With recent advances in ionization sources and instrumentation, ion mobility spectrometers (IMS) have transformed from a detector for chemical warfare agents and explosives to a widely used tool in analytical and bioanalytical applications. This increasing measurement task complexity requires higher and higher analytical performance and especially ultra-high resolution. In this review, we will discuss the currently used ion mobility spectrometers able to reach such ultra-high resolution, defined here as a resolving power greater than 200. These instruments are drift tube IMS, travelling wave IMS, trapped IMS and field asymmetric or differential IMS. The basic operating principles and the resulting effects of experimental parameters on resolving power are explained and compared between the different instruments. This allows understanding the current limitations of resolving power and how ion mobility spectrometers may progress in the future.

Ion mobility spectrometry provides ion separation in the gas phase mainly based on differing ion-neutral collision cross sections, enabling powerful analysis of many isomers. However, the separation also has a miniscule mass dependence due to the acceleration and collision properties. In this work, we show for the first time that using a compact ultra-high-resolution ion mobility spectrometer with a resolving power of 250 and an UV ionization source enables the separation of isotopologues with ion mobility spectrometry. This is demonstrated for regular and perdeuterated acetone, benzene, and toluene as well as toluene-C in nitrogen and in purified air as drift gas. The observed peak shifts in the ion mobility spectrum agree with the basic ion mobility equation when using nitrogen as drift gas and also agree with a combination of this equation with Blanc's law when using purified air as drift gas. For benzene and toluene, a reduction in the ion-neutral collision cross section of the isotopically replaced species is observed. Furthermore, a third peak formed from regular and perdeuterated acetone is observed, which can most likely be attributed to the exchange of a methyl group.

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The online hyphenation of chip-based high-performance liquid chromatography (chip-HPLC) with ion mobility spectrometry (IMS) via fully integrated electrospray emitters is introduced. A custom-built drift tube IMS with shifted potentials was developed in order to keep the IMS orifice electrically grounded, allowing for a robust coupling with chip-HPLC. Proof-of-concept studies with the newly developed analytical setup revealed the suitability of IMS as a promising and powerful detection concept for chip-based separation techniques. Comparison of IMS with fluorescence detection and electrospray ionization-mass spectrometry (ESI-MS) allowed a more detailed characterization of the IMS as a new detection method for chip-HPLC. Moreover, the analysis of a mixture consisting of three isobaric antidepressants demonstrated the performance of chip-HPLC/IMS as a miniaturized two-dimensional separation technique.

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