Mineral Composition of Rice, Carrots, and Chayote after Microwave-Assisted Decomposition using Diluted Nitric Acid

Abstract

Carrots, chayote, and rice are foods widely consumed by the Brazilian population, so developing analytical methods that allow elemental characterization becomes extremely important. In this work, the sample decomposition method is proposed to prepare samples (raw and cooked) of rice, carrots, and chayote using dilute nitric acid. The samples underwent microwave-assisted acid decomposition (HNO3/H2O2) for subsequent determination by ICP OES for metals Al, Cd, Cr, Cu, Fe, and Pb and flame spectrophotometer for K and Na. Two decomposition methods were used. Method 1: 260 and 500 mg of sample, 3.5 mL of HNO3 1 mol L-1, 1.0 mL of 30% m/m H2O2 and 3.5 mL of ultrapure water. Method 2: 260 and 500 mg of sample, 5.0 mL of 1 mol L HNO3-1, 2.5 mL of 30% m/m H2O2 and 0.5 mL of ultrapure water. A factorial design (22) was also performed to know the effects of some variables on digestion efficiency. Based on the accuracy study using certified reference materials and the analysis of residual acidity and dissolved organic carbon, the condition that presented the best decomposition results was using 5 mL of HNO3 0.5 mol L-1 and 2.5 mL of 30% H2O2 for 100 mg of sample. Quantification limits ranged from 0.14 (Cr) 3.4 mg kg-1 (Pb), and the correlation coefficient was close to one in all cases. It was also observed that the cooking process increased the concentration of Al, Cd, and Cr and decreased Cu and K for all foods.