Mass spectrometry of negative ions of pyridine N-oxides

The dissociative capture of electrons by N-oxides of pyridine and α-, β-, and γ-picolines was investigated by mass spectrometry [1]. The mass spectra of the negative ions were studied. The curves of the effective yields of the negative ions as a function of the energies of the electrons were obtained.     

Pyrolysis mass spectrometry of terephthalate polyesters using negative ionization

The usual method of studying thermal degradation mechanisms of polymers in vacuo is to use electron ionization pyrolysis mass spectrometry. This can lead to mass spectral fragmentation from the 70 eV electrons used. Low energy electrons (10–15 eV) produce a low abundance of positive ions. However, if a molecule is prone to capture a thermal energy electron, then negative ions are produced in high abundance. This report describes the negative ion pyrolysis mass spectrometry of polyethylene terephthalate and polybutylene terephthalate.     

Positive and negative ions of the amino acid histidine formed in low‐energy electron collisions

Histidine is an aromatic amino acid crucial for the biological functioning of proteins and enzymes. When biological matter is exposed to ionising radiation, highly energetic particles interact with the surrounding tissue which leads to efficient formation of low‐energy electrons. In the present study, the interaction of low‐energy electrons with gas‐phase histidine is studied at a molecular level in order to extend the knowledge of electron‐induced reactions with amino acids. We report both on the formation of positive ions formed by electron ionisation and negative ions induced by electron attachment. The experimental data were complemented by quantum chemical calculations. Specifically, the free energies for possible fragmentation reactions were derived for the τ and the π tautomer of histidine to get insight into the structures of the formed ions and the corresponding neutrals. We report the experimental ionisation energy of (8.48 ± 0.03) eV for histidine which is in good agreement with the calculated vertical ionisation energy. In the case of negative ions, the dehydrogenated parent anion is the anion with the highest mass observed upon dissociative electron attachment. The comparison of experimental and computational results was also performed in view of a possible thermal decomposition of histidine during the experiments, since the sample was sublimated in the experiment by resistive heating of an oven. Overall, the present study demonstrates the effects of electrons as secondary particles in the chemical degradation of histidine. The reactions induced by those electrons differ when comparing positive and negative ion formation. While for negative ions, simple bond cleav ages prevail, the observed fragment cations exhibit partly restructuring of the molecule during the dissociation process.     

Free energies of electron ejection of aromatic hydrocarbon negative ions in ethers at room temperature

The free energies of electron ejection of some aromatic hydrocarbon mono- and di-negative ions in DME and 2-MTHF at room temperature are estimated from existing experimental data. The formation of free solvated electrons requires about the same amount of free energy whether the negative ion is present as a loosely bound or a tightly bound associate with the sodium counter ion. When the ejected electrons are trapped by a solvent cavity in the vicinity of the sodium ion the reaction requires appreciably less energy for the tightly bound than for the loosely bound associates. The data are essential for the study of the photo-ionization of aromatic hydrocarbon negative ions in ethers.     

Effective shell charge of electrons on a sphere

By ignoring the radial motions of the electrons in the valence shell of an atom one formally obtains the problem of electrons constrained to move on a sphere. This sphere will be attracted by the core as if it had an effective charge equal to the number of electrons on it minus a certain quantity resulting from the mutual repulsion of the electrons. This “effective shell charge” is a very simple, but still precise and quantitative concept which provides a good understanding of many empirical facts about atoms and ions, most notably Hund's rules. Implications for negative ions are discussed and chemical bonding is touched briefly. A qualitative difference in physical behavior for small and large sphere radii is pointed out.     

Mass spectra of negative ions of anabasine and esters of lupinine

Conclusions The mass spectra of negative ions of anabasine derivatives and lupinine esters have been studied. The resonance capture of slow electrons to form both fragmentary and molecular ions strongly depends on the donor-acceptor properties of the substituents.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2249–2253, October, 1981.     

Negative chemical ionization in quadrupole ion trap mass spectrometry: Effects of applied voltages and reaction times

The effects of applied voltages and reaction times on negative ion chemical ionization in the quadrupole ion trap are investigated. Mass-selected ejection of undesired reagent ions and selective mass storage of only negative ions are required for practical negative ion chemical ionization. This is achieved by application of rf and dc voltages to the ring electrode to control the mass-to-charge ratios one polarity) of ions stored, as well as by application of a supplemental rf voltage applied across the endcap electrodes to selectively eject ions of a particular mass-to-charge ratio. Even with careful control of these parameters, negative chemical ionization is not as sensitive as electron ionization and positive chemical ionization because of the lack of thermal electrons in the ion trap. Mass selection of the hydroxide anion as a reagent ion and exclusion of all positive ions provide [M ? H]^? ions with little or no fragmentation for a wide variety of compounds.     

Ion formation mechanism in laser desorption ionization of individual nanoparticles

Covariance mapping is used to study ion formation mechanisms in laser desorption ionization of individual 50 or 220 nm diameter particles having compositions similar to ambient aerosol. Single particle mass spectra are found to vary substantially from particle to particle. This variation is systematic--the energetically preferred ions (e.g., lowest ionization energy, highest electron affinity) are positively correlated with each other and negatively correlated with less preferred ions. For the compositions studied, the average positive ion yield is two to five times greater than the negative ion yield, indicating that free electrons are the main negatively charged species. For many particles, typically 20% to 40% of those analyzed, only positive ions are detected. Smaller particles give fewer negative ions, presumably because the plume is less dense and electron capture is less likely. The results suggest that ion formation occurs by a two stage process. In the first stage, photoionization of laser desorbed neutrals gives cations and free electrons. In the second stage, collisions in the plume cause electron capture and competitive charge transfer. When the particle ablates in a manner giving a dense plume with many collisions, the energetically preferred positive and negative ions are dominant. When the particle ablates in a manner giving a less dense plume with fewer collisions, the less preferred ions are able to survive and the energetically preferred ions constitute a lower fraction of the total ion signal. Systematic particle to particle variations of relative signal intensities can complicate ambient particle classification efforts by spreading a single particle composition over several classes.     

Mass-spectrometric study of the cyclization of diazo compounds. 10. 2-Diazomalonic acid amides. Mass spectra of the negative ions

The mass spectra of the negative ions of the dissociative resonance capture of electrons of diazo amides and the isomeric triazoles were studied. The molecular negative ions of these compounds are unstable and do not undergo interisomerization. The principal fragmentation process involves the elimination of a molecule of nitrogen and transformation of the resulting [M-N₂]^– ions to the heterocyclic form, which is the same for the two isomers.See [1] for communication 9.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 941–944, July, 1987.     

Formation of negative ions by interaction of electrons with heterocyclic compounds

Curves are derived for the effective yield of negative ions from the interaction of electrons with thiophene, 2-methylthiophene, 2-propylthiophene, 3-methylthiophene, 3-propylthiophene, furan, and selenophene. It is found that there are two quasi-stationary states of the molecular negative ions having lifetimes of about 10^–14 and 4 · 10^–15 sec. The cross sections for formation of these states are estimated, and also the cross sections for resonant elastic scattering of electrons. The states are shown to be related to excited states of the molecule. It is found that alkyl substitution has the following effects on resonant electron capture: a) reduces the probability of ring breakage and b) reduces the energy levels of the quasi-stationary state. Some aspects of dissociative ionization are discussed.     

Mass spectra of negative ions from thiophane and its alkyl derivatives

The mass spectra of the negative ions from the dissociative capture of electrons were obtained for thiophane and its alkyl derivatives. With a few exceptions, all of these negative ions of the spectrum contain sulfur atoms: electron capture is not observed in cyclic compounds — cyclopentane and methylcyclopentane. The principle routes of dissociation during electron capture were isolated for thiophane and alkylthiophanes and explain the greater portion of the observed lines.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1631–1632, December, 1971.     

Design of a high-sensitivity negative ion source time-of-flight mass analyzer assembly created by cylindrical electrodes with a common axis

The new design incorporates the negative ion source and the mass analyzer, both constructed from cylindrical electrodes. The ion source is formed by three gridded cylindrical electrodes: a pulsed grid, the intermediate grid and the final accelerating grid. During a first time lapse, the electrons penetrate through the pulsed grid into the retarding field between this grid and the intermediate grid. The electrons are turning at some depth inside this intergrid space, where the attachment to neutral molecules most probably occurs. Next, the pulsed grid becoming strongly negative and ions are extracted towards the final acceleration grid. The ions from the cylindrical surface where they were created concentrate on the common axis of the electrodes (lateral focusing). The source lateral and time focus are coincident. A cylindrical electrostatic mirror is fitted to the source. The design, with a single stage, ensures also lateral focusing of the ions diverging from the common axis of the electrodes. The mirror electric and geometric parameters were selected to ensure both lateral and time focusing on the final detector with subsequent high luminosity. The basic parameters of the specific negative ion source time-of-flight mass analyzer design proposed here, are ion source final acceleration, intermediate, pulsed cylindrical grid radii 10, 20 and 30 mm, respectively, electrostatic mirror earthed grid and ion turning points surface radii 0.6 and 0.8 m, respectively. Ion packet smearing by the ion energy spread (resulting from the initial electron energy spread as electrons are turning at different depths inside the ionization region, from the moment when ions were created, being accelerated towards the pulsed grid during ionization) and by the turnaround time inside the cylindrical field was accounted for. Maintaining very high sensitivity, a resolution of the order of 100 is expected.     

Electron capture ion mobility spectrometry for the selective detection of chlorinated and brominated species after capillary gas chromatography

This paper reports the first investigation of electron capture ion mobility spectrometry as a detection method for capillary gas chromatography. In previous work with negative ion mobility detection after gas chromatography, the principal reactant ion species were O₂^? or hydrated O₂^? due to the presence of oxygen in the drift gas. These molecular reactant ions have a mobility similar to chloride and bromide ions, which are the principal product ions formed by most halogenated organics via dissociative ion-molecule reactions. Oxygenated reactant ions thus interfere with the selective detection of chloride and bromide product ions. A recently described ion mobility detector design efficiently eliminated ambient impurities, including oxygen, from infiltrating the ionization region of the detector; consequently, in the negative mode of operation, the ionization species with N₂ drift gas were thermalized electrons. Thermalized electrons have a high mobility and their drift time occupies a region of the ion mobility spectrum not occupied by chloride, bromide, or other product ions. The result was improved selectivity for halogenated organics which ionize by dissociative electron capture. This was demonstrated by the selective detection of 4,4′-dibromobiphenyl from the components of a polychlorinated biphenyl mixture (Aroclor 1248).     

Kinetic energy of free electrons affects MALDI positive ion yield via capture cross-section

A method for enhancing positive analyte ion signal in MALDI is described. The idea is based on influencing the kinetic energy of free electrons emitted from the organic/metal interface. It has been recently shown that free electrons in MALDI have energies around 1 eV. This energy is close to the maximum capture cross-section of most common MALDI matrices, leading to the efficient formation of negative matrix ions. This results in the reduction of the positive analyte ion yield. The effect can be minimized by shifting the kinetic energy of the electrons away from the maximum of the matrix capture cross-section by choosing a different substrate material.     

Resonant electron capture by some amino acids and their methyl esters

Resonant electron capture mass spectra of aliphatic and aromatic amino acids and their methyl esters show intense [M-H](-) negative ions in the low-energy range. Ion formation results from a predissociation mechanism mediated by the low-energy pi*oo resonant state. Methylation in general has little influence on the electronic structure according to quantum chemical calculations, but the corresponding ions from the methyl esters, [M-Me](-), could be ascertained to arise only at higher resonance energies. Aromatic amino acids are characterized by an additional low-energy fragmentation channel associated with the generation of negative ions with loss of the side chain. The complementary negative ions of the side chains are more efficiently produced at higher energies. The results have significant implications in biological systems as they suggest that amino acids can serve as radiation protectors since they have been found to efficiently thermalize electrons.     

Radiation damage of biosystems mediated by secondary electrons: resonant precursors for uracil molecules

Calculations are presented for the energy locations and spatial structures of low-energy resonant states describing transient negative ions (TNIs) of the uracil molecule in the gas phase. The resonant states are modeled using scattering calculations of low energy electrons interacting with isolated molecules in their equilibrium geometry. The interaction forces used in this model are described in detail. Examination of the spatial densities of the excess resonant electrons for the various TNIs found by the calculations allows one to associate the metastable anions with specific features of the experimentally observed fragmentation patterns.     

激光作用磷化铟所产生的负离子质谱及其分析

以激光蒸发结合超声分子束膨胀,近年来已成为产生与研究原子簇的一种重要手段。在以这一方式产生的Ga_xAs_y的光电离质谱中,发现含奇数个原子的簇离子的信号强度相对较高。最近我们在自制的装置上,于高真空中直接以脉冲激光作用于GaAs、GaP、InP等多种半导体材料,在所记录的负离子质谱中也都观察到类似的奇强偶弱的现象,其中尤以In_xP_y~-最为显著。实验用的激光离子源飞行时间质谱计的构造已有另文详细介绍。该装置通过朝相反方向分别加速正负离子而可同时记录激光等离子体的正负离子质谱。实验采用Nd~(3+):YAG激光器的调Q倍频输出(532nm),聚焦后作用于样品表面的激光功率密度约为10~8W·cm~(-2)。质谱计的加速电压1kV,正负离子的无场漂移长度约1.15m,数据的模数转换速度为2×10~7s~(-1)。实验所用的磷化铟是片状的高纯度半     

Negative ion-containing plasma in parallel-plate radio-frequency discharge in oxygen

The properties of a plasma in a parallel-plate radio frequency (rf) symmetric discharge of 13.56 MHz in oxygen have been investigated. The plasma contains negative ions. The temperature and density of the negative ions have been determined from the slope and height of the semilog-plotted second derivative of the probe characteristics. The laser photodetachment technique is applied, and data indicating the existence of negative ions are obtained. The negative ion densities deduced by both methods are comparable. The negative ion temperature is found to be from 0.1 to 0.6 eV, depending on the pressure and rf power density. The electron energy distribution f(E) deviates from Maxwellian and has a depletion of low-energy parts, which may be due to the loss of low-energy electrons in attachments. A shifted annular-shaped distribution is fitted to the measured f(E), and the data analysis is made according to this form of f(E).     

Electric field assisted thermal desorption ionization using an infrared laser

A new technique has been developed for the formation of gas-phase electrons and ions by electric field assisted thermal desorption ionization at atmospheric pressure. Experiments were carried out using a sharp tungsten wire tip coated with a thin solid sample film which was irradiated by a 10.6 μm infrared laser. By applying a strong electric field on the laser-irradiated tungsten wire tip, abundant ions such as alkali ions, halide ions, and also multiply charged negative ions S(2)O(6)(2-) and S(2)O(8)(2-), were formed. Copyright 1999 John Wiley & Sons, Ltd.     

Negative matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry fragmentation of synthetic analogs of the O-specific polysaccharide of Vibrio cholerae O:1 in the presence of anionic dopants

Oligosaccharides (tri- to hexamers) that represent terminal epitopes of O-antigens of Vibrio cholerae O:1, serotypes Ogawa and Inaba, have been studied by negative matrix-assisted laser desorption/ionization time of flight/time-of-flight mass spectrometry (MALDI ToF/ToF MS). The [M - H(+)](-) ions are formed after expulsion of a proton from molecules studied under condition of MALDI MS analysis in the negative mode. Several ammonium salts (chloride, nitrate, hydrogencarbonate and hydrogensulfate) were used as additives to increase the formation of negative ions from saccharides. The most efficient was the addition of ammonium hydrogencarbonate, which increased the amount of [M - H(+)](-) ions more than six times. Between three fragmentation pathways, the new conjugated transfer of electrons within the second downstream unit of oligosaccharides was discovered. Production of these ions, which has not been observed in any other kinds of measurement, distinguishes substances belonging to Ogawa and Inaba serotypes. The negative MALDI ToF/ToF mass spectra are simpler and, at the same time, more informative, as compared with positive and negative electrospray ionization ion trap as well as with positive MALDI ToF/ToF analysis.