Laser-Ablation Sampling for Accurate Analysis of Sulfur in Edible Salts

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.

Sulfur determination in laser-induced breakdown spectroscopy combined with resonance Raman scattering

Sulfur is an essential element in industry, but it is difficult to be detected by laser-induced breakdown spectroscopy (LIBS). In this work, the disulfide radical Raman scattering was observed in sulfur plasma by combining LIBS with resonance Raman scattering (LIBS-RRS). Sulfur has been ablated by a focused laser beam to generate plasma, in which some sulfur atoms were combined to form disulfide radicals. The disulfide radical resonance Raman was excited by a 306.4 nm wavelength laser and observed at 710 and 1420 cm^-1 Raman shift. Using different contents of sulfur mixed with alumina (Al₂O₃) powder, both LIBS and LIBS-RRS calibrations were obtained at the same ablation laser energy. The calibration curve of sulfur atomic emission S I 921.28 nm was set up, and the linear coefficient (R²) was 0.285 and the detection limit (LoD) was 13.092 wt %. While the R² was 0.966 and LoD was 0.118 wt % for S₂ 710 cm^-1 in LIBS-RRS. The results indicate that disulfide radical Raman scattering by LIBS-RRS is promising for the determination of sulfur content and the diagnosis of molecular evolution in plasma.

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.

Laser-induced breakdown spectroscopy (LIBS): a novel technology for identifying microbes causing infectious diseases

With the advent of improved experimental techniques and enhanced precision, laser-induced breakdown spectroscopy (LIBS) offers a robust tool for probing the chemical constituents of samples of interest in biological sciences. As the interest continues to grow rapidly, the domain of study encompasses a variety of applications vis-à-vis biological species and microbes. LIBS is basically an atomic emission spectroscopy of plasma produced by the high-power pulsed laser which is tightly focused on the surface of any kinds of target materials in any phase. Due to its experimental simplicity, and versatility, LIBS has achieved its high degree of interest particularly in the fields of agricultural science, environmental science, medical science, forensic sciences, and biology. It has become a strong and sensitive elemental analysis tool as compared to the traditional gold standard techniques. As such, it offers a handy, rapid, and flexible elemental measurement of the sample compositions, together with the added benefits of less cumbersome sample preparation requirements. This technique has extensively been used to detect various microorganisms, extending the horizon from bacteria, molds, to yeasts, and spores on surfaces, while also being successful in sensing disease-causing viruses. LIBS-based probe has also enabled successful detection of bacteria in agriculture as well. In order for good quality processing of food, LIBS is also being used to detect and identify bacteria such as Salmonella enteric serovar typhimurium that causes food contamination. Differences in soil bacteria isolated from different mining sites are a very good indicator of relative environmental soil quality. In this connection, LIBS has effectively been employed to discriminate both the inter- and intra-site differences of the soil quality across varying mining sites. Therefore, this article summarizes the basic theory and use of LIBS for identifying microbes causing serious agricultural and environmental infectious diseases.