Investigation of decomposition products of microwave digestion of food samples

The involatile residues remaining after closed-vessel microwave digestion of various food samples, using nitric acid, with and without post-digestion treatment with hydrogen peroxide, have been studied. Decomposition products were found to include aliphatic and aromatic acids, nitro-compounds, oxalates and inorganic nitrates and phosphates. Measures of digestion completeness were provided by appearance, carbon content, infrared spectra and thin layer chromatograms of the residues, enabling a comparison of different digestion methods and sample types. Residual carbon levels varied linearly with the relative amounts of carboxylic acid and inorganic nitrate, as measured by infrared spectrometry of the residues. The formation of calcium oxalate was also a function of the degree of decomposition: the carboxylic acid: oxalate ratio increased in a logarithmic fashion with increasing residual carbon content.Particularly high levels of residual carbon from nitric acid digestion of milk powder, due largely to carboxylic acid residues, were substantially decreased by post-digestion treatment with hydrogen peroxide. However, nitrobenzoic acids, which proved major interferents in electrochemical analysis, were only removed by treatment with perchloric acid.