New technology for in situ visualization, monitoring and controlling microwave chemical reaction progress using a focused microwave high pressure–temperature closed-vessel digestion system

An experimental model has been developed for studying microwave energy transfer to a sample/solution placed in a high-pressure closed-vessel focused microwave-heated digestion system. The absorption of microwave radiation with a frequency of 2.45 GHz in sample solutions was studied as a function of the efficiency of sample decomposition by measuring the incident and reflected power, temperature and pressure. Experimental measurements of incident/reflected power were carried out with a microwave detector for one type of reagent classically used in wet digestion techniques, namely nitric acid. HNO3 had been chosen as a digesting reagent to reduce the complexity of the observations to a single power-absorbing component. Results obtained by examining pressure, temperature, incident and reflected power graphs were compared with treatment efficiencies determined by measuring total organic carbon variation, arsenobetaine and arsenic content as well as trace element (Cu, Fe, Mn, Zn) recovery in the digests. The reflected power of a sample after microwave-assisted digestion was successfully used as an indicator to evaluate the digestion efficiency. Methodology was developed using the powdered biological certified reference material (DORM-2).