Microwave-induced combustion for petroleum coke digestion: a promising sample preparation strategy for subsequent elemental determination

The increasing demand for the elemental determination of petrochemical samples by inductively coupled plasma techniques requires the development of suitable sample preparation methods that permit high sample throughput and are in accordance with green chemistry recommendations. For this aim, the development of microwave-induced combustion (MIC) method has been attempted to completely digest petroleum coke samples and address quantitative determination of elements, such as Ag, Al, Ba, Ca, Cd, Co, Cu, Fe, Mg, Mn, Mo, Na, Ni, Pb, Sr, V and Zn, by inductively coupled plasma optical emission spectrometry (ICP OES). A sequential procedure to evaluate the sample mass and the solution to retain the analytes quantitatively was first conducted. Satisfactory results were obtained (residual carbon content in final solutions was lower than 0.5%) by digesting 400 mg of samples using 20 bar of oxygen and analytes were quantitatively retained in a dilute acid solution such as 4.2 mol L-1 HNO3. An agreement better than 87% was achieved using certified reference materials and the performance of the proposed method was also compared to that of the standard procedure recommended by the American Society of Testing and Materials ASTM D 5600. Despite both methods producing similar results, the proposed method by MIC was much simpler, offered a higher sample throughput and the digests were compatible with ICP OES measurements avoiding interferences in the determination step compared the ASTM D 5600 method. These results clearly demonstrated that the proposed MIC method in combination with ICP OES could be a promising alternative to obtain quantitative information about the elemental composition of petroleum coke.