Feasibility of nut digestion using single reaction chamber for further trace element determination by ICP-OES

Abstract

A method for nut digestion using single reaction chamber (SRC, UltraWave™ system) was proposed. High sample masses (up to 1.5g) were used in order to improve the limits of detection for trace elements. Nuts with high fat content (Brazil nut, cashew nut and walnut) were used to evaluate the efficiency of the proposed method. Different oxidant conditions were evaluated (only concentrated HNO3 and mixtures of HNO3 and H2O2). Comparison with Multiwave™ system was also carried out. Efficiency of digestion for both systems was evaluated by the determination of residual carbon content (RCC) and residual acidity of final digestates. The determination of Al, As, C, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn was performed using inductively coupled plasma optical emission spectrometry (ICP-OES). Carbon related effects on analytical response for emission lines used in ICP-OES determination were also evaluated. The best efficiency of digests was obtained using UltraWave™ system with 1.5g of nut, concentrated HNO3 (without the addition of H2O2) and a temperature and pressure of 275°C and 180bar, respectively. In this condition the RCC was 4.6% (corresponding to 2350mgL−1 of C in digestates) and residual acidity was lower than 5% (equivalent to 0.3mL of concentrated HNO3 diluted up to 25mL). On the other hand, for Multiwave™ system only 0.5g of nuts could be digested. Carbon related effects on analytical response on ICP-OES determination were not observed in these conditions. Significant differences were not observed by the comparison between the results obtained by UltraWave™ system and certified reference values (t test, 95% confidence level). Therefore, using UltraWave™ system that allowed a relatively high sample mass to be digested it was possible to determine trace elements at low concentration even using ICP-OES in conventional mode.