Doubly-charged Ions Research Articles

Expansion of the radioactive ion beam program at Argonne

The Argonne Tandem Linear Accelerator System (ATLAS) at Argonne National Laboratory (ANL) provides a wide range of stable ion beams and radioactive beams which have contributed to our understanding of nuclear structure and reactions. Until now, most radioactive ion beams at ATLAS were produced in flight using light-ion reactions such as (p, n), (d, n), (d, p), (d,3He), and (3He,n). Within the next few months, the radioactive ion beam program at ATLAS will acquire much extended, new capabilities with the commissioning of a new facility: the CAlifornium Rare Isotope Breeder Upgrade (CARIBU). CARIBU will supply ion beams of 252Cf fission fragments, which are thermalized in a gas catcher. The singly- and doubly-charged ions extracted from the gas catcher will be mass-separated and either delivered to a low-energy experimental area, or charge bred with a modified ECR source and subsequently reaccelerated by the ATLAS facility. Properties of hundreds of these neutron-rich nuclides will be investigated using ion traps, decay stations, the newly commissioned HELical Orbit Spectrometer (HELIOS), and other available experimental equipment such as Gammasphere and the FMA. HELIOS was constructed to take advantage of rare ion beams, such as those provided by CARIBU, through light-ion transfer reactions in inverse kinematics, and represents a new approach to the study of direct reactions in inverse kinematics which avoids kinematic broadening. Experiments are currently being conducted with HELIOS, and first results with the d(28Si,p) and d(12B,p) reactions have shown excellent energy resolution.

785 nm femtosecond laser ablation for improved precision and reduction of interferences in Sr isotope analyses using MC-ICP-MS

The femtosecond (fs) laser ablation (LA) MC-ICP-MS Sr isotope analysis of calcium-rich materials is reported. Independent Sr isotope ratio determination using solution nebulization (SN) MC-ICP-MS after chromatographic separation of Sr was also conducted. Material studied includes a marine carbonate saggital otolith certified (for trace metals) reference material FEBS-1, and igneous fluorite. Analyses of the FEBS-1 using LA analyses at repetition rates of 5 to 200 Hz with energies from 0.1 to 0.6 mJ pulse−1 and SN analyses demonstrate that fs LA Sr isotope analysis produced similar within-run precision to the SN Sr isotope analysis when the same total number of ions was collected in an analysis. The results demonstrate that the precision for fs LA Sr isotope analyses is more controlled by Poisson counting statistics than for nanosecond (ns) LA analyses, and improvements (≥ 2 fold) in external precision, within-run precision, and accuracy can be obtained in comparison with reported ns LA-based results. Good external precision (1SD, n = 17) ± 0.000007, good within-run precision (1SE) ± 0.000010, as well as good accuracy (0.000006) determinations of Sr isotope ratios in carbonate materials can be obtained in situ with a spatial resolution of 60 to 90 μm when using a Faraday detector signal intensity at mass 88 of greater than 10 V and a total integration time of 151 s. In this contribution, it is further shown that an accurate 84Sr/86Sr ratio of 0.056560 ± 0.000015 to 0.056525 ± 0.000007 can be measured in fs LA Sr isotope analysis of FEBS-1. Previously reported Ca argide and Ca dimer polyatomic ion interferences were not observed to be significant in this study. In addition, this study reports the criteria necessary for effective interference corrections from doubly-charged ions of Er and Yb in LA Sr isotope analysis of fluorite samples, which have Sr concentrations > 650 ppm, and a rare-earth element (REE) Er/Sr or Yb/Sr concentration ratio of less than 0.01.

Gas-Phase Fragmentation Characteristics of Benzyl-Aminated Lysyl-Containing Tryptic Peptides

The fragmentation characteristics of peptides derivatized at the side-chain epsilon-amino group of lysyl residues via reductive amination with benzaldehyde have been examined using collision-induced dissociation (CID) tandem mass spectrometry. The resulting MS/MS spectra exhibit peaks representing product ions formed from two independent fragmentation pathways. One pathway results in backbone fragmentation and commonly observed sequence ion peaks. The other pathway corresponds to the unsymmetrical, heterolytic cleavage of the C(zeta)-N(epsilon) bond that links the benzyl derivative to the side-chain lysyl residue. This results in the elimination of the derivative as a benzylic or tropylium carbocation and a (n - 1)(+)-charged peptide product (where n is the precursor ion charge state). The frequency of occurrence of the elimination pathway increases with increasing charge of the precursor ion. For the benzyl-modified tryptic peptides analyzed in this study, peaks representing products from both of these pathways are observed in the MS/MS spectra of doubly-charged precursor ions, but the carbocation elimination pathway occurs almost exclusively for triply-charged precursor ions. The experimental evidence presented herein, combined with molecular orbital calculations, suggests that the elimination pathway is a charge-directed reaction contingent upon protonation of the secondary epsilon-amino group of the benzyl-derivatized lysyl side chain. If the secondary epsilon-amine is protonated, the elimination of the carbocation is observed. If the precursor is not protonated at the secondary epsilon-amine, backbone fragmentation persists. The application of appropriately substituted benzyl analogs may allow for selective control over the relative abundance of product ions generated from the two pathways.

Artificial neural networks for the prediction of peptide drift time in ion mobility mass spectrometry

BackgroundThere is an increasing usage of ion mobility-mass spectrometry (IMMS) in proteomics. IMMS combines the features of ion mobility spectrometry (IMS) and mass spectrometry (MS). It separates and detects peptide ions on a millisecond time-scale. IMS separates peptide ions based on drift time that is determined by the collision cross-section of each peptide ion in a given experiment condition. A peptide ion's collision cross-section is related to the ion size and shape resulted from the peptide amino acid sequence and their modifications. This inherent relation between the drift time of peptide ion and peptide sequence indicates that the drift time of peptide ions can be used to infer peptide sequence and therefore, for peptide identification.ResultsThis paper describes an artificial neural networks (ANNs) regression model for the prediction of peptide ion drift time in IMMS. Each peptide in this work was represented using three descriptors (i.e., molecular weight, sequence length and a two-dimensional sequence index). An ANN predictor consisting of four input nodes, three hidden nodes and one output node was constructed for peptide ion drift time prediction. For the model training and testing, a 10-fold cross-validation strategy was employed for three datasets each containing different charge states. Dataset one contains 212 singly-charged peptide ions, dataset two has 306 doubly-charged peptide ions, and dataset three has 77 triply-charged peptide ions. Our proposed method achieved 94.4%, 93.6% and 74.2% prediction accuracy for singly-, doubly- and triply-charged peptide ions, respectively.ConclusionsAn ANN-based method has been developed for predicting the drift time of peptide ions in IMMS. The results achieved here demonstrate the effectiveness and efficiency of the prediction model. This work can enhance the confidence of protein identification by combining with current database search approaches for protein identification.

Voltammetric determination of paraquat at DNA–gold nanoparticle composite electrodes

A novel electroanalytical method for the detection of paraquat using DNA modified gold nanoparticles immobilized at a gold electrode is demonstrated. The electrode surface was first modified using the self-assembly of gold nanoparticles (NPs) followed by the simple adsorption of DNA onto the NPs, which was straightforward, fast and cost effective. The DNA–nanoparticle composite sensor was then characterized in terms of electrochemical responses both in the absence and in the presence of paraquat using cyclic voltammetry, differential pulse voltammetry and square wave voltammetry. The DNA–NPs composite electrode proved to work adequately as a biosensor for the quantitative analysis of paraquat concentrations, taking advantage of utilizing both the modified gold nanoparticles and the interaction between DNA with paraquat molecules. In addition, the NPs modified electrode demonstrated good sensitivity and stability towards the first reversible reduction step of the double charged paraquat ion. Good linearity between paraquat concentration and peak current was observed for the concentration range of 5.0 × 10 −6 to 1.0 × 10 −3 M when using differential pulse voltammetry. The use of modified electrodes improves the performance of the biosensor in the presence of interfering species in particular when square wave voltammetry is used.

Two-electron emissions on atomic photoabsorption studied by multi-electron coincidence spectroscopy

Simultaneous two-electron emissions from core-valence double photoionization in Ne and upon core-electron excitation in Ar have been studied using a very efficient multi-electron coincidence method. The former case demonstrates that the population ratio of singlet and triplet doubly-charged ion states reveals the double photoionization dynamics. The latter case shows that a weakly-bound electron can be used as a probe of the double Auger dynamics of a core-hole. In both cases, the inadequacy of the shake-off model is revealed by the experimental observations alone and the necessity to describe better electron correlation for two-electron emission, as done in the knock-out model.

Measurement of Carrier-Envelope Phase and Field Strength of a Few-Cycle Pulse by Non-sequential Double Ionization

We propose a method to measure the carrier-envelop phase (CEP) and the intensity of a few-cycle pulse by controlling the non-sequential double ionization (NSDI) process. By using an additional static electric field, we can change the momentum distribution of the double-charged ions parallel to the laser polarization from an asymmetrical double-hump structure to a nearly symmetrical one. It is found that the ratio between the strength of the static electric field and that of the laser field is sensitive to the CEP but robust against the intensity fiuctuation. Therefore we can determine the CEP of a few-cycle pulse precisely by measuring the static electric field. Furthermore, if the CEP of the few-cycle pulse is fixed at a certain value, we can also calibrate the intensity of the laser pulse by the static electric field.

Spectroscopic Study of a Single Vacuum-Arc Cathode Spot

The cathode-spot plasma in spark and arc stages of a vacuum discharge was studied spectroscopically. A single spot was generated with high reproducibility in a gap under ultrahigh-vacuum conditions with a liquid-metal cathode of GaIn alloy. The self-breakdown of the vacuum gap resulting in a short discharge of less than 100 ns as well as a discharge over and up to 3 mus has been considered. The combination of a 0.5-m spectrograph with a streak camera enabled observation of spot evolution with a time resolution in the nanosecond range. Applying the streak camera as an image converter, time-integrated spectra resolved in the direction along the arc axis have been obtained. Limits concerning wavelength and time resolution as well as the emission intensity are discussed. Spectral lines of Ga and In atoms and single- and double-charged ions have been observed simultaneously. At the beginning of the discharge, ionic lines of higher charge state and wide line broadening dominate the spectrum. With a delay of several hundreds of nanoseconds, atomic lines appear and fall down in intensity to a much lesser degree than the ionic lines. Hence, atomic lines finally dominate in the arc stage of the discharge.

Spectral Accuracy of Molecular Ions in an LTQ/Orbitrap Mass Spectrometer and Implications for Elemental Composition Determination

In addition to mass accuracy, the ability of a mass spectrometer to faithfully measure the isotopic distribution of an ion, defined as spectral accuracy, is also important. Although time-of-flight mass spectrometers are reported to possess high spectral accuracy capability compared with other mass spectrometers, the Orbitrap has not yet been investigated. Ten natural products (moxidectin, erythromycin, digoxin, rifampicin, amphotericin B, rapamycin, gramicidin S, cyclosporin A, vancomycin, and thiostrepton) ranging in molecular weight from 639 to 1663 Da were measured on an LTQ/Orbitrap mass spectrometer with resolving power settings of 7.5, 15, 30, 60, and 100 K. The difference in the observed profile isotope pattern compared with the theoretical calculation after peak shape calibration, denoted spectral error, was calculated using the program MassWorks (Cerno Bioscience, Danbury, CT, USA). Spectral errors were least at 7.5 K resolving power (< or = 3%) but exceeded 10% for some compounds at 100 K. The increasing spectral error observed at higher resolving power for compounds with complex fine structure might be explained by the phenomena of isotopic beat patterns as observed in FTICR. Several compounds with prominent doubly charged ions allowed comparison of spectral accuracies of singly- versus doubly-charged ions. When using spectral error to rank elemental compositions with formula constraints (C(0-100)H(0-200)N(0-50)O(0-50)Cl(0-5)S(0-5)) and a mass tolerance < or = 2 parts-per-million, the correct formula was ranked first 35% of the time. However, spectral error considerations eliminated >99% of possible elemental formulas for compounds with molecular weight >900 Da.

Angular distributions of electrons in sequential two-photon double ionization of atoms by free electron laser radiation

Calculated results for angular distribution and angular correlation of electrons produced in the process of two-photon double ionization of Xe are presented. In particular, a peculiar dependence of the angular distribution of the second step electrons on the alignment of the intermediate ionic state is predicted. The angular correlations of the electrons qualitatively depend on the term of the residual double charged ion. The results can be used in ongoing and future experiments with free-electron laser.

Ion energy and mass distributions of the plasma during modulated pulse power magnetron sputtering

Modulated pulse power (MPP) sputtering is a variation of high power pulsed magnetron sputtering (HPPMS) that overcomes the rate loss issue and achieves enhanced plasma ionization through modulation of the pulse shape, intensity, and duration. In this study, the principle and characteristics of MPP/HPPMS technique are first introduced. An electrostatic quadrupole plasma mass spectrometer installed parallel to the target surface has been used to examine the plasma properties, including time averaged ion energy and mass distributions of the positive ions, generated during sputtering a metal Cr target in pure Ar and Ar/N 2 atmospheres using MPP and continuous dc power sources in a closed field unbalanced magnetron sputtering system. It was found that the MPP plasma exhibits a low ion energy peak at 1–2 eV and a short ion energy tail with the maximum ion energy affected by the peak current and power utilized on the cathode. A significantly increased numbers of single and double charged Cr and Ar ions were identified in the MPP plasma as compared to the dc plasma in pure Ar. The number of ions (ion flux) increased when the peak target power and current were increased. Besides single and double charged Cr, Ar and N ions, N 3 +, N 4 +, CrN + and CrN 2 + ion species were also identified in the MPP discharge with the introduction of N 2 into the system. The ion energy distributions of ion species for the MPP plasma in Ar/N 2 atmosphere exhibit similar peak values and tail distributions to those of the MPP plasma in pure Ar atmosphere. However, the energy tail extended toward higher energies due to the increased peak current and power on the cathode as the N 2 flow rate percentage was increased in the system.

Prediction of the doubly charged ion pattern by modelling the high- and low-resolution mass spectra of isotopomeric forms

The presence of doubly charged ions in mass spectra is detected only occasionally because their clusters are observed more rarely than singly charged ones. The patterns connected with doubly charged ions are located in the spectrum below M/2. The narrow shapes of such patterns as well as overlapping with other bands generate significant problems in their interpretation. The method described here is based on modelling of the isotopomeric form of single- and double-charged mass ion clusters. The present work attempts to explain the generation of the double charge disotopomeric patterns of high- as well as low-resolution spectra. Predicting the high-resolution mass cluster is simpler than calculations of the low-resolution cluster. The high-resolution cluster may represent the initial form of low-resolution pattern formation.

Sorption of silicic acid from non-saturated aqueous solution by a complex of zinc ions with poly(vinylamine)

Silicifying organisms (diatom algae, sponges, etc.) build multifarious silica and composite nano-structures which attracts rapt attention of specialists in nanotechnology. These organisms can capture monomeric silicic acid from diluted solutions (<1 mM) and molecular mechanism of this process is still unknown. Some hypotheses include mediatorial role of zinc ions coordinated with a special protein transporter. We have studied several basic hydrogels and their complexes with double charged metal ions in sorption of silicic acid from non-saturated solutions. Cross-linked poly(vinylamine) coordinated with Zn 2+ was able to capture silicic acid from 0.1 to 1.7 mM solutions. The other ions (Cu 2+, Fe 2+, Ni 2+, Co 2+, Cd 2+ and Mn 2+) and less basic poly(1-vinylimiazole) were not active in the sorption. A mechanism included oligomerization of silicic acid and formation of triple complex poly(vinylamine)-Zn 2+-oligosiloxane was proposed. FTIR, NMR and EDS data confirm this conclusion. The found capture of silicic acid from diluted solutions with polyamine–zinc complex shows possibility in principle of the zinc-mediated transmembrane transport of Si(OH) 4 in silicifying organisms. On the other hand, the ability of zinc ions immobilized on the polyamine support to capture oligosilicates could be used in directed assembly of nanocomposite structures by the “bottom-up” approaches.

Carrier-envelope phase dependence of non-sequential double ionization in few-cycle pulses

The influence of carrier-envelope phase (CEP) of a few-cycle laser pulse on non-sequential double ionization (NSDI) of an atom is investigated by applying the three-dimensional semi-classical re-scattering method. The asymmetric momentum distribution of the double-charged ion parallel to the laser polarization, which depends on the CEP, is explained by comparing the contributions from several half-cycles in the laser pulse. The ionization rate and the returning kinetic energy of the first electron dramatically affect the contributions from each half-cycles, and play different roles in different laser intensity regimes.

Plasma parameters in electrospark deposition of silver coatings

Spectroscopic measurements and mathematical simulations are conducted to study the composition and parameters of spark discharge in depositing coatings with silver electrodes using an Elitron-22 generator. The effective temperature and electron concentration are measured at the maximum pulse current. It is shown that the intensity of spectral lines of atoms and ions is proportional to the erosion rates of Ag anode and Ag-C composite anode. Pressure in the plasma channel is calculated taking into account the pinch effect (~1 MPa). It is shown that the temperature of the material in the anode spot reaches the boiling point of Ag under about 1 MPa (~3200-3300 K). A mathematical model is used to study the influence of erosion rate, power, and pressure on the plasma parameters and the intensity of spectral lines of anode materials. The method proposed allows evaluating the efficiency of material transfer in the vapor phase during spark deposition of coatings. A method for quantitative spectral determination of carbon in Ag-C material using the spectral lines of doublecharged ions of Ag and C is developed.

Inter-annual stability of oligopeptide patterns of Planktothrix rubescens blooms and mass mortality of Daphnia in Lake Hallwilersee

During mass developments of Planktothrix rubescens, the biomass of this cyanobacterium was collected over a period of four consecutive years (2002–2005) from Lake Hallwilersee, Switzerland. To avoid any shifts in analytical separation and sensitivity, the biomasses were extracted with 60% aqueous methanol at the end of the investigation period and were analysed within 1 week by LC-ESMS. A new mass spectrometric method to quantify oligopeptides was introduced. The sum of all major molecular species (quasi-molecular ion, double charged ion, adducts, dimers and molecular ions that had lost a water molecule) rather than just the signal of the quasi-molecular ion was used to determine the total abundance of oligopeptides. This procedure has become necessary because the variable presence of inorganic ions and the varying conditions of the mass spectrometric source strongly affect the formation of the different molecular species. Several anabaenopeptins, oscillapeptins and planktocyclins were found. [Asp 3, Dhb 7]microcystin-RR was the major microcystin. The oligopeptide patterns were relatively stable over the investigation period of 4 years. In June 2005, a mass mortality of Daphnia was observed. The dead Daphnia, which floated on the surface of the lake, were collected and analysed for oligopeptides. Planktocyclin and planktocyclin sulfoxide, which belong to the major cyclic peptides in P. rubescens, were found in the carcasses of Daphnia, but microcystins were missing. Live zooplankton of the epilimnion was represented by both Daphnia and copepods, while the patches of dead zooplankter on the lake surface were free of copepods and contained only Daphnia. Protease inhibitors rather than microcystins are discussed as the major bioactive compounds for grazer defence of P. rubescens.

The Effect of Phosphorylation on the Electron Capture Dissociation of Peptide Ions

The effect of site and frequency of phosphorylation on the electron capture dissociation of peptide ions has been investigated. The ECD of a suite of synthetic peptides (APLSFRGSLPKSYVK; one unmodified, three singly-phosphorylated, three-doubly phosphorylated, and one triply-phosphorylated); two tryptic phosphopeptides (YKVPQLEIVPNpSAEER, α-casein and FQpSEEQQQTEDELQDK, β-casein) and their unmodified counterparts, were determined over a range of ECD cathode potentials. The results show that, for doubly-charged precursor ions, the presence of phosphorylation has a deleterious effect on ECD sequence coverage. The fragmentation patterns observed suggest that for peptides with multiple basic residues, the phospho-groups exist in their deprotonated form and form salt-bridges with protonated amino acid side chains. The fragmentation observed for the acidic tryptic peptides suggested the presence of noncovalent interactions, which were perturbed on phosphorylation. Increasing the ECD electron energy significantly improves sequence coverage. Alternatively, improved sequence coverage can be achieved by performing ECD on triply-charged precursor ions. The findings are important for the understanding of gas-phase fragmentation of phosphopeptides.

Naturally Occurring Disulfide-bound Dimers of Three-fingered Toxins

Disulfide-bound dimers of three-fingered toxins have been discovered in the Naja kaouthia cobra venom; that is, the homodimer of alpha-cobratoxin (a long-chain alpha-neurotoxin) and heterodimers formed by alpha-cobratoxin with different cytotoxins. According to circular dichroism measurements, toxins in dimers retain in general their three-fingered folding. The functionally important disulfide 26-30 in polypeptide loop II of alpha-cobratoxin moiety remains intact in both types of dimers. Biological activity studies showed that cytotoxins within dimers completely lose their cytotoxicity. However, the dimers retain most of the alpha-cobratoxin capacity to compete with alpha-bungarotoxin for binding to Torpedo and alpha7 nicotinic acetylcholine receptors (nAChRs) as well as to Lymnea stagnalis acetylcholine-binding protein. Electrophysiological experiments on neuronal nAChRs expressed in Xenopus oocytes have shown that alpha-cobratoxin dimer not only interacts with alpha7 nAChR but, in contrast to alpha-cobratoxin monomer, also blocks alpha3beta2 nAChR. In the latter activity it resembles kappa-bungarotoxin, a dimer with no disulfides between monomers. These results demonstrate that dimerization is essential for the interaction of three-fingered neurotoxins with heteromeric alpha3beta2 nAChRs.

Serum proteomic profiling of obese patients: correlation with liver pathology and evolution after bariatric surgery

Objective:Chronic liver diseases, including cirrhosis, may develop in obese patients. Steatosis and non-alcoholic steatohepatitis (NASH) are risk factors for progression to fibrosis. To date, diagnosis of steatosis and NASH relies...

Electron Impact Ionization/Dissociation of Molecules: Production of Energetic Radical Ions and Anions

In order to provide quantitative information on electron collision processes involving various plasma constituents (in particular hydrocarbons) and to elucidate the properties of cations and anions produced we have carried out the past years a series of studies with a variety of techniques in our laboratory in Innsbruck. In the present review we will present some recent results on electron impact ionization and attachment in order to illustrate recent progress in this field in particular concerning the production of energetic fragment cations for hydrocarbons and differences in the attachment of isomers for nitro-organics. Using a Nier type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene, propene and other hydrocarbons have been measured for electron energies up to 1000 eV. Discrimination factors for ions have been determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover charge separation reactions (for instance in case of acetylene the Coulomb explosion of the doubly-charged parent ions C2H2++ into the fragment ions C2H+ and H+) are investigated by means of a number of metastable mass spectrometry methods and the associated mean kinetic energy release is deduced. Free electron attachment to the three different isomers of mono-nitrotoluene molecules in the gas phase is studied using two different crossed electron-molecule beams technique. In contrast to previous studies for a large number of negative ions in nitro-organic compounds the presently measured relative cross section curves are recorded with an electron energy resolution of better than 100 meV. For several product anions including the nitro anion NO2− remarkable differences for the three isomers are observed. For almost all fragment anion efficiency curves the 2-nitrotoluene exhibits pronounced differences compared to the two other isomers. In contrast, 3- and 4-nitrotoluene disagree only slightly in a few fragment anions from each other.