Ion formation in droplet-assisted ionization.

Area of Science:

• Analytical Chemistry
• Mass Spectrometry
• Ionization Techniques

Background:

• Droplet-assisted ionization (DAI) generates intact molecular ions from aerosol droplets via a temperature-controlled capillary inlet.
• This study investigates ion formation by examining analyte mass flow, droplet solvent, and capillary temperature effects on ion signal intensity.

Purpose of the Study:

• To explore and optimize ion formation mechanisms in droplet-assisted ionization (DAI).
• To determine the impact of key experimental parameters on ion signal intensity for aerosol analysis.

Main Methods:

• A modified Waters SYNAPT G2-S mass spectrometer was utilized with a custom temperature-controlled capillary inlet for DAI.
• Analyte droplets were generated by atomization and passed through the inlet, with optional diffusion drying.
• Aerosol size distributions were measured to determine analyte mass flow.

Main Results:

• Significantly higher analyte signal intensities were observed with protic solvents (e.g., water) compared to aprotic solvents (e.g., acetonitrile).
• Optimal DAI performance was achieved at capillary inlet temperatures exceeding 500°C, though analyte-dependent.
• Adding 3-nitrobenzonitrile matrix to water-acetonitrile droplets enhanced ion formation to levels comparable to pure water.

Conclusions:

• Ion formation mechanisms in DAI appear similar to other inlet ionization techniques.
• Optimized ion yields for DAI are in the 10⁻⁵ to 10⁻⁶ range, suitable for various aerosol applications.