Guanghui Niu

Combined Laser-Induced Breakdown with Raman Spectroscopy: Historical Technology Development and Recent Applications

Abstract: A modern trend in the development of laser-based analytical instruments is to integrate more functions in a single device, which could supply complementary information and improve the capability to identify different components. This review presents historical development of the combined analysis by laser-induced breakdown spectroscopy (LIBS) and Raman Spectroscopy, which briefly describes basic instrumental principles and technological aspects of the hyphenated technique, as well as the requirements in the design of interactive instruments, including laser systems, optical systems for laser beam delivery, emission collection devices, and spectrum measurement systems. Various configurations of the combined instrumentation have been designed and used depending on the different applications, such as pigment analysis in the cultural heritage field, detection of explosives, analysis of geological samples, and applications in future planetary missions. In addition, the feasibility, applicability, and prospective aspects of the hybrid technology are discussed in detail.

Laser-induced breakdown spectroscopy for solution sample analysis using porous electrospun ultrafine fibers as a solid-phase support

The major application of the laser-induced breakdown spectroscopy (LIBS) technique had been in the analysis of solid samples because the measurement of LIBS for liquid samples experiences some experimental difficulties, such as splashing, a quenching effect, and a shorter plasma lifetime. In the present work, electrospun ultrafine fibers were explored and used for the first time as a solid-phase support to quantify chromium (Cr) and copper (Cu) in aqueous solutions by LIBS. The liquid sample was first transferred to an ultrafine fiber surface, which could minimize the drawbacks of liquid sample analysis with LIBS. Due to the special micro-porous structure, the electrospun ultrafine fibers could hold a larger liquid sample and also the liquid sample was easy to evaporate. On the other hand, as a polymer substrate, the porous electrospun ultrafine fibers contributed to the minimal blank since there was no other unwanted heavy metal matrix that affected the detection during the liquid LIBS analysis. Meanwhile, the large sampling spot to fiber diameter ratio will minimize the potential influence generated in the liquid sample distribution process. With this pre-treated sample technique, the sensitivities of LIBS for liquid samples are improved considerably and the detection limits for Cr and Cu reached 1.8 ppm and 1.9 ppm, respectively. Therefore, the present strategy definitely paves the way for a wider application of LIBS in liquid sample analysis.

Classification of iron ores by laser-induced breakdown spectroscopy (LIBS) combined with random forest (RF)

Laser-induced breakdown spectroscopy (LIBS) integrated with random forest (RF) was developed and applied to the identification and discrimination of ten iron ore grades. The classification and recognition of the iron ore grade were completed using their chemical properties and compositions. In addition, two parameters of the RF were optimized using out-of-bag (OOB) estimation. Finally, support vector machines (SVMs) and RF machine learning methods were evaluated comparatively on their ability to predict unknown iron ore samples using models constructed from a predetermined training set. Although results show that the prediction accuracies of SVM and RF models were acceptable, RF exhibited better predictions of classification. The study presented here demonstrates that LIBS–RF is a useful technique for the identification and discrimination of iron ore samples, and is promising for automatic real-time, fast, reliable, and robust measurements.

Quantitative analysis of sedimentary rocks using laser-induced breakdown spectroscopy: comparison of support vector regression and partial least squares regression chemometric methods

Laser Induced Breakdown Spectroscopy (LIBS) is attracting more and more attention in geology fields because of its unique advantages of on-line and in situ analysis and the availability of portable even handheld instruments due to the development of laser sources and mini-spectrometers. However, parameters such as accuracy and precision of the instrument are still essential for field application. In this paper, two algorithms to determine the concentrations of five main elements (Si, Ca, Mg, Fe and Al) in sedimentary rock samples are proposed based on support vector regression (SVR) and partial least squares regression (PLSR). The proposed comparison demonstrates that the SVR model performed better with more satisfactory accuracy and precision under the optimized conditions. For SVR quantitative analysis, the spectral features (20 lines) without principal component analysis (PCA) were selected as the input variables. The optimized penalty parameter C and the key parameter of the radial basis function (RBF)-σ obtained by genetic algorithm (GA) were 4.63 and 0.9159, respectively. Also, the best number of the principal components of PLSR was optimized to be 8 by 10-fold cross-validation (CV) testing. Furthermore, the accuracy corresponding to the average relative standard deviations (RSDs) and the precision related to the root mean square error (RMSE) were calculated according to the performance of the two regression models. A significant enhancement of accuracy, of up to 43.50 times, and of precision, of 7.19 times, for the SVR model was obtained, which can eliminate the self-absorption of plasma efficiently compared with the linear machine learning method PLSR. In conclusion, the chemometric method of SVR with better accuracy and precision can be successfully applied for the quantitative analysis of complex geological samples using the LIBS technique.

Exploration of a 3D nano-channel porous membrane material combined with laser-induced breakdown spectrometry for fast and sensitive heavy metal detection of solution samples

In this paper, a nano-channel material was combined with laser induced breakdown spectroscopy (LIBS) to achieve sensitive and quick detection of metal ions in liquid samples. A 3D anodic aluminum oxide porous membrane (AAOPM) was selected as a novel substrate for the first time, which showed excellent potential for liquid analysis. It is worth mentioning that the LIBS signal of the target elements in aqueous solution dropped on the 3D AAOPM was increased by up to 19 times in comparison with that on the tablet sample made of aluminium oxide powder. The attractive results are mainly attributed to the peculiar structure of the 3D AAOPM. Firstly, an abundant strong coordination metal–oxygen bond between hydroxyl groups and metal ions existed on the surface of the novel substrate. Secondly, the extremely high aspect ratio of the 3D AAOPM could supply a much larger contact area between the matrix and analytes. Thirdly, the special nano-channel distribution could make efficient coupling of a laser beam with the materials. Finally, the sample pervasion and volatilization could be finished within a very short time because of the micrometer level thickness and porosity of the 3D AAOPM. The calibration curves with linearity ranges (1–100 μg mL−1) and good linearity (R squared better than 0.983 for all of the four target elements) were established, and the limits of detection (LODs) obtained were 0.18 μg mL−1, 0.12 μg mL−1, 0.081 μg mL−1, and 0.11 μg mL−1 for Cu2+, Ag+, Pb2+, and Cr3+, respectively. In real sample analyses, the recoveries of three elements at different concentration levels were all in the range of 92.5–107.4%, with the relative standard deviations of parallel samples around 10.0%. This novel method showed a fast, simple and super sensitive monitoring tool for liquid sample analysis compared with the traditional LIBS method.

Characterization of local thermodynamic equilibrium in a laser-induced aluminum alloy plasma.

The electron temperature was evaluated using the line-to-continuum ratio method, and whether the plasma was close to the local thermodynamic equilibrium (LTE) state was investigated in detail. The results showed that approximately 5 μs after the plasma formed, the changes in the electron and excitation temperatures, which were determined using a Boltzmann plot, overlapped in the 15% error range, which indicated that the LTE state was reached. The recombination of electrons and ions and the free electron expansion process led to the deviation from the LTE state. The plasmas expansion rate slowed over time, and when the expansion time was close to the ionization equilibrium time, the LTE state was almost reached. The McWhirter criterion was adopted to calculate the threshold electron density for different species, and the results showed that experimental electron density was greater than the threshold electron density, which meant that the LTE state may have existed. However, for the nonmetal element N, the threshold electron density was greater than the value experimental value approximately 0.8 μs after the plasma formed, which meant that LTE state did not exist for N.

Laser-induced breakdown spectroscopy technique for quantitative analysis of aqueous solution using matrix conversion based on plant fiber spunlaced nonwovens.

In the present work, laser-induced breakdown spectroscopy (LIBS) was applied to detect concentrations of chromium and nickel in aqueous solution in the form of matrix conversion using plant fiber spunlaced nonwovens as a solid-phase support, which can effectively avoid the inherent difficulties such as splashing, a quenching effect, and a shorter plasma lifetime during the liquid LIBS analysis. Drops of the sample solution were transferred to the plant fiber spunlaced nonwovens surface and uniformly diffused from the center to the whole area of the substrate. Owing to good hydrophilicity, the plant fiber spunlaced nonwovens can hold more of the liquid sample, and the surface of this material never wrinkles after being dried in a drying oven, which can effectively reduce the deviation during the LIBS analysis. In addition, the plant fiber spunlaced nonwovens used in the present work are relatively convenient and low cost. Also, the procedure of analysis was simple and fast, which are the unique features of LIBS technology. Therefore, this method has potential applications for practical and in situ analyses. To achieve sensitive elemental detection, the optimal delay time in this experiment was investigated. Under the optimized condition, the limits of detection for Cr and Ni are 0.7 and 5.7  μg·mL(-1), respectively. The results obtained in the present study show that the matrix conversion method is a feasible option for analyzing heavy metals in aqueous solutions by LIBS technology.

Dehydrated Carbon Coupled with Laser-Induced Breakdown Spectrometry (LIBS) for the Determination of Heavy Metals in Solutions.

In this article, a novel and alternative method of laser-induced breakdown spectroscopy (LIBS) analysis for liquid sample is proposed, which involves the removal of metal ions from a liquid to a solid substrate using a cost-efficient adsorbent, dehydrated carbon, obtained using a dehydration reaction. Using this new technique, researchers can detect trace metal ions in solutions qualitatively and quantitatively, and the drawbacks of performing liquid analysis using LIBS can be avoided because the analysis is performed on a solid surface. To achieve better performance using this technique, we considered parameters potentially influencing both adsorption performance and LIBS analysis. The calibration curves were evaluated, and the limits of detection obtained for Cu2+, Pb2+, and Cr3+ were 0.77, 0.065, and 0.46 mg/L, respectively, which are better than those in the previous studies. In addition, compared to other absorbents, the adsorbent used in this technique is much cheaper in cost, easier to obtain, and has fewer or no other elements other than C, H, and O that could result in spectral interference during analysis. We also used the recommended method to analyze spiked samples, obtaining satisfactory results. Thus, this new technique is helpful and promising for use in wastewater analysis and management.

Design and evaluation of a new bench-top instrument for laser-induced breakdown spectroscopy

ABSTRACT A newly designed and developed bench-top instrument is reported for laser-induced breakdown spectroscopy offering compact size and relatively low cost. A miniature laser was designed in the laboratory with a small volume and a light weight. The spectrometer was controlled using laboratory-written software. The instrument is suitable for the direct and rapid analysis of solid samples after simple pretreatment. Good stability and accuracy were achieved for quantitative analysis. The performance of the instrument was evaluated by qualitative and quantitative analyses of nickel ore for Mn, Al, Fe, Cr, Zn, Mg, Si, and Ca. The quantitative analysis showed linear calibration graphs and small relative standard deviations. The results show that the new instrument is suitable for elemental analysis.

Combination of support vector regression (SVR) and microwave plasma atomic emission spectrometry (MWP-AES) for quantitative elemental analysis in solid samples using the continuous direct solid sampling (CDSS) technique

Sample introduction has always been a significant issue in the research of plasma emission spectrometry. In this paper, the continuous direct solid sampling (CDSS) technique, a novel and alternative technique for direct solid analysis based on microwave plasma atomic emission spectrometry (MWP-AES), was firstly proposed. By allowing the plasma column to directly interact with the solid sample surface, the sample was heated and melted, and the elements in the sample were atomized and excited continuously. With the help of multivariate analysis, elements including Corg, Cu, Pb and Cr in geological samples were qualitatively and quantitatively determined. To construct a robust and accurate support vector regression (SVR) model, spectral bands consisting of characteristic lines and adjacent noise zones were selected as input variables. The penalty parameter C and the key parameter of the radial basis function (RBF) were optimized to be 16.0453 and 2.9008, respectively. The optimized model showed satisfactory results with a linearity correlation coefficient R2 better than 0.99 and RSD lower than 11.08% for all target elements. The proposed CDSS technique can be used to successfully conduct direct solid analysis, avoiding the use of chemical reagents, which makes it green and environmentally friendly. The total analysis time can cost less than half a minute, indicating that the proposed method can be promising in high throughput and rapid analysis. What is more, the CDSS technique combines sampling, atomization and excitation together, which eliminates the use of an electrothermal vaporization or laser ablation unit when analyzing solid samples directly using MWP-AES. The proposed method opens a promising and alternative door for plasma spectrometry especially for MWP-AES due to the advantages of continuous direct solid analysis, rapid analysis speed, no or minimal sample pretreatment, and a simplified analytical system, which is suitable for in situ and field analysis.