a C–H molecular band was clearly detected in the oil-contaminated soil, while no C–H band was detected in the clean soil. Linear calibration curve of the C–H molecular band was successfully made by using a soil sample containing various concentrations of oil. The limit of detection of the C–H band in the soil sample was 0.001 mL/g. Furthermore, the emission spectrum of the contaminated soil clearly displayed titanium(Ti) lines, which were not detected in the clean soil. The existence of the C–H band and Ti lines in oil-contaminated soil can be used to clearly distinguish contaminated soil from clean soil. For comparison, the emission spectra of contaminated and clean soil were also obtained using scanning electron microscope-energy dispersive X-ray(SEM/EDX) spectroscopy,showing that the spectra obtained using LIBS are much better than using SEM/EDX, as indicated by the signal to noise ratio(S/N ratio).     

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

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.     

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

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 MgⅡ 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.     

Feasibility of Trace Alcohol Congener Detection and Identification Using Laser-Induced Breakdown Spectroscopy

In this paper, a feasible scheme is reported for the detection and identification of trace alcohol congeners that have identical elemental composition using laser-induced breakdown spectroscopy （LIBS）. In the scheme, an intensive pulsed laser is used to break down trace alcohol samples and the optical emission spectra of the induced plasma are collected for the detection and identifq ication of alcohol molecules. In order to prepare trace alcohol samples, pure ethanol or methanol is bubbled by argon carrier gas and then mixed into matrix gases. The key issue for the scheme is to constitute indices from the LIBS data of the alcohol samples. Two indices are found to be suitable for alcohol detection and identification. One is the emission intensity ratio （denoted as H/C） of the hydrogen line （653.3 nm） to the carbon line （247.9 nm） for identification and the other is the ratio of the carbon line （as C/Ar） or the hydrogen line （as H/Ar） to the argon lines （866.7 nm） for quantitative detection. The calibration experiment result shows that the index H/C is specific for alcohol congeners while almost being independent of alcohol concentration. In detail, the H/C keeps a specific constant of 34 and 23 respectively for ethanol and methanol. In the meanwhile, the C/Ar and H/Ar indices respond almost linearly to the alcohol concentration below 1300 ppm, and are therefore competent for concentration measurement. With the indices, trace alcohol concentration measurement achieves a limit of 140 ppm using a laser pulse energy of 300 mJ.     

Determination of Iron in Water Solution by Time-Resolved Femtosecond Laser-Induced Breakdown Spectroscopy

The influence of the energy of femtosecond laser pulses on the intensity of Fe I (371.99 nm) emission line and the continuous spectrum of the plasma generated on the surface of Fe3+ water solution by a Ti: sapphire laser radiation with pulse duration <45 fs and energies up to 7 mJ is determined. A calibration curve was obtained for Fe3+ concentration range from 0.5 g/L to the limit of detection in water solution, and its saturation was detected for concentrations above 0.25 g/L, which is ascribed to self-absorption. The 3σ-limit of detection obtained for Fe in water solution is 2.6 mg/L in the case of 7 mJ laser pulse energy. It is found that an increase of laser pulse energy insignificantly affects on LOD in the time-resolved LIBS and leads to a slight improvement of the limit of detection.     

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

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 significant.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.     

A Hydrogel's Formation Device for Quick Analysis of Liquid Samples Using Laser-Induced Breakdown Spectroscopy

The laser-induced breakdown spectroscopy technique has irreplaceable advantages in the field of detection due to its multi-phase specimen detection ability.The development of the LIBS technique for liquid analysis is obstructed by its inherent drawbacks like the surface ripples and extinction of emitted intensity,which make it unpractical.In this work,an in-situ hydrogel formation sampling device was designed and used the hydrogel as the detection phase of LIBS for Cu,Cr and Al in an aqueous solution.With the measured amount of resin placed in the device,the formed hydrogel could be obtained within 20 s after putting the device into water solution.The formed hydrogel could be directly analyzed by LIBS and reflect the elemental information of the water sample.The prominent performance made this hydrogel's formation device especially suitable for quick in-situ environmental liquid analysis using LIBS.     

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

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.     

Effect of Melting Iron-Based Alloy Temperature on Carbon Content Observed in Laser-Induced Breakdown Spectroscopy

Our recent work has determined the carbon content in a melting ferroalloy by laser-induced breakdown spectroscopy (LIBS). The emission spectrum of carbon that we obtained in the laboratory is suitable for carbon content determination in a melting ferroalloy but we cannot get the expected results when this method is applied in industrial conditions: there is always an unacceptable error of around 4% between the actual value and the measured value. By comparing the measurement condition in the industrial condition with that in the laboratory, the results show that the temperature of the molten ferroalloy samples to be measured is constant under laboratory conditions while it decreases gradually under industrial conditions. However, temperature has a considerable impact on the measurement of carbon content, and this is the reason why there is always an error between the actual value and the measured value. In this paper we compare the errors of carbon content determination at different temperatures to find the optimum reference temperature range which can fit the requirements better in industrial conditions and, hence, make the measurement more accurate. The results of the comparative analyses show that the measured value of the carbon content in molten state (1620 K) is consistent with the nominal value of the solid standard sample (error within 0.7%). In fact, it is the most accurate measurement in the solid state. Based on this, we can effectively improve the accuracy of measurements in laboratory and can provide a reference standard of temperature for the measurement in industrial conditions.     

Selection of Spectral Data for Classification of Steels Using Laser-Induced Breakdown Spectroscopy

Principal component analysis (PCA) combined with artificial neural networks was used to classify the spectra of 27 steel samples acquired using laser-induced breakdown spec?troscopy. Three methods of spectral data selection, selecting all the peak lines of the spectra, selecting intensive spectral partitions and the whole spectra, were utilized to compare the influ?ence of different inputs of PCA on the classification of steels. Three intensive partitions were selected based on experience and prior knowledge to compare the classification, as the partitions can obtain the best results compared to all peak lines and the whole spectra. We also used two test data sets, mean spectra after being averaged and raw spectra without any pretreatment, to verify the results of the classification. The results of this comprehensive comparison show that a back propagation network trained using the principal components of appropriate, carefully se?lected spectral partitions can obtain the best results. A perfect result with 100% classification accuracy can be achieved using the intensive spectral partitions ranging of 357-367 nm.     

Development of a Laboratory Cement Quality Analysis Apparatus Based on Laser-Induced Breakdown Spectroscopy

Determination of the chemical composition of cement and ratio values of clinker plays an important role in cement plants as part of the optimal process control and product quality evaluation. In the present paper, a laboratory laser-induced breakdown spectroscopy (LIBS) apparatus mainly comprising a sealed optical module and an analysis chamber has been designed for possible application in cement plants for on-site quality analysis of cement. Emphasis is placed on the structure and operation of the LIBS apparatus, the sealed optical path, the temperature controlled spectrometer, the sample holder, the proper calibration model established for minimizing the matrix effects, and a correction method proposed for overcoming the ‘drift’ obstacle. Good agreement has been found between the laboratory measurement results from the LIBS method and those from the traditional method. The absolute measurement errors presented here for oxides analysis are within 0.5%, while those of ratio values are in the range of 0.02 to 0.05. According to the obtained results, this laboratory LIBS apparatus is capable of performing reliable and accurate, composition and proximate analysis of cement and is suitable for application in cement plants.     

气体采样滤膜中铀含量的激光诱导击穿光谱分析方法研究

为对核设施尾气监测中气体采样滤膜中铀元素的含量进行测量分析,本文开展了激光诱导击穿光谱（LIBS）定量分析气体采样滤膜中铀含量的方法研究。通过优化LIBS测量延迟时间参数,最终选出了铀的定量分析有效特征谱线,并得到了浓度范围较低的工作曲线。结果表明,在选择的实验条件下,采用标准样品对筛选后的特征谱线进行实验验证,测量值的相对偏差绝对值小于20%,RSD小于10%。     

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

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 classification of rocks and soils,and is thus useful for the detection of geological materials.     

Parameters Optimization of Laser-Induced Breakdown Spectroscopy Experimental Setup for the Case with Beam Expander

Improvement of measurement precision and repeatability is one of the issues cur?rently faced by the laser-induced breakdown spectroscopy (LIBS) technique, which is expected to be capable of precise and accurate quantitative analysis. It was found that there was great poten?tial to improve the signal quality and repeatability by reducing the laser beam divergence angle using a suitable beam expander (BE). In the present work, the influences of several experimental parameters for the case with BE are studied in order to optimize the analytical performances: the signal to noise ratio (SNR) and the relative standard deviation (RSD). We demonstrate that by selecting the optimal experimental parameters, the BE-included LIBS setup can give higher SNR and lower RSD values of the line intensity normalized by the whole spectrum area. For validation purposes, support vector machine (SVM) regression combined with principal component analysis (PCA) was used to establish a calibration model to realize the quantitative analysis of the ash content. Good agreement has been found between the laboratory measurement results from the LIBS method and those from the traditional method. The measurement accuracy presented here for ash content analysis is estimated to be 0.31%, while the average relative error is 2.36%.     

气体采样滤膜中钚的激光诱导击穿光谱定量分析研究

在核设施排放物的监测中,为确保环境安全及实现核材料衡算,需针对通风管气体采样滤膜中钚的含量进行分析。为探索利用激光诱导击穿光谱（LIBS）技术定量分析滤膜中钚元素含量的可行性,开展了某核设施热室内含钚气体采样滤膜的LIBS分析研究。通过分析含钚气体采样滤膜中钚的LIBS特征光谱信号,筛选出钚的有效发射谱线,并优化LIBS测量延迟时间参数,最终得到了浓度范围较低情况下的定量标准曲线,并进行了技术验证分析。研究结果显示,针对滤膜中的钚元素可采用有效特征谱线进行LIBS定量分析,能用于定量分析钚的可分析有效特征谱线为PuⅡ 443.298 nm、PuⅠ 460.721 nm、PuⅡ 466.389 nm。分析结果表明,LIBS分析结果的精度好于3%。     

Quantitative Analysis of Carbon Content in Bituminous Coal by Laser-Induced Breakdown Spectroscopy Using UV Laser Radiation

The carbon content of bituminous coal samples was analyzed by laser-induced break?down spectroscopy. The 266 nm laser radiation was utilized for laser ablation and plasma gener?ation in air. The partial least square method and the dominant factor based PLS method were used to improve the measurement accuracy of the carbon content of coal. The results showed that the PLS model could achieve good measurement accuracy, and the dominant factor based PLS model could further improve the measurement accuracy. The coefficient of determination and the root-mean-square error of prediction of the PLS model were 0.97 and 2.19%, respectively; and those values for the dominant factor based PLS model were 0.99 and 1.51%, respectively. The results demonstrated that the 266 nm wavelength could accurately measure the carbon content of bituminous coal.