Method development for the determination of nickel in petroleum using line-source and high-resolution continuum-source graphite furnace atomic absorption spectrometry

Several sample preparation methods have been investigated for the direct determination of nickel in crude oil using graphite furnace atomic absorption spectrometry (GF AAS). Xylene was found unsuitable as solvent because of the poor long-term stability of the solutions and the resulting contamination of the equipment. Isobutyl methyl ketone (IBMK) solutions exhibited better stability, but the sensitivity of the organic nickel salt used for the standard solutions showed a high day-to-day variability. An oil-in-water emulsion using Triton X-100 as surfactant gave the best results. Using high-resolution continuum-source (HR-CS) GF AAS, it could be observed that up to 50% of the nickel in crude oil, most likely low molecular weight nickel porphyrins, were lost already at pyrolysis temperatures >400 degreesC, whereas the rest of the nickel as well as the nickel standard were stable up to 1300 degreesC. The nickel absorption at a secondary line at 232.138 nm was recorded simultaneously with that at the primary line at 232.003 nm, expanding the dynamic working range by an order of magnitude. The best characteristic mass obtained was m(0)=27 pg and the limit of detection was around 0.07 mug g(-1) Ni in oil, based on an emulsion of 2 g oil in 10 ml. The accuracy of the procedure was verified by analyzing the certified reference material (CRM) NIST SRM 1634c, Trace Metals in Residual Fuel Oil. (C) 2004 Elsevier B.V. All rights reserved.