Molecular and stable isotopic compositions of crude oils and their variations during weathering experiments

Abstract

Crude oil is a complex mixture of thousands of different organic compounds formed from a variety of organic materials that are chemically converted under differing geological conditions over long periods of time. Oils from different sources have distinct chemical compositions, which can be used to identify the source of oil pollution. The chemical compositions of spilled oils, however, can be altered in the environment by a number of physicochemical and biological factors including evaporation, dissolution, photooxidation and microbial degradation, making source identification difficult. In this study, compound compositions and stable isotope ratios of six crude oils were analyzed using GC/MS, GC/FID and GC/IRMS. Parent PAHs, akylated PAHs and n-alkane profiles of individual crude oil showed significant differences among crude oils. Diagnostic ratios of source-specific PAH compounds were also varied clearly. The stable isotopic compositions of n-alkane in crude oils exhibited significant differences (27.5‰ - 37.2‰) among crude oils. However, compound compositions of crude oils were altered during the weathering experiment. Low molecular weight PAHs compounds decreased rapidly and short-chain n-alkanes also decreased. However, the stable isotopic compositions of n-alkanes in weathered oil did not show significant changes. This study shows that stable isotope profile of n-alkanes c