Evaluation of sample preparation methods for the determination of As, Cd, Pb, and Se in rice samples by GF AAS

This study presents a comparative evaluation of three different methods for sample preparation in rice for As, Cd, Pb, and Se determination by graphite furnace atomic absorption spectrometry (GF AAS). Raw and cooked white rice, parboiled rice, and brown rice samples were studied with the following procedures: i) HNO3 decomposition in open system with cold finger, ii) HNO3 decomposition in ultrasonic bath, and iii) alkali solubilization with TMAH. The sample preparation using reflux system was optimized and the best conditions were: 750 mg of sample, 10 mL of HNO3, 120 degrees C (heating), and 90 min in the digester block. In rice samples, medium 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 validations were assured by analysis of certified reference material (NIST 1568a), and results were in accordance with the certified values. The recovery test was carried out and results ranged between 80 and 115% for all analytes in the 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 secure. Such characteristics are necessary for the application in routine analysis with medium detection limits of 77.2; 1.0; 35.5 and 192.1 ng g(-1), for As, Cd, Pb and Se, respectively. (C) 2015 Published by Elsevier B.V.