Hybrid CFD-DSMC Simulation of Ion Transport in Photoionization Mass Spectrometry: From Atmospheric Pressure to High Vacuum

Abstract

Rationale

The ion motion within a mass spectrometer is governed by the coupling of gas dynamics and electric fields. Therefore, a comprehensive understanding of ion transport from atmospheric pressure to the high-vacuum mass analyzer is crucial.

Methods

In this work, a hybrid computational fluid dynamics–direct simulation Monte Carlo (CFD-DSMC) strategy was employed to accurately resolve the cross-scale flow fields spanning from the continuum to the rarefied regime in the mass spectrometer. Subsequently, a multiphysics model integrating gas dynamics, electric fields, and ion trajectories was developed to achieve high-precision predictions of ion transport behavior.

Results

To validate the accuracy of the theoretical simulation, ion transmission efficiencies of acetone and toluene were experimentally measured using a homemade low-pressure photoionization time-of-flight mass spectrometer (LPPI-TOF-MS). The experimental results demonstrated a strong correlation with the simulation predictions, achieving a maximum Pearson correlation coefficient (r) of 0.96 for toluene and 0.90 for acetone in the radio-frequency-only quadrupole (RFQ) region, confirming the model's reliability.

Conclusions

This study provides a robust theoretical tool for elucidating ion transport mechanisms and guiding the optimization of ion optics in mass spectrometers.