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

Abstract

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 ex of 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.

Effect of Isopropanol on the Fluorescence of Crude Oil-in-Water Dispersions

“…The spectra will still be affected by both inner filter effects and energy transfer especially important in multicomponent systems. 14 Similar to the excitation spectra in Figure 3, the SFS intensities increase with increasing isopropanol content. However, it is observed that there is a significant apparent redistribution of contributions to the spectrum while the source of fluorophores is the same, which can be due to changes to the polarity of both water phase and oil phase and redistribution of the partitioning between the two resulting phases.…”

“…In very complex mixtures as crude oil, the variability among molecular structures is large, but substitution to pure PAHs normally only leads to displacement of the spectrum by 10 to 20 nm. The spectra will still be affected by both inner filter effects and energy transfer especially important in multicomponent systems …”

“…The use of isopropanol is suggested because it does not in itself fluoresce, and it stabilizes the oil-in-water emulsions or dispersions. However, the fluorescence intensity of crude oil can be extensively affected by factors other than the concentration of fluorophores, such as inner filter effects and fluorescence resonance energy transfer, which lead to a red-shift and quenching of the emission band. , Even at very low oil concentrations, crude oil-in-water dispersions have a complex fluorescence profile due to the concentration of aromatic hydrocarbons in the continuous phase being different from that inside the droplets. Those concentrations change dynamically until the dispersion reaches physical stability or undergoes phase separation.…”

2022 Pioneers in Energy Research: Oliver Mullins

“…39 The importance of understanding limitations of each physical technique and assumptions in data interpretation is demonstrated by an original article by Prof. Andersen from the Technical University of Denmark, highlighting the importance of spectral correction in the fluorescence spectroscopic analysis of crude oils and asphaltenes. 40 These experimental studies are followed by two theoretical studies. Virtual experiments on real asphaltene molecules from non-contact atomic force microscopy (nc-AFM) were conducted by Prof. Janesko of Texas Christian University and Prof. Brothers of Texas A&M University at Qatar to predict bulk properties and confirm hypotheses.…”

“…New insights into the asphaltene structure and aggregations were obtained by Prof. Goual at the University of Wyoming using high-resolution electron microscopy . The importance of understanding limitations of each physical technique and assumptions in data interpretation is demonstrated by an original article by Prof. Andersen from the Technical University of Denmark, highlighting the importance of spectral correction in the fluorescence spectroscopic analysis of crude oils and asphaltenes . These experimental studies are followed by two theoretical studies.…”