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At least 181 records · Page 10Linked to original sources
PubMed1994

Wide-mass-range axialization for high-resolution Fourier-transform ion cyclotron resonance mass spectrometry of externally generated ions.

Ions generated in both continuous and pulsed-mode external ion sources are injected into an open-ended cylindrical Fourier-transform ion cyclotron resonance ion trap by means of an electrostatic ion guide consisting of concentric cylinder and wire electrodes to which a potential difference is applied. Broadband axialization during ion injection improves ion trapping efficiency by reducing the ion off-axis displacement. Significant improvements in both signal-to-noise ratio and mass resolving power, along with high mass selectivity are demonstrated for ions generated by external Cs+ secondary ion mass spectrometry or by matrix-assisted laser desorption/ionization.

Cesium↗
PubMed2001

Probing reactive sites for ion-molecule reactions of anthraquinones with dimethyl ether using an external source ion trap tandem mass spectrometer and computational chemistry.

Gas-phase ion-molecule reactions of anthraquinone derivatives with dimethyl ether (DME) were investigated using an external source ion trap mass spectrometer. Semi-empirical calculations were executed to determine possible reactive sites for the product ions. Collision activated dissociation (CAD) was successfully performed for very low abundance of ion-molecule products. Even for product ions with a relative intensity below 1%, CAD experiments can be successfully performed. Significantly more structural information could be elucidated based on this special feature. Importantly, the CAD spectra of very minor ions could be measured by this ion trap instrument, which significantly enhances the future role of the ion trap as a powerful analytical instrument. CAD of all product ions on anthraquinone compounds typically eliminates neutral molecules such as CO or H(2)O. A hydration phenomenon in the CAD processes resulting from the precursor ions incorporating one molecule of H(2)O and then eliminating one molecule of CO was observed in this study.

Anthraquinones↗
PubMed2000

Metal Ion Catalysis in the Hydrolysis of Esters of 2-Hydroxy-1,10-phenanthroline: The Effects of Metal Ions on Intramolecular Carboxyl Group Participation.

Rate constants have been determined for hydrolysis of the acetate, glutarate, and phthalate monoesters of 2-hydroxy-1,10-phenanthroline in water at 30 degrees C and &mgr; = 0.1 M with KCl. The hydrolysis reactions of the esters are hydroxide ion catalyzed at pH > 9. The phthalate and glutarate monoesters have in addition pH-independent reactions from pH 5.5 to 9 that involve intramolecular participation by the neighboring carboxylate anion. The pH-independent reaction of the glutarate monoester is approximately 5-fold faster than that of the phthalate monoester. The plots of log k(obsd) vs pH for hydrolysis of the carboxyl substituted esters are bell shaped at pH < 5, which indicates a rapid reaction of the zwitterionic species (carboxyl anion and protonated phenanthroline nitrogen). The divalent metal ions, Cu(2+), Ni(2+), Zn(2+), and Co(2+), complex strongly with the esters; saturation occurs at metal ion concentrations less than 0.01 M. The 1:1 metal ion complexes have greatly enhanced rates of hydrolysis; the second-order rate constants for the OH(-) reactions are increased by factors of 10(5) to 10(8) by the metal ion. The pH-rate constant profiles for the phthalate and glutarate ester metal ion complexes have a sigmoidal region below pH 6 that can be attributed to a metal ion-promoted carboxylate anion nucleophilic reaction. The carboxyl group reactions are enhanced 10(2) - to 10(3) -fold by the metal ions, which allows the neighboring group reaction to be competitive with the favorable metal ion-promoted OH(-) reaction at pH < 6, but not at pH > 6. The half-lives of the pH-independent neighboring carboxyl group reactions of the Cu(II) complexes at 30 degrees C are approximately l2 s. The other metal ion complexes are only slightly less reactive (half-lives vary from 2.5 to 40 s). These are the most rapid neighboring carboxyl group reactions that have been observed in ester hydrolysis. Copyright 2000 Academic Press.

Journal Article↗
PubMed2004

Alloy liquid metal ion sources and their application in mass separated focused ion beams.

For special purposes like writing ion implantation or ion mixing in the micrometer- or sub-micrometer range different ion species are needed. Therefore alloy liquid metal ion sources (LMISs) are used. The energy distribution of the ions from an alloy LMIS is one of the determining factors for the performance of a FIB column. Different source materials like Au(73)Ge(27), Au(82)Si(18), Au(77)Ge(14)Si(9), Co(36)Nd(64), Er(69)Ni(31), and Er(70)Fe(22)Ni(5)Cr(3) were investigated with respect to the energy spread of the different ion species as a function of emission current, ion mass and emitter temperature. The alloy LMISs discussed above have been used in the Rossendorf FIB system IMSA especially for writing implantation to fabricate sub-micrometer pattern without any lithographic steps. A Co-FIB was applied for the ion beam synthesis of CoSi(2) micro-structures. Additionally, the possibility of varying the current density with the FIB by changing the pixel dwell time was used for radiation damage investigations in Si and SiC at elevated implantation temperatures. Furthermore, a broad spectrum of ions was employed to study the sputtering process depending on temperature, angle of incidence and ion mass on a couple of target materials using the volume-loss method. Especially this technique was used for the fabrication of various kinds of micro-tools.

Journal Article↗
PubMed1997

Twitch potentiation induced by caffeine in the mouse diaphragm depends on external calcium ions in the absence of potassium ions.

1. Removal of external K+ ions increases the amplitude of directly elicited twitch contractions of the mouse diaphragm (Nishimura et al., 1996). This increase depends on external Ca2+ ions. 2. We examined the effect of caffeine (2 mM) on this increase in twitch amplitude. The mouse diaphragm muscle was directly stimulated in the presence of d-tubocurarine (10 microM). 3. Caffeine increased the amplitude of twitches in a standard bathing solution. This effect was maintained in a solution without either K+ or Ca2+ ions but was abolished in a solution from which both ions were absent. Readdition of Ca2+ ions restored the potentiating effect of caffeine. 4. In the presence of caffeine, removal of both K+ and Ca2+ ions decreased the resting membrane potentials of muscle fibers to about -53 mV. The readdition of 2 mM Ca2+ ions restored the membrane potentials. 5. Twitch potentiation in the absence of external K+ ions was attenuated by 10 microM bepridil but not by 3 microM verapamil or 10 microM Cd2+ ions. 6. These results support the hypothesis that Na(+)-Ca2+ exchange can support twitch contraction during the inhibition of Na(+)-K(+)-ATPase activity. The influx of Ca2+ ions into the cells might be stored in the sarcoplasmic reticulum.

Animals↗
PubMed2001

Atmospheric pressure ion trapping in a tandem FAIMS-FAIMS coupled to a TOFMS: studies with electrospray generated gramicidin S ions.

A tandem FAIMS-FAIMS system for ion trapping at room temperature and atmospheric pressure is described. The first FAIMS device consisted of a side-to-side configuration (sFAIMS) suitable for ion separation, whereas the second FAIMS device was appropriate for ion trapping (tFAIMS). Ions pre-selected by the sFAIMS entered the tFAIMS and were captured by virtual trapping fields at the hemispherical tip of the inner electrode. The use of the sFAIMS, with wider electrode diameters, and consequently better ion separation efficiency than the tFAIMS, lowered the number of background ions captured in the trapping region of tFAIMS, and thus reduced the space charge effects in the trap. This tandem device was coupled to a laboratory built time-of-flight mass spectrometer and was evaluated using the electrospray generated [M + 2H]2+ ion of gramicidin S. The half-time (t1/2) of the exponential decay of the ion cloud in tFAIMS, determined by monitoring the residual intensity of ions extracted from the ion trapping region of tFAIMS after various delay times, was about 2 s.

Algorithms↗
PubMed2002

A secondary ion microprobe ion trap mass spectrometer.

An ion trap mass analyzer has been attached to an organic secondary ion microprobe. A pressure differential >100 can be maintained between the ion trap and microprobe. The well-focused secondary ion beam can transit a small (2 mm) diameter tube, but gas flow from ion trap to microprobe is impeded. This pressure differential allows the microprobe to retain imaging capability. Ion trap and microprobe data systems are integrated by taking advantage of the highly reproducible periodicity of the ion trap operating in resonant ejection mode and asynchronous signal and data acquisition afforded by commercially available interface cards. Secondary ion mass spectra and images obtained indicate an approximately 10-fold improvement in sensitivity, although preliminary evidence indicates low (<1%) trapping efficiency. Image data acquisition using the ion trap for mass analysis requires at least 10 times as much time compared to using a quadrupole mass filter because the mass-selected instability mode is used for mass analysis, i.e., mass resolution in the ion trap is not continuous as it is in the quadrupole.

Choline↗
PubMed2003

Detection of arginine dimethylated peptides by parallel precursor ion scanning mass spectrometry in positive ion mode.

Dimethylation at arginine residues has been shown to be central in cellular processes such as signal transduction, transcription activation, and protein sorting. The two methyl groups are either placed symmetric or asymmetric on the zeta standing nitrogen atoms of the arginine side chain. Here, we introduce a novel method that enables the localization of dimethylarginine (DMA) residues in gel-separated proteins at a level of sensitivity of better than 1 pmol and that allows one to distinguish between the isomeric symmetric and asymmetric position of the methyl groups. The method utilizes two side-chain fragments of DMA, the dimethylammonium ion (m/z 46.06) and the dimethylcarbodiimidium ion (m/z 71.06), for positive ion mode precursor ion scanning. Dimethylcarbodiimidium ions (m/z 71.06) are produced by symmetric as well as asymmetric dimethylarginine but are observed more strongly for symmetric DMA. It is utilized here in the precursor of m/z 71 scan to indicate the presence of DMA in a peptide. The dimethylammonium ion (m/z 46.06) is specific for asymmetric DMA and is utilized here in the precursor of m/z 46 scan. The positive ion mode allows for the identification of the protein by peptide sequencing and simultaneous detection and localization of the modified residues. The analysis can be conducted on any mass spectrometer capable of precursor ion scanning. However, the high resolution of a quadrupole TOF instrument is beneficial to assign the accurate charge state of the often highly charged precursors. Using the precursor of m/z 71 scan, we found FUS/TLS and Sam68 to be DMA-containing proteins. We discovered at least 20 DMA sites in FUS/TLS. In MS/MS, we observed neutral loss of dimethylamine (m/z 45.05) from which it follows that the dimethylation in FUS/TLS is asymmetric. Monitoring in parallel the fragments m/z 46.06 and 71.06 in precursor ion scans and peptide sequencing, we identified at least nine asymmetric DMA modifications in Sam68. The parallel monitoring of fragments in precursor ion scans is a versatile tool to specify the nature of protein modifications in cases where a single fragment is not conclusive.

Amino Acid Sequence↗
PubMed1999

Hypochlorite Ion Decomposition: Effects of Temperature, Ionic Strength, and Chloride Ion.

The effects of pH, hypochlorite and chloride ion concentration, temperature, and ionic strength on the kinetics and mechanism of decomposition of concentrated hypochlorite ion and the formation of chlorate ion and oxygen in the pH 9-14 region has been investigated. In the absence of catalytic levels of transition-metal ions, the rate of chlorate ion formation is 8.7 times faster than the rate of oxygen formation for the concentration range of 0.7-3.0 M hypochlorite ion with various levels of chloride ion in 0.001-1.0 M OH(-) at 15-55 degrees C. Under these same conditions, the equation that describes the effect of temperature and ionic strength on the decomposition of OCl(-) is the following: log k(2) = 0.149&mgr; + log[2.083 x 10(10)T exp(-1.018 x 10(5)/RT) exp(-56.5/R)] where k(2) has units of M(-)(1) sec(-)(1) and &mgr; is between 1 and 6 molar. Numerical simulation of the decomposition of OCl(-) was carried out while taking into account the effect of each of the above experimental variables. Evidence is presented suggesting chloride ion catalyzed decomposition of hypochlorite ion in the pH 9-10 region that is in addition to the contribution of chloride ion to the ionic strength.

Journal Article↗
PubMed1985

Secondary ion mass spectrometry for sulfoglycolipids: application of negative ion detection.

A series of underivatized sulfoglycolipids (SM4g, lyso-SM4g, SM4s, SM3, SM2, SB2, and SB1a) from various tissues were analyzed by both positive (POS-SI-MS) and negative (NEG-SI-MS) secondary ion mass spectrometry. By POS-SI-MS were detected the molecular ions of sulfoglycolipids in the form with sodium or potassium together with some fragment ions useful for the carbohydrate sequence determination. The analysis of monosulfogangliotriaosyl- or monosulfogangliotetraosylceramide and bis-sulfoglycolipid was difficult due to noise in the high mass region. On the other hand, NEG-SI-MS of sulfoglycolipids gave more intense signals from molecular ion of (M-H)- for monosulfoglycolipids and [M-H+Na)-H)- for bis-sulfoglycolipid. Many fragment ions useful for the elucidation of the carbohydrate sequences were also obtained with significant intensities. The fragmentation was assessed to occur at the glycosidic linkages to form ions of the oligosaccharides with or without ceramide. These ions were useful for sugar sequencing and also for distinguishing the differences in the position of the sulfate group. The intensities of saccharide ions without sulfate were lower than those with sulfates. In the case of SB2 and SB1a, containing 2 mol of sulfate ester groups, the molecular ion was detected as [M-H+Na)-H)-. Also, fragment ions with 2 mol of sulfate were detected as the sodium-additive form. It was concluded that NEG-SI-MS is a very useful technique for the structural elucidation of higher sulfoglycolipids.

Animals↗
PubMed1973

Gibberellic Acid and Ion Release from Barley Aleurone Tissue: Evidence for Hormone-dependent Ion Transport Capacity.

The release of potassium, magnesium, and phosphate ions from aleurone cells of barley (Hordeum vulgare L. cv. Himalaya) is a gibberellic acid-dependent process. The release of these ions is preceded by a lag period of 6 to 8 hours after gibberellic acid addition. The effect of gibberellic acid on the release of ions is not mediated through an effect on ion solubilization. Thus, gibberellic acid does not apreciably affect the sum of extracted and released ions relative to controls. Rather, the effect of the hormone is on the release process itself. Inhibitors of oxidative phosphorylation when added with gibberellic acid or at times up to 6 hours after gibberellic acid inhibition release. When these inhibitors are added after ion release has begun, however, rapid efflux of ions occurs. These results suggest a strong correlation between energy levels and ion transport capacity. Inhibitors of RNA and protein synthesis also inhibit gibberellic acid-stimulated ion release. Evidence suggests that RNA and protein synthesis are required to establish and maintain ion release capacity of aleurone cells.

Journal Article↗
PubMed1987

Stochastic theory of ion movement in channels with single-ion occupancy. Application to sodium permeation of gramicidin channels.

The electrodiffusion equations were solved for the one-ion channel both by the analytical method due to Levitt and also by Brownian dynamic simulations. For both types of calculations equilibration of ion distribution between the bath and the ends of the channel was assumed. Potential profiles were found that give good fits to published data on Na+ permeation of gramicidin channels. The data were best fit by profiles that have no relative energy maximum at the mouth of the channel. This finding suggests that alignment of waters or channel charged groups inside the channel in response to an ion's approach may provide an energetically favorable situation for entry sufficient to overcome the energy required for removing bulk waters of hydration. An alternative possibility is that the barrier to ion entry is situated outside the region restricted to single-ion occupancy. Replacement of valine with more polar amino acids at the No. 1 location was found to correspond to a deepening of the potential minima near the channel mouths, an increase in height of the central barrier to ion translocation across the channel, and possibly a reduction in the mobility of the ion-water complex in the channel. The Levitt theory was extended to calculate passage times for ions to cross the channel and the blocking effects of ions that entered the channel but didn't cross. These quantities were also calculated by the Brownian dynamics method.

Gramicidin↗
PubMed2003

H/D exchange of gas phase bradykinin ions in a linear quadrupole ion trap.

The gas phase H/D exchange reaction of bradykinin ions, as well as fragment ions of bradykinin generated through collisions in an orifice skimmer region, have been studied with a linear quadrupole ion trap (LIT) reflectron time-of-flight (rTOF) mass spectrometer system. The reaction in the trap takes only tens of seconds at a pressure of few mTorr of D2O or CD3OD. The exchange rate and hydrogen exchange level are not sensitive to the trapping q value over a broad range, provided q is not close to the stability boundary (q = 0.908). The relative rates and hydrogen exchange levels of protonated and sodiated +1 and +2 ions are similar to those observed previously by others with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer system. The doubly and triply protonated ions show multimodal isotopic distributions, suggesting the presence of several different conformations. The y fragment ions show greater exchange rates and levels than a or b ions, and when water or ammonia is lost from the fragment ions, no exchange is observed.

Bradykinin↗
PubMed2005

High-sensitivity ion mobility spectrometry/mass spectrometry using electrodynamic ion funnel interfaces.

The utility of ion mobility spectrometry (IMS) for separation of mixtures and structural characterization of ions has been demonstrated extensively, including in biological and nanoscience contexts. A major attraction of IMS is its speed, several orders of magnitude greater than that of condensed-phase separations. Nonetheless, IMS combined with mass spectrometry (MS) has remained a niche technique, substantially because of limited sensitivity resulting from ion losses at the IMS-MS junction. We have developed a new electrospray ionization (ESI)-IMS-QTOF MS instrument that incorporates electrodynamic ion funnels at both front ESI-IMS and rear IMS-QTOF interfaces. The front funnel is of the novel "hourglass" design that efficiently accumulates ions and pulses them into the IMS drift tube. Even for drift tubes of 2-m length, ion transmission through IMS and on to QTOF is essentially lossless across the range of ion masses relevant to most applications. The rf ion focusing at the IMS terminus does not degrade IMS resolving power, which exceeds 100 (for singly charged ions) and is close to the theoretical limit. The overall sensitivity of the present ESI-IMS-MS system is comparable to that of commercial ESI-MS, which should make IMS-MS suitable for analyses of complex mixtures with ultrahigh sensitivity and exceptional throughput.

Electrochemistry↗
PubMed2005

Through the channel and around the channel: Validating and comparing microscopic approaches for the evaluation of free energy profiles for ion penetration through ion channels.

Microscopic calculations of free energy profiles for ion transport through biological ion channels present a very serious challenge to modern simulation approaches. The main problem is due to the major convergence problems associated with the heterogeneous landscape of the electrostatic environment in ion channels and with the need to evaluate the profile associated with the transfer of the ion from bulk water to the channel environment. This problem is compounded by the lack of reliable and relevant benchmarks that can discriminate between alternative approaches. The present study is aimed at reducing the above problems by defining benchmarks that are directly relevant to ion channels and can also give converging results. This is done by constructing a series of models of a truncated gramicidin channel with different numbers of water molecules and by comparing the profiles for going around the channel and through the channel. These discriminating models are then used to validate and compare the adiabatic charging free energy perturbation (FEP) approach combined with an umbrella sampling approach (Warshel, A. J. Phys. Chem. 1982, 86, 2218) and the potential of mean force (PMF) approach used frequently in studies of ion channels. It is found that both approaches work quite well until one moves to the case of the fully solvated channel. In this limit, the PMF approach may give different results for the overall work of going through the channel and around the channel, while the FEP approach gives physically consistent results. The present benchmark also indicates that the weighted histogram analysis method (WHAM) approach does not offer a significant advantage over earlier approaches at least as much as studies of ion channels are concerned. Finally, it is concluded that the FEP approach may be more useful in evaluating the overall barrier for moving ions from water to ion channels and that in some cases it might be beneficial to use the FEP approach for selective points along the channel and then to connect these points by PMF calculations.

Algorithms↗
PubMed2006

Selective coulometric release of ions from ion selective polymeric membranes for calibration-free titrations.

Coulometry belongs to one of the few known calibration-free techniques and is therefore highly attractive for chemical analysis. Titrations performed by the coulometric generation of reactants is a well-known approach in electrochemistry, but suffers from limited selectivity and is therefore not generally suited for samples of varying or unknown composition. Here, the selective coulometric release of ionic reagents from ion-selective polymeric membrane materials ordinarily used for the fabrication of ion-selective electrodes is described. The selectivity of such membranes can be tuned to a significant extent by the type and concentration of ionophore and lipophilic ion-exchanger and is today well understood. An anodic current of fixed magnitude and duration may be imposed across such a membrane to release a defined quantity of ions with high selectivity and precision. Since the applied current relates to a defined ion flux, a variety of non-redox active ions may be accurately released with this technique. In this work, the released titrant's activity was measured with a second ionophore-based ion-selective electrode and corresponded well with expected dosage levels on the basis of Faraday's law of electrolysis. Initial examples of coulometric titrations explored here include the release of calcium ions for complexometric titrations, including back titrations, and the release of barium ions to determine sulfate.

Calcium↗
PubMed2001

Ion transport in roots: measurement of fluxes using ion-selective microelectrodes to characterize transporter function.

The transport of mineral ions into and out of tissues and cells is central to the life of plants. Ion transport and the plasma membrane transporters themselves have been studied using a variety of techniques. In the last 15 years, measurement of specific ion fluxes has contributed to the characterization of transport systems. Progress in molecular genetics is allowing gene identification and controlled expression of transporter molecules. However the molecular expression of transporter gene products must be characterized at the functional level. The ion-selective microelectrode technique to measure specific ion fluxes non-invasively is ideally suited to this purpose. This technique, its theory, its links with others and its application and prospects in plant science, are discussed. Ions studied include hydrogen, potassium, sodium, ammonium, calcium, chloride and nitrate. Applications discussed include: solute ion uptake by roots; gravitropism and other processes in the root cap, meristematic and elongation zones; Nod factor effect on root hairs; osmotic and salt stresses; oscillations; the effects of light and temperature. Studies have included intact roots, leaf mesophyll and other tissues, protoplasts and bacterial biofilms. A multi-ion capability of the technique will greatly assist functional genomics, particularly when coupled with imaging techniques, patch clamping and the use of suitable mutants.

Cell Membrane↗
PubMed2005

Retention profiles and mechanism of anion separation on latex-based pellicular ion exchanger in ion chromatography.

A retention model based on stoichiometric approach has been developed in order to describe analyte retention of anions on latex-based pellicular ion exchanger. The chromatographic process entails two stepwise and complex equilibria, first is ion-pair forming of analyte or eluent ion with ion-exchange sites under the effect of electrostatic forces due to the sulfonic layer behind the aminated functional groups of stationary phase. Second component is the ion-exchange between the analyte and eluent ions. As a new parameter of the fractional electrostatic coefficient of the ion exchange capacity was introduced to develop retention profiles of anions. Analysis of the dependence of the capacity factors on the eluent concentrations at different values of fractional coefficient shed light on the possible complex mechanism. Extensive experimental retention data were obtained for 14 anions (formate, acetate, propionate, pyruvate, lactate, chloride, nitrate, oxalate, malonate, succinate, tartarate, fumarate, maleate, sulphate) using hydroxide eluents of varying concentration. The ion-pair formation and ion-exchange selectivity constants for analyte and eluent species are determined using derived retention equation from experimental data by nonlinear iterative calculation. The model was utilized to predict retention data under elution conditions of practical importance. The predicted and obtained retention factors are in good agreement, which confirms the predictive power of the model.

Anion Exchange Resins↗