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

Area of Science:

• Analytical Chemistry
• Mass Spectrometry

Background:

• Selective ionization of polycyclic aromatic hydrocarbons (PAHs) in complex matrices is challenging for direct ionization mass spectrometry, especially under atmospheric pressure ionization (API).
• Existing methods often struggle with the ionization of weakly acidic and neutral PAHs due to ion suppression and limited deprotonation.

Purpose of the Study:

• To evaluate tetramethylammonium hydroxide (TMAH) as a solvent modifier for enhancing PAH ionization in negative-ion mode.
• To investigate the fundamental effects of TMAH on ion suppression, deprotonation, and gas-phase ion chemistry in mass spectrometry.
• To demonstrate the application of TMAH-assisted electrospray ionization (ESI) for analyzing complex hydrocarbon mixtures.

Main Methods:

• Systematic assessment using equimolar mixtures of neutral and acidic model compounds.
• High-resolution Orbitrap mass spectrometry (MS) with tetramethylammonium hydroxide (TMAH) as a solvent modifier.
• Application of TMAH-assisted ESI to crude oil samples for hydrocarbon analysis.

Main Results:

• TMAH significantly improved the detection of weakly acidic and neutral PAHs by promoting efficient gas-phase deprotonation.
• The method overcame limitations of conventional ESI, enhancing ionization efficiency.
• TMAH-assisted ESI enabled the detection of previously inaccessible hydrocarbon species in crude oil, including low-alkylated fluorene derivatives.

Conclusions:

• TMAH is a simple and effective strategy to expand the analytical scope of ESI-MS for nonpolar and weakly acidic hydrocarbons.
• This approach provides a valuable tool for advanced molecular characterization of complex organic mixtures.
• The study highlights a novel method for improving PAH analysis in challenging matrices.