Analytical advanced techniques in the molecular-level characterization of Brazilian crude oils

Abstract

The study focused on the analytical characterization at the molecular level of twenty-five Brazilian oils by applying two analytical advanced techniques: comprehensive bidimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOFMS) and negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI(−)-FT-ICR MS), using crude oil samples directly with minimal sample preparation and without any fractionation steps. First, individual compounds present in the classes were detected, identified and semi-quantified using a polar/non-polar column set, which has proved to be more effective in separating compounds. Later, the ESI(−)-FT-ICR MS was optimized for crude oil analysis and characterization of the less volatile polar compounds (oxygen-, nitrogen- and sulfur-containing compounds). Many classes were identified via GC×GC-TOFMS, such as n-alkanes, branched alkanes, alkyl-monocyclic alkanes, alkyl-bicyclic alkanes, saturated polycyclic, alkyl-benzenes, alkyl-naphthalenes, alkyl-phenanthrenes, alkyl-chrysene, alkyl-indanes, alkyl-tetrahydro-naphthalenes, pyrenes, fluorenes and sulfur compounds. The high chromatographic resolution power of GC×GC-TOFMS (for volatile compounds) with the high dynamic m/z range of FT-ICR MS allowed the chemical characterization of the crude oils with the identification of polar, volatile and nonvolatile compound classes. ESI(−)-FT-ICR MS data suggest that many heteroatomic compounds were detected, with m/z values ranging from 200 to 800, resulting in percentages of assignable peaks higher than 75% (composed of O3, N2, O2, NO2, O and N heteroatom classes). All assignable signals showed ultrahigh resolution and accuracy mass. Detailed information on the composition of these oils is very important for predicting the physical and chemical characteristics of the crude oil (e.g., total acid number) and its products using small sample amounts, and offers new possibilities for crude oil fingerprinting.