When API Mass Spectrometry Meets Super Atmospheric Pressure Ion Sources

Area of Science:

• Analytical Chemistry
• Mass Spectrometry
• Physical Chemistry

Background:

• The free-jet technique and electrospray ionization (ESI) have been explored for gas-phase ionization at pressures exceeding atmospheric levels.
• Early ESI mass spectrometers utilized counter-flow curtain gas sources operating at super-atmospheric pressures (e.g., 1050 Torr).
• Theoretical analyses suggest super-atmospheric operation is preferable for gaseous ionization in space-charge-limited scenarios.

Purpose of the Study:

• To review ion sources for mass spectrometry that operate at pressures significantly above 1 atmosphere.
• To discuss the advantages and historical context of high-pressure gas-phase ionization techniques.
• To highlight research and laboratory applications that utilize super-atmospheric pressure ion sources.

Main Methods:

• Review of existing literature on high-pressure ionization sources for mass spectrometry.
• Discussion of theoretical considerations for super-atmospheric operation in ESI.
• Examination of practical implementations and challenges of running ion sources at elevated pressures.

Main Results:

• High-pressure operation (up to 1000 Torr and above) has been demonstrated to be advantageous in various ionization techniques, including ESI.
• Despite theoretical benefits, most commercial instruments operate at atmospheric pressure due to safety, convenience, and maintenance concerns.
• Several research groups, including the authors' laboratory, have successfully implemented and studied ion sources operating at pressures above 1 atm.

Conclusions:

• Operating ion sources at pressures above atmospheric pressure is feasible and offers potential advantages for specific mass spectrometry applications.
• Further research and development may lead to wider adoption of super-atmospheric pressure ion sources, overcoming current practical limitations.
• This review provides insights into the historical development and ongoing exploration of high-pressure gas-phase ionization for enhanced mass spectrometry analysis.