A Simple Laser-Induced Breakdown Spectroscopy Method for Quantification and Classification of Edible Sea Salts Assisted by Surface-Hydrophilicity-Enhanced Silicon Wafer Substrates

Salt, one of the most commonly consumed food additives worldwide, is produced in many countries. The chemical composition of edible salts is essential information for quality assessment and origin distinction. In this work, a simple laser-induced breakdown spectroscopy instrument was assembled with a diode-pumped solid-state laser and a miniature spectrometer. Its performances in analyzing Mg and Ca in six popular edible sea salts consumed in South Korea and classification of the products were investigated. Each salt was dissolved in water and a tiny amount of the solution was dropped and dried on the hydrophilicity-enhanced silicon wafer substrate, providing homogeneous distribution of salt crystals. Strong Mg II and Ca II emissions were chosen for both quantification and classification. Calibration curves could be constructed with limits-of-detection of 87 mg/kg for Mg and 45 mg/kg for Ca. Also, the Mg II and Ca II emission peak intensities were used in a k-nearest neighbors model providing 98.6% classification accuracy. In both quantification and classification, intensity normalization using a Na I emission line as a reference signal was effective. A concept of interclass distance was introduced, and the increase in the classification accuracy due to the intensity normalization was rationalized based on it. Our methodology will be useful for analyzing major mineral nutrients in various food materials in liquid phase or soluble in water, including salts.

Acoustic shock waves emitted from two interacting laser generated plasmas in air

We present an acoustic detection technique to study the interaction of two shock waves emitted by two nearby, simultaneous, laser-induced air-breakdown events that resembles the phenomenon of interaction of fluids. A microphone is employed to detect the acoustic shock wave (ASW) from the interaction zone. The experiments were done at various separation distances between the two plasma sources. The incident laser energy of the sources is varied from 25 to 100 mJ in ratios from 1:1 to 1:4. The peak sound pressure of the ASW was compared between the single and dual plasma sources, showing that the pressures are higher for the dual plasma source than that of the single plasma. The evolution of peak sound pressures is observed to depend on (a) the pulse energy of the sources and (b) the plasma separation distance, d. For the equal energy sources, the peak sound pressures increased linearly up to a certain plasma separation distance d, beyond which the pressures saturated and decayed. For the case of unequal energy sources, the peak sound pressures showed an interesting response of increase, saturation, decay, and further increase with plasma separation distance d. These observations indicate the dynamics of acoustic wave interactions across the interaction zone of the two sources depend on the input laser pulse energy as well as the plasma separation distance d.