Speciation analysis of volatile and non-volatile vanadium compounds in Brazilian crude oils using high-resolution continuum source graphite furnace atomic absorption spectrometry

A method is proposed that makes possible determining total and 'thermally stable' vanadium in crude oil without prior separation, and to calculate 'volatile' vanadium by difference. The volatile fraction is believed to be largely vanadyl porphyrine complexes. The method is based on the unsurpassed background correction capability of high-resolution continuum source graphite furnace atomic absorption spectrometry (HR-CS GF AAS), which allows pyrolysis temperatures as low as 300 deg. C to be used. The samples were prepared as oil-in-water emulsions, and aqueous standards emulsified in the same way were used for calibration. Total vanadium has been determined using a pyrolysis temperature of 400 deg. C, and 'thermally stable' vanadium using a pyrolysis temperature of 800 deg. C. The content of total vanadium in 12 Brazilian crude oil samples was found to be between less than 0.04 and about 30 mg kg-1. The volatile fraction was between 5 and 51% of the total content, and there was no correlation between the total and the volatile vanadium content. The limits of detection and quantification were 0.04 and 0.12 mg kg-1 of V in crude oil, respectively, based on a mass of 2 g of oil in 10 mL of emulsion. The precision was better than 4% at the 3 mg kg-1 level and better than 1.5% at the 30 mg kg-1 level of V in crude oil