Avaliação de estratégias para a determinação de constituintes inorgânicos em sopas instantâneas por técnicas espectrométricas de emissão por plasma.

Abstract

This Doctoral Thesis work describes the evaluation of strategies for inorganic chemical determination by microwave-induced plasma optical emission spectrometry (MIP OES) and inductively coupled plasma optical emission spectrometry (ICP OES) in instant soups. In the first study, analysis by MIP OES using an N2 plasma was evaluated for the determination of Cu, K, Mg, Mn, P and Zn in soups, after acid decomposition assisted by microwave radiation. The sample preparation procedure consisted of the use of dilute nitric acid solution, 2 mol L-1 , and 1 mL of hydrogen peroxide. All determinations were performed using external calibration. The accuracy of the method was evaluated by analyzing three certified reference materials: apple leaves (NIST 1515), rice flour (NIST 1568b) and soy flour (NIST 3234). The percentages of agreement ranged from 83.3 to 107.5% for the analytes. Addition and recovery tests were also performed, and the percentages of recoveries obtained ranged from 86.0 to 106.0%. The detection limits obtained for Cu, K, Mg, Mn, P and Zn were 0.09; 4.9; 1.0; 0.04; 5.4 and 0.9 mg kg-1 , respectively. The concentration ranges obtained for Cu, K, Mg, Mn, P and Zn in 19 samples of instant soups were: 1.31-13.8, 1135-17824, 172-567, 1.72-12.1, 608-1826 and 2.90- 12.1 mg kg-1 , respectively. The use of microwave induced N2 plasma proved to be a low-cost alternative for routine chemical analysis. In another study, we evaluated the development and applicability of an extraction procedure for the determination of Ca, Cu, Fe, K, Mg, Mn, P, Sr and Zn in instant soups by ICP OES. Initially, 8% hydrochloric acid extracting solution (v v-1 ) was used for the studies. The following parameters were evaluated: (1) proportion of sample mass and volume of extracting solution and (2) use of heating in a water bath. The results were evaluated based on the extraction efficiencies, in %, and were not satisfactory, with values below 80% and/or above 120%. The evaluation of possible matrix effects was performed using the addition and recovery technique. In view of the results obtained, the use of internal standardization (IS) and standard addition (SA) techniques was investigated compared to external calibration (EC). A different strategy was then adopted for the extraction procedure, the use of an alkaline extracting solution of tetramethylammonium hydroxide (TMAH, 25 % m v -1 ). The operational parameters of the plasma, such as nebulization gas flow and radiofrequency power, were optimized. The results were also evaluated by the extraction efficiencies (%) of the analytes, which were below 80% for some analytes and not very repetitive. In this way, and possibly due to the different compositions of the soup samples, the proposal of a single extraction procedure for all analyzed samples was not achieved.