Molecular dynamics simulation of ion mobility in gases

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Analytical Chemistry

Background:

• Ion mobility spectrometry (IMS) is a powerful analytical technique.
• Accurate theoretical prediction of ion mobility and collision cross sections (CCSs) is crucial for IMS.
• Existing methods may not fully capture the complexities of ion-gas interactions.

Purpose of the Study:

• To develop a novel force field molecular dynamics method for simulating ion drift in buffer gases under electric fields.
• To enable direct calculation of ion mobility and CCSs relevant to IMS.
• To assess the accuracy and efficiency of the developed method.

Main Methods:

• A force field molecular dynamics approach was employed to simulate ion drift.
• Simulations were conducted in high-density buffer gases (∼50 bars) and high electric fields (∼10⁷ V/m).
• Element-based Lennard-Jones parameters were used without specific tuning for different molecules.

Main Results:

• The method accurately simulates ion drift, yielding ion mobility and CCS values.
• Simulated CCS values for 15 small molecular ions showed good agreement with experimental data (mean unsigned error of 2.6 Ų for He, 4.4 Ų for N₂).
• The method can achieve ±1%-2% convergence for CCS values within 5-19 hours of simulation time.

Conclusions:

• The developed molecular dynamics method provides a direct and efficient way to simulate ion drift and determine key IMS parameters.
• This approach simultaneously accounts for ion internal dynamics and ion-gas collisions, outperforming trajectory methods.
• The findings support the utility of this method for predicting CCSs and advancing IMS applications.