Evaluation of microwave-assisted ultraviolet digestion method for rice and wheat for subsequent spectrometric determination of As,Cd, Hg and Pb

The aim of this study was to optimize a sample preparation method for rice and wheat using microwave-assisted ultraviolet digestion (MW-UV) for subsequent determination of toxic elements. Cadmium and Pb were determined by sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS), while As and Hg were determined by chemical vapor generation coupled to atomic absorption spectrometry (CVG-AAS). A systematic evaluation of the microwave heating program, nitric acid concentrationand sample mass was performed to optimize the MW-UV digestion method for rice and wheat samples. The relationship between nitric acid concentration and sample mass was monitored by determination of the residual carbon content (RCC) and residual acidity (RA) in order to obtain a high efficiency of digestion. The MW-UV method was successful at digesting up to 1100 mg of rice and wheat using 4 mol L⁻¹ HNO₃ with RCC and RA lower than 1.5 %, and 10 %, respectively. Recovery results ranging from 88 % to 117 % and agreement with certified reference values (t-test, 95 % confidence level) were obtained after digestion using the MW-UV method for spiked samples and certified reference materials (peach leaves-NIST 1547 and tomato leaves-NIST 1573), respectively. The optimized method was suitable for analysis of toxic elements in rice and wheat in compliance with the maximum levels reported in official directives.