Ion Mobility-Mass Spectrometry: Time-Dispersive Instrumentation

The field of ion mobility-mass spectrometry (IM-MS) has grown with significant momentum in recent years in both fundamental advances and pioneering applications. A search of the terms “ion mobility” and “mass spectrometry” returns more than 2 000 papers, with over half of these being published in the past 4 years (Figure ​(Figure1,1, left). This increased interest has been motivated in large part by improved technologies which have enabled contemporary IM-MS to be amendable to a variety of samples in biology and medicine with high sensitivity, resolving power, and sample throughput. Figure 1 (Left) Histogram of the number of publications published per year in ion mobility and ion mobility-mass spectrometry. Note that the scale is truncated at 300 to highlight the number of publications specifically utilizing IM-MS. Further distinction is ... Highlights of the historical development of the field are presented in Figure ​Figure1,1, right. Ion mobility and mass spectrometry trace their foundations to the X-ray experiments of Thomson and Rutherford in the late 1800s,1 with Tyndall making significant improvements in the analytical capabilities of ion mobility around the 1930s.2,3 During this early era of discovery, a variety of ion mobility experimental parameters were explored, including differences in pressure,4,5 temperature,6,7 electric field,8 and the ion residence time (age) in the drift region.9 Hybrid IM-MS instruments of various configurations were developed by several groups in the 1960s to study gas-phase ion chemistry.10−12 Ion mobility measurements were used by Dole in the earliest development of electrospray ionization (ESI).13,14 Following commercialization,15 ion mobility instrumentation was used for structure-based characterization16 and differentiation of chemical isomers.17,18 In 1982, laser ionization was demonstrated with ion mobility as a means of generating simplified mobility spectra based on protonated species.19 The features which define modern IM-MS, namely, high resolution, high sensitivity, and broad sample compatibility, were developed in the 1990s and coincided with the rapid development of MS in response to the introduction of ESI and MALDI sample ionization.20−22 The last 2 decades saw significant improvements made in the coupling of IM to MS, notably the use of electrodynamic fields to confine, transfer, and focus ions across disparate pressure regions into high vacuum. An interesting observation to be made in this historical analysis is that many of the features we associate with contemporary ion mobility technology were key aspects of early ion mobility instrument design. Several noteworthy reviews of IM-MS have been published, which cover many aspects of the IM-MS technique and range of applications.23−28 A number of influential books covering various aspects of the ion mobility field are also available.29−34 Of particular relevance is Mason and McDaniel’s Transport Properties of Ions in Gases,35 which was recently republished by the American Society for Mass Spectrometry in their classic books series. Though last revised in 1988, this book is still widely considered the seminal treatment of the motion of ions in gases. The technologies and application areas which IM-MS now encompasses has expanded to such a breadth that new reviews covering IM-MS and related areas now appear every few years in the literature. A comprehensive and critical review of the field as a whole is no longer appropriate nor tractable, and as such it is the intent of this review to focus primarily on recent developments made with regard to temporally dispersive ion mobility techniques (drift tubes and traveling wave separators), with an emphasis on their use specifically in IM-MS instrumentation and methods. This focus is selected because of the recent commercial offerings in this regard that have become widely used in many research environments. The present review is not intended to be comprehensive of the ion mobility advances but rather focuses on time-dispersive IM-MS instrumentation over the past few years.