Palladium as chemical modifier for the stabilization of volatile nickel and vanadium compounds in crude oil using graphite furnace atomic absorption spectrometry

Abstract

It has been observed in previous work using high-resolution continuum-source atomic absorption spectrometry that up to 50% of the nickel and vanadium in crude oil, most likely non-polar nickel and vanadyl porphyrin complexes, was already lost at pyrolysis temperatures >400 °C, whereas the rest of the analyte was stable up to at least 1200 °C. In this work we were investigating the use of a chemical modifier in order to avoid these low-temperature losses and also to make possible a differential determination of the volatile analyte fractions. Oil-in-water emulsions, using Triton X-100 as surfactant, were used for easy sample handling. A mass of 20 μg of Pd, introduced into the graphite tube and thermally pretreated prior to the injection of the emulsion, was found to efficiently prevent any low-temperature losses of nickel and vanadium from crude oil samples up to pyrolysis temperatures of 1200 °C and 1450 °C, respectively. The best characteristic mass obtained was m₀ = 16 pg for Ni and m₀ = 33 pg for V. The limits of detection and quantification were 0.02 μg g⁻¹ and 0.065 μg g⁻¹, respectively, for Ni, and 0.06 μg g⁻¹ and 0.17 μg g⁻¹, respectively, for V in oil, based on an emulsion of 2 g of oil in 10 mL. The precision expressed as relative standard deviation (n = 9) for a crude oil containing about 5 μg g⁻¹ Ni and 3 μg g⁻¹ V was 2.0% and 3.1% for Ni and V, respectively. The accuracy of the procedure was verified by analyzing the certified reference material NIST SRM 1634c, trace metals in fuel oil, and the research material RM 8505, vanadium in crude oil.