Maxwell-Wagner Effect Applied to Microwave-Induced Self-Ignition: A Novel Approach for Carbon-Based Materials.

A new method for analytical applications based on the Maxwell-Wagner effect is proposed. Considering the interaction of carbonaceous materials with an electromagnetic field in the microwave frequency range, a very fast heating is observed due to interfacial polarization that results in localized microplasma formation. Such effect was evaluated in this work using a monomode microwave system, and temperature was recorded using an infrared camera. For analytical applications, a closed reactor under oxygen pressure was evaluated. The combination of high temperature and oxidant atmosphere resulted in a very effective self-ignition reaction of sample, allowing its use as sample preparation procedure for further elemental analysis. After optimization, a high sample mass (up to 600 mg of coal and graphite) was efficiently digested using only 4 mol L-1 HNO3 as absorbing solution. Several elements (Ba, Ca, Fe, K, Li, Mg, Na, and Zn) were determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Accuracy was evaluated by using a certified reference material (NIST 1632b). Blanks were negligible, and only a diluted solution was required for analytes absorption preventing residue generation and making the proposed method in agreement with green chemistry recommendations. The feasibility of the proposed method for hard-to-digest materials, the minimization of reagent consumption, and the possibility of multi elemental analysis with lower blanks and better limits of detection can be considered as the main advantages of this method.

Microwave-induced combustion: Thermal and morphological aspects for understanding the mechanism of ignition process for analytical applications.

In the present work, for the first time a systematic study was performed using an infrared camera and scanning electron microscopy (SEM) coupled to energy dispersive X-ray spectrometry (EDS) to evaluate the mechanisms involved in microwave-induced combustion method, which has been extensively used for sample preparation. Cellulose and glass fiber discs, wetted with the igniter solution (6molL-1 NH4NO3), were evaluated under microwave field in a monomode system. The temperature of the discs surface was recorded during microwave irradiation and the effect of NH4NO3 concentration and irradiation time on cellulose oxidation was evaluated. The morphology of the discs surface was characterized by SEM before and after irradiation in an inert atmosphere. According to the results, the surface temperature of the discs increased near to 100°C and remained in this temperature for few seconds while water evaporate. After that, temperature increased over 200°C due to the thermal decomposition of NH4NO3 salt, releasing a large amount of energy that accelerates cellulose oxidation. The higher the igniter concentration, the shorter was the microwave irradiation time for cellulose oxidation. The SEM images revealed that cellulose disc was more porous after microwave irradiation, enhancing oxygen diffusion within the paper and making easier its ignition. The EDS spectrum of cellulose and glass fiber discs showed that signal intensity for nitrogen decreased after microwave irradiation, showing that NH4NO3 was consumed during this process. Therefore, it was demonstrated that the ignition process is the result of synergic interaction of NH4NO3 thermal decomposition and organic matter oxidation (cellulose) releasing heat and feeding the chain reaction.

Microwave-induced combustion coupled to flame furnace atomic absorption spectrometry for determination of cadmium and lead in botanical samples.

A procedure based on microwave-induced combustion coupled to flame furnace (FF) atomic absorption spectrometry (FF-AAS) was used for analysis of solid samples. Botanical samples were prepared as pellets and introduced into a quartz holder device. This device was fitted to a glass chamber that was used for the combustion step. The complete device was coupled to the flame furnace by using poly(tetrafluoroethylene) and quartz tubes. The glass chamber was placed inside a microwave oven in a position previously set to receive the higher power of microwave radiation. Ignition was performed by microwave radiation using a small piece of paper wetted with NH4NO3 solution. An oxygen flow was used to assist the sample combustion and also to transport the combustion products up to the heated FF positioned above an air/ acetylene burner. Flame furnace temperature, oxygen flow rate, flame stoichiometry, and sample mass range were evaluated. Cadmium and lead were determined in botanical samples as examples to demonstrate the potential of the proposed procedure for trace analysis. Sample masses up to 60 mg could be used, allowing a limit of detection as low as 0.003 and 0.24 microg g(-1) for Cd and Pb, respectively. Integrated absorbance was used with an integration time of 30 s. Background signals were always low, and relative standard deviation (n = 5) was below 9% for Cd and 11% for Pb. The throughput was 20 determinations/h, including the weighing step. Accuracy was between 94 and 105%, and calibration was performed using standard solutions. The combustion device could be easily adapted to conventional atomic absorption spectrometers.