Application of a microwave oven for drying and nitric acid extraction of mercury and selenium from fish tissue

The drying and nitric acid digestion of fish tissue using a microwave oven for the determination of Hg and Se were studied. For the drying process, the method was compared with the freeze-drying method. The water content found in the muscle of tilapia, Oreochromis mossanicus, by using a freeze dryer at -40°C and a pressure of ≤10 μmHg for 24 h, and a microwave oven (convection mode) at 70°C for 3 h was 78.16 and 78.09% m/m, respectively. The Hg and Se levels found in fish tissues dried by the two methods were consistent with a 95% confidence level. The efficiency of dissolution of Hg and Se from dried muscle of tilapia, using concentrated HNO₃ in the closed vessel microwave digéstion method was found to be much greater than that of traditional open vessel digestion. The microwave heating digestion method was tested satisfactorily using two certified reference materials. The recovery of added standards of Hg and Se was found to be 84.8 and 96.6%, respectively.