Asphaltene profiling of crude and heavy fuel oils to assist in forensic oil spill investigations

When oil is spilt, investigators compare the chemical profile of the spilt oil with suspected source oils through a process commonly referred to as oil fingerprinting. Oil fingerprints are currently generated exclusively from volatile fractions of oil whilst the non-volatile fraction is discarded. The non-volatile fraction contains asphaltenes, which are defined as the oil fraction that is not soluble in short chain n-alkanes. There is sufficient evidence in the literature to suggest that asphaltenes may offer probative information if they were included in oil spill investigations. This PhD research has investigated whether asphaltene profiling methods can in fact provide probative information that warrants their use alongside current oil fingerprinting methods. Asphaltene profiling methods were designed primarily as screening tools for the exclusion of obvious non-related oils prior to conducting conventional oil fingerprinting. A range of spectroscopic and thermal degradation methods were developed and tested using ten different crude and heavy fuel oils. The infrared spectroscopy and pyrolysis-gas chromatography-mass spectrometry methods were found to be the most suitable methods for oil spill investigations. Both methods were probative and capable of differentiating non-related oils, whilst grouping together oils from the same origin. A blind study was also conducted using these two methods. The original sample-set and five additional oils were analysed, and asphaltene profiles were interpreted. The four pairs of duplicated oils that were included in the blind study were correctly grouped together. The majority of non-related oils were differentiated except for two pairs of very closely-related oils which were not differentiated. Taking into account that asphaltene profiling was designed as a screening tool for use only in conjunction with conventional oil fingerprinting, the false inclusion of two pairs of non-related oils was not problematic. Conventional oil fingerprinting successfully differentiated these two pairs of oils following asphaltene profiling

An FTIR method for the analysis of crude and heavy fuel oil asphaltenes to assist in oil fingerprinting

A proof-of-concept spectroscopic method for crude and heavy fuel oil asphaltenes was developed to complement existing methods for source determination of oil spills. Current methods rely on the analysis of the volatile fraction of oils by Gas Chromatography (GC), whilst the non-volatile fraction, including asphaltenes, is discarded. By discarding the non-volatile fraction, important oil fingerprinting information is potentially lost. Ten oil samples representing various geographical regions were used in this study. The asphaltene fraction was precipitated from the oils using excess n-pentane, and analysed by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). Based on visual interpretation of FTIR spectra along with peak height ratio comparisons, all ten oil samples could be differentiated from one another. Furthermore, ATR-FTIR was not able to differentiate a weathered crude oil sample from its source sample, demonstrating significant potential for the application of asphaltenes in oil fingerprinting

Pyrolysis-GC-MS analysis of crude and heavy fuel oil asphaltenes for application in oil fingerprinting

A Pyrolysis – Gas Chromatography – Mass Spectrometry (Py-GC-MS) method was developed for the analysis and profiling of crude and heavy fuel oil asphaltenes, for application in oil fingerprinting. Asphaltenes were precipitated from ten geographically different oils using n-pentane, and analysed by Py-GC-MS. Alkane profiles and sulphur/aromatic profiles were used to compare the oils, and to correctly differentiate oils from different geographical regions. Py-GC-MS could not differentiate a weathered oil sample and a fresh oil sample from the same source. The results of this study support the findings from a previously developed FTIR method for asphaltene profiling