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  • Analytical Chemistry and Spectroscopy  (12)
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  • Analytical Chemistry and Spectroscopy  (12)
  • Chemistry  (12)
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  • 1
    Electronic Resource
    Electronic Resource
    Various energy minima and corresponding fragmentation processes: Alkylsilanes (1993)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 28 (1993), S. 1491-1497 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Fragmentation of alkylsilanes, in particular trimethylethylsilane, were studied by mass-analysed ion kinetic energy (MIKE) and collision-induced decomposition MIKE techniques. Ab initio and semi-empirical molecular orbital calculations were applied to explain the main fragmentation processes. These calculations indicate that more than one minimum can be located on the potential energy surface of a given ground-state molecule ion. These differ from each other mainly in the length of the silicon-carbon bonds. The structures can be adequately described as complexes of a trivalent silyl ion and an alkyl radical. Each of these complexes fragments by the loss of the weakly bound alkyl radical. The calculated energetics of these reactions were found to be in good agreement with the observed energy dependence of the mass spectra.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210281221
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  • 2
    Electronic Resource
    Electronic Resource
    Electron impact-induced fragmentation of 2,3-dihydro-1,5-benzothiazepin-4(5H)-ones (1994)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 29 (1994), S. 303-308 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electron impact-induced fragmentations of 2,3-dihydro-1,5-benzothiazepin-4(5H)-ones and some related compounds were investigated. On the basis of low- and high-resolution measurements, metastable ion studies by means of mass-analysed ion kinetic energy spectroscopy and collision-induced dissociation experiments, the main fragmentation pathways were established. The effect of methyl and phenyl substituents at the C(2) and C(3) positions of the heterocyclic ring on the fragmentations was also studied. The (1,3) ring splitting was investigated in some detail, using semi-empirical molecular orbital calculations.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210290607
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  • 3
    Electronic Resource
    Electronic Resource
    Reactive ion-surface collisions: Application of ionized acetone-d6, DMSO-d6 and pyridine-d5 as probes for the characterization of self-assembled monolayer films on gold (1993)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 28 (1993), S. 1665-1673 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Low-energy collisions of ionized acetone-d6, DMSO-d6 and pyridine-d5 with alkanethiolate self-assembled monolayer (SAM) surfaces prepared on gold were investigated. These experiments can be used to monitor routinely the condition of the surfaces and to determine whether a surface is acceptable for a given application. In addition to the surface-induced dissociation of the molecular ions, intense peaks corresponding to the addition of H (or D) to the ionized molecules are observed in the ion-surface collision spectra. The relative abundance of the [M + H(D)]+ reaction products was found to be dependent on several variables, including (i) the chemical composition of the monolayers (alkanethiolate, perdeuteroalkanethiolate, fluorinated alkanethiolate), (ii) the chain length of the alkanethiolate monolayers, (iii) the time for which the vapor-deposited Au surfaces are exposed to the thiol solution during surface preparation and (iv) the degree of deliberate damage of the SAM film (70 eV Ar+· beam bombardment). The variation in [M + H(D)]+ abundance with induced surface changes illustrates the utility of these adduct ions for the characterization of the quality of the monolayers. The general trend observed is an increase in the [M + H]+ peak with the increased presence of hydrocarbon adsorbates, which in turn is believed to be directly related to increased disorder (inherent or induced) of the monolayer film. The increase in [M + H]+ is seen with a decrease in chain length, with shorter monolayer preparation times, and with an increase in surface damage. In general, a fluorinated surface that has been intentionally damaged by an Ar+· beam retains its highenergy deposition characteristics and is appropriate for use as a collision target for many types of projectiles.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210281244
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  • 4
    Electronic Resource
    Electronic Resource
    Influence of different ionization conditions on the mass spectrometric behaviour of some fluorinated acyclic nucleoside analogs (1995)
    New York, NY : Wiley-Blackwell
    Rapid Communications in Mass Spectrometry 9 (1995), S. 1127-1134 
    Details
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The behaviour of three fluorinated acyclic nucleoside analogs has been examined under different ionization conditions, i.e. by electron ionization, fast-atom bombardment and positive-ion chemical ionization using methane, isobutane and ammonia as reactant gases. The protonated molecules have been studied by collisional spectroscopy. In general, protonation takes place on the functional groups exhibiting a proton affinity higher than that of the reactant species but some discrepancies are present. They have been explained by the presence of quasi S—H—F structures implicating proton bridging between sulphur and fluorine atoms.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/rcm.1290091210
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  • 5
    Electronic Resource
    Electronic Resource
    Thermal decomposition kinetics of protonated peptides and peptide dimers, and comparison with surface-induced dissociation (1995)
    New York, NY : Wiley-Blackwell
    Rapid Communications in Mass Spectrometry 9 (1995), S. 829-836 
    Details
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Rate constants for the unimolecular decomposition of peptide monomer and dimer ions by thermal and surface-induced dissociation (SID) are measured and compared. Rate constants for thermal dissociation are measured in a heated wide-bore capillary flow reactor attached in front of the capillary leading into the mass spectrometer. Thermal decomposition of the leucine enkephalin ion (YGGFL)H+ is observed between 600 and 680 K with rate constants of 20-200 s-1, and yields many of the same fragments as SID at 35 eV, although with different relative intensities. The thermal decomposition yields the Arrhenius parameters Ea = 38.3 kcal/mol, log A = 15.7. The decomposition of the monomer and dimer ions are also observed by using SID on C18 and fluorinated hydrocarbon surfaces, with rate constants of 2 × 104 to 40 × 104 s-1. The SID activated monomer ions are assigned equivalent temperatures of 710-840 K by extrapolation of the thermal activation parameters. The protonated dimer ion (YGGFL)2 H+ decomposes thermally at 500-540 K to yield the monomer ion. The dimer also decomposes by SID at low collision energies 10-20 eV on both surfaces to yield the monomer ion, and at much higher energies of 60-80 eV to yield fragments identical to the decomposition of the monomer. The large energy requirement for fragmentation from the dimer is due to energy deposition into more degrees of freedom plus the additional energy required for dissociation of the dimer to the monomer. It is assumed that the energy deposition is linear with collision energy up to 80 eV, and that the energy becomes randomized throughout the dimer, including energy flow through the hydrogen bond(s). These mechanistic assumptions are supported quantitatively by the SID energy relations between monomer and dimer fragmentation. Thermal decomposition of the larger, multiply protonated melittin ion [M + 3H]3+ occurs at substantially higher temperatures, between 810-840 K, than those required for thermal decomposition of (YGGFL)H+, to yield many of the same sequence ions as produced by SID at 135 eV on a fluorinated surface.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/rcm.1290090922
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  • 6
    Electronic Resource
    Electronic Resource
    On the structure of [C9H6O]+· ions originating by electron impact induced decomposition of chalcone (1991)
    New York, NY : Wiley-Blackwell
    Rapid Communications in Mass Spectrometry 5 (1991), S. 569-573 
    Details
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The electron impact induced fragmentations of chalcone have been studied with the aid of high resolution measurements, using metastable decompositions by linked scans and mass-analysed ion kinetic energy spectroscopy through collisional experiments by ion-trap mass spectrometry and labellng by 13C deuterium. Based on the data obtained, two different mechanisms for the formation of m/z 130 ions have beenproposed.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/rcm.1290051115
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  • 7
    Electronic Resource
    Electronic Resource
    Internal energy distribution of benzene molecular ions in surface-induced dissociation (1995)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 30 (1995), S. 212-217 
    Details
    ISSN: 1076-5174
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The so-called ‘deconvolution’ method has been used to determine the internal energy distribution of molecular ions of an organic compound, benzene, excited by collisions with self-assembled monolayer surfaces formed on gold. The average internal energy was found to increase linearly with the laboratory collision energy. The kinetic energy-internal energy (T-V) conversion was 17% for the octadecanethiolate monolayer and 28% for the 2-(perfluorooctyl)-ethanethiolate monolayer surface. The results are similar to those obtained for metal carbonyl projectiles, though they indicate somewhat higher energy conversion. In addition, excitation of the projectile ion well beyond 20 eV internal energy is observed.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jms.1190300132
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  • 8
    Electronic Resource
    Electronic Resource
    Use of bond orders and valences for the description and prediction of primary fragmentation processesPaper presented at the Ninth Informal Meeting on Mass Spectrometry at Padua, Italy. 22-24 April 1991. (1991)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 26 (1991), S. 936-938 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Some approaches and examples are shown to demonstrate the usefulness of inherently ‘classical’ parameters, such as bond orders and valences derivable from quantum mechanical wavefunctions, in the description and prediction of mass spectral primary fragmentation processes.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210261104
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  • 9
    Electronic Resource
    Electronic Resource
    Stereochemical effects in the mass spectra of some cyclopropylsilanes explained by the MNDO molecular orbital method. I. Silicon-oxygen interactions (1992)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 27 (1992), S. 869-875 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Large differences have been found between the mass spectra of cis- and trans-1-carbomethoxy-2-trimethylsilylcyclopropane. To help in explaining these differences, quantum chemical calculations were performed by the MNDO method on the neutral molecules, on the vertical and adiabatic molecular ions and also the [M - methyl]+ ions. To evaluate the results, heats of formation, bond orders, molecular geometry, spin and charge densities and orbital coefficients were used. These indicate the formation of a very strong bonding interaction between silicon and oxygen in the molecular ion of the cis but not the trans isomer, leading to spontaneous methyl loss and a stabilization energy of ∼ 2.6 eV.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210270805
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  • 10
    Electronic Resource
    Electronic Resource
    Studies on structure characterization of tryptophan melanin: Comparison between filament and curie-point pyrolysis gas chromatography/mass spectrometry (1992)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 27 (1992), S. 1216-1219 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structural studies of a synthetic melanin, obtained by means of performic acid oxidation of tryptophan, were carried out by pyrolysis gas chromatography/mass Spectrometry (Py-GC/MS). To identify the best pyrolysis conditions, both Curie-point pyrolysis and filament pyrolysis were employed and the effects of pyrolysis temperatures and times were studied. Using the first approach, various compounds were identified: toluene, ethylbenzene, styrene, indole, methylindole, ethylindole, phenol, cresol and ethylphenol. Using filament pyrolysis some interesting differences could be observed. Whereas toluene, ethylbenzene, phenol, cresol and methylindole were found, neither indole nor ethylindole was detected. Instead, new pyrolysis products were evident, such as methylpyrrole and indolin-2-one. Hence filament pyrolysis seems to activate different thermal decomposition pathways of the melanin under study. It is suggested that tryptophan melanin is a polymer containing indole and hydroxyindole derivatives as subunits.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210271111
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