Study of different decomposition strategies of silicate-rich samples for subsequent trace elements determination by spectrometric methods
Name: RAFAEL MANTOVANELI SOUSA
Type: PhD thesis
Publication date: 19/09/2022
Advisor:
Name |
Role |
|---|---|
| MARIA TEREZA WEITZEL DIAS CARNEIRO LIMA | Advisor * |
Examining board:
| Name |
Role |
|---|---|
| ANA RITA DE ARAUJO NOGUEIRA | External Examiner * |
| CLEOCIR JOSÉ DALMASCHIO | Internal Examiner * |
| GEISAMANDA PEDRINI BRANDÃO ATHAYDE | Internal Examiner * |
| JOAQUIM ARAÚJO NÓBREGA | Co advisor * |
| MARIA TEREZA WEITZEL DIAS CARNEIRO LIMA | Advisor * |
Pages
Summary: Sample preparation is one of the most critical steps in the analytical sequence. Depending on the complexity of the matrix and the interest of the chemical analysis, it may be necessary to carry out acid assisted decomposition, using highly toxic reagents, bringing risks both to the analyst and to the environment. Thus, the present study aims to develop and verify alternative sample preparation methods that could be used instead of conventional methods. One of the proposed methods does use of diluted HNO3 for the partial decomposition of marine sediment and subsequent determination of Co, Cr, Cu, Fe, Mn, Ni, P, Pb, V and Zn by ICP OES (inductively coupled plasma with optical emission spectrometry). A Mixture Design of Experiments was carried out to optimize the optimal proportion among the reagents used in the decomposition, with the optimal volumes being: 5.0 mL of HNO3 10 mol L-1 , 2.0 mL of HClconcentrated and 3.0 mL of H2O2 30 % v v-1 . Analytical figures of merit were established to verify the method, with good selectivity, wide linear working range, low quantification limits (0.023 6.39 mg kg-1 ), good accuracy (71 to 115% agreement with CRM, certified reference material) and good accuracy (relative standard deviation, RSD<5%) for all analytes. The proposed method was compared with EPA (Environmental Protection Agency) 3051A, and no statistical differences were observed between the concentrations determined by the two methods, corroborating the quality of the proposed method. In a second method, the decomposition of marine sediment samples was proposed, performing the in loco synthesis of HBF4 from the reaction between HF and H3BO3. This method has the main advantage of eliminating an additional step for the neutralization of excess F- . Reagent volumes were optimized univariately, reaching the optimal condition of 6.0 mL of HClconcentrated, 3.0 mL of HNO3concentrated, 1.5 mL of HFconcentrated and 0.6 g of H3BO3 P.A. REE (rare-earth elements) were determined by ICP-MS (inductively coupled plasma with mass spectrometry). Analytical figures of merit were established to verify the proposed method, and found a wide linear range of work, low limits of quantification (0.0009 0.05 mg kg-1 ), good accuracy (agreements 78 to 115% and recoveries 91 115%) and good accuracy (RSD Comparing the proposed method with EPA 3052, no statistically significant differences were observed between the concentrations determined in these methods, indicating the applicability of the proposed method. In another proposed method, the replacement of HF by HBF4 was investigated for the decomposition of estuarine sediments. A Mixture Design of Experiments was carried out, and the optimal proportion among the volumes of HNO3, HCl and HBF4 was investigated, reaching the working condition of 5.6 mL of HNO3concentrated, 3.0 mL of HClconcentrated and 1.5 mL of HBF4concentrated. REE were determined by ICP-MS Analytical figures of merit were established to verify the proposed method, with a wide linear working range, low quantification limits (0.11 13.6 ng kg-1 ), good accuracy (agreements 86 113% and recoveries 81 110%, except Sc Comparing the concentrations determined by the proposed method with the EPA 3052, no statistical differences were found between the concentrations, indicating the quality of the proposed method. The last method proposes performing mechanochemical synthesis of sediment samples to form new compounds that are more easily soluble in the presence of chemical modifiers. High efficiency grinding at times of 15 and 60 minutes was investigated using a disc mill. The presence of chemical modifiers Na2CO3, NaF and Li2B4O7, in proportions of 1:2 and 1:4, were also investigated. After the grinding, the samples were decomposed using a nitric-hydrochloric acid mixture, according to the EPA 3051 method and REE were determined by ICP-MS. The preliminary results obtained had an increase in the concentration of analytes with the increase in grinding time, indicating that the mechanochemical synthesis favored the availability of the analytes. Comparing the concentrations found in the samples after 60 minutes of grinding with the concentrations determined in the decomposed sample using HF, a good agreement between the values was observed. The chemical and mineralogical characterization, carried out by the FT-IR (Fourier-transform infrared spectroscopy) and XRD (X Ray Diffraction) techniques, showed changes in the chemical structures and mineralogical phases after milling, corroborating the results obtained in the REE determinations.
