Abstract
Ion mobility spectrometers (IMS) are versatile gas analyzers. Due to their small size and robustness, combined with a very high sensitivity, they are often used in gas sensing applications such as environmental monitoring. In order to improve the selectivity, they are typically combined with a mass spectrometer (MS). Since IMS works at atmospheric pressure, and MS works at vacuum, a special interface reducing the pressure over normally two stages has to be used. In this paper the influence of this coupling of different pressure areas on the IMS signal will be analyzed with help of finite elements method simulations.
Highlights
Ion mobility spectrometry is a well known technique in order to analyze gases
The ions are separated according to their mobility, which allows them to reach different velocities when colliding with the ambient air molecules and to reach the detector after substance-specific drift times
The air currents have been simulated by solving typical computational fluid dynamics (CFD) equations for the k-H-turbulence model [9]
Summary
Ion mobility spectrometry is a well known technique in order to analyze gases. The corresponding spectrometers are small, robust, sensitive and fast. Since IMS devices do not use vacuum (one factor that makes their size small), the ions constantly collide with ambient air molecules on the way to the detector It is their mobility that decides how fast they can reach the detector while drifting towards it [1]. One version is e.g. the time-of-flight mass spectrometry, where the ions, similar to IMS, travel a certain distance to a detector Since they travel in vacuum, it is not the mobility and the collisions with air molecules that determine their velocity and their travel time, but their kinetic energy provided by the electric acceleration field and their mass. In this paper we have analyzed with help of finite elements method (FEM) simulations the change of the ion motion through the IMS when directly combined with an intermediate vacuum stage of a few mbar pressure. After a short description of the set up, the paper will show how the IMS response is directly affected by the pressure change caused by the interface to the reduced pressure stage
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