Targeted ion mobility analysis, whereby predefined compounds are analyzed by their ion mobilities, has become an important tool in modern analytical chemistry. It is particularly useful for analyzing complex samples containing large number of components and interferences. This targeted mobility strategy enhances analytical performance of quantitation with reduced noise, enriched analyte signals, expanded dynamic range and reduced analysis time. Numerous advancements in IMS instrumentation technologies for targeted analysis have emerged, achieving substantial enhancements in critical performance metrics such as resolution, sensitivity, and duty cycle. Through integration with mass spectrometry (MS), ion mobility-mass spectrometry (IM-MS) has evolved into a powerful analytical tool for characterizing complex real-world samples. This review highlights these innovations in IMS instrumentation for targeted analysis along with IMS-MS application-specific contexts for the last ten years. Special emphasis is placed on operational modes involving mobility-based selection or filtering mechanisms for isolating targeted ion species.

The spread of illicit drug abuse is a major public health issue in most countries. Law enforcement personnel on drug abuse need a tool to confiscate and block illicit trade of illicit drug or monitoring drug impaired driving. Therefore, affordable and portable instruments for illegal drug detection are in critical need. In this study, we developed a portable periodic focusing differential ion mobility (PFDMA) spectrometer for in situ motoring illicit drugs. The device comprised an ionization source, drift cell, air blower, voltage divider board and electronic control board. In general, an ion mobility spectrometer (IMS) is able to work with air as a buffer gas at atmospheric pressure. There is no need to install heavy mechanical and/or expensive turbo molecular pumps. For our instrument, a small air blower was used to generate laminar counterflow. The whole device is compact with a size of 150 x 200 x 280 (WLH, mm) and a weight of 2.5 kg. In general, most commercial drift tube ion mobility spectrometers use radioactive or corona discharge ionization. In this work, we utilized a different approach to analyze liquid samples. The device was equipped with a nanoelectrospray ionization source. To avoid memory effects, a disposable micropipette was used. An aluminum-coated tip with an ID of 2 mm was applied with a high voltage to achieve nanoelectrospray ionization without sheath gas. Advantages of PFDMA are to isomer separation and better sensitivity compared to other types of ion mobility spectrometers. Most illicit drugs could be detected by this homemade device. The detection limit is lower than the level allowed by law. With our device, the detection limit of methamphetamine is approx 10 ng/ml.(c) 2022 Published by Elsevier B.V.

In recent years, there has been a rapid increase in the use of counter-flow-type ion mobility spectrometers (IMS) due to their improved resolution and functionality. In this study, we developed a new type of counter-flow ion mobility device named the U-shaped mobility analyzer (UMA) for coupling with a mass spectrometer, where the analyte ions could travel along a gas flow in the first channel of the UMA device and then against a gas flow in the second channel of the device. Hence, a mobility band-pass filter was formed by setting different electric fields in the two channels, which enables high-resolution mobility selection of analyte ions. A resolution of ca. 180 was achieved for singly charged small organic molecules, and a resolution of up to ca. 370 was achieved for multiply charged +15 myoglobin. It was thus demonstrated that this filtering function can greatly enhance the dynamic range of an IMS-MS instrument, particularly favoring targeted analysis in complex matrices. Alternatively, the analyte ions could be operated in a so-called trap-scan mode in which ions were trapped first in one of the channels and released sequentially for mobility analysis with an even higher resolution (ca. 210 for singly charged small organic molecules and ca. 590 for +15 myoglobin). Overall, this new UMA device would enable many new applications in omics studies with its high resolution and dynamic range, especially when using the filter-scan mode for scrutinizing analytes with very low concentrations under high chemical backgrounds.

In this work, we present the detection sensitivity improvement of electrospray ionization (ESI) mass spectrometry of neutral saccharides in a positive ion mode by the addition of various amino acids. Saccharides of a broad molecular weight range were chosen as the model compounds in the present study. Saccharides provide strong noncovalent interactions with amino acids, and the complex formation enhances the signal intensity and simplifies the mass spectra of saccharides. Polysaccharides provide a polymer-like ESI spectrum with a basic subunit difference between multiply charged chains. The protonated spectra of saccharides are not well identified because of different charge state distributions produced by the same molecules. Depending on the solvent used and other ions or molecules present in the solution, noncovalent interactions with saccharides may occur. These interactions are affected by the addition of amino acids. Amino acids with polar side groups show a strong tendency to interact with saccharides. In particular, serine shows a high tendency to interact with saccharides and significantly improves the detection sensitivity of saccharide compounds. Graphical Abstract ᅟ.

Ion mobility-mass spectrometry (IMMS) is a very attractive method for studies in structural biology because of the ability of rapid isolation by nearly simultaneous m/z characterization and size separation, leading to an emergence of IMMS as a complimentary biochemical tool. Earlier, we developed a method based on varying the protein concentration in solution prior to electrospray ionization (ESI) with subsequent m/z selection and dissociation of protein multimers by IMMS of cytochrome c. The focus of this work will be to correctly distinguish truly different ion conformations formed by ESI versus homomultimeric complexes with the same m/z for well-studied proteins bovine ubiquitin and insulin. These proteins were chosen due to their large difference in solution phase structures: insulin tightly bound by disulfide linkages, and ubiquitin—a protein that may adopt a range of states from compact to extended. Our preliminary results, as with cytochrome c reveal false negatives for protein oligomer formation and false positives for protein conformational states. In addition, these results will be couched in terms of the need for quantification of IMMS analysis of proteins given the total area under IMMS peaks can also distinguish conformation versus aggregation as higher order oligomers have more mass per ion. This article is part of the themed issue ‘Quantitative mass spectrometry’.

Ion mobility spectrometry (IMS) as a stand-alone technique has become increasingly important for applications in security, defense, and environmental monitoring, and also in biological applications such as molecular structure and -omic analysis when combined with mass spectrometry. Yet, the majority of these devices are drift cell based and limited by low duty cycles because of ion gating. Differential Mobility Analyzers (DMAs) are attractive alternatives due to their continuous ion transmission and success in analyzing aerosol particles in real time environmental tests. But, the resolution of a DMA is low due to difficulties in achieving laminar gas flow, low sample gas flow to sheath gas flow ratio, and high velocity sheath gas using small pumps, if portability is a concern. To overcome these challenges, we will introduce a new ion mobility spectrometer that increases the amount of work done on the ions during separation by introducing an electric field opposing the gas flow direction while simultaneously preserving laminar gas flow. The development of the Periodic Focusing Differential Mobility Analyzer (PFDMA) can lead to a portable device that exhibits both high resolution and sensitivity, to meet the needs of today's expanding applications.

Triboelectric spray ionization (TESI) is a variation of electrospray ionization (ESI) using common instrumental components, including gas flow, solvent flow rate and heat, the only difference being the use of a high-voltage power supply for ESI or a static charge for TESI. The ionization of solvent or analyte is due to the electrostatic potential difference formed between the spray electrode and counter electrode. The ion source contains a pneumatic spray operated over a range of flow rates (0.15-1.5 µl/min) and gas pressures (0-100). This new design contains a standalone spray assembly and an optional metal mesh in front of the spray. There are several parameters that affect the performance during ionization of molecules including the flow rate of solvent, gas pressure, temperature, solvent acidity, distance and potential difference between emitter and counter electrode. A variable electrostatic potential can be applied for higher ionization efficiency. The new ionization method was successfully applied to solutions of various proteins under different conditions. The same charge-state distributions compared to other ESI techniques are observed for all the protein samples. The unique feature of TESI is very efficient spraying by using a natural electrostatic potential even at the potential that a human body can produce. This provides very gentle ionization efficiency of peptides and proteins in different solvents.

The generally accepted view of protein structure in the gas-phase is that protein ions produced by electrospray ionization (ESI) exist in a number of different states, and the resulting charge state distribution (CSD) and ion mobility spectrum is interpreted as evidence for protein ions retaining some memory of solution-phase conformation. Even with the inclusion of ion mobility information, reports of protein ion structure in the gas-phase are oftentimes in disagreement not only within the discipline but also as interpreted by other gas-phase techniques. The focus of this work will be to correctly distinguish truly different ion conformations formed by ESI versus homomultimeric complexes with the same m/z. The concentration of cytochrome c in solution was varied over a wide range, and the multiply charged multimers (MCMs) present in the ion mobility/mass spectrum were unambiguously assigned by m/z selection and dissociation prior to ion mobility/mass spectrometry analysis. The results revealed false negatives for protein oligomer formation and false positives for protein conformational states and no evidence that gas-phase cytochrome c ions retain memory of solution-phase conformation, characteristics of great importance for structural biology. The results also suggest that the total IM-MS distribution for a protein is the complex result of individual MCMs either surviving until detection (undissociated) or dissociating into lower order multimers or a number of product ions for each m/z.

A novel self-powered dual spray ionization source has been developed for applications in mass spectrometry. This new source does not use any power supply and produces both positive and negative ions simultaneously. The idea behind this ionization source comes from the Kelvin water dropper. The source employs one or two syringes, two pneumatic sprays operated over a range of flow rates (0.15-15 μL min(-1)) and gas pressures (0-150 psi), and two double layered metal screens for ion formation. A variable electrostatic potential from 0 to 4 kV can be produced depending on solvent and gas flow rates that allow gentle ionization of compounds. There are several parameters that affect the performance during ionization of molecules including the flow rate of solvent, gas pressure, solvent acidity, position of spray and metal screens with respect to each other and distance between metal screens and the counter electrode. This ionization method has been successfully applied to solutions of peptides, proteins and non-covalent complexes. In comparison with ESI, the charge number of the most populated state is lower than that from ESI. It indicates that this is a softer ionization technique and it produces more protein ions with folded structures. The unique features of Kelvin spray ionization (KeSI) are that the method is self-powered and ionization occurs at very low potentials by providing very low internal energy to the ions. This advantage can be used for the ionization of very fragile molecules and investigation of non-covalent interactions.

The implementation of surface-induced dissociation (SID) to study the fast dissociation kinetics (sub-microsecond dissociation) of peptides in a MALDI TOF instrument has been reported previously. Silicon nanoparticle assisted laser desorption/ionization (SPALDI) now allows the study of small molecule dissociation kinetics for ions formed with low initial source internal energy and without MALDI matrix interference. The dissociation kinetics of N(CH3)4 and N(CD3)4 were chosen for investigation because the dissociation mechanisms of N(CH3)4 have been studied extensively, providing well-characterized systems to investigate by collision with a surface. With changes in laboratory collision energy, changes in fragmentation timescale and dominant fragment ions were observed, verifying that these ions dissociate via unimolecular decay. At lower collision energies, methyl radical (CH3) loss with a submicrosecond dissociation rate is dominant, but consecutive H loss after CH3 loss becomes dominant at higher collision energies. These observations are consistent with the known dissociation pathways. The dissociation rate of CH3 loss from N(CH3)4 formed by SPALDI and dissociated by an SID lab collision energy of 15 eV corresponds to log k 8.1, a value achieved by laser desorption ionization (LDI) and SID at 5 eV. The results obtained with SPALDI SID and LDI SID confirm that (1) the dissociation follows unimolecular decay as predicted by RRKM calculations; (2) the SPALDI process deposits less initial energy than LDI, which has advantages for kinetics studies; and (3) fluorinated self-assembled monolayers convert about 18% of laboratory collision energy into internal energy. SID TOF experiments combined with SPALDI and peak shape analysis enable the measurement of dissociation rates for fast dissociation of small molecules. (J Am Soc Mass Spectrom 2009, 20, 957–964) © 2009 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry

The resolution of ion mobility spectrometry (IMS) is of paramount importance for both post-ionization separations and structural characterization of ions that have similar ion-neutral collision cross sections; however, the instrumental features that lead to increased resolution also decrease ion transmission through the drift cell. The periodic-focusing DC ion guide (PDC IC) drift cell provides increased ion transmission with minimal loss in resolution.In earlier work we showed that the electrode geometry (inner diameter, thickness, and spacing) strongly affects ion focusing and ion transmission. Here, we critically evaluate the effect of the electrode geometry of a PDC IC drift cell on both ion transmission and resolution. In this study we examine two drift cells that differ in length (63 and 125 cm) and electrode configuration. We also examine the effects of applied voltage and pressure in an attempt to maximize both resolution and ion transmission. Experimental data obtained with fullerene and model peptide ions are compared with calculated ion trajectories using SIMION 8.0 simulations. Published by Elsevier B.V.

Collision induced dissociation (CID) combined with matrix assisted laser desorption ionization-ion mobility-mass spectrometry (MALDI-IM-MS) is described. In this approach, peptide ions are separated on the basis of mobility in a 15 cm drift cell. Following mobility separation, the ions exit the drift cell and enter a 5 cm vacuum interface with a high field region (up to 1000 V/cm) to undergo collisional activation. Ion transmission and ion kinetic energies in the interface are theoretically evaluated accounting for the pressure gradient, interface dimensions, and electric fields. Using this CID technique, we have successfully fragmented and sequenced a number of model peptide ions as well as peptide ions obtained by a tryptic digest. This instrument configuration allows for the simultaneous determination of peptide mass, peptide-ion sequence, and collision-cross section of MALDI-generated ions, providing information critical to the identification of unknown components in complex proteomic samples.

A novel ion mobility (IM)-surface-induced dissociation (SID)-mass spectrometer consisting of two independent time-of-flight (TOF) mass analyzers is described. The dual TOF instrument configuration facilitates high-throughput post-ionization separation and mass analysis of precursor and fragment ions, and the utility of 3D data acquisition is demonstrated for top-down proteomics by performing simultaneous acquisition of peptide mass maps and amino acid sequence determination.

The implementation of surface-induced dissociation (SID) to study the fast dissociation kinetics (sub-microsecond dissociation) of peptides in a MALDI TOF instrument has been reported previously. Silicon nanoparticle assisted laser desorption/ionization (SPALDI) now allows the study of small molecule dissociation kinetics for ions formed with low initial source internal energy and without MALDI matrix interference. The dissociation kinetics of N(CH(3))(4)(+) and N(CD(3))(4)(+) were chosen for investigation because the dissociation mechanisms of N(CH(3))(4)(+) have been studied extensively, providing well-characterized systems to investigate by collision with a surface. With changes in laboratory collision energy, changes in fragmentation timescale and dominant fragment ions were observed, verifying that these ions dissociate via unimolecular decay. At lower collision energies, methyl radical (CH(3)) loss with a sub-microsecond dissociation rate is dominant, but consecutive H loss after CH(3) loss becomes dominant at higher collision energies. These observations are consistent with the known dissociation pathways. The dissociation rate of CH(3) loss from N(CH(3))(4)(+) formed by SPALDI and dissociated by an SID lab collision energy of 15 eV corresponds to log k = 8.1, a value achieved by laser desorption ionization (LDI) and SID at 5 eV. The results obtained with SPALDI SID and LDI SID confirm that (1) the dissociation follows unimolecular decay as predicted by RRKM calculations; (2) the SPALDI process deposits less initial energy than LDI, which has advantages for kinetics studies; and (3) fluorinated self-assembled monolayers convert about 18% of laboratory collision energy into internal energy. SID TOF experiments combined with SPALDI and peak shape analysis enable the measurement of dissociation rates for fast dissociation of small molecules.

Collision cross-sections of gas-phase (CsI)n = (1-7)Cs(+) cluster ions formed by pulsed-UV laser (355 nm) desorption ionization are measured by ion mobility-mass spectrometry. Experimental collision cross-sections are compared with calculated cross sections of candidate structures generated from a search for the lowest energy structures at the DFT/B3LYP/LACV3P** and MP2/LACVP3P** levels. The relative stabilities of these candidate structures are examined by IM-CID-MS, and the experimental results are compared to theoretical predictions. Analysis of (CsI)n = (1-7)Cs(+) cluster ion dissociation energies shows that the lower fragmentation thresholds are observed for cluster ions with the lower predicted stability.

An ion mobility-mass spectrometer (IM-MS) interface is described that can be employed to perform collisional activation and/or collision-induced dissociation (CID) with good transmission of mobility separated ions to the MS analyzer. The IM-MS interface consists of a stacked-ring ion guide design, where the field strength and pressure ratio can be operated such that structural rearrangement reactions and/or CID are achieved as a function of the effective ion temperature. The ion dynamics and collisional activation processes in the IM-MS interface are described as a function of the ion-neutral collisions, ion kinetic energies, and effective ion temperature. The applicability of the IM-CID-MS methodology to studies of peptide ion fragmentation is illustrated using a series of model peptides.

Matrix-assisted laser desorption/ionization (MALDI) coupled with ion mobility–mass spectrometry (IM–MS) provides a rapid (μs–ms) means for the two-dimensional (2D) separation of complex biological samples (e.g., peptides, oligonucleotides, glycoconjugates, lipids, etc.), elucidation of solvent-free secondary structural elements (e.g., helices, β-hairpins, random coils, etc.), rapid identification of post-translational modifications (e.g., phosphorylation, glycosylation, etc.) or ligation of small molecules, and simultaneous and comprehensive sequencing information of biopolymers. In IM–MS, protein-identification information is complemented by structural characterization data, which is difficult to obtain using conventional proteomic techniques. New avenues for enhancing the figures of merit (e.g., sensitivity, limits of detection, dynamic range, and analyte selectivity) and optimizing IM–MS experimental parameters are described in the context of deriving new information at the forefront of proteomics research.

Ion mobility-mass spectrometry (IM-MS) data is interpreted as evidence that gas-phase bradykinin fragment 1-5 ( BK1-5 , RPPGF) [M + H] + ions exist as three distinct structural forms, and the relative abundances of the structural forms depend on the solvent used to prepare the matrix-assisted laser desorption ionization (MALDI) samples. Samples prepared from organic rich solvents (90% methanol/10% water) yield ions having an ion mobility arrival-time distribution (ATD) that is dominated by a single peak; conversely, samples prepared using mostly aqueous solvents (10% methanol/90% water) yield an ATD composed of three distinct peaks. The BK1-5 [M + H] + ions were also studied by gas-phase hydrogen/deuterium (H/D) exchange ion-molecule reactions and this data supports our interpretation of the IM-MS data. Plausible structures for BK1-5 ions were generated by molecular dynamics (MD). Candidate MD-generated structures correlated to measured cross-sections suggest a compact conformer containing a β -turn whereas a more extended, open form does not contain such an interaction. This study illustrates the importance of intra-molecular interactions in the stabilization of the gas-phase ions, and these results clearly illustrate that solution-phase parameters (i.e., MALDI sample preparation) greatly influence the structures of gas-phase ions.

The influence of field strength on the separation of tryptic peptides by drift tube-based ion mobility-mass spectrometry is reported. Operating the ion mobility drift tube at elevated field strengths (expressed in V cm−1 torr−1) reduces separation times and increases ion transmission efficiencies. Several accounts in the literature suggest that performing ion mobility separation at elevated field strength can change the selectivity of ion separation. To evaluate the field strength dependant selectivity of ion mobility separation, we examined a data set of 65 singly charged tryptic peptide ion signals (mass range 500–2500 m/z) at six different field strengths and four different drift gas compositions (He, N2, Ar, and CH4). Our results clearly illustrate that changing the field strength from low field (15 V cm−1 torr−1) to high field (66 V cm−1 torr−1) does not significantly alter the selectivity or peak capacity of IM-MS. The implications of these results are discussed in the context of separation methodologies that rely on the field strength dependence of ion mobility for separation selectivity, e.g., high-field asymmetric ion mobility spectrometry (FAIMS).