Alkyl carbon number limits the ring-number separation of polycyclic aromatic compounds in petroleum analysis

•Application of a normal-phase liquid chromatographic method for the separation of a crude oil according to the number of aromatic rings.•Three mechanisms responsible for inefficient separation of alkylated aromatic compounds according to ring numbers were discussed.•The counteractive role of size exclusion mechanism in normal-phase chromatography during ring-number separation was revealed.•Experimental findings were corroborated through molecular modeling calculations.