Characterization of ion transmission in UV-FAIMS by incorporating ion recombination

Abstract

High-field asymmetric waveform ion mobility spectrometry (FAIMS) is a powerful technique for the separation and characterization of gas-phase ions. However, the quantitative model for this technique is incomplete because previous studies on ion loss have mostly focused on ion neutralization and diffusion in the separation process and neglected ion annihilation owing to ion recombination in the transmission process. In this work, an ion recombination model of UV-FAIMS was described and validated using a broad range of experimental gas flow rates (0–800 L/h) and concentrations (10 ppb–100 ppm). The theoretical analysis provided a new solution method for important parameters and indicated the existence of a linear growth interval and a point of saturated signal intensity, both of which were explained by the introduced recombination loss factor and validated experimentally. In particular, the existence of saturation was verified for the first time. Based on this theory, ionization efficiencies of 4.56 × 10−5, 5.41 × 10−5, and 4.47 × 10−6 were calculated for 1,3-butadiene, acetone, and ammonia, respectively, at atmospheric pressure, with corresponding the recombination coefficients of 2.56 × 10−7, 3.74 × 10−7, and 1.89 × 10−8 cm3s−1. Further, an optimized quantitative model of UV-FAIMS was proposed by incorporating ion recombination theory, providing the optimum quantitative interval and a parameter optimization method for UV-FAIMS, which were also validated using a wide range of experimental conditions. This work provides a basis for parameter optimization for UV-FAIMS and a new method of determining the ionization efficiency and recombination coefficient of a gas-phase analyte. Furthermore, the incorporation of ion recombination theory is instructive for the construction of quantitative models of atmospheric-pressure photoionization mass spectrometry and UV-ion mobility spectrometry.