Establishing Atmospheric Pressure Chemical Ionization Efficiency Scale

Area of Science:

• Analytical Chemistry
• Mass Spectrometry

Background:

• The ionization mechanisms in atmospheric pressure chemical ionization (APCI) sources are not fully understood.
• Existing assumptions about APCI may oversimplify its complex chemical processes.

Purpose of the Study:

• To develop the first ionization efficiency (IE) scale for an APCI source.
• To investigate the factors influencing compound ionization in APCI.
• To compare APCI ionization behavior with electrospray ionization (ESI).

Main Methods:

• Creation of an ionization efficiency scale spanning over 5 logIE units.
• Inclusion of 40 diverse compounds with varied chemical and physical properties.
• Experimental analysis of ionization efficiency across different compound classes.

Main Results:

• The ionization efficiency order for most compounds in APCI mirrors that observed in ESI.
• Larger molecules, such as tetraalkylammonium cations, are efficiently ionized in APCI.
• Low-polarity hydrocarbons like pyrene and anthracene are ionized by APCI but not ESI.

Conclusions:

• APCI likely involves multiple parallel ionization mechanisms.
• A non-evaporation-based mechanism plays a more significant role in APCI than previously thought.
• The findings necessitate a revised understanding of APCI ionization processes.