Source Identification of Oil Spills Based on the Isotopic Composition of Individual Components in Weathered Oil Samples

Abstract

Correlation of crude oils and refined hydrocarbon products spilled in the environment with their respective sources is commonly undertaken using techniques such as gas chro matography (GC) or gas chromatography/mass spectrometry (GC/MS) or bulk parameters such as the isotopic composition of the aliphatic or aromatic fractions. Under certain circumstances extensive weathering of the samples, through evaporation, water-washing, or biodegradation, may make such correlations extremely difficult and the results somewhat tenuous. Results are presented in this paper from an investigation to study the use of gas chromatography/isotope ratio mass spectrometry (GC/IRMS) as a complimentary correlation technique to GC and GC/MS, particularly for samples that have undergone extensive weathering. In the study, a variety of oils and refined hydrocarbon products, weathered both artificially and naturally, were analyzed by GC, GC/MS and GC/IRMS. In cases where samples have lost their more volatile n-alkanes as a result of weathering, the isotopic compositions of the individual compounds were not found to be extensively affected. Hence GC/IRMS can be particularly useful for correlation of refined products dominated by n-alkanes in the C10−C20 region and containing none of the biomarkers more commonly used for correlation purposes. For extensively weathered crude oils that may have lost all of their n-alkanes, it has been demonstrated that isolation and pyrolysis of the asphaltenes followed by GC/IRMS of the individual pyrolysis products can be used for correlation purposes with their unaltered counterparts. In summary, it is not proposed that GC/IRMS be used as a stand-alone correlation tool but in conjunction with existing techniques such as GC and GC/MS. It is proposed however that it could be particularly useful in situations where existing techniques do not provide unambiguous correlations. In certain cases, particularly with lighter refined products, GC/IRMS can provide valuable correlation data in the absence of biomarker data. Finally, characterization of asphaltene pyrolysis products by GC/IRMS provides an alternative and useful method for correlating extensively degraded samples with their non-degraded sources.