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

Bizzi, Cezar A., Cruz, Sandra M., Schmidt, Lucas, Burrow, Robert A., Barin, Juliano S., Paniz, Jose N. G., Flores, Erico M. M.
Analytical chemistry 2018 v.90 no.7 pp. 4363-4369
absorption, atomic absorption spectrometry, barium, calcium, cameras, coal, detection limit, electromagnetic field, graphene, green chemistry, heat, iron, lithium, magnesium, microwave treatment, nitric acid, oxidants, oxygen, potassium, sodium, temperature, zinc
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.