Identification of crude oils by synchronous fluorescence techniques
Fluorescence spectroscopy is a highly sensitive and specific method for oil identification since petroleum oils contain a mixture of many fluorescent aromatic hydrocarbons. Because of this complex mixture, the spectral distribution and the intensity of the fluorescence will vary with the chemical composition of the oil. Therefore at fixed instrumental conditions and a given concentration of oil, the fluorescence spectrum results in a characteristic "fingerprint" of a given oil. The first part of this work involved the development of a library of fluorescence spectra for model aromatic compounds common in environmental samples. The techniques utilized were based on synchronous fluorescence whose advantages in oil identification have been demonstrated. Extensive studies were carried out using the relatively new and little used technique of variable angle synchronous fluorescence (v.a.s.f.). Non-linear v.a.s.f., where the angle of scan trajectory is altered during a scan. was developed as a means of obtaining desired "sections" through the excitation-emission matrix. Good selectivity and calibration linearity were observed for mixtures of compounds. The results obtained demonstrate the potential of the technique in oil identification. Derivative spectroscopy was applied to enhance minor spectral details in the comparison of closely related samples. The second part of the work involved the application of the previous techniques to real-world samples in the form of crude oils and tar balls. The "fingerprinting principle" is based on the fact that environmental samples of oils can be correlated with possible sources by comparing their stable compositional features. Hence fluorescence studies were performed on several oil samples before and after laboratory weathering. By observing spectral regions where weathering occurs, it was noted that these fluorescence techniques for oil identification could be further improved. Effects of cyclodextrins and micelles. and the process of deoxygenation by nitrogen bubbling and sodium sulphite addition, were investigated as further techniques for discriminating between similar samples and enhancing certain spectral details. Finally, other techniques that have been used in oil identification such as infra-red, thin-layer chromatography, and gas chromatography were briefly examined and their use in a multi-method oil analysis approach discussed.