Expanding the detectable molecular weight range of crude oil heteroatomic compounds via normal-phase LC-MS

Crude oil is an extremely complex mixture characterized by a wide boiling point range and broad mass distribution. Direct mass spectrometric analysis, particularly with electrospray ionization (ESI), often exhibits severe ionization suppression toward less polar or high-molecular-weight components, which hinders comprehensive molecular characterization. Here, we demonstrate that coupling normal-phase liquid chromatography (NPLC) with high-resolution mass spectrometry (HRMS) effectively alleviates this suppression via separation based on polarity and functional groups. This pre-separation ensures that compounds entering the ESI source possess similar ionization efficiencies, thereby expanding the detectable dynamic range of the ionization source. Furthermore, by constructing multi-ion extracted ion chromatograms (MI-EIC), we were able to resolve critical structural information. For instance, O1 class compounds (phenols) were resolved into multiple distinct isomer series, while the O2 class revealed not only classical naphthenic acids but also phenolic and aromatic acid subclasses. Additionally, the retention behaviors of multi-heteroatom compounds (N1S1, N1O1, N1O2) provided new insights into their functional group assignments, showcasing NPLC-HRMS as a powerful tool in advanced petroleomic characterization.