Aggregation of petroporphyrins and fragmentation of porphyrin ions: Characterized by TIMS-TOF MS and FT-ICR MS

Abstract : Vanadyl porphyrins in the heavy crude oil were enriched and characterized by trapped ion mobility spectrometry-time of flight mass spectrometry (TIMS-TOF MS) and Fourier transform ion cyclotron resonance mass spectrometry. Two series of mobiligram peaks were present in the extracted ion mobility spectra of the petroporphyrins. Collision induced dissociation (CID) revealed that one peak series was [M + H]+, while the other peak series was cluster ions, which may contain an amine in the ions. Model compounds were used to investigate the aggregation and fragmentation behavior of porphyrin ions. It was found that all the porphyrins tend to form adduct ions in the electrospray ionization, such as [2 M]+, [2 M + H]+, [2 M + Na]+ and cluster ions of porphyrin with other compounds. This implies the strong molecular aggregation potential of porphyrins with themselves and other petroleum molecules. The porphyrin cluster ions generated in ionization source could decompose and continuously disaggregate during the transmission in the MS. The [M]+ and [M + H]+ were dissociated into fragment ions and the cluster ions were disaggregated into [M]+ and [M + H]+ simultaneously with high collision energy. The CID analysis of Zn-OEP porphyrin showed that [M + H]+ has a lower dissociation energy than [M]+. The results provide instructive information for the understanding of aggregation and fragmentation behavior of petroporphyrins as well petroleum asphaltenes.