Multivariate Analysis: Use of Doehlert Design (DD) for Optimization of a Simple Sample Preparation Procedure for the Determination of Inorganic Constituents in Cocoa Beans Employing Microwave Acid Digestion and ICP OES

This study presents a microwave-assisted procedure for sample preparation of cocoa beans using diluted HNO3 with H2O2 for the determination of Ba, Ca, Cu, K, Mg, Mn, P, S, Sr and Zn by inductively coupled plasma optical emission spectrometry (ICP OES). Doehlert modified design was used for optimization of experimental condition, and the volume of H2O2 (0, 0.5, 1.75 and 3 mL) and concentration of HNO3 (0.5, 3.75 and 7 mol L-1) were evaluated. The best experimental conditions were chosen considering lower residual acidity (RA), lower dissolved organic carbon (DOC) and the acceptable trueness range (80-120%), were 5 mL of 3.75 mol L-1 HNO3 and 1.75 mL of 30% v v-1 H2O2. The procedure was validate using certified reference materials (CRMs), digested using the optimized conditions. The proposed procedure was applied in twelve samples of cocoa beans, and the concentrations (mg kg-1) ranged from: 7.6-27 (Ba), 520-1955 (Ca), 11-26 (Cu), 6212-10855 (K), 1677-2516 (Mg), 13-29 (Mn), 4679-6104 (P), 1499-1898 (S), 7-32 (Sr) and 34-46 (Zn). A Pearson correlation plot was proposed to evaluate the correlation between elements, revealing high correlations between Mg and K; K and P; Cu and Mn; Cu and Zn; Cu and S.