Separation and characterization of petroleum asphaltene fractions by ESI FT-ICR MS and UV-vis spectrometer

Using heptane, toluene, and tetrahydrofuran (THF) as eluant, asphaltenes were fractionated into five fractions based on their polarity and solubility. The molecular composition of polar heteroatom species in both asphaltene and its fractions were analyzed by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). The application of UV-vis spectrometer in characterizing asphaltene composition and measuring asphaltene concentration was discussed. About 11.9 wt% asphaltene components adsorbed permanently on silica gel in the extrography column after excessive elution with various solvents. In negative FT-ICR MS, the mass spectra show that acidic and neutral nitrogen-containing compounds such as N₁ and N₁S₁ mainly existe in the first three less polar fractions, while oxygen-containing compounds such as O₂, O₂S, O₂S₂, O₃, and O₄ show high relative abundance in more polar fractions. These results suggest oxygen-containing compounds have stronger adsorption ability with silica gel. It was observed that the double bond equivalence (DBE) distribution of N₁ class species in the fractions shifted to higher values while the carbon number shifted to smaller numbers as polarity of fractions increased. This indicates that acidic and neutral N₁ compounds with longer carbon chain and less aromaticity have less polarity compared with those with shorter carbon chain and stronger aromaticity. UV-vis absorbance indicats that fractions containing the most aromatic and most polar asphaltene have better absorbance at long wavelength, while the fractions that consist of least aromatic and least polar asphatlenes show high absorbance at short wavelength.