STUDY OF SHRIMP SAMPLE DIGESTION ASSISTED BY INFRARED RADIATION FOR TRACE ELEMENT ANALYSIS BY ICP OES

Recent work in the literature reports the use of infrared radiation (IR) as a heating source for the effective extraction of trace elements in organic samples1. The present work proposes to study the compounds of the digestion of shrimp sample submitted to the IR heating system. The IR system prototype was developed in our laboratory and the evaluation of its efficiency is under study. A full factorial design in 2 levels with central point was used for optimization of the experimental conditions. The desirability function was used to establish optimal system conditions. The sample preparation was performed using 1.0 g of freeze-dried shrimp sample and adding 8 mL of HNO3 65% w w-1. The mixture was subjected to the IR heating system for 10 min (500 W). The final solution was filtered and diluted to 20 mL. The diluted solutions were used for quantification of trace elements by ICP OES and analysis of dissolved organic carbon. An aliquot of the solution was taken to dryness and afterwards for analysis by Fourier Transform infrared spectrometry to elucidate the compounds found in the residue. The infrared spectrum of the residue of the solution was compared to the spectrum of the original sample, prior to the extraction procedure using the IR system. Comparing both infrared spectra we notice an evident decrease in α-helices structures of the proteins with consequent increase of β-helices structures. This change is related to denaturation of the protein with consequent rearrangement to a new 3D conformation2. The residual carbon analysis shows levels around 1.45 ± 0.2 % w w-1. The analysis of organic nitrogen before and after the extraction in the IR system presents residual nitrogen content around 38.15 ± 0.3 % w w-1. This relatively high value of residual organic nitrogen explains the infrared spectrum obtained with an intense band in 1000 cm-1 region, which is related to phenylalanine residues2 and the nitrogenous compounds of this amino acid that can be easily found in organic residues submitted to decomposition with HNO33. The analyses of As, Cd, Co, Cu, Fe, Mn, Ni, Se, V and Zn were performed by ICP OES and showed standard deviations lower than 10 % and good recovery (> 85 %) for all elements (NIST 1566b – Oyster Tissue). The procedures using the IR system as a heating source presents as a good, low cost alternative for the preparation of organic samples, guaranteeing good efficiency, is less time-consuming and also uses a small amount of concentrated ac