Determination of Fe, Ni and V in asphaltene by ICP OES after extraction into aqueous solutions using sonication or vortex agitation

A simple procedure for the extraction of Fe, Ni and V from asphaltene into acid solutions prior to the ICP OES determination is proposed. Either sonication or vortex agitation was used to disperse the organic sample into concentrated nitric acid, achieving efficient analyte extraction into the aqueous acid solution. Both procedures were compared and the advantage of the ultrasound-assisted procedure was evaluated. In both cases, pre-heating of sample–acid mixtures was necessary to enable quantitative extractions. Optimized conditions for the ultrasonic bath were established: 1 L of water at room-temperature, 20 min of sonication and tubes (maximum of eight) in vertical position. By using the vortex, 50 min of vigorous mechanical agitation was needed for each one of the tubes containing samples. The acid solution obtained after extraction was directly aspirated into the ICP in order to perform quantification by optical emission. Calibration was made with aqueous analyte standards containing Sc as internal standard. The procedures were tested using certified fuel oil (SRM NIST 1634c) and the method was applied for the determination of Fe, V and Ni in one asphaltene sample obtained by the fractionation of the SRM NIST 1634c. Recoveries above 90% were achieved and limits of quantification in the asphaltene sample were 1.5, 15 and 1.0 μg g^− 1 estimated for Fe, Ni and V, respectively.