Feasibility of Quantitative Analysis of Magnesium and Calcium in Edible Salts Using a Simple Laser-Induced Breakdown Spectroscopy Device

Highly Precise Laser-Induced Breakdown Spectroscopy Analysis of Major Mineral Nutrients in Edible Salts Using Miniaturized Salt Ponds and Alternating Laser-Ablation Data Sampling

In this work, we applied a hydrophilicity-enhanced solid substrate and an alternating laser-ablation data sampling (ALADS) scheme to improve laser-induced breakdown spectroscopy (LIBS) measurement precision and demonstrated the performance in analyzing K, Mg, Ca, and S contained in commercially available edible salt products. Five edible salt products from Australia, Bolivia, France, and South Korea were dissolved in water and a tiny volume of each solution was dropped on the solid substrate, that is, a miniaturized salt pond. After being dried, the residual salt crystals distributed still inhomogeneously, but the homogeneity could be significantly improved in comparison with that from typical drop-and-dry methods. The ALADS scheme was applied to extract three precise measurements from 9798 single-shot LIBS spectra covering the entire salt pond. The measurements obtained by ALADS were found to agree well with one another regardless of the inhomogeneous distribution of salt crystals. As a result, the measurement precision was proved remarkably. Limits of detection for K, Mg, Ca, and S were estimated to be 0.64, 1.7, 14, and 530 mg/kg, respectively, which are enough to analyze those elements contained in salts typically at the level of 100 parts per million (ppm) to ∼3 wt% for the purpose of salt quality assessment.

Feasibility of a Low-Power, Low-Resolution Laser-Induced Breakdown Spectroscopy Instrument for Analysis of Nickel Alloys: Quantification of the Major Alloying Elements and Classification

A simple cost-effective laser-induced breakdown spectroscopy (LIBS) instrument was used for quantification of major elements in several nickel alloys and also sorting them. A compact low-power diode-pumped solid-state laser and a miniature low-resolution spectrometer were assembled for the LIBS instrument. Material properties of the nickel alloys depend mainly on the composition of the major elements, Ni, Cr, and Fe, ranging from a few to ∼60 wt%. The emission peaks at 547.7 nm, 520.4 nm, and 438.1 nm for Ni, Cr, and Fe, respectively, were chosen for this analysis. The analytical performance was found to be enough for the quantification of Ni, Cr, and Fe in the nickel alloys. Limits of detection and accuracy were estimated to be a few weight percent (wt%) and measurement precisions were less than 10% in terms of relative standard deviation. The calibration performance of this intensity-based method was compared with that of the "ratio method" which is used in conventional optical emission spectroscopy analyses. The comparison indicates that the intensity-based method is more appropriate with the low-performance LIBS instrument that detects emission peaks of only a few major elements. Also, multivariate modeling of the six different nickel alloy samples based on the emission peak intensities of Ni, Cr, and Fe was performed using k-nearest neighbors (KNN) and linear discriminant analysis (LDA). The KNN and ordinary LDA models showed 95.0% and 98.3% classification correctness for the separate test data set, respectively. To improve classification performance further, the two-step LDA model was trained. In this approach, the two closest sample classes responsible for the decrease in the classification correctness were separately modeled in the second step to exploit their difference effectively. The two-step LDA model showed 100% correctness in classifying the test objects. Our results indicate that such a low-performance LIBS instrument can be effectively utilized for quantitative analysis of the major elements in the nickel alloys and their rapid identification or sorting in combination with an appropriate multivariate modeling algorithm.

Two-Step Partial Least Squares-Discriminant Analysis Modeling for Accurate Classification of Edible Sea Salt Products Using Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been widely applied to material classification in various fields, and partial least squares-discriminant analysis (PLS-DA) is one of the frequently used classical multivariate statistics to construct classification models based on the LIBS spectra. However, classification accuracy of the PLS-DA model is sensitive to the number of classes and their similarities. Considering this characteristic of PLS-DA, we suggest a two-step PLS-DA modeling approach to improve the classification accuracy. This strategy was demonstrated for a six-class problem in which six commercial edible sea salts produced in Japan, South Korea, and France are classified using their LIBS spectra. At the first step, test spectra were sorted into four classes and one extended class, composed of the two other most confusing classes, and then the test spectra in the extended class were further classified into each of the two constituent classes which were modeled separately from the other four classes. This two-step classification has been found to remarkably improve the PLS-DA classification accuracy by maximizing the difference between the confusing classes in the second-step modeling.

Characteristics of laser induced breakdown investigated by a compact, nongated optical multichannel analyzer system and its potential application

Laser induced breakdown is a highly temporally and spatially dynamic phenomenon, normally studied using a highly temporally resolved optical detector system. In this work, a compact, low cost optical multichannel analyzer (OMA) system without a built-in temporal gating device and thus operated under a free running mode was used to investigate the characteristics of laser induced plasma. A Nd-YAG laser beam was used as the excitation source from several samples, namely, copper, zinc, and aluminum plates. The characteristics of the plasma emission produced under various experimental parameters, including the pulse energy, surrounding gas pressure, and collection fiber position, were examined. It was found that the essential features of emission spectra can be investigated even using the ungated, compact OMA system even without a highly temporally resolved gating system. The plasma emission characteristics critically depend on the experimental parameters. A quality emission spectrum, featuring a high intensity with a low background, can be obtained using the ungated, compact OMA system under optimized conditions, namely, a pulse energy of approximately 8 mJ, a surrounding gas pressure of 10 Torr, and a collection fiber position of more than 5 mm above the surface of the sample. The features of the emission spectra detected under optimized conditions are only similar to those obtained using a sophisticated, gated OMA system. The characteristics of the emission spectra are in good agreement with the previous assumption of the shockwave role in plasma excitation. Having quality emission spectra under the optimized conditions, a preliminary practical laser induced breakdown spectroscopy (LIBS) analysis using the ungated, compact OMA system was performed on several samples, such as standard brass, commercial pure gold, and natural stone samples. The aluminum emission lines are strongly detected from the standard brass sample (C1118) containing aluminum at 2.8%. The LIBS system also unequivocally revealed a qualitatively abandoned impurity presence in the purportedly pure commercial gold sample. It also effectively confirmed qualitatively a Cu presence in the blinking spots of the natural stone collected from a traditional mining site in Aceh. This result implies the effectiveness of the LIBS using the ungated, compact OMA system for quick, practical analysis.

Laser-induced breakdown spectroscopy for rapid accurate analysis of Mg, Ca, and K in edible sea salts

A compact laser-induced breakdown spectroscopy (LIBS) instrument and a simple sample preparation method were developed for rapid on-site analysis of Mg, Ca, and K in edible sea salt products. The LIBS instrument was assembled using a small diode-pumped solid-state laser and a handheld spectrometer. Aqueous solutions of salts were prepared and sampled by using pieces of filter papers. The dried filter paper was attached on the flat surface of a silicon wafer and then analyzed by LIBS. Calibration curves were obtained using binary mixtures of ${\rm NaCl} {-} {{\rm MgSO}_4}$NaCl-MgSO₄, ${\rm NaCl} {-} {{\rm CaCl}_2}$NaCl-CaCl₂, and NaCl-KCl and used to estimate the concentrations of Mg, Ca, and K in 13 edible sea salt products. Matrix effects on the results from LIBS were identified in comparison with those from inductively coupled plasma optical emission spectroscopy. This indicates that the matrix of sea salt samples is significantly different from that of the binary mixture standards. The sea salts with known concentrations of Mg, Ca, and K were employed to match the matrices of samples and standards. This improved analysis accuracy remarkably. Furthermore, an alternative indirect method for estimating the concentration of K was suggested on the basis of the strong positive correlations observed between the concentrations of Mg and K in the sea salt samples.