bluuebunny/crossref_metadata_2025 huggingface dataset:
Calibration of ion spray mass spectra using cluster ions
Authors: Anacleto J. F. , Pleasance S. , Boyd R. K.
Published: May 2005

Mass calibration for ion spray mass spectrometry can be achieved by using cluster ions formed by flow injection of solutions of alkali metal salts in aqueous acetonitrile into the liquid flowing to the ion spray needle. Source contamination is thereby reduced to a minimum. For quadrupole mass analyzers, sodium iodide provides an ideal compromise between undesirable spectral complexity and spacings between calibrant mass peaks sufficiently close that interpolation errors are negligible. When much closer spacings are required, protonated water clusters provide an excellent calibration up to about m/z 1000. If higher mass ranges are required with a large number of calibrant peaks, a solution of mixed alkali metal iodides does provide the expected spectra but intensities are poor at higher m/z values. For liquid chromatography with on‐line mass spectrometry (LC/MS) the mass calibration may be checked without changing the mobile phase by post‐column flow injection of a cesium carbonate solution, since the carbonate anion is wholly displaced by the anion of the mobile phase acid modifier, resulting in no mixed clusters. The metal salt calibrants have the additional advantage of being useful over a wide range of tuning parameters in the atmospheric pressure ionization source, covering those appropriate to both relative molecular mass determinations of large proteins and to LC/MS of small analyte species.
Plasma desorption mass spectrometry of non‐molecular materials: Positive and negative ions formed from [Cd(SC6H4‐2‐CH3)2]∞
Authors: Craig A. Grey , Kinny David , Garbutt Robert , Dance Ian G. , Derrick Peter J.
Published: May 2005

Both positive and negative plasma desorption mass spectra of the three‐dimensionally non‐molecular material [Cd(SC6H4‐2‐CH3)2]∞ contain fragment ions directly related to the lattice structure of the material. Fragment ions are observed which correspond with [CdSC7H7]+˙ and [CdS2C7H7]−˙ which do not involve proton transfer reactions. In contrast, a fragment ion [CdS2C14H13]−˙ would appear to involve a deprotonation reaction. The scarcity of protonated positive ion fragments may be a consequence of the sample structure and/or constraints of the analysis system.
Mass spectrometric study of structure and stereochemistry of some stereoisomeric furanosidic derivatives
Authors: Borges C. , Ferreira M. A. Almoster , Jennings K. R.
Published: May 2005

A mass spectrometric study of eight new carbohydrate derivatives has been carried out to determine the influence of structure on the fragmentation mechanisms of stereoisomers. Results indicate that the relative intensities of peaks due to fragment ions of each pair of isomers are sometimes very different, especially those observed in the collision‐induced decomposition mass spectra of [MH]+ ions. Metastable decompositions related to identical fragmentation processes of two isomers release different amounts of kinetic energy, however. It is usually observed that the kinetic energy released is larger for the S‐configuration than for the R‐configuration of the two epimers of a particular species.
Studies on structure characterization of tryptophan melanin: Comparison between filament and curie‐point pyrolysis gas chromatography/mass spectrometry
Authors: Vèkey Karoly , Tamás József , Somogyi Arpád , Bertazzo Antonella , Costa Carlo , Allegri Graziella , Seraglia Roberta , Traldi Piero
Published: May 2005

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.
Proferrioxamine siderophores of Erwinia amylovora. A capillary liquid chromatographic/electrospray tandem mass spectrometric study
Authors: Feistner Gottfried J. , Stahl Douglas C. , Gabrik Ann H.
Published: May 2005

On‐line capillary liquid chromatography–electrospray mass spectrometry and tandem mass spectrometry, in combination with precursor feeding and acetylation studies, were used to identify and characterize the siderophores of the fire blight pathogen Erwinia amylovora. Proferrioxamines D2, E, G1, G2, X1, and X2 were found, with D2 and E being the major siderophores for all strains of E. amylovora studied. These proferrioxamines have previously been observed with other microbial species. In addition, eleven additional hydroxamate compounds were found, nine of which occur naturally and two which were induced by directed fermentation. The novel natural proferrioxamines included three cyclic tetrahydroxamates, designated T1–3. The most abundant of these, T1, consists of four 5‐succinylamino‐1‐hydroxyaminopentane residues. In T2 and T3, one of these residues is substituted by 4‐succinylamino‐1‐hydroxyaminobutane and 3‐succinylamino‐1‐hydroxyaminopropane, respectively. E. amylovora also incorporated 3‐succinylamino‐1‐hydroxyaminopropane into a novel trihydroxamate, which has been designated X7. The novel finding that proferrioxamines may partly be comprised of 1,3‐diaminopropane residues is noteworthy because previous feeding studies with Streptomyces olivaceus Tü 2718 suggested that 1,3‐diaminopropane is biosynthetically not tolerated. A truncated proferrioxamine G1 was also identified, which is referred to as G1t, and, on feeding of diaminobutane, tetrahydroxamates T7 and T8, which are derived from T1 by substitution of two and three diaminopentane with diaminobutane residues. The structures of five other hydroxamates, with molecular masses of 530 (X8), 544 (X9), 622 (T4), 719 (T5) and 733 (T6), are not yet known. The results prove that the biosynthetic capabilities of E. amylovora for proferrioxamine production are flexible. E. amylovora may therefore be a good organism for the fermentative production of novel proferrioxamines that may be useful for reversing or avoiding aluminum and iron intoxications in man, for tumor diagnosis and therapy with radiolabeled antibodies and for use as DNA‐cleaving reagents. On the other hand, the production of all the different proferrioxamines involves common biosynthetic steps, some of which are not used in higher plants or vertebrates. Interference with the biosynthesis of proferrioxamines may therefore provide an opportunity for the development of alternative fire blight control agents.
Computer‐aided interpretation of mass spectra using databases with spectra and structures. I. Structure searches
Authors: Henneberg D. , Weimann B. , Zalfen U.
Published: May 2005

For databases containing spectra and structures of the reference compounds, structural descriptors (‘fragments’) have been derived that are used for structure searches in the databases. The 190 fragments have been defined according to the contents of the Wiley/NBS Mass Spectral Database and to fragmentation behaviour. A search for structures with defined fragments (absence or presence of certain fragments) retrieves certain classes of compounds. An application for checking a reference spectrum is discussed. A search for structures similar to a target structure has been developed to be used in cases where a structure can be proposed for the unknown compound. The most closely related structures existing in the database will be selected, the respective spectra often being the key for interpretation or structure elucidation, as illustrated by an example.
Spectroscopy and fragmentation of alkylamines investigated by photoionization,
Authors: Moritz F. , Grotemeyer J.
Published: May 2005

The fragmentation of some alkylamines and their deuterated analogues were investigated by single‐ and multiphoton ionization. The internal energies of the molecular ions were varied prior to ionization through different expansions in a supersonic beam. Further, it is demonstrated that the difference between nano‐ and femtosecond pulses used for multi‐photon ionization has an influence on the fragmentation pattern of the amines due to a change in the activation scheme. Spetroscopy and the fragmentation reactions were investigated with respect to the internal energy of the ions. It is shown that large differences are observed in the mass spectra depending of the ionization method used. Photoionization yields intense molecular ions whereas electron impact ionization leads to strong fragmentation.
Formation of isomeric ethylenehalonium and α‐haloethyl cations and the role of ion–neutral complexes in the decomposition of protonated β‐haloethanols
Authors: Heck Albert J. R. , de Koning Leo J. , Nibbering Nico M. M.
Published: May 2005

Decomposition of protonated β‐haloethanols was found to proceed predominantly via the loss of water. The mechanism was studied in the decomposition of metastable and collisionally activated isotopically labelled protonated β‐haloethanols. The study involved the characterization of the resulting fragment ions by collisionally activated decomposition and by low‐pressure ion/molecule probe reactions. The results indicate that loss of water from protonated β‐haloethanols proceeds via two competing mechanisms. The competition between these mechanisms appears to be governed by the nature of the halogen and by the internal energy of the decomposing protonated β‐haloethanol. In the first mechanism, loss of water is considered to be initiated by a concerted 1,2‐elimination of H3O+ resulting in an ion–molecule complex between H3O+ and haloethene. Rapid and reversible proton transfers between the two species in the complex is followed by a relatively slow dissociation ofthe intermediate complex resulting in the formation of an water molecule and an α‐haloethyl cation. In competition with this mechanism, loss of water can be initiated by carbon–oxygen bond cleavage, which is assisted by halogen–carbon bond formation resulting in the formation of a water molecule and cyclic ethylenehalonium ion.
Mass spectrometry with extraction of dissolved ions at atmospheric pressure for prostaglandin identification in biological materials
Authors: Isay S. V. , Kafanova T. V. , Matveeva E. R. , Mirgorodskaya O. A.
Published: May 2005

Prostaglandins (PGA2, PGE1, PGE2, PGF2α) and their methyl esters served as standards to show the possibility of application of mass spectrometry with extraction of dissolved ions at atmospheric pressure (EDIAP) for prostaglandin identification. Applying EDIAP mass spectrometry to the soft coral Plexaura homomalla, all the prostaglandins and their derivatives described in the literature were identified.
Comparison of keV and eV collision‐induced dissociation of some protonated isomeric cyclopropane derivatives
Authors: Hamdan M. , Curcuruto O. , Tarzia G. , Balsamini C.
Published: May 2005

A number of protonated isomeric cyclopropane derivatives formed by isobutane chemical ionization (CI) were mass selected and their respective collision‐induced dissociation (CID) spectra were investigated at ion kinetic energies of 30 and 150 eV and 6 keV. Qualitative differences which may stem from different molecular structures and/or different internal excitation which may accompany the ionization process were observed in some CI mass spectra and in the low‐energy CID spectra but not in their keV counterparts. This experimental observation and the dissociation of polyatomic ions in two different energy ranges are discussed.
Photodissociation studies of small peptide ions by Fourier transform mass spectrometry
Authors: Gorman Greg S. , Amster I. Jonathan
Published: May 2005

Fourier transform mass spectrometry was used to study the 193 nm photodissociation of laser‐desorbed peptide ions. The output of an excimer laser was split and the two beams were directed sequentially at a sample probe to form ions, and through an aperture in the analyzer cell, to cause photodissociation of trapped ions. A solenoid‐driven shutter that is capable of rapid switching (30 ms) between the open and closed states allowed a single laser to be used for both desorption/ionization and photodissociation. Multiple laser pulses were required to fragment a significant fraction of the precursor ions (five pulses yielded 40% dissociation). Collisionally activated dissociation (CAD) mass spectra of the peptides were recorded and compared with the photodissociation mass spectra. Photo‐activation of peptide ions produced lower yields of fragment ions formed by loss of small molecules (H2O, NH3, CO, CO2) than did CAD, and produced higher yields of structurally informative sequence ions. For most peptides, the efficiency of ion fragmentaton by photodissociation paralleled the condensed‐phase molar absorptivity of the peptide at 193 nm. A comparison of these data with those of previous studies suggested that both the ionization method and the observation time (dictated by the type of mass spectrometer) influence the appearance of the photodissociation mass spectrum.
Formation and collision‐induced dissociation behaviour of doubly charged gas‐phase fullerene anions C, C and higher homologues. Collision‐induced electron‐stripping process
Authors: Mandelbaum A. , Etinger A.
Published: May 2005

Doubly charged molecular anions M2− of fullerenes are formed in the gas phase under chemical ionization conditions with isobutane as the reagent gas. The efficiency of double electron attachment increases with increasing size of the fullerenes: the C ion is the most abundant doubly charged anion in the negative‐ion CI mass spectrum, although the concentration of C70 was about 12% in the fullerene mixture examined. Under low‐energy collision‐induced dissociation conditions an electron is ejected from the doubly charged C ion resulting in the singly charged molecular anion C˙. This process appears to be the first report of the ejection of an electron (electron stripping) from a doubly charged anion in the gas phase:

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In situ enzymatic digestion of oligopeptides on a direct insertion probe tip combined with fast atom bombardment and tandem mass spectrometry
Authors: Mák Marianna , Tamás József
Published: May 2005

The applicability of an approach to establish the sequence of oligopeptides involving on‐probe‐tip enzymatic fragmentation and subsequent direct fast atom bombardment (FAB) mass spectrometry (MS) and FAB‐MS/MS of the subpeptide mixture was tested using two model peptides consisting of 19 and 25 amino acid residues. The method was found to be both time and sample sparing and flexible enough to use even subsequent digests, if necessary, with enzymes of different specificity. It provided in both cases complete information, not only for the sequence of enzymic fragments but also for their order in the original oligopeptide. Consequently, this approach to the sequence analysis of oligopeptides can be a method of choice when the FAB‐MS/MS technique itself is unsuitable for obtaining detailed structural information. The method seems to be applicable to enzymically digestible peptides with an upper mass limit of about 3500.
Electron impact mass spectra of phosphine sulphides and selenides
Authors: Miranda René , Salas Ismael , Salceda Catalina , Velasco Luis , Cabrera Armando , Salmón Manuel
Published: May 2005

An electron impact mass spectral study of phosphine sulphides and selenides established the effect of the heteroatom and the role of the ortho substituent in the fragmentation pattern.
Mass displacements in quadrupolar field analysers
Authors: Traldi Pietro , Favretto Donata , Catinella Silvia , Bortolini Olga
Published: May 2005

The influence of different instrumental parameters on the mass displacements observed in an ion trap and a quadrupole mass filter was investigated. In ion trap measurements it was observed that isolated ions (obtained by both ‘apex’ and ‘two‐step’ isolation methods) show the same mass displacements determined in the usual mass spectra; isobaric ions of different structure irradiated with supplementary a.c. fields are ejected at different a.c. voltages, and different cooling times do not affect the mass value. In the quadrupole mass filter, differences in mass were found among isobaric ions of different structure, although to a smaller extent than those observed in the ion trap. All the data obtained are in agreement with the hypothesis of ion–r.f. field interactions originating from the polarizability of the ion.
Fullerenes from kerogen by laser ablation fourier transform ion cyclotron resonance mass spectrometry
Authors: Rose Harry R. , Smith Derek R. , Fisher Keith J. , Dance Ian G. , Willett Gary D. , Wilson Michael A.
Published: May 2005

Raw oil shale, kerogen (demineralized shale) and carbonaceous residues from kerogen pyrolysis in the range 350–700°C (at 50°C intervals) were studied by laser ablation Fourier transform ion cyclotron resonance mass spectrometry using the fundamental frequency of Nd: YAG laser (1064 nm). Normally, pyrolysis of the raw materials produces oil and the resulting residues have decreased hydrogen to carbon ratios and exhibit relative increases in aromatic carbons. Raw shale and kerogen give positive‐ion spectra with mainly protonated species of m/z 100–400. Laser ablation positive‐ion mass spectra of the pyrolysis products of the kerogen show the presence of C60, C70 and other fullerene ions with a distribution of higher mass fullerene ions up to m/z 4000. Using high laser powers (100–3000 MW cm−2), the residue from pyrolysis at 350°C initially did not produce any fullerene ions (apart from traces of C60 and C70), but after continued ablation a cavity was formed in the target and a wide distribution of fullerene ions was obtained with subsequent laser pulses. Residues obtained from the pyrolysis of kerogen at 400–500°C produced fullerene ions at both low (4–200 kW cm−2) and high laser powers. The 550°C pyrolysis residue gave only small amounts of C60 and C70 positive ions at low laser power whereas residues from the pyrolysis of kerogen above 550°C did not give fullerene ions over a wide range of laser powers. It is proposed from the above results that the changes in the aromatic nature of the kerogen residues with increasing pyrolysis temperature are directly related to the ease of fullerene formation. This is possibly due to the formation of large polycyclic aromatic systems at pyrolysis temperatures above 400°C, formed in the residues. It should be noted that the shale samples (raw or pyrolysed) did not generate fullerene ions under any of the conditions employed in these experiments.
Metastable fractions in fullerenes
Authors: Lifshitz Chava , Laskin Julia , Peres Tikva
Published: May 2005

Metastable fractions were determined for the series of reaction Cn+ → C + C2 in fulleren samples. A reverse‐geometry double‐focusing instrument was employed with electron impact ionization and high energy resolution. Reactions were studied in the second field‐free region and corrections were made for artifact peaks due to decompositions in the first field‐free region. The metastable fractions D were converted into apparent decay constant k̄ and the results were compared with previous data from sector and time‐of‐flight instruments, respectively. The differences in the behaviour of evaporative ensembles from laser ablation of graphite and of ionized fullerenes are discussed.
The search for the gas‐phase negative‐ion pinacol rearrangement. 2—aryl rearrangements
Authors: Dua Suresh , Alexander Margaret J. , Bowie John H.
Published: May 2005

Deprotonated phenyl ethylene glycols, on collisional activation, lose water by a negative ion pinacol rearrangement involving phenyl migration. For example, deprotonated benzpinacol undergoes the following reactions: Ph2C(OH)C(O−)Ph2 → [PhCOCPh3 – H]− + H2O. It is proposed that this rearrangement proceeds from an eclipsed (or near eclipsed) conformer which is stabilized by the HO and O− groups forming a strong intramolecular hydrogen bond.
Unimolecular reactions of isolated organic ions: The chemistry of the oxonium ions CH3CH2CH2CH2+O = CH2 and CH3CH2CH2CH = O+CH3
Authors: Bowen Richard D. , Derrick Peter J.
Published: May 2005

The reactions of the metastable oxonium ions CH3CH2CH2CH2+O = CH2 and CH3CH2CH2 = O+ CH3 are reported and discussed. Both these isomers of C5H11O+ expel predominantly CH2O (75–90% of the metastable ion current), a moderate amount of C3H6 (5–15%), a minor amount of CH3OH (2–8%) and a very small proportion of H2O (0.5–3%). All these processes give rise to Gaussian metastable peaks. The kinetic energy releases associated with fragmentation of these oxonium ions are similar, but slightly larger for dissociation of CH3CH2CH2CH = O+CH3. The behaviour of labelled analogues confirm that the reactions of CH3CH2CH2CH = O+CH3 are closely related, but subtly different. Elimination of CH2O and C3H6 is intelligible by means of mechanisms involving CH3CH+CH2CH2OCH3. This open‐chain cation is accessible to CH3CH2CH2 +O = CH2 by a 1,5‐H shift and to CH3CH2CH2‐CH = O+CH3 by two consecutive 1,2‐H shifts (or, possibly, a direct 1,3‐H shift). The rates of these 1,2‐, 1,3‐ and 1,5‐H shifts are compared with one another and also with the rates of CH2O and C3H6 loss from each of the two oxonium ions. The 1,5‐H shift that converts CH3CH+CH2CH2OCH3 formed from CH3CH2CH2CH = O+ CH3 into CH3CH2CH2+O = CH2 prior to CH2O elimination is essentially unidirectional. In contrast, the corresponding step converting C5H11O+ ions generated as CH3CH2CH2CH2+O = CH2 into CH3CH+ CH2CH2OCH3 competes effectively with expulsion of CH2O and C3H6. The implications of the latter finding for the degree of concert in the hydrogen transfer and carbon‐carbon bond fission steps in alkene losses from oxonium ions via routes that are formally isoelectronic with the retro ‘ene’ pericyclic process are emphasized.
Dissociation mechanisms of energy‐selected chlorobutane ions: Experiment and theory
Authors: Ferreira M. A. Almoster , Cabral B. J. Costa , Oliveira M. C. , Baer T.
Published: May 2005

The breakdown diagrams and photoionization efficiency curves of isomeric chlorobutane ions were obtained by energy‐selected molecular ions with the photoelectron–photoion coincidence (PEPICO) technique. The loss of HCl from 1‐ and 2‐chlorobutane molecular ions was found to be fast, and accurate dissociation onsets were determined from the crossover energy in the breakdown diagrams and appropriately corrected for the precursor thermal energy. In spit of the low activation energy, the isochlorobutane molecular ion was found to dissociate slowly near the dissociation limit, the time‐of‐flight distribution having been analyzed in terms of two exponential decay rates. The dissociation reaction involved a large kinetic energy release that pointed to a 1,2‐elimination leading to a stable C4H ion. AB initio calculations indicated that the reaction path involved an H‐atom transfer through a barrier, which is a favorable case for a tunneling model to explain the slow decomposition rate. The kinetic energy release of chlorine radical loss from tert‐chlorobutane was determined as a function of the ion internal energy. At low energies the kinetic energy release was nearly statistical but the dissociation from the excited electronic states resulted in a large and non‐statistical kinetic energy release.
Mechanism, energetics, kinetics and dynamics of the reaction C2H6+˙ → C2H4+˙ + H2
Authors: Bråten Svein Magne , Helgaker Trygve , Uggerud Einar , Vulpius Tore
Published: May 2005

The distributiosn of relative translational energy released during (i) loss of H2 from metastable CH3CH3+˙ ions, (ii) loss of HD from metastable CH3CD3+˙ ions and (iii) loss of D2 from metastable CD3CD3+˙ ions were measured. The relevant parts of the potential energy surface of the reaction C2H6+˙ → C2H4+˙ + H2 (including isotopic variants) were investigated using various theoretical methods (high‐level ab initio quantum chemistry, RRKM calculations, dynamic reaction trajectory calculations). A consistent reaction model is presented which reproduces available experimental data. Quantum mechanical barrier tunnelling is found to be important, leading to an activation energy (on the microsecond time‐scale) which is approximately 0.1 eV below the critical energy.
Oxirane as a chemical ionization gas: Reactivity towards different classes of compounds
Authors: Lange C.
Published: May 2005

Oxirane chemical ionization (CI) gives numerous ions, including C2H3O+ and C2H5O+. These ions react with organic molecules through various specific ion–molecule reactions such as hydride abstraction, protonation, additions or cycloadditions. Oxirane CI allows discrimination between unsaturated compounds with [M + 43]+ and [M + 57]+ adduct ions and heteroatom functions with [M + 45]+ adduct ion. All are diagnostic ions. Oxirane CI permits selectivity during the ionization process of a mixture and discrimination of isomers.
The role of ion–neutral complexes in the reactions of onium ions and related species
Authors: Bowen Richard D.
Published: May 2005

An account is given of the development of the proposal that ion–neutral complexes are involved in the unimolecular reactions of onium ions (R1R2CZ+R3; Z = O, S, NR4; R1, R2, R3, R4 = H, CnH2n + 1), with particular emphasis on the informative C4H9O+ oxonium ion system (Z = O; R1, R2 = H; R3 = C3H7). Current ideas on the role of ion‐neutral complexes in cation rearrangements, hydrogen transfer processes and more complex isomerizations are illustrated by considering the behaviour of isomeric CH3CH2CH2X+ and (CH3)2CHX+ species [X = CH2O, CH3CHO, H2O, CH3OH, NH3, NH2CH3, NH(CH3)2, CH2NH, CH2NCH3, CO, CH3˙, Br˙ and I˙]. Attention is focused on the importance of four energetic factors (the stabilization energy of the ion–neutral complex, the energy released by rearrangement of the cationic component, the enthalpy change for proton transfer between the partners of the ion neutral complex and the ergicity of recombination of the components) which influence the reactivity of the complexes. The nature and extent of the chemistry involving ion‐neutral complexes depend on the relative magnitudes of these parameters. Thus, when the magnitude of the stabilization energy exceeds the energy released by cation rearrangement, the ergicity of proton transfer is small, and recombination of the components in a new way is energetically favourable, extensive complex‐mediated isomerizations tend to occur. Loss of H2O from metastable CH2O+C3H7 ions is an example of such a reaction. Conversely, if the stabilization energy is small compared with the magnitude of the energy released by eation rearrangement, the opportunities for complex‐mediated processes to become manifest are decreased, especially if proton transfer is endoergic. Thus, CH3CH2CH2CO+ expels CO, with an increased kinetic energy release, after rate‐limiting isomerization of CH3CH2CH2+CO to (CH3)2CH+CO has taken place. When proton transfer between the components of the complex is strongly exoergic, fragmentation corresponding to single hydrogen transfer occurs readily. The proton‐transfer step is often preceded by cation rearrangement for CH3CH2CH2X+ species. In such circumstances, the involvement of ion–neutral complexes can be detected by the observation of unusual site selectivity in the hydrogen‐transfer step. Thus, C3H6 loss from CH2N+(R1)CH2CH2CH3 (R1 = H, CH3, C3H7) immonium ions is found by 2H‐labelling experiments to proceed via preferential α‐and γ‐hydrogen transfer; this finding is explained if the incipient +CH2CH2CH3 ion isomerizes to CH3CH+CH3 prior to proton abstraction. In contrast, the isomeric CH2N+(R1)CH(CH3)2 species undergo specific β‐hydrogen transfer because the developing CH3CH+CH3 cation is stable with respect to rearrangements involving a 1,2‐H shift.
Methoxymethyl cation [CH3OCH2]+ revisited: Experimental and theoretical study
Authors: Audier H. E. , Bouchoux G. , McMahon T. B. , Milliet A. , Vulpius T.
Published: May 2005

The metastable dissociation of the methoxymethyl cation and a number of its deuterium and 13C variants was examined using a reverse‐geometry double‐focusing mass spectrometer. The loss of methane from the methoxymethyl cation clearly showed a composite peak shape which, when deconvoluted, revealed a bimodal kinetic energy release distribution in the resulting formyl cations. Labelling experiments revealed that the two carbon atoms and all hydrogens become equivalent on the time‐scale of the unimolecular dissociation lifetime of the decomposing ion. A small deuterium isotope effect was found which can be rationalized on the basis of zero point energy effects. The bimodal kinetic energy release distribution was shown, with the aid of a four‐sector instrument, to be due to the production of both formyl cation (with a large kinetic energy release) and isoformyl cation (with a much smaller kinetic energy release). The methoxymethyl cation was also prepared with a precisely defined amount of internal energy in a Fourier transform ion cyclotron resonance (FTICR) spectrometer by the reaction of methyl cation with formaldehyde. Experiments with 13C and deuterium labelling revealed that the dissociation to formyl cation of the methoxymethyl cations formed in the low‐pressure FTICR cell by reaction of methyl cation with formaldehyde is accompanied by complete scrambling of the carbons and incomplete scrambling of the hydrogens. Ab initio calculations were carried out which identified and characterized each of the stable minima and transition states for the appropriate reactions. The calculations were fully consistent with the mechanism deduced on the basis of the experimental data.
Simple determination of relative hydrogen atom affinities by supercritical fluid chromatography/atmospheric pressure chemical ionization mass spectrometry
Authors: Matsumoto Kozo
Published: May 2005

Coupled supercritical fluid chromatography and atmospheric pressure chemical ionization mass spectrometry was applied to estimate hydrogen atom affinities. The intensities of the M+ and MH+ peaks in the mass spectra were measured at various drift voltages (collision energies). The ratios of the two peaks were plotted against the drift voltages to obtain threshold drift voltages. The threshold drift voltages of polycyclic aromatic hydrocarbons were plotted and found to be proportional to the values of reported hydrogen atom affinities. The value for acenaphthylene was estimated to be 292.9 kJ mol−1 from this relationship.
Field‐dependent [M – 2H]+. formation in the field desorption mass spectrometric analysis of hydrocarbon samples
Authors: Heine Curt E. , Geddes Marie M.
Published: May 2005

High‐boiling hydrocarbons often yield abundant [M — 2H]+. ions under the conditions of field desorption mass spectrometry (FDMS). This work evaluated [M — 2H]+. formation from various saturated and aromatic compounds. The most intense [M — 2H]+. signals observed resulted from the analysis of saturated compounds containing long and branched chains. The presence of an aromatic ring in a molecule, however, strongly diminished [M — 2H]+. formation during FDMS. Experiments involving manipulation of the applied potential between the FD anode and cathode reveal the strong field dependence of this phenomenon. At a potential difference of 10 kV, strong [M — 2H]+. formation occurred for samples including squalane, hexatriacontane (C36 n‐alkane) and a Polywax 655 mixture. Analyses of the same samples conducted at a potential difference of 4 kV produced only weak (if measurable) [M — 2H+.] signals. The magnitude of [M — 2H]+. formation also decreased as the sample quantity decreased.
Disparity between solution‐phase equilibria and charge state distributions in positive‐ion electrospray mass spectrometry
Authors: Wang Guangdi , Cole Richard B.
Published: May 2005

Two peptides, bradykinin and gramicidin S, were used to investigate the relationship between protonation in the solution phase and charge state distribution observed in electrospray ionization (ES) mass spectra. The degree of protonation in solution was estimated using acid‐base equilibrium calculations where possible. Protonation in solution was varied by adjusting pH, solvent composition and peptide concentration. Major disparities were observed between calculated solution‐phase peptide protonation and the charge state distributions observed in ES mass spectra. The [(M + 2H)2+]/[(M + H)+] ratio calculated in solution was larger than the abundance ratio (M + 2H)2+ /(M + H)+ in the ES mass spectra of all acidic aqueous (pH < 6.5) and non‐aqueous solutions; in basic aqueous solutions (pH > 9.5) the opposite was true. At high pH, electrophoretic droplet charging may reduce the activity of OH− in positively charged droplets. The results at low pH imply the existence of supplementary factors in the ES ionization process which largely attenuate the degree of charging in the gas phase as compared with solution. Factors such as the increasing intra‐ and intermolecular coulombic repulsion between charge carriers (protons) and increasing attractive forces between protonated sites and counterions at progressively later stages of charged droplet evaporation were hypothesized to be chiefly responsible for this effect. Non‐aqueous solvents of high basicity compete with analytes to some extent for available protons, forming protonated solvent molecules while decreasing the sensitivity and the degree of multiple charging of peptides.
Reaction of ionized propene with methanol: Comparison with CH3OH2+ formation from ionized 2‐methylpropanol
Authors: McAdoo David J. , Zhao Guohong , Ahmed Mohammed S. , Hudson Charles E. , Giam C. S.
Published: May 2005

Methanol exchanges its hydroxyl hydrogen with deuteriums in ionized propene during the formation of protonated methanol. This exchange is attributed to the interconversion [CH3CHCH2+. HOCH3⇌][CH2 ⋅CHCH2 +H2OCH3] in single collisions. The exchange in this ion‐molecule reaction is analogous to that observed in the formation of protonated methanol from ionized 2‐methylpropanol, supporting the intermediacy of ion‐neutral complexes in the final step of the latter reaction. Ion‐molecule reactions were studied using a Fourier transform mass spectrometer.
A tandem mass spectrometric study of C2H5I+CH3 ions: A search for the non‐classical ethyl cation stabilized by methyl iodide
Authors: Zagorevskii Dmitri V. , Holmes John L.
Published: May 2005

The results of this study have shown that the C2H5I+CH3 ion containing classical and non‐classical forms of the ethyl group can be generated by gas‐phase ion—molecule reactions. The classical ions fragment by CH4 and CH3⋅ losses, the latter without loss of positional identity of H atoms in the ethyl group. Hydrogen scrambling, however, precedes ethene loss, taking place in an ion of non‐classical form. The latter is produced directly from ion—molecule reactions and from energy‐rich classical ions. The relative energies of the classical and non‐classical ions could not be determined, although the former was proposed to be the global minimum. Finally, the classical ion serves as a source for the ylid ion CH3ICH2+., which previously had eluded preparation.
Water sorption characteristics of grp composite: Effect of outdoor weathering
Authors: Blaga A.
Published: May 2005

The complete sorption‐desorption equilibrium isotherms determined at 23° C indicate that glass fiber‐reinforced polymer (GRP) sheets weathered outdoors for 3 years absorb larger amounts of water vapor than the non‐weathered sheets. Sorption and desorption Kinetic results at 23° and two levels of relative pressure (p/p0 = 0.80 and p/p0 = 0.975) show that the weathered sheets absorb water vapor at considerably higher rates than the non‐weathered sheets. Larger amounts of sorbed water, and faster sorption and desorption will result in respectively greater magnitudes and higher rates of dimensional changes (swelling and shrinking). This accelerates the overall effects of environmentally exerted stress‐fatigue on the GRP composite.
Low‐velocity impact damage in graphite‐fiber reinforced epoxy laminates
Authors: Rhodes Marvin D. , Williams Jerry G. , Starnes James H.
Published: May 2005

An experimental investigation was conducted to identify the failure mechanism and to understand damage propagation in compression‐loaded composite structures. The tests were conducted on several laminates of different ply orientation with thicknesses that ranged from 0.56 to 0.79 cm. The panels were damaged by 1.27‐cm‐diameter aluminum spheres propelled normal to the specimen surface at velocities ranging from 30 m/s to 140 m/s. Results indicate that there is significant internal laminate damage due to low‐velocity impact with no surface damage. The internal damage consists of delamination and intraply cracking. Three damage propagation modes were identified as causing specimen failure; delamination, axial load‐lateral deformation coupling, and local shear failure.
Flexural fatigue of marine laminates reinforced with woven roving of E‐glass and of Kevlar® 49 aramid
Authors: Zweben Carl , Wardle Marilyn W.
Published: May 2005

A comparison of the flexural fatigue characteristics of E‐glass and Kevlar® 49 aramid in polyester and vinyl ester resin hand lay‐ups typical of boat hull laminates is presented. Data on unidirectional E‐glass and aramid composites from epoxy preimpregnated tapes are reported for comparison. The data indicate that while the initial flexural strength of E‐glass woven roving laminates is greater than that of aramid laminates, the runout stress of aramid laminates at 106fatigue cycles is similar or superior to glass. S‐N curves for aramid laminates are flatter indicating better flexural fatigue resistance.
Fatigue of glass and carbon fiber reinforced engineering thermoplastics
Authors: Mandell J. F. , Huang D. D. , McGarry F. J.
Published: May 2005

Cyclic tension fatigue S‐N curves are given for injection moleded Nylon 6/6, polycarbonate, polysulfone, polyphenylene sulfide, and poly(amide‐imide) matrices with glass and carbon fibers as well as for unreinforced material. The S‐N curves for most composites appear linear, with no evidence of a fatigue limit up to 106cycles. Some nonlinearity is evident with the Nylon 6/6 composities, and these appear to fail at a cumulative strain similar to the ultimate static strain. The remainder of the composites appear to fail by a crack propagation mechanism. The glass reinforced materials all degrade at a similar rate in fatigue, while the carbon reinforced materials with brittle matrices degrade more slowly than do those with ductile matrices. The latter effect may be due to greater integrity of the cracked regions for brittle matrix systems.
The resin matrix: Its influence on sheet molding compound mechanical properties
Authors: Pritchard Geoffrey , Rhoades Gary V.
Published: May 2005

A series of sheet molding compound (SMC) materials have been prepared in which the only variable was the polyester resin used as the matrix. The polyester resins were all synthesized from maleic anhydride, propylene glycol and 1,2 propane diol. Systematic variations were made in the molar ratio of maleic of to phthalic anhydride used, and also in the hydroxyl values of the polyesters. The resins were also separately crosslinked with styrene, and the fracture toughness of the cast sheets measured. The toughness of the SMC was assumed to be measurable by linear elastic fracture mechanics (LEFM) methods. Results indicated that the resin matrix toughness was inversely related to polyester unsaturation but the SMC toughness was directly related to it. The hydroxyl values of the resins were determined by nuclear magnetic resonance (NMR) of the silyated derivatives. The tensile properties of the SMC were investigated. Modulus increased steadily with resin hydroxyl value, but tensile strength showed a maximum at 50 mg. These observations indicate the importance of hydroxyl value in SMC resin formulation, arising from its effect on fiber‐resin bond strength.
Physical aging in graphite/epoxy composites
Authors: Kong Eric S. W. , Lee Susanna M. , Nelson Howard G.
Published: May 2005

Matrix‐dominated mechanical behavior of a graphite/epoxy composite has been found to be affected by sub‐Tg annealing. Postcured (±45°)4S specimens of Thornel 300 graphite/Narmco 5208 epoxy were quenched from above Tg and given a sub‐Tg annealing at 140°C for times up to 105 min. The ultimate tensile strength, strain‐to‐break, and toughness of the composite material were found to decrease as functions of sub‐Tg annealing time. No weight loss was observed during the sub‐Tg annealing. The time‐dependent change in mechanical behavior is explained on the basis of free‐volume changes that are related to the physical aging of the nonequilibrium glassy network‐epoxy. The results imply possible changes in composite properties with service time.
Effect of water and isooctane absorption on the flexural fatigue strength of a sheet molding compound
Authors: Hoa S. V. , Ngo A. D. , Sankar T. S.
Published: May 2005

The effect of water and isooctane absorption on the flexural fatigue strength of a sheet molding compound (SMC‐R30) is investigated. Absorption of water reduces the fatigue life of specimens of SMC‐R30 as compared to the fatigue life of specimens tested in air. Absorption of isooctane reduces the fatigue life much more than absorption of water.
Optimum strength of composites of ceramic particles in poly(methyl methacrylate)
Authors: Atsuta M. , Turner D. T.
Published: May 2005

Ceramic particles were sedimented in methyl methacrylate, which was then polymerized by exposre to γ‐rays. After annealing, specimens were cut and tested, and the ceramic content determined by ashing. Diamond indentation hardness was found to increase monotonically with increase in ceramic content. In contrast, both Young's modulus and, more markedly, flexural strength were found to give optimum (peak) values for a content of lithium aluminum silicate near 80 wt‐percent. The higher silicate contents were reached with use of a silane coupling agent and by centrifuging, prior to polymerization. Examination of fracture surfaces by scanning electron microscopy indicated that the decrease in mechanical properties was due to the presence of voids in the polymeric matrix. It is suggested that void formation becomes serious when ceramic particles are initially too close‐packed to respond to conraction, during polymerization, by shifting closer together. Consistent with this suggestion, both void formation and peaking in strength can be prevented by continuously centrifuging the composite throughout the polymerization reaction.
Fatigue damage studies in high strength sheet‐molding compound fiberglass composites
Authors: Kundrat R. , Joneja S. , Broutman L. J.
Published: May 2005

The materials considered in this investigation were oriented‐hybrid‐glass‐fiber‐reinforced vinyl ester resin sheet‐molding compound (SMC) composites. The composition of one material was 60 percent continuous glass and 5 percent random fibers, while the other was 45 percent continuous glass and 20 percent random fibers. The S‐N curves for the two materials were determined in the continuous fiber direction. The fatigue stress levels ranged from approximately 20 to 80 percent of each material's respective ultimate tensile strength. The residual tensile strengths of the fiber‐reinforced composites were not seriously reduced until near final fracture. The moduli of the composites in the fiber direction are also not reduced in any detectable manner. Damping measurements have shown that the damping increases during fatigue and that it is a sensitive measure of matrix and interface damage, generally debonding and cracking. It is shown by damping measurements that damage accumulates differently for two materials studied. The material with the higher percent of continuous glass appeared to be more cycle‐dependent, while the other was more stress‐dependent.
Monitoring cure viscosity of epoxy composite
Authors: Tajima Yuji A.
Published: May 2005

Viscosity is an important process variable that should be monitored and controlled to ensure the consistent and reliable production of graphite‐fiber‐reinforced composite parts by the autoclave/vacuum bag process. Unfortunately, no technique is currently known for the direct measurement of viscosity in this process. However, d.c. conductivity and a.c. phase lag appear to correlate quite well with viscosity, and either electrical property may be followed in order to indirectly monitor viscosity. As the method of measuring d.c. conductivity is so inexpensive, simple, and almost free of electrical interference compared to the method of measuring a.c. phase lag, it appears to be the method of first choice for application in an autoclave environment.
Use of wood fibers in thermoplastic composites
Authors: Kokta Bohuslav V. , Chen Rubie , Daneault Claude , Valade Jacques L.
Published: May 2005

Wood fibers from aspen and spruce have been used for filler and reinforcement of polystyrene. The wood fibers used were in the form of refined wood. In order to improve compatibility of wood fibers with polymeric matrices, fibers have been modified by copolymerization with styrene. The kanthate method of grafting employing the ferrous‐hydrogen peroxide catalytic system was used for fiber treatment. The following properties of composites have been measured: elastic‐modulus, tensile strength, and energy absorbed at break. In summary, it has been found that the composites from grafted fibers showed superior mechanical properties to those with original fibers. In general, as a filler, the aspen fibers were superior to spruce, and the shorter fibers superior to longer ones. Mechanical properties of composites as compared to polystyrene were improved as follows: elastic modulus +37 percent; tensile strength +35 percent and energy at breakup by 43.5 percent. The best composites properties have been achieved at 40 percent of fiber fraction present.
Evaluation of surface treatments for fibers in composite materials
Authors: Fraser W. A. , Ancker F. H. , Dibenedetto A. T. , Elbirli B.
Published: May 2005

An embedded single filament test has been developed that measures the cumulative distribution of critical fibers lengths for a given fiber‐polymer system. The mean value of critical lengths and the dispersion of values are a function of both the shear transmission characteristics of the interface and the strength characteristics of the fibers. A computer model of the stochastic fracture process has been developed that incorporates a mixed‐element, weakest‐link model to describe the strength properties of the fibers, a random generator to describe the stochastics of multiple fiber fracture, and a shear‐lag analysis to describe the shear transmission across the interface. From the measurement of critical fiber lengths, a value for the magnitude of the shear transmission across the fiber‐polymer interface, τ, can be determined. From measured values fiber strength, the ability of a surface treatment to protect a given fiber can be determined. A variety of surface treatments for E‐glass are evaluated in a number of thermoplastic polymer materials. The experimental technique and the associated analysis are shown to be valuable in separating the coupling and sizing effects of the fiber surface treatment formulations. This should be of particular value in the optimization of commercial fiber treatment formulations.