Ion mobility spectrometer with heated sample inlet - Solution to the issue of temperature effect on resolving power

One major challenge in detecting less volatile compounds with an ion mobility spectrometer (IMS) is preventing condensation of target molecules in the sampling line and ionization region to allow for fast response and recovery. Heating the entire device including the sampling line can of course mitigate condensation of such compounds, but this comes at the cost of reduced resolving power and compromised detection limits. Furthermore, a considerable amount of additional power and an IMS design with temperature-resistant components are needed.

In this work, a different approach has been investigated, with a heated sample inlet in combination with a directed sample gas flow through the ionization region, but with the drift region at lower temperature. While this approach effectively addresses the issue of condensation, it results in an inhomogeneous temperature distribution within the drift region. Simulations and experimental data reveal that this uneven temperature distribution can significantly distort the peaks in the ion mobility spectrum, depending on the IMS orientation. However, positioning the IMS vertically, with the detector facing down, significantly minimizes temperature-induced peak distortion, thereby maintaining high resolving power. In this orientation, the IMS used in this work shows a resolving power of 80, while the IMS sample inlet and outlet are heated to 423 K. In addition, a directed sample gas flow in the ionization region is used to further reduce condensation in the ionization region.

The approach and findings revealed in this work allow the construction of IMS with a heated sample inlet to prevent condensation of less volatile compounds while also maintaining the high resolving power of a drift tube at room temperature. The results on the influence of the orientation of the IMS can also be applied to even higher temperatures.