Declustering and fragmentation of protein ions from an electrospray ion source

A triple quadrupole mass spectrometer with a high pressure collision cell has been used to explore the declustering and fragmentation processes that may occur in the vacuum interface region of an electrospray or ionspray ion source. Using apomyoglobin as a model protein compound, collisional processes in Q2 were used to elucidate possible mechanisms which could occur in the orifice-skimmer region to affect the observed charge state distribution. The results indicate that charge loss or gain through collisional loss of a proton or electron does not occur; rather, higher collision energy results in better declustering of lower charge state ions, and fragmentation of higher charge state ions. The net result is an apparent shift toward lower charge state as the collision energy in the free jet region is increased. In addition, the data suggest that a mixture of heavily clustered monomers and possibly dimers and multimers are present in the expansion from ion source into vacuum, and it is this mixture which is acted on by the declustering field to produce the observed mass spectrum. The presence of these “superclusters” needs to be considered in any theory of ion desorption and transport processes in the source and interface region.     

Quantum dynamics simulations of interfacial electron transfer in sensitized TiO2 semiconductors

Ab initio DFT molecular dynamics simulations are combined with quantum dynamics calculations of electronic relaxation to investigate the interfacial electron transfer in catechol/TiO(2)-anatase nanostructures under vacuum conditions. It is found that the primary process in the interfacial electron-transfer dynamics involves an ultrafast (tau(1) approximately 6 fs) electron-injection event that localizes the charge in the Ti(4+) surface ions next to the catechol adsorbate. The primary event is followed by charge delocalization (i.e., carrier diffusion) through the TiO(2)-anatase crystal, an anisotropic diffusional process that can be up to an order of magnitude slower along the [-101] direction than carrier relaxation along the [010] and [101] directions in the anatase crystal. It is shown that both the mechanism of electron injection and the time scales for interfacial electron transfer are quite sensitive to the symmetry of the electronic state initially populated in the adsorbate molecule. The results are particularly relevant to the understanding of surface charge separation in efficient mechanisms of molecular-based photovoltaic devices.     

Charge inversion mass spectrometry: dissociation of resonantly neutralized molecules

Charge inversion mass spectrometry is an MS/MS method in which the electric charge of the precursor ions is opposite to that of the secondary product ions. Charge inversion mass spectrometry is classified into four types depending on the electric charge and time scale of collisions. Charge inversion mass spectrometry using collisions with gaseous targets in the keV energy collision range has provided insights into the structures and reactions of ions and neutral molecules. The characteristics of charge inversion experiments are presented in terms of the reaction endothermicities and the cross sections and their dependence on the target species. In the case of rare-gas or simple molecular targets, double-electron transfer in one collision is effective to form positive ions from negative ions, while, in the case of alkali metal targets, successive single-electron transfers in two collisions is effective to form negative ions from positive ions. On the basis of the observed target-density dependence of the product ion intensity and thermochemical considerations for internal energy distribution using thermometer molecules, the charge inversion processes using alkali metal targets have been confirmed to occur by electron transfers in successive collisions and the dissociation processes are found to occur in energy-selected neutral species formed from near-resonant neutralization with alkali metal targets. While collisionally activated dissociation (CAD) is due to dissociation of activated ions with broad internal energy distributions, the charge inversion process using alkali metal targets is due to dissociation of energy-selected neutral species with narrow internal energy distributions. The charge inversion/alkali metal spectra provide clear differentiation of the isomeric cations of C(2)H(2), C(3)H(4) and dichlorobenzenes. The CAD spectra of these isomeric cations are similar.     

In situ Raman spectroscopy of surfaces modified by ion soft landing

The design and characterization of a system for in situ Raman analysis of surfaces prepared by ion soft landing (SL) is described. The performance of the new high vacuum compatible, low cost, surface analysis capability is demonstrated with surface enhanced Raman spectroscopy (SERS) of surfaces prepared by soft landing of ions of crystal violet, Rhodamine 6G, methyl orange and copper phthalocyanine. Complementary in situ mass spectrometric information is recorded for the same surfaces using a previously implemented secondary ion mass spectrometer (SIMS). Imaging of the modified surfaces is achieved using 2D Raman imaging as demonstrated for the case of Rhodamine 6G soft landing. The combination of the powerful molecular characterization tools of SERS and SIMS in a single instrument fitted with in-vacuum sample transport capabilities, facilitates in situ analysis of surfaces prepared by ion SL. In particular, information is provided on the charge state of the soft landed species. In the case of crystal violet the SERS data suggest that the positively charged ions being landed retain their charge state on the surface under vacuum. By contrast, in the case of methyl orange which is landed as an anion, the SERS spectra suggest that the SL species has been neutralized.     

New perspective of electron transfer chemistry

A new perspective of electron transfer chemistry is described for fine control of electron transfer reactions including back electron transfer in the charge separated state of artificial photosynthetic compounds and its synthetic application. Fundamental electron transfer properties of suitable components of efficient electron transfer systems are described in light of the Marcus theory of electron transfer, in particular focusing on the Marcus inverted region, and they are applied to design multi-step electron transfer systems which can well mimic the function of a photosynthetic reaction center. Both intermolecular and intramolecular electron transfer processes are finely controlled by complexation of radical anions, produced in the electron transfer, with metal ions which act as Lewis acids. Quantitative measures to determine the Lewis acidity of a variety of metal ions are given in relation to the promoting effects of metal ions on the electron transfer reactions. The mechanistic viability of metal ion catalysis in electron transfer reactions is demonstrated by a variety of examples of chemical transformations involving metal ion-promoted electron transfer processes as the rate-determining steps, which are made possible by complexation of radical anions with metal ions.     

Topography and field effects in secondary ion mass spectrometry Part II: insulating samples

A study is conducted on the effects of sample topography on the secondary ion mass spectrometry (SIMS) analysis of insulating samples, using poly(ethylene terephthalate) fibres (100 µm diameter) as a model system and simulations of the ion extraction field using finite element analysis. We focus on two significant issues: topographic field effects caused by the penetration of the extraction field into the sample, and the effect of charge compensation on the secondary ion images. Guidance is provided for setting the reflector voltage correctly for insulating fibres in reflectron SIMS instruments. The presence of the topographic sample distorts the extraction field, causing the secondary ions to be deflected laterally. This results in the severe loss of ion signals from the sides of the fibres because of the limited angular acceptance of the analyser. Strategies to reduce topographic field effects, including alternative sample mounting methods, are discussed. We also find that, in general, insulating samples are charged by the flood gun electrons resulting in a negative surface potential. This causes large variations in the SIMS images depending on the electron current, electron energy, raster mode and secondary ion polarity. Recommendations are given for analysts to obtain more reproducible images and reduce the effect of differential electron charging, for example by using a lower electron flood beam energy. © 2011 Crown copyright.     

A Mechanism for Ionization of Nonvolatile Compounds in Mass Spectrometry: Considerations from MALDI and Inlet Ionization

Mechanistic arguments relative to matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) address observations that predominately singly charged ions are detected. However, recently a matrix assisted laser ablation method, laserspray ionization (LSI), was introduced that can use the same sample preparation and laser as MALDI, but produce highly charged ions from proteins. In MALDI, ions are generated from neutral molecules by the photon energy provided to a matrix, while in LSI ions are produced inside a heated inlet tube linking atmospheric pressure and the first vacuum region of the mass spectrometer. Some LSI matrices also produce highly charged ions with MALDI ion sources operated at intermediate pressure or high vacuum. The operational similarity of LSI to MALDI, and the large difference in charge states observed by these methods, provides information of fundamental importance to proposed ionization mechanisms for LSI and MALDI. Here, we present data suggesting that the prompt and delayed ionization reported for vacuum MALDI are both fast processes relative to producing highly charged ions by LSI. The energy supplied to produce these charged clusters/droplets as well as their size and time available for desolvation are determining factors in the charge states of the ions observed. Further, charged droplets/clusters may be a common link for ionization of nonvolatile compounds by a variety of MS ionization methods, including MALDI and LSI.     

Highly charged ion induced nanostructures at surfaces by strong electronic excitations

Nanostructure formation by single slow highly charged ion impacts can be associated with high density of electronic excitations at the impact points of the ions. Experimental results show that depending on the target material these electronic excitations may lead to very large desorption yields in the order of a few 1000 atoms per ion or the formation of nanohillocks at the impact site. Even in ultra-thin insulating membranes the formation of nanometer sized pores is observed after ion impact. In this paper, we show recent results on nanostructure formation by highly charged ions and compare them to structures and defects observed after intense electron and light ion irradiation of ionic crystals and graphene. Additional data on energy loss, charge exchange and secondary electron emission of highly charged ions clearly show that the ion charge dominates the defect formation at the surface.     

The ion sensitivity of surface conductive single crystalline diamond

Charge build-up at the solid/aqueous interface is a ubiquitous phenomenon that determines the properties of interfacial electrical double layers. Due to its unique properties, the surface of diamond offers an attractive platform to investigate charging mechanisms in aqueous solutions. We investigate the surface charge by studying the ion sensitivity of H-terminated single crystalline diamond surface conductive layers. The effect of monovalent and divalent salts has been probed at different pH values. For a pH above 3.5, increasing the ionic strength results in a decrease of the surface conductivity, in contrast to the results obtained for pH below 3.5. Electrokinetic experiments are in good agreement with the surface conductivity measurements, showing an isoelectric point at pH 3.5 for the H-terminated diamond surface. We discuss the results in terms of the Coulombic screening by electrolyte ions of the surface potential, which is induced by a pH-dependent surface charge. The origin of this surface charge is discussed in terms of charge regulation by amphoteric hydroxyl surface groups and unsymmetrical adsorption of hydroxide and hydronium ions induced by the hydrophobic nature of the H-terminated diamond surface. This surface charge can have important consequences for processes governed by the diamond/aqueous interface, such as electron transfer to charged redox molecules, adsorption of charged molecules and proteins, and ion sensitivity.     

HRTEM-EELS cross-sectional structural analyses of glassy carbon substrate irradiated by platinum ions using a coaxial arc plasma gun

A glassy carbon (GC) substrate was irradiated by Pt ions using a coaxial arc plasma gun in vacuum. The structure of the substrate was studied in the atomic scale using high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) with a thin foil specimen prepared by cross-sectioning using focused ion beam milling. HRTEM combined with the EELS measurements of the cross-sectioned sample indicated a disordering of the GC substrate surface and the detection of phenol-type species, carbon atoms in aliphatic chain molecules, and carboxyl groups, which are considered to have been formed by chemical reaction between the carbon atoms of the GC substrate surface and hydrogen or oxygen atoms. Penetrated Pt atoms were observed inside the GC substrate, and the surface position of the GC substrate was located from the depth profiles of the EELS spectra. STEM-EELS analyses of a pristine GC substrate without Pt ion irradiation were also conducted for reference.     

Charging effects in an electron bombarded Ar matrix and the role of chemiluminescence-driven relaxation

The relaxation processes in pure and doped Ar films preirradiated by an electron beam are studied with the focus on charging effects. Correlated real time measurements have been performed applying current and optical activation spectroscopy methods. Thermally stimulated exoelectron emission and thermally stimulated luminescence are detected in the vacuum ultraviolet and visible range. An appreciable accumulation of electrons in the matrix is found, and prerequisites for negative space charge formation are ascertained. The part played by pre-existing and radiation-induced defects as well as dopants is considered and the temperature range of the electron trap stability is elucidated. It is shown that laser-induced electron detachment from O(-) centers results in an enhancement of electron detrapping via the chemiluminescence mechanism, viz. neutralized and thermally mobilized O atoms recombine. Formation of O(2)* results in the emission of visible photons. These photons act as a stimulating factor for electron release and transport, terminating in exoelectron emission and charge recombination. Chemiluminescence therefore plays an important role in the decay of charged centers.     

A time-dependent molecular orbital approach to electron transfer in ion–metal surface collisions

A time-dependent molecular orbital method has been developed to study charge transfer in collisions of ions with metal surfaces at energies between 1 and 100 au. A set of localized basis functions consisting of generalized Wannier functions for the surface and s- and p-atomic functions for the ion, is used to separate the system into primary and secondary regions. An effective Hamiltonian and time-dependent equations for the electron density matrix are obtained in the primary region, where most charge transfer occurs. The equations for the electron density matrix are solved with a linearization scheme. The method is suitable to study atomic orbital orientation for collisions of ions and surfaces. A model calculation for Na⁺ + W(110) collisions with a prescribed trajectory is presented. The interaction potentials between the W(110) surface and Na⁺ 3s and 3p orbitals are calculated from Na⁺ pseudopotentials. Results show that the yield of neutralized atoms in 3p states changes as the collision energy is lowered.     

Characterization and surface-reactivity of nanocrystalline anatase in aqueous solutions

The chemical and electrostatic interactions at mineral-water interfaces are of fundamental importance in many geochemical, materials science, and technological processes; however, the effects of particle size at the nanoscale on these interactions are poorly known. Therefore, comprehensive experimental and characterization studies were completed, to begin to assess the effects of particle size on the surface reactivity and charging of metal-oxide nanoparticles in aqueous solutions. Commercially available crystalline anatase (TiO2) particles were characterized using neutron and X-ray small-angle scattering, electron microscopy, and laser diffraction techniques. The 4 nm primary nanoparticles were found to exist almost exclusively in a hierarchy of agglomerated structures. Potentiometric and electrophoretic mobility titrations were completed in NaCl media at ionic strengths from (0.005 to 0.3) mol/kg, and 25 degrees C, with these two experimental techniques matched as closely as the different procedures permitted. The pH of zero net proton charge (pHznpc, from potentiometric titration) and isoelectric point pH value (pHiep, from electrophoretic mobility titrations) were both in near perfect agreement (6.85 +/- 0.02). At high ionic strengths the apparent pHznpc value was offset slightly toward lower pH values, which suggests some specific adsorption of the Na+ electrolyte ions. Proton-induced surface charge curves of nanocrystalline anatase were very similar to those of larger rutile crystallites when expressed relative to their respective pHznpc values, indicating that the development of positive and negative surface charge away from the pHznpc for nanocrystalline anatase is similar to that of larger TiO2 crystallites.     

The role of conformation on electron capture dissociation of ubiquitin

Effects of protein conformation on electron capture dissociation (ECD) were investigated using high-field asymmetric waveform ion mobility spectrometry (FAIMS) and Fourier-transform ion cyclotron resonance mass spectrometry. Under the conditions of these experiments, the electron capture efficiency of ubiquitin 6+ formed from three different solution compositions differs significantly, ranging from 51 +/- 7% for ions formed from an acidified water/methanol solution to 88 +/- 2% for ions formed from a buffered aqueous solution. This result clearly indicates that these protein ions retain a memory of their solution-phase structure and that conformational differences can be probed in an ECD experiment. Multiple conformers for the 7+ and 8+ charge states of ubiquitin were separated using FAIMS. ECD spectra of conformer selected ions of the same charge states differ both in electron capture efficiency and in the fragment ion intensities. Conformers of a given charge state that have smaller collisional cross sections can have either a larger or smaller electron capture efficiency. A greater electron capture efficiency was observed for ubiquitin 6+ that has the same collisional cross section as one ubiquitin 7+ conformer, despite the lower charge state. These results indicate that the shape of the molecule can have a greater effect on electron capture efficiency than either collisional cross section or charge state alone. The cleavage locations of different conformers of a given charge state were the same indicating that the presence of different conformers in the gas phase is not due to difference in where charges are located, but rather reflect conformational differences most likely originating from solution. Small neutral losses observed from the singly- and doubly-reduced ubiquitin 6+ do not show a temperature dependence to their formation, consistent with these ions being formed by nonergodic processes.     

Charge separation and ion-pair formation in cations subsequent to electron-impact

Investigation of metastable ion transitions shows, that spontaneous charge separations (loss of an electron from a cation) and ion-pair formations (loss of negative ions from positive ones)are taking place in conventional mass spectrometry. Such processes may be important factors in formation of doubly charged ions.     

Interpretation of static secondary ion spectra

A model is presented in which electron impact (EI)/electronic excitation plays a pivotal role in the formation of secondary ions in the SIMS experiment, especially those originating from discrete molecular species. Positive ions are formed by electron loss whereas negative ions are formed by electron capture. Collisions of the new ions with the surface and with other species directly above the sample, along with metastable decay events, reduce the number of odd electron ions detected and produce the changes that make SIMS spectra so different from EI mass spectra. Primary support for this model is gathered from static SIMS spectra themselves, which can be rationalized to a large degree by assuming that the same rearrangement and fragmentation mechanisms that are invoked to explain EI mass spectra take place at the surface after kiloelectron‐volt ion impacts. The static secondary ion spectra of a variety of simple discrete molecular species, of simple hydrocarbons, of monofunctional organic species and of more complicated multifunctional organic species are analyzed in this way and the utility of this model is demonstrated. Copyright © 2004 John Wiley & Sons, Ltd.     

Inelastic collision processes of low-energy protons in liquid water

Production probabilities of ions and excited particle species along the proton beam track in liquid water are estimated around the Bragg peak region, taking into account charge-changing processes and energetic secondary electron (δ-ray) behavior. Ionization and excitation processes are divided into two categories in this study: primary processes associated with direct proton (or hydrogen) interaction and secondary processes arising from the electrons ejected by the primary process. We show that the number of events in the secondary processes producing ions and excited particles is larger than that of the primary processes around the Bragg peak while neutralized protons (i.e., hydrogen) with low energy have a large contribution to direct ionization. Effects of charge-changing processes on ionization and excitation are also discussed.     

一种新型二次离子质谱的一次离子源及其离子光学系统

本研究设计了一种新型用于二次离子质谱的一次离子源及其离子光学系统.通过此一次离子源,大气压下产生的一次离子可以被加速、聚焦并传输到位于真空条件下的样品表面并电离样品得到可供质谱仪分析的二次离子.通过理论模拟结合实验系统研究了此一次离子源的主要组成部分——离子光学系统的原理、结构和性能.以电喷雾电离源为例,成功地将大气压...     

The time of flight static secondary negative ion mass spectra of poly(methylmethacrylate), poly(ethylmethacrylate), and poly(methylmethacrylate-co-ethylmethacrylate). Ion structures and quantification

Negative secondary ion mass spectra of poly(methylmethacrylate) (PMMA), poly(ethyl-methacrylate) (PEMA), and two copolymers of methylmethacrylate (MMA) and ethylmethacrylate (EMA) are presented. The spectra were obtained with a time of flight-static secondary ion mass spectrometer without charge compensation. The structures of typical ions in the mass range 20–220 amu are given, and the intensities of these ions were used to obtain some quantitative information about the ion formation processes. It is shown that quantification of a simple two-component system like the copolymers mentioned above is rather straightforward.     

Radiation-Induced Injection Currents in Polymer Layers Exposed to Radiation on Their Outer Side

Radiation-induced currents in polymer layers exposed to radiation on their outer side were theoretically analyzed under conditions when the role of secondary emission processes in establishing steady-state charging regimes is very insignificant (the potential of the open surface does not exceed one tenth of electron beam energy). The cases of both electron and hole conductivity of polymers were considered. The determining role of the frequency factor for the events of thermal detrapping of charge carriers in the choice of a model for describing the phenomenon in question was revealed. Previously published data on the issue are discussed and partly criticized.