Avaliação de Métodos de Preparo de Amostra para a Determinação de As, Cd, Pb e Se em Arroz por Espectrometria de Absorção Atômica com Forno de Grafite

Abstract

This study presents a comparative evaluation of three different methods for sample preparation of rice for As, Cd, Pb, and Se determination by graphite furnace atomic absorption spectrometry. Raw and cooked white rice, parboiled rice, and brown rice samples were studied with the following procedures: i) HNO3 decomposition in an open system with “cold finger”, ii) HNO3 dissolution in ultrasonic bath, and iii) alkaline solubilization with TMAH. Sample preparation using the reflux system was optimized: 750 mg of sample, 10 mL of HNO3, heating at 120 ºC, and 90 minutes of decomposition time. In rice samples from Brazil, average concentrations of 12.0 ± 4.0 and 177.0 ± 64.0 ng g-1 for Cd and Pb were found, respectively, while for As and Se, the concentrations were below the limit of detection of the studied methods. After cooking, samples presented a maximum decrease of 61% in Cd concentration. The method accuracy was assured by the analysis of a certified reference material (NIST 1568a), and results were in accordance with the certified values. A recovery test was carried out and results ranged between 80 and 115% for all analytes in rice samples, before and after cooking. All evaluated procedures presented accurate and precise results; however, the decomposition with reflux system proved to be more effective, simple and safe. Such characteristics are necessary for application in routine analysis with detection limits of 77.2; 1.0; 35.5 and 192.1 ng g-1 for As, Cd, Pb and Se, respectively.