Structure characterization of petroleum vacuum residues by in-beam EI Fourier transform ion cyclotron resonance mass spectrometry

A n-heptane soluble fraction (AH-VR-Sa) of Arabian heavy vacuum residues (AH-VR) has been analyzed by in-beam electron ionization Fourier transform ion cyclotron resonance mass spectrometry (EI FT-ICR MS). The probe temperature and the ionization energy of in-beam EI were independently adjusted to expand the detectable range of peaks originating from components of AH-VR-Sa and to prevent fragmentation of the molecular ions. The upper mass limit of the peaks was extended to m/z = 700 when the probe temperature and the ionization energy were set to 300degreesC and 30 eV, respectively. The average molecular weight of the spectrum increased by an increase in the probe temperature. The molecular formulas of compounds in AH-VR-Sa were determined essentially based on accurate mass measurements. Major components of being characterized were hydrocarbons both with and without one sulfur atom. The compound type estimated on the basis of hydrogen-deficiency index Z (CnH2n+Z) was discussed. The strongest peaks of hydrocarbons with sulfur atom showed the Z value of - 10. The compound type analysis indicates that sulfur atom may be located at an aromatic ring but not at a side chain. The effect of experimental conditions on the detectable compound type distribution was also discussed. The present results demonstrate that the characterization of vacuum residues by in-beam El FF-ICR MS is advantageous over the conventional EI Fr-ICR MS in the detectable range of peaks and in the versatility for samples. In-beam El FT-ICR MS is useful as a complementary method to characterize vacuum residues by electrospray ionization (ESI) FT-ICR MS, since ESI cannot ionize hydrocarbons with high Z values. (Int J Mass Spectrom 221 (2002) 93-105) (C) 2002 Elsevier Science B.V. All rights reserved.