Suggested “New Method for Determination of Dispersity in Petroleum Systems” Is Based on Trivial Experimental Artifacts

Evdokimov, Igor N.; Losev, Aleksandr P.

doi:10.1021/ef800100z

Cited by 3 publications

(2 citation statements)

References 18 publications

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“…These spectral characteristics are substantially affected by the mode of condensation in nonlinearly condensed aromatic chromophores and substitution. As is obvious from Figure , the fluoresecence spectra are all continua, practically without any significant features, and it is not possible to make any assignments except for the bands due to porphyrins. ,,, …”

Section: Discussionmentioning

confidence: 98%

Cause of Asphaltene Fluorescence Intensity Variation with Molecular Weight and Its Ramifications for Laser Ionization Mass Spectrometry

2009

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The molecular-weight (MW)-dependent fluorescence of asphaltene discovered over 20 years ago has been re-examined from a theoretical point of view. It is concluded here that the dependence is a result of the sensitivity of the jump between the non-intersecting potential energy surfaces involved in the S 1 f S 0 internal conversion to the excitation energy of the S 1 state, the energy gap, ∆E. The excitation energy of organoaromatic molecules, in general, depends upon the size of the aromatic chromophore which, in turn, has been shown to increase with the MW of the gel-permeation-chromatographic (GPC)-separated fractions of asphaltene. The larger the chromophore size, the lower the excitation energy of the S 1 state. This then leads to an increase of the vibrational Franck-Condon factor governing the rate of internal conversionwhere f v is the Franck-Condon factor and R is a proportionality constant. As ∆E decreases, the rate of the S 1 f S 0 transition increases in competition with the rate of fluorescence, resulting in the suppression of fluorescence with an increasing MW of the asphaltene. Thus, the vibrational Franck-Condon factor, f v , has the desirable feature that it is not directly reliant on chemical composition but only the excitation energy of the chromophore that matches the requirement for applicability to asphaltene. This recognition revealed the possibility of a critical role of IC in the laser ionization process applied in mass spectroscopy. Here, photoexcitation leads through a vertical transition to a higher lying electronic state, which, upon internal conversion to the S 1 state, S n > 1 f S 1 , generates some excess vibrational energy in the S 1 state. This IC is a fast process, with k(IC) ∼ 10 13 s -1 , much faster than the S 1 f S 0 IC, with k ∼ 10 6 -10 9 s -1 . However, the excess vibrational energy has a marked accelerating effect on the latter rate, which becomes an exponential function of the excess vibrational energy. With an increasing size of the aromatic chromophore (MW), ∆E decreases and the excess vibrational energy, E v , increases, and as a result, both increase the rate of k(IC) S 1 f S 0 up to a level commensurate with k(S n > 1 f S 1 ). Increased rates for k(IC) S 1 f S 0 mean an increased loss of photons and a consequently lower ion yield in the MS. The effect occurs in parallel with MW, and at a critical MW, the ion yield may drop below the detection limit. Thus, it is suggested that the extremely low value found for the MW of asphaltene in most laser ionization mass spectrometry (LIMS) experiments is due to this inherent problem in the photophysics of asphaltene. Laser photo-ionization is a complicated process that has its limitations, and its non-critical use can lead to misleading results.

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Section: Discussionmentioning

confidence: 98%

Cause of Asphaltene Fluorescence Intensity Variation with Molecular Weight and Its Ramifications for Laser Ionization Mass Spectrometry

2009

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“…In [12][13][14], the spectroscopy of natural circular dichroism was applied for the first time in studies of structural pecu liarities of oil systems. However, these publications caused controversy [15], which was related to certain drawbacks of the experimental technique for studying circular dichroism. The spectroscopy of magnetic cir cular dichroism is devoid of these drawbacks.…”

Section: Introductionmentioning

confidence: 99%

Magneto-optical activity of crude oil and its heavy fractions

et al. 2012

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INTRODUCTIONElectron spin resonance (ESR) and optical absorp tion spectroscopy methods have been widely used to reveal particular features of the molecular structure of oil and its high molecular components, primarily res ins and asphaltenes. The ESR method makes it possi ble to study the state of paramagnetic ions, in particu lar, V 4+ ions, which, as a rule, are contained in oil as porphyrin complexes of vanadyl VO 2+ [1,2]. In the ground state, the V 4+ ion has the 3d 1 configuration with the spin 1/2. A strong covalent bond, which is deter mined by a short V-O distance, leads to an axial dis tortion of the nearest environment of the vanadium ion and, as a consequence, to a strong dependence of optical and magnetic resonance spectra on the proper ties of the compound that contains VO 2+ complexes. This particular feature allowed researchers to success fully use the ESR method and the optical absorption spectroscopy for obtaining information on the local symmetry, degree of covalence, structure of electron levels, presence of defects in the matrix, etc., in com pounds that contain vanadyl complexes (e.g., [3][4][5]).In studies of the properties of oils and their heavy fractions, special attention is paid to the investigation of their IR spectra [6], which contain a large number of characteristic narrow peaks that make interpreta tion of molecular structures easier [6]. In most cases, the visible and UV spectra are described by rather smooth curves [3], which do not contain characteristic peaks and, therefore, are considered to be less infor mative. However, the absence itself of characteristic features in these spectra is directly related to the prop erties of heavy oil fractions. A specific brownish color of resins and asphaltenes is one of their characteristic features [7,8] and is determined by the occurrence of broad electronic absorption bands in the visible and UV ranges. The absorption spectra in these ranges have been studied in [9][10][11], in which the use of low concentration oil solutions made it possible to reveal a number of resonant bands in the UV and visible spec tral ranges. These works demonstrated the potential of the UV and visible spectroscopy for studying samples of crude oil.Methods of polarization spectroscopy that study natural optical activity and magneto optical effects could significantly complement the absorption spec troscopy. In both situations, the main effect under study is circular dichroism, more specifically, natural circular dichroism and magnetic circular dichroism. The natural optical activity of oil is mainly related to carbon atoms that occupy asymmetric positions, i.e., are related to four different atoms or radicals. The optical activity is very sensitive to any changes in the structure of the substance and to intermolecular inter action; therefore, it can yield valuable information on the nature of substituents in molecules (both organic and complex inorganic compounds) and on their con formations, internal rotation, etc. The magneto opti cal activity is related to t...

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Optical Interrogation of Petroleum Asphaltenes: Myths and Reality

Evdokimov

2017

Analytical Characterization Methods for Crude Oiland Related Products

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Suggested “New Method for Determination of Dispersity in Petroleum Systems” Is Based on Trivial Experimental Artifacts

Cited by 3 publications

References 18 publications

Cause of Asphaltene Fluorescence Intensity Variation with Molecular Weight and Its Ramifications for Laser Ionization Mass Spectrometry

Cause of Asphaltene Fluorescence Intensity Variation with Molecular Weight and Its Ramifications for Laser Ionization Mass Spectrometry

Magneto-optical activity of crude oil and its heavy fractions

Optical Interrogation of Petroleum Asphaltenes: Myths and Reality

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