Characterization of aromatic hydrocarbons and sulfur heterocycles in Saudi Arabian heavy crude oil by gel permeation chromatography and ultrahigh resolution mass spectrometry

Abstract

The characterization of petroleum crude oil samples remains an overwhelming task for any single analytical technique. Additionally, the compositional complexity of crude oil typically increases with increase in sulfur content and decrease in API gravity. In the present investigation we employed gel permeation chromatography (GPC) for separating Arabian Heavy crude oil into clearly defined fractions, and atmospheric pressure photo ionization Fourier transform-ion cyclotron resonance mass spectrometry (APPI FT-ICR MS) for elucidating the structural commonalities through in-depth characterization of the fractions. The work involved the development of a GPC method by the selection of suitable columns, mobile phase, and optimization of separation conditions. Toluene was found to be suitable as a mobile phase for the analysis of nonpolar aromatic compounds as well as compatible with the subsequent APPI FT-ICR MS measurement. The GPC fractionated aromatic compounds based on the number of carbon atoms in alkyl chains. Compounds with a higher number of carbon atoms in their alkyl chains elute earlier in the GPC separation. This observation was true for aromatic hydrocarbons (HC), mono-sulfur (S1) and di-sulfur (S2) containing compounds. Using respective planar aromatic cores as parent structures, we systematically calculated the average number of alkyl carbon atoms present in each GPC fraction. The average number of alkyl carbon atoms were found to be 36, 27, 17, 9 and 5 in successive GPC fractions (from early to late elution), with only two alkyl carbon atoms overlap between successive fractions.