Extraction induced by emulsion breaking for the determination of As, Co, Cr, Mn, Mo and Pb in heavy and extra-heavy crude oil samples by ICP-MS

Abstract

An extraction induced by emulsion breaking (EIEB) procedure was evaluated for the determination of As, Co, Cr, Mn, Mo and Pb in heavy and extra-heavy crude oil samples by ICP-MS. Optimization of the method was performed by studying the effect of some parameters such as Triton X-100 and HNO₃ concentrations and kind of emulsion breaking (heating and centrifugation). In the optimized method, 0.5 g of xylene was mixed with 0.5 g of crude oil sample. Then, the emulsion was formed by adding 3.0 g of emulsifying solution containing 10% m/v of Triton X-100 and 20% v/v of HNO₃. The emulsion was broken by heating at 90 °C and then, 1.0 g of the aqueous extract was mixed with internal standard solution (Rh) and deionized water to complete 3.0 g for ICP-MS analysis. However, this procedure exhibited a matrix effect in the determination of As (high ionization potential element) because of the migration of Triton X-100 towards the organic phase in the emulsion breaking step. Therefore, the optimized condition for As was to carry out the emulsion breaking by centrifugation or use selenium as the internal standard. Accuracy was evaluated by comparison with certified values of As and Co in SRM 1634c and good agreement was obtained according to Student's t-test. The EIEB procedure was compared with direct introduction of emulsions in the analysis of 7 crude oil samples without significant differences for most of the elements according to the Wilcoxon test. There was significant difference for Mo, probably due to the low extraction of Mo by EIEB. Limits of detection were between 0.15 and 0.31 μg kg⁻¹ and practical limits of quantification, considering the dilution factor of the procedure, were between 9 and 18 μg kg⁻¹.