Importance of Spectral Correction in Fluorescence Spectroscopic Studies of Crude Oils and Asphaltenes

Fluorescence spectroscopy has been widely used in biochemistry to elucidate molecular interactions. Likewise, it has also been applied to petroleum systems and, in particular, asphaltene association by concentration effects. It is also a common method for online measurements of oil in water in the offshore oil and gas industry. As a result of the complexity of crude oils and asphaltenes, many assumptions and corrections are necessary in an adequate analysis, especially of concentration effects. Unfortunately, many studies in the literature lack appropriate incorporations of simple effects, such as reabsorption effects (inner filter) and energy transfer [fluorescence resonance energy transfer (FRET)], among molecules leading to apparent red shifts. The latter has wrongly been reported as an effect of molecular association. In the present paper, we analyze the "PetroPhase 2017" asphaltenes and crude oils using emission spectra (λ_exof 300 and 400 nm) and synchronous spectra as a function of concentrations in toluene between 0.2 and 1000 ppm in both front face and right angle irradiation geometries. Inner filter effects are seen in both configurations, leading to complete loss of fluorescence at a short wavelength. After corrections, we observe that linearity in intensity-concentration relations is almost restored, but some red shifts remain. This is mainly due to FRET and, hence, not related to true molecular aggregation. We further explore a number of Stern-Volmer (SV)-based approaches to elucidate mechanisms of molecular interaction using fluorescence spectroscopy. The SV plots indicate that fluorophores could be shielded at quite high concentrations in agreement with asphaltene aggregation. The presence of FRET indicates that some molecules contain several covalently bonded but independent fluorophores. The conclusion is that caution is needed in interpretation of concentration effects in fluorescence spectroscopy studies of petroleum fractions and all possible corrections should be made before concluding on molecular interactions, such as critical nanoaggregate concentration phenomena.