Collision Cross Sections Research Articles

Heavy-flavor hadron decays producing muon cross sections in pp collisions at high transverse momentum

The focus of this work was on investigating and comparing two methods for calculating the cross sections of muons produced by the decay of heavy-flavor hadrons in pp collisions, which cover the interval 2<pT< 20 GeV/c at a center-of-mass energy of s = 7 TeV. One method involves using the fixed-order plus next-to-leading logarithms (FONLL) approach to scale the measured pT-differential cross section of muons from heavy-flavor hadron decay in pp collisions at a center-of-mass energy of s = 5.02 TeV. The other method involves using FONLL calculations to extrapolate the published measured pT-differential cross section at s = 7 TeV to higher pT. In these two methods, we can use the published measured pT-differential cross section at s = 5.02 TeV and s = 7 TeV, which cover the interval 2<pT< 20 GeV/c and 2<pT< 12 GeV/c, respectively. In terms of consistency within uncertainties, the results from both methods were compared not only to pQCD-based FONLL calculations but also to each other. Because they provide the necessary pp reference, these calculations are critical for determining the nuclear modification factor of muons from heavy flavor decays in heavy-ion collisions.

Charge Inversion Ion/Ion Reactions Coupled to Ion Mobility/Mass Spectrometry: Oligosaccharides.

Various ion mobility-based separation techniques and instruments have been recently developed to increase the operational resolution of ion mobility separations, especially of isomers and isobars. In addition to developments in instrumentation, different covalent and noncovalent derivatization techniques have helped achieve effective separations by magnifying minor differences in collision cross section. Among these methodologies is host-guest complex formation and, a new development presented herein, charge inversion ion-ion reactions coupled to ion mobility separations. We used these methods to enable formation of complexes between isomeric deprotonated oligosaccharides and alkaline earth metals (in solution) and alkaline earth metal-trisphenanthroline complexes (in vacuo), observing minor shifts in ion mobility arrival times for the charge inversion reaction products as well as unique mobility fingerprints indicative of separations of α/β anomers of disaccharides. For example, we have demonstrated separations between reducing disaccharides such as lactose and lactulose and nonreducing disaccharides. We also observed separations based on the pyranose/furanose configurations of the isomers. These results suggest the potential for ion/ion reactions to enable isomer separation of biomolecules from various compound classes using ion mobility-mass spectrometry (IM-MS).

Collisional excitation of N+(3P) in interstellar clouds

ABSTRACT The detection of the N+ ion in ionized interstellar regions, as well as its use as a tracer for atomic nitrogen (N) in interstellar media, makes it an interesting ion from the astrophysical point of view. Accurate determination of the N+ abundance in the interstellar clouds requires detailed modelling of its excitation conditions. Hence, collisional excitation induced by the dominant interstellar species has to be studied. In this paper, we study the collisional excitation of N+ by He and H2. We provide new potential energy surfaces describing the interactions between N+ and He and between N+ and H2 considered as a structureless collider. Quantum calculations of the excitation cross-sections for collisions of N+ with He and H2 are provided, as well as the first excitation rate coefficients for N+ induced by these two species. The results are compared to collisional data for the excitation induced by electrons. It is shown that the excitation of N+ by a neutral collider can play a significant role in weakly ionized regions as well as in warm molecular clouds.

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

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

Expanding Orbitrap Collision Cross-Section Measurements to Native Protein Applications Through Kinetic Energy and Signal Decay Analysis.

The measurement of collision cross sections (CCS, σ) offers supplemental information about sizes and conformations of ions beyond mass analysis alone. We have previously shown that CCSs can be determined directly from the time-domain transient decay of ions in an Orbitrap mass analyzer as ions oscillate around the central electrode and collide with neutral gas, thus removing them from the ion packet. Herein, we develop the modified hard collision model, thus deviating from the prior FT-MS hard sphere model, to determine CCSs as a function of center-of-mass collision energy in the Orbitrap analyzer. With this model, we aim to increase the upper mass limit of CCS measurement for native-like proteins, characterized by low charge states and presumed to be in more compact conformations. We also combine CCS measurements with collision induced unfolding and tandem mass spectrometry experiments to monitor protein unfolding and disassembly of protein complexes and measure CCSs of ejected monomers from protein complexes.

Effects of charge on protein ion structure: Lessons from cation-to-anion, proton-transfer reactions.

Collision cross-section values, which can be determined using ion mobility experiments, are sensitive to the structures of protein ions and useful for applications to structural biology and biophysics. Protein ions with different charge states can exhibit very different collision cross-section values, but a comprehensive understanding of this relationship remains elusive. Here, we review cation-to-anion, proton-transfer reactions (CAPTR), a method for generating a series of charge-reduced protein cations by reacting quadrupole-selected cations with even-electron monoanions. The resulting CAPTR products are analyzed using a combination of ion mobility, mass spectrometry, and collisional activation. We compare CAPTR to other charge-manipulation strategies and review the results of various CAPTR-based experiments, exploring their contribution to a deeper understanding of the relationship between protein ion structure and charge state.

Effects of structural characteristics of (un)conjugated steroid metabolites in their collision cross section value.

In this work, the collision cross section (CCS) value of 103 steroids (including unconjugated metabolites and phase II metabolites conjugated with sulfate and glucuronide groups) was determined by liquid chromatography coupled to traveling wave ion mobility spectrometry (LC-TWIMS). A time of flight (QTOF) mass analyzer was used to perform the analytes determination at high-resolution mass spectrometry. An electrospray ionization source (ESI) was used to generate [M+H]+, [M+NH4]+ and/or [M-H]- ions. High reproducibility was observed for the CCS determination in both urine and standard solutions, obtaining RSD lower than 0.3% and 0.5% in all cases respectively. CCS determination in matrix was in accordance with the CCS measured in standards solution showing deviations below 2%. In general, CCS values were directly correlated with the ion mass and allowed differentiating between glucuronides, sulfates and free steroids although differences among steroids of the same group were less significant. However, more specific information was obtained for phase II metabolites observing differences in the CCS value of isomeric pairs concerning the conjugation position or the α/β configuration, which could be useful in the structural elucidation of new steroid metabolites in the anti-doping field. Finally, the potential of IMS reducing interferences from the sample matrix was also tested for the analysis of a glucuronide metabolite of bolasterone (5β-androstan-7α,17α-dimethyl-3α,17β-diol-3-glucuronide) in urine samples.

R-matrix atomic data for application in astrophysics and fusion

A selection of electron-impact excitation R-matrix calculations completed by the collision group at Queens University Belfast are discussed. The atomic data calculated are the foundation of applications in a variety of fields, a sample of which are presented here. They include an analysis of Fe ii emission in active galactic nuclei, the possible presence of Pt and Au in the spectra of binary neutron star mergers and the viability of W ions as a diagnostic tool in magnetically confined tokamak plasmas. The radiative atomic data were computed using the most recent version of the fully relativistic structure code grasp0 and the collision cross sections were evaluated using the recently developed pdarcR-matrix package.Graphic abstract

Cross-calibration of atomic pressure sensors and deviation from quantum diffractive collision universality for light particles

The room-temperature, velocity-averaged, total cross section for atom–atom and atom–molecule collisions can be approximated using a universal function depending only on the magnitude of the leading order dispersion coefficient, C 6. This feature of the total cross section together with the universal function for the energy distribution transferred by glancing angle collisions ( (Booth et al 2019 New J. Phys. 21 102001)) can be used to empirically determine the total collision cross section and realize a self-calibrating, vacuum pressure standard. This was previously validated for Rb+N2 and Rb+Rb collisions. However, the post-collision energy distribution is expected to deviate from in the limit of small C 6 and small reduced mass. Here we observe this deviation experimentally by performing a direct cross-species loss rate comparison for Rb+H2 and Li+H2 collisions. We measure a velocity averaged total collision cross section ratio of . Based on an ab initio computation of m3 s−1, we deduce m3 s−1, in agreement with a Rb+H2 ab initio value of . By contrast, fitting the Rb+H2 loss rate as a function of trap depth to the universal function we find m3 s−1. This work demonstrates the utility of sensor-atom cross-calibration experiments to check the validity of theoretical computations to extend and enhance the performance of cold atom based pressure sensors.

Simulation Chemical Reactions in Faddeev Approach

The main features of resonance in scattering are described and resonances are determined on the basis of the theory of collisions in a two-body system, as well as resonances emerging as a result of collisions in a few-body system. Regularities in the emergence of such resonances and their characteristics are analyzed. The results of calculations of these resonant processes occurring during collisions of electrons with diatomic molecules, made on the basis of the quantum theory of scattering in a few body system, based on Faddeev-Yakubovsky equations are discussed. The results of calculations of the resonant cross sections of electron and atom collisions with molecules are presented. Obtained results are compared with the available experimental data and with the results of calculations based on other approximations. In addition some biological applications (e.g. properties biopolymer molecules) are presented.

Validation of Field-Dependent Ion-Solvent Cluster Modeling via Direct Measurement of Cluster Size Distributions.

Ion mobility spectrometry is widely used in analytical chemistry, either as a stand-alone technique or coupled to mass spectrometry. Ions in the gas phase tend to form loosely bound clusters with surrounding solvent vapors, artificially increasing the collisional cross section and the mass of the ion. This, in turn, affects ion mobility and influences separation. Further, ion-solvent clusters play an important role in most ionization mechanisms occurring in the gas phase. Consequently, a deeper understanding of ion-solvent cluster association and dissociation processes is desirable to guide experimental design and interpretation. A few computational models exist, which aim to describe the amount of clustering as a function of the reduced electric field strength, bath gas pressure and temperature, and the chemical species probed. It is especially challenging to model ion mobility under high reduced electrical field strengths due to the nonthermal conditions created by the field. In this work, we aim to validate a recently proposed first-principles model by comparing its predictions with direct measurements of cluster size distributions over a range of 20-120 Td as observed using a High Kinetic Energy Ion Mobility Spectrometer coupled to a mass spectrometer (HiKE-IMS-MS). By studying H+(H2O)n, [MeOH + H + n(H2O)]+, [ACE + H + n(H2O)]+, and [PhNH2 + H + n(H2O)]+ as test systems, we find very good agreement between model and experiment, supporting the validity of the computational workflow. Further, the detailed information gained from the modeling yields important insights into the cluster dynamics within the HiKE-IMS, allowing for better interpretation of the measured ion mobility spectra.

Rotational state-changes in C5N− by collisions with He and H2

ABSTRACT The anion C5N− is one of the largest linear (C,N)-bearing chains detected in the interstellar medium. Here we present and discuss the general features of new ab initio potential energy surfaces describing the interaction of this linear anion with He and H2. We employ a Legendre Polynomials expansion representation for the former and an artificial neural network fit for the latter. We then carry out quantum scattering calculations to yield rotationally inelastic cross-sections for collisions with He and H2, using relative translational energy values in the range of 0.1–300 cm−1. We then obtained the corresponding inelastic rate coefficients as a function of temperature covering the range from 1 to 100 K. The results for these two systems are compared with each other, as well as with the earlier results on the C3N− colliding with the same partners. We found that the final inelastic rate coefficients for this anion are all fairly large, those from collisions with H2 being the largest. The consequences of such findings on their non-equilibrium rotational populations in interstellar environments are discussed in our conclusions.

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

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

Cryogenic ion mobility-mass spectrometry for the study of conformations of host-guest complexes

Variable-temperature cryogenic ion mobility-mass spectrometry (VT-Cryo-IM-MS) was developed to examine the conformations of host-guest complexes. The VT-Cryo-IM-MS consists of an electrospray ionization source, a quadrupole ion trap, a cryogenic ion drift tube, and a time-of-flight mass spectrometer. The ion drift tube was cooled to 83 K with liquid nitrogen. Two sets of ion funnels are installed at the source and the drift tube to focus the ion beam. We used Li+-encapsulated fullerene (Li+@C60) as a reference ion to examine the ion-mobility resolution of the VT-Cryo-IM-MS apparatus because this ion has a rigid structure with a single conformer. The temperature dependence of collision cross section (CCS) of Li+@C60 and the resolution of the apparatus were determined at each temperature. We examined the conformations of host-guest complexes of a series of metal ions (M = Ca2+, Na+, Ag+, K+, Rb+, and Cs+) with dibenzo-24-crown-8 (DB24C8) by Cryo-IM-MS. All CCS distributions were fitted with two Gaussian functions which are assigned to closed and open conformers. These closed and open conformers have different distances between the two benzene rings of DB24C8. The abundance ratio of closed conformers for these M(DB24C8) complex ions depends on the guest M ion with an order, Ca2+ < Na+ < Ag+ < Cs+ < Rb+ < K+. The stability of the closed conformer of K+(DB24C8) originates from the benzene ring positional relationship which is quite close to that of the free benzene dimer. This special benzene dimer-like conformation produces the strong π-π interaction between the benzene rings of K+(DB24C8).

Electron-neutral collision cross sections for H2O: II. Anisotropic scattering and assessment of the validity of the two-term approximation

This work proposes a complete and consistent set of cross sections (CS) for electron collisions with water gas molecules to be published in the IST-Lisbon database on LXCat. The set is validated by the electron swarm analysis technique. The anisotropic angular distribution of electrons in rotational collisions is considered by means of the Born approximation in a two-term Boltzmann solver (LisbOn KInetics two-term Boltzmann solver (LoKI-B)) and a Monte Carlo simulations code (LoKI-MC), both freely available as open-source codes. The inclusion of electron anisotropic scattering in rotational collisions significantly improves the agreement between calculations and measurements of the electron drift velocity, reduced mobility, characteristic energy, reduced Townsend ionisation coefficient, reduced effective Townsend coefficient and reduced attachment coefficient. The MC simulations are deemed more accurate and shown to yield similar results as LoKI-B with the proposed set. The use of LoKI-MC also validates the set of CSs against parameters that cannot be obtained by LoKI-B, such as the longitudinal diffusion coefficient or the bulk transport coefficients.

Ab Initio Double-Differential Ionization Cross-Section Calculations in Antiproton–Helium Collisions

We present ionization cross-sections for antiproton and helium collisions based on an ab initio time-dependent coupled channel method. In our calculations, a finite basis set of regular helium Coulomb wave packets and Slater function were used. The semiclassical approximation was applied with the time-dependent Coulomb potential to describe the antiproton–electron interaction. Three different projectile energies were considered as 10, 50 and 100 keV. We found clear evidence for the formation of the anti-cusp in the differential distributions.

Charge transfer and ionization cross-sections in collisions of singly charged lithium ions with helium and nitrogen atoms

We present a non-perturbative classical treatment of the charge transfer and ionization processes in collisions between singly charged lithium ions with helium and nitrogen atomic targets. Single capture and single ionization total cross sections are calculated using a three-body classical trajectory Monte Carlo (CTMC) method in which the interaction between the collision partners is described by a Garvey-type model potential. The cross sections are evaluated for collision energies between 20 keV and 100 MeV. In particular, we found excellent agreement between our results and the available experimental data for the case of the single capture of He(1s) by Li+ ions. In addition, our CTMC results are in a reasonable agreement with the experimental results for collision energies higher than 200 keV for single capture of N(2p) atoms by Li+. Furthermore, we present single ionization cross sections for both collision systems.

Kinetic Theory of Instability of the Electron Beam-Plasma System Taking into Account the Elastic Electron-Atomic Collisions for an Arbitrary Scattering Indicatrix

A kinetic theory of stability of a fast electron beam-gas-discharge plasma system under conditions when the electron mean free path relative to elastic collisions with atoms is of the order of the plasma dimension along the direction of the beam propagation is developed. It is assumed that the cross section of elastic electron-atom collisions is considerably higher than the cross section of inelastic processes with electrons. The dispersion equations for an arbitrary indicatrix of elastic electron-atom scattering are obtained. Despite the smallness of the cross section for inelastic scattering in comparison with elastic one, it is also necessary to consider inelastic processes when the plasma Knudsen numbers are less than 0.3. As the Knudsen number decreases beneath 0.3 the growth rate considerably decreases in the range of instability, because the role of elastic scattering of electrons by atoms increases.

Size-Dependent Geometrical Structures of Platinum Oxide Cluster Cations Studied by Ion Mobility–Mass Spectrometry

Structures of platinum oxide cluster cations (PtnOm+) were studied by ion mobility-mass spectrometry in combination with theoretical calculations. Structures of oxygen-equivalent PtnOn+ (n = 3-7) clusters were discussed from the comparison between their collision cross sections (CCSs) obtained by mobility measurement and simulated CCSs of their structural candidates from structural optimization calculations. Assigned structures of PtnOn+ were found to be composed of Pt frameworks and bridging O atoms, which follows the previous theoretical prediction on the neutral clusters. The structures change from planar (n = 3 and 4) to three-dimensional (n = 5-7) with increasing cluster size by deforming platinum frameworks. Comparison with other group-10 metal oxide cluster cations (MnOn+; M = Ni and Pd) showed that the PtnOn+ structures have a similar tendency to PdnOn+ rather than NinOn+.

Regarding the Influence of Additives and Additional Plasma-Induced Chemical Ionization on Adduct Formation in ESI/IMS/MS.

Ion mobility spectrometers (IMS) separate ions based on their ion mobility, which depends mainly on collision cross-section, mass, and charge of the ions. However, the performance is often hampered in electrospray ionization (ESI) by the appearance of multiple ion mobility peaks in the spectrum for the same analyte due to clustering and additional sodium adducts. In this work, we investigate the influence of solvents and buffer additives on the detected ion mobility peaks using ESI. Additionally, we investigate the effects of an additional chemical ionization (CI) induced by plasma ionization on the ions formed by electrospray. For this purpose, we coupled our high-resolution IMS with a resolving power of Rp = 100 to a time-of-flight mass spectrometer. Depending on the analyte and the chosen additives, the ionization process can be influenced during the electrospray process. For the herbicide isoproturon, the addition of 5 mM sodium acetate results in the formation of the sodium adduct [M + Na]+, which is reflected in the ion mobility K0 of 1.22 cm2/(V·s). In contrast, the addition of 5 mM ammonium acetate yields the protonated species [M + H]+ and a correspondingly higher K0 of 1.29 cm2/(V·s). In some cases, as with the herbicide pyrimethanil, the addition of sodium acetate can completely suppress ionizations. By carefully choosing the solvent additive for ESI-IMS or additional CI, the formation of different ion mobility peaks can be observed. This can facilitate the assignment of ions to ion mobility peaks using IMS as a compact, stand-alone instrument, e.g., for on-site analysis.