Resolving structural isomers of monosaccharide methyl glycosides using drift tube and traveling wave ion mobility mass spectrometry

Differentiation of regioisomers of sulfobenzoic acid by traveling-wave ion mobility mass spectrometry

An ion mobility mass spectrometry (IM-MS) investigation using a Synapt G2 mass spectrometer was conducted to separate anions generated from the three regioisomers of sulfobenzoic acid. The results revealed that the differences in arrival time distributions (ATDs) were inadequate to differentiate the isomers unambiguously. However, the ATD profiles of the product ions, generated by fragmenting the respective mass-selected m/z 201 precursor ions in the Trap region of the three-compartment traveling-wave ion guide of the Synapt G2 mass spectrometer, were distinctly different, enabling definitive differentiation of the isomers. An arrival-time peak for an ion of m/z 157 resulting from the loss of CO2 from the respective precursors was common to all three mobilograms. However, only the profile recorded from the para-isomer exhibited a unique arrival-time peak for an ion of m/z 137, originating from an SO2 loss. Such a peak corresponding to an SO2 loss was absent in the ATD profiles of the ortho- and meta-isomers. Additionally, the mobilogram of the meta-isomer displayed a unique peak at 3.42 ms. Based on its product ion spectrum, this peak was attributed to the bisulfite anion (m/z 81; HSO3-). Previously, this meta-isomer specific m/z 81 ion had been proposed to originate from a two-step process involving the intermediacy of an m/z 157 ion formed by CO2 loss. However, our detailed tandem mass spectrometric experiments suggest that the m/z 81 is not a secondary product but rather an ion that originated from a direct elimination of a benzyne derivative from the m/z 201 precursor ion.

Developments in Trapped Ion Mobility Mass Spectrometry to Probe the Early Stages of Peptide Aggregation

Ion mobility mass spectrometry (IM-MS) has proven to be an excellent method to characterize the structure of amyloidogenic protein and peptide aggregates, which are formed in coincidence with the development of neurodegenerative diseases. However, it remains a challenge to obtain detailed structural information on all conformational intermediates, originating from the early onset of those pathologies, due to their complex and heterogeneous environment. One way to enhance the insights and the identification of these early stage oligomers is by employing high resolution ion mobility mass spectrometry experiments. This would allow us to enhance the mobility separation and MS characterization. Trapped ion mobility spectrometry (TIMS) is an ion mobility technique known for its inherently high resolution and has successfully been applied to the analysis of protein conformations among others. To obtain conformational information on fragile peptide aggregates, the instrumental parameters of the TIMS-Quadrupole-Time-of-Flight mass spectrometer (TIMS-qToF-MS) have to be optimized to allow the study of intact aggregates and ensure their transmission toward the detector. Here, we investigate the suitability and application of TIMS to probe the aggregation process, targeting the well-characterized M307-N319 peptide segment of the TDP-43 protein, which is involved in the development of amyotrophic lateral sclerosis. By studying the influence of key parameters over the full mass spectrometer, such as source temperature, applied voltages or RFs among others, we demonstrate that by using an optimized instrumental method TIMS can be used to probe peptide aggregation.

Structural characterization and analysis of different epimers of neutral glycosphingolipid LcGg4 by ion mobility spectrometry-mass spectrometry

LcGg4, a neutral glycosphingolipid (GSL) and cancer antigen, its epimers GalNAc-LcGg4 and GlcNAc-LcGg4, and three lipid forms of GalNAc-LcGg4 were studied by mass spectrometry (MS). It was found that different forms of GalNAc-LcGg4 carrying homologous (d16:1/18:0) and (d18:1/18:0) lipids were easily separated and identified using liquid chromatography (LC)-MS. In addition, like gangliosides, homologous lipid forms of GalNAc-LcGg4 showed the same fragmentation pattern, except for a uniform shift of their glycolipid product ions by a certain m/z number determined by the varied lipid structure. It was also disclosed that LcGg4 and its epimers GalNAc-LcGg4 and GlcNAc-LcGg4, which are different only in the C4-configuration of their non-reducing end sugar residues, gave the same MS/MS product ions in similar relative intensities, as well as the same LC retention time, suggesting the challenge to differentiate epimeric GSLs by LC-MS. However, ion mobility spectrometry (IMS)-MS was able to efficiently separate and distinguish these epimers. This study has demonstrated the promise of IMS-MS for isomeric GSL characterization and the IMS-MS and LC-MS/MS combination for natural GSL analysis.

Field-Switching Repeller Flowing Atmospheric-Pressure Afterglow Drift Tube Ion Mobility Spectrometry

In this work, a field-switching (FS) technique is employed with a flowing atmospheric pressure afterglow (FAPA) source in drift tube ion mobility spectrometry (DTIMS). The premise is to incorporate a tip-repeller electrode as a substitute for the Bradbury-Nielsen gate (BNG) so as to overcome corresponding disadvantages of the BNG, including the gate depletion effect (GDE). The DTIMS spectra were optimized in terms of peak shape and full width by inserting an aperture at the DTIMS inlet that was used to control the neutral molecules' penetration into the separation region, thus preventing neutral-ion reactions inside. The FAPA and repeller's experimental operating conditions including drift and plasma gas flow rates, pulse injection times, repeller positioning and voltage, FAPA current, and effluent angle were optimized. Ion mobility spectra of selected compounds were captured, and the corresponding reduced mobility values were calculated and compared with the literature. The 6-fold improvements in limit of detection (LOD) compared with previous work were obtained for 2,6-DTBP and acetaminophen. The enhanced performance of the FS-FAPA-DTIMS was also investigated as a function of the GDE when compared with FAPA-DTIMS containing BNG.

Paired Derivatization Approach with H/D-Labeled Hydroxylamine Reagents for Sensitive and Accurate Analysis of Monosaccharides by Liquid Chromatography Tandem Mass Spectrometry

Monosaccharides play important roles in biological processes. Sensitive and accurate analyses of monosaccharides remain challenging because of their high hydrophilicities and poor ionization efficiencies. Here, we developed a paired derivatization approach with H/D-labeled hydroxylamines for simultaneous quantification of 12 monosaccharides by liquid chromatography tandem mass spectrometry (LC-MS/MS). O-(4-Methoxybenzyl)hydroxylamine hydrochloride (4-MOBHA·HCl) showed higher derivatization efficiency for monosaccharides compared to six other hydroxylamine analogues. The derivatization of monosaccharides was readily achieved in an aqueous solution. Furthermore, the deuterium-labeled isotope reagent, d₃-4-MOBHA·HCl, was newly synthesized to stably label monosaccharides to improve its accuracy and precision in complex matrix analysis. As a result, 12 monosaccharides were rapidly detected by LC-MS/MS within 16 min with significant improvements in chromatographic separation and retention time. The detection sensitivity increased by 83 to 1600-fold with limits of quantitation ranging from 0.25 to 3.00 fmol. With the paired derivatization strategy, the monosaccharides could be accurately quantified with good linearity (R² > 0.99) and satisfactory accuracy (recoveries: 85-110%). Using this method, we achieved sensitive and accurate quantification of the monosaccharide composition of herbal polysaccharides and the change in monosaccharide levels in human cell lines under physiopathological conditions. More importantly, the developed method was able to differentiate between the levels of the monosaccharides in fecal samples of human ulcerative colitis (UC) patients and UC mice compared to their respective controls. The differential monosaccharides determined in human feces provided a good diagnostic performance in distinguishing the UC patients from healthy individuals, showing potential for clinical application.

Simultaneous Analysis of Fatty Alcohols, Fatty Aldehydes, and Sterols in Thyroid Tissues by Electrospray Ionization-Ion Mobility-Mass Spectrometry Based on Charge Derivatization

In this work, we developed a rapid and high-sensitivity method for simultaneous analyses of fatty alcohols, fatty aldehydes, and sterols by combining the optimized derivatization reaction with electrospray ionization-ion mobility-mass spectrometry (ESI-IM-MS). Pyridine and thionyl chloride were used as derivatization reagents as they were easily removed after the derivatization reaction and could generate permanently charged tags on different functional groups including hydroxyls and aldehydes. Through this one-step derivatization reaction, the sensitivity of detection for fatty alcohols, fatty aldehydes, and sterols was significantly increased. Moreover, the introduction of ion mobility spectrometry (IMS), offering an additional resolution power, ensured more sensitive and accurate detection of derivative products without increasing analytical time. Being connected with high-performance liquid chromatography, more than 15 kinds of compounds were analyzed within 4 min. Relative quantification using peak intensity ratios between d₀-/d₅-labeled ions were subsequently applied for analyzing these 15 kinds of compounds in human thyroid carcinoma and para-carcinoma tissues. The results showed significant differences in content of some analytes between these two kinds of tissues (p < 0.05). The correlations between most of the analytes in thyroid carcinoma tissues are better than the correlations in para-carcinoma tissues.

Rapid Differentiation of Asian and American Ginseng by Differential Ion Mobility Spectrometry-Tandem Mass Spectrometry Using Stepwise Modulation of Gas Modifier Concentration

This study reports a rapid and robust method for the differentiation of Asian and American ginseng samples based on differential ion mobility spectrometry-tandem mass spectrometry (DMS-MS/MS). Groups of bioactive ginsenoside/pseudo-ginsenoside isomers, including Rf/Rg₁/F₁₁, Rb₂/Rb₃/Rc, and Rd/Re, in the ginseng extracts were sequentially separated using DMS with stepwise changes in the gas modifier concentration prior to MS analysis. The identities of the spatially separated ginsenoside/pseudo-ginsenoside isomers were confirmed by their characteristic compensation voltages at specific modifier loading and MS/MS product ions. As expected, Asian ginseng samples contained some Rf and an insignificant amount of F₁₁, whereas American ginseng samples had a high level of F₁₁ but no Rf. The origin of the whole and sliced ginseng could further be confirmed using the quantitative ratios of three sets of ginsenoside markers, namely, Rg₁/Re, Rb₁/Rg₁, and Rb₂/Rc. Based on our results, new benchmark ratios of Rg₁/Re < 0.15, Rb₁/Rg₁ > 2.15, and Rb₂/Rc < 0.26 were proposed for American ginseng (as opposed to Asian ginseng).

Advancing Solutions to the Carbohydrate Sequencing Challenge

Carbohydrates possess a variety of distinct features with stereochemistry playing a particularly important role in distinguishing their structure and function. Monosaccharide building blocks are defined by a high density of chiral centers. Additionally, the anomericity and regiochemistry of the glycosidic linkages carry important biological information. Any carbohydrate-sequencing method needs to be precise in determining all aspects of this stereodiversity. Recently, several advances have been made in developing fast and precise analytical techniques that have the potential to address the stereochemical complexity of carbohydrates. This perspective seeks to provide an overview of some of these emerging techniques, focusing on those that are based on NMR and MS-hybridized technologies including ion mobility spectrometry and IR spectroscopy.

Ion mobility-mass spectrometry analysis of diarylquinoline diastereomers: Drugs used for tuberculosis treatment

Mycobacterium tuberculosis infection results in more than two million deaths per year and is the leading cause of mortality in people infected with HIV. A new structural class of antimycobacterials, the diarylquinolines, has been synthesized and is being highly effective against both M. tuberculosis and multidrug-resistant tuberculosis. As diarylquinolines are biologically active only under their ( R,S) stereoisomeric form, it is essential to differentiate the stereoisomers ( R,S) and ( R,R). To achieve this, tandem mass spectrometry and ion mobility spectrometry-mass spectrometry have been performed with 10 diarylquinoline diastereomers couples. In this study, we investigated cationization with alkali metal cations and several ion mobility drift gases in order to obtain diastereomer differentiations. We have shown that diastereomers of the diarylquinolines family can be differentiated separately by tandem mass spectrometry and in mixture by ion mobility spectrometry-mass spectrometry. However, although the structure of each diastereomer is close, several behaviors could be observed concerning the cationization and the ion mobility spectrometry separation. The ion mobility spectrometry isomer separation efficiency is not easily predictable; it was however observed for all diastereomeric couples with a significant improvement of separation using alkali adducts compared to protonated molecules. With the use of drift gas with higher polarizability only an improvement of separation was obtained in a few cases. Finally, a good correlation of the experimental collision cross section (relative to three-dimensional structure of ions) and the theoretical collision cross section has been shown.

Characterization of Electrospray Ionization (ESI) Parameters on In-ESI Hydrogen/Deuterium Exchange of Carbohydrate-Metal Ion Adducts

The conformations of glycans are crucial for their biological functions. In-electrospray ionization (ESI) hydrogen/deuterium exchange-mass spectrometry (HDX-MS) is a promising technique for studying carbohydrate conformations since rapidly exchanging functional groups, e.g., hydroxyls, can be labeled on the timeframe of ESI. However, regular application of in-ESI HDX to characterize carbohydrates requires further analysis of the in-ESI HDX methodology. For instance, in this method, HDX occurs concurrently to the analyte transitioning from solution to gas-phase ions. Therefore, there is a possibility of sampling both gas-phase and solution-phase conformations of the analyte. Herein, we differentiate in-ESI HDX of metal-adducted carbohydrates from gas-phase HDX and illustrate that this method analyzes solvated species. We also systematically examine the effects of ESI parameters, including spray solvent composition, auxiliary gas flow rate, sheath gas flow rate, sample infusion rate, sample concentration, and spray voltage, and discuss their effects on in-ESI HDX. Further, we model the structural changes of a trisaccharide, melezitose, and its intramolecular and intermolecular hydrogen bonding in solvents with different compositions of methanol and water. These molecular dynamic simulations support our experimental results and illustrate how an individual ESI parameter can alter the conformations we sample by in-ESI HDX. In total, this work illustrates how the fundamental processes of ESI alter the magnitude of HDX for carbohydrates and suggest parameters that should be considered and/or optimized prior to performing experiments with this in-ESI HDX technique. Graphical Abstract ᅟ.

Rapid Separation and Quantitation of Cocaine and its Metabolites in Human Serum by Differential Mobility Spectrometry-tandem Mass Spectrometry (DMS-MS-MS)

Cocaine continues to be one of the most widespread abused illicit drugs in the USA. Rapid methods are needed for the identification and quantitation of cocaine and its metabolites, benzoylecgonine (BE), ecgonine methyl ester (EME) and cocaethylene (CE), in biological specimens by clinical and forensic toxicology laboratories. Presented is a differential ion mobility spectrometry-tandem mass spectrometry (DMS-MS-MS) method for the analysis of cocaine and its major metabolites in human serum that requires minimal sample preparation and no column chromatography. A Shimadzu Nexera X2 ultra-high performance liquid chromatography system was used to infuse the samples into the DMS cell at a rate of 30 μL/min. Separation of cocaine and its metabolites were performed in a SelexION DMS component from Sciex coupled to a QTRAP 6500 with an IonDrive Turbo V source for TurbolonSpray® using acetonitrile as a chemical modifier. Analysis consisted of ramping the CoV from -35 V to -6 V while monitoring the multiple reaction monitoring (MRM) transitions of each analyte. The assay was evaluated for linearity, bias, precision, carryover, interferences and stability. Calibration curves ranged from 10 to 1,000 ng/mL with linear regression correlation coefficients (r2) of 0.9912 or greater for each analyte. The limit of quantitation was set at 10 ng/mL. Intra-day precision (%CV) ranged from 0% to 15% for cocaine, 1% to 19% for BE, 1% to 17% for EME and 0% to 18% for CE. Inter-day precision ranged from 9% to 14% for cocaine, 2% to 17% for BE, 5% to 11% for EME and 5% to 15% for CE. No carryover or interferences were detected. Bland-Altman analysis of previously analyzed specimens by UPLC-MS-MS showed variability of 30% or less. The method demonstrates the applicability of DMS-MS-MS for high throughout analysis of drugs and their metabolites in clinical and forensic toxicology laboratories.

Untargeted Molecular Discovery in Primary Metabolism: Collision Cross Section as a Molecular Descriptor in Ion Mobility-Mass Spectrometry

In this work, we established a collision cross section (CCS) library of primary metabolites based on analytical standards in the Mass Spectrometry Metabolite Library of Standards (MSMLS) using a commercially available ion mobility-mass spectrometer (IM-MS). From the 554 unique compounds in the MSMLS plate library, we obtained a total of 1246 CCS measurements over a wide range of biochemical classes and adduct types. Resulting data analysis demonstrated that the curated CCS library provides broad molecular coverage of metabolic pathways and highlights intrinsic mass-mobility relationships for specific metabolite superclasses. The separation and characterization of isomeric metabolites were assessed, and all molecular species contained within the plate library, including isomers, were critically evaluated to determine the analytical separation efficiency in both the mass ( m/ z) and mobility (CCS/ΔCCS) dimension required for untargeted metabolomic analyses. To further demonstrate the analytical utility of CCS as an additional molecular descriptor, a well-characterized biological sample of human plasma serum (NIST SRM 1950) was examined by LC-IM-MS and used to provide a detailed isomeric analysis of carbohydrate constituents by ion mobility.

DMS as an orthogonal separation to LC/ESI/MS/MS for quantifying isomeric cerebrosides in plasma and cerebrospinal fluid

Cerebrosides, including glucosylceramides (GlcCers) and galactosylceramides (GalCers), are important membrane components of animal cells with deficiencies resulting in devastating lysosomal storage disorders. Their quantification is essential for disease diagnosis and a better understanding of disease mechanisms. The simultaneous quantification of GlcCer and GalCer isomers is, however, particularly challenging due to their virtually identical structures. To address this challenge, we developed a new LC/MS-based method using differential ion mobility spectrometry (DMS) capable of rapidly and reproducibly separating and quantifying isomeric cerebrosides in a single run. We show that this LC/ESI/DMS/MS/MS method exhibits robust quantitative performance within an analyte concentration range of 2.8-355 nM. We further report the simultaneous quantification of nine GlcCers (16:0, 18:0, 20:0, 22:0, 23:0, 24:1, 24:0, 25:0, and 26:0) and five GalCers (16:0, 22:0, 23:0, 24:1, and 24:0) molecular species in human plasma, as well as six GalCers (18:0, 22:0, 23:0, 24:1, 24:0 and 25:0) and two GlcCers (24:1 and 24:0) in human cerebrospinal fluid. Our method expands the potential of DMS technology in the field of glycosphingolipid analysis for both biomarker discovery and drug screening by enabling the unambiguous assignment and quantification of cerebroside lipid species in biological samples.

Applications of Ion Mobility-Mass Spectrometry in Carbohydrate Chemistry and Glycobiology

Carbohydrate analyses are often challenging due to the structural complexity of these molecules, as well as the lack of suitable analytical tools for distinguishing the vast number of possible isomers. The coupled technique, ion mobility-mass spectrometry (IM-MS), has been in use for two decades for the analysis of complex biomolecules, and in recent years it has emerged as a powerful technique for the analysis of carbohydrates. For carbohydrates, most studies have focused on the separation and characterization of isomers in biological samples. IM-MS is capable of separating isomeric ions by drift time, and further characterizing them by mass analysis. Applications of IM-MS in carbohydrate analysis are extremely useful and important for understanding many biological mechanisms and for the determination of disease states, although efforts are still needed for higher sensitivity and resolution.

Analysis of alkaloids from Peganum harmala L. sequential extracts by liquid chromatography coupled to ion mobility spectrometry

An orthogonal two dimensional analysis method based on high performance liquid chromatography (HPLC) separation and electrospray ionization-ion mobility spectrometry (ESI-IMS) detection was developed for the analysis of alkaloid compounds from Peganum harmala L. seeds. Reverse phase (RP) and hydrophilic interaction chromatography (HILIC) were compared for the most optimal performance using three different chromatographic columns. The experimental results suggest that HILIC mode is a better option for combining with the ESI-IMS system for higher sensitivity and ease in hyphenating. Under optimized conditions, alkaloids from different extraction phases were determined by means of the established HPLC-IMS method. More compounds from Peganum harmala L. seed extracts were differentiated on the HPLC-ESI-IMS system by their retention time and drift time than by HPLC or ESI-IMS alone, and thirteen alkaloids were tentatively identified based on m/z and fragment ions using ultra-high-performance liquid chromatography tandem mass-spectrometry (UPLC-MS/MS). Hence, our results indicate that this method can be considered to be advantageous over traditional absorbance detection methods for resolving complex mixtures because of complementary separation steps, elevated peak capacity, and higher sensitivity.

Collision Cross Sections and Ion Mobility Separation of Fragment Ions from Complex N-Glycans

Ion mobility mass spectrometry (IM-MS) holds great potential for structural glycobiology, in particular in its ability to resolve glycan isomers. Generally, IM-MS has largely been applied to intact glycoconjugate ions with reports focusing on the separation of different adduct types. Here, we explore IM separation and report the collision cross section (CCS) of complex type N-glycans and their fragments in negative ion mode following collision-induced dissociation (CID). CCSs of isomeric fragment ions were found, in some cases, to reveal structural details that were not present in CID spectra themselves. Many fragment ions were confirmed as possessing multiple structure, details of which could be obtained by comparing their drift time profiles to different glycans. By using fragmentation both before and after mobility separation, information was gathered on the fragmentation pathways producing some of the ions. These results help demonstrate the utility of IM and will contribute to the growing use of IM-MS for glycomics. Graphical Abstract ᅟ.

Isomer Information from Ion Mobility Separation of High-Mannose Glycan Fragments

Extracted arrival time distributions of negative ion CID-derived fragments produced prior to traveling-wave ion mobility separation were evaluated for their ability to provide structural information on N-linked glycans. Fragmentation of high-mannose glycans released from several glycoproteins, including those from viral sources, provided over 50 fragments, many of which gave unique collisional cross-sections and provided additional information used to assign structural isomers. For example, cross-ring fragments arising from cleavage of the reducing terminal GlcNAc residue on Man₈GlcNAc₂ isomers have unique collision cross-sections enabling isomers to be differentiated in mixtures. Specific fragment collision cross-sections enabled identification of glycans, the antennae of which terminated in the antigenic α-galactose residue, and ions defining the composition of the 6-antenna of several of the glycans were also found to have different cross-sections from isomeric ions produced in the same spectra. Potential mechanisms for the formation of the various ions are discussed and the estimated collisional cross-sections are tabulated. Graphical Abstract ᅟ.

Adding a new separation dimension to MS and LC-MS: What is the utility of ion mobility spectrometry?

Ion mobility spectrometry is an analytical technique known for more than 100 years, which entails separating ions in the gas phase based on their size, shape, and charge. While ion mobility spectrometry alone can be useful for some applications (mostly security analysis for detecting certain classes of narcotics and explosives), it becomes even more powerful in combination with mass spectrometry and high-performance liquid chromatography. Indeed, the limited resolving power of ion mobility spectrometry alone can be tackled when combining this analytical strategy with mass spectrometry or liquid chromatography with mass spectrometry. Over the last few years, the hyphenation of ion mobility spectrometry to mass spectrometry or liquid chromatography with mass spectrometry has attracted more and more interest, with significant progresses in both technical advances and pioneering applications. This review describes the theoretical background, available technologies, and future capabilities of these techniques. It also highlights a wide range of applications, from small molecules (natural products, metabolites, glycans, lipids) to large biomolecules (proteins, protein complexes, biopharmaceuticals, oligonucleotides).

Anomeric memory of the glycosidic bond upon fragmentation and its consequences for carbohydrate sequencing

Deciphering the carbohydrate alphabet is problematic due to its unique complexity among biomolecules. Strikingly, routine sequencing technologies-which are available for proteins and DNA and have revolutionised biology-do not exist for carbohydrates. This lack of structural tools is identified as a crucial bottleneck, limiting the full development of glycosciences and their considerable potential impact for the society. In this context, establishing generic carbohydrate sequencing methods is both a major scientific challenge and a strategic priority. Here we show that a hybrid analytical approach integrating molecular spectroscopy with mass spectrometry provides an adequate metric to resolve carbohydrate isomerisms, i.e the monosaccharide content, anomeric configuration, regiochemistry and stereochemistry of the glycosidic linkage. On the basis of the spectroscopic discrimination of MS fragments, we report the unexpected demonstration of the anomeric memory of the glycosidic bond upon fragmentation. This remarkable property is applied to de novo sequencing of underivatized oligosaccharides.Establishing generic carbohydrate sequencing methods is both a major scientific challenge and a strategic priority. Here the authors show a hybrid analytical approach integrating molecular spectroscopy and mass spectrometry to resolve carbohydrate isomerism, anomeric configuration, regiochemistry and stereochemistry.

Laser-ablation electrospray ionization mass spectrometry with ion mobility separation reveals metabolites in the symbiotic interactions of soybean roots and rhizobia

Technologies enabling in situ metabolic profiling of living plant systems are invaluable for understanding physiological processes and could be used for rapid phenotypic screening (e.g., to produce plants with superior biological nitrogen-fixing ability). The symbiotic interaction between legumes and nitrogen-fixing soil bacteria results in a specialized plant organ (i.e., root nodule) where the exchange of nutrients between host and endosymbiont occurs. Laser-ablation electrospray ionization mass spectrometry (LAESI-MS) is a method that can be performed under ambient conditions requiring minimal sample preparation. Here, we employed LAESI-MS to explore the well characterized symbiosis between soybean (Glycine max L. Merr.) and its compatible symbiont, Bradyrhizobium japonicum. The utilization of ion mobility separation (IMS) improved the molecular coverage, selectivity, and identification of the detected biomolecules. Specifically, incorporation of IMS resulted in an increase of 153 differentially abundant spectral features in the nodule samples. The data presented demonstrate the advantages of using LAESI-IMS-MS for the rapid analysis of intact root nodules, uninfected root segments, and free-living rhizobia. Untargeted pathway analysis revealed several metabolic processes within the nodule (e.g., zeatin, riboflavin, and purine synthesis). Compounds specific to the uninfected root and bacteria were also detected. Lastly, we performed depth profiling of intact nodules to reveal the location of metabolites to the cortex and inside the infected region, and lateral profiling of sectioned nodules confirmed these molecular distributions. Our results established the feasibility of LAESI-IMS-MS for the analysis and spatial mapping of plant tissues, with its specific demonstration to improve our understanding of the soybean-rhizobial symbiosis.

Vibrational Signatures of Isomeric Lithiated N-acetyl-D-hexosamines by Gas-Phase Infrared Multiple-Photon Dissociation (IRMPD) Spectroscopy

Three lithiated N-acetyl-D-hexosamine (HexNAc) isomers, N-acetyl-D-glucosamine (GlcNAc), N-acetyl-D-galactosamine (GalNAc), and N-acetyl-D-mannosamine (ManNAc) are investigated as model monosaccharide derivatives by gas-phase infrared multiple-photon dissociation (IRMPD) spectroscopy. The hydrogen stretching region, which is attributed to OH and NH stretching modes, reveals some distinguishing spectral features of the lithium-adducted complexes that are useful in terms of differentiating these isomers. In order to understand the effect of lithium coordination on saccharide structure, and therefore anomericity, chair configuration, and hydrogen bonding networks, the conformational preferences of lithiated GlcNAc, GalNAc, and ManNAc are studied by comparing the experimental measurements with density functional theory (DFT) calculations. The experimental results of lithiated GlcNAc and GalNAc show a good match to the theoretical spectra of low-energy structures adopting a ⁴ C ₁ chair conformation, consistent with this motif being the dominant conformation in condensed-phase monosaccharides. The epimerization effect upon going to lithiated ManNAc is significant, as in this case the ¹ C ₄ chair conformers give a more compelling match with the experimental results, consistent with their lower calculated energies. A contrasting computational study of these monosaccharides in their neutral form suggests that the lithium cation coordination with Lewis base oxygens can play a key role in favoring particular structural motifs (e.g., a ⁴ C ₁ versus ¹ C ₄ ) and disrupting hydrogen bond networks, thus exhibiting specific IR spectral features between these closely related lithium-chelated complexes. Graphical Abstract ᅟ.

Identification of Lewis and Blood Group Carbohydrate Epitopes by Ion Mobility-Tandem-Mass Spectrometry Fingerprinting

Glycans have several elements that contribute to their structural complexity, involving a range of monosaccharide building blocks, configuration of linkages between residues and various degrees of branching on a given structure. Their analysis remains challenging and resolving minor isomeric variants can be difficult, in particular terminal fucosylated Lewis and blood group antigens present on N- and O-glycans. Accurately characterizing these isomeric structures by current techniques is not straightforward and typically requires a combination of methods and/or sample derivatization. Yet the ability to monitor the occurrence of these epitopes is important as structural changes are associated with several human diseases. The use of ion mobility-mass spectrometry (IM-MS), which separates ions in the gas phase based on their size, charge and shape, offers a new potential tool for glycan analysis and recent reports have demonstrated its potential for glycomics. Here we show that Lewis and blood group isomers, which have identical fragmentation spectra, exhibit very distinctive IM drift times and collision cross sections (CCS). We show that IM-MS/MS analysis can rapidly and accurately differentiate epitopes from parotid gland N-glycans and milk oligosaccharides based on fucosylated fragment ions with characteristic CCSs.

Ion Mobility Collision Cross Section Compendium

In this review, we focus on an important aspect of ion mobility (IM) research, namely the reporting of quantitative ion mobility measurements in the form of the gas-phase collision cross section (CCS), which has provided a common basis for comparison across different instrument platforms and offers a unique form of structural information, namely size and shape preferences of analytes in the absence of bulk solvent. This review surveys the over 24,000 CCS values reported from IM methods spanning the era between 1975 to 2015, which provides both a historical and analytical context for the contributions made thus far, as well as insight into the future directions that quantitative ion mobility measurements will have in the analytical sciences. The analysis was conducted in 2016, so CCS values reported in that year are purposely omitted. In another few years, a review of this scope will be intractable, as the number of CCS values which will be reported in the next three to five years is expected to exceed the total amount currently published in the literature.

Squeezing of Ion Populations and Peaks in Traveling Wave Ion Mobility Separations and Structures for Lossless Ion Manipulations Using Compression Ratio Ion Mobility Programming

In this work we report an approach for spatial and temporal gas-phase ion population manipulation, wherein we collapse ion distributions in ion mobility (IM) separations into tighter packets providing higher sensitivity measurements in conjunction with mass spectrometry (MS). We do this for ions moving from a conventional traveling wave (TW)-driven region to a region where the TW is intermittently halted or "stuttered". This approach causes the ion packets spanning a number of TW-created traveling traps (TT) to be redistributed into fewer TT, resulting in spatial compression. The degree of spatial compression is controllable and determined by the ratio of stationary time of the TW in the second region to its moving time. This compression ratio ion mobility programming (CRIMP) approach has been implemented using "structures for lossless ion manipulations" (SLIM) in conjunction with MS. CRIMP with the SLIM-MS platform is shown to provide increased peak intensities, reduced peak widths, and improved signal-to-noise (S/N) ratios with MS detection. CRIMP also provides a foundation for extremely long path length and multipass IM separations in SLIM providing greatly enhanced IM resolution by reducing the detrimental effects of diffusional peak broadening and increasing peak widths.

Ion-Mobility-Derived Collision Cross Section as an Additional Identification Point for Multiresidue Screening of Pesticides in Fish Feed

Ion mobility spectrometry allows for the measurement of the collision cross section (CCS), which provides information about the shape of an ionic molecule in the gas phase. Although the hyphenation of traveling-wave ion mobility spectrometry (TWIMS) with high-resolution quadrupole time-of-flight mass spectrometry (QTOFMS) has been mainly used for structural elucidation purposes, its potential for fast screening of small molecules in complex samples has not yet been thoroughly evaluated. The current work explores the capabilities of ultrahigh-performance liquid chromatography (UHPLC) coupled to a new design TWIMS-QTOFMS for the screening and identification of a large set of pesticides in complex salmon feed matrices. A database containing TWIMS-derived CCS values for more than 200 pesticides is hereby presented. CCS measurements showed high intra- and interday repeatability (RSD < 1%), and they were not affected by the complexity of the investigated matrices (ΔCCS ≤ 1.8%). The use of TWIMS in combination with QTOFMS was demonstrated to provide an extra-dimension, which resulted in increased peak capacity and selectivity in real samples. Thus, many false-positive detections could be straightforwardly discarded just by applying a maximum ΔCCS tolerance of ±2%. CCS was proposed as a valuable additional identification point in the pesticides screening workflow. Several commercial fish feed samples were finally analyzed to demonstrate the applicability of the proposed approach. Ethoxyquin and pirimiphos-methyl were identified in most of the analyzed samples, whereas tebuconazole and piperonil butoxide were identified for the first time in fish feed samples.

Travelling-wave ion mobility mass spectrometry and negative ion fragmentation of hybrid and complex N-glycans

Nitrogen collisional cross sections (CCSs) of hybrid and complex glycans released from the glycoproteins IgG, gp120 (from human immunodeficiency virus), ovalbumin, α1-acid glycoprotein and thyroglobulin were measured with a travelling-wave ion mobility mass spectrometer using dextran as the calibrant. The utility of this instrument for isomer separation was also investigated. Some isomers, such as Man₃ GlcNAc₃ from chicken ovalbumin and Man₃ GlcNAc₃ Fuc₁ from thyroglobulin could be partially resolved and identified by their negative ion fragmentation spectra obtained by collision-induced decomposition (CID). Several other larger glycans, however, although existing as isomers, produced only asymmetric rather than separated arrival time distributions (ATDs). Nevertheless, in these cases, isomers could often be detected by plotting extracted fragment ATDs of diagnostic fragment ions from the negative ion CID spectra obtained in the transfer cell of the Waters Synapt mass spectrometer. Coincidence in the drift times of all fragment ions with an asymmetric ATD profile in this work, and in the related earlier paper on high-mannose glycans, usually suggested that separations were because of conformers or anomers, whereas symmetrical ATDs of fragments showing differences in drift times indicated isomer separation. Although some significant differences in CCSs were found for the smaller isomeric glycans, the differences found for the larger compounds were usually too small to be analytically useful. Possible correlations between CCSs and structural types were also investigated, and it was found that complex glycans tended to have slightly smaller CCSs than high-mannose glycans of comparable molecular weight. In addition, biantennary glycans containing a core fucose and/or a bisecting GlcNAc residue fell on different mobility-m/z trend lines to those glycans not so substituted with both of these substituents contributing to larger CCSs. Copyright © 2016 John Wiley & Sons, Ltd.

Greatly Increasing Trapped Ion Populations for Mobility Separations Using Traveling Waves in Structures for Lossless Ion Manipulations

The initial use of traveling waves (TW) for ion mobility (IM) separations using structures for lossless ion manipulations (SLIM) employed an ion funnel trap (IFT) to accumulate ions from a continuous electrospray ionization source and was limited to injected ion populations of ∼106 charges due to the onset of space charge effects in the trapping region. Additional limitations arise due to the loss of resolution for the injection of ions over longer periods, such as in extended pulses. In this work a new SLIM "flat funnel" (FF) module has been developed and demonstrated to enable the accumulation of much larger ion populations and their injection for IM separations. Ion current measurements indicate a capacity of ∼3.2 × 108 charges for the extended trapping volume, over an order of magnitude greater than that of the IFT. The orthogonal ion injection into a funnel shaped separation region can greatly reduce space charge effects during the initial IM separation stage, and the gradually reduced width of the path allows the ion packet to be increasingly compressed in the lateral dimension as the separation progresses, allowing efficient transmission through conductance limits or compatibility with subsequent ion manipulations. This work examined the TW, rf, and dc confining field SLIM parameters involved in ion accumulation, injection, transmission, and IM separation in the FF module using both direct ion current and MS measurements. Wide m/z range ion transmission is demonstrated, along with significant increases in the signal-to-noise ratios (S/N) due to the larger ion populations injected. Additionally, we observed a reduction in the chemical background, which was attributed to more efficient desolvation of solvent related clusters over the extended ion accumulation periods. The TW SLIM FF IM module is anticipated to be especially effective as a front end for long path SLIM IM separation modules.

Ultra-High Resolution Ion Mobility Separations Utilizing Traveling Waves in a 13 m Serpentine Path Length Structures for Lossless Ion Manipulations Module

We report the development and initial evaluation of a 13 m path length Structures for Lossless Manipulations (SLIM) module for achieving high resolution separations using traveling waves (TW) with ion mobility (IM) spectrometry. The TW SLIM module was fabricated using two mirror-image printed circuit boards with appropriately configured RF, DC, and TW electrodes and positioned with a 2.75 mm intersurface gap. Ions were effectively confined in field-generated conduits between the surfaces by RF-generated pseudopotential fields and moved losslessly through a serpentine path including 44 "U" turns using TWs. The ion mobility resolution was characterized at different pressures, gaps between the SLIM surfaces, and TW and RF parameters. After initial optimization, the SLIM IM-MS module provided about 5-fold higher resolution separations than present commercially available drift tube or traveling wave IM-MS platforms. Peak capacity and peak generation rates achieved were 246 and 370 s(-1), respectively, at a TW speed of 148 m/s. The high resolution achieved in the TW SLIM IM-MS enabled, e.g., isomeric sugars (lacto-N-fucopentaose I and lacto-N-fucopentaose II) to be baseline resolved, and peptides from an albumin tryptic digest were much better resolved than with existing commercial IM-MS platforms. The present work also provides a foundation for the development of much higher resolution SLIM devices based upon both considerably longer path lengths and multipass designs.

Identification of structurally closely related monosaccharide and disaccharide isomers by PMP labeling in conjunction with IM-MS/MS

It remains particularly difficult for gaining unambiguous information on anomer, linkage, and position isomers of oligosaccharides using conventional mass spectrometry (MS) methods. In our laboratory, an ion mobility (IM) shift strategy was employed to improve confidence in the identification of structurally closely related disaccharide and monosaccharide isomers using IMMS. Higher separation between structural isomers was achieved using 1-phenyl-3-methyl-5-pyrazolone (PMP) derivatization in comparison with phenylhydrazine (PHN) derivatization. Furthermore, the combination of pre-IM fragmentation of PMP derivatives provided sufficient resolution to separate the isomers not resolved in the IMMS. To chart the structural variation observed in IMMS, the collision cross sections (CCSs) for the corresponding ions were measured. We analyzed nine disaccharide and three monosaccharide isomers that differ in composition, linkages, or configuration. Our data show that coexisting carbohydrate isomers can be identified by the PMP labeling technique in conjunction with ion-mobility separation and tandem mass spectrometry. The practical application of this rapid and effective method that requires only small amounts of sample is demonstrated by the successful analysis of water-soluble ginseng extract. This demonstrated the potential of this method to measure a variety of heterogeneous sample mixtures, which may have an important impact on the field of glycomics.

Multidimensional Separation of Natural Products Using Liquid Chromatography Coupled to Hadamard Transform Ion Mobility Mass Spectrometry

A high performance liquid chromatograph (HPLC)was interfaced to an atmospheric drift tube ion mobility time of flight mass spectrometry. The power of multidimensional separation was demonstrated using chili pepper extracts. The ambient pressure drift tube ion mobility provided high resolving powers up to 166 for the HPLC eluent. With implementation of Hadamard transform (HT), the duty cycle for the ion mobility drift tube was increased from less than 1% to 50%, and the ion transmission efficiency was improved by over 200 times compared with pulsed mode, improving signal to noise ratio 10 times. HT ion mobility and TOF mass spectrometry provide an additional dimension of separation for complex samples without increasing the analysis time compared with conventional HPLC. Graphical Abstract ᅟ.

Separation and Identification of Isomeric Glycans by Selected Accumulation-Trapped Ion Mobility Spectrometry-Electron Activated Dissociation Tandem Mass Spectrometry

One of the major challenges in structural characterization of oligosaccharides is the presence of many structural isomers in most naturally occurring glycan mixtures. Although ion mobility spectrometry (IMS) has shown great promise in glycan isomer separation, conventional IMS separation occurs on the millisecond time scale, largely restricting its implementation to fast time-of-flight (TOF) analyzers which often lack the capability to perform electron activated dissociation (ExD) tandem MS analysis and the resolving power needed to resolve isobaric fragments. The recent development of trapped ion mobility spectrometry (TIMS) provides a promising new tool that offers high mobility resolution and compatibility with high-performance Fourier transform ion cyclotron resonance (FTICR) mass spectrometers when operated under the selected accumulation-TIMS (SA-TIMS) mode. Here, we present our initial results on the application of SA-TIMS-ExD-FTICR MS to the separation and identification of glycan linkage isomers.

Ion-neutral collisional cross sections of carbohydrate isomers as divalent cation adducts and their electron transfer products

As the gravity of glycoscience continues to amass, a commensurate demand for rapid, sensitive, and structurally comprehensive glycoanalytical tools has arisen. Among the most elusive but desirable analytical capabilities is the ability to expeditiously and unambiguously detect, distinguish, and resolve carbohydrates that differ only in their constitutional isomerism and/or stereoisomerism. While ion mobility spectrometry (IMS) has proven a highly promising tool for such analyses, the facility of this method to discriminate larger oligosaccharides is still somewhat limited. In an effort to expand the capabilities of IMS to discriminate among carbohydrate isomers, the present investigation was focused on IMS studies of four trisaccharide isomers, four pentasaccharide isomers, and two hexasaccharide isomers, each as group II metal ion adducts and their corresponding gas-phase electron transfer (ET) products. These studies were also evaluated in the context of previously investigated group I metal ion adducts of the same saccharides. The orientationally averaged ion-neutral collisional cross sections (CCSs) of the various carbohydrate/metal ion adducts were found to be dependent on the structures of specific carbohydrate isomers, sensitive to the electronic characteristics of the bound cation, and responsive to the attachment of an additional electron (in the case of divalent metal ion adducts). Overall, these results underscore the utility of metal ions for probing carbohydrate structure in concert with IMS, and the capacity of gas-phase ion chemistry to expand the menu of such probes.

Ion manipulations in structures for lossless ion manipulations (SLIM): computational evaluation of a 90° turn and a switch

The process of redirecting ions through 90° turns and 'tee' switches utilizing Structures for Lossless Ion Manipulations (SLIM) was evaluated at 4 Torr pressure using SIMION simulations and theoretical methods. The nature of pseudo-potential in SLIM-tee structures has also been explored. Simulations show that 100% transmission efficiency in SLIM devices can be achieved with guard electrode voltages lower than ∼10 V. The ion plume width in these conditions is ∼1.6 mm while at lower guard voltages lead to greater plume widths. Theoretical calculations show marginal loss of ion mobility resolving power (<5%) during ion turn due to the finite plume widths (i.e. race track effect). More robust SLIM designs that reduce the race track effect while maximizing ion transmission are also reported. In addition to static turns, the dynamic switching of ions into orthogonal channels was also evaluated both using SIMION ion trajectory simulations and experimentally. Simulations and theoretical calculations were in close agreement with experimental results and were used to develop more refined SLIM designs.

Fragmentation and ion mobility properties of negative ions from N-linked carbohydrates: Part 7. Reduced glycans

RATIONALE

Negative ion collision-induced dissociation (CID) spectra of released N-glycans provide very informative structural information relating to branching patterns and location of residues such as fucose. For some structural studies, particularly those involving chromatography, glycans are often reduced to avoid production of multiple peaks from α- and β-anomers. We examined the effect of reduction on the production of diagnostic fragment ions and on the ion mobility properties of N-glycans.

METHODS

Released N-glycans from the glycoproteins bovine fetuin, ribonuclease B, chicken ovalbumin, and porcine thyroglobulin were reduced with sodium cyanoborohydride and both negative ion CID spectra and ion mobility properties of their phosphate adducts were examined with a Waters Synapt G2Si travelling-wave ion mobility mass spectrometer with electrospray sample introduction. Estimated collisional cross sections were measured with dextran as the calibrant,

RESULTS

Fragment ions were similar to those from the unreduced glycans with the exception that the prominent (2,4) A cleavage ion from the reducing terminus was replaced by a prominent [M-H3 PO4](-) ion. Other ions arising from the chitobiose core were of lower relative abundance than those from the unreduced glycans. Estimated collisional cross sections were similar to those of the unreduced compounds but with symmetrical arrival time distribution (ATD) profiles, unlike those of the unreduced glycans whose peaks often contained prominent asymmetry. This observation showed that this asymmetry was due to anomer separation.

CONCLUSIONS

Reduction of the reducing terminal GlcNAc residue resulted in fewer diagnostic ions from the chitobiose core but fragmentation of the remainder of the molecules generally paralleled that of the unreduced glycans. Thus, most structural information, with the exception of the linkage position of fucose on the core GlcNAc, was available. ATD peaks were symmetrical with the result that cross sections were more appropriate for data-base searching than those from the non-reduced compounds where asymmetry produced lower precision in the measurement.

Travelling-wave ion mobility and negative ion fragmentation of high-mannose N-glycans

The isomeric structure of high-mannose N-glycans can significantly impact biological recognition events. Here, the utility of travelling-wave ion mobility mass spectrometry for isomer separation of high-mannose N-glycans is investigated. Negative ion fragmentation using collision-induced dissociation gave more informative spectra than positive ion spectra with mass-different fragment ions characterizing many of the isomers. Isomer separation by ion mobility in both ionization modes was generally limited, with the arrival time distributions (ATD) often showing little sign of isomers. However, isomers could be partially resolved by plotting extracted fragment ATDs of the diagnostic fragment ions from the negative ion spectra, and the fragmentation spectra of the isomers could be extracted by using ions from limited areas of the ATD peak. In some cases, asymmetric ATDs were observed, but no isomers could be detected by fragmentation. In these cases, it was assumed that conformers or anomers were being separated. Collision cross sections of the isomers in positive and negative fragmentation mode were estimated from travelling-wave ion mobility mass spectrometry data using dextran glycans as calibrant. More complete collision cross section data were achieved in negative ion mode by utilizing the diagnostic fragment ions. Examples of isomer separations are shown for N-glycans released from the well-characterized glycoproteins chicken ovalbumin, porcine thyroglobulin and gp120 from the human immunodeficiency virus. In addition to the cross-sectional data, details of the negative ion collision-induced dissociation spectra of all resolved isomers are discussed.

Correcting the fundamental ion mobility equation for field effects

Cross sections measured by ion mobility spectrometry are corrected for collision frequency and cooling/heating-controlled momentum transfer.

Self-assembly of triangular metallomacrocycles using unsymmetrical bisterpyridine ligands: isomer differentiation via TWIM mass spectrometry

Three unsymmetrical, 60°-bended bisterpyridine ligands with varying phenylene spacer lengths have been synthesized via the Suzuki-Miyaura coupling reactions. Their self-assembly processes were found to be strongly dependent on the ligand geometry. Upon complexation with Zn(II) ions, only 2,4''-di(4'-terpyridinyl)-1,1':4',1''-terphenyl underwent self-selection to give a trinuclear metallomacrocycle with perfect heteroleptic connectivity and the other two afforded a mixture of constitutional isomers. The metallosupramolecular assemblies were characterized by NMR spectroscopy, electrospray mass spectrometry (ESI MS), and single-crystal X-ray diffraction. In particular, the identification of isomeric architecture was accomplished using tandem mass spectrometry (MS(2)) coupled with traveling wave ion mobility mass spectrometry (TWIM MS).

Application of ion mobility tandem mass spectrometry to compositional and structural analysis of glycopeptides extracted from the urine of a patient diagnosed with Schindler disease

RATIONALE

Schindler disease is caused by the deficient activity of α-N-acetylgalactosaminidase, which leads to an abnormal accumulation of O-glycopeptides in tissues and body fluids. In this work the Schindler condition is for the first time approached by ion mobility (IMS) tandem mass spectrometry (MS/MS), for determining urine glycopeptide fingerprints and discriminate isomeric structures.

METHODS

IMS-MS experiments were conducted on a Synapt G2s mass spectrometer operating in negative ion mode. A glycopeptide mixture extracted from the urine of a patient suffering from Schindler disease was dissolved in methanol and infused into the mass spectrometer by electrospray ionization using a syringe-pump system. MS/MS was performed by collision-induced dissociation (CID) at low energies, after mobility separation in the transfer cell. Data acquisition and processing were performed using MassLynx and Waters Driftscope software.

RESULTS

IMS-MS data indicated that the attachment of one or two amino acids to the carbohydrate backbone has a minimal influence on the molecule conformation, which limits the discrimination of the free oligosaccharides from the glycosylated amino acids and dipeptides. The structural analysis by CID MS/MS in combination with IMS-MS of species exhibiting the same m/z but different configurations demonstrated for the first time the presence of positional isomers for some of the Schindler disease biomarker candidates.

CONCLUSIONS

The IMS-MS and CID MS/MS platform was for the first time optimized and applied to Schindler disease glycourinome. By this approach the separation and characterization of Neu5Ac positional isomers was possible. IMS CID MS/MS showed the ability to determine the type of the glycopeptide isomers from a series of possible candidates.

Distinguishing isobaric phosphated and sulfated carbohydrates by coupling of mass spectrometry with gas phase vibrational spectroscopy

An original application of the coupling of mass spectrometry with vibrational spectroscopy, used for the first time to discriminate isobaric bioactive saccharides with sulfate and phosphate functional modifications, is presented. Whereas their nominal masses and fragmentation patterns are undifferentiated by sole mass spectrometry, their distinctive OH stretching modes at 3595 cm(-1) and 3666 cm(-1), respectively, provide a reliable spectroscopic diagnostic for distinguishing their sulfate or phosphate functionalization. A detailed analysis of the 6-sulfated and 6-phosphated d-glucosamine conformations is presented, together with theoretical scaled harmonic spectra and anharmonic spectra (VPT2 and DFT-based molecular dynamics simulations). Strong anharmonic effects are observed in the case of the phosphated species, resulting in a dramatic enhancement of its phosphate diagnostic mode.

Ion Mobility Mass Spectrometry for Ion Recovery and Clean-Up of MS and MS/MS Spectra Obtained from Low Abundance Viral Samples

Many samples of complex mixtures of N-glycans released from small amounts of material, such as glycoproteins from viruses, present problems for mass spectrometric analysis because of the presence of contaminating material that is difficult to remove by conventional methods without involving sample loss. This study describes the use of ion mobility for extraction of glycan profiles from such samples and for obtaining clean CID spectra when targeted m/z values capture additional ions from those of the target compound. N-glycans were released enzymatically from within SDS-PAGE gels, from the representative recombinant glycoprotein, gp120 of the human immunodeficiency virus, and examined by direct infusion electrospray in negative mode followed by ion mobility with a Waters Synapt G2 mass spectrometer (Waters MS-Technologies, Manchester, UK). Clean profiles of singly, doubly, and triply charged N-glycans were obtained from samples in cases where the raw electrospray spectra displayed only a few glycan ions as the result of low sample concentration or the presence of contamination. Ion mobility also enabled uncontaminated CID spectra to be obtained from glycans when their molecular ions displayed coincidence with ions from fragments or multiply charged ions with similar m/z values. This technique proved to be invaluable for removing extraneous ions from many CID spectra. The presence of such ions often produces spectra that are difficult to interpret. Most CID spectra, even those from abundant glycan constituents, benefited from such clean-up, showing that the extra dimension provided by ion mobility was invaluable for studies of this type.