Simulation of Sensitivity in Quadrupole Mass Spectrometer at Low Vacuum

There are many chemical measurement scenarios that benefit from hand portable mass spectrometer. However, the vacuum system requirement has been the primary challenge in reducing the size and weight. If mass analysis is arranged at low vacuum condition, the bulky turbo pump can be eliminated which will minimize size, weight and power of portable MS. In order to acquire the ion operation mode suiting for low vacuum mass spectrometer, aiming at QMF, we set up a simulation platform in COMSOL Multiphysics based on Langevin collision model which was superior to the damping force model used before. By using this platform, six parameters were evaluated which may affect the sensitivity of the quadrupole MS. The parameters were consist of the pressure of the buffer gas, the mass to charge of ions, the mass of background gas, the initial velocity of the ions, the radius of the quadrupole and the temperature. The Langevin collision model simulation results showed that three states of ions in low vacuum quadrupole could be acquired which formed a sharp contract to the two states by applying the damping force model. This means that the results acquired by Langevin collision model accords better to the reality. At low vacuum condition, the increasing of collisions between ions and background gas molecule decreased the sensitivity of MS. Nevertheless, according to the simulation results, increasing the initial kinetic energy of ions, enlarging the radius of quadruple rods, raising the temperature and using the small mass background gas would contribute to a higher sensitivity at low vacuum condition. Besides, ions with greater mass were easier to get a higher sensitivity. It can be concluded that by using a proper ion operation mode, the decreasing sensitivity caused by pressure will regain. This provides a guidance for the design of low vacuum MS which can significantly reduce the pumping requirements required for miniature MS. The next work is to design an actual quadrupole MS instrument according to the results of the simulation and to evaluate the enhancement of sensitivity.