Measurements of asphaltenes in vacuum residues by laser desorption ionization mass spectroscopy

Molecular weight distribution measurements of asphaltenes in five types of petroleum vacuum residues (VR) were performed by laser desorption ionizatio n mass spectroscopy (LDI-MS). The mass spectrum was obtained by optimizing the contents of the solution a nd laser power. For Arabian Heavy VR (AH-VR) asphaltene, its mass spectrum showed a large peak up to m/z 6000 with a bi-modal profile, that is, a smal l peak near m/z 400 and a large and broad peak near m/z 1500 for the others . Similar distribution profiles were obtained for those in another type of VR. The average molecular weight obtained by mass spectrum was always small er than that obtained by vapor pressure osmometry or by gel permeation chro matography. The mass spectra for the three fractions, saturate, aromatic, and resin, se parated from maltene in AH-VR were also obtained. The mass range of the sma ller peak at near m/z 400 for the asphaltene was almost the same as that of aromatic and/or resin in the maltene. Based on the mass spectra of four fractions separated by solvent extraction and GPC for AW-VR asphaltene, the peak near m/z 400 would correspond to so me components in the resin drawn at the solvent extraction. By increasing the laser power of LDI-MS for AH-VR asphaltene, sharp homogen eous peaks classified in different types of aromatic hydrocarbons were obse rved at the interval of m/z 24 over a range from 700 to 5000, accompanied w ith many fragmentary peaks. It suggests that AW-VR asphaltene consists of v arious groups of homologous derivatives attached to a given condensed aroma tic ring system. Although similar periodic peaks for SL-VR to those for AH-VR are observed, a different distribution was shown by group analysis. The LDI-MS is applica ble to characterization and structural analysis of asphaltene.