Simulation Study of a Magnetic Focusing Drift Tube for Proton Transfer Reaction Mass Spectrometry

Proton transfer reaction mass spectrometry (PTR-MS) is widely used for online detection of volatile organic compounds. Sensitivity enhancement is an important development trend. The drift tube is a critical component that affects sensitivity. Ion transmission efficiency in the conventional direct current (DC) drift tube is low. Radio frequency focusing technology is commonly used in the drift tube to improve ion transmission efficiency, but there is still a problem of low-mass discrimination. We propose a novel magnetic focusing drift tube to avoid low-mass discrimination and improve ion transmission efficiency. Ion focusing transmission is achieved by introducing magnetic rings and magnetic mesh inside and outside the drift tube. Combined with SIMION simulation, the influence of magnet geometric parameters and magnetic flux density on ion focusing transmission was systematically studied. The simulation results show that the greater the difference in magnetic flux density between the inner and outer magnets, the higher the ion transmission efficiency. At a difference of 14.99 T, the ion transmission efficiency of m/z 19, 55, and 100 is 92.2%, 94.3%, and 93.6%, which is 83.3, 52.3, and 55.1 times higher than that of the conventional DC drift tube, respectively. Under a practically achievable magnetic flux density of 1-2 T, the proposed design still delivers a significant enhancement in sensitivity compared to the conventional DC drift tube. Moreover, the similar transmission efficiency of the 10 ions indicates no low-mass discrimination. In addition, the mean kinetic energy of the magnetic focusing drift tube is lower than that of the conventional DC drift tube, retaining the characteristic of soft ionization. The magnetic focusing drift tube not only provides a new technology for high-sensitivity PTR-MS but also has the potential to be used in other mass spectrometry to improve ion transmission efficiency.