Low-Vacuum Quadrupole Mass Spectrometry for Marine Applications

Performing mass spectrometry (MS) in a low-vacuum environment not only substantially reduces the cost, size, weight and power of the instrument but also expands its applicability, making it is more suitable for field applications such as marine applications. As it is a complicated instrument, simulation is a critical step in the development process of a mass spectrometer. The Langevin collision model describes the interaction between low-energy particles. By introducing the Langevin collision theory to the simulation of low-vacuum ion motion, the movement of discrete ions can be more accurately described and the influence of different operational models on the performance of MS can be evaluated. Based on Langevin collision model, a low-vacuum simulation platform of cylindrical ion trap (CIT) was built in COMSOL Multiphysics. The influence of frequency of the radio frequency (RF) voltage V, amplitude of the radio frequency (RF) voltage ω and size of the ion trap r₀ on the sensitivity and resolution of MS were investigated. The simulation results showed that by applying a high-frequency RF voltage, a spectral peak with a narrower width and a larger signal strength could be obtained and thus higher sensitivity and resolution were obtained. This is because when the frequency of the RF voltage is enhanced, the ion acquires more energy from the quadrupole within a unit time, which increases the moving velocity and enhances the collisional capability of the ions. When ω increases, in order to keep Mathieu parameter q unchanged, V must be increased with the square of ω or decrease r₀ proportionally. The value of V is determined by the electric circuit, and it is very difficult to increase it significantly. Moreover, a high voltage can cause discharge. Therefore, from this perspective, in order to use a larger RF frequency ω, the size of the ion trap r₀ must be reduced. Thus, it is concluded that by increasing the frequency of the radio frequency voltage while reducing the size of the ion trap, mass analysis can be conducted under a low vacuum. Additionally, the results was applied to experiments and obtained excellent spectral graphs of organic compounds under a maximum pressure of 2 Pa in another paper, which proved the effectiveness of the conclusions obtained from the simulation.