Combustão iniciada por microondas em sistema fechado para a decomposição de amostras biológicas

Abstract

A procedure for sample decomposition was proposed in this study based on sample combustion assisted by microwave radiation. Combustion is started by microwave radiation in the presence of oxygen under pressure using ammonium nitrate as an aid for ignition. The system was adapted to commercial sample preparation quartz vessels. Three models of quartz holders were evaluated, and the chosen one was suspended at the top of the vessel, being used simultaneously as a sample holder and to protect the polytetrafluorethylene cap of the vessel from the flame generated in the combustion process. Sample was pressed into a pellet and placed on a disc paper in the holder and a solution of ammonium nitrate was added in order to start the combustion. Nitric acid or water was placed inside the vessel and it was used for analytes absorption. The system was closed and placed inside a commercial microwave oven. About 3 s of microwave irradiation were necessary to start the combustion. The microwave irradiation time of the procedure was 30 s and the combustion process took about 10 s. The system was used to digestion of bovine liver and milk powder samples and furter determination of copper and zinc. Bovine liver, milk powder, non-fat milk powder and oyster tissue certified reference materials were used to evaluate the accuracy of the procedure. Good agreement for zinc was obtained from bovine liver certified reference material when microwave combustion (95 to 104%) and microwave combustion followed by reflux, were used to sample decomposition, even if water was used for absorption of analyte. For copper, the values were between 93 and 96% for microwave combustion (not followed by reflux) in samples of bovine liver and oyster tissue, when HNO3 2 mol L-1, or concentrated, was used for analyte absorption. However, for combustion followed by reflux, the agreement was 96 to 100%. Good agreement for zinc was obtained (100%) for milk powder and non-fat milk powder, when only microwave combustion was used and HNO3 concentrate was used for analyte absorption. Results from the proposed procedure were also compared to those from conventionally used procedures for biological samples decomposition, such as wet digestion in open vessels and microwave-assisted closed vessels digestion. The advantages of this procedure include the complete sample decomposition in less time than other procedures and acid consumption (acid was used as absorption solution) always lower than 2%. Another advantage is the low residual carbon content, less of 1.3% without reflux and less than 0.4% with the reflux step.