Residence Time Of Ions Research Articles

Biological Membranes in Extreme Conditions: Anionic Tetraether Lipid Membranes and Their Interactions with Sodium and Potassium.

Archaea such as Sulfolobus acidocaldarius tolerate extreme temperatures and high acidity and can grow in the presence of toxic metals and low concentrations of Na+ or K+. It is believed that their unique tetraether membranes protect them from harsh environments and allow their survival under such conditions. We used molecular dynamics simulations to study membranes comprising glycerol dialkylnonitol tetraether lipids, which are the main component of S. acidocaldarius membranes, in solutions containing different concentrations of NaCl and KCl or with Na+ or K+ counterions (trace cations, 0 M). Anionic binding sites on the membranes were almost 50% occupied in the presence of counterions. The free energy of cation-phosphate complexation and the residence times of ions near the membranes were found to be both ion- and concentration-dependent. Sodium ions had more favorable interactions with the membranes and a longer residence time, whereas higher cation concentrations led to shorter ion residence times. When only counterions were present in the solutions, large residence times suggested that the membrane may function as a cation-attracting reservoir. The results suggested that the ions can be easily transferred to the cytoplasm as needed, explaining the growth curves of S. acidocaldarius under different salinities and pH.

Possibility of operating quadrupole mass filter at high resolution

A new possibility of improving the resolving power of quadrupole mass filters has been studied theoretically in this work. The results show that with the use of two AC excitations, in addition to the main RF supply, it is possible to modify the first stability diagram for mass filtering by creating a narrow and long band of stability along the X boundary near the tip of first stability region. These newly developed stability regions (the X-band) are similar to higher stability regions, and offer high mass resolution and fast mass separation features. This approach overcomes the many limitations of the normal operation of quadrupole analyzers, while retaining the advantages of using the first stability region. The new operation mode could achieve up to 10,000 mass resolving power with the ion residence time of only 100 RF cycles. In addition, the ion transmission efficiency with the use of the X-band is not only compromised, but is greater than in the normal operation mode. Furthermore, the new mode features one-dimensional mass filtering (in the X direction only) that is not sensitive to nonlinear field distortions, which are particularly problematic for quadrupole mass filters which built with circular rods. Faster mass separation has been confirmed in simulations and theoretical computations of the exponential increment of the trajectory instability. Due to the location of the X-band near the tip of the first stability region, the new operation mode can still have the benefits of traditional techniques (delayed DC ramp) for overcoming the negative effects of fringe fields and improving the ion transmission efficiency. The theoretical simulations show that the method of improving the performance of quadrupole mass filters does not require any modifications of mechanical structures, and only needs different and a little more sophisticated method of electric applications.

Dynamics Study of Lithium Ion Diffusion in Super Lithium Ion Conductors, Li10GeP2S12 (LGPS)

Fast ionic conduction in solid electrolytes plays a key role in feasibility of the all-solid-state battery system. Among the lithium ion conductors, the Li10GeP2S12 (LGPS) system shows the conductivity comparable to organic liquid electrolytes [1]. The recent study focused on the synthesis of solid solutions of this material, which might introduce either lithium vacancy or interstitial lithium ions and might affect its ionic conductivity and electrochemical stability. The crystal structures of the solid solution were studied using neutron diffraction technique. Their conduction pathway was estimated using Maximum Entropy Method (MEM) based on the structure information. These results indicate the lithium conduction pathway in LGPS is one-dimensional pathway along c axis at room temperature, and three-dimensional one at higher temperature [2]. However, the results of MEM provided only the space area of possible lithium ion conduction. Then, the study of lithium diffusive behaviors is necessary to understand lithium ionic conductivity in more details. The dynamics of lithium ion diffusions can be obtained directly by using the Quasielastic Neutron Scattering (QENS) technique because the quasielastic scattering spectrum is a broadening of elastic peak caused by the diffusion of atoms or ions within a material. When ions cause diffusion motions on a fixed sublattice, the quasielastic scattering spectra can exhibit a Q-dependence, which provides information on the dynamical structure of the ions diffusion. In this study, the QENS measurements were performed using the high-resolution Si crystal analyzer TOF type near-backscattering spectrometer, DNA, at MLF/J-PARC, Tokai, Ibaraki, Japan, at the energy resolution of 3.6 µeV [3]. S(Q,ω) spectra from 150 to 640 K were collected. After the analyzed those using jump-diffuse model [4,5], self-diffusion constants, jump length and mean residence time of conduction lithium ions in LGPS system were determined. The jump length was 1.73 Å at 473K and 2.72 Å at 640 K respectively. The jump length at 473 K was consistent with conduction along caxis. The jump length at 640 K also corresponded to the three dimensional conduction model. In this presentation, the mechanism of lithium ion diffusion in LGPS system will be discussed.

Solvation structure and dynamics of ions in concentrated urea solution

The solvation structure and dynamics of ions and a neutral hydrophobic solute in aqueous urea solutions are investigated using classical molecular dynamics simulations. Our results are analyzed in terms of varying concentrations ranging from pure water to ~6.9M solution of urea. It is found that except K+, other ions prefer to interact with water molecules in the solution irrespective of their charge and size whereas the neutral solute does not prefer to interact either with water or urea. The calculated diffusion coefficient values show comparatively slower dynamics for ions than the neutral solute in aqueous urea mixtures. The residence time of ions in water is found to be increased with increasing urea concentration whereas for the neutral solute it remains same for the entire range of urea concentration. There is no significant change observed for the structural relaxation time and lifetime of hydrogen bonded water molecules in the solution.

Ion-molecule adduct formation in tandem mass spectrometry.

Nowadays most LC-MS methods rely on tandem mass spectrometry not only for quantitation and confirmation of compounds by multiple reaction monitoring (MRM), but also for the identification of unknowns from their product ion spectra. However, gas-phase reactions between charged and neutral species inside the mass analyzer can occur, yielding product ions at m/z values higher than that of the precursor ion, or at m/z values difficult to explain by logical losses, which complicate mass spectral interpretation. In this work, the formation of adduct ions in the mass analyzer was studied using several mass spectrometers with different mass analyzers (ion trap, triple quadrupole, and quadrupole-Orbitrap). Heterocyclic amines (AαC, MeAαC, Trp-P-1, and Trp-P-2), photo-initiators (BP and THBP), and pharmaceuticals (phenacetin and levamisole) were selected as model compounds and infused in LCQ Classic, TSQ Quantum Ultra AM, and Q-Exactive Orbitrap (ThermoFisher Scientific) mass spectrometers using electrospray as ionization method. The generation of ion-molecule adducts depended on the compound and also on the instrument employed. Adducts with neutral organic solvents (methanol and acetonitrile) were only observed in the ion trap instrument (LCQ Classic), because of the ionization source on-axis configuration and the lack of gas-phase barriers, which allowed inertial entrance of the neutrals into the analyzer. Adduct formation (only with water) in the triple quadrupole instruments was less abundant than in the ion trap and quadrupole-Orbitrap mass spectrometers, because of the lower residence time of the reactive product ions in the mass analyzer. The moisture level of the CID and/or damper gas had a great effect in beam-like mass analyzers such as triple quadrupole, but not in trap-like mass analyzers, probably because of the long residence time that allowed adduct formation even with very low concentrations of water inside the mass spectrometer.

A theoretical study of nanostructure membranes for separating Li+ and Mg2+ from Cl−

In this paper, molecular dynamics simulation was used to investigate the separation of Li+, Mg2+ and Cl− ions from aqueous solution through armchair carbon and boron nitride nanotubes under the application of electric fields. The investigated systems included (7,7) and (8,8) carbon and boron nitride nanotubes embedded in a silicon nitride membrane in the aqueous ionic solution of LiCl+MgCl2. For separating ions, a specific range of electric fields was applied to the simulated systems. The following properties were meticulously examined: potential mean force, ionic current, water density, number of hydrogen bonds, normalized transport rate of water with respect to the number of separated ions, the residence time of ions in the nanotubes and the radial distribution functions of ions–water molecules. The results of the study indicate that the permeation of ions across nanotubes depends upon the diameter of the considered nanotubes. Under the applied electric fields, (7,7) carbon and boron nitride nanotubes were able to extract Li+ and Mg2+ ions from the aqueous solution; however, Cl− ions permeated through (8,8) nanotubes. The current generated in the system by the passage of ions through nanotubes in different electric fields was measured by Ohm’s law. The findings of the present study indicate that the examined systems can be utilized as a model in ion separation devices.

Determination of residence times of ions in a resistive glass selected ion flow-drift tube using the Hadamard transformation.

Selected ion flow tube mass spectrometry, SIFT-MS, used for trace gas analyses has certain fundamental limitations that could be alleviated by adding a facility that allows reaction times and ion interaction energies to be varied. Thus, a selected ion flow-drift tube, SIFDT, has been created to explore the influence of an embedded electric field on these parameters and on reaction processes. The new SIFTD instrument was constructed using a miniature resistive glass drift tube. Arrival times of ions, t, analysed by a downstream quadrupole mass spectrometer over the m/z range 10-100 were studied by modulating the injected ion current using a gate lens. Single pulse modulation was compared with pseudorandom time multiplexing exploiting the Hadamard transformation. A simple model involving analysis of ethanol and water vapour mixture in air was used to explore the advantages of the SIFDT concept to SIFT-MS analysis. It is shown that the resistive glass drift tube is suitable for SIFDT experiments. The Hadamard transformation can be used to routinely determine reagent ion residence time in the flow-drift tube and also to observe differences in arrival times for different product ions. Two-dimensional data combining arrival time and mass spectra can be obtained rapidly. The calculated ion drift velocities vary with the reduced field strength, E/N, and the calculated ion mobilities agree with theoretical and previous literature values. This study has provided evidence that the SIFDT-MS technique can be implemented in a miniature and low-cost instrument and two- or three-dimensional data can be obtained (product ion count rates as functions of m/z, t and E/N) using the Hadamard transformation thus providing exciting possibilities for further analytical additions and extensions of the SIFT-MS technique. Copyright © 2015 John Wiley & Sons, Ltd.

Removal of a hazardous heavy metal from aqueous solution using functionalized graphene and boron nitride nanosheets: Insights from simulations

A computer simulation was performed to investigate the removal of Zn2+ as a heavy metal from aqueous solution using the functionalized pore of a graphene nanosheet and boron nitride nanosheet (BNNS). The simulated systems were comprised of a graphene nanosheet or BNNS with a functionalized pore containing an aqueous ionic solution of zinc chloride. In order to remove heavy metal from an aqueous solution using the functionalized pore of a graphene nanosheet and BNNS, an external voltage was applied along the z-axis of the simulated box. For the selective removal of zinc ions, the pores of graphene and BNNS were functionalized by passivating each atom at the pore edge with appropriate atoms. For complete analysis systems, we calculated the potential of the mean force of ions, the radial distribution function of ion–water, the residence time of ions, the hydrogen bond, and the autocorrelation function of the hydrogen bond.

Development of an ion activation stage for atmospheric pressure ionization sources.

The ion-molecule chemistry in typical atmospheric pressure ion sources is essentially thermodynamically controlled. Methods relying on gas-phase protonation reactions, e.g. atmospheric pressure chemical ionization (APCI), thus suffer from the low reactivity of the equilibrated reagent ion population, which is mostly [H + (H2O)n](+). Reagent ion activation to yield reactive species such as H3O(+) is largely uncontrolled in commercial API mass spectrometers. The ion activation stage (IAS) is realized as an ion 'tunnel' device. The 30-electrode geometry has an octagonal cross section and an inner diameter of 10 mm. The tunnel is mounted in a vacuum chamber, which directly attaches to the first pumping stage of API mass spectrometers. The effluent from a typical inlet capillary is expanding into the IAS. Reagent ions are generated at atmospheric pressure. Mass spectrometric analysis is performed with quadrupole and time-of-flight instruments. The performance of the IAS is demonstrated by the controlled activation of the initially equilibrated proton-bound water cluster system. It is shown that a gradual increase in the RF voltage amplitude applied to the electrode structure leads to a reproducible shift of the cluster distribution along with clearly discernible protonation thresholds of selected analytes. Increasing the radiofrequency (RF) voltage from zero to maximum values does not change the average ion residence time within the IAS. We have developed an IAS for operation in the intermediate (1-10 mbar) regime in the ion transfer region of API mass spectrometers. This stage is fully compatible with the recently introduced cAPCI method, which relies on the operation of a liquid point electrode generating very clean and stable thermal distributions of [H + (H2O)n] clusters. The IAS allows controlled ion activation by increasing the ion temperature, which is demonstrated by selective analyte protonation.

Insight into the Li Ion Dynamics in Li12Si7: Combining Field Gradient Nuclear Magnetic Resonance, One- and Two-Dimensional Magic-Angle Spinning Nuclear Magnetic Resonance, and Nuclear Magnetic Resonance Relaxometry

A comprehensive picture of the rather complex Li ion dynamics in the binary silicide Li12Si7 is presented. Long-range Li dynamics is probed by field gradient NMR methods. The obtained macroscopic tracer-diffusion coefficients are in good agreement with a jump process characterized by an activation energy of approximately 0.2 eV which was previously measured in 7Li NMR relaxometry—a microscopic method which probes ion dynamics on an atomic scale. From high-resolution magic-angle spinning (MAS) NMR, it can be concluded that 9 of the 13 crystallographically independent sites take part in this fast diffusion process. Li ions on the residual four sites are bound more tightly to the silicide Zintl anions with activation barriers ranging from 0.32 to 0.55 eV. Accordingly, the mean residence times of Li ions on these sites are considerably longer, which makes it possible to test their dynamics with 1D/2D MAS exchange NMR methods. We present a series of mixing-time-dependent 2D MAS exchange NMR measurements. The extracted Li jump rates are in very good agreement with those of dynamic processes investigated by NMR relaxometry. The data is interpreted in relation to the results of two recently published NMR studies on Li12Si7 and structural assignments based on one- and two-dimensional 29Si{7Li} heteronuclear correlation spectroscopy. These experiments assist in attributing different ionic mobilities to the different crystallographic lithium sites, suggesting preferred ion conduction pathways. Combining all these methods and their respective results, a consistent picture of the Li ion dynamics in Li12Si7 is obtained. Overall lithium species in the vicinity of the Si4 star unit tend to be more mobile than lithium species interacting with the Si5 ring units of the silicon framework structure.

Ion-neutral gas reactions in a collision/reaction cell in inductively coupled plasma mass spectrometry: Correlation of ion signal decrease to kinetic rate constants

Reaction gas flow rate dependent Ar2+ and Ar+ signals are correlated to fundamental kinetic rate coefficients. A simple calculation, assuming that gas exits the reaction cell due only to effusion, is described to estimate the gas pressure in the reaction cell. The value of the product of the kinetic rate constant and the ion residence time in the reaction cell can be determined from experimental measurement of the decrease in an ion signal as a function of reaction gas flow rate. New kinetic rate constants are determined for the reaction of CH3F with Ar+ and Ar2+.

Photoelectron spectroscopy of size-selected cluster ions using synchrotron radiation

A new experimental setup for photoelectron spectroscopy of size-selected cluster ions using synchrotron VUV radiation as generated by the Swiss Light Source is presented. An intense positively charged cluster ion beam is produced in a high-intensity magnetron sputter source. The clusters are subsequently mass selected in a sector magnet. To maximize the residence time of the cluster ions in the ionization region of the velocity map imaging spectrometer, the cluster ion beam is decelerated where it crosses the light beam. First experiments on (MoO3)n+ (n = 69 and 59) cluster cations show that the approach is capable of delivering photoelectron spectra of size-selected transition metal cluster ions.

Electrodialysis reversal desalination: optimal voltage application rate, linear velocity, and mean ion residence time for maximal ion removal rate

Optimal voltage application rate, linear velocity, and mean ion residence time (MIRTd) in dilute stream were searched for electrodialysis reversal desalination by using two different feed waters containing TDS 3330 and 7190 mg/L NaCl. Two sets of literature data were used in the analyses. First, the relation among demineralization degree, voltage application rate, and MIRTd were derived from theory. Then, the findings were analyzed from the first set of literature data and validated with the second set of data. The analyses show that the maximal demineralization degree does not appear at the highest voltage application alone and that demineralization degree increases with the increasing voltage application rate to a certain critical value only. The maximal demineralization degree occurs at the combination of lower voltage application and longer MIRTd or vice versa. Data show the maximal demineralization degree does not always present the maximal ion removal rate per effective area of cell pairs per power. Therefore, both the maximal demineralization degree and maximal ion removal rate per effective area of cell pair per power were used to gauge optimal voltage application, linear velocity, and MIRTd. Optimal voltage application, linear velocity, and MIRTd were found to be 45–70 V, 15.1–12.3 cm/s, and 2.3–2.7 min for feed water 3330–7190 mg/L NaCl in Aquamite I membrane stack with Mark I spacer in 135 different operational conditions.

Separation of protein conformers by differential ion mobility in hydrogen-rich gases.

Proteins in solution or the gas phase tend to exhibit multiple conformational families, each comprising distinct structures. Separation methods have generally failed to resolve these, with their convolution producing wide peaks. Here, we report full separation of >10 conformers for most ubiquitin charge states by the new approach of differential ion mobility spectrometry (field asymmetric waveform ion mobility spectrometry, FAIMS) employing H2/N2 gas mixtures with up to 85% H2. The resolving power (up to 400) is five times the highest previously achieved (using He/N2 buffers), greatly increasing the separation specificity. The peak widths match the narrowest obtained by FAIMS for any species under the same conditions and scale with the protein charge state (z) and ion residence time (t) as z(-1/2) and t(-1/2), as prescribed for instrumental (diffusional) broadening. This suggests resolution of specific geometries rather than broader ensembles.

Effect of particle size of drinking-water treatment residuals on the sorption of arsenic in the presence of competing ions

Arsenite [As(III)] and arsenate [As(V)] sorption by Fe- and Al-based drinking-water treatment residuals (WTR) was studied as a function of particle size at different pHs, and in the presence of competing ligands, namely, phosphate, citrate, and oxalate. Both WTRs showed high affinity for As oxyanions. However, Al-WTR showed higher As(III) and As(V) sorption capacity than Fe-WTR because of their greater surface area. The effect of particle size on As sorption was pronounced on Fe-WTR, where the smaller fraction sorbed more As(III) and As(V) than the larger fractions, whereas relatively minor effects of particle size on As sorption was observed for Al-WTR. Arsenite sorption on both WTRs increased with increasing pH up to circum-neutral pHs and then decreased at higher pHs, whereas As(V) sorption decreased steadily with increasing pH. The capacity of competing ligands to inhibit sorption was greater for As(III) than As(V) on both WTRs (particularly on Al-WTR) following the sequence: oxalate<citrate<phosphate. It was also a function of As ion residence time on the WTR surfaces: the longer the residence time, the less effective were the competing ligands in As desorption.

Direct Visualization of Single Ions in the Stern Layer of Calcite

Calcite is among the most abundant minerals on earth and plays a central role in many environmental and geochemical processes. Here we used amplitude modulation atomic force microscopy (AFM) operated in a particular regime to visualize single ions close to the (1014) surface of calcite in solution. The results were acquired at equilibrium, in aqueous solution containing different concentrations of NaCl, RbCl, and CaCl(2). The AFM images provide a direct and atomic-level picture of the different cations adsorbed preferentially at certain locations of the calcite-water interface. Highly ordered water layers at the calcite surface prevent the hydrated ions from directly interacting with calcite due to the energy penalty incurred by the necessary restructuring of the ions' solvation shells. Controlled removal of the adsorbed ions from the interface by the AFM tip provides indications about the stability of the adsorption site. The AFM results show the familiar "row pairing" of the carbonate oxygen atoms, with the adsorbed monovalent cations located adjacent to the most prominent oxygen atoms. The location of adsorbed cations near the surface appears better defined for monovalent ions than for Ca(2+), consistent with the idea that Ca(2+) ions remain further away from the surface of calcite due to their larger hydration shell. The precise distance between the different hydrated ions and the surface of calcite is quantified using MD simulation. The preferential adsorption sites found by MD as well as the ion residence times close to the surface support the AFM findings, with Na(+) ions dwelling substantially longer and closer to the calcite surface than Ca(2+). The results also bring new insights into the problem of the Stern and electrostatic double layer at the surface of calcite, showing that parameters such as the thickness of the Stern layer can be highly ion dependent.

Long-lived Ar-Hg plasma in the afterglow of a high-current pulsed discharge

High-density (n > 1012 cm−3) argon-mercury plasma produced by a short (t ∼ 20 μs) high-power pulsed discharge in argon with an admixture of mercury vapor at a discharge current of ∼50 A, an argon pressure of ∼4 mm Hg, and a mercury vapor pressure of ∼10−3 mm Hg was studied using optical spectroscopy and radio physics methods. It is found that the lifetime of this plasma after the end of the discharge pulse is up to 10−2 s. It is shown that such an abnormally long lifetime of such an afterglow plasma, as compared to the plasma of an argon discharge without an admixture of mercury vapor, is related to the long residence time of atoms and ions of both argon and mercury in highly excited states due to chemi-ionization processes involving long-lived metastable argon ions. It is suggested that dissociative recombination of highly excited molecular ions of argon play an important role in the transfer of excitation to argon atoms and ions that are close to autoionization states.

Concentration dependence of NaCl ion distributions around DPPC lipid bilayers

We study the coordination of excess NaCl to zwitterionic DPPC lipid bilayers using molecular dynamics simulations. We find that Na ions directly coordinate with the DPPC lipid carbonyl groups. As the number of excess ions increases, the number of coordinated ions increases, until it reaches a plateau at a ratio near 1 ion per every four lipids at 310 K, and 1 ion per every six lipids at 323 K. The area per lipid decreases as the number of excess ions is increased. For low number of ions per lipids (1:16 and 1:8), most Na ions are bound to the lipid carbonyls, while the Cl form an ionic cloud around the lipid choline groups. As a result of the Na binding, the lipid has an effective positive charge density. The residence time of Na ions bound to the lipid is longer than 40 ns, while Cl ions exchange faster than the nanoseconds timescale. We find that the bound Na ions replace ordered water around the carbonyls. The net linear charge density near the carbonyl groups stays positive, regardless of the presence of excess salt in the solution.

Tandem Ion Trap Design with Enhanced Mass Analysis Capabilities for Large Populations of Ions

A new arrangement consisting of two separate radio frequency (rf) quadrupole ion traps is used to analyze large populations of ions over a wide mass-to-charge (m/z) range. The setup consists of an "accumulation" trap that is maintained at a higher pressure than the second high-performance "analyzer" trap. The two traps are scanned simultaneously, with a mass difference between that determines the residence time and mass range of ions in the analytical trap. Initially, all ions are trapped in the accumulation trap and then mass-selectively ejected into the analyzer trap. As ions arrive in the analyzer trap, they cool through collisions with the buffer gas and then are mass selectively ejected toward the detector. This concurrent linked mass scanning reduces the total number of ions present in the analyzer trap during mass analysis, thereby reducing space charge effects and leading to improved resolution and mass accuracy of analytical spectra.

An ion trap CIMS instrument for combined measurements of atmospheric OH and H 2SO 4: First test measurements above and inside the planetary boundary layer

Gaseous sulfuric acid (GSA) plays an important role in atmospheric secondary aerosol formation. To quantify its contribution to these processes long-term measurements of GSA using a very sensitive detection method are required. OH is the most important oxidant in the atmosphere and oxidizes, e.g., SO 2. A powerful detection method is chemical ionization mass spectrometry (CIMS). We have built a novel CIMS instrument composed of an ion trap mass spectrometer (ITMS) and a selective ion source suitable to measure gaseous H 2SO 4 and OH and to do fragmentation studies. The probe air inlet and sampling line were optimized to minimize wall losses of H 2SO 4 and OH. An ion source was developed to minimize artificial signals and to create reagent ions (NO 3 − (HNO 3) h (H 2O) n ) which react highly selectively with GSA. An H 2SO 4/OH calibration source based on water vapor photolysis was developed to overcome uncertainties in remaining wall losses, ion residence time, reaction rate coefficient and mass discrimination. Field measurements of GSA in the planetary boundary layer (several weeks at northern and middle latitudes) and high-altitude mountain-site measurements of GSA and OH were carried out. The ITMS instrument offers a detection limit as low as 1 × 10 5 molecules per cm 3 and a time resolution of about 2 min .