The effect of hydration on the mobility of halogen ions in drift tubes and differential mobility spectrometers

The ion transport mechanism is a key factor influencing the analytical properties of detectors used in ion mobility spectrometry. In this study, the transport properties of hydrated chloride, bromide, and iodide ions were investigated using drift tube ion mobility spectrometer (DT IMS) and differential mobility spectrometer (DMS) under controlled humidity and temperature conditions. The objective of this work was to compare the effects of temperature and electric field strength on the transport properties of hydrated halide ions. DT IMS measurements were performed in nitrogen at atmospheric pressure over the temperature range of 318-363 K and water vapor concentrations up to approximately 1000 ppm. For all halide ions, increasing humidity resulted in systematic decreases in reduced mobility, whereas increasing temperature promoted declustering and increased mobility. DMS measurements revealed positive field-dependent mobility behavior for all hydrated halide ions. Increasing electric field strength causes declustering, leading to mobility changes analogous to those observed in DT IMS with increasing temperature. The results indicate that the commonly used concept of effective temperature does not adequately describe the transport of hydrated ions in DT IMS and DMS, where clustering and declustering processes modify the composition of the ion population. Comparison of DT IMS and DMS data indicates that a more appropriate description can be obtained using the concept of an equivalent temperature defined by the degree of ion hydration. Because hydration equilibria depend on water vapor concentration, the equivalent temperature is also humidity-dependent and cannot be represented by a single universal value.