Selected Charge States Research Articles

The impact of the break-up of molecules in the stripper on the AMS performance

The break-up of molecules in the stripper of an AMS facility induces an energy and angular spread in the beam. This can cause beam losses, thereby reducing the performance of an AMS facility. With an appropriate design, these effects can be minimised and the efficiency enhanced. As an example, the break-up of BeO molecules is discussed here. First the basic physics is described and corresponding models and computer codes are explained. Then experimental data are shown and compared with the models. For gas stripping, beam profiles are available for small and large facilities. All show tails which can be attributed to the dissociation of the molecules. The size of the effect depends on various parameters such as beam energy, selected charge state, stripping gas type and ion optics. Foil stripping leads to energy distributions which are much wider, and show typically a double peak.

Charge-state independent anomalous transport for a wide range of different impurity species observed at Wendelstein 7-X

In this paper, the plasma volume averaged impurity confinement of selected charge states and impurity species has been characterized for the Stellarator Wendelstein 7-X (W7-X), covering a wide range of atomic charges (Z = 12–44) and atomic masses (M = 28–184). A comparison of the experimental findings to theoretical neoclassical and turbulent transport expectations suggests, aside from/in addition to the neoclassical transport, an additional significant anomalous transport mechanism, which is not inconsistent with the predictions of a turbulence dominated impurity transport and is in agreement with the experimental results from recent transport studies based on the direct measurements of impurity diffusion profiles, performed at W7-X.

Valet Parking for Protein Ion Charge State Concentration: Ion/Molecule Reactions in Linear Ion Traps.

There are several analytical applications in which it is desirable to concentrate analyte ions generated over a range of charge states into a single charge state. This has been demonstrated in the gas phase via ion/ion reactions in conjunction with a technique termed ion parking, which can be implemented in electrodynamic ion traps. Ion parking depends upon the selective inhibition of the reaction of a selected charge state or charge states. In this work, we demonstrate a similar charge state concentration effect using ion/molecule reactions rather than ion/ion reactions. The rates of ion/molecule reactions cannot be affected in the manner used in conventional ion parking. Rather, to inhibit the progression of ion/molecule proton transfer reactions, the product ions must be removed from the reaction cell as they are formed and transferred to an ion trap where no reactions occur. This is accomplished here with mass-selective axial ejection (MSAE) from one linear ion trap to another. The application of MSAE to inhibit ion/molecule reactions is referred to as "valet parking" as it entails the transport of the ions of interest to a remote location for storage. Valet parking is demonstrated using model proteins to concentrate ion signal dispersed over multiple charge states into largely one charge state. Additionally, it has been applied to a simple two-protein mixture of cytochrome c and myoglobin.

Charge stripping at high energy heavy ion Linacs

For each heavy-ion accelerator facility charge stripping is a key technology – the stripping charge state, its efficiency to produce ions in the selected charge state, and the beam quality after stripping substantially determine the entire accelerator performance. Modern heavy ion accelerator facilities such as the future Facility for Antiproton and Ion Research (FAIR) at GSI, Darmstadt, Germany provide for high-intensity heavy-ion beams beyond 200 MeV/u. Heavy ions generated in an ion source at comparatively low charge states are pre-accelerated in a Linac to a few MeV/u, after charge stripping the average charge state is increased and one (or several) ion charge states are selected for further acceleration. This enables more efficient acceleration up to the final beam energy, compared to acceleration of ions with a low charge state. C-foil stripping allows for highest mean charge state and best stripping efficiency into the desired charge state. Therefore minimum acceleration voltage could be expected utilizing C-foil stripping. Due to the high power deposited by the ions in the stripping media and radiation damages if solids are used, self-recovering stripper media must be used in any case. First layout scenarios for a 200 MeV/u heavy ion Linac considering efficient heavy ion stripping will be presented.

Gas-Phase Photoluminescence and Photodissociation of Silver-Capped Hexagold Clusters.

We report on the radiative and nonradiative deactivation pathways of selected charge states of the stoichiometric hexagold phosphine-stabilized ionic clusters, [(C)(AuDppy)6Ag2·(BF4) x](4- x)+ with x = 2 and 3 (Dppy = diphenylphosphino-2-pyridine), combining gas-phase photoluminescence and photodissociation with quantum chemical computations. These clusters possess an identical isostructural core made of a hyper-coordinated carbon at their center octahedrally surrounded by six gold ions, and two silver ions at their apexes. Their luminescence and fragmentation behavior upon photoexcitation was investigated under mass and charge control in an ion trap. The experimental and computational results shed light on the electronic states involved in the optical transitions as well as on their core, ligand, or charge transfer character. Gas-phase results are discussed in relation with condensed phase measurements, as well as previous observations in solution and on metal-organic frameworks. The monocationic species ( x = 3) is found to be less stable than the dicationic one ( x = 2). In the luminescence spectrum of the monocationic species, a shoulder at short wavelength can be observed and is assigned to fragment emission. This fragment formation appears to be favored for the monocation by the existence of a low lying singlet state energetically overlapping with the triplet state manifold, which is populated quickly after photoexcitation.

Structural characterization of nucleotide 5'-triphosphates by infrared ion spectroscopy and theoretical studies.

The molecular family of nucleotide triphosphates (NTPs), with adenosine 5'-triphosphate (ATP) as its best-known member, is of high biochemical importance as their phosphodiester bonds form Nature's main means to store and transport energy. Here, gas-phase IR spectroscopic studies and supporting theoretical studies have been performed on adenosine 5'-triphosphate, cytosine 5'-triphosphate and guanosine 5'-triphosphate to elucidate the intrinsic structural properties of NTPs, focusing on the influence of the nucleobase and the extent of deprotonation. Mass spectrometric studies involving collision induced dissociation showed similar fragmentation channels for the three studied NTPs within a selected charge state. The doubly charged anions exhibit fragmentation similar to the energy-releasing hydrolysis reaction in nature, while the singly charged anions show different dominant fragmentation channels, suggesting that the charge state plays a significant role in the favorability of the hydrolysis reaction. A combination of infrared ion spectroscopy and quantum-chemical computations indicates that the singly charged anions of all NTPs are preferentially deprotonated at their β-phosphates, while the doubly-charged anions are dominantly αβ-deprotonated. The assigned three-dimensional structure differs for ATP and CTP on the one hand and GTP on the other, in the sense that ATP and CTP show no interaction between nucleobase and phosphate tail, while in GTP they are hydrogen bonded. This can be rationalized by considering the structure and geometry of the NTPs where the final three dimensional structure depends on a subtle balance between hydrogen bond strength, flexibility and steric hindrance.

Measurements of the spatial magnetic field distribution in a z-pinch plasma throughout the stagnation process

We report on measurements, made for the first time for an imploding plasma at its stagnation, of the magnetic field spatial distribution. Utilized for the measurements is a spectroscopic technique based on simultaneous recording of each of the left- and right-handed circularly polarized Zeeman emissions. While this method allows for overcoming the Stark- and Doppler-broadenings that obscure the Zeeman splitting, it requires a line of sight that is parallel to the magnetic field lines. To this end, the radial charge-state distribution in the stagnating z-pinch plasma was measured, and the method was employed for emission lines of several selected charge states located in various radial locations. This also allowed for measuring the time-resolved magnetic field radial distribution in a single discharge, which is advantageous for high-energy-density plasma experiments. These distributions were measured at various locations along the pinch axis.

Conducting ion tracks generated by charge-selected swift heavy ions

Conducting ion tracks in tetrahedral amorphous carbon (ta-C) thin films were generated by irradiation with swift heavy ions of well-defined charge state. The conductivity of tracks and the surface topography of the films, showing characteristic hillocks at each track position, were investigated using conductive atomic force microscopy measurements. The dependence of track conductivity and hillock size on the charge state of the ions was studied using 4.6MeV/u Pb ions of charge state 53+, 56+ and 60+ provided by GANIL, as well as 4.8MeV/u Bi and Au ions of charge state from 50+ to 61+ and 4.2MeV/u 238U ions in equilibrium charge state provided by UNILAC of GSI. For the charge state selection at GSI, an additional stripper-foil system was installed at the M-branch that now allows routine irradiations with ions of selected charge states. The conductivity of tracks in ta-C increases significantly when the charge state increases from 51+ to 60+. However, the conductivity of individual tracks on the same sample still shows large variations, indicating that tracks formed in ta-C are either inhomogeneous or the conductivity is limited by the interface between ion track and Si substrate.

Dual stoichiometry and subunit organization in the ClpP1/P2 protease from the cyanobacterium Synechococcus elongatus.

The Clp protease is conserved among eubacteria and most eukaryotes, and uses ATP to drive protein substrate unfolding and translocation into a chamber of sequestered proteolytic active sites. To investigate the proteolytic core of the ClpXP1/P2 protease from the cyanobacterium Synechococcus elongatus we have used a non-denaturing mass spectrometry approach. We show that the proteolytic core is a double ring tetradecamer consisting of an equal number of ClpP1 and ClpP2 subunits with masses of 21.70 and 23.44 kDa, respectively. Two stoichiometries are revealed for the heptameric rings: 4ClpP1 + 3ClpP2 and 3ClpP1 + 4ClpP2. When combined in the double ring the stoichiometries are (4ClpP1 + 3ClpP2) + (3ClpP1 + 4ClpP2) and 2 × (3ClpP1 + 4ClpP2) with a low population of a 2 × (4ClpP1 + 3ClpP2) tetradecamer. The assignment of the stoichiometries is confirmed by collision-induced dissociation of selected charge states of the intact heptamer and tetradecamer. Presence of the heterodimers, heterotetramers and heterohexamers, and absence of the mono-oligomers, in the mass spectra of the partially denatured protease indicates that the ring complex consists of a chain of ClpP1/ClpP2 heterodimers with the ring completed by an additional ClpP1 or ClpP2 subunit.

10Be detection at the new AMS beam line at CEDAD: Performance tests and first results

A new AMS (Accelerator Mass Spectrometry) beam line has been recently installed at CEDAD (Centre for Dating and Diagnostics)-university of Salento, Lecce, Italy for the detection by AMS of rare isotopes such as 10Be, 26Al, 129I and actinides. In this paper the performances of the new line for the measurement of 10Be are presented. Different tests were carried out in order to determine proper operating parameters and experimental conditions such as acceleration voltage, selected charge states and silicon nitride degrader thickness. The obtained results allow to assess the good performances of the new spectrometer in terms of boron suppression and background levels.

Electron capture and deprotonation processes observed in collisions between Xe8+and multiply protonated cytochrome-C

Electron-transfer processes in interaction between highly charged ions and multiply protonated proteins have been studied. Collisions between Xe${}^{8+}$ at 96 keV and protonated cytochrome-C at selected charge state ($q$ from 15+ to 19+) result in mass spectra composed mainly of intact molecular ions. From the spectra, single and double electron capture processes by Xe${}^{8+}$ from the protonated molecular ions were identified and the relative cross sections were measured. An unexpected process, the deprotonation process, was also observed. It is tentatively attributed to the loss of a proton induced by the strong electric field carried by the projectile ion in long-distance collisions. Upon charge variation of the molecular target from 15 to 19, the single and double electron capture cross sections remain nearly constant, while the relative cross section of the deprotonation process increases dramatically from 0.8% (\ifmmode\pm\else\textpm\fi{}0.1%) to 17% (\ifmmode\pm\else\textpm\fi{}1%). This strong charge dependency is explained by the decrease of the proton affinities with the charge. This proton removal process has not been observed previously. It seems to be specific to the long-distance Coulomb interactions between protons bound along the protein chain and the highly charged atomic ions.

Extracted fragment ion mobility distributions: A new method for complex mixture analysis

A new method is presented for constructing ion mobility distributions of precursor ions based upon the extraction of drift time distributions that are monitored for selected fragment ions. The approach is demonstrated with a recently designed instrument that combines ion mobility spectrometry (IMS) with ion trap mass spectrometry (MS) and ion fragmentation, as shown in a recent publication [J. Am. Soc. Mass Spectrom. 22 (2011) 1477–1485]. Here, we illustrate the method by examining selected charge states of electrosprayed ubiquitin ions, an extract from diesel fuel, and a mixture of phosphorylated peptide isomers. For ubiquitin ions, extraction of all drift times over small mass-to-charge ( m/ z) ranges corresponding to unique fragments of a given charge state allows the determination of precursor ion mobility distributions. A second example of the utility of the approach includes the distinguishing of precursor ion mobility distributions for isobaric, basic components from commercially available diesel fuel. Extraction of data for a single fragment ion is sufficient to distinguish the precursor ion mobility distribution of cycloalkyl-pyridine derivatives from pyrindan derivatives. Finally, the method is applied for the analysis of phosphopeptide isomers (LFpTGHPESLER and LFTGHPEpSLER) in a mixture. The approach alleviates several analytical challenges that include separation and characterization of species having similar (or identical) m/ z values within complex mixtures.

Catching Proteins in Liquid Helium Droplets

An experimental approach is presented that allows for the incorporation of large mass-to-charge ratio selected ions in liquid helium droplets. It is demonstrated that droplets can be efficiently doped with a mass-to-charge ratio selected amino acid as well as with the much bigger m ≈ 12,000 amu protein cytochrome C in selected charge states. The sizes of the ion-doped droplets are determined via electrostatic deflection. Under the experimental conditions employed, the observed droplet sizes are very large and range, depending on the incorporated ion, from 10¹⁰ helium atoms for protonated phenylalanine to 10¹² helium atoms for cytochrome C. As a possible explanation, a simple model based on the size and internal energy dependence of the pickup efficiency is given.

‘Top-down’ characterization of site-directed mutagenesis products of Staphylococcus aureus dihydroneopterin aldolase by multistage tandem mass spectrometry in a linear quadrupole ion trap

The gas-phase fragmentation reactions of a series of site-directed mutagenesis products of Staphylococcus aureus dihydroneopterin aldolase have been examined by multistage tandem mass spectrometry (MS/MS and MS(3)) in a linear quadrupole ion trap in order to explore the utility of this instrumentation for routine 'top-down' recombinant protein characterization. Following a rapid low resolution survey of the fragmentation behavior of the precursor ions from the wild type (WT) protein, selected charge states were subjected to detailed structural characterization by using high resolution 'zoom' and 'ultrazoom' resonance ejection MS/MS product ion scans. Dissociation of the [M + 18H](18+) charge state yielded a range of product ions from which extensive sequence information could be derived. In contrast, dissociation of the [M + 20H](20+) charge state resulted in a single dominant y(96) product ion formed by fragmentation between adjacent Ile/Gly residues, with only limited sequence coverage. Further extensive sequence information was readily obtained however, by MS(3) dissociation of this initial product. From the combined MS/MS and MS(3) spectra an overall sequence coverage of 66.9%, with fragmentation of 85 of the 127 amide bonds within the WT protein, was obtained. MS/MS and MS(3) of three of the four site-directed mutagenesis products (E29A), (Y61F) and (E81A) were found to yield essentially identical product ion spectra to the WT protein, indicating that these modifications had no significant influence on the fragmentation behavior. The specific site of modification could be unambiguously determined in each case by characterization of product ions resulting from fragmentation of amide bonds on either side of the mutation site. In contrast, MS/MS and MS(3) of the K107A mutant led to significantly different product ion spectra dominated by cleavages occurring N-terminal to proline, which restricted the ability to localize the modification site to within only an 8 amino acid region of the sequence. This work highlights the need for further studies to characterize the charge state, sequence and structural dependence to the low energy collision induced dissociation reactions of multiply protonated intact protein ions.

Charge-state selective fragmentation analysis for protonated peptides in infrared multiphoton dissociation

We investigate the charge-state selective cleavage in gas-phase protonated peptides by using electrospray ionization (ESI) Fourier-transform ion cyclotron resonance (FTICR) mass spectrometer (MS). The singly and multiply protonated peptides (Angiotensin II, Angiotensin I, Urotensin II, Bradykinin, Substance P) at the selected charge state were cleaved with the techniques of infrared multiphoton dissociation (IRMPD) in FTICR MS. Systematically changing IR laser power, the fragment ions were assessed to determine the cleaved amino bonds in the peptides at the selected charge state, then to quantitatively obtain the dissociation efficiency for each fragment. The results show that the fragment ions are observed at the selective cleavage of Asp-Xaa in singly charged Angiotensin I, Angiotensin II and Urotensin II ions that contain an acidic residue (Asp) and a basic residue (Arg or Lys), while the fragment ions arising from cleavage at Xaa-Pro dominate in those doubly charged peptides. It is observed that the dissociation channel of Asp-Xaa requires higher energy than that of Xaa-Pro. The charge selective fragmentation is not seen for Bradykinin and Substance P containing strong basic amino residues without an acidic residue.

Ion parking during ion/ion reactions in electrodynamic ion traps.

Under appropriate ion density conditions, it is possible to selectively inhibit rates of ion/ion reactions in a quadrupole ion trap via the application of oscillatory voltages to one or more electrodes of the ion trap. The phenomenon is demonstrated using dipolar resonance excitation applied to the end-cap electrodes of a three-dimensional quadrupole ion trap. The application of a resonance excitation voltage tuned to inhibit the ion/ion reaction rate of a specific range of ion mass-to-charge ratios is referred to as "ion parking". The bases for rate inhibition are (i) an increase in the relative velocity of the ion/ion reaction pair, which reduces the cross section for ion/ion capture and, at least in some cases, (ii) reduction in the time of physical overlap of positively charged and negatively charged ion clouds. The efficiency and specificity of the ion parking experiment is highly dependent upon ion densities, trapping conditions, ion charge states, and resonance excitation conditions. The ion parking experiment is illustrated herein along with applications to the concentration of ions originally present over a range of charge states into a selected charge state and in the selection of a particular ion from a set of ions derived from a simple protein mixture.

A novel tracer-gas injection system for scrape-off layer impurity transport and screening experiments

The design, operation, and initial results of a novel tracer-gas injection system is described. This system has been developed to diagnose local impurity transport and screening properties as a function of depth into the scrape-off layer (SOL) of Alcator C-Mod. A fast-scanning Langmuir–Mach probe has been outfitted with a capillary gas-feed and an inertially activated valve. Impurity injection plumes lasting 5–10 ms can be formed at any depth into the SOL, up to the separatrix location. The local dispersion of selected charge state impurities in the SOL can be followed by camera imaging and Doppler spectroscopy. In principle, the local screening properties of the SOL can also be directly assessed by injecting gas at different depths into the SOL and observing the resultant impurity concentrations in the core.

A new technique for decomposition of selected ions in molecule ion reactor coupled with ortho-time-of-flight mass spectrometry.

A molecule ion reactor (MIR), i.e. a gas filled radio-frequency only quadrupole with a longitudinal electrical field (RFQLEF), is used as an atmospheric pressure ionization interface for an orthogonal time-of-flight mass spectrometer (O-TOFMS). A new phenomenon of selective ion 'heating' in a MIR near Mathieu's instability threshold was found and confirmed by computer simulation. The 'heating' in collisions with buffer gas molecules leads to ion decomposition. In the case of multicharged ions, fragments with an m/z value higher than that of the parent ion have a stable motion and can be analysed by an O-TOFMS. Fragmentation of bradykinin and beta-endorphin molecular ions having a selected charge state is demonstrated. The spectra show clear 'ladder' structure. The phenomenon may be used as an alternative to tandem mass spectrometry (MS/MS) for molecule structure analysis.

Absolute total, one-, two-, and three-electron-transfer cross sections forArq+(8≤q≤16) on Ar at 2.3qkeV

Absolute values for the total, one-, two-, and three-electron-transfer cross sections for ${\mathrm{Ar}}^{\mathit{q}+}$ ions (8\ensuremath{\le}q\ensuremath{\le}16) colliding with argon at 2.3q keV laboratory energy were measured by the growth-rate method. The ${\mathrm{Ar}}^{\mathit{q}+}$ ions were produced by the Cornell superconducting solenoid, cryogenic electron-beam ion source (CEBIS) and extracted at 2.3 kV. Selected charge states traversed a gas cell, after which they were detected and charge-state analyzed by the energy-retardation method and by a \ensuremath{\pi}/ \ensuremath{\surd}2 cylindrical electrostatic analyzer. The target gas pressure in the cell was measured directly by the orifice flow method used for absolute pressure-gauge calibration. The overall error in the ${\mathrm{Ar}}^{\mathit{q}+}$ on Ar cross-section measurements is \ifmmode\pm\else\textpm\fi{}10%.

Absolute total and one- and two-electron transfer cross sections for Arq+ (8

Absolute values for the total and one- and two-electron transfer cross sections for ${\mathrm{Ar}}^{\mathit{q}+}$ ions (8\ensuremath{\le}q\ensuremath{\le}16) colliding with helium and molecular hydrogen at 2.3q keV laboratory energy were measured by the growth-rate method. The He and ${\mathrm{H}}_{2}$ total cross sections as a function of Ar-projectile L-shell occupation number increase monotonically from ${\mathrm{Ar}}^{8+}$, 2p shell full, to ${\mathrm{Ar}}^{16+}$, 2s shell empty. The ${\mathrm{H}}_{2}$ one-electron capture cross section scales approximately as [${\mathit{E}}_{\mathrm{IP}}$ (He)/${\mathit{E}}_{\mathrm{IP}}$(${\mathrm{H}}_{2}$)${]}^{2}$ times the corresponding He cross section, but the shapes of the two cross sections differ in detail. The ${\mathrm{Ar}}^{\mathit{q}+}$ ions were produced by the Cornell superconducting-solenoid, cryogenic electron-beam ion source (CEBIS) and extracted at 2.3 kV. Selected charge states traversed a gas cell, after which they were detected and charge-state analyzed by the energy-retardation method and by a \ensuremath{\pi}/ \ensuremath{\surd}2 cylindrical electrostatic analyzer. The target-gas pressure in the cell was measured directly by the orifice-flow method used for absolute-pressure gauge calibration. The overall error in the ${\mathrm{Ar}}^{\mathit{q}+}$ on ${\mathrm{H}}_{2}$ cross-section measurements is \ifmmode\pm\else\textpm\fi{}10%, and \ifmmode\pm\else\textpm\fi{}15% in the He measurements.