Distinguishing enantiomeric amino acids with chiral cyclodextrin adducts and structures for lossless ion manipulations

SLIM-Based High-Resolution Ion Mobility Reveals New Structural Insights into Isomeric Vitamin D Metabolites and their Isotopologues

Testing for vitamin D deficiency remains a high-volume clinical assay, much of which is done using mass spectrometry-based methods to alleviate challenges in selectivity associated with immunoassays. Ion mobility-mass spectrometry (IM-MS) has been proposed as a rapid alternative to traditional LC-MS/MS methods, but understanding the structural ensemble that contributes to the ion mobility behavior of this molecular class is critical. Herein we demonstrate the first application of high-resolution Structures for Lossless Ion Manipulations (SLIM) IM separations of several groups of isomeric vitamin D metabolites. Despite previous IM studies of these molecules, the high resolving power of SLIM (Rp ∼ 200) has revealed additional conformations for several of the compounds. The highly similar collision cross sections (CCS), some differing by as little as 0.7%, precluded adequate characterization with low-resolution IM techniques where, in some cases, wider than expected peak widths and/or subtle shoulders may have hinted at their presence. Importantly, these newly resolved peaks often provided a unique mobility that could be used to separate isomers and provides potential for their use in quantification. Lastly, the contribution of isotopic labeling to arrival time distribution for commonly used 13C- and deuterium-labeled internal standards was explored. Minor shifts of ∼0.2-0.3% were observed, and in some instances these shifts were specific to the conformer being measured (i.e., "closed" vs "open"). Accounting for these shifts is important during raw data extraction to ensure reproducible peak area integration, which will be a critical consideration in future quantitative applications.

Separation and Characterization of Synthetic Cannabinoid Metabolite Isomers Using SLIM High-Resolution Ion Mobility-Tandem Mass Spectrometry (HRIM-MS/MS)

Synthetic cannabinoids, a subclass of new psychoactive substances (NPS), are laboratory-made substances that are chemically similar to those found naturally in the cannabis plant. Many of these substances are illicitly manufactured and have been associated with severe health problems, prompting a need to develop analytical methods capable of characterizing both known and previously undetected compounds. This work focuses on a novel Structures for Lossless Ion Manipulations (SLIM) IM-MS approach to the differentiation and structural characterization of synthetic cannabinoid metabolites, specifically MDA-19/BUTINACA, JWH-018, and JWH-250 isomer groups. These different compound classes are structurally very similar, differing only in the position of one or a few functional groups; this yielded similarity in measured collision cross section (CCS) values. However, the high resolution of SLIM IM provided adequate separation of many of these isomers, such as sodiated JWH-250 metabolites N-4-OH, N-5-OH, and 5-OH, which displayed CCS of 187.5, 182.5, and 202.3 Å2, respectively. In challenging cases where baseline separation was precluded due to nearly identical CCS, such as for JWH-018 isomers, simple derivatization by dansyl chloride selectively reacted with the 6-OH compound to provide differentiation of all isomers using a combination of CCS and m/z. Finally, the opportunity to use this method for structural elucidation of unknowns was demonstrated by using SLIM IM mobility-aligned MS/MS fragmentation. Different MDA-19/BUTINACA isomers were first mobility separated and could then be individually activated, yielding unique fragments for both targeted identification and structural determination. Overall, the described SLIM IM-MS/MS workflow provides significant potential as a rapid screening tool for the characterization of emerging NPS such as synthetic cannabinoids and their metabolites.

Noncovalent Host-Guest Complexes of Artemisinin with α-, β-, and γ- Cyclodextrin Examined by Structural Mass Spectrometry Strategies

Cyclodextrins (CDs) are a family of macrocyclic oligosaccharides with amphiphilic properties, which can improve the stability, solubility, and bioavailability of therapeutic compounds. There has been growing interest in the advancement of efficient and reliable analytical methods that assist with elucidating CD host-guest drug complexation. In this study, we investigate the noncovalent ion complexes formed between naturally occurring dextrins (αCD, βCD, γCD, and maltohexaose) with the poorly water-soluble antimalarial drug, artemisinin, using a combination of ion mobility-mass spectrometry (IM-MS), tandem MS/MS, and theoretical modeling approaches. This study aims to determine if the drug can complex within the core dextrin cavity forming an inclusion complex or nonspecifically bind to the periphery of the dextrins. We explore the use of group I alkali earth metal additives to promote the formation of various noncovalent gas-phase ion complexes with different drug/dextrin stoichiometries (1:1, 1:2, 1:3, 1:4, and 2:1). Broad IM-MS collision cross section (CCS) mapping (n > 300) and power-law regression analysis were used to confirm the stoichiometric assignments. The 1:1 drug:αCD and drug:βCD complexes exhibited strong preferences for Li+ and Na+ charge carriers, whereas drug:γCD complexes preferred forming adducts with the larger alkali metals, K⁺, Rb⁺, and Cs⁺. Although the ion-measured CCS increased with cation size for the unbound artemisinin and CDs, the 1:1 drug:dextrin complexes exhibit near-identical CCS values regardless of the cation, suggesting these are inclusion complexes. Tandem MS/MS survival yield curves of the [artemisinin:βCD + X]⁺ ion (X = H, Li, Na, K) showed a decreased stability of the ion complex with increasing cation size. Empirical CCS measurements of the [artemisinin:βCD + Li]⁺ ion correlated with predicted CCS values from the low-energy theoretical structures of the drug incorporated within the βCD cavity, providing further evidence that gas-phase inclusion complexes are formed in these experiments. Taken together, this work demonstrates the utility of combining analytical information from IM-MS, MS/MS, and computational approaches in interpreting the presence of gas-phase inclusion phenomena.

General Method to Obtain Collision Cross-Section Values in Multipass High-Resolution Cyclic Ion Mobility Separations

In recent years, ion mobility spectrometry-mass spectrometry (IMS-MS) has advanced the field of omics-based research, especially with the development of high-resolution platforms; however, these separations have generally been qualitative in nature. The rotationally averaged ion neutral collision cross section (CCS) is one of the only quantitative metrics available for aiding in characterizing biomolecules in IMS-MS. However, determining the CCS of an ion for multipass IMS systems, such as in cyclic ion mobility-mass spectrometry (cIMS-MS) and structures for lossless ion manipulations, has been challenging due to the lack of methods available for calculating CCS when more than a single pass is required for separation as well as the laborious nature of requiring calibrants and unknown compounds to be subjected to identical number of passes, which may not be possible in certain instances because of peak splitting, high levels of diffusion, etc. Herein, we present a general method that uses average ion velocities for calculating CCS values in cIMS-MS-based separations. Initially, we developed calibration curves using common CCS calibrants [i.e., tetra-alkylammonium salts, polyalanine, and hexakis(fluoroalkoxy)phosphazines] at different traveling wave (TW) conditions and the calculated cIMS CCS values were within ∼1% error or less compared to previously established drift tube IMS CCS measurements. Since it has been established that glycans can split into their α/β anomers, we utilized this method for two glycan species, 2α-mannobiose and melibiose. Both glycans were analyzed at the same TW conditions as the calibrants, and we observed anomer splitting at pathlengths of 20 m for 2α-mannobiose and 40 m for melibiose and thus assigned two unique CCS values for each glycan, which is the first time this has ever been done. We have demonstrated that the use of average ion velocities is a robust approach for obtaining CCS values with good agreement to CCS measurements from the previous literature and anticipate that this methodology can be applied to any IMS-MS platform that utilizes multipass separations. Our future work aims to incorporate this methodology for the development of a high-resolution CCS database to aid in the characterization of human milk oligosaccharides.

Assessing the Use of Host-Guest Chemistry in Conjunction with Cyclic Ion Mobility Separations for the Linkage-Specific Characterization of Human Milk Oligosaccharides

Human milk oligosaccharides (HMOs) are a class of glycans that are highly abundant in human milk and contribute to the healthy growth of an infant's immune system. While new advancements in analytical methodologies have been made in glycomics, the high degree of isomeric heterogeneity and lack of authentic standards have made the high-resolution separation and accurate characterization of linkage positioning of all HMO species very challenging. Herein, we present an evaluation of the use of host-guest chemistry in conjunction with cyclic ion mobility spectrometry-mass spectrometry (cIMS-MS)-based separations for the identification of linkage positioning in three pairs of di-, tetra-, and hexasaccharide HMO isomers that only differ in the positioning of one glycosidic linkage (β1,3 versus β1,4). Suitable hosts, such as α/β cyclodextrins, cucurbit[n]urils (n = 5, 7), crown ethers, cyclic peptides, and an ionophore, were used to assess host-guest inclusion complex formation as well as linkage-specific cIMS-MS trends. Our results indicated a linkage-specific trend for the [M + 2α + 2H]2+ cyclodextrin-based host-guest inclusion complexes where the β1,3 linkage-containing isomers were always higher mobility than the β1,4 linkage-containing ones as well one for the [M + α + β + 2H]2+ complexes where the β1,4 linkage-containing isomers were always higher mobility than the β1,3 linkage-containing ones. We also observed diagnostic mobility fingerprints for the cucurbituril-based complexes. We anticipate that linkage-specific and mobility fingerprint trends can potentially aid in identifying linkage positioning for other HMO isomers as well as in complex human milk samples.

Chiral derivatization-enabled discrimination and on-tissue detection of proteinogenic amino acids by ion mobility mass spectrometry

The importance of chiral amino acids (AAs) in living organisms has been widely recognized since the discovery of endogenous d-AAs as potential biomarkers in several metabolic disorders. Chiral analysis by ion mobility spectrometry-mass spectrometry (IMS-MS) has the advantages of high speed and sensitivity but is still in its infancy. Here, an N α-(2,4-dinitro-5-fluorophenyl)-l-alaninamide (FDAA) derivatization is combined with trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) for chiral AA analysis. For the first time, we demonstrate the simultaneous separation of 19 pairs of chiral proteinogenic AAs in a single fixed condition TIMS-MS run. The utility of this approach is presented for mouse brain extracts by direct-infusion TIMS-MS. The robust separation ability in complex biological samples was proven in matrix-assisted laser desorption/ionization (MALDI) TIMS mass spectrometry imaging (MSI) as well by directly depositing 19 pairs of chiral AAs on a tissue slide following on-tissue derivatization. In addition, endogenous chiral amino acids were also detected and distinguished. The developed methods show compelling application prospects in biomarker discovery and biological research.

Cardio-Onco-Metabolism - Metabolic vulnerabilities in cancer and the heart

Cancer and cardiovascular diseases (CVDs) are the leading cause of death worldwide. Metabolic remodeling is a hallmark of both cancer and the failing heart. Tumors reprogram metabolism to optimize nutrient utilization and meet increased demands for energy provision, biosynthetic pathways, and proliferation. Shared risk factors for cancer and CVDs suggest intersecting mechanisms for disease pathogenesis and progression. In this review, we aim to highlight the role of metabolic remodeling in cancer and its potential to impair cardiac function. Understanding these mechanisms will help us develop biomarkers, better therapies, and identify patients at risk of developing heart disease after surviving cancer.

Quantitation of Enantiomeric Excess in an Achiral Environment Using Trapped Ion Mobility Mass Spectrometry

We present a novel, straightforward method to determine the enantiomeric excess (ee) of tryptophan (Trp) and N-tert-butyloxycarbonyl-O-benzylserine (BBS) solutions without chiral additives. For this, lithium carbonate, sodium carbonate, or silver acetate was added to solutions of Trp or BBS. Singly negatively charged dimer and trimer clusters were then formed by electrospray ionization and analyzed using trapped ion mobility spectrometry (TIMS) and time-of-flight mass spectrometry. When a solution contains both enantiomers, homo- and heterochiral clusters are generated which can be separated in the TIMS-tunnel based on their different mobilities using a nitrogen buffer gas. The ratio of homochiral to heterochiral clusters shows a binomial distribution and can be calibrated with solutions of known ee to yield ee measurements of samples with better than 1% accuracy. Samples can be prepared rapidly, and measurements are completed in less than 5 min. Current instrumental limitations restrict this method to rigid molecules with large functional groups adjacent to the chiral centers. Nevertheless, we expect this method to be applicable to many pharmaceuticals and provide the example of 1-methyltryptophan to demonstrate this.

Ion mobility mass spectrometry in the omics era: Challenges and opportunities for metabolomics and lipidomics

Researchers worldwide are taking advantage of novel, commercially available, technologies, such as ion mobility mass spectrometry (IM-MS), for metabolomics and lipidomics applications in a variety of fields including life, biomedical, and food sciences. IM-MS provides three main technical advantages over traditional LC-MS workflows. Firstly, in addition to mass, IM-MS allows collision cross-section values to be measured for metabolites and lipids, a physicochemical identifier related to the chemical shape of an analyte that increases the confidence of identification. Second, IM-MS increases peak capacity and the signal-to-noise, improving fingerprinting as well as quantification, and better defining the spatial localization of metabolites and lipids in biological and food samples. Third, IM-MS can be coupled with various fragmentation modes, adding new tools to improve structural characterization and molecular annotation. Here, we review the state-of-the-art in IM-MS technologies and approaches utilized to support metabolomics and lipidomics applications and we assess the challenges and opportunities in this growing field.

Evaluating the Utility of Temporal Compression in High-Resolution Traveling Wave-Based Cyclic Ion Mobility Separations

Ion mobility spectrometry coupled to mass spectrometry (IMS-MS) is slowly becoming a more integral part in omics-based workflows. With the recent technological advancements in IMS-MS instrumentation, particularly those involving traveling wave-based separations, ultralong pathlengths have become readily available in commercial platforms (e.g., Select Series Cyclic IMS from Waters Corporation and MOBIE from MOBILion). However, a tradeoff exists in such ultralong pathlength separations: increasing peak-to-peak resolution at the cost of lower signal intensities and thus poorer sensitivity of measurements. Herein, we explore the utility of temporal compression, where ions are compressed in the time domain, following high-resolution cyclic ion mobility spectrometry-mass spectrometry-based separations on a commercially available, unmodified platform. We assessed temporal compression in the context of various separations including those of reverse sequence peptide isomers, chiral noncovalent complexes, and isotopologues. From our results, we demonstrated that temporal compression improves IMS peak intensities by up to a factor of 4 while only losing ∼5 to 10% of peak-to-peak resolution. Additionally, the improvement in peak quality and signal-to-noise ratio was evident when comparing IMS-MS separations with and without a temporal compression step performed. Temporal compression can readily be implemented in existing traveling wave-based IMS-MS platforms, and our initial proof-of-concept demonstration shows its promise as a tool for improving peak shapes and peak intensities without sacrificing losses in resolution.

On the separation of enantiomers by drift tube ion mobility spectrometry

Racemic mixtures of twelve common α-amino acids and three chiral drugs were tested for the separation of their enantiomers by drift tube ion mobility spectrometry (IMS)-quadrupole mass spectrometry (QMS) by introducing chiral selectors into the buffer gas of the IMS instrument. (R)-α-(Trifluoromethyl)benzyl alcohol, (L)-ethyl lactate, methyl (S)-2-chloropropionate, and the R and S enantiomers of 2-butanol and 1-phenyl ethanol were evaluated as chiral selectors. Experimental conditions were varied during the tests, including buffer gas temperature, concentration and type of chiral selectors, analyte concentration, electrospray (ESI) voltage, ESI solvent pH, and buffer gas flow rate. The individual enantiomers yielded the same drift times and the racemic mixtures could not be separated. Energy calculations of the chiral selector-ion interactions showed that these separations are unlikely using 2-butanol as a chiral selector but they might be theoretically feasible depending on the chiral selector nature and the type of enantiomers. Several plausible explanations for not achieving separations were analyzed. Recommendations for potential enantiomer separations by IMS are proposed.

Exploiting Self-Association to Evaluate Enantiomeric Composition by Cyclic Ion Mobility–Mass Spectrometry

The characterization of enantiomers is an important analytical challenge in the chemical and life sciences. Thorough evaluation of the purity of chiral molecules is particularly required in the pharmaceutical industry where safety concerns are paramount. Assessment of the enantiomeric composition is still challenging and time-consuming, meaning that alternative approaches are required. In this study, we exploit the formation of dimers as diastereomeric pairs of enantiomers to affect separation by high resolution cyclic ion mobility–mass spectrometry. Using the example of (R/S)-thalidomide, we show that even though this is not an enantiomer separation, we can determine which enantiomer is in excess and obtain quantitative information on the enantiomer composition without the need for a chiral modifier. Further examples of the approach are presented, including d/l-tryptophan and (R/S)-propanolol, and demonstrate the need for mobility resolving power in excess of 400 (CCS/ΔCCS).

“Targeted Glucocorticoid Analysis using Ion Mobility-Mass Spectrometry (IM-MS)”

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Liquid chromatography-ion mobility-mass spectrometry (LC-IM-MS) for glucocorticoids.

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Determination of collision cross sections (CCS) for isomers.

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Different cation adducts shifted mobility and improved IM separation.

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Changing drift gas (He, Ar, CO₂) shifted mobility and improved resolution.

Introduction

Ion mobility-mass spectrometry (IM-MS) is an emerging technique in the -omics fields that has broad potential applicability to the clinical lab. As a rapid, gas-phase structure-based separation technique, IM-MS offers promise in isomer separations and can be easily combined with existing LC-MS methods (i.e., LC-IM-MS). Several experimental conditions, including analyte cation adducts and drift composition further provide a means to tune separations for global and/or targeted applications.

Objectives

The primary objective of this study was to demonstrate the utility of IM-MS under a range of experimental conditions for detection of glucocorticoids, and specifically for the separation of several isomeric pairs.

Methods

LC-IM-MS was used to characterize 16 glucocorticoids including three isomer pairs: cortisone/prednisolone, betamethasone/dexamethasone, and flunisolide/triamcinolone acetonide. Collision cross section (CCS) values were measured for all common adducts (e.g., protonated and sodiated) using both step-field and single-field methods. Alternative alkali, alkaline earth, and transition metals were introduced, such that their adducts could also be measured. Finally, four different drift gases (helium, nitrogen, argon, and carbon dioxide) were compared for their relative separation capability.

Results

LC-IM-MS offered a robust, multidimensional separation technique that allowed for the 16 glucocorticoids to be analyzed and separated in three-dimensions (retention time, CCS, and m/z). Despite the relatively modest resolution of isomer pairs under standard conditions (i.e., nitrogen drift gas, sodiated ions, etc.), improvements were observed for alkaline earth and transition metals (notable barium adducts) and in carbon dioxide drift gas.

Conclusion

In summary, LC-IM-MS offers potential as a clinical method due to its ease of coupling with traditional LC-MS methods and its promise for tuning separations to better resolve targeted and/or global isomers in complex biological samples.

Toward Routine Analysis of Anabolic Androgenic Steroids in Urine Using Ion Mobility-Mass Spectrometry

Anabolic androgenic steroids (AAS) make up one of the most prevalent classes of performance-enhancing drugs banned by the World Anti-Doping Agency (WADA) due to the competitive advantage they can afford athletes. Mass spectrometry-based methods coupled with chromatographic separations have become the gold standard for AAS analysis because of the superior sensitivity and selectivity provided. However, emerging analytical techniques including ion mobility spectrometry (IMS) have been demonstrated in recent applications as a means to further characterize and identify potential unknowns while simultaneously delivering improved sensitivity by filtering noise. Herein we outline the next crucial steps in bringing IMS to the routine drug testing workflow by combining it with established chromatographic and mass spectrometry methods (i.e., LC-IM-MS) for the detection of AAS in human urine. In addition to robust measurement of collision cross sections which can be used for identification purposes, functional group microtrends provide a structural basis on which to elucidate the structure of future novel anabolic agents. Lastly, the developed workflow is tested by analysis of testosterone in a realistic matrix (human urine) and demonstrates a limit of detection of 524 pg/mL, which surpasses the WADA Minimum Required Performance Levels for anabolic steroids. This work is expected to pave the way toward routine incorporation of IMS into analytical drug testing workflows to augment both qualitative and quantitative measure of performance enhancing drugs in the future.

Implementation of Ion Mobility Spectrometry-Based Separations in Structures for Lossless Ion Manipulations (SLIM)

Structures for Lossless Ion Manipulations (SLIM) is a powerful variant of traveling wave ion mobility spectrometry (TW-IMS) that uses a serpentine pattern of microelectrodes deposited onto printed circuit boards to achieve ultralong ion path lengths (13.5 m). Ions are propelled through SLIM platforms via arrays of TW electrodes while RF and DC electrodes provide radial confinement, establishing near lossless transmission. The recent ability to cycle ions multiple times through a SLIM has allowed ion path lengths to exceed 1000 m, providing unprecedented separation power and the ability to observe ion structural conformations unobtainable with other IMS technologies. The combination of high separation power, high signal intensity, and the ability to couple with mass spectrometry places SLIM in the unique position of being able to address longstanding proteomics and metabolomics challenges by allowing the characterization of isomeric mixtures containing low abundance analytes.

Powerful Steroid-Based Chiral Selector for High-Throughput Enantiomeric Separation of α-Amino Acids Utilizing Ion Mobility-Mass Spectrometry

Stereospecific recognition of amino acids (AAs) plays a crucial role in chiral biomarker-based diagnosis and prognosis. Separation of AA enantiomers is a long and tedious task due to the requirement of AA derivatization prior to the chromatographic or electrophoretic steps which are also time-consuming. Here, a mass-tagged chiral selector named [d₀]/[d₅]-estradiol-3-benzoate-17β-chloroformate ([d₀]/[d₅]-17β-EBC) with high reactivity and good enantiomeric resolution in regard to AAs was developed. After a quick and easy chemical derivatization step of AAs using 17β-EBC as the single chiral selector before ion mobility-mass spectrometry analysis, good enantiomer separation was achieved for 19 chiral proteinogenic AAs in a single analytical run (∼2 s). A linear calibration curve of enantiomeric excess was also established using [d₀]/[d₅]-17β-EBC. It was demonstrated to be capable of determining enantiomeric ratios down to 0.5% in the nanomolar range. 17β-EBC was successfully applied to investigate the absolute configuration of AAs among peptide drugs and detect trace levels of d-AAs in complex biological samples. These results indicated that [d₀]/[d₅]-17β-EBC may contribute to entail a valuable step forward in peptide drug quality control and discovering chiral disease biomarkers.

Design of Cyclodextrin-Based Functional Systems for Biomedical Applications

Cyclodextrins (CDs) are a family of α-1,4-linked cyclic oligosaccharides that possess a hydrophobic cavity and a hydrophilic outer surface with abundant hydroxyl groups. This unique structural characteristic allows CDs to form inclusion complexes with various guest molecules and to functionalize with different substituents for the construction of novel sophisticated systems, ranging from derivatives to polymers, metal-organic frameworks, hydrogels, and other supramolecular assemblies. The excellent biocompatibility, selective recognition ability, and unique bioactive properties also make these CD-based functional systems especially attractive for biomedical applications. In this review, we highlight the characteristics and advantages of CDs as a starting point to design different functional materials and summarize the recent advances in the use of these materials for bioseparation, enzymatic catalysis, biochemical sensing, biomedical diagnosis and therapy.

Direct and simultaneous recognition of the positional isomers of aminobenzenesulfonic acid by TIMS-TOF-MS

Positional isomer recognition is a challenging scientific issue. Fast and accurate detection of isomers is required for understanding their chemical properties. Here, we describe a method for simultaneous recognition of three positional isomers of 2-aminobenzenesulfonic acid (2-ABSA), 3-ABSA, and 4-ABSA using trapped ion mobility spectroscopy-time-of-flight mass spectrometry (TIMS-TOF-MS). The three ABSA positional isomers were recognized by measuring the different ion mobility of the ternary complexes of [β-cyclodextrin (CD)+ABSA + Li]⁺ or [λ-CD + ABSA + Na]⁺, because their different collision cross-sections or different spatial conformations. The collision-induced dissociation mechanism of the different complexes of [β-CD + ABSA + Li]⁺ and [λ-CD + ABSA + Na]⁺ using tandem mass spectrometry exhibited the same dissociation process with slightly different dissociation energies, which the smaller cross-section requires higher collision energy that means the smaller complex with tighter and more stable conformation than a larger complex for the ABSA complexes. In addition, relative quantification of the ABSA isomers was studied by measuring any two of the three ABSA isomer complexes at different molar ratio of 10:1 to 1:10 in the μM range, good linearity (R² > 0.99) with precision between 2.14% and 2.58%, and accuracy ≥ 97.1% were obtained. The method for fast determination and recognition of ABSA positional isomers by combination with CD and alkali metal ions possesses the advantages of being simple, direct, rapid, sensitive, cost-effective, and needs no chemical derivatives or chromatographic separation before analysis. Therefore, the proposed method would be a powerful tool for the analysis of ABSA isomers or even other positional isomers.

Evaluation of Waveform Profiles for Traveling Wave Ion Mobility Separations in Structures for Lossless Ion Manipulations

Structures for lossless ion manipulations (SLIM) have recently enabled a powerful implementation of traveling wave ion mobility spectrometry (TWIMS) for ultrahigh resolution separations; however, experimental parameters have not been optimized, and potential significant gains may be feasible. Most TWIMS separations have utilized square-shaped waveforms applied by time-dependent voltage stepping across repeating sets of electrodes, but alternative waveforms may provide further improvements to resolution. Here, we characterize five waveforms (including square and sine) in terms of their transmission efficiency, IMS resolution, and resolving power, and explore the effects of TW amplitude and speed on the performance of each. We found, consistent with previous work, separations were generally improved with higher TW amplitudes, moderately improved by lower speeds (limited by ion "surfing" with the waves), and found decreases in signal intensity at the extremes of operating conditions. The triangle and asymmetric "ramp forward" shaped profiles were found to provide modestly greater resolution and resolving power, an observation we tentatively attribute to their relatively uniform fields and minimal low-field regions.

Noncovalent Complexes of Cyclodextrin with Small Organic Molecules: Applications and Insights into Host-Guest Interactions in the Gas Phase and Condensed Phase

Cyclodextrins (CDs) have drawn a lot of attention from the scientific communities as a model system for host–guest chemistry and also due to its variety of applications in the pharmaceutical, cosmetic, food, textile, separation science, and essential oil industries. The formation of the inclusion complexes enables these applications in the condensed phases, which have been confirmed by nuclear magnetic resonance (NMR) spectroscopy, X-ray crystallography, and other methodologies. The advent of soft ionization techniques that can transfer the solution-phase noncovalent complexes to the gas phase has allowed for extensive examination of these complexes and provides valuable insight into the principles governing the formation of gaseous noncovalent complexes. As for the CDs’ host–guest chemistry in the gas phase, there has been a controversial issue as to whether noncovalent complexes are inclusion conformers reflecting the solution-phase structure of the complex or not. In this review, the basic principles governing CD’s host–guest complex formation will be described. Applications and structures of CDs in the condensed phases will also be presented. More importantly, the experimental and theoretical evidence supporting the two opposing views for the CD–guest structures in the gas phase will be intensively reviewed. These include data obtained via mass spectrometry, ion mobility measurements, infrared multiphoton dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations.

Rapid and Simultaneous Characterization of Drug Conjugation in Heavy and Light Chains of a Monoclonal Antibody Revealed by High-Resolution Ion Mobility Separations in SLIM

Antibody-drug conjugates (ADCs) have recently gained traction in the biomedical community due to their promise for human therapeutics and an alternative to chemotherapy for cancer. Crucial metrics for ADC efficacy, safety, and selectivity are their drug-antibody ratios (DARs). However, DAR characterization (i.e., determining the average number of conjugated drugs on the antibody) through analytical methods remains challenging due to the heterogeneity of drug conjugation as well as the numerous post-translational modifications possible in the monoclonal antibody. Herein, we report on the use of high-resolution ion mobility spectrometry separations in structures for lossless ion manipulations coupled to mass spectrometry (SLIM IMS-MS) for the rapid and simultaneous characterization of the drug load profile (i.e., stoichiometric distribution of the number of conjugated drugs present on the mAb), determination of the weighted average DAR in both the heavy and light chains of a model antibody-drug conjugate, and calculation of the overall DAR of the ADC. After chemical reduction of the ADC and a subsequent 31.5 m SLIM IMS separation, the various drug-bound antibody species could be well resolved for both chains. We also show significantly higher resolution separations were possible for these large ions with SLIM IMS as compared to ones performed on a commercially available (1 m) drift tube IMS-MS platform. We expect high-resolution SLIM IMS separations will augment the existing toolbox for ADC characterization, particularly to enable the rapid optimization of DAR for a given ADC and thus better understand its potential toxicity and potency.

Traveling-Wave-Based Electrodynamic Switch for Concurrent Dual-Polarity Ion Manipulations in Structures for Lossless Ion Manipulations

We describe the development of a dual-polarity traveling-wave (TW) structures for lossless ion manipulations (SLIM) ion mobility spectrometry (IMS) device capable of switching both positive and negative ions that are traveling simultaneously along the same path to different regions of the SLIM. Through simulations, the routing efficiency of the SLIM TW switch was compared to a SLIM direct-current-based (DC) switch developed previously for IMS-MS. We also report on the initial experimental evaluation of a dual-polarity SLIM platform, which uses the TW-based ion switch to achieve higher resolution multipass serpentine ultralong path with extended routing (SUPER) IMS separations. Overall, these results show that the dual-polarity TW switch is not only as effective as DC switching in terms of routing efficiency but also is agnostic to the polarity of the ions being routed.

Ion Mobility Spectrometry: Fundamental Concepts, Instrumentation, Applications, and the Road Ahead

Ion mobility spectrometry (IMS) is a rapid separation technique that has experienced exponential growth as a field of study. Interfacing IMS with mass spectrometry (IMS-MS) provides additional analytical power as complementary separations from each technique enable multidimensional characterization of detected analytes. IMS separations occur on a millisecond timescale, and therefore can be readily nested into traditional GC and LC/MS workflows. However, the continual development of novel IMS methods has generated some level of confusion regarding the advantages and disadvantages of each. In this critical insight, we aim to clarify some common misconceptions for new users in the community pertaining to the fundamental concepts of the various IMS instrumental platforms (i.e., DTIMS, TWIMS, TIMS, FAIMS, and DMA), while addressing the strengths and shortcomings associated with each. Common IMS-MS applications are also discussed in this review, such as separating isomeric species, performing signal filtering for MS, and incorporating collision cross-section (CCS) values into both targeted and untargeted omics-based workflows as additional ion descriptors for chemical annotation. Although many challenges must be addressed by the IMS community before mobility information is collected in a routine fashion, the future is bright with possibilities.

SLIM Ultrahigh Resolution Ion Mobility Spectrometry Separations of Isotopologues and Isotopomers Reveal Mobility Shifts due to Mass Distribution Changes

We report on separations of ion isotopologues and isotopomers using ultrahigh-resolution traveling wave-based Structures for Lossless Ion Manipulations with serpentine ultralong path and extended routing ion mobility spectrometry coupled to mass spectrometry (SLIM SUPER IMS-MS). Mobility separations of ions from the naturally occurring ion isotopic envelopes (e.g., [M], [M+1], [M+2], ... ions) showed the first and second isotopic peaks (i.e., [M+1] and [M+2]) for various tetraalkylammonium ions could be resolved from their respective monoisotopic ion peak ([M]) after SLIM SUPER IMS with resolving powers of ∼400-600. Similar separations were obtained for other compounds (e.g., tetrapeptide ions). Greater separation was obtained using argon versus helium drift gas, as expected from the greater reduced mass contribution to ion mobility described by the Mason-Schamp relationship. To more directly explore the role of isotopic substitutions, we studied a mixture of specific isotopically substituted (15N, 13C, and 2H) protonated arginine isotopologues. While the separations in nitrogen were primarily due to their reduced mass differences, similar to the naturally occurring isotopologues, their separations in helium, where higher resolving powers could also be achieved, revealed distinct additional relative mobility shifts. These shifts appeared correlated, after correction for the reduced mass contribution, with changes in the ion center of mass due to the different locations of heavy atom substitutions. The origin of these apparent mass distribution-induced mobility shifts was then further explored using a mixture of Iodoacetyl Tandem Mass Tag (iodoTMT) isotopomers (i.e., each having the same exact mass, but with different isotopic substitution sites). Again, the observed mobility shifts appeared correlated with changes in the ion center of mass leading to multiple monoisotopic mobilities being observed for some isotopomers (up to a ∼0.04% difference in mobility). These mobility shifts thus appear to reflect details of the ion structure, derived from the changes due to ion rotation impacting collision frequency or momentum transfer, and highlight the potential for new approaches for ion structural characterization.

Opening new paths for biological applications of ion mobility - mass spectrometry using Structures for Lossless Ion Manipulations

Ion mobility separations coupled to mass spectrometry (IM-MS) have received much attention for their ability to provide complementary structural information to solution-phase-based separations, as well as to aid in the identification of unknown compounds. While IM-MS is an increasingly powerful analytical technique, significant bottlenecks related to the resolution of measurements have kept it from becoming broadly applied for biological analyses. Presently, IM-MS-based measurements also remain limited in terms of their sensitivity as compared to state of the art MS-based approaches alone. Structures for Lossless Ion Manipulations (SLIM)-based IM separations provide a basis for overcoming these bottlenecks, addressing issues associated with resolution and sensitivity in the omics, and potentially opening the door to much broader application.

Separation of β-Amyloid Tryptic Peptide Species with Isomerized and Racemized l-Aspartic Residues with Ion Mobility in Structures for Lossless Ion Manipulations

Accumulation of β-amyloid (Aβ) is one of the hallmarks of Alzheimer's disease. The deposition of β-amyloid plaques is likely to start years in advance of manifestation of clinical symptoms, although the exact timing is unknown. Over the years, Aβ peptides undergo both post-translational modification and stereoisomerization. Analysis of the resulting stereoisomers is particularly challenging because of their identical elemental composition and similar physicochemical properties. Herein, we have utilized our recently developed structures for lossless ion manipulations ion mobility-mass spectrometry platform (SLIM IM-MS), in conjunction with serpentine ultralong path with extended routing (SUPER), to baseline resolve four distinct sets of Aβ17-28 tryptic peptide epimers on a rapid (∼1 s) time scale. We discovered that sodium adduct ions, [M + H + Na]2+, allowed baseline SLIM SUPER IM resolution for all Aβ epimer sets assessed, while such baseline separations were unachievable for their [M + 2H]2+ doubly protonated ions.