Are Most Micrometer Droplets (>10 μm) Wasted in Electrospray Ionization? An Insight from Real-Time High-Speed Imaging

Area of Science:

• Analytical Chemistry
• Mass Spectrometry

Background:

• Electrospray ionization (ESI) is a primary technique for mass spectrometry (MS) analysis of nonvolatile compounds.
• ESI is inherently disordered, producing polydisperse droplets from a Taylor cone and jet.
• A significant portion of analyte signal in ESI-MS may originate from a small subset of droplets.

Purpose of the Study:

• To develop a system for sectioning electrospray plumes into discrete packets for MS analysis.
• To investigate the droplet size distribution and contribution to MS signals within these packets.
• To understand the fundamental processes governing analyte ionization in ESI-MS.

Main Methods:

• Development of a system to section electrospray plumes into millisecond and submillisecond lifetime packets.
• Utilizing a high-speed camera (10,000 frames per second) to capture images of electrospray packets.
• Correlating high-speed imagery with MS signals of acetaminophen to analyze droplet characteristics and their ionization efficiency.

Main Results:

• Quantified droplet numbers (<300), average diameters (10-20 μm), and volumes (tens of picoliters) in individual electrospray packets.
• Demonstrated that micrometer droplets (>10 μm) contribute minimally to MS signals.
• Confirmed the hypothesis that nanodroplets are the primary carriers of analyte molecules for ionization.

Conclusions:

• The majority of MS signals in ESI originate from nanodroplets, not larger microdroplets.
• Only a small fraction of emitted microdroplets effectively transport ionizable analyte molecules.
• Future ESI strategies could enhance efficiency by 'recharging' initial microdroplets.