Unlocking PAH Ionization in Negative-Mode ESI Orbitrap MS Using Tetramethylammonium Hydroxide: A Petroleomic Strategy

The selective ionization of cyclopentadiene-derived polycyclic aromatic hydrocarbons (PAHs) in complex matrices remains a persistent challenge in direct ionization mass spectrometry, particularly under atmospheric pressure ionization (API) conditions. In this study, tetramethylammonium hydroxide (TMAH) was evaluated as a solvent modifier to enhance the ionization efficiency of PAHs in negative-ion mode using high-resolution Orbitrap MS. A systematic assessment with equimolar mixtures of neutral and acidic model compounds was performed to elucidate the fundamental effects of TMAH on ion suppression, deprotonation mechanisms, and gas-phase ion chemistry. The incorporation of TMAH significantly improved the detection of weakly acidic and neutral PAHs by promoting efficient gas-phase deprotonation, thus overcoming conventional ESI limitations. To demonstrate the method's analytical robustness, TMAH-assisted ESI was subsequently applied to crude oil samples, enabling the detection of hydrocarbon species otherwise inaccessible under standard conditions, with a particular focus on low-alkylated fluorene derivatives. These results establish a simple and effective strategy for expanding the analytical scope of ESI-MS toward nonpolar and weakly acidic hydrocarbons, offering a valuable tool for the advanced molecular characterization of complex organic mixtures.