Optimized column chromatography method for the separation of polycyclic aromatic compounds in petroleum

The separation and preparation of petroleum components are fundamental steps for molecular and isotopic analyses. Due to the compositional complexity and structural similarity of hydrocarbons, especially among aromatic isomers, conventional fractionation techniques may not exhibit sufficient selectivity for Compound-Specific Isotope Analysis (CSIA). This limitation is relevant for polycyclic aromatic hydrocarbons (PAHs), whose isomers can present distinct molecular and isotopic characteristics associated with the source, depositional environment, and thermal maturity. CSIA has been employed to determine the isotopic composition of individual aromatic hydrocarbons and to improve the characterization of petroleum sources. The origin of organic matter is one of the main factors controlling the δ¹³C values of low molecular weight aromatic hydrocarbons (LMWAHs). Isotopic variations in alkylphenanthrenes and alkylnaphthalenes have been associated with organic matter sources; however, thermal maturity can also influence these signatures. Literature reports that naphthalene and phenanthrene exhibit smaller isotopic variations with increasing maturity than substituted PAHs. Thus, the isotopic composition of individual aromatic compounds can assist in source discrimination and oil–source rock correlation. To obtain reliable isotopic measurements, efficient chromatographic separation and appropriate preparation of target compounds are required. An approach was proposed for the separation of low molecular weight PAHs from the aromatic fraction of crude oils. Initially, the aromatic fraction of an oil sample was obtained, followed by its separation on an alumina column. The obtained fractions were characterized by GC-MS to evaluate the efficiency of the process. The results indicate that the optimized method provided a separation of naphthalene- and phenanthrene-type aromatic compounds, as well as their alkyl homologs. Hexane/dichloromethane mixtures allowed for obtaining fractions enriched in these groups, respectively. This fractionation is relevant for CSIA because the enrichment of target compounds reduces coelution and chromatographic interferences, thereby increasing the reliability of isotopic measurements. Therefore, the combination of selective chromatographic separation and CSIA constitutes an efficient strategy to investigate molecular and isotopic characteristics and to evaluate organic matter sources, thermal maturity, depositional environments, and oil–source rock correlations.