Determinação de elementos terras raras em petróleo e frações por amostragem direta e ICP-MS com vaporização eletrotérmica

Abstract

In this work, a method for the determination of rare earth elements (REE) present in low concentrations in crude oil, atmospheric distillation residue (RAT) and vacuum distillation residue (VR) using direct sampling and inductively coupled plasma mass spectrometry (ICP-MS) with electrothermal vaporization (ETV) was developed. Due to the facility of REE in forming refractory compounds inside the graphite furnace during the heating step, the use of a modifier gas (Freon R-12) was necessary to promote the vaporization and transport of these elements to the ICP-MS equipment. The use of 6 mg of citric acid was suitable for external calibration with aqueous reference solution. The heating program was evaluated based on the pyrolysis and vaporization curves for reference solution and samples. Pyrolysis and vaporization temperatures of 700 and 2200 °C were established, respectively. Considering that the transport efficiency of analytes to the plasma is related to the bypass, carrier and modifier (Freon R-12) gases, the flow rates of these gases for the aqueous reference solution and the samples were evaluated. The flow rates of 0.40 L min-1, 0.40 L min-1 e 2.0 mL min-1 were selected for the bypass, carrier and Freon R-12 gases, respectively. Using the operational conditions optimized for the ETV-ICP-MS system, the applicability of the method for the determination of REE in synthetic crude oil emulsions with different water contents and NaCl concentrations was investigated. It was observed that synthetic crude oil emulsions with water content and NaCl concentration up to 10% and 2000 μg g-1, respectively, can be analyzed by ETV-ICP-MS. The influence of sample mass was evaluated and relatively high masses (up to 18 mg of crude oil, 5 mg of RAT and 6 mg of RV) can be employed in the proposed method. The accuracy was evaluated by the comparison of results obtained by ETV-ICP-MS with those obtained by ICP-MS with ultrasonic nebulizer (USN) after microwave assisted wet digestion (MAWD) using a mixture of concentrated HNO3 and H2O2. No statistical difference (t-Student, 95% of confidence level) was observed between the results obtained for REE in crude oil (water content from 0.12 to 2%), RAT and RV by ETV-ICP-MS and USN-ICP-MS after MAWD. The limits of quantification for REE by ETV-ICP-MS were lower (between 0.02 and 2.7 ng g-1) than the values obtained by USN-ICP-MS after MAWD (between 0.6 and 3.7 ng g-1). The ETV-ICP-MS method was proposed for the first time for the determination of REE in low concentrations in crude oil and distillation residues showing to be a promising alternative for routine analysis.