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Abstract

Electrospray ionization (ESI) is the most common ionization method in atmospheric pressure ionization mass spectrometry because of its easy use and handling and because a diverse range of components can be effectively ionized from high to medium polarity. Usually, ESI is not employed for the analysis of non-polar hydrocarbons, but under some circumstances, they are effectively ionized. Polyaromatic hydrocarbons and aromatic heterocycles can form radical ions and protonated molecules after ESI, which were detected by Fourier transform ion cyclotron resonance mass spectrometry. The highly condensed aromatic structures are obtained from a heavy crude oil, and the results show class distribution from pure hydrocarbons up to more non-basic nitrogen-containing species. By using different solvent compositions [toluene/methanol (50/50 v/v), dichloromethane/methanol (50/50 v/v), dichloromethane/acetonitrile (50/50 v/v) and chloroform], the results show that the lack of proton donor agent helps to preserve the radical formation that was created at the metal/solution interface inside the electrospray capillary. The results demonstrate that with an appropriate selection of solvent and capillary voltage, the ratio between the detected radical ion and protonated molecule form can be manipulated. Therefore, ESI can be expanded for the investigation of asphaltene and other polyaromatic systems beyond the polar constituents as non-polar hydrocarbons can be efficiently analyzed.