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.