Microwave induced plasma optical emission spectrometry (MIP OES) and laser-induced breakdown spectroscopy (LIBS) for multi-element determination and location in ceramic tableware

Heavy Metals in Foods and Beverages: Global Situation, Health Risks and Reduction Methods

Heavy metals are chemical elements with a toxic effect on the human body. The expansion of industries has led to significant increasing levels of these constituents in the environment. Intensive agriculture can also lead to an increased concentration of heavy metals as a result of using different fertilizers and pesticides. Heavy metal accumulation in soil and plants represents a serious issue because of the potential risks to consumers. There are several methods available for the removal of these toxic components from different substrates (chemical precipitation, electrodialysis, coagulation and flocculation, photocatalytic removal, and adsorption-based processes), but most procedures are expensive and difficult to perform. Thus, more research is needed on the development of low-cost methods in foods. This work represents a review on the heavy metal presence in different food substrates (such as fruits and vegetables, milk and dairy products, meat and meat derivatives, oils, and alcoholic beverages) and provides an overview of the current situation worldwide, taking into account the fact that risks for human health are induced by the intensification of industry and the high degree of pollution. Considering that the toxicological quality of food affects its acceptability, this work provides valuable data regarding the actual situation on the proposed topic.

Schlieren imaging and spectroscopic approximation of the rotational-vibrational temperatures of a microwave discharge igniter with a resonating cavity

A microwave discharge igniter (MDI) with a resonating cavity was developed and optimized for practical applications in an internal combustion engine. In contrast to the typical microwave ignition, the resonating cavity of the MDI induces a discharge through dielectric resonance. Its source of microwave (MW) is a 2.45 GHz semiconductor oscillator that is capable of numerous oscillation patterns. To verify and demonstrate the optimum ignition performance and combustion, we varied the oscillation parameters (signal factors) of the MW to optimize the performance of the MDI using the Taguchi method. Plasma spectroscopy was used for ignition condition analysis. Two sets of microwave pulses, a first pulse followed by a second set of pulse bursts, were used to ignite a propane-air fuel. The flame kernel growth rate and O I species generation were used as the response outputs, which were obtained, respectively from Schlieren imaging and emission spectroscopy experiments. The extended pulse periods and higher MW pulse numbers of the second set of pulses improved the response outputs of the MDI. To further analyze the effect of MW oscillation patterns on plasma properties and performance, measurements were done on MW superimposed operation with the high-voltage ignition from spark plugs. The MW transmission on a typical spark plug enhanced the air plasma ignition. Higher input MW energy and more extended MW pulse widths results in increased spectral intensity and radical generation of OH, O, and N ₂. An inverse relation between the temperature and spectral intensity functions of the MW pulse width was observed, which was attributed to the cutoff density of the MW-enhanced plasma.

Atmospheric air plasma sustainment by semiconductor microwave for hydroxyl radical production and powder metal element analysis

A semiconductor microwave device that generates a series of burst microwaves at a sub-microsecond duration has been successfully used in a breakdown plasma spectrometer in atmospheric conditions. Microwave delivery has been changed to couple the microwave with laser sparks and electric sparks which are typical plasma ignition sources in laser-induced breakdown spectroscopy (LIBS) and spark-induced breakdown spectroscopy (SIBS). A helical antenna was used for the laser spark, while a coaxial antenna was considered more appropriate for the electric spark. The weak and transient sparks in LIBS and SIBS are enlarged by the microwaves which are stably sustained in the air. The microwave's output power and pulse duration are easily controllable, resulting in tunable plasma intensity and sustained production of hydroxyl radicals (OH radicals). Even in continuous-wave operation by microwave, the low-energy system prevented the formation of high-temperature thermal plasma (>10,000 K) without any mechanical cooling system. The microwave-enhanced LIBS (MW-LIBS) and microwave-enhanced SIBS (MW-SIBS) could be applied to optical emission spectroscopy analyses. In analytical applications, MW-SIBS produces no shockwave in contrast with MW-LIBS which is a great advantage in powdered samples. The MW-SIBS successfully analyzed the direct introduction of copper metal powders.

Antenna Characteristics of Helical Coil with 2.45 GHz Semiconductor Microwave for Microwave-Enhanced Laser-Induced Breakdown Spectroscopy (MW-LIBS)

A copper helical coil antenna was developed, characterized, and optimized for 2.45 GHz operations supplied by a microwave semiconductor oscillator. The application field of interest is laser-induced breakdown spectroscopy enhanced by microwave. Simulations using the Ansys HFSS demonstrate the superior localized E-field strength of the helical coil antenna, compared with other antenna-type structures. Simulation results show that E-field strength at the tip of the antenna has a logarithmic trend for increasing the coil pitch. The optimum pitch is 5 mm for a coil diameter of 6.5 mm upon consideration of the system compactness. Despite the antenna’s open-circuit end, the presence of target samples does not interfere with the E-field and H-field distribution of the antenna and the surrounding environment. Applications in microwave-enhanced laser-induced breakdown spectroscopy (MWLIBS) confirm the importance of the antenna reflector. The electric field strength was over 100 times higher than the previous capacitor-like antenna. The antenna configuration angle was then experimentally optimized for maximum enhancement effects in the spectrochemical analysis of Al₂O₃. The antenna angle of 60° from the laser beam propagation achieved maximum enhancement in the emission signal of Al I.