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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Measuring a mixture of acetone and perdeuterated acetone (acetone-d6) with an ultra-high resolution drift time ion mobility spectrometer (resolving power of Rp = 235) and ultraviolet ionization (10.6 eV) at ambient pressure reveals three separated peaks. Two of the peaks can easily be associated with acetone and perdeuterated acetone. In a former publication several findings indicated an exchange of a methyl group and the formation of a H3COCD3 related peak. In this work the formed ion species were analyzed with a high resolution drift time ion mobility time of flight mass spectrometer. The mass spectra clearly show the formation of three proton-bound dimer peaks whereas the peak between acetone and acetone-d6 is a proton-bound mixed dimer consisting of one acetone and one acetone-d6 molecule.

We report the first hyphenation of chip-electrochromatography (ChEC) with ion mobility spectrometry (IMS). This approach combines the separation power of two electrokinetically driven separation techniques, the first in liquid phase and the second in gas phase, with a label-free detection of the analytes. For achieving this, a microfluidic glass chip incorporating a monolithic separation column, a nanofluidic liquid junction for providing post-column electrical contact, and a monolithically integrated electrospray emitter was developed. This device was successfully coupled to a custom-built high-resolution drift tube IMS with shifted potentials. After proof-of-concept studies in which a mixture of five model compounds was analyzed in less than 80 s, this first ChEC−IMS system was applied to a more complex sample, the analysis of herbicides spiked in the wine matrix. The use of ChEC before IMS detection not only facilitated the peak allocation and increased the peak capacity but also enabled analyte quantification. As both, ChEC and IMS work at ambient conditions and are driven by high voltages, no bulky pumping systems are needed, neither for the hydrodynamic pumping of the mobile phase as in high-performance liquid chromatography nor for generating a vacuum system as in mass spectrometry. Accordingly, the approach has great potential as a portable analytical system for field analysis of complex mixtures.

We introduce the coupling of droplet microfluidics and ion mobility spectrometry (IMS) to address the challenges of label-free and chemical-specific detection of compounds in individual droplets. In analogy to the established use of mass spectrometry, droplet−IMS coupling can be also achieved via electrospray ionization but with significantly less instrumental effort. Because IMS instruments do not require high-vacuum systems, they are very compact, cost-effective, and robust, making them an ideal candidate as a chemical-specific end-of-line detector for segmented flow experiments. Herein, we demonstrate the successful coupling of droplet microfluidics with a custom-built high-resolution drift tube IMS system for monitoring chemical reactions in nL-sized droplets in an oil phase. The analytes contained in each droplet were assigned according to their characteristic ion mobility with limit of detections down to 200 nM to 1 μM and droplet frequencies ranging from 0.1 to 0.5 Hz. Using a custom sheath flow electrospray interface, we have further achieved the chemical-specific monitoring of a biochemical transformation catalyzed by a few hundred yeast cells, at single droplet level.