Structure characterization of polyaromatic hydrocarbons in Arabian mix vacuum residue by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Abstract

Molecular formulas of constituents in vacuum residue were characterized by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Evaluation of electrospray ionization (ESI) ability for hydrocarbons by using model compounds indicates that aromatic compounds having more than two fused rings without functional group are detectable as molecular ions, while that basic nitrogen-containing compounds produce protonated ions in the ESI solvent of methanol/chloroform. Thus, even peaks appear for both hydrocarbons and nitrogen-containing compounds in methanol/chloroform. Although basic nitrogen compound detected selectively in mixture of equal molar concentration of hydrocarbons, hydrocarbon could be observed almost the same intensity when the concentration of nitrogen compounds was adjusted as low as that of Arabian mix vacuum residue (AM-VR: N 0.4 wt.%). When ESI solvent of methanol/chloroform/trifluoroacetic acid (TFA) was used, protonated hydrocarbons produced predominantly as odd peaks. Thus, it was revealed that peaks originating from nitrogen-containing compounds and hydrocarbons can be clearly distinguished by changing composition of ESI solvents. In application to AM-VR, protonated nitrogen-containing compounds ([C n H 2 n+ Z N + H] + and [C n H 2 n+ Z NS + H] +; even masses) were observed selectively in methanol/chloroform, and both protonated nitrogen-containing compounds and protonated or sodium-cationized hydrocarbons ([M + H] + or [M + Na] +; odd masses) were observed simultaneously in the solvent composition of methanol/chloroform/TFA.