Molecular Characterization of Non-Basic Nitrogen Compounds in Crude Oils with High-Acid Number by HPLC-Orbitrap MS

Non-basic nitrogen compounds, primarily comprising carbazoles, benzocarbazoles, and their alkyl-substituted homologues, represent a significant class of polar heteroatomic constituents in crude oils. Their molecular composition and structural characteristics provide crucial theoretical insights and practical guidance for designing high-efficiency petroleum processing catalysts and developing advanced refining technologies. High-resolution mass spectrometry (HRMS) equipped with an electrospray ionization (ESI) source is widely recognized as a key analytical technique for identifying and quantifying these compounds in crude oils due to its high sensitivity and mass accuracy, enabling unambiguous molecular formula assignment. However, in ultrahigh-acid crude oils, the ionization of non-basic nitrogen compounds is often strongly suppressed by acidic oxygen-containing species such as naphthenic acids, a phenomenon that severely compromises detection accuracy and leads to incomplete characterization of their molecular composition. This suppression arises from the competitive nature of ESI, in which compounds with higher ionization efficiency, such as naphthenic acids, preferentially consume available charges and dominate the mass spectrum. In this study, normal-phase high-performance liquid chromatography coupled with Orbitrap mass spectrometry (HPLC-Orbitrap MS) was employed to address this limitation. The pre-separation function of normal-phase HPLC, utilizing a polar amide column and a gradient of nonpolar to polar solvents, effectively isolates non-basic nitrogen compounds from interfering acids based on polarity differences, thereby significantly mitigating ionization suppression. Additionally, a post-column solvent delivery system was implemented to introduce methanol and ammonium hydroxide, facilitating efficient ionization of the separated analytes under stable conditions. This method enables the characterization of non-basic nitrogen compounds in ultrahigh-acid crude oils. A total of 1 144 nitrogen-containing compounds with carbon numbers ranging from 10 to 90 were successfully detected, including carbazoles, benzocarbazoles, and their alkylated homologues, covering a wide range of double bond equivalents (DBEs) from 6 to 20. Notably, compounds with carbon numbers exceeding 50, which are typically underrepresented in direct infusion analysis due to their lower ionization efficiency and suppression by more polar species, are readily detectable following chromatographic separation. The detected compound classes also include less abundant species such as indoles (DBEs 6-8), which are particularly susceptible to ionization suppression and rarely observed in MS analysis via direct infusion of high-acid crude oils. This method offers a reliable and efficient analytical approach for characterizing polar nitrogen compounds in complex crude oil systems and contributes to a deeper understanding of the geochemical behavior and transformation mechanisms during petroleum formation and refining, with potential applications in geochemical correlation studies and process optimization in refineries.