Metastable Ions Research Articles

Complementary b/y fragment ion pairs from post-source decay of metastable YahO for calibration of MALDI-TOF-TOF-MS/MS

Complementary b/y fragment ion pairs from post-source decay (PSD) of metastable YahO protein ion were evaluated for use in the calibration of MALDI-TOF-TOF for tandem mass spectrometry (MS/MS). The yahO gene from pathogenic Escherichia coli O157:H7 strain EDL933 was cloned into a pBAD18 plasmid vector and transferred to a yahO gene knockout (ΔyahO) of non-pathogenic Escherichia coli strain K-12. YahO protein was over-expressed by overnight culturing of the K-12 mutant on Luria-Bertani agar (LBA) supplemented with arabinose. MS/MS-PSD of the YahO protein ion results in polypeptide backbone cleavage on the C-terminal side of seven aspartic acid (D) residues of the mature protein sequence resulting in a series of prominent b/y fragment ion pairs. Interestingly, we observe an unusual oscillatory pattern in the fragmentation efficiency of cleavage sites across the linear polypeptide chain of YahO. The instrument was calibrated for MS/MS-PSD using at least six of twelve of the most intense b- and y-type fragment ions of YahO. Calibration was tested on the disulfide bond-reduced B-subunit of Shiga toxin 2 from Escherichia coli O157:H7 strain EDL933 cultured overnight on LBA supplemented with ciprofloxacin. Two non-simultaneous calibrant/analyte acquisition scenarios were evaluated: 1. YahO and B-subunit on separate (but adjacent) spot locations; 2. YahO and the B-subunit co-located on the same spot. We observed no significant difference in mass accuracy for co-location vs. adjacent spot analysis in spite of a significant drop in ion intensity of both calibrant and analyte for co-location analysis.

Silver Ion Polyelectrolyte Container as a Sensitive Quartz Crystal Microbalance Gas Detector.

A polyelectrolyte film containing metastable silver ions was applied as a quartz crystal microbalance (QCM) gas detector. The polyelectrolyte film was obtained by immersing a polyelectrolyte with numerous amine groups in a metal ion solution. The QCM detector with silver ions responded to a very low methylmercaptan gas concentration (20 ppb) but did not respond to ammonia, volatile amines, aromatic compounds, or alcohols. The response speed of the QCM detector increased gradually with increasing methylmercaptan concentrations. The highly sensitive and selective response is promoted by a ligand substitution reaction caused by the formation of coordinative bonds between a metastable silver ion and amine groups in the polyelectrolyte film. To the best of our knowledge, this system has the highest sensitivity among reported QCM gas detectors. Such high-sensitivity among reported QCM gas detectors. Such high-sensitivity gas detectors for volatile sulfur compounds have wide ranging applications in areas such as volcanic eruption prediction, food inspection, environmental analysis, and medical diagnostics.

Where Does the Electron Go? Stable and Metastable Peptide Cation Radicals Formed by Electron Transfer.

Electron transfer to doubly and triply charged heptapeptide ions containing polar residues Arg, Lys, and Asp in combination with nonpolar Gly, Ala, and Pro or Leu generates stable and metastable charge-reduced ions, (M + 2H)+●, in addition to standard electron-transfer dissociation (ETD) fragment ions. The metastable (M + 2H)+● ions spontaneously dissociate upon resonant ejection from the linear ion trap, giving irregularly shaped peaks with offset m/z values. The fractions of stable and metastable (M + 2H)+● ions and their mass shifts depend on the presence of Pro-4 and Leu-4 residues in the peptides, with the Pro-4 sequences giving larger fractions of the stable ions while showing smaller mass shifts for the metastables. Conversion of the Asp and C-terminal carboxyl groups to methyl esters further lowers the charge-reduced ion stability. Collisional activation and photodissociation at 355 nm of mass-selected (M + 2H)+● results in different dissociations that give sequence specific MS3 spectra. With a single exception of charge-reduced (LKGLADR + 2H)+●, the MS3 spectra do not produce ETD sequence fragments of the c and z type. Hence, these (M + 2H)+● ions are covalent radicals, not ion-molecule complexes, undergoing dramatically different dissociations in the ground and excited electronic states. The increased stability of the Pro-4 containing (M + 2H)+● ions is attributed to radicals formed by opening of the Pro ring and undergoing further stabilization by hydrogen atom migrations. UV-VIS photodissociation action spectroscopy and time-dependent density functional theory calculations are used in a case in point study of the stable (LKGPADR + 2H)+● ion produced by ETD. In contrast to singly-reduced peptide ions, doubly reduced (M + 3H)+ ions are stable only when formed from the Pro-4 precursors and show all characteristics of even electron ions regarding no photon absorption at 355 nm or ion-molecule reactions, and exhibiting proton driven collision induced dissociations. Graphical Abstract ᅟ.

Carbon dust particles in a beam-plasma discharge

This paper focuses on dynamics of micro-sized carbon dust grains in beam-plasma discharge (BPD) plasmas. It was demonstrated that injected dust particles can be captured and transported along the discharge. Longitudinal average velocity of the particles in the central area of the plasma column was 17 m/sec, and 2 m/sec in the periphery. Dust injection caused a decrease of emission intensity of metastable nitrogen molecular ion. This effect is suggested for a spectroscopy method for particles’ potential measurements. Five-micron radius carbon dust grains obtained potential above 500 V in the experiments on PR-2 installation, proving the feasibility of BPDs for the charging of fine dust particles up to high potential values, unattainable in similar plasma conditions.

Porous Transition of Polyelectrolyte Film through Reaction-Induced Phase Separation Caused by Interaction with Specific Metal Ions.

We describe a novel method for the simple and eco-friendly fabrication of porous polyelectrolyte films. A polyelectrolyte with many amine groups undergoes structural transformation from a dense to a porous structure upon immersion in a specific metal ion solution. The porous transition was the result of a reaction-induced phase separation, which was caused by the formation of new bonds between the polyelectrolyte and metal ions. This method enables control of the pore size of the porous structure in the nanoscale (54 nm) to microscale (1.63 μm) range through variation of the concentration or type of metal ions in the solution. To the best of our knowledge, this is the first report illustrating wide-range control of the pore size of a porous polyelectrolyte structure achieved by metal ions. These porous polyelectrolyte films with adjustable pore size and metastable metal ions can be employed in applications such as adsorption and catalysis.

2-Benzylindane radical cations in the gas phase (Part II): Substituent effects on regio- and stereoselective progressive hydrogen scrambling occurring prior to unimolecular fragmentation

In the present and in the preceding article, the unimolecular fragmentation of the radical cations of 2-benzylindane and eleven derivatives bearing meta- or para-substituents at the benzylic moiety have been studied with respect to the hydrogen exchange that precedes their fragmentation by the McLafferty reaction. Extensive regio- and stereospecific deuterium labelling confirmed that the hydrogen exchange involves the two cis-oriented H atoms at the benzylic C-1 and C-3 positions of the indanyl moiety and the two ortho-H atoms of the benzyl group. This 4-H exchange is perfectly stereoselective for the most electron-rich congeners [X=OCH3,N(CH3)2 and 3′,5′-(OCH3)2] but less stereoselective for the other analogs. Energy- and lifetime-dependent measurements with the deuterium-labelled isotopologs revealed that the extent of H/D exchange progresses with the ions’ lifetime much faster in the meta- than in the para-isomers. With exception of the meta-dimethylamino derivative, equilibration (complete scrambling) of the four H atoms is reached in the long-lived, metastable ions, whereas the para-dimethylamino isomer does not undergo any exchange at all. The relative rate of the H/D exchange is rationalized on the basis of the energy profiles calculated for the reversible γ-H transfer that generates the corresponding distonic arenium-ion intermediates and for the overall fragmentation by the McLafferty reaction. The ipso-protonolysis of the benzyl residue giving rise to the loss of the corresponding arenes specifically from some of the para-isomers was also found to reflect the regio- and stereoselective 4-H exchange.

Simulation of nonstationary phenomena in atmospheric-pressure glow discharge

Nonstationary processes in atmospheric-pressure glow discharge manifest themselves in spontaneous transitions from the normal glow discharge into a spark. In the experiments, both so-called completed transitions in which a highly conductive constricted channel arises and incomplete transitions accompanied by the formation of a diffuse channel are observed. A model of the positive column of a discharge in air is elaborated that allows one to interpret specific features of the discharge both in the stationary stage and during its transition into a spark and makes it possible to calculate the characteristic oscillatory current waveforms for completed transitions into a spark and aperiodic ones for incomplete transitions. The calculated parameters of the positive column in the glow discharge mode agree well with experiment. Data on the densities of the most abundant species generated in the discharge (such as atomic oxygen, metastable nitrogen molecules, ozone, nitrogen oxides, and negative oxygen ions) are presented.

Behavior of molecules and molecular ions near a field emitter**This work is a partial contribution of the US Government and therefore is not subject to copyright in the United States.

The cold emission of particles from surfaces under intense electric fields is a process which underpins a variety of applications including atom probe tomography (APT), an analytical microscopy technique with near-atomic spatial resolution. Increasingly relying on fast laser pulsing to trigger the emission, APT experiments often incorporate the detection of molecular ions emitted from the specimen, in particular from covalently or ionically bonded materials. Notably, it has been proposed that neutral molecules can also be emitted during this process. However, this remains a contentious issue. To investigate the validity of this hypothesis, a careful review of the literature is combined with the development of new methods to treat experimental APT data, the modeling of ion trajectories, and the application of density-functional theory simulations to derive molecular ion energetics. It is shown that the direct thermal emission of neutral molecules is extremely unlikely. However, neutrals can still be formed in the course of an APT experiment by dissociation of metastable molecular ions.

Development of a Laser Induced Fluorescence (LIF) system on thePlasma Material Interaction System (PLAMIS-II) device

A laser induced fluorescence (LIF) system has been developed for the plasma material interaction system (PLAMIS-II) device, which is equipped with a unique plasma gun composed of a LaB6 cathode and two anodes with electromagnets to generate a focused dense plasma. PLAMIS-II simulates the interactions of plasma with different materials and is to be used for the test of plasma facing components of fusion devices. The LIF system is composed of a seed laser with Littmann/Metcalf cavity and a master oscillator power amplifier to pump 3d4F7/2 metastable argon ion to 4p4D5/2 level at the wavelength of 668.61 nm, which has the following input parameters: laser power = 20 mW, line width < 100 kHz, and a mode-hop free tuning range > 70 GHz. For in-situ measurement of laser wavelength, the wavelength spectrum of an iodine cell was measured by a photo-transistor during LIF measurement. To measure argon ion temperature (Ti) and drift velocity (vd) in PLAMIS-II, the fluorescence light with the wavelength of 442.72 nm, emitted from 4p4D5/2 level to 4s4P3/2 level and passing through 1 nm band-width filter, was collected by the photomultiplier tube combined with a lock-in amplifier and a chopper with frequency of 3 kHz. Initial data of Ti and vd were analysed in terms of gas flow rate and applied power.

N-Acetylglycine Cation Tautomerization Enabled by the Peptide Bond.

We present a combined experimental and theoretical study of the ionization of N-acetylglycine molecules by 48 keV O(6+) ions. We focus on the single ionization channel of this interaction. In addition to the prompt fragmentation of the N-acetylglycine cation, we also observe the formation of metastable parent ions with lifetimes in the microsecond range. On the basis of density functional theory calculations, we assign these metastable ions to the diol tautomer of N-acetylglycine. In comparison with the simple amino acids, the tautomerization rate is higher because of the presence of the peptide bond. The study of a simple biologically relevant molecule containing a peptide bond allows us to demonstrate how increasing the complexity of the structure influences the behavior of the ionized molecule.

Exploring the metastability and the pathways for polyanionic isomerization in the dianions and trianions of doubly- and triply-deprotonated benzene

Polyanions of the aromatic origin such as dianion of benzene (C6H6 2−) are generally metastable having negative electron affinity (EA). However, the de-protonated species of these anions such as doubly-deprotonated dianions of benzene (C6H4 2−) can have higher proton affinity compared to the magnitude of the negative EA, which can trigger an intra-molecular proton transfer besides the electron auto-detachment. Such species though may exhibit resonances as in the temporary anions and cannot be treated using the conventional variational methods of the quantum mechanics. In this work, we discuss the metastability, through a nuclear-charge stabilization method, for the poly-deprotonated dianions and trianions of benzene, while exploring the intra-molecular proton-transfer reaction pathways using the conventional quantum mechanical methods, for the isomerization between the isomeric doubly-deprotonated benzene dianions, namely the ortho-, meta-, and para-C6H4 2−, as well as between the isomeric triply-deprotonated benzene trianions, namely the 1,2,3-, 1,2,4-, and 1,3,5-C6H3 3− in their singlet state. For this, the potential energy surfaces of the polyanionic species, in particular those of the doubly-deprotonated dianions, are successfully explored through an automated and systematic search performed using the global reaction route mapping method. The computations performed at the levels of the MP2, DFT with and without dispersion, and CCSD(T) theories, identified elusive reaction pathways for the isomerization between the anionic species of the poly-deprotonated benzene. Notably, a single-step isomerization between the ortho- and para-C6H4 2− dianions is observed to proceed via a direct 1, 3-proton shift in an unconventional transition state involving a puckered benzene ring in a nearly half-boat conformation. However, for the isomerization between C6H3 3− benzene trianions, the present work was able to reveal pathway only between 1,2,4-C6H3 3− and 1,3,5-C6H3 3− trianions, indicating the problems like variational collapse inherent with the conventional methods while dealing with the molecular polyanions. However, the lifetime of the metastable molecular ions investigated in this work is estimated to be of the order of only a few femtoseconds, an order of magnitude higher than the proton-transfer rate along the proposed isomerization pathways, but this study is able to provide valuable insights into the mechanism of proton transfer in the presence of a competing electron auto-detachment process.

Origin of the sequential dissociation of (CO2)3+ by heavy ion impact

SynopsisAn experimental investigation of sequential dissociation of CO2 by Ne4+ ion impact at incident energies 1.12 MeV was performed. Two initial states of (CO2)3+ ions were observed in the first fragmentation step. But for the second fragmentation step, our result suggests that both the metastable ions (CO)2+ are populated in 3Σ+ states.

Isomerization of Metastable Amine Radical Cations by Dissociation—Recombination

The metastable molecular ions of primary aliphatic amines branched at C2 can isomerize by cleavage-recombination, thereby facilitating fragmentation reactions that require less energy than simple cleavage of the initial molecular ion. This process complements the reactions described by Audier to account for the conspicuous absence of the conventional a-cleavage among the major fragmentation reactions of the metastable molecular ions of primary amines.

Supergene gold mobility in orogenic gold deposits, Otago Schist, New Zealand

Gold has been chemically mobilised by groundwater from host sulphide minerals in orogenic gold deposits of Otago. Mobilisation occurred near the Cenozoic Otago Schist erosional surface beneath a sedimentary cover. Initial Au mobilisation, on a scale of micrometres, occurred when solid solution and microparticulate gold in pyrite and arsenopyrite grains were liberated by sulphide oxidation to iron oxyhydroxide pseudomorphs. Larger-scale mobilisation involved leaching of gold from up to 100-m-thick zones which were the target of historic mining, with up to 10× enrichment of Au in reprecipitation zones. Gold in the supergene zones is commonly crystalline with octahedral shapes and nuggety forms which fill cavities and coat prismatic quartz crystals. This gold retains some or all of the Ag (typically 2–8 wt%) from the primary source gold. Oxidised groundwaters that have interacted with sulphides become enriched in dissolved sulphate, but retain high pH (7–8.5). Under these conditions, metastable thiosulphate ions can dissolve and transport Au and Ag to be precipitated later by either oxidation or reduction.

Direct observation of hole shift and characterization of spin states in radical ion pairs generated from photoinduced electron transfer of (phenothiazine)(n)-anthraquinone (n = 1, 3) dyads.

Photoinduced intramolecular electron transfer of dyad PTZ3-PTZ2-PTZ1-B-AQ consisting of phenothiazine trimer (PTZ3-PTZ2-PTZ1), bicyclo[2.2.2]octane (B), and anthraquinone (AQ) was investigated. After excitation (∼20 ps) of the AQ moiety in THF, a metastable radical ion pair (RIP) PTZ3-PTZ2-PTZ1(+)-B-AQ(-) appeared at ∼620 nm. From 500 ps to 6 ns the spectrum changed to a new absorption (∼950 nm), which was assigned to the hole-shifted stable RIP state PTZ3-PTZ2(+)-PTZ1-B-AQ(-). The time constant of the hole-shift process was determined to be 6.0 ns. The hole-shifted RIP state had a lifetime (τ) of 250 ns and was characterized by spin-polarized signals as a spin-correlated radical pair (SCRP) by means of time-resolved ESR. These results were compared with those for the phenothiazine monomer analog PTZ-B-AQ, which also produced the RIP state PTZ(+)-B-AQ(-) with τ = 1.9 μs. Time-resolved ESR showed an all emission signal pattern showing the triplet mechanism of PTZ-B-(3)AQ* → (3)[PTZ(+)-B-AQ(-)]. The origin of the difference in the lifetimes between the trimer and the monomer RIP states was discussed from various points of view, including free energy difference in the RIP states, reorganization energy difference in the charge recombination process, and the spin-state difference. Of these, the spin-state difference effect provided the most reasonable explanation.

Metastable atomic ion production in electron-O2 collisions

Pulsed electron impact ionization, time-of-flight (TOF) spectroscopy and a novel detector are used to study the production of metastable O+ in e-O2 collisions in the electron energy range from threshold to 450eV. Measured fragment ion kinetic energies are in the range 2–20eV. The ions are identified as being in the 2P state within the ground 2p3 configuration. Peak metastable ion production occurs at an electron energy of ∼130eV in agreement with the energy for maximum production of all O+ ions but the fall off in ionization efficiency toward higher incident electron energies is faster in the metastable case.

Valence-shell photoionization of Ag-like Xe7+ ions: experiment and theory

We report on experimental and theoretical results for the photoionization of Ag-like xenon ions, Xe7+, in the photon energy range 95–145 eV. The measurements were carried out at the Advanced Light Source at an energy resolution of ΔE = 65 meV with additional measurements made at ΔE = 28 meV and 39 meV. Small resonance features below the ground-state ionization threshold, at about 106 eV, are due to the presence of metastable Xe7+ ions in the ion beam. On the basis of the accompanying theoretical calculations using the Dirac atomic R-matrix codes (DARC), an admixture of only a few percent of metastable ions in the parent ion beam is inferred, with almost 100% of the parent ions in the ground level. The cross section is dominated by a very strong resonance associated with excitation and subsequent autoionization. This prominent feature in the measured spectrum is the resonance located at (122.139 ± 0.01) eV. An absolute peak cross section of 1.2 Gigabarns was measured at 38 meV energy resolution. The experimental natural width Γ = 76 ± 3 meV of this resonance compares well with the theoretical estimate of 88 meV obtained from the DARC calculation with 249 target states. Given the complexity of the system, overall satisfactory agreement between theory and experiment is obtained for the photon energy region investigated.

Recombination in nonideal ion gas discharge afterglow plasma

Recombination rates in ion plasma are presented that are obtained by processing experimental data on gas discharge afterglow in fluorine and sulfur hexafluoride media. A theoretical explanation is given to the slowing down of the recombination rate of plasma in such media as the degree of Coulomb nonideality of the media increases. An additional factor for the suppression of recombination—solvation of ions—is considered. It is shown that, in the case of discharge afterglow plasma in a fluorine medium, the process of recombination proceeds in two stages with the formation of intermediate metastable ion pairs produced by ion-molecule clusters. In a sulfur hexafluoride medium, the formation of such pairs does not have a significant effect on the recombination rate.

Direct analysis of complex mixtures by mass spectrometry

Mixture analysis can provide information on individual components if the sample is first subjected to chromatographic separation. Two critical capabilities, soft ionization and the ability to mass-select and then dissociate ions of a particular m/z, coalesced in 1975, allowing direct analysis of complex mixtures by mass spectrometry. Chemical ionization was used as the soft ionization method and mass-analysis used the ion kinetic energy spectrometer (MIKES). Soft molecular ionization produces a set of ions that are structural surrogates of the neutral molecules; they can be mass-selected and allowed to spontaneously dissociate (i.e. as metastable ions) or fragmented upon energy transfer e.g. in the course of collision-induced dissociation (CID). The second stage of mass analysis provides information about the atomic connectivity in the precursor ion and by implication in the original molecule. This review focuses on the development of complex mixture analysis by mass spectrometry and allied topics. Discussion of the activation techniques associated with (collision-induced dissociation, surface-induced dissociation and metastable ion dissociation) emphasizes the importance of energy transfer phenomena and the internal energies distributions of ions to explain the observed mass spectra. The translational to internal energy transfer in collisions is readily accessed in the MIKES where the second stage mass analyzer is a kinetic energy/charge analyzer. Collisions in the keV range can also be used to change ion the charge state via the processes of charge exchange, electron stripping, or charge inversion. New ionization sources for analysis of non-volatile compounds that were introduced during the time period (1975–1990) of this review included secondary ion mass spectrometry, plasma desorption and field ionization and they are briefly discussed. Several types of scans were developed to rapidly access information of the individual components, including chemically specific scans (e.g. neutral loss scans of mass 30 for nitro compounds).

Controllable synthesis of Mg–Fe layered double hydroxide nanoplates with specific Mg/Fe ratios and their effect on adsorption of As(v) from water

Layered double hydroxides (LDHs) have been studied comprehensively for adsorptive application due to their outstanding ability to exchange anions. An easy thermal method is presented to synthesize Mg–Fe LDH nanoplates with the controlled Mg/Fe molar ratios (3 : 1, 4 : 1, 5 : 1). Their ability to remove arsenate from water is investigated. Through comparative analysis, the results indicate that the Mg/Fe molar ratio strongly affects adsorption of As(V) on the as-prepared LDHs. The LDHs with a Mg/Fe molar ratio of 4 : 1 exhibited the fastest adsorption rate and the highest adsorption capacity among the LDHs with three different Mg/Fe molar ratios, which was attributed to the higher BET surface area and metastable iron ions in the brucite-like sheets of the LDHs.