Investigation of temperature, electric field and drift-gas composition effects on the mobility of NH4+ ions in He, Ar, N2, and CO2

• In this work the effect of temperature, electric field strength, and drift gas composition was investigated on the reduced ion mobility of NH4+ simultaneously.
• The experimental data have shown that an inverse relationship exists between the reduced ion mobility and the mass and polarizability of the neutral drift gas. The drift of NH4+ is faster in gases with low mass and polarizability (e.g., helium) than in drift gases with large mass and high polarizability (e.g., carbon dioxide).
• Although the small ranges of the electric field strength does not have effect on the reduced ion mobility of NH4+ in Ar, N2 and CO2 but the reduced ion mobility of NH4+ decrease moderately with increasing the electric field in He drift gas.
• Despite decreasing of reduced ion mobility of large protonated amines in helium with increasing temperature the reduced ion mobility of NH4+ increases with increasing temperature in helium. We have investigated the changes of reduced ion mobility of NH4+ as the result of breaking down clusters at high temperature is more important than heavy ions.

The aim of this work is to investigate the effects of various parameters on the mobility of NH4+ by use of an ion mobility spectrometer that is equipped with a corona discharge ionization source at atmospheric pressure. The reduced ion mobility values of NH4+ were measured in carbon dioxide, argon, nitrogen and helium as a function of temperature and E/N, the ratio of the electric field strength to the gas number density. The drift time of the ion in the different drift gases depends upon the ion-neutral interaction potentials that determine the collision cross-sections of the ion-drift gas systems, which includes a dependence upon the polarizabilities of the drift gases. The results of this research show that, under the same drift cell conditions, the reduced ion mobilities of NH4+ in the drift gases decrease in the order: He ≫ Ar ≥ N2 > CO2. We also investigated temperatures between 25 and 225 °C to study the effect on the reduced ion mobilities of NH4+ in the different drift gases. The reduced mobility was increased by increasing the temperature, which is attributed to break down of clusters at high temperature. We found the reduced ion mobility of NH4+ does not change with increasing E/N in Ar, N2 and CO2, while it decreases in He.

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