Oxidative Ionization Under Certain Negative-Ion Mass Spectrometric Conditions

Area of Science:

• Analytical Chemistry
• Mass Spectrometry
• Physical Chemistry

Background:

• Phenolic compounds are prevalent in various chemical and biological systems.
• Understanding their ionization mechanisms in mass spectrometry is crucial for accurate analysis.
• Existing methods may not fully elucidate the behavior of compounds like 1,4-hydroquinone.

Purpose of the Study:

• To investigate the negative-ion mass spectrometric behavior of 1,4-hydroquinone.
• To differentiate between oxidative ionization and deprotonation pathways.
• To explore the role of superoxide radical-anion (O2•−) in ionization.

Main Methods:

• Negative-ion mass spectrometry using helium-plasma ionization (HePI).
• Comparison of spectra from 1,4-hydroquinone and 1,4-benzoquinone.
• Analysis under varying electrospray ionization (ESI) conditions and capillary voltages.
• Monitoring ion intensities to estimate atmospheric pressure chemical ionization (APCI) contribution.

Main Results:

• 1,4-Hydroquinone generates (M - 2)•− radical-anions via oxidative ionization, not deprotonation, under HePI.
• Spectra from HePI of 1,4-hydroquinone and 1,4-benzoquinone were nearly identical.
• O2•− plays a critical role, forming an adduct with 1,4-hydroquinone.
• ESI predominantly yields (M - 1)− ions, but higher voltages shift towards oxidative ionization.

Conclusions:

• Oxidative ionization, involving O2•−, is a significant pathway for 1,4-hydroquinone under specific mass spectrometry conditions.
• The ionization mechanism is dependent on the source conditions, particularly voltage.
• This study provides insights into distinguishing ionization pathways for phenolic compounds.