Alpha function of FAIMS: Modeling uncertainty and experimental findings

Abstract

High-field asymmetric waveform ion mobility spectrometry (FAIMS) is a specific ion mobility device used to detect and characterize gas-phase ions under ambient conditions. This device separates ions based on differences in their mobilities, which are characterized by alpha dependence (α(E/N)). However, no effective theoretical model currently exists to describe the accuracy and optimize experimental parameters. Therefore, this paper describes a model of uncertainty to provide fast and effective guidance for optimally selecting experimental parameters and accurately evaluating the detection results of FAIMS. This model reveals how the deviations of α2 and α4 depend on the dispersion voltage (DV) and other parameters. Further, we evaluate three meaningful findings. The first is the relationship between α2 and α4. The second is the optimal sequence of DV, which does not generally utilize an arithmetic progression, but only contains two components (1 and 0.758 for α2, and 1 and 0.749 for α4), where the measurement times required for the optimal sequence of DV were approximately 50 % less than that needed for an arithmetic progression. Finally, we determine that the optimal values of the rectangular asymmetric waveform field ratio are 1/3 for α2 and 0.377 for α4. This analytical model and its inferences were verified extensively using a home-made tandem FAIMS device, which worked by selecting ions in the first stage and analyzing them in the second stage, thus eliminating the interference from noise signals and obtaining accurate results.