Exploring Fluoroquinolone Protomer Populations Formed with MALDI and Plasma Post-Ionization Mass Spectrometry

Area of Science:

• Analytical Chemistry
• Mass Spectrometry
• Physical Chemistry

Background:

• Molecules with multiple protonation sites can form isomers (protomers) in mass spectrometry.
• Protonation site changes can alter collision-induced dissociation spectra, hindering analyte identification.
• Understanding protomer formation is crucial for accurate mass spectrometry workflows.

Purpose of the Study:

• To investigate the formation of protomers for ten fluoroquinolone antibiotics.
• To compare protomer formation in conventional matrix-assisted laser desorption ionization (MALDI) versus atmospheric pressure MALDI plasma postionization (AP-MALDI-PPI) sources.
• To assign protonation sites using trapped-ion mobility spectrometry (TIMS) and collision-induced dissociation (CID).

Main Methods:

• Analysis of ten fluoroquinolone antibiotics using conventional MALDI and AP-MALDI-PPI.
• Separation of fluoroquinolone protomers using TIMS.
• CID experiments to generate fragment ions for protonation site assignment.
• Comparison of experimental collisional cross sections (CCS) with theoretical calculations.

Main Results:

• Conventional MALDI primarily forms the most stable protomer, indicating a thermodynamically driven process.
• AP-MALDI-PPI generates 1-3 protomers, suggesting a different, kinetically influenced protonation mechanism.
• Protomer populations can be manipulated by matrix choice and plasma doping (methanol/acetonitrile).
• TIMS-CID successfully separated and characterized fluoroquinolone protomers, assigning protonation sites.

Conclusions:

• Ionization source significantly impacts fluoroquinolone protomer formation and ratios.
• Sample preparation and analyte structure also influence protomer populations and ion mobility distributions.
• Accurate analyte confirmation in mass spectrometry requires consideration of protomer behavior and ionization conditions.