Theoretical Collision Cross Sections Research Articles

Using ion mobility spectrometry to understand signal dilution during tandem mass spectrometry sequencing of digital polymers: Experimental evidence of intramolecular cyclization.

Optimizing the structure of digital polymers is an efficient strategy to ensure their tandem mass spectrometry (MS/MS) readability. In block-truncated poly(phosphodiester)s, homolysis of C-ON bonds in long chains permits the release of smaller blocks amenable to sequencing. Yet the dissociation behavior of diradical blocks was observed to strongly depend on their charge state. Polymers were ionized in negative mode electrospray and activated in-source so that blocks released as primary fragments can be investigated using ion mobility spectrometry (IMS) or sequenced in the post-IMS collision cell. Collision cross sections (CCS) were derived from arrival times using a calibration procedure developed for polyanions using the IMSCal software. A multistep protocol based on quantum methods and classical molecular dynamics was implemented for molecular modeling and calculation of theoretical CCS. Unlike their triply charged homologues, dissociation of diradical blocks at the 2- charge state produces additional fragments, with +1m/z shift for those holding the nitroxide α-termination and -1m/z for those containing the carbon-centered radical ω-end. These results suggest cyclization of these diradical species, followed by H• transfer on activated reopening of this cycle. This assumption was validated using IMS resolution of the cyclic/linear isomers and supported by molecular modeling. Combining IMS with molecular modeling provided new insights into how the charge state of digital blocks influences their dissociation. These results permit to define new guidelines to improve either ionization conditions or the structural design of these digital polymers for best MS/MS readability.

Precise analysis of thyroxine enantiomers in pharmaceutical formulation by mobility difference based on cyclodextrin

Identification and determination of chiral pharmaceutical residues is still a challenging analytical puzzle. In this work, a simple, rapid, and effective method for chiral D/L-tetraiodothyronine (T4) separation and quantitative was developed based on host–guest recognition using ion mobility spectrometry-mass spectrometry (IMS-MS). The D/L-T4 enantiomers were mobility separated by their diastereomeric complexes through mixing with cyclodextrin (CD) and metal ions. D/L-T4 was first separated by complexing with host molecule (α-, β-, γ-CD), observing weak peak-to-peak resolution (Rp-p) by the formed binary complex [CD + D/L-T4-H]+, and the Rp-p decreased with the CD size increasing. However, the separation effect of D/L-T4 was much improved with the addition of divalent metal ions (G2+) by the formed ternary complex [CD + D/L-T4 + G]2+. In comparison, α-CD related complexes can possess the best separation effect for D/L-T4 in most cases. Considering the high selectivity, non-toxic, and chemically stable of β-CD, [β-CD + D/L-T4 + Ca]2+ was selected for D/L-T4 analysis (RP-P = 0.764). Whereafter, chemical theoretical conformations for [β-CD + D/L-T4 + H]+ and [β-CD + D/L-T4 + Ca]2+ were optimized, discovering similar micro-interaction modes between [β-CD + D-T4 + H]+ and [β-CD + L-T4 + H]+; while with the addition of Ca2+, significantly different interaction modes were observed between [β-CD + D-T4 + Ca]2+ and [β-CD + L-T4 + Ca]2+. And theoretical collision cross section (CCS) trends for the complexes were consistent with that of the experimental results. Additionally, calibration curves were linear within 1.00 to 104 ng mL−1 with coefficient (R2 > 0.99), gaining the limit of detection (LODs) calculation of 0.11 ng mL−1, and the detection range between D-T4 and L-T4 of 45.6:1 to 1:59.8. Finally, the method was applied for D/L-T4 detection in Levothyroxine tablets, the detection content has good consistency on drug labeling. Because the proposed method exhibited good analytical performance in terms of speed, selectivity, sensitivity, and reproducibility of the measurements, that can be a promising strategy for effective D/L-T4 detection in pharmaceutical industries or other practical samples.

Separation of cinchona alkaloid stereoisomers and analogues by ion mobility and chemical theoretical calculation

In this study, a simple and accurate method for the separation and quantitative analysis of cinchona alkaloid stereoisomers and analogues was developed using trapped ion mobility spectrometry (TIMS)-mass spectrometry (MS) and theoretical calculations. Stereoisomer studied were N-(9-Deoxy-epi-cinchonin-9-yl) picolinamide (N-CNP)/N-(9-Deoxy-epi-cinchonidin-9-yl) picolinamide (N-CDP), N-Benzylcinchoninium (BCN)/N-Benzylcinchonidinium (BCN), and Cinchonine (CCN)/Cinchonidine (CCD). TIMS results revealed slight mobility differences in the protonated stereoisomers, [CCD + H]+/[CCN + H]+ complexes showed the most effective separation, with a resolution (RP–P) of 0.431. When the macrocyclic antibiotics rifamycin (Rif) and natamycin (Nat) were introduced as separation reagents, the binary diastereomer complexes [Nat/Rif + CCD/CCN + H]+, [Rif + N-CDP/N-CNP + H]+, and [Rif + BCD/BCN + H]+ were formed by non-covalent interaction with the stereoisomers. The TIMS analysis revealed that the stereoisomers were separated by the formed binary complexes: CCD/CCD was separated by [CCD/CCN + Nat + H]+ (RP–P = 1.771), N-CDP/N-CNP was separated by [N-CDP/N-CNP + Rif + H]+ (RP–P = 0.914), and the BCD/BCN was separated by [BCD/BCN + Rif]+ (RP–P = 0.713). The conformations of the binary complexes were optimized using chemical theoretical calculations which revealed the microscopic interaction between the stereoisomers was different, and that the theoretical collision cross section (CCS) was consistent with the observed experimental results. An additional quantitative analysis of the stereoisomers yielded a limit of detection (LOD) ≦ 0.408 μg L−1 and satisfactory linearities (R2 > 0.99). The proposed method is simple, highly selective, highly sensitive, and without the need for chromatographic separation, and is therefore a promising option for the separation and analysis of cinchoninestereoisomers or analogues, as well as in desirable figures-of-merit for forensics.

Linking Gas-Phase and Solution-Phase Protein Unfolding via Mobile Proton Simulations.

Native mass spectrometry coupled to ion mobility (IM-MS) combined with collisional activation (CA) of ions in the gas phase (in vacuo) is an important method for the study of protein unfolding. It has advantages over classical biophysical and structural techniques as it can be used to analyze small volumes of low-concentration heterogeneous mixtures while maintaining solution-like behavior and does not require labeling with fluorescent or other probes. It is unclear, however, whether the unfolding observed during collision activation experiments mirrors solution-phase unfolding. To bridge the gap between in vacuo and in-solution behavior, we use unbiased molecular dynamics (MD) to create in silico models of in vacuo unfolding of a well-studied protein, the N-terminal domain of ribosomal L9 (NTL9) protein. We utilize a mobile proton algorithm (MPA) to create 100 thermally unfolded and coulombically unfolded in silico models for observed charge states of NTL9. The unfolding behavior in silico replicates the behavior in-solution and is in line with the in vacuo observations; however, the theoretical collision cross section (CCS) of the in silico models was lower compared to that of the in vacuo data, which may reflect reduced sampling.

Applications of ion mobility-mass spectrometry in the chemical analysis in traditional Chinese medicines

离子淌度质谱(IM-MS)是一种将离子淌度分离与质谱分析相结合的新型分析技术。IM-MS的主要优势不仅是在质谱检测前提供了基于气相离子形状、大小、电荷数等因素的多一维分离,而且能够提供碰撞截面积、漂移时间等质谱信息进而辅助化合物鉴定。近年来,随着IM-MS技术的不断发展,该技术在中药化学成分分析中受到越来越多的关注。首先,IM-MS已成功应用于改善中药复杂成分尤其是同分异构体或等量异位素等成分的分离;其次,IM-MS可通过多重碎裂模式辅助高质量中药小分子质谱信息的获取;此外,IM-MS提供的高维质谱数据信息还可促进中药复杂体系多成分的整合分析。该文在对IM-MS分类和基本原理进行概述的基础上,从分离能力及分离策略、多重碎裂模式、多维质谱数据处理策略3个方面,重点综述了IM-MS在中药化学成分分析中的应用,以期为IM-MS在中药化学成分研究提供参考。

Simultaneous Differentiation of C═C Position Isomerism in Fatty Acids through Ion Mobility and Theoretical Calculations.

Fatty acids play a pivotal role in biological processes and have many isomers, particularly at the C═C position, that influence their biological function. Distinguishing between isomers is crucial to investigating their role in health and disease. However, separating the isomers poses a significant analytical challenge. In this study, we developed a simple and rapid strategy combining ion mobility spectrometry and theoretical chemical calculations to differentiate and quantify the C═C positional isomers in 2-/3-butenoic acid (BA), 2-/3-/4-pentenoic acid (PA), and 2-/3-/5-hexenoic acid (HA). C═C positional isomerism was mobility-differentiated by simple complexation with crown ethers (12C4, 15C5, and 18C6) and divalent metal ions (Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Zn2+, Sr2+, and Ba2+), that is, converting C═C positional isomers with small structural differences into complexes with large structural differences through the interaction with metal ions and crown ethers. Metallized isomers were formed but could not be differentiated due to their complex and overlapping extracted ion mobiliograms (EIMs). Binary crown ether-isomer complexes were not observed, indicating that C═C positional isomers could not be separated by simple mixing with crown ethers. However, significant EIM differences were obtained for the formed ternary complexes, allowing baseline separation for the isomers. Notably, all crown ethers and metal ions have a separation effect with the isomers, with a calculated separation resolution (Rp-p) of 0.07-2.44. Theoretical chemical calculations were performed to provide in-depth structural information for the complexes and explain the separation principle. Theoretical conformational space showed that the divalent metal ions act as a bridge connecting the crown ether and the isomer. Additionally, the ternary complex becomes more compact as the distance between C═C and -COOH increases. Theoretical results can reflect the features of mobility experiments, with relative errors between the experiment collision cross-section (CCS) and theoretical CCS of no more than ±8.06%. This method was also evaluated in terms of quantification, accuracy, and precision repeatability. Overall, this study establishes that the crown ether-metal ion pair can function as a robust unit for differentiating C═C positional isomerism.

Structural Characterization of Dendriplexes In Vacuo: A Joint Ion Mobility/Molecular Dynamics Investigation.

The combination between ion mobility mass spectrometry and molecular dynamics simulations is demonstrated for the first time to afford valuable information on structural changes undergone by dendriplexes containing ds-DNA and low-generation dendrimers when transferred from the solution to the gas phase. Dendriplex ions presenting 1:1 and 2:1 stoichiometries are identified using mass spectrometry experiments, and the collision cross sections (CCS) of the 1:1 ions are measured using drift time ion mobility experiments. Structural predictions using Molecular Dynamics (MD) simulations showed that gas-phase relevant structures, i.e., with a good match between the experimental and theoretical CCS, are generated when the global electrospray process is simulated, including the solvent molecule evaporation, rather than abruptly transferring the ions from the solution to the gas phase. The progressive migration of ammonium groups (either NH4+ from the buffer or protonated amines of the dendrimer) into the minor and major grooves of DNA all along the evaporation processes is shown to compact the DNA structure by electrostatic and hydrogen-bond interactions. The subsequent proton transfer from the ammonium (NH4+ or protonated amino groups) to the DNA phosphate groups allows creation of protonated phosphate/phosphate hydrogen bonds within the compact structures. MD simulations showed major structural differences between the dendriplexes in solution and in the gas phase, not only due to the loss of the solvent but also due to the proton transfers and the huge difference between the solution and gas-phase charge states.

Molecular networking and collision cross section prediction for structural isomer and unknown compound identification in plant metabolomics: a case study applied to Zhanthoxylum heitzii extracts

Mass spectrometry-based plant metabolomics allow large-scale analysis of a wide range of compounds and the discovery of potential new active metabolites with minimal sample preparation. Despite recent tools for molecular networking, many metabolites remain unknown. Our objective is to show the complementarity of collision cross section (CCS) measurements and calculations for metabolite annotation in a real case study. Thus, a systematic and high-throughput investigation of root, bark, branch, and leaf of the Gabonese plant Zhanthoxylum heitzii was performed through ultra-high performance liquid chromatography high-resolution tandem mass spectrometry (UHPLC-QTOF/MS). A feature-based molecular network (FBMN) was employed to study the distribution of metabolites in the organs of the plants and discover potential new components. In total, 143 metabolites belonging to the family of alkaloids, lignans, polyphenols, fatty acids, and amino acids were detected and a semi-quantitative analysis in the different organs was performed. A large proportion of medical plant phytochemicals is often characterized by isomerism and, in the absence of reference compounds, an additional dimension of gas phase separation can result in improvements to both quantitation and compound annotation. The inclusion of ion mobility in the ultra-high performance liquid chromatography mass spectrometry workflow (UHPLC-IMS-MS) has been used to collect experimental CCS values in nitrogen and helium (CCSN2 and CCSHe) of Zhanthoxylum heitzii features. Due to a lack of reference data, the investigation of predicted collision cross section has enabled comparison with the experimental values, helping in dereplication and isomer identification. Moreover, in combination with mass spectra interpretation, the comparison of experimental and theoretical CCS values allowed annotation of unknown features. The study represents a practical example of the potential of modern mass spectrometry strategies in the identification of medicinal plant phytochemical components.

Uncovering the behaviour of ions in the gas-phase to predict the ion mobility separation of isomeric steroid compounds

Bile acids are steroid compounds involved in biological mechanisms of neurodegenerative diseases making them potential biomarkers for diagnosis or treatment. These compounds exist as structural and conformational isomers, which hinder distinguishing them in physiological processes. We aimed to develop tandem mass spectrometry-ion mobility spectrometry (MS/MS-IMS) methodologies to explore and understand the behaviour of isomeric steroids in the gas-phase and rapidly separate them. Unlike previously published ion mobility data, various isomers were investigated in mixtures to better mimic complex (pre-) clinical samples. The experimental collision cross sections (CCS)s were compared to the theoretical CCS values for an in-depth analysis of isomeric ions' behaviour in the gas-phase. Based on density-functional theory, we identified the impact of adduct positioning on the 3D conformation of enantiomers, diastereomers and structural isomers. The curling of the large side chains hedged the small differences among the isomers and lowered the CCS values. On the other hand, fragmenting off the identical side branches as well as imposing the bending of the steroid ring resulted in ion mobility differentiation. Careful data evaluation revealed the tendency of isomers to form homo-cluster in the mixture solutions and assist the separation. Our fundamental and experimental findings enable the ion mobility separation of isomeric steroids to be predicted. The introduced rapid and optimal MS/MS-IMS analytical methodology can be applied to distinguish isomeric bile acids both in a solution and potentially in patients’ tissue samples, and consequently, reveal their molecular pathways.

Structural analysis of petroporphyrins from asphaltene by trapped ion mobility coupled with Fourier transform ion cyclotron resonance mass spectrometry.

Molecular characterization of compounds present in highly complex mixtures such as petroleum is proving to be one of the main analytical challenges. Heavy fractions, such as asphaltenes, exhibit immense molecular and isomeric complexity. Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) with its unequalled resolving power, mass accuracy and dynamic range can address the isobaric complexity. Nevertheless, isomers remain largely inaccessible. Therefore, another dimension of separation is required. Recently, ion mobility mass spectrometry has revealed great potential for isomer description. In this study, the combination of trapped ion mobility and Fourier transform ion cyclotron resonance mass spectrometry (TIMS-FTICR) is used to obtain information on the structural features and isomeric diversity of vanadium petroporphyrins present in heavy petroleum fractions. The ion mobility spectra provided information on the isomeric diversity of the different classes of porphyrins. The determination of the collision cross section (CCS) from the peak apex allows us to hypothesize about the structural aspects of the petroleum molecules. In addition, the ion mobility signal full width at half maximum (FWHM) was used as a measure for isomeric diversity. Finally, theoretical CCS determinations were conducted first on core structures and then on alkylated petroporphyrins taking advantage of the linear correlation between the CCS and the alkylation level. This allowed the proposal of putative structures in agreement with the experimental results. The authors believe that the presented workflow will be useful for the structural prediction of real unknowns in highly complex mixtures.

Characterization of Steroids through Collision Cross Sections: Contribution of Quantum Chemistry Calculations.

A wide range of collision cross section (CCS) databases for different families of compounds have recently been established from ion mobility mass spectrometry (IM-MS) measurements. Nevertheless, the need to validate these new data sets to provide the necessary confidence about the use of this parameter is increasingly expressed by the scientific community. If such a validation requires that complementary mass spectrometry experiments are conducted, it also appears that alternative strategies can contribute to the validation of such empirical data. In particular, in silico approaches are relevant to compute theoretical CCS values, to be compared to experimental ones. A recently published CCS database for 300 steroids allowed one to observe experimentally significant deviations of the expected CCS versus m/z correlations for some compounds. The present work attempts to rationalize such deviations with Density Functional Theory (DFT) calculations. MN15/6-311++G(d,p) investigations have been carried out, starting with a conformational analysis of a sample of 20 selected steroids and the determination of their preferred gas-phase ionization site. CCS values were then computed and compared to the experimental data. This approach allowed one to rationalize the experimental trends, providing an accurate description of the key properties of the various steroids considered.

Conformational Characterization of Polyelectrolyte Oligomers and Their Noncovalent Complexes Using Ion Mobility-Mass Spectrometry.

Poly-l-lysine (PLL), polystyrenesulfonate (PSS), and a mixture of these polyelectrolytes were investigated by electrospray ionization ion mobility mass spectrometry. The IM step confirmed the formation of noncovalent (i.e., supramolecular) complexes between these polyelectrolytes, which were detected in various charge states and stoichiometries in the presence of their constituents. Experimental and theoretical collision cross sections (CCSs) were derived for both PLL and PSS oligomers as well as their noncovalent assemblies. PSS chains showed higher compactness with increasing size as compared to PLL chains, indicating that the intrinsic conformations of the polyelectrolytes depend on the nature of the functional groups on their side chains. The CCS data for the noncovalent complexes further revealed that assemblies with higher PLL content have higher CCS values than other compositions of similar mass and that PLL-PSS complex formation is accompanied by significant size contraction.

Structures of Magnesium Oxide Cluster Cations Studied Using Ion Mobility Mass Spectrometry.

Structural assignments of gas-phase magnesium oxide cluster cations, MgnOn+ (n ≤ 24), have been achieved from a comparison of experimental collision cross sections (CCSs) measured using ion mobility mass spectrometry and theoretical CCSs calculated for equilibrium structures optimized by quantum chemical calculations. Various structures based on rock-salt and hexagonal-tube structures were assigned for n = 5-13 and 15. On the other hand, only rock-salt type structures were assigned for n = 4, 14, 16-21, and 24. The CCS values and the total energies were close for the hexagonal-tube and rock-salt structures of a given size in the small size range (n ≤ 15 except for 4 and 14). The hexagonal-tube structures of the cluster ions with n ≥ 16 were less stable and had larger CCSs than the rock-salt structures. These results indicate that the structures of the MgnOn+ clusters were changed from mixtures of rock-salt and hexagonal-tube structures to pure rock-salt structures with the growth of the cluster size. All atoms in the hexagonal-tube structures are located on the surface of the clusters, even if the cluster size increases. In contrast, the assigned rock-salt structures with n = 13, 14, and n ≥ 17 had atoms inside the clusters, which means that the average coordination numbers are substantially higher for the rock-salt structures than for the hexagonal tube structures. The structural change from the mixed structures to pure rock-salt with an increase in size n can be attributed to this difference in the coordination number.

Compositions and Isomer Separation of Palladium Oxide Cluster Cations Studied by Ion Mobility Mass Spectrometry

Geometric structures of gas-phase palladium oxide cluster cations, PdnOm+, were investigated for stable compositions by ion mobility mass spectrometry (IMMS) and quantum chemical calculations. Pure metallic (m = 0) and oxygen-deficient (m < n) cluster cations were preferentially obtained from the mass spectra as a result of collision-induced dissociation. Structures of cluster series, Pd3Om+ (m = 1–6), Pd4Om+ (m = 2–8), and Pd5Om+ (m = 3–8), were determined by comparing experimental collision cross sections obtained by IMMS and theoretical collision cross sections of optimized structures by density functional theory calculations. As for the Pd3Om+ cluster cations, structural transition was observed from one-dimensional chains to two-dimensional (2D) branched/2D sheets and finally to three-dimensional (3D) compact structures with increasing m. These 2D and 3D isomers were found to retain their triangular metal-core configuration. 2D sheets and 3D compact isomers that maintain a tetrahedral metal-core confi...

Separation, Quantification and Structural Study of (+)‐Catechin and (–)‐Epicatechin by Ion Mobility Mass Spectrometry Combined with Theoretical Algorithms

Summary of main observation and conclusionTwo catechin epimers and their non‐covalent complexes with γ‐cyclodextrin were studied by using ion mobility coupled with mass spectrometry (IM‐MS). Rapid separation of complexes was achieved with the peak‐to‐peak resolution reaching 0.86 after optimization of IM condition. Collision cross section (CCS) was measured to explore the structural difference of complexes. A gap of 11.75 Å2 between two complexes was found. Molecular modeling and theoretical CCS calculation were adopted to explain the measurement results. Two binding ways of both complexes were found and the calculated CCS corresponds accurately to the measured CCS. Quantification of catechins in mixtures was performed and the relative error was less than 15%, indicating the effectiveness of quantification by IM‐MS.

Structures of stoichiometric sodium oxide cluster cations studied by ion mobility mass spectrometry

Structures of stable compositions of sodium oxide cluster cations (NanOm+, n≤11) have been investigated by ion mobility mass spectrometry. Stoichiometric compositions series, Na(Na2O)(n−1)/2+ (n=3, 5, 7, 9, and 11), were observed as stable composition series, and NaO(Na2O)(n−1)/2+ series (n=5, 7, 9, and 11) were observed as secondary stable series in the mass spectra. To assign the structures of these cluster ion series, collision cross sections between the ions and helium buffer gas were determined experimentally from the ion mobility measurements. Theoretical collision cross sections were also calculated for optimized structures of these compositions. Finally, the structures of Na(Na2O)(n−1)/2+ and NaO(Na2O)(n−1)/2+ were assigned to those having similar structural frames for each n except for n=9. All bonds in the assigned structures of Na(Na2O)(n−1)/2+ were between sodium and oxygen. On the other hand, there was one O−O bond in addition to Na−O bonds in NaO(Na2O)(n−1)/2+. This result indicates that NaO(Na2O)(n−1)/2+ have a peroxide ion (O22−) as a substitute for an oxide ion (O2−) of Na(Na2O)(n−1)/2+. As a result, both stable series, Na(Na2O)(n−1)/2+ and NaO(Na2O)(n−1)/2+, are closed-shell compositions. These closed-shell characteristics have a strong influence on the stability of sodium oxide cluster cations.

A strategy for absolute quantitation of isomers using high performance liquid chromatography-ion mobility mass spectrometry and material balance principle

Accurate characterization of isomers is a quite challenge and time-consuming work. A major bottleneck is the lack of relevant reference standards. In this work, we conducted a proof-of-concept study to develop a standard-free method for absolute quantitation of isomers by using high performance liquid chromatography-ion mobility mass spectrometry and material balance principle. The isomer structures were characterized by matching the rank order of experimental collision cross sections (CCSs) to that of theoretical CCSs. Then a time-dependent hydrolysis protocol derived from material balance equation was used for the calculation of the relative correction factor (F). Finally, the multi-level normalized matrix approach was developed to absolutely and precisely quantitate isomeric compounds. To assess this method, quercetin-liver microsome reaction pool was applied. With the developed method, four isomeric metabolites of quercetin were accurately analyzed, and metabolic profiling of quercetin isomeric metabolites was firstly reported with the absolute quantitation results. This is an accurate yet simple method for the absolute quantitation of positional isomers of interest.

Fullerene-Functionalized Monolayer-Protected Silver Clusters: [Ag29(BDT)12(C60) n]3- ( n = 1-9).

We report the formation of supramolecular adducts between monolayer-protected noble metal nanoclusters and fullerenes, specifically focusing on a well-known silver cluster, [Ag29(BDT)12]3-, where BDT is 1,3-benzenedithiol. We demonstrate that C60 molecules link with the cluster at specific locations and protect the fragile cluster core, enhancing the stability of the cluster. A combination of studies including UV-vis, high-resolution electrospray ionization mass spectrometry, collision-induced dissociation, and nuclear magnetic resonance spectroscopy revealed structural details of the fullerene-functionalized clusters, [Ag29(BDT)12(C60) n]3- ( n = 1-9). Density functional theory (DFT) calculations and molecular docking simulations affirm compatibility between the cluster and C60, resulting in its attachment at specific positions on the surface of the cluster, stabilized mainly by π-π and van der Waals interactions. The structures have also been confirmed from ion mobility mass spectrometry by comparing the experimental collision cross sections (CCSs) with the theoretical CCSs of the DFT-optimized structures. The gradual evolution of the structures with an increase in the number of fullerene attachments to the cluster has been investigated. Whereas the structure for n = 4 is tetrahedral, that of n = 8 is a distorted cube with a cluster at the center and fullerenes at the vertices. Another fullerene, C70, also exhibited similar behavior. Modified clusters are expected to show interesting properties.

Structural characterization of small molecular ions by ion mobility mass spectrometry in nitrogen drift gas: improving the accuracy of trajectory method calculations.

The investigation of ion structures based on a combination of ion mobility mass spectrometry (IM-MS) experiments and theoretical collision cross section (CCS) calculations has become important to many fields of research. However, the accuracy of current CCS calculations for ions in nitrogen drift gas limits the information content of many experiments. In particular, few studies have evaluated and attempted to improve the theoretical tools for CCS calculation in nitrogen drift gas. In this study, based on high-quality experimental measurements and theoretical modeling, a comprehensive evaluation of various aspects of CCS calculations in nitrogen drift gas is performed. It is shown that the modification of the ion-nitrogen van der Waals (vdW) interaction potential enables accurate CCS predictions of 29 small ions with ca. 3% maximum relative error. The present method exhibits no apparent systematic bias with respect to ion CCS (size) and dipole moment, suggesting that the method adequately describes the long-range interactions between the ions and the buffer gas. However, the method shows limitations in reproducing experimental CCS at low temperatures (<150 K) and for macromolecular ions, and calculations for these cases should be complemented by CCS calculation methods in helium drift gas. This study presents an accurate and well-characterized CCS calculation method for ions in nitrogen drift gas that is expected to become an important tool for ion structural characterization and molecular identification. The experimental values reported here also provide a foundation for future studies aiming at developing more efficient computational tools.

Geometrical Structures of Gas Phase Chromium Oxide Cluster Anions Studied by Ion Mobility Mass Spectrometry

Structural assignments of gas phase chromium oxide cluster anions, CrmOn- (m = 1-7), have been achieved by comparison between experimental collision cross sections measured by ion mobility mass spectrometry and theoretical collision cross sections of optimized structures by quantum chemical calculations. In the mass spectrum, significant magic behavior between the numbers m and n was not observed for CrmOn-, while wide ranges of compositions were observed around CrmO2m+2- to (CrO3)m- as reported previously. The (CrO3)m- (m = 3-7) ions were assigned to have monocyclic-ring structures for m = 3-5 and bicyclic rings for m = 6 and 7. In addition, gradual structural change from these cyclic structures of (CrO3)m- to three-dimensional structures of CrmO2m+2- was found for m = 4-7. The energy levels of molecular orbitals of a calculated monocyclic structure of Cr5O15- were also found to be consistent with previous results of photoelectron spectroscopy, although those of the bicyclic isomer exhibited a different behavior. Moreover, the observation of abundant ions generated by collision induced dissociations at the inlet of the ion drift cell indicates that the larger sized (CrO3)m- (m > 5) series were unstable and easily dissociated to smaller ions.