Mass spectrometry of acoustically levitated droplets

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Spectroscopy

Background:

• Containerless sample handling, like acoustic levitation, avoids surface adsorption issues in mass spectrometry.
• Interfacing levitated droplets with mass spectrometers is crucial for realizing these benefits.
• Understanding air-droplet interface phenomena and ionization processes is essential.

Purpose of the Study:

• To develop and demonstrate an effective interface for acoustically levitated droplets with mass spectrometry.
• To utilize a novel charge and matrix-assisted laser desorption/ionization (CALDI) technique for analyzing levitated droplets.
• To investigate ion generation and extraction from levitated droplets.

Main Methods:

• Acoustic levitation was used to suspend a 5-microL droplet containing peptides and an ionic matrix.
• A four-ring electrostatic lens and corona needle were employed for droplet charging.
• Charge and matrix-assisted laser desorption/ionization (CALDI) with a 337-nm laser pulse was used for ion generation.
• Atmospheric sampling mass spectrometry detected the analyte ions.
• Delayed ion extraction experiments were performed.

Main Results:

• Mass spectra were successfully obtained from acoustically levitated droplets using the CALDI technique.
• Both positive and negative ions were produced, similar to atmospheric pressure matrix-assisted laser absorption/ionization.
• No ion signal was detected without droplet charging, indicating its necessity.
• The ion generation and extraction cycle operated at 20 Hz.

Conclusions:

• The developed CALDI technique effectively interfaces acoustically levitated droplets with mass spectrometry.
• Droplet charging is crucial for ion generation, likely by enhancing the surface electric field and reducing charge recombination.
• This method offers a promising approach for containerless analysis and studying interfacial phenomena.