Design and Simulation of Focusing Drift Tube for Proton Transfer Reaction Mass Spectrometer

Drift tube is one of the core components of proton transfer reaction mass spectrometer (PTR-MS). Technical innovation of the drift tube is one of the important ways to improve the performance of the instrument. In recent years, focusing drift tube (FDT) has been paid more and more attention because of its high ion utilization ratio. In this work, the design and simulation of a FDT combined with a radio frequency quadrupole and a resistor glass tube was reported. The quadrupole was used to provide an RF focusing field, and the resistor glass tube was used to generate a uniformly distributed DC gradient electric field. Based on the designed 3D structure, the simulation model was built in SIMION 8.1 and the parameters such as the diameter of the quadrupole, the frequency and the voltage amplitude of the RF field were simulated systematically with three kinds of ions (m/z 19, 55 and 100). The results showed that the introduction of the RF field was very effective in reducing the radius of the ion beam, and the rod with larger diameter was beneficial to improve the ion focusing efficiency, especially for the ions with low mass-charge ratio. When the frequency was about 2 MHz, the focusing efficiency was the highest. With the increase of the voltage amplitude, the radius of ion beam decreased rapidly and finally tended to smooth. In addition, more than 85% of the ions could successfully pass through the sampling orifice with a diameter of 1.0 mm, and the transport efficiency was about 28 times higher than that of the traditional drift tube. In this study, structural design, SIMION modeling and ion transmittance of RF quadrupole focusing drift tube (RFQ-FDT) were described in detail, hoping to provide some technical support for follow-up research.