Double-bond equivalence linear equations for structural interpretation of fossil hydrocarbons

Abstract

The structural elucidation of hydrocarbons still remains a challenge for petroleomics characterization using ultrahigh-resolution mass spectrometry (UHRMS). Double-bond equivalence (DBE) linear equations were proposed for better predicting and understanding the molecular structure of hydrocarbons in crude oils. They were defined as the linear equation in the Cartesian coordinate system with carbon number as the abscissa and DBE as the ordinate, where y-intercepts represented the starting core structures and slopes described the structural increments. According to the magnitude of the slope determined by ΔDBE/ΔC (carbon number), DBE linear equations were divided into three types: alkyl homologous equation, naphthenic homologous equation, and aromatic homologous equation. Any structure of fossil hydrocarbons can be represented by the combination of these three types of equations, as all typical types of structural increments and their adding modes were considered. Furthermore, a distribution map of aromatic hydrocarbons was constructed based on DBE linear equations, which could provide an intuitionistic illustration of the ring structures, especially the aromatic structures of fossil hydrocarbons. The feasibilities of the distribution map to distinguish naphthenic and aromatic rings and to classify the numbers of aromatic rings were verified by experimental results obtained by on-line coupling of normal phase liquid chromatography and ultrahigh-resolution mass spectrometry. In real sample analysis, combination of DBE linear equations and tandem MS was demonstrated to be a superior way for structural elucidation, because DBE linear equations could provide a confined database for effective structural search.