Simulation Study of a Novel Cylindrical Direct Current-Ion Funnel to Improve Ion Transmission

Proton transfer reaction mass spectrometry (PTR-MS) is a widely used technique for on-line detection of volatile organic compounds (VOCs). Sensitivity enhancement is an important development direction. Although traditional radio frequency (RF) enhancement technology can suppress ion radial diffusion through dynamic electric fields, they inevitably introduce issues such as RF electric field complexity, ion fragmentation risk.

In this work, we proposed a novel cylindrical direct current (DC)-ion funnel drift tube, consisting of 20 conventional ring electrodes and 5 new cylindrical DC-focusing electrodes (DC-FEs) for use in PTR-MS. The dimensions of the DC-FEs, voltage, and pressure were simulated using ion trajectory simulation to analyze the ion transmission efficiency and the kinetic energy distribution.

The results show that the optimum ion transmission efficiency in the novel DC-ion funnel is 81.4% (m/z 100), which is a fivefold improvement compared to the reported spherical DC-ion funnel. Furthermore, the novel technique can be extended to the atmospheric ion guide. The simulation results show that the initial ion inside a circle with a diameter of 11.8 mm is gradually compressed toward the central axis, and all the ions can reach the last electrode within a diameter of 1 mm. The optimal ion transmission efficiency is 90.3% in the atmospheric ion guide.

The new cylindrical DC-ion funnel developed in this study has improved sensitivity solely through a DC electric field, achieving technological innovation without RF. Moreover, the novel structure proposed in this paper has a simpler structure and higher ion transmission efficiency than the reported spherical DC-ion funnel, making it highly suitable as the drift tube of PTR-MS. In addition, the novel cylindrical DC-ion funnel is highly expandable and is expected to be used in other mass spectrometry techniques for sensitivity enhancement.