CHEMICAL BASIS OF FLUORESCENCE ALTERATION OF CRUDE OILS AND KEROGENS .2. FLUORESCENCE AND INFRARED MICRO-SPECTROMETRIC ANALYSIS OF VITRINITE AND LIPTINITE

Samples of pure vitrinite, sporinite and cutinite have been studied to understand the photochemical basis of their temporal fluorescence alteration (TFA), under constant u.v. irradiation, in air. TFA is the result of photo-oxidation reactions mainly affecting the extractable fraction; two different mechanisms compete: (1) Rapid oxidation releases hydrocarbons from the kerogen macromolecular structure and consumes aliphatic chains in the mobile phase. The dominant FTIR response is a rise of C=O groups. Fluorescence declines and undergoes a red shift by oxygen and concentration quenching (negative TFA). (2) Depending on the mobile phase polarity, slow formation of a "loose molecular network" occurs through oxygen bridging (alkyl- and aryl-ethers, aromatics substitution by aliphatics and carbonyl groups). Fluorescence increases by dilution of fluorophores, sensitized fluorescence and phosphorescence (positive TFA). Natural samples exhibit intermediate behaviors depending on their own maturity and chemical composition. TFA spectra give valuable information both on the extractable fraction and oxidation state of the macerals.