Development of an isotope dilution laser ablation ICP-MS method for multi-element determination in crude and fuel oil samples

Abstract

An inductively coupled plasma isotope dilution mass spectrometric (ICP-IDMS) method with direct introduction of the isotope-diluted sample into the plasma by laser ablation was developed for accurate, sensitive, fast, and simultaneous determination of trace metals in different oil samples. Metallo-organic solutions of isotope spikes (⁵⁰V, ⁵³Cr, ⁶⁵Cu, ⁵⁷Fe, ⁶²Ni, ⁶⁸Zn, ¹¹³Cd, ¹¹⁷Sn, and ²⁰⁶Pb) were prepared from corresponding aqueous stock solutions by using liquid–liquid extraction of complexed metal ions in isobutyl methyl ketone. The isotope-diluted sample was absorbed by a cellulose material, which was fixed in a special PTFE holder for ablation, using a laser system with high ablation rates. Under these conditions, no time-dependent spike/analyte fractionation was observed for the metallo-organic spike/oil mixtures and the measured isotope ratios of the isotope-diluted samples remained constant over the whole ablation time, which is a necessary precondition for accurate results with the isotope dilution technique. The accuracy of LA-ICP-IDMS determinations was demonstrated by analyzing the fuel oil reference material BCR 1634c, certified for vanadium and nickel, a commercially available oil based standard (SCP-21) for nine metals, and by comparing LA-ICP-IDMS results with those obtained by ICP-IDMS using microwave-assisted sample digestion. Detection limits in the range of 0.02 µg g⁻¹ (V) to 0.2 µg g⁻¹ (Fe) were obtained for LA-ICP-IDMS by total analysis times per sample of only 10 min. Three crude oil samples of different origin with vanadium, nickel, and iron concentrations in the range of 2–60 µg g⁻¹ were also analyzed, indicating iron heterogeneities by the relatively high standard deviation.