Doubly Charged Molecular Ions in GC-MS with Cold EI

Area of Science:

• Physical Chemistry
• Chemical Physics
• Mass Spectrometry

Background:

• Electron ionization (EI) is a common technique for molecular analysis.
• Understanding ion formation mechanisms is crucial for interpreting mass spectra.
• Previous studies focused on smaller molecules or less controlled vibrational states.

Purpose of the Study:

• To investigate the formation of multiply charged molecular ions in large molecules using cold electron ionization.
• To explore the influence of vibrational cooling on ion abundance and fragmentation.
• To elucidate novel ionization mechanisms in large organic compounds.

Main Methods:

• Utilizing supersonic molecular beams (SMB) for vibrational cooling of molecules.
• Employing electron ionization (EI) on vibrationally cold molecules.
• Analyzing ion abundance and fragmentation patterns using mass spectrometry.

Main Results:

• Doubly charged molecular ions were observed in various large molecules, including hydrocarbons.
• Triply charged molecular ions were detected in large polycyclic aromatic hydrocarbons (PAHs) like decacyclene and ovalene.
• Doubly charged ion abundance significantly increased with enhanced vibrational cooling and molecular size.
• A new ionization mechanism involving two ionization events on the same molecule was proposed to explain observations.

Conclusions:

• Vibrational cooling plays a critical role in the formation of multiply charged molecular ions.
• The size of the molecule influences the dominant ionization mechanisms.
• Electron ionization mass spectrometry of large molecules involves at least three distinct processes: single ionization, double ionization, and single ionization with subsequent internal excitation.