Electron Impact Excitation Of Ions Research Articles

Enhancing Metabolome Annotation by Electron Impact Excitation of Ions from Organics-Molecular Networking.

Liquid chromatography-high-resolution mass spectrometry (LC-HRMS) is widely used in untargeted metabolomics, but large-scale and high-accuracy metabolite annotation remains a challenge due to the complex nature of biological samples. Recently introduced electron impact excitation of ions from organics (EIEIO) fragmentation can generate information-rich fragment ions. However, effective utilization of EIEIO tandem mass spectrometry (MS/MS) is hindered by the lack of reference spectral databases. Molecular networking (MN) shows great promise in large-scale metabolome annotation, but enhancing the correlation between spectral and structural similarity is essential to fully exploring the benefits of MN annotation. In this study, a novel approach was proposed to enhance metabolite annotation in untargeted metabolomics using EIEIO and MN. MS/MS spectra were acquired in EIEIO and collision-induced dissociation (CID) modes for over 400 reference metabolites. The study revealed a stronger correlation between the EIEIO spectra and metabolite structure. Moreover, the EIEIO spectral network outperformed the CID spectral network in capturing structural analogues. The annotation performance of the structural similarity network for untargeted LC-MS/MS was evaluated. For the spiked NIST SRM 1950 human plasma, the annotation coverage and accuracy were 72.94 and 74.19%, respectively. A total of 2337 metabolite features were successfully annotated in NIST SRM 1950 human plasma, which was twice that of LC-CID MS/MS. Finally, the developed method was applied to investigate prostate cancer. A total of 87 significantly differential metabolites were annotated. This study combining EIEIO and MN makes a valuable contribution to improving metabolome annotation.

Electron Impact Excitation of Ions from Organics on Singly Protonated Peptides with and without Post-Translational Modifications.

We report on the dissociation of singly protonated peptides by electrons using electron-activated dissociation (EAD), which comprises electron impact excitation of ions from organics (EIEIO), electronic-excitation dissociation (EED), and electron ionization dissociation (EIoD). Various singly protonated peptides were dissociated using a recently reported fast EAD device. The dissociation can be induced through two pathways: (i) vibrational dissociation similar to collision-activated dissociation (CAD, or collision-induced dissociation, CID) by relaxation from a molecular electronic excited state to high vibrational states; and (ii) radical-induced dissociation where molecular electronic excitation is followed by homolytic cleavage. EAD is complementary to CAD as additional molecular information can be obtained; e.g., fragile PTM moieties, such as glycosylation and sulfation, can be localized. Simultaneously, the energetic production of radical z• fragments enables Leu and Ile discrimination, like in a hot ECD process. Using the fast EAD device, LC-EIEIO-time-of-flight mass spectrometry was applied to a tryptic monoclonal antibody digest containing short singly protonated peptides.

Dissociation of Biomolecules by an Intense Low-Energy Electron Beam in a High Sensitivity Time-of-Flight Mass Spectrometer

We report the progress on an electron-activated dissociation (EAD) device coupled to a quadrupole TOF mass spectrometer (QqTOF MS) developed in our group. This device features a new electron beam optics design allowing up to 100 times stronger electron currents in the reaction cell. The electron beam current reached the space-charge limit of 0.5 μA at near-zero electron kinetic energies. These advances enable fast and efficient dissociation of various analytes ranging from singly charged small molecules to multiply protonated proteins. Tunable electron energy provides access to different fragmentation regimes: ECD, hot ECD, and electron-impact excitation of ions from organics (EIEIO). The efficiency of the device was tested on a wide range of precursor charge states. The EAD device was installed in a QqTOF MS employing a novel trap-and-release strategy facilitating spatial mass focusing of ions at the center of the TOF accelerator. This technique increased the sensitivity 6-10 times and allows for the first time comprehensive structural lipidomics on an LC time scale. The system was evaluated for other compound classes such as intact proteins and glycopeptides. Application of hot ECD for the analysis of glycopeptides resulted in rich fragmentation with predominantly peptide backbone fragments; however, glycan fragments attributed to the ECD process were also observed. A standard small protein ubiquitin (8.6 kDa) was sequenced with 90% cleavage coverage at spectrum accumulation times of 100 ms and 98% at 800 ms. Comparable cleavage coverage for a medium-size protein (carbonic anhydrase: 29 kDa) could be achieved, albeit with longer accumulation times.

Analysis of circular polarization of the emitted photon in plasmas

In this paper, a new relativistic distorted wave approximation for cross section describing longitudinally-polarized electron impact excitation of ions in the presence of a strongly coupled plasma is proposed and implemented based on the flexible atomic code. The uniform electron gas model is used to incorporate the plasma effects, which includes the effect of static plasma screening on nuclear charge and the effect of confinement due to the neighbouring ions. For the bound and continuum wavefunctions, the Dirac equation with the ion-sphere potential is solved to account for relativistic effects. In order to check the consistency of the modifications, a self consistent theoretical attempt using the multiconfiguration Dirac-Fock method is also carried out to get the energy levels accounting for the plasma environment within the ion-sphere model. For the collision dynamics, as examples of applications, we investigate the total cross sections, the magnetic sublevels cross sections, and the degrees of circular polarizations of fluorescence radiation after 1s2s01→1s2ss03 and 1s2s01→1s2pp13,1 excitations of Helium-like Fe XXV ion by longitudinally-polarized electron impact with the ion-sphere potential. It is shown that the behavior of the cross sections for a pure Coulomb potential changes rapidly due to the plasma screening effects described by the ion-sphere potential in the process of calculating scattering matrix elements. A detailed, quantitative, comparison between the present results and other available theoretical values is presented for the plasma-free cases, showing good agreement. Our results contribute to a better understanding of the fundamentals of fluorescence polarization in strongly coupled plasmas. Results obtained in this work are beneficial for the diagnostic determination of laboratory plasmas.

Electron-impact excitation of ions within a quantum plasma

We develop a new relativistic model which can describe the dynamics on electron-impact excitation (EIE) of ions within a quantum plasma environment. An exemplary excitation 1s1/2 → 2p3/2 of Fe XXVI by electron impact with the exponential cosine-screened Coulomb potential (ECSCP) is presented, i.e., 25 times ionized atoms of Fe, exist in laser-produced plasmas, astrophysical objects, and electron beam ion trap experiments. For the atomic structure of target ion, using the same potential, a Ritz variational method with a trial wave function that contains unknown variational parameters is also proposed for comparison purposes. In this method, an analytical formula of bound energy is derived from the energy equation, and the variational parameters are determined by the minimization of the energy. The energies obtained from our two methods are compared with each other and other theoretical predictions based on the non-relativistic model and found to be in good agreement. For the collision dynamics, the continuum wave function is determined by solving the modified Dirac equations, in which the ECSCP is included for the solution of the continuum orbital. The total cross sections, magnetic sublevel cross sections, and the degrees of linear polarization of emitted x-rays are calculated as a function of the incident energy and screening parameter by using the distorted-wave method in the relativistic frame. Since there are no experiments or other calculations for EIE cross sections on this ion, the obtained results have been compared with those for an isolated ion. The effect of quantum plasma leads to a reduction in the total and magnetic sublevel cross-sections and thus reduces the degrees of linear polarization of emitted x-rays. Comparative analysis allows to estimate the quality of the proposed approximations. Our dynamic model is practicable and easy to apply, which may provide an alternative route towards information with fluorescence polarization in a quantum plasma environment.

Polarization of x-ray radiation after electron-impact excitation of ions in plasmas

A new relativistic model for calculating (magnetic sublevel) electron-impact excitation process in plasmas is proposed in this paper, which is essential to predict the polarization property of x-ray radiation. Our model focuses on weakly coupled plasmas (assuming that the plasma is of low density and very hot), where we use a Debye-Hückel potential to represent the interactions among the charged particles, in order to calculate plasma shielding effects. As a practical example, we present results of accurate calculations of transition energy, cross section, and linear polarization of the photon emission following EIE of 1s2→1s2p3,1P1 lines of He-like Ar16+ ion over a broad range of energies and Debye lengths, i.e., 16 times ionized atom of Ar. Such highly-ionized species can exist in many environments, such as in laser-produced plasmas, x-ray laser, astrophysics, and inertial confinement fusion experiments. The energies and cross sections are found to be decreased with decreasing Debye length, these general characteristics also lead to a reduction in the linear polarization of the photon emission. The present approach compares well, with existing methods and experimental data. Apart from other applications, results obtained in this work should be useful in plasma diagnostic.

Polarization of fluorescence radiation following electron impact excitation of ions immersed in strongly coupled plasmas

An investigation on the electron impact excitation and subsequent radiative decay process of He-like Fe24+ ions immersed in strongly coupled plasmas is made, using the 1s2p P3,11 → 1s2 S10 characteristic lines as an example. The shielded nuclear potential of the uniform electron gas model (UEGM) type experienced by the electron is parameterized by the ion-sphere radius. For the target structure, accuracies of wave functions are justified by evaluating the energies of the relevant states. To serve as an independent check of the results, self-consistent calculations are carried out using the multiconfiguration Dirac-Fock relativistic configuration interaction method incorporating the same potential. For the impact excitation process, the UEGM potential is also employed to screen the projectile electron from the nucleus and target electrons. A distorted-wave method in the framework of the relativistic theory is developed to include the effect of plasma background, in which the continuum wave function of the projectile electron is evaluated by solving numerically the modified Dirac equations. The plasma effects on the total excitation cross section, magnetic sublevel cross section, and linear polarization of fluorescence radiation are investigated. The comparison with the available theoretical and experimental results is satisfactory.

Quantitative structural multiclass lipidomics using differential mobility: electron impact excitation of ions from organics (EIEIO) mass spectrometry

We report a method for comprehensive structural characterization of lipids in animal tissues using a combination of differential ion mobility spectrometry (DMS) with electron-impact excitation of ions from organics (EIEIO) mass spectrometry. Singly charged lipid ions in protonated or sodiated forms were dissociated by an electron beam having a kinetic energy of 10 eV in a branched radio-frequency ion trap. We established a comprehensive set of diagnostics to characterize the structures of glycerophospholipids, sphingolipids, and acylglycerols, including glycosylated, plasmalogen, and ester forms. This EIEIO mass spectrometer was combined with DMS as a separation tool to analyze complex lipid extracts. Deuterated quantitative standards, which were added during extraction, allowed for the quantitative analysis of the lipid molecular species in various lipid classes. We applied this technique to the total lipids extracted from porcine brain, and we structurally characterized over 300 lipids (with the exception of cis/trans double-bond isomerism in the acyl chains). The structural dataset of the lipidomes, whose regioisomers were distinguished, exhibit a uniquely defined distribution of acyl chains within each lipid class; that is, sn-1 and sn-2 in the cases of glycerophospholipids or sn-2 and (sn-1, sn-3) in the cases of triacylglycerols.

Distinguishing Cis and Trans Isomers in Intact Complex Lipids Using Electron Impact Excitation of Ions from Organics Mass Spectrometry.

We present a mass spectrometry-based method for the identification of cis and trans double-bond isomers within intact complex lipid mixtures using electron impact excitation of ions from organics (EIEIO) mass spectrometry. EIEIO involves irradiating singly charged lipid ions with electrons having kinetic energies of 5-16 eV. The resulting EIEIO spectra can be used to discern cis and trans double-bond isomers by virtue of the differences in the fragmentation patterns at the carbon-carbon single bonds neighboring the double bonds. For trans double bonds, these characteristic fragments include unique closed-shell and open-shell (radical) products. To explain this fragmentation pattern in trans double bonds, we have proposed a reaction mechanism involving excitation of the double bond's π electrons followed by hydrogen atom rearrangement. Several lipid standards were analyzed using the EIEIO method, including mixtures of these standards. Prior to EIEIO, some of the lipid species in these mixtures were separated from their isomeric forms by using differential mobility spectrometry (DMS). For example, mixed cis and trans forms of triacylglycerols and phosphatidylcholines were identified by this DMS-EIEIO workflow. With this combined gas-phase separation and subsequent fragmentation, we could eliminate the need for authentic standards for identification. When DMS could not separate cis and trans isomers completely, as was the case with sphingomyelins, we relied upon the aforementioned diagnostic EIEIO fragment peaks to determine the relative contribution of the trans double-bond isomer in the mixed samples. We also applied the DMS-EIEIO methodology to natural samples extracted from a ruminant (bovine), which serve as common origins of trans fatty acids in a typical Western diet that includes dairy products.

Development of a Branched Radio-Frequency Ion Trap for Electron Based Dissociation and Related Applications.

Collision-induced dissociation (CID) is the most common tool for molecular analysis in mass spectrometry to date. However, there are difficulties associated with many applications because CID does not provide sufficient information to permit details of the molecular structures to be elucidated, including post-translational-modifications in proteomics, as well as isomer differentiation in metabolomics and lipidomics. To face these challenges, we are developing fast electron-based dissociation devices using a novel radio-frequency ion trap (i.e., a branched ion trap). These devices have the ability to perform electron capture dissociation (ECD) on multiply protonated peptide/proteins; in addition, the electron impact excitation of ions from organics (EIEIO) can be also performed on singly charged molecules using such a device. In this article, we review the development of this technology, in particular on how reaction speed for EIEIO analyses on singly charged ions can be improved. We also overview some unique, recently reported applications in both lipidomics and glycoproteomics.

脂質異性体の同定は電子を用いた解離：EIEIOによりどこまで可能となったか

脂質異性体の同定は電子を用いた解離：EIEIOによりどこまで可能となったか

Structural identification of triacylglycerol isomers using electron impact excitation of ions from organics (EIEIO)

Electron-induced dissociation or electron impact excitation of ions from organics (EIEIO) was applied to triacylglycerols (TAGs) for in-depth molecular structure analysis using MS. In EIEIO, energetic electrons (∼10 eV) fragmented TAG ions to allow for regioisomeric assignment of identified acyl groups at the sn-2 or sn-1/3 positions of the glycerol backbone. In addition, carbon-carbon double bond locations within the acyl chains could also be assigned by EIEIO. Beyond the analysis of lipid standards, this technique was applied to edible oils and natural lipid extracts to demonstrate the power of this method to provide in-depth structural elucidation of TAG molecular species.

In-depth sphingomyelin characterization using electron impact excitation of ions from organics and mass spectrometry

Electron impact excitation of ions from organics (EIEIO), also referred to as electron-induced dissociation, was applied to singly charged SM molecular species in the gas phase. Using ESI and a quadrupole TOF mass spectrometer equipped with an electron-ion reaction device, we found that SMs fragmented sufficiently to identify their lipid class, acyl group structure, and the location of double bond(s). Using this technique, nearly 200 SM molecular species were found in four natural lipid extracts: bovine milk, porcine brain, chicken egg yolk, and bovine heart. In addition to the most common backbone, d18:1, sphingosines with a range of carbon chain lengths, sphingadienes, and some sphinganine backbones were also detected. Modifications in natural SMs were also identified, including addition of iodine/methanol across a carbon-carbon double bond. This unparalleled new approach to SM analysis using EIEIO-MS shows promise as a unique and powerful tool for structural characterization.

Electron-impact ionization-excitation of the neon valence shell studied by high-resolution electron-momentum spectroscopy

We present experimental and theoretical results for electron-impact-induced ionization and excitation of the neon valence shell by using a highly sensitive and high-resolution ($e, 2e$) spectrometer. The controversies in previous electron momentum spectroscopic results have been resolved. The satellites at 55.83 and 71.50 eV are mainly attributed to a ${\phantom{\rule{4pt}{0ex}}}^{2}P$ manifold, and mixed up with some ${}^{2}S$, not a pure ${}^{2}S$ as were assigned by the earlier electron-momentum spectroscopy. Our experimental momentum distribution for the satellite at 55.83 eV is consistent with that reported by Watanabe et al. [J. Electron Spectrosc. Relat. Phenom 142, 325 (2005)], but not consistent with Samardzic et al.'s results [Phys. Rev. A 48, 4390 (1993)]. The difference between the observed momentum distribution of the satellite at 55.83 eV and the calculations with the plane-wave impulse approximation suggests the need for further improvement of the theoretical methods.

Near-complete structural characterization of phosphatidylcholines using electron impact excitation of ions from organics.

Although lipids are critical components of many cellular assemblies and biological pathways, accurate descriptions of their molecular structures remain difficult to obtain. Many benchtop characterization methods require arduous and time-consuming procedures, and multiple assays are required whenever a new structural feature is probed. Here, we describe a new mass-spectrometry-based workflow for enhanced structural lipidomics that, in a single experiment, can yield almost complete structural information for a given glycerophospholipid (GPL) species. This includes the lipid's sum (Brutto) composition from the accurate mass measured for the intact lipid ion and the characteristic headgroup fragment, the regioisomer composition from fragment ions unique to the sn-1 and sn-2 positions, and the positions of carbon-carbon double bonds in the lipid acyl chains. Here, lipid ions are fragmented using electron impact excitation of ions from organics (EIEIO)--a technique where the singly charged lipid ions are irradiated by an electron beam, producing diagnostic product ions. We have evaluated this methodology on various lipid standards, as well as on a biological extract, to demonstrate this new method's utility.

Integral cross sections for electron-impact ionization–excitation of laser-excited barium

We have measured electron-impact ionization–excitation (IE) cross sections for the Ba+ (…6p 2P) level out of the excited (…6s5d 1, 3D1, 2) and (…6s6p 1P1) levels of the neutral Ba atom in the energy range of 20–50 eV. Cross sections for IE out of the (…6s6p 1P1) level are approximately 50% larger than those from the (…6s2 1S0) ground state, while cross sections for IE out of the D levels are approximately 50% that of IE out of the ground state. The dependence of these cross sections on the initial P state alignment has been found to be small.

Electron-impact ionization excitation of helium in the quasiphoton regime

The triply differential cross section of ionization excitation of helium, leaving the residual ion in the n = 2 excited states, is evaluated for the kinematics considered experimentally by Dupre et al. [J. Phys. B 25, 259 (1992)]. The interaction of the incident electron with the target is described at the first order, while the interaction of the ejected electron with the residual ion is treated very accurately within the formalism of the Jacobi matrix method. In the quasiphoton limit and for low ejected electron energies, the presence of series of doubly excited states, mainly below the n = 3 single ionization threshold in helium, makes the triply differential cross sections extremely sensitive to both the energy and the emission angle of the ejected electron. We show that the convolution of our results with a Gaussian energy profile, in which the full width at half-maximum corresponds to the energy resolution in the experiment, has a significant effect. Our results suggest that it is also important to account for the finite resolution on the measurement of the scattering angle when the experimental data are compared to the theoretical predictions. Comparison of our theoretical results convoluted both in energy and in angle with the experimental data demonstrates the importance of an accurate description of the helium spectrum. A possible two-step mechanism involving single ionization of the target followed by excitation of the core electron is proposed to explain the remaining discrepancies.

Electron impact ionization-excitation and double-ionization dynamics of He at large momentum transfer

We report a collision dynamics study on electron-impact ionization-excitation and double-ionization of He at large momentum transfer. The symmetric noncoplanar (e,2e) and (e,3-1e) cross sections have been measured for transitions to the He+ n=2 excited state and He2+doubly ionized state at impact energies of 1.2, 2.1, and 4.3 keV. An investigation of impact energy dependence of the cross sections provides definite evidence that noticeable higher-order effects are involved even at the rather high impact energies used. It has been found that second Born approximation calculations satisfactorily account for the experimental results, indicating that the second-order two-step mechanisms play crucial roles in the ionization-excitation and double-ionization processes under the kinematical conditions considered here.

Electron-impact ionization-excitation dynamics at large momentum transfer: (e,2e) studies on He and H2

We report an (e,2e) study on simultaneous ionization-excitation processes of He and H2 at large momentum transfer. The symmetric noncoplanar (e,2e) cross sections have been measured for transitions to excited ion states. It has been found that noticeable higher-order effects are involved even at the rather high impact energies employed, which ranged between 1.2 and 4.3 keV. Second Born approximation calculations satisfactorily account for the experimental results, indicating that the second-order two-step mechanisms serve as a key to understanding the ionization-excitation dynamics. Roles of the two-step mechanisms will be fully discussed.

Electron-impact ionization-excitation of atoms

As a highly correlated process, simultaneous electron-impact ionization-excitation presents a major challenge to experimentalists and theorists alike. Taking advantage of significant improvements in multi-hit detector technology and the rapid increase of computational power, a number of advances have recently been reported. The current state-of the-art in this field is illustrated by a few key examples.