Dynamic Response of Ion Mobility Spectrometry

Ion mobility spectrometers (IMS) are widely used in the security industry as well as in analytical measurements. Nevertheless, the IMS input is dynamic in nature because the sampling input can vary over time. Similar to most instruments, IMS is unable to respond immediately to variations in the input sample. Therefore, the measurements are made under transient conditions, which may affect the results. This work investigates the dynamic response of an IMS to different inputs, including steps, pulsed, exponential, and Gaussian functions. A theoretical model was developed based on two phenomena: the accumulation or dilution of a sample in the ionization region and the adsorption or desorption inside the injection port. Both processes have a charging/discharging nature. Thus, a mathematical expression was derived that takes into account two RC circuits in series. Fitting the output signal of the experimental data to the expression obtained from the model gave reasonable time constants of 2-4 s and 15-20 s for the dilution and the desorption processes, respectively. The model performance was evaluated by comparing the output with the experimental results, which were in excellent correlation. IMS was also found to behave in a manner similar to a second-order instrument, in which the output is related to the input via a second-order differential equation. These results are applicable to GC-IMS and IMS-based detectors.