Structurally Informative Isotopic Shifts in Ion Mobility Spectra for Heavier Species

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.

Isolating the Contributions from Moments of Inertia in Isotopic Shifts Measured by High-Resolution Cyclic Ion Mobility Separations

The unexpected finding that isotopomers (i.e., isotopic isomers) can be separated with high-resolution ion mobility spectrometry-mass spectrometry (IMS-MS) has raised new structural considerations affecting an ion's mobility, namely its center of mass (CoM) and moments of inertia (MoI). Unfortunately, thus far, no studies have attempted to experimentally isolate either CoM or MoI, as they are intrinsically linked by their definitions, where MoI is calculated in relation to CoM. In this study, we designed and synthesized four isotopically labeled tetrapropylammonium (TAA3) ions, each with a unique mass distribution. Three of the synthesized TAA3 ions were labeled symmetrically, thus having identical CoM but differing MoI, which we verified using density functional theory (DFT) calculations. Consequently, we were able to isolate the effect of MoI changes in high-resolution IMS-MS separations. Cyclic ion mobility spectrometry-mass spectrometry (cIMS-MS) separations of the isotopically labeled TAA3 variants revealed isotopic mobility shifts attributable solely to changes in MoI. A 60-m cIMS-MS separation demonstrated that two nominally isobaric TAA3 pseudoisotopomers could be partially resolved, showcasing potential feasibility for isotopomer separations on commercially available IMS-MS platforms. With our previously established collision cross section (CCS) calibration protocol, we also quantified the relationship between MoI and CCS. Our results represent the first demonstration of IMS-MS separations based solely on MoI differences. We believe these findings will contribute important evidence to the growing body of literature on the physical nature of isotopic shifts in IMS-MS separations and work toward more accurate CCS predictions.

Coupling Isotopic Shifts with Collision Cross-Section Measurements for Carbohydrate Characterization in High-Resolution Ion Mobility Separations

Herein, we introduce a two-dimensional strategy to better characterize carbohydrate isomers. In a single experiment, we can derive cyclic ion mobility-mass spectrometry (cIMS-MS)-based collision cross-section (CCS) values in conjunction with measuring isotopic shifts through the relative arrival times of light and heavy isotopologues. These isotopic shifts were introduced by permethylating carbohydrates with either light, CH3, or heavy, CD3, labels at every available hydroxyl group to generate a light/heavy pair of isotopologues for every individual species analyzed. We observed that our calculated CCS values, which were exclusively measured for the light isotopologues, were orthogonal to our measured isotopic shifts (i.e., relative arrival time values between heavy and light permethylated isotopologues). Our permethylation-induced isotopic shifts scaled well with increasing molecular weight, up to ∼m/z 1300, expanding the analysis of isotopic shifts to molecules 3-4 times as large as those previously studied. Our presented use of coupling CCS values with the measurement of isotopic shifts in a single cIMS-MS experiment is a proof-of-concept demonstration that our two-dimensional approach can improve the characterization of challenging isomeric carbohydrates. We envision that our presented 2D approach will have broad utility for varying molecular classes as well as being amenable to many forms of derivatization.

High-Definition Differential Ion Mobility Spectrometry with Structural Isotopic Shifts for Anionic Compounds

Differential ion mobility spectrometry (FAIMS) had emerged in the 2000s as a novel tool for postionization separations in conjunction with mass spectrometry (MS). High-definition FAIMS introduced a decade ago has enabled resolution of peptide, lipid, and other molecular isomers with minute structural variations and recently the isotopic shift analyses where the spectral pattern for stable isotopes fingerprints the ion geometry. Those studies, including all isotopic shift analyses, were in the positive mode. Here, we achieve the same high resolution for anions exemplified by phthalic acid isomers. The resolving power and magnitude of isotopic shifts are in line with the metrics for analogous haloaniline cations, establishing high-definition negative-mode FAIMS with structurally specific isotopic shifts. Different shifts (including the new ¹⁸O) remain additive and mutually orthogonal, demonstrating the generality of those properties across the elements and charge states. Expanding to common (not halogenated) organic compounds is a key step toward the broad use of FAIMS isotopic shift methodology.

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.

Hydrogen-Deuterium-Exchange-Based Mass Distribution Shifts in High-Resolution Cyclic Ion Mobility Separations

The mass distribution of ions influences separations in ion mobility spectrometry-mass spectrometry (IMS-MS). Herein, we introduce a method to induce mass distribution shifts for various analytes using hydrogen-deuterium exchange (HDX) immediately prior to ionization using a dual syringe approach. By replacing labile hydrogens on analytes with deuteriums, we were able to differentiate isomers using separations of isotopologues. For each analyte studied, every possible level of deuteration (from undeuterated to fully deuterated) was generated and then separated using cyclic ion mobility spectrometry-mass spectrometry (cIMS-MS). The information gained from such separations (relative arrival times; t_Rel. values) was found to be orthogonal to conventional IMS-MS separations. Additionally, the observed shifts were linearly additive with increasing deuteration, suggesting that this methodology could be extended to analytes with a larger number of labile hydrogens. For one isomer pair, as few as two deuteriums were able to produce a large enough mass distribution shift to differentiate isomers. In another experiment, we found that the mass distribution shift was large enough to overcome the reduced mass contribution, resulting in a "flipped" arrival time where the heavier deuterated isotopologue arrived before the lighter one. In this work, we present a proof-of-concept demonstration that mass-distribution-based shifts, t_Rel. values, could potentially act as an added dimension to characterize molecules in IMS-MS. We anticipate, along with future work in this area, that mass-distribution-based shifts could enable the identification of unknown molecules through a database-driven approach in an analogous fashion to collision cross section (CCS) measurements.

High-Definition Ion Mobility/Mass Spectrometry with Structural Isotopic Shifts for Nominally Isobaric Isotopologues

We had reported the isotopic envelopes in differential IMS (FAIMS) separations depending on the ion structure. However, this new approach to distinguish isomers was constrained by the unit-mass resolution commingling all nominally isobaric isotopologues. Here, we directly couple high-definition FAIMS to ultrahigh-resolution (Orbitrap) MS and employ the resulting platform to explore the FAIMS spectra for isotopic fine structure. The peak shifts therein for isotopologues of halogenated anilines with ¹⁵N and ¹³C (split by 6 mDa) in N₂/CO₂ buffers dramatically differ, more than for the ¹³C, ³⁷Cl, or ⁸¹Br species apart by 1 or 2 Da. The shifts in FAIMS space upon different elemental isotopic substitutions are orthogonal mutually and to the underlying separations, forming fingerprint multidimensional matrices and 3-D trajectories across gas compositions that redundantly delineate all isomers considered. The interlocking instrumental and methodological upgrades in this work take the structural isotopic shift approach to the next level.

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.

Isomer and Conformer-Specific Mass Distribution-Based Isotopic Shifts in High-Resolution Cyclic Ion Mobility Separations

Herein, we present the use of mass distribution-based isotopic shifts in high-resolution cyclic ion mobility spectrometry-mass spectrometry (cIMS-MS)-based separations to characterize various isomeric species as well as conformers. Specifically, by using the observed relative arrival time values for the isotopologues found in the isotopic envelope after long pathlength cIMS-MS separations, we were able to distinguish dibromoaniline, dichloroaniline, and quaternary ammonium salt isomers, as well as a pair of 25-hydroxyvitamin D3 conformers based on their respective mass distribution-based shifts. Our observed shifts were highly reproducible and broadly applied to the isotopologues of various atoms (i.e., Cl, Br, and C). Additionally, through a control experiment, we determined that such shifts are indeed pathlength-independent, thus demonstrating that our presented methodology could be readily extended to other high-resolution IMS-MS platforms. These results are the first characterization of conformers using mass distribution-based IMS-MS shifts, as well as the first use of a commercial cIMS-MS platform to characterize isomers via their mass distribution-based shifts. We anticipate that our methodology will have broad applicability for biological analytes and that mass distribution-based shifts could potentially act as an added dimension of analysis in existing IMS-MS workflows in omics-based research. Specifically, we envision that the development of a database of these mass distribution-based shifts could, for example, enable the identification of unknown metabolites in complex matrices.

Experimental Measurements of Relative Mobility Shifts Resulting from Isotopic Substitutions with High-Resolution Cyclic Ion Mobility Separations

Herein, we report on the experimental measurements for estimated relative mobility shifts caused by changes in mass distribution from isotopic substitutions in isotopologues and isotopomers with high-resolution cyclic ion mobility separations. By utilizing unlabeled and fully labeled isotopologues with the same isotopic substitutions (i.e., ²H or ¹³C), we created a highly precise mobility scale for each set analyzed to determine the magnitude of such mass distribution shifts and thus calculate estimated deviations from expected, theoretical reduced mass contributions. We observed relative mobility shifts in various isotopologues (e.g., hexadecyltrimethylammonium, sucrose, and palmitic acid species) that deviated from reduced mass theory, according to the Mason-Schamp relationship, ranging in estimated magnitude from ∼0.007% up to ∼0.1% in relative mobility. More interestingly, it was found that two deuterated palmitic acid isotopomers also differed by ∼0.03% from one another in their respective relative mobility shifts. Our results are the first report of isotopologue and isotopomer separations on a commercially available cyclic ion mobility spectrometry-mass spectrometry platform. We envision that our presented mobility scale methodology will have broad applicability in studying the effect of mass distribution changes from isotopic substitutions in other biomolecules and help pave the way for the improvement of ion mobility theory and collision cross section calculators.

Measurement and Theory of Gas-Phase Ion Mobility Shifts Resulting from Isotopomer Mass Distribution Changes

The unanticipated discovery of recent ultra-high-resolution ion mobility spectrometry (IMS) measurements revealing that isotopomers─compounds that differ only in the isotopic substitution sites─can be separated has raised questions as to the physical basis for their separation. A study comparing IMS separations for two isotopomer sets in conjunction with theory and simulations accounting for ion rotational effects provides the first-ever prediction of rotation-mediated shifts. The simulations produce observable mobility shifts due to differences in gas-ion collision frequency and translational-to-rotational energy transfer. These differences can be attributed to distinct changes in the moment of inertia and center of mass between isotopomers. The simulations are in broad agreement with the observed experiments and consistent with relative mobility differences between isotopomers. These results provide a basis for refining IMS theory and a new foundation to obtain additional structural insights through IMS.

Delineation of Isomers by the 13C Shifts in Ion Mobility Spectra

Mass spectrometry (MS) and isotopes were intertwined for a century, with stable isotopes central to many MS identification and quantification protocols. In contrast, the analytical separations including ion mobility spectrometry (IMS) largely ignored isotopes, partly because of insufficient resolution. We recently delineated various halogenated aniline isomers by structurally specific splitting in FAIMS spectra. While this capability hinges on the ¹³C shifts, all preceding studies leveraged ³⁷Cl or ⁸¹Br to enhance the differentiation. However, such abundant heavy isotopes are absent from typical organic compounds. With single I isotope, iodinated organics generate similar isotopic envelopes dominated by the ¹³C atoms. Here, we distinguish the three monoiodoaniline isomers based on the shifts solely for one or two ¹³C atoms. The differentiation may be somewhat improved using multipoint peak position descriptions for more reproducible shifts. The interisomer order of shifts differs from those for chlorinated or brominated analogues, showcasing the specificity of approach. We also investigated the mass scaling of isotopic shifts, encountering divergent trends for different structural families.

Identification and quantification of amino acids and related compounds based on Differential Mobility Spectrometry

A new metabolite descriptor allowing fast quantification for the diagnosis of metabolic diseases.