External Ionization Research Articles

Gas-phase basicity of (CH3)3N+-C6H4-COO- zwitterions: a new class of organic super bases.

Gas-phase basicity of (CH3)3N+-C6H4-COO- zwitterions: a new class of organic super bases.

Study of gas-phase reactivity of positive and negative even-electron ions prepared from diethylmethylphosphonate ester in an external chemical ionization source of orthogonal tandem quadrupole/ion trap instrument

Gas phase reactivity of the protonated and deprotonated diethylmethylphosphonate (DEMP), prepared under positive chemical ionization (PICI) and negative chemical ionization (NICI) conditions in an external high pressure source, was investigated using a recent homemade quadrupole filter/ion trap tandem with an orthogonal ion beam transmission. Ion chemical study with regard to unimolecular dissociation is described, especially with regard to the ion stability at different injection rf levels. The selective injection of the DEMPH + and [DEMP − H] − ions seems to be a very critical step that depends upon the helium buffer gas pressure and the injection low m/z cutoff values (LMCO). Indeed, if the parent ion abundance rises up to 50% of total ion current at a low injection rf level (LMCO of 25 Th), as LMCO is increased extend fragmentations are favored. Such decompositions mainly involved 1–5 proton transfers leading to competitive and/or consecutive losses of ethylene and water. The ion abundance versus the injection rf level presents real energy resolved dependence. Ion/molecule reactions initiated by reactive fragments of externally prepared DEMPH + [e.g. PO +, P(OH) 2 +, H 2CP(OH) 2 +, OP(OH) 2 +, H 3C–POEt +, H 3C–PO(OCH 3) +, H 3C(OH) 2POH +, H 3C–PO(OEt) +, CH 3(OEt)HPOH +, CH 3(OH)(CH 2CHO)POH +, CH 3(OH)(OEt)POH +, CH 3(OCH 3)(CH 2CHO)POH +] have been investigated. These fragment ions, selected using a quadrupole filter, induce selective ion/molecule reactions with the DEMP molecules directly introduced into the ion trap cell. The exothermic reactions observed are (1) a proton transfer process and (2) proton-bound dimer formation, as well as stepwise addition/elimination reactions in which the first electrophilic attack at the phosphorous site of the DEMP is followed by ethylene loss. On the other hand, by selecting and injecting negative ions like alkoxide species (externally formed from aliphatic alcohols in NICI), nucleophilic attacks such as a S N 2-P pathway on DEMP can occur. However, most of the ions are generated by elimination and/or deprotonation mechanisms. Alkoxide size and alcohol nucleophilic character significantly influence the orientation of the various ion/molecule reactions.

Detection of pharmaceutical compounds in tissue by matrix-assisted laser desorption/ionization and laser desorption/chemical ionization tandem mass spectrometry with a quadrupole ion trap

A novel quadrupole ion trap mass spectrometer laser microprobe instrument with an external ionization source was constructed and used to investigate the matrix-assisted laser desorption/ionization (MALDI) detection of pharmaceutical compounds in intact tissue. In addition to MALDI, laser desorption coupled with chemical ionization (LD/CI) was investigated. MALDI, using 2,5-dihydroxybenezoic acid (DHB) as a matrix, was employed to detect the anticancer drug paclitaxel from a thin section of rat liver tissue which had been incubated in a solution of paclitaxel. The results of that experiment showed that the ability to perform tandem mass spectrometry (MS/MS) with the quadrupole ion trap was crucial in the identification of drug compounds at trace levels in the complex tissue matrix. MALDI MS/MS was then used to detect the presence of paclitaxel in a human ovarian tumor at a concentration of approximately 50 mg/kg. Finally, the drug spiperone was detected in incubated rat liver tissue at an approximate level of 25 mg/kg using LD/CI (no MALDI matrix). Again, the MS/MS capability of the quadrupole ion trap was crucial in the identification of the drug at trace levels in the complex tissue matrix.

Langmuir oscillations under nonequilibrium conditions

The influence of external ionization sources on the characteristics of Langmuir oscillations in a partially ionized plasma is considered. It is demonstrated that in the case of continuous sources of ionization the nonlinear Langmuir oscillations are nonharmonic and are determined by the Airy equation.

Gas-phase bovine ubiquitin cation conformations resolved by gas-phase hydrogen/deuterium exchange rate and extent

The gas-phase hydrogen/deuterium exchange of [M + nH] n+ ( n = 5–13) ions of bovine ubiquitin with the H/D exchange reagent D 2O are examined by electrospray ionization Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. All of the odd or all of the even charge states were isolated by stored waveform inverse Fourier transform excitation and simultaneously reacted with D 2O leaked steadily into the ICR cell for reaction periods ranging from 1 s to 1 h. Different gas-phase protein conformations could be resolved according to difference in extent of H/D exchange. The 5+ and 6+ charge states display broad distributions of conformations ranging from 0–80% deuterium incorporation. In contrast, each of the higher charge states, 7–11+ and 13+, displays a single major isotopic distribution, whereas the 12+ charge state separates into two isotopic distributions of comparable abundance. In general, H/D exchange rates decrease with increasing charge state. External electrospray ionization source conditions (capillary current and external accumulation period) were varied while observing the conformational distribution of the 7+ charge state: increased heating in either region reduced the number of slow-exchanging conformations. At 9.4 T, it is possible to trap a large number of ions for a long reaction period (up to 1 h) at relatively high pressure (2 × 10 −7 Torr). These results demonstrate the capability of FT-ICR mass analysis following gaseous H/D exchange of electrosprayed proteins to disperse different gas-phase protein conformations for subsequent isolation and characterization.

Binding Energies of Hexahydrated Alkaline Earth Metal Ions, M2+(H2O)6, M = Mg, Ca, Sr, Ba: Evidence of Isomeric Structures for Magnesium

Binding Energies of Hexahydrated Alkaline Earth Metal Ions, M²⁺(H₂O)₆, M = Mg, Ca, Sr, Ba: Evidence of Isomeric Structures for Magnesium

The role of cationic and covalent active centers in the polymerization of 2-methyl-2-oxazoline initiated with benzyl bromide

It was shown that in the polymerization of 2-methyl-2-oxazoline initiated with benzyl bromide in nitrobenzene the equilibrium constant of the interconversion of cationic and covalent active centers could be measured over a wide range of temperatures, thus making the thermodynamic parameters of this reaction available. The “temperature jump” method was used to measure the rate constants and the activation parameters of the reversible transformation of the ionic active centers into covalent ones. The apparent rate constant of the chain growth in this polymerization was measured, and individual rate constants of chain growth on ionic and covalent centers were approximated using systems in which one kind of active centers strongly prevails. The obtained data allowed for a complete description of the individual reactions and for an estimation of the contribution of these reactions to chain growth. Because of the higher reactivity of the ionic species and higher rate of ionization without monomer participation (thus, through intramolecular ionization), there are no conditions at which the covalent species may prevail in the chain growth. Even if there is a large excess of covalent species, propagation proceeds on the ionic ones. The effective monomer concentration is approximately 103 mol/L, indicating that at 1 mol/L of monomer the internal ionization proceeds 103 times faster than the external ionization (with monomer).

The role of cationic and covalent active centers in the polymerization of 2-methyl-2-oxazoline initiated with benzyl bromide

It was shown that in the polymerization of 2-methyl-2-oxazoline initiated with benzyl bromide in nitrobenzene the equilibrium constant of the interconversion of cationic and covalent active centers could be measured over a wide range of temperatures, thus making the thermodynamic parameters of this reaction available. The “temperature jump” method was used to measure the rate constants and the activation parameters of the reversible transformation of the ionic active centers into covalent ones. The apparent rate constant of the chain growth in this polymerization was measured, and individual rate constants of chain growth on ionic and covalent centers were approximated using systems in which one kind of active centers strongly prevails. The obtained data allowed for a complete description of the individual reactions and for an estimation of the contribution of these reactions to chain growth. Because of the higher reactivity of the ionic species and higher rate of ionization without monomer participation (thus, through intramolecular ionization), there are no conditions at which the covalent species may prevail in the chain growth. Even if there is a large excess of covalent species, propagation proceeds on the ionic ones. The effective monomer concentration is approximately 103 mol/L, indicating that at 1 mol/L of monomer the internal ionization proceeds 103 times faster than the external ionization (with monomer).

Feasibility Study of Magnetohydrodynamics Acceleration of Unseeded and Seeded Airflows

Nonequilibrium, reacting, ionized gas flow modeling is used to study the feasibility of magnetohydrodynamics (MHD) acceleration of airflows for the energy addition wind tunnel. The kinetic model incorporates equations of one-dimensional magnetogasdynamics, the master equation for vibrational level populations of diatomic species, equations of chemical and ionization kinetics, and the Boltzmann equation for electrons. The model is validated by comparison with the experiments in MHD accelerators. Calculations are made for two accelerator schemes, the first using an electron beam to sustain nonequilibrium ionization in unseeded air and the second using alkali-seeded air. Although at low pressures external ionization allows substantial increase of the flow total enthalpy, the obtained test section pressure is much lower than required, and the flow quality is poor. Calculations for alkali-seeded flows predict test section flow parameters closer to the target values, with O atom and NO concentrations lower than in the e-beam-controlled flows. Flow stability is analyzed using the linear stability theory. A thermodynamic energy addition criterion is used to demonstrate the advantage of direct kinetic energy increase in MHD acceleration over thermal energy addition methods

Gas-phase dimerization of dimethylaniline in an external electrospray Fourier transform mass spectrometer

N,N-Dimethylaniline (DMA) is a common starting material in graft copolymerization and photopolymerization processes when combined with an organic acid. Several anodic oxidation studies of DMA, in an acidic medium, have been performed. Numerous electrochemical studies imply that DMA has a tendency to dimerize, forming the tail-to-tail coupling product N,N,N',N'-tetramethylbenzydine (TMB). Here, external electrospray ionization Fourier transform mass spectrometry was used to investigate the dimerization and charging characteristics of DMA in an acidic medium in the absence of an applied electrochemical technique. DMA forms both tail-to-tail and head-to-tail dimer products, and the different linkage types were determined by employing tandem mass spectrometry for structure elucidation. In addition, it was found that desolvated protonated DMA ions accumulated and stored in the hexapole ion guide for 1-10 s before transport to the trapped ion cell can undergo gas-phase ion-molecule reactions to form dimeric products, including doubly charged species. Hence the hexapole storage region can be used to conduct ion-molecule reactions away from the low-pressure ion trap.

Quantification of biomolecules by external electrospray ionization Fourier transform mass spectrometry.

Fourier transform mass spectrometry (FTMS) is well-known for its capabilities in structural characterization of molecules. Recent developments in radio frequency excitation, linearized trapping, and accumulation of ions generated from external sources have improved the potential of FTMS for quantitative analysis. Here, a commercial external electrospray ionization FTMS, employing a linearized ion trap (the Infinity Cell) and an ion accumulation procedure in which ions are deflected off-axis and injected into the trap, is evaluated as an analytical method for quantifying amino acids, peptides, and proteins. Linear response over approximately 2-3 orders of magnitude is observed for singly-charged ions with low coefficients of variation (generally < 10%), and the calibration curves generated can be used to quantify structurally similar analytes with < 4% relative error, as shown here for quantification of leucine enkephalin from curves generated by methionine enkephalin. Similar precision is obtained for multiply-charged lysozyme, but over only 1.5 orders of magnitude. Some m/z discrimination is observed as a function of trap accumulation potential for a two-component cytochrome c/lysozyme mixture. The results are promising because they suggest that quantification using liquid chromatography coupled to electrospray FTMS is possible.

Double ionization of atoms by multiply charged ions and a strong electromagnetic field: Effects of correlation in the continuum

A nonstationary theory of double ionization of two-electron atoms in collisions with multiply charged ions or by an intense electromagnetic field is developed. An approach that permits investigating both problems by a single method is formulated. A two-electron continuum wave function that takes into account the interaction of the electrons with the atomic nucleus and the external ionizer as well as with one another is obtained as a product of Coulomb waves with modified Sommerfeld parameters. The computational results obtained for the double ionization of helium atoms by multiply charged ions are in good quantitative agreement with the existing experimental data.

X‐Ray Ionization of Protoplanetary Disks

In the light of new observations of star forming regions by the Advanced Satellite for Cosmology and Astrophysics (ASCA) and by ROSAT, we assess the ability of young stars to ionize their own circumstellar disks with stellar coronal X-rays. Although stellar winds may absorb soft X-rays, hard X-rays can penetrate to large column densities and, until they are absorbed, produce ionization rates greater than standard estimates for Galactic cosmic rays. As in previous studies of the external ionization of protoplanetary disks by cosmic rays, we find that X-ray ionization produces a surface layer that is well coupled to disk magnetic fields at ~AU distances. The properties of the surface layer depend on the characteristics of the X-ray source and, thus, on the evolutionary status of the central star. Even if Galactic cosmic rays are efficiently excluded by magnetized winds, stellar X-ray irradiation alone may provide sufficient ionization for disks to accrete via the Balbus-Hawley instability. The resulting vertically layered structure of disks at ~AU distances (a well-coupled surface layer overlying a poorly coupled, deeper layer) may lead to divergent dynamical evolution in the two regions. While accretion in the surface layer contributes to the buildup of the mass of the star, the quiescent conditions in the poorly coupled, deeper layer appear to be conducive to the formation of planets.

The Hourglass Nebulae of Sher 25 and SN 1987A: Two of a Kind?

We have performed a detailed study of the morphology and kinematics of the hourglass-shaped nebula around the blue supergiant Sher 25 in the galactic giant H II region NGC 3603. Near-infrared high-resolution adaptive optics images in the Brγ line and HST/NICMOS images in the He I 1.08 μm line were compared with isovelocity maps in the Hα and [N II] lines. The adaptive optics observations clearly resolved the width of the ring (09, i.e., 0.027 pc), yielding δR/R = 1/8. We show that the Hα and [N II] lines trace the entire silhouette of the hourglass. The bipolar lobes of the hourglass expand at 70 km s-1, whereas the ring around the waist of the hourglass expands at 30 km s-1. Both the ring and the bipolar lobes have about the same dynamical age, indicating a common origin and a major outburst and mass-loss event 6630 yr ago. The ionized mass within the hourglass is between 0.3 and 0.6 M☉—quite comparable to the total mass suggested for the expanding (presupernova) shell around SN 1987A. The hourglass structure around Sher 25 is similar to that of SN 1987A in spatial extent, mass, and velocity. The major differences between these two nebulae might arise from environmental effects. Both internal and external ionization sources are available for Sher 25's nebula. Furthermore, Sher 25 and its hourglass-shaped nebula appear to be moving to the southwest with respect to the ambient interstellar medium, and ram pressure has apparently deformed the hourglass. We conclude that the circumstellar nebulae around SN 1987A and Sher 25 are very similar and define a new class of nebulae around blue supergiants in their final evolutionary stage.

Current–voltage response of anodic plasma contactors with external ionization

A spherical, steady-state model that considers the combined influence of plasma emission and external ionization of neutral gas on the operation of electron-collecting contactors is presented. Ionization by both Maxwellian and streaming electrons is investigated. A core (or quasineutral cloud) and a no-core mode are considered and the transition conditions from one to another are derived. The collected current is determined in terms of the main plasma and contactor parameters. The current-voltage response reproduces the ignited regime observed in experiments. The transition to the ignited regime and the transition to the core mode are not equivalent. It is shown that ionization by the electron beam is the main driver of ignition. The Langmuir law for the double layer causes a self-regulation of the core that affects the collection of current and leads to a multiplicity of solutions. Different neutral gas profiles are analyzed. It is uncertain whether ignition is reached when the neutral gas is provided by the contactor exclusively. The appearance of nonmonotonic potential profiles for low plasma emissions is noted.

Kinetics of high pressure beam-produced heavy inert gas plasmas

The calculations of quasistationary Kr and Xe beam-produced plasma parameters were performed in the range of gas densities Na from 1019 up to 1021 cm−3 and ionization degrees Ne/Na from 10−5 up to 10−3. The values of ion and electron densities, populations of excited states and electron temperature Te depending on external ionization rate were obtained in the way of proper balance equation system solving. The main mechanisms responsible for BPP parameters forming were analyzed. Obtained results (specifically, Te from 0.4 up to 0.8 eV) are distinguished with available information in publications.

Recent Progress in Organic Analysis Using Liquid Chromatograph Mass Spectrometer (LC-MS).

Combined liquid chromatograph mass spectrometer (LC-MS) for organic micro analysis was described. The review of the historical aspect on the development LC-MS interface is important to understand recent advanced methods for the analysis. Several problems found in the earlier developed interface are stated. The moving belt interface was the first successful desolvation method. The simplified method, such as direct liquid introduction (DLI) as well as thermospray (TSP), are discussed. Fast atom bombardment (FAB) ionization is available in the frit interface. The compositions of various spray methods are presented for the LC-MS interface including particle beam (PB), electrospray (ES), atmospheric pressure chemical ionization (APCI), sonic spray, droplet electrospray and ionspray. Several applications using these interfaces are shown. Recent mass spectrometers using Fourier transform (FT) technique which exhibits high mass resolving power and accuracy is one of the best analyzer for LC-MS work. Dual ion source equipped with matrix assisted laser desorption ionization (MALDI) and external ionization source are commonly available for FTMS. The new topics on the recent developed mass spectrometry are introduced.

Development of an “in-source” thermospray-type interface for on-line capillary liquid chromatography-mass spectrometry

Abstract The design and the construction of an “in-source” thermospray-type capillary interface for LC-MS experiments on a bench-top quadrupole mass spectrometer is described. The interface consists of a small diameter fused capillary tube which is introduced directly into the CI source through the solid probe inlet and is oriented in the beam axis. In order to produce average linear velocities comparable to those used in conventional thermospray and compatible with the optimum operating conditions of capillary LC-MS systems, a fused-silica transfer capillary of 10 μm is used. Heating of the interface is provided by the ion source and ions are generated using external mode ionization (filament-on). The effect on performance of several operating parameters, such as source temperature, mobile phase composition, sampling distance in the source and flow-rate have been investigated. The results obtained in experiments conducted on the effect of interface temperature and flow-rate demonstrate that the system is very stable under optimal operating conditions which are compatible with the optimum performance of capillary liquid chromatography systems. The interface developed can be used with a series of mobile phases (normal and reversed) and the ionization features in terms of sensitivity are in all cases essentially comparable. The system yields reproducible results and allows picogram range sensitivity to be achieved in flow injection mass spectrometric or liquid chromatographic experiments involving low-molecular-mass polar organic compounds.

Enhanced accumulated trapping efficiency using an auxiliary trapping electrode in an external source Fourier transform ion cyclotron resonance mass spectrometer

A scheme for the enhanced accumulation of ions injected into the trapped ion cell of a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer from an external electrospray ionization source is described. This method utilizes an auxiliary electrode positioned outside the trapped ion cell which increases the effective collisional path length for ions traversing the trapped ion cell. A comparison made between pressure-assisted accumulated trapping in a symmetric trapping well, an asymmetric trapping well, and a symmetric trapping well with that for the addition of an auxiliary electrode indicates that trapping efficiency can be significantly improved when an auxiliary electrode trapping configuration is utilized.

The identification of in vitro metabolites of bupropion using ion trap mass spectrometry

AbstractWe have identified in vitro metabolites of bupropion (Wellbutrin®) from incubations with human liver S9 fraction and human liver microsomes based on molecular weight information from full scan experiments using a liquid chromatograph coupled to a quadrupole ion trap mass spectrometer capable of multi‐stage operation (LC/MSn). Preliminary experiments have shown that this instrument provides comparable sensitivity to conventional LC‐coupled triple quadrupole instruments for metabolic studies, while allowing detailed structural studies using MSn experiments and routine on‐line coupling with high performance liquid chromatography via an external atmospheric pressure chemical ionization (APCI) source. The LC/MS analysis of human S9 showed the presence of three isomeric monohydroxylated metabolites of bupropion. These were further characterized in a series of MS/MS experiments which gave characteristic spectra for the three isomers. A minor dihydroxylated species was also identified in the human S9 sample and further characterized in a series of MSn experiments. Detailed structural information was generated by the use of on‐line LC/MSn type experiments. We have followed the fragmentation pathways of several molecular ion species in a series of sequential LC/MSn experiments, extending as far as MS6 with scan cycle times of less than 1.5 s. Such experiments have provided insights into the structure of specific fragment ions. Additional metabolic products were identified in the rat liver microsomes incubation sample.