DETERMINATION OF TRACE-METALS IN TEA USING BOTH MICROWAVE DIGESTION AT ATMOSPHERIC-PRESSURE AND INDUCTIVELY-COUPLED PLASMA-ATOMIC EMISSION-SPECTROMETRY

A microwave digestion method for tea leaves has been developed using an open focused microwave digester. The optimized procedure involves initial reaction with nitric acid followed by perchloric acid to give a clear, colourless solution, Digestions are complete in 35 min, thus offering a substantial time saving on the conventional hot-plate procedure which may take in excess of 2 h. The optimized method has been employed to digest a standard reference material, a number of tea samples of different origins, four commercial blends and a herbal tea. Tea liquors were also prepared. The subsequent determination of Al, Ba, Ca, Cu, Mg, Mn and Zn was achieved using inductively coupled plasma atomic emission spectrometry. The results obtained for the standard tea were in excellent agreement with the certified values. A large variation in the trace element composition of the tea samples was observed and these results are discussed with reference to both existing literature values and country of origin.