Collisional focusing effects in radio frequency quadrupoles

Area of Science:

• Analytical Chemistry
• Mass Spectrometry
• Ion Optics

Background:

• Conventional radiofrequency-only quadrupoles are typically operated at low pressures.
• Understanding ion behavior at higher pressures is crucial for advanced mass spectrometry applications.

Purpose of the Study:

• To investigate ion transmission through a radiofrequency-only quadrupole at high operating pressures.
• To characterize the phenomenon of "collisional focusing" and its dependence on ion mass and pressure.
• To explore the potential applications of high-pressure quadrupole operation.

Main Methods:

• Experimental measurements of ion signals (mass-resolved and total ion currents) at pressures from 5 × 10⁻⁴ to 1 × 10⁻² torr.
• Systematic variation of initial ion injection energy (1-30 eV).
• Monte Carlo simulations to model ion energy loss.

Main Results:

• Ion transmission through a quadrupole exit aperture initially increases with pressure, peaking at approximately 8 × 10⁻³ torr, before decreasing.
• "Collisional focusing" was observed, enhancing transmission at higher pressures, contrary to classical scattering theory.
• Collisional focusing increases with ion mass (up to 16,950 u) and is accompanied by significant axial kinetic energy loss.

Conclusions:

• High-pressure operation of radiofrequency-only quadrupoles can lead to enhanced ion transmission via collisional focusing.
• The observed phenomenon is analogous to effects in 3D ion traps and suggests potential for improved ion sampling from high-pressure sources.
• Monte Carlo simulations support the observed energy loss mechanisms, validating the experimental findings.