Effect of Humidity on the Mobilities of Small Ions in Ion Mobility Spectrometry

Ions in the ion mobility spectrometry (IMS) are mostlyhydrated.A single peak in the drift time spectrum is usually generated by amixture of ions differing in the number of attached water molecules.Under real IMS detector operating conditions, ions change their compositionduring movement in the drift region due to the changes in the numberof water molecules attached to the ion. The impact of water vaporon the drift times of small ions at different temperatures was studiedexperimentally using an ion mobility spectrometer. The experimentswere carried out for hydronium, ammonium, oxygen, chloride, bromide,and iodide ions. A theoretical model was developed, allowing us tocalculate the effective mobility of ions for a given concentrationof water vapor and temperature. The basic assumption adopted in thismodel was the linear dependence of the effective mobility coefficienton the mobility of ions with a certain degree of hydration. The weightingfactors in this relationship are the abundances of individual typesof ions. These parameters were determined by calculations based onthe thermodynamics of the formation and disintegration of ionic clusters.From the known values of temperature, pressure, and humidity, thevalues of effective mobilities can be predicted quite accurately.The dependencies of reduced mobilities on the average degree of hydrationwere also determined. For these dependencies, the measurement pointson the graphs are gathered along specific lines. This means that theaverage degree of hydration unambiguously determines the value ofreduced mobility for a given type of ions.