Investigation of aging processes in bitumen at the molecular level with high-resolution fourier-transform ion cyclotron mass spectrometry and two-dimensional gas chromatography mass spectrometry.

Bitumen is a highly viscous and chemically complex petroleum-derived material, which is applied as a binder in road construction. However, the asphalt undergoes hardening, cracking, and embrittlement not only due to oxidative short-term aging during the mixing and paving process but also due to long-term aging during the service time of the pavement. In this study, chemical changes occurring during short-term aging, mimicked by a prolonged rotating flask procedure, are investigated for an artificial bitumen model at the molecular level. The model bitumen enables the application of two complementary analytical techniques for obtaining a comprehensive insight into the aging effects: high-resolution Fourier-transform ion cyclotron mass spectrometry (FT-ICR MS) coupled to thermogravimetry was applied to investigate the aging effects on polar to semipolar high-molecular-weight compounds ionized by atmospheric pressure chemical ionization. Aromatic core structures were analyzed by alternating collision-induced dissociation. In order to support structural assignments from FT-ICR MS data in the semivolatile region, comprehensive two-dimensional gas chromatography mass spectrometry (GC × GC-HRTOFMS) with electron ionization at 70 eV was applied for the group-type analysis and the investigation of particular chemical functionalities. Oxidation processes were revealed to be the prevalent reactions caused by short-term aging of the hydrocarbons (CH-class) and sulfur-containing classes. Aromatic species with low steric hindrance or activated carbon positions as well as high aromatic core structures are favorably oxidized, forming carbonyl functionalities. For molecules with one sulfur atom (S1-class), nonaromatic species such as tetrahydrothiophenes decrease, whereas aromatic S1-compounds remain constant. Nonaromatic S1O1-species tend to further oxidize, while higher aromatic species are formed with ongoing aging time. Moreover, this study highlights the aging behavior of nitrogen-containing compounds, such as carbazoles. A significant reduction of the N-classes was observed during aging, indicating thermal-induced condensation reactions as well as favored oxidation of highly aromatic core structures