The preliminary evaluation of stable carbon-isotope ratio mass spectrometry (IRMS) as a novel, alternative method for screening residual heavy or weathered petroleum wastes is presented. Seven well-characterised middle-to high-boiling range oils (predominant boiling range: 150-600°C) and their isolated class fractions were analysed by IRMS. Whole oil δ13C values (which express the ratio of13C to12C within each sample) showed a small but significant decrease with decreasing oil asphaltene content, from -26.8% for heavier oils to -28.8% for lighter, predominantly paraffinic oils. In agreement with the conventionally observed trend δ13C values of isolated class fractions were found to increase in the following order δ13C(sat) ( ≃ α13C(oil)) < δ13C(aro) < δ13C(pol) < δ13C(asp), with δ13C(sat) up to 2.5% more negative than δ13C(pol) and δ13C(asp). However, this variation was much less pronounced for the heavier oil samples. As a result, isotope type curves exhibited a clear distinction between heavier oils, which produced much flatter type curves, and lighter oils, for which type curves were characteristically sloping, from these preliminary results, IRMS would appear to have application as a potential method for identifying a predominance of asphaltene class frcations in residual hydrocarbon wastes and for distinguishing between heavy and tight oily contaminants.
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