Desenvolvimento de métodos de preparo de amostras de petróleo e posterior determinação de elementos terras raras por ICP-MS

Abstract

In this work, the determination of rare earth elements by ICP-MS was evaluated using different systems for nebulization step (concentric and desolvation using microconcentric and ultrasonic nebulizers) and nebulizer gas flow rate and RF power. The performance of the nebulizers was investigated in the presence of different concentration of HNO3, Ba and NaCl. For crude oil digestion, eight sample preparation methods were investigated: microwave-assisted acid digestion (MAWD), microwave-assisted-UV digestion (MAWD), microwave-assisted acid digestion under oxygen pressure (MW-O2-AD), microwave-induced combustion (MIC), focused-microwave-induced combustion (FMIC), dry ashing and microwave (MAE) and ultrasound (USAE) assisted extraction. These methods were optimized based on some parameters and further applied for ten crude oil samples with different densities (API 10,8 to 23,5) in order to evaluate the digestion efficiency. With the use of ultrasonic nebulizer, better limits of detection were obtained in comparison with concentric and APEX nebulizers. Using HNO3 concentration up to 1.4 mol l-1, no interferences were observed in the determination step when the nebulizer with desolvation systems were applied. Concerning the sample preparation method evaluated, using MW-O2-AD and FMIC, crude oil digestion was not suitable and operational risks were observed during the sample treatment. With the use of extraction methods based on microwaves and ultrasound, analytes recoveries were not quantitative and these methods were not suitable for rare earth elements extraction from crude oil samples, even after optimization of the operational conditions. For the methods based on MAWD, MW-UV-AD, MIC and dry ashing, crude oil digestion was complete and low values of residual carbon content were obtained. Concerning the MW-UV-AD, MIC and dry ashing methods, it was possible to use diluted acid for digestion of high samples masses that is an important aspect to reduce reagent consumption and residues generation, and also results in low blank values, minimizing interferences in the determination step. Sample masses in the range of 0.2 to 0.5 were decomposed by MIC and MAWD and MW-UV-AD, respectively. Using dry ashing, crude oil digestion was complete with samples masses up to 3 g and temperature set at 500 ºC and when high temperatures were used, analyte losses were observed, mainly for samples with high chlorine content. The MAWD, MW-UV-AD, MIC and dry ashing were suitable for crude oil digestion with different properties for further rare earth elements determination. Accuracy was evaluated by comparing the results obtained with those using neutron activation analysis (NAA). Agreement better than 95% was obtained comparing the results of NAA with those obtained after crude oil digestion by MAWD, MW-UV-AD, MIC and dry ashing and analytes determination by ICP-MS. Analyte recovery tests were also performed and quantitative recoveries were obtained for all analytes.