Positive Molecular Ions Research Articles

Study of the admixture influence on the oxygen spectra properties

The spectra of oxygen from two different producers and oxygen with small admixture of rare gases or nitrogen were studied. The spectra were generated by positive column of the glow discharge. The spectral lines (wavelength of 844.7 and 777.4 nm) and the bands of oxygen (atmospheric A-band of molecular oxygen and first negative system of molecular positive oxygen ion) were analyzed. It was found that in very pure oxygen (Linde production) the first negative system is practically not developed. Small admixture of another gas causes appearance of this system. The influence of the nitrogen was more significant. It is possible to conclude that existence or non-existence of the full first negative system is dependent on the presence of the admixture with energetic levels near to oxygen ones.

The calculation of time-resolved negative-ion-to-neutral-to-positive-ion-spectra with an application to iron-carbonyl

We investigate “negative-ion-to-neutral-to-positive-ion” (NENEPO) pump-probe spectroscopy as proposed by Wöste and co-workers [Wolf et al., Phys. Rev. Lett. 74, 4177 (1995)] where the dynamics of wave packets in a neutral molecule produced by photodetachment from the negative ion is probed by time-delayed photoionization and detection of positive molecular ions or photoelectrons. Approximations are introduced for an effective numerical treatment of the double-ionization process. The organometallic compound FeCO is used as an example.

Ions and electrons of the lower‐latitude D region

A positive and negative ion chemical steady state model which combines about 400 ion‐chemical and photochemical reactions with a two‐dimensional neutral model for minor atmospheric constituents is presented in the height range 50–84 km, for latitudes −50° to +50° in January and September. The ion model yields concentrations of the main positive and negative ions and of electrons. Two important parameters for the description of the lower ionosphere were deduced, namely, the ratio λ of electrons to negative ions and the ƒ+ parameter, describing the ratio of the positive cluster ions to positive molecular ions. In addition, the parameterization of ƒ+ indicates a strong dependency on the water vapor concentration. At the transition height (ƒ+ = 1) at midlatitudes the water concentration can be expressed as a function of the electron density and the temperature: [H2O] = 1.09 × 10−6[ρ]0.6T−3[e], with the units of the water vapor density [H2O], the air density [ρ], and the electron concentration [e] in cm−3 and T in degree kelvin.

Limits on stability of positive molecular ions in a homogeneous magnetic field

The problem of stability of positive diatomic molecular ions with the nuclear chargesZ1 andZ2 andN electrons in a homogeneous magnetic fieldB is studied forZ1,Z2,N,B→∞. The conditions of instability are obtained for different relations amongZ1,Z2,N andB. A new version of the HVZ theorem for systems in a homogeneous magnetic field is proved.

Laser desorption/ionization time-of-flight mass spectrometry of nitrated polycyclic aromatic hydrocarbons.

Mass spectra of four nitrated polycyclic aromatic hydrocarbons (nitro-PAHs), 9-nitroanthracene, 1-nitropyrene, 2-nitro-9-fluorenone, and 2-nitrofluorene, have been investigated using single-step laser desorption/ionization time-of-flight mass spectrometry. Pulsed UV radiation at 266 or 213 nm was used for desorption and ionization of solid samples deposited on an aluminum probe. The positive molecular ion which was observed for each nitro-PAH was of greater relative intensity when 213 nm radiation was used. A strong [M - NO]+ peak was observed in all spectra, and an intense NO+ signal accompanied the [M - NO]+ signal when 213 nm was used but was only weakly present when 266 nm was used. Comparison of the various spectra suggests that nitro-PAHs undergo an excited state nitro-nitrite rearrangement, followed by loss of NO. Multiphoton ionization of the NO fragment appears to be the principal route of formation of NO+ during laser desorption/ionization when 213 nm radiation is used. The presence of the carbonyl group in 2-nitro-9-fluorenone leads to unique and prominent fragments involving losses of CO from the carbonyl bridge.

Direct mass spectrometric analysis of ozonides: application to unsaturated glycerophosphocholine lipids.

The reaction of ozone with double bonds present in glycerophosphocholine lipids results in formation of ozonides that can be directly analyzed by mass spectrometry as either positive or negative molecular ion species generated by electrospray ionization. Polyunsaturated fatty acyl groups esterified to the phospholipid yielded a mixture of ozonide species with the maximum number of ozone molecules added equal to the total number of double bonds. Ozonide decomposition resulted in omega-aldehyde and omega-carboxylic acid products as revealed by ESI-MS. Collisional activation of the ozone adducts for mono- and polyunsaturated phospholipids gave rise to fragment ions indicative of the position of the double bonds in these molecules. The major decomposition pathway for either positive or negative ozonide ion species involved charge remote fragmentation of the ozonide initiated by homolytic cleavage of the peroxide bridge followed by rearrangement to form the omega-aldehyde and omega-carboxylate acyl species. The reaction of ozone with phospholipids containing polyunsaturated fatty acyl groups is a useful method to probe the position of double bonds by electrospray ionization mass spectrometry.

Gas-phase fast-atom bombardment mass spectrometry

Gas-phase fast-atom bombardment (FAB) mass spectra of fullerene C 60, polycyclic aromatic hydrocarbons, n-hydrocarbons, heterocyclic compounds, and others, by using high energy He, Ar, Xe, or isobutane beams, have been compared with the electron impact ionization (EI) and/or charge exchange ionization (CEI) mass spectra. The positive ion gas-phase FAB mass spectra measured show a fragmentation pattern characterized by a high intensity of lower-mass fragment ions and considerable fragmentation, the extent of which exceeded that in the He CEI mass spectra, and an intense peak corresponding to molecular ions M ·+. From the results obtained it was suggested that the internal energy distribution of M ·+ ions formed under gas-phase FAB conditions ranges widely from the non-fragmenting lower energy to considerable fragmenting higher energy. The Massey criterion for the He (8 keV) beam indicated that a single collision between a fast atom A fast and an analyte molecule M gas brings about the formation of singly and/or doubly charged molecularions as the most likely energy deposition process. The formation of multiply charged molecular and fragment ions. M ·+ and m 2+ ( z = 1–4), and the extent of fragmentation seemed to be dependent on the structure of the analyte molecules, as well as the bombarding particles used. The mechanisms of formation of positive and negative molecular ions M +, M -, and [M-H] -, under gas-phase FAB conditions, has been discussed on the basis of the positive and negative ion mass spectra obtained.

Structural Characterization of Phospholipids by Matrix-Assisted Laser Desorption/Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance mass spectrometry provides for structural analysis of the principal biological phospholipids: glycerophosphatidylcholine, -ethanolamine, -serine, and -inositol. Both positive and negative molecular or quasimolecular ions are generated in high abundance. Isolated molecular ions may be collisionally activated in the source side of a dual trap mass analyzer, yielding fragments serving to identify the polar head group (positive ion mode) and fatty acid side chains (negative ion mode). Azimuthal quadrupolar excitation following collisionally activated dissociation refocuses productions close to the solenoid axis; subsequent transfer of product ions to the analyzer ion trap allows for high-resolution mass analysis. Cyro-cooling of the sample probe with liquid nitrogen greatly reduces matrix adduction encountered in the negative ion mode.

The CID spectra of hydrated oxygen ions

AbstractA steady production of positive hydrated oxygen molecular ions has been achieved by the introduction of H2O vapour into the ion source of a mass spectrometer under chemical ionization conditions using O2 reagent gas. Collision‐induced dissociation, mass‐analyzed ion kinetic energy (CID MIKE) spectra (R. G. Cooks, J. H. Beynon, R. M. Caprioli and G. R. Lester, Metastable Ions, Elsevier, Amsterdam, 1973) of the m/z=50 ions, which correspond to the H2O·O ions, show a prominent loss of a water molecule, but no significant hydrogen loss. By contrast, the CID MIKE spectra of the H2O ions obtained under identical conditions indicate an important successive loss of hydrogen and oxygen atoms, giving rise to an intense H2O+· peak. Conversely, with H2O·O, a relatively loosely‐bound adduct of the two stable moieties is indicated, a ‘cluster’ that fragments into its constituents with a preferential charge retention on the species of lower ionization energy.

Evidence for strongly enhanced yields of negative molecular secondary ions due to bombardment with SF n cluster ions

We have performed the first comparative study of the effect of molecular ion bombardment on the yield of positive and negative molecular ions sputtered from adsorbed layers. Mass spectra of differently treated silicon samples were measured by TOF-SIMS using 27–33 keV Xe + and SF − 6 primary ions at impact angles between 56° and 66°. The spectra of positively charged secondary ions are dominated by characteristic peaks due to C n H + m , ( m, n = 0, l, 2, …) ions and, prior to etching, by the well-known plasticizer peak at mass number 149 u. The secondary ion yields Y + are higher for SF − 6 than for Xe + impact, the yield ratio R + = Y + (SF − 6)/ Y + (Xe +) amounting to a factor of about 2 at low masses ( M < 35 u) and a factor of 4 to 6 at higher masses. This moderate yield enhancement may tentatively be attributed to a corresponding difference in the cross sections for ion induced desorption. The spectra of negatively charged secondary ions, observed under SF − 6 bombardment, exhibit high yields of H −, O −, OH −, C n H − m ( n = 0, 1, 2, …; m = 0 and 1), (SiO 2) − n and (SiO 2) n X −, where X can be H, O and OH. For M < 35 u the yield ratio R − (≈ 3) is similar to R +, but with increasing molecular ion mass R − increases rapidly to become as large as ≈ 100 for M > 80. Comparative studies using primary ion beams of Xe +, SF + 5 and SF − 5 showed that the strong yield enhancement observed in the negative SIMS mode is due to the molecular character of the projectile and only marginally (if any) related to the projectile charge. In an application of this cluster bombardment technique we show that changes in composition of the topmost layers, brought about by etching of the sample, can be studied in detail by comparing the yields of bulk-type high-mass molecular ions like (SiO 2) n X −( X = H or OH).

A coincidence counting study of spontaneous desorption

The “spontaneous” emission of ions and photons from surfaces was studied with coincidence counting time-of-flight mass spectrometry. Photons were produced by the spontaneous desorption process with an experimental yield of about 0.01%. Both positive and negative secondary ion yields were measured under various conditions. Results imply classes of desorption events. This may indicate that several types of primary ions are being produced during a single experiment. There was no detectable change in the types of primary ions involved in the spontaneous desorption process as a function of sample potential within the range examined. Positive and negative valine molecular ion yields were found to be similar.

The reactions of positive and negative halogen ions with Cl2 and Br2

A selected ion flow tube study has been carried out at 300 K of the reactions of some atomic and molecular positive and negative halogen ions with Cl2 and Br2 from which the rate coefficients k and ion product distributions have been determined. For the energetic F+ ion reactions, dissociative charge transfer is the dominant process, while for the Cl+ ions, only nondissociative charge transfer occurs. For the less energetic Br+ and I+ reactions, dihalogen molecular ions are important products. All these positive ion reactions proceed quite efficiently, i.e., the k are appreciable fractions of kc, their respective collisional rate coefficients, except for the reactions of Cl2 with the lower energy ions of the spin–orbit triplet of I+, i.e., I+(3P1,0), for which k∼0.07kc, this being due to the endothermicities of the reactions. The molecular ion Cl2+ undergoes rapid nondissociative charge transfer with Br2, a process which is, of course, endothermic for the reaction of Br2+ with Cl2 and so no reaction is observed. The less-energetic atomic negative ion reactions proceed—via atom exchange—in which the atomic negative ion of the reactant molecular species and a dihalogen molecule are produced. For those reactions that are exothermic, the k are, within error, equal to (2/3)kc, implying that they proceed via complexes which separate statistically back to reactants (1/3) and forward to products (2/3). Both the Br−+Cl2 and Cl−+Br2 reactions are somewhat less efficient (i.e., k&amp;lt;2/3kc), a result of the slight endothermicities of the reactions. Of the molecular negative ion reactions, electron transfer is the major process in the Cl2− reaction with Br2, whereas the reaction of Br2− with Cl2 proceeds relatively slowly producing the triatomic ion BrCl2−.

On molecular ion formation by hydride abstraction in plasma desorption mass spectrometry

Positive molecular ion formation in plasma desorption (PD) by hydride abstraction leading to [M − H] + molecular ions is not a frequently observed phenomenon. An example of a compound that exhibits an [M − H] + molecular ion in its PD spectrum is the neutral lipid cholesterol. Data on the initial radial velocity distribution of the [M − H] + ion of cholesterol are presented and compared to data on the radial velocity distribution of the M + molecular ion of a structurally similar compound (ergosterol). The distribution for [M − H] + cholesterol ions is broader and its centroid is closer to zero radial velocity than the corresponding distribution for ergosterol M + ions. These data support a model suggesting formation of [M − H] + by a “self” chemical ionization type reaction.

Kinetic energy measurements of molecular ions ejected into an electric field by matrix‐assisted laser desorption

Measurements of kinetic energy distributions of molecular ions ejected into an extraction field by matrix-assisted laser desorption are reported. The measurements were made in a time-of-flight mass spectrometer with an electrostatic mirror by measuring the reflected signal as a function of the difference between the accelerating voltage and the voltage applied to the mirror. The molecular ions were found to have less kinetic energy than the extraction field alone would normally provide, i.e., we observed an energy deficit. Under conditions typical for a matrix-assisted laser desorption experiment, the deficit is about 24 eV for molecular ions of insulin. The size of the deficit increases with the intensity of the molecular ion signal, and the molecular weight of the protein; it is also larger for negative molecular ions than for positive molecular ions.

Absence of discrete spectrum of the Schr�dinger operators of positive molecular ions

For multiatomic positive molecular ions withN electrons and chargesZ i of the nuclei it is shown that there are no bound states forZ i >γN for some γ. It is also shown that the discrete spectrum of the Schrodinger operator of a molecular ion is empty if for at least some of the nuclei the conditionZ i ≫N is satisfied, whereas the charges of the other nuclei can be small.

Prediction of the electronic energy levels of doubly charged positive molecular ions with the aid of the multiple scattering Xα method

Approximate electronic structure calculations with the multiple scattering Xα method have been employed to predict the energy differences, for NH 3 and N 2O, between the neutral molecule ground state and energetically low-lying states of the doubly charged positive molecular ion with the neutral molecule's geometric structure. Comparison with experimental values of such vertical double ionization energies, observed with double charge transfer spectroscopy, indicates that direct application of the method gives unsatisfactory results. Instead a combination of an interaction energy calculated with the method and experimentally determined single ionization energies can predict double ionization energies to an accuracy sufficient to aid the analysis of double charge transfer spectra.

Secondary-ion and electron production from surfaces bombarded by large polyatomic ions.

Heavy molecular ions with energies in the range 10-20 keV and masses from 276 u to 132,000 u, produced by matrix-assisted laser desorption, were used as primary projectiles to produce secondary-ion spectra from a variety of surfaces in a tandem time-of-flight mass spectrometer. In the negative mode the ratio of electron emission to secondary-ion emission was found to decrease rapidly with increasing projectile mass. Ion emission was found to dominate for primary ions larger than approximately 10,000 u. Positive or negative molecular ions and cations were observed from several organic targets of masses up to 1140 u (gramicidin S) for incident projectiles up to mass 132,000 u, i.e., for projectile speeds down to approximately 7000 m/s. Other ions characteristic of the target were also observed for these projectiles. Thus, large polyatomic ions can cause secondary-ion desorption even at very low velocity. The background ions of both polarities are similar to those found in keV particle bombardment by monatomic projectiles. The same ions are observed for all the projectiles; most can be identified with hydrocarbon background. The relative intensities of the background positive ions are largely independent of projectile, and for both polarities the ratio of the ions characterizing the target to those forming the background is approximately constant for all the projectiles. These results strongly suggest that the background ions come from the usual layer of organic impurities attached to the target surface. No direct evidence for surface-induced dissociation was observed in this mass and energy range.

Relative sensitivity factors for positive atomic and molecular ions sputtered from Si and GaAs

Secondary ion mass spectrometry (SIMS) relative sensitivity factors (RSFs) are tabulated for atomic and molecular ions sputtered from Si and GaAs matrices using oxygen primary ions and positive secondary ions. The RSFs are inversely proportional to the secondary ion yields. RSFs are plotted versus atomic number to establish patterns and trends. The data for molecular ions aid in the resolution of certain mass interferences and also have implications for the SIMS ionization process.

Thin-layer chromatography-mass spectrometry with ionization by californium-252 fission products used for studying molecular mass distribution of oligomeric polystyrenes

The potential of a time-of-flight mass spectrometer with ionization by californium-252 fission products has been studied with regard to identification of oligomeric polystyrenes (PS) by means of their positive molecular ions M +. The optimized procedure for sample preparation prior to mass spectrometry consists of evaporation of PS on a substrate prepared by electrodispersion of a cellulose nitrate solution. The sensitivity of the method depends on the molecular mass of the oligomeric polymerhomologues and on the sample mass, but is in any case high enough for being used for investigating fractions obtained by thin-layer chromatography (TLC) and having molecular masses of up to 2 × 10 3, without the necessity of a prior sample concentration. The method enables one to identify oligomeric polymerhomologues extracted from the TLC plate and to determine the mass of the monomer unit and of the end groups from the difference in the molar mass of the ions M + of neighbouring oligomers. These results, along with those of quantitative thin-layer chromatography (and also chromatograms obtained by means of other chromatographic methods) contain all information necessary for establishing the molecular mass distribution (MMD) of the oligomers, i.e. comprise an absolute method for determining the MMD of oligomers.

MacProMass: A computer program to correlate mass spectral data to peptide and protein structures

A program known as MacProMass has been written for Macintosh computers to assist in the analysis of mass spectral data of peptides and proteins. The program employs a user friendly, graphical interface and accommodates a variety of protein structures including cyclic peptides and multiple chain proteins. In addition to molecular mass calculations for positive and negative molecular ions, MacProMass also calculates elemental composition, amino acid composition, isoelectric point, surface free energy, and high-performance liquid chromatography index values for whole structures and peptide fragments resulting from enzymatic or chemical degradation. Users can program their own amino acid residues and terminal groups. In addition to search routines for both mass and sequence, theoretical fragment ions for peptide mass spectra can be calculated. Analysis of variant proteins is facilitated with a subroutine that systematically catalogs single amino acid substitutions that correspond to mass differences between observed and expected molecular ions. Interchain and intrachain disulfide bonds and other types of linkages are maintained throughout the chemical and enzymatic degradation operations.