bluuebunny/crossref_metadata_2025 huggingface dataset:
Wavelength‐dependent fragmentation in resonance‐enhanced multiphoton ionization mass spectrometry
Authors: Kinsel Gary R. , Segar Kimberly R. , Johnston Murray V.
Published: May 2005

Wavelength‐dependent effects in the resonance‐enhanced multiphoton ionization/fragmentation mass spectra of p‐chloroaniline and diphenyl ether are presented. For both molecules, the formation of low‐energy fragments can be discriminated against in favor of higher‐energy fragments by using ‘low’‐energy radiation (290 nm region) for ionization/fragmentation. The same low‐energy fragments become dominant when higher‐energy radiation (266 nm) is used. This unique behavior is explained in terms of the narrow distribution of parent ion internal energies created through resonance‐enhanced multiphoton ionization/fragmentation and the competing kinetic processes accessed by the parent ion as it absorbs each successive photon.
Hydrogen interchange prior to the fragmentation of protonated molecules
Authors: Harrison Alex G.
Published: May 2005

Protonated ethanol shows extensive interchange of hydrogen between the carbons and oxygen prior to fragmentation to either [C2H5]++H2O or to [H3O]++C2H4. Protonated isopropanol shows only a small amount of interchange of hydrogens between carbon and oxygen prior to fragmentation, while protonated alkyl amines show practically no interchange of hydrogens between carbon and nitrogen prior to fragmentation. These results, and earlier results (Terlouw et al., Org. Mass Spectrom. 21, 665 (1986)) showing hydrogen interchange between carbon and oxygen in protonated methyl isopropyl ether, arc discussed in relation to the energetics for fragmentation of the protonated species [RHXH]+. It is concluded that as long as the proton affinity of XH is greater than the proton affinity of the olefin R by no more than ∼50 kJ mol−1 hydrogen interchange between the alkyl group and the heteroatom will occur.
Reduction of a quaternary phenanthridinium salt in a chemical ionization source when introduced by direct injection in acetonitrile
Authors: Arpino Patrick J. , Schmitter Jean‐Marie
Published: May 2005

Reduction of phenanthridinium methiodide to its dihydroderivative was observed when a solution of the sample in acetonitrile was injected in the chemical ionization source of a combined liquid chromatograph/mass spectrometer equipped with a direct liquid introduction interface and a polarized desolvation chamber.
Energy‐resolved collisional activation of dimethyl phosphonate and dimethyl phosphite ions in a quadrupole ion trap and a triple quadrupole mass spectrometer
Authors: Brodbelt Jennifer S. , Kenttämaa Hilkka I. , Cooks R. Graham
Published: May 2005

Collision‐activated dissociation spectra of dimethyl phosphonate and dimethyl phosphite ions were measured as a function of the amplitude of a supplementary AC voltage applied across the end‐caps of an ion‐trap mass spectrometer. These spectra yield breakdown graphs which bear a close resemblance to those obtained by varying collision energy in a triple‐quadrupole mass spectrometer operating under multiple‐collision conditions. Variation in the time of excitation at the resonance frequency provides an alternative route to breakdown graphs. The results demonstrate that energy deposition occurs via multiple activating collisions in the ion trap. Maximum energy deposition observed is somewhat smaller under normal operating conditions in the ion trap than in the triple‐quadrupole mass spectrometer.
Analysis of natural products by tandem mass spectrometry employing reactive collisions with ethyl vinyl ether
Authors: Pachuta Robin R. , Kenttämaa Hilkka I. , Cooks R. Graham , Zennie Thomas M. , Ping Cai , Chang Ching‐Jer , Cassady John M.
Published: May 2005

Representative natural products from the diterpene dilactone, psorospermin and quabalactone classes were protonated, mass‐selected and reacted with ethyl vinyl ether in a triple‐quadrupole mass spectrometer. Minor differences in the structures of the compounds led to different reactivities toward the reagent, as indicated by the relative abundances of products such as the ethylated and vinylated compounds. Additional information is obtained from the dissociation products formed upon non‐reactive, inelastic collisions with the neutral reagent. The daughter spectra have excellent signal: noise ratios and good reproducibility. The results demonstrate that the use of reactive collisions may supplement collision‐activated dissociation in chemical analysis of large organic molecules by tandem mass spectrometry.
Low‐resolution accurate mass measurement in self‐chemical ionization mass spectrometry
Authors: Caldwell Gary W. , Masucci John A. , Inners Ruth R.
Published: May 2005

Low‐resolution (2000, 10% valley definition) accurate mass measurements using self‐chemical ionization (self‐CI) have been evaluated as an alternative to the conventional chemical ionization (CI) method. In conventional CI experiments a high pressure of reagent gas is required to induce ionization while in self‐CI no reagent gas is used and the self‐CI is produced presumably by molecular/fragment ion–molecule reactions. Nine compounds ranging in mass from 50–500 daltons were examined. Results obtained by the self‐CI method indicate that the elemental composition assignment can be obtained simultaneously for the protonated molecule and/or molecular ion. It is also shown that perfluorokerosene can be used routinely in self‐CI as an internal reference standard over a broad mass range (50–500 daltons). It is sometimes difficult to use a single reference standard in conventional low‐resolution CI accurate mass spectrometry over a similar mass range.
The structures of [C4H4]+˙ ions by photodissociation in an ion cyclotron resonance spectrometer
Authors: van der Hart W. J.
Published: May 2005

The photodissociation of [C4H4]+˙ fragment ions at the ion cyclotron resonance time‐scale competes with relaxation of the internal energy by infrared emission. As a result the fraction of photodissociating ions increases with light intensity. The experiments indicate that [C4H4]+˙ from benzene and 1,5‐hexadiyne consists of a mixture of 60% vinyl acetylene ions, 10% butatriene ions and 30% cyclic ions. This confirms previous conclusions from studies of the ion‐molecule reactions of [C4H4]+˙ with benzene.
Reduction of methylene blue during the ionization process
Authors: Burinsky David J. , Dilliplane Robert L. , DiDonato Gerald C. , Busch Kenneth L.
Published: May 2005

Methylene blue is readily reduced during the process of ionization in methods as diverse as electron ionization, direct exposure probe electron ionization, direct exposure probe chemical ionization, solid sample secondary ion mass spectrometry, liquid sample secondary ion mass spectrometry and fast atom bombardment mass spectrometry. The reduction of methylene blue need not initially involve the abstraction of hydrogen from the solvent present in the latter two methods.
Charge neutralization of ions from benzene
Authors: Sedgwick J. B. , Nelson P. R. , Steiner P. A. , Moran T. F.
Published: May 2005

Cross‐sections have been measured for the charge neutrilization if ions from benzene in kiloelectron‐volt collisions with benezene target molecules. Measured values range from 65 Å2 for the symmetric [C6H6]+C6H6 resonant reactions to 8 Å2 for [C3H3]+C6H6 reactions. Cross‐sections computed using a simple resonance charge transfer model compare favourably with experimental data for the symmetric reactions. The cross‐sections for asymmetric reactions are smaller that those for they symmetric system and magnitudes of the asymmetric cross‐sections are correlated with recombination energies of the respective ions.
On the mechanism of the ethyl elimination from the molecular ion of 6‐methoxy‐1‐hexene
Authors: Molenaar‐Langeveld Tineke A. , Fokkens Roel H. , Nibbering Nico M. M.
Published: May 2005

It is shown by 13C and D labelling that the ethyl radical elimination from the molecular ion of 6‐methoxy‐1‐hexene is a very complex process involving at least two different channels. The major channel (80%) is induced by an initial 1,5‐hydrogen shift in the molecular ion from C(5) to C(l) leading via a series of steps to methoxy‐cyclohexnne, which then undergoes a ring contraction to 2‐methyl‐1‐methoxycyclopentane, being the key intermediate for the ethyl loss. The same key intermediate is formed in the other, minor channel (20%) by ring closure directly following an initial 1,6‐hydrogen shift in the molecular ion of 6‐methoxy‐1‐hexene from C(6) to C(l). Collision‐induced dissociation experiments on the [M − ethyl]+ ion from 6‐methoxy‐1‐hexene have further established that it has the unique structure of oxygen methyl cationized 2‐methyIpropen‐2‐al. This ion is also generated by ethyl loss from the molecular ion of 2‐methyl‐1‐methoxycyclopentane itself, as shown by collision‐induced dissociation experiments, thus confirming the key role of the intermediate mentioned.
Molecular states formed by ion beam neutralization of [C2H2]+ and [C2H3]+ on metal targets
Authors: Hudgins Douglas M. , Raksit Asit B. , Porter Richard F.
Published: May 2005

A combination of charge‐stripping and beam‐scattering techniques has been used to study the molecular states formed when a fast beam of [C2H2]+ and [C2H3]+ in several isotopic forms are neutralized by electron transfer from metal target atoms (K, Na, Mg and Zn). For [C2H3]+ the isotopic compositions and relative abundances of product states were found to be insensitive to the method of ion preparation (electron impact and chemical ionization). Ground state neutrals are formed in partial abundance when Mg or Zn is used as a target atom. With low ionization potential targets (K and Na) excitel dissociative states of C2H2 and C2H3 are formed as major beam constituents. For these states decomposition products have been identified and fragmentation energies measured. The excited states of C2H2 and C2H3 lie alout 6.8 eV and 2.9 eV, respectively, above their stable ground states. The discussion focuses on the possible identity of the excited states and their structural relations to the precursor ions.
Peptides investigated by laser desorption—multiphoton ionization mass spectrometry
Authors: Grotemeyer J. , Schlag E. W.
Published: May 2005

Laser evaporation of intact neutral molecules into a supersonic beam combined with multiphoton ionization (MUPI) is used to study the fragmentation behaviour of peptides. Owing to the separation of desorption and ionization, an optimization of these processes can be applied to the sample. The investigation of mixtures containing hydrophobic and hydrophilic peptides shows equal probabilities for detection of the two molecules, demonstrating that the neutral yield of both classes of compound is equal in the desorption process. A loss of sensitivity is not observed. By employing the feature of tunable fragmentation, it is possible to sequence peptides in the gas phase. At low laser intensities only the molecular ions are formed. By increasing photon intensities, fragmentation reactions are induced. Owing to the nature of the multiphoton ionization, these mass spectra (at moderate laser powers) contain few and only structurally dependent signals. The molecular ion of the sample investigated is detected in every case.
Neutralization–reionization efficiencies for gaseous [C3Hn]+ Ions
Authors: Wesdemiotis Chrysostomos , Feng Rong
Published: May 2005

A neutralization‐reionization study of [C3Hn]+ ions, n = 6−0, shows that neutralization efficiencies are larger for hydrocarbon radical cations than they are for even‐electron carbocations with one more H atom. Reionization efficiencies to positive ions do not depend appreciably on the electronic properties of the neutral species undergoing reionization. However, reionization efficiencies to negative ions increase dramatically with the degree of unsaturation, by nearly two orders of magnitude, from C3H6 to C3. All C3Hn neutrals studied survive intact neutralization to produce a considerable fraction of stable, non‐dissociating [C3Hn]+ upon cationization. All C3Hn radicals (n = 5, 3, 1) can be transferred collisionally to stable even‐electron anions. For the C3Hn hydrocarbon molecules studied (n = 6, 4, 2, 0) the stability of the molecular radical anion increases substantially with the degree of unsaturation.
The functional relationship scan in tandem mass spectrometry
Authors: Vincenti M. , Schwartz J. C. , Cooks R. G. , Wade A. P. , Enke C. G.
Published: May 2005

A type of tandem mass spectrometric scan is described in which m1 +, m2 + and m3, defined in terms of the process m1 + → m2 + + m3, are all varied while maintaining some prescribed relationship between them. The scan can be viewed as a variant upon the neutral loss scan in which the constant mass difference between parent and daughter ion masses is replaced by some other functional relationship. Specific examples of this scan are provided in two simple applications: (i) symmetrical proton‐bound dimers are recognized using the functional scan m2 = (m1 + 1)/2; and (ii) symmetrical Diels–Alder adducts comprised of diene and dienophile of equal masses are recognized in mixtures using the scan function m2 = (1/2)m1.
Mass spectrometry of arenediazonium salts by fast atom bombardment
Authors: Ambroz Hanna B. , Jennings Keith R. , Kemp Terence J.
Published: May 2005

Contrary to expectation, thermally labile arenediazonium salts (M) afford sizeable peaks due to ArN2 + and (ArN2 +)M ions under fast atom bombardment; the main fragment ion is usually the highly reactive aryl cation Ar+, the production of which is independent of whether its ground state is singlet or triplet.
A mass spectral rearrangement of 2‐(alkylamino)benzoic acid derivatives and related heterocyclic systems
Authors: Barbuch Robert J. , Peet Norton P.
Published: May 2005

The behavior of a series of 2‐(alkylamino)benzoic acid derivatives and related heterocylic systems has been investigated using electron impact mass spectrometry and fragmentation pathways have been formulated. Deuterium‐labeled and 13C‐labeled compounds were used in this study. An interesting rearrangement of the alkylamino compounds, in which the alkyl substituent is incorporated into a fulvene ion, was observed. This rearrangement had previously been observed for 1‐alkylisatins and l‐alkyl‐3‐arylimino‐2‐indolinones.
Negative ion chemical ionization mass spectrometry—binding of molecules to bromide and iodide anions
Authors: Caldwell G. W. , Masucci J. A. , Ikonomou M. G.
Published: May 2005

The analytical potential of negative ion chemical ionization (NICI) mass spectrometry utilizing dibromodifluoro‐methane (CF2Br2) and iodomethane (CH3I)/methane (CH4) as reagent gases is examined. The NICI mass spectrum of CF2Br2 contains Br−, [HBr2]− and [CF2Br3]− anions. Weak acids (i.e. those acids with approximately ΔH°(acid) values between 1674 and 1464 kJ mol−1) react with Br− to produce minor yields of the hydrogen−bonded bromide attachment [MH + Br]− anion or are unreactive. Strong acids (i.e. those acids with approximately ΔH°(acid) > 1464 kJ mol−1) produce primarily [MH + Br]− anions with a minor yield of proton transfer [M  H]− anion. The NICI spectrum of CH3I/CH4 is dominated by I−. Weak acids react with I− to yield minor amounts of [MH + 1]− or are unreactive. Strong acids produce only [MH + l]− anions. From a consideration of the gas‐phase basicity of the halide anion and the binding energy of the hydrogen‐bonded halide attachment adduct, thermochemical data are used as a potential guide to rationalize or predict the ions observed in NICI mass spectra.
One and two hydrogen atom migrations in the retro‐diels‐alder fragmentation of norbornene and bicyclo [2.2.2] octene derivatives under electron impact
Authors: Weisz Adrian , Mandelbaum Asher
Published: May 2005

Norbornene derivatives with one or two carbonyl‐containing substituents at positions 5 and 6 (anhydrides, esters, amides, cyclic ketones) undergo an electron impact induced rerro‐Diels‐Alder fragmentation accompanied by the migration of one hydrogen atom (RDA + H) giving rise to the [dienophile + H]+ ions. Bicyclo[2.2.2]octene derivatives substituted at C(5) and C(6) by a ring with two carbonyl groups (anhydrides, imides, diketones) undergo an RDA fragmentation accompanied by the transfer of two hydrogen atoms (RDA + 2H). Bicyclo[2.2.2]octene 5,6‐diesters undergo both the RDA + H and RDA + 2H fragmentations, and the relative abundance of the resulting [dienophile + H]+ and [dienophile + 2H]+˙ ions is strongly affected by configuration.
Substituent effects in the mass spectra of diethyl phenyl phosphates
Authors: Kascheres Concetta , Sintoni Mirtes
Published: May 2005

The mass spectra of diethyl phenyl phosphates show substituent effects with electron‐donating groups favouring the molecular ion M+˙, and the [MC2H4]+˙, [M – 2C2H4]+˙ and [XPhOH]+˙ ions. The [PO3C2H6]+ (m/z 109) and [PO3H2]+ (m/z 81) ions are favoured by electron‐withdrawing groups. Results suggest that the formation of the [XPhC2H3]+˙ ion involves rearrangement of C2H3 to the position ortho to the phosphate group. Ortho effects are also observed.
Isomerization and fragmentation of methyl benzofuran ions and chromene ions in the gas phase
Authors: Spilker Rüdiger , Grützmacher Hans‐Fr.
Published: May 2005

The rearrangement of the molecular ions of the isomeric 2‐ and 3‐methyl benzofurans (1 and 2), 2H‐chromene (3) and 4H‐chromene (4) has been studied as a further example of the isomerization of oxygen‐heteroaromatic radical cations via a ring expansion/ring contraction mechanism well documented for molecular ions of alkyl benzenes. The ions 1+˙−4+˙ fragment mainly by H loss into identical chromylium ions a. The process exhibits consistently a large kinetic energy release and an isotope effect kH/kD, which arise from a rate‐determining energy barrier of the last dissociation step. Differences of the kinetic energy releases, the isotope effects and the appearance energies of the methyl benzofuran ions and the chromene ions indicate a large energy barrier also for the initial hydrogen migration during the rearrangement of the methyl benzofuran ions. This is substantiated by an MNDO calculation of the minimum energy reaction path. In contrast to the behaviour of alkyl benzene ions, a unidirectional isomerization of the methyl benzofuran ions by ring expansion takes place but no mutual interconversion of the molecular ions of methyl benzofurans and chromenes.
Fragmentation reactions of the enolate ions of 2‐pentanone
Authors: Donnelly Allen , Chowdhury Swapan , Harrison Alex. G.
Published: May 2005

The reaction of [OH]− with 2‐pentanone produces two enolate ions, [CH3CH2CH2COCH2]− and [CH3COCHCH2CH3]−, by proton abstraction from C(1) and C(3), respectively. Using deuterium isotopic labelling the fragmentation reactions of each enolate have been delineated for collisional activation at both high (8 keV) and low (5–100 eV) collisional energies. The primary enolate ion fragments mainly by elimination of ethene. Two mechanisms operate: elimination of C(4) and C(5) with hydrogen migration from C(5), and elimination of C(3) and C(4) with migration of the C(5) methyl group. Minor fragmentation of the primary enolate also occurs by elimination of propane and elimination of C2H5; the latter reaction involves specifically the terminal ethyl group. The secondary enolate ion fragments mainly by loss of H2 and by elimination of CH4; for the latter reaction four different pathways are operative. Minor elimination of ethene also is observed involving migration of a C(5) hydrogen to C(3) and elimination of C(4) and C(5) as ethene.
Differentiation of the isomeric 1,2‐cyclopentanediols by ion‐molecule reactions in a triple‐quadrupole mass spectrometer
Authors: Meyerhoffer William J. , Bursey Maurice M.
Published: May 2005

Collisionally activated decompositions and ion‐molecule reactions in a triple‐quadrupole mass spectrometer are used to distinguish between cis‐ and trans‐1,2‐cyclopentanediol isomers. For ion kinetic energies varying from 5 eV to 15 eV (laboratory frame of reference), qualitative differences in the daughter ion spectra of [MH]+ are seen when N2 is employed as an inert collision gas. The cis −1,2‐cyclopentanediol isomer favors H2O elimination to give predominantly [MH‐ H2O]+. In the trans isomer, where H2O elimination is less likely to occur, the rearrangement ion [HOCH2CHOH]+ exists in significantly greater abundance. Ion‐molecule reactions with NH3 under single‐collision conditions and low ion kinetic energies can provide thermochemical as well as stereochemical information. For trans −1,2‐cyclopentanediol, the formation of [NH4]+ by proton transfer is an exothermic reaction with the maximum product ion intensity at ion kinetic energies approaching 0 eV. The ammonium adduct ion [M + NH4]+ is of greater intensity for the trans isomer. In the proton transfer reaction with the cis isomer, the formation of [NH4]+ is an endothermic process with a definite translational energy onset. From this measured threshold ion kinetic energy, the proton affinity of cis −1,2‐cyclopentanedioi was estimated to be 886 ± 10 kJ mol−1.
Effect of ammonium formate as ionizing additive in thermospray liquid chromatography‐mass spectrometry for the determination of triazine, phenylurea and chlorinated phenoxyacetic acid herbicides
Authors: Barceló D.
Published: May 2005

A comparison between the use of ammonium acetate and ammonium formate in thermospray liquid chromatography‐mass spectrometry with positive and negative ion modes using ‘filament‐on’ mode has been applied for the determination of simazine, atrazine, propazine, monuron, diuron, linuron, 2,4,‐D, 2,4,5‐T and silvex. By using ammonium formate, the positive ion mode showed for triazine and phenylurea herbicides [M + H]+ and [M + NH4]+, respectively, and the formation of other adduct ions different from ammonium acetate. In the negative ion mode, chlorinated phenoxyacetic acid herbicides exhibited [M + acetate]− or [M + formate]−, depending on the ionizing additive. Applications are reported for the determination of triazine and chlorinated phenoxyacetic acid herbicides in spiked soil and water samples, respectively.
Electron ionization mass spectrometry of systematically modified ligands in chromium(III) β‐diketonates
Authors: Schultze Peter , Wenclawiak Bernd W.
Published: May 2005

The positive ion mass spectra of several chromium(III) β‐diketonates with aliphatic α‐substituents have been investigated. Metastable peaks in the spectra confirm that ions containing substituents with γ‐H atoms undergo propene loss. This implies a McLafferty rearrangement of an open‐chain ligand structure. Ethyl radicals are lost from n‐butyl substituents; methyl groups are cleaved from the molecular ions of complexes formed from methyl‐substituted ligands. The main fragment is, as expected, [ML2]+; however, its further fragmentation is different from that of [ML3]+. Electron donating substituents stabilize doubly charged molecular ions.
Cyclization of 1,2,3,5‐polysubstituted pyrroles in electron impact
Authors: He Meiyu , Yu Zhenpei , Hua Wenting
Published: May 2005

Mass spectra of eight new 1,2,3,5‐polysubstituted pyrroles were studied by methods of electron impact, mass‐analysed ion kinetic energy and high‐resolution data. It has been found that five‐membered or six‐membered cyclic ions were formed and other rearrangement ions were observed in the mass spectra.
Fast atom bombardment and secondary ion mass spectra of thiamine hydrochloride
Authors: Brown Stephen M. , Busch Kenneth L. , Burinsky David J.
Published: May 2005

The ratio of the fragment ions at m/z 122 and m/z 123 in the positive ion fast atom bombardment or secondary ion mass spectra of thiamine hydrochloride varies with sample preparation and experimental conditions. For all mass spectra that contained significant abundances of matrix (S) ions [S + H]+, the fragment at m/z 123 is the more abundant of the two ions. If [S + H]+ ions are not observed in the mass spectrum under the conditions selected, the ion at m/z 122 is more abundant. This correlation suggests that hydrogen transfer to the fragment ion occurs in the gas phase, with the composition of the ion‐solvent cluster ions in the selvedge region being the key factor. The ratio of the fragment ions at m/z 123 and m/z 122 is thus an indicator of the extent of protonation in the selvedge, the region immediately above the solvent surface created by primary particle bombardment.
Experimental determination of the double ionization energies of the perhalomethanes, CF3Br, CF2Br2, CFBr3 and CF2ClBr
Authors: Griffiths W. J. , Harris F. M.
Published: May 2005

The double ionization energies of the perhalomethanes CF3Br, CF2Br2, CFBr3 and CF2ClBr have been determined using the technique of double‐charge‐transfer spectroscopy. The values found are compared with those of other perhalomethane molecules also determined in this laboratory.
Ortho effects in organic molecules on electron impact: 19—Competing oxygen transfers from nitro group to sulphur and acetylinic carbons in 2‐nitrophenylphenylethynylsulphides
Authors: Ramana D. V. , Krishna N. V. S. Rama
Published: May 2005

Oxygen transfers to both the acetylenic carbons and sulphur are noticed in parallel fragmentation pathways during the electron‐impact induced decompositions of 2‐nitrophenylphenylethynylsulphides. Single oxygen transfer to acetylinic carbons leads to the most abundant ion corresponding to the benzoyl cation whilst double oxygen transfers to both the acetylenic carbons followed by the ejection of two CO units from the M+˙ ion afford another abundant fragment corresponding to the phenothiazine radical cation. However, the oxygen transfers to sulphur yield a less abundant [M  SO2H]+ ion. The proposed fragmentation pathways and the ion structures are sup ported by high‐resolution data, collision‐induced dissociation Linked‐scan spectra and chemical substitution.
Reactivity in the gas phase. Behaviour of isoxazoles under negative ion chemical ionization conditions
Authors: Guarna Antonio , Moneti Gloriano
Published: May 2005

[M  H+]− ions of isoxazole (la), 3‐methylisoxazole (1b), 5‐methylisoxazole (1c), 5‐phenylisoxazole (1d) and benzoylacetonitrile (2a) are generated using NICI/OH− or NICI/NH2− techniques. Their fragmentation pathways are rationalized on the basis of collision‐induced dissociation and mass‐analysed ion kinetic energy spectra and by deuterium labelling studies. 5‐Substituted isoxazoles 1c and 1d, after selective deprotonation at position 3, mainly undergo N  O bond cleavage to the stable α‐cyanoenolate NC  CH  CR  O− (R = Me, Ph) that fragments by loss of RCN, or RH, or H2O. The same α‐cyanoenolate anion (R = Ph) is obtained from 2a with OH−, or NH2−, confirming the structure assigned to the [M  H+]− ion of 1d, On the contrary, 1b is deprotonated mainly at position 5 leading, via NO and C(3)C(4) bond cleavages, to HC ≡ C O − and CH3CN. Isoxazole (1a) undergoes deprotonation at either position and subsequent fragmentations. Deuterium labelling revealed an extensive exchange between the hydrogen atoms in the ortho position of the phenyl group and the deuterium atom in the α‐cyanenolate NC  CD = CPh  O−.
The electron impact mass spectra of some cevine orthoacetates
Authors: Waight Eric S.
Published: May 2005

The most important fragmentation in the electron impact mass spectra of dihydrocevine orthoacetate and some analogues involves loss of an acetoxyl radical from the molecular ion, and is associated with strain in the orthoacetate group bridging a six‐membered ring. The process is much less important in the less‐strained dihydrocevine isoorthoacetate, which gives the ion m/z 112, characteristic of cevine alkaloids, as the most abundant. A further important difference concerns an ion involving the loss of C(20) and its substituents together with a hydrogen atom from M+˙, also relatively unimportant in the isoorthoacetate.
Application of on‐line HPLC‐FAB mass spectrometry to the analysis of enzymatic digests of ribonuclease B
Authors: Mock K. , Firth J. , Cottrell J. S.
Published: May 2005

A tryptic digest of bovine ribonuclease B was analysed by on‐line HPLC‐FAB mass spectrometry to verify the primary sequence. Peptides obtained by digestion with trypsin before and after treatment with N‐glycanase were separated on a slurry‐packed fused silica LC column. The entire eluent was introduced into the mass spectrometer source via a continuous flow interface. A single injection of 100 pinoles of material provided data covering over 70% of the sequence.
Negative ions generated by reactions with oxygen in the chemical ionization source. II. The use of wall‐catalyzed oxidation reactions for differentiation of polycyclic aromatic hydrocarbons and their methyl derivatives
Authors: Stemmler Elizabeth A. , Buchanan Michelle V.
Published: May 2005

Polycyclic aromatic hydrocarbons (PAH) undergo wall‐catalyzed oxidation reactions, in conjunction with electron‐and ion‐molecule reactions, when oxygen is present in the chemical ionization source. Under conditions that promote these wall‐catalyzed oxidation reactions (addition of oxygen, elevated ion source temperature) oxidized products are easily detected by measurement of the negative ion chemical ionization (NICI) mass spectra. The NICI mass spectra are dominated by ions that result from electron‐capture ionization of oxidation products such as ketones, quinones and anhydrides. In this study the NICI mass spectra of 30 PAH and 46 methyl‐substituted PAH were measured. These spectra can be used to differentiate certain isomeric PAH based upon differences in molecular structure. Of particular interest are ions that appear in the spectra of methyl‐substituted PAH, which can provide information regarding the position of methyl substituents.
Primary‐particle‐beam‐induced reduction of silver acetate in glycerol solutions to form a silver mirror
Authors: Dean L. K. L. , Busch Kenneth L.
Published: May 2005

The liquid SIMS mass spectra of silver acetate dissolved in a glycerol matrix is discussed, with emphasis on the formation of a ‘silver mirror’ on the surface of the glycerol droplet owing to reduction of the silver acetate. Silver clusters containing up to three silver atoms have been observed from this mirrored surface; Ag3+ cluster ions are not observed in the spectrum when conditions are such that the mirror is not formed. For example, use of a slightly oxidizing matrix (o‐nitrophenyl octyl ether or m‐nitrobenzylalcohol) prevents formation of the ‘mirror’; only Ag+ is sputtered from this surface.
Unusual fragmentation of trimethylsilylated enols derived from m‐ and p‐hydroxyacetophenones
Authors: Kraus Rupert , Spiteller Gerhard
Published: May 2005

When 4‐hydroxyacetophenone is treated with MSTFA the corresponding bis‐trimethylsilylated enol ether (1a) is obtained. The mass spectrum of la is characterized by a [M – 1]+ base peak. Extensive deuteration experiments revealed that the hydrogen is mainly removed from a ring position, but originates also to some extent from the side chain (methylidene group) and even to a very small amount from the hydrogens of the methyl groups of the enolic trimethylsilyl group. A mechanism for this fragmentation behaviour is formulated.
Dissociation dynamics of halotoluene ions, production of tolyl, benzyl and tropylium ([C7H7]+)ions
Authors: Olesik Susan , Baer Tomas , Morrow J. C. , Ridal Jeffrey J. , Buschek John , Holmes John L.
Published: May 2005

The dissociation rates and energetics of the loss of halogen atoms from energy‐selected halotoluene ions were investigated by photoelectron photoion coincidence (PEPICO) and collisional activation (CA) mass spectrometric experiments. Dissociation onsets, determined from the dissociation rates measured as a function of the internal energy of the parent ion, revealed the formation of three [C7H7]+ isomers, which were identified, on the basis of the CA data, as the tolyl, benzyl and tropylium ions. All of the ions investigated produced a mixture of isomeric ions. Only iodotoluene ions produced any tolyl product ions by a direct bond cleavage. The bromo‐ and chlorotoiuene ions produced mixtures of benzyl and tropyl ions. The observed two‐component decay rates of the iodotoluene ions revealed the participation of a lower energy [C7H7I]+ ˙ isomer in the dissociation process. The identity of this isomer is not known but it probably does not have the cycloheptatriene ion structure because considerable kinetic energy was released in this dissociation.
Determination of the fragmentation mechanisms of organophosphorus ions by H2O and D2O atmospheric‐pressure ionization tandem mass spectrometry
Authors: Snyder A. Peter , Harden Charles S.
Published: May 2005

Dimethylmethyl phosphonate (DMMP), dimethyl phosphite (DMPI), trimethyl phosphite (TMPI) and trimethyl phosphate (TMP) were investigated using H2O and D2O atmospheric‐pressure ionization (API) tandem mass Spectrometry. All daughter ions could be explained by losses of one or a successive number of stable molecules as opposed to losses of radicals such as the hydride, methyl and methoxy species. Losses of neutral methanol and dimethyl ether and of protonated methanol and formaldehyde ions from all four organophosphorus pseudo‐molecular ions were observed. The DMMP and DMPI MH+ pseudomolecular ions produced the losses of neutral C2H6 and water, respectively. Formaldehyde loss was not observed for the MH+ ions, but it was well represented in the decomposition pathways of daughter ions. The D2O reagent gas highlighted the role of the ionizing proton/ deuteron in the various daughter ions, including m/z 95, 79, 65, 49, 33, 31 and 47. The last ion was found to be isobaric in that m/z 47 and 48 both appeared with similar abundances in the D2O‐API daughter ion mass spectra of TMPI and TMP.
Mass spectrometry of some methyl esters of N‐4‐pyrimidinylamino acids: Rearrangements of the ions occurring on electron‐impact ionization
Authors: Plaziak Adam S. , Celewicz Lech , Spychala Jaroslaw , Golankiewicz Krzysztof
Published: May 2005

The electron‐impact‐induced mass fragmentations of methyl esters of N‐(1H‐2‐oxo‐4‐pyrimidinyl)ainino acids are reported. The loss of a ·COOCH3 radical and the elimination of a methanol molecule were observed and possible mechanisms of these processes are proposed. The loss of a ·COOH radical and elimination of water were established, which suggests the rearrangement of the investigated esters into isomeric N‐4‐pyrimidinylamino acids.
Odd‐electron molecular ion and loss of toluene in fast atom bombardment mass spectra of some carotenoids
Authors: Caccamese Salvatore , Garozzo Domenico
Published: May 2005

Positive‐ion fast atom bombardment (FAB) and B/E linked scan FAB mass spectra of seven carotenoids are reported. In all cases the M +· ions are observed, and the [M + H]+ ions are absent in the hydrocarbons and weak in the oxygenated compounds. The usefulness of B/E linked scan FAB mass spectra to distinguish isomers and to attribute the loss of toluene from the M +· to an ionic fragmentation and not to a thermal process is discussed.
Formation of β‐carbolines from 1‐[(1′ ‐carboxy‐2′‐indol‐3′‐yl‐ethyl)amino]‐1‐deoxy‐D‐fructose under electron impact conditions
Authors: Yaylayan Varoujan , Paré J. R. Jocelyn , Laing Richard , Sporns Peter
Published: May 2005

Interest in β‐carbolines has recently increased owing to their detection in human tissue and in some food products; one of the possible sources of their formation in food is the Amadori rearrangement product (ARP) of tryptophan with glucose, l‐[(1′‐carboxy‐2′‐indol‐3′‐ylethyl)amino] ‐1‐deoxy‐D‐fructose. Based on the intermediates identified in the high‐resolution electron impact spectrum of the tryptophan Amadori product and by B/E and B2/E linked‐scan experiments, a mechanism is proposed for their formation under electron impact conditions. The proposed mechanism is an intramolecular nucleophilic substitution reaction of the dehydrated tryptophan ARP, leading to the formation of 3‐substituted β‐carbolines; this mechanism might shed some light on the origin of their abundant formation in food.
H–D Exchange in some macrocyclic complexes of nickel(II) under electron impact
Authors: Palii Sergiu P. , Indrichan Konstantin M. , Gerbeleu Nikolai V. , Arion Vladinir B. , Zagorevskii Dmitrii V. , Nekrasov Yurii S.
Published: May 2005

The electron impact mass spectra of coordination compounds of nickel(II) containing 14‐membered hexaaza macrocyclic ligands composed of S‐substituted isothiosemicarbazide and β‐diketone units with cis and trans arrangement of SR groups were studied. The molecular ions exhibited the most in tense peaks in all cases. The loss of peripheral groups is a common feature of the decomposition of the molecular ions for the compounds studied. Exchange between the hydrogen atoms of the alkylthio groups and the hydrogen at γ‐carbon atoms of the β‐diketonate fragments was observed in the mass spectra of compounds with trans‐SR groups. The molecular ions of the complexes with cis‐SR groups selectively eliminated the radical R˙.
Mndo study of fragmentations in mass spectrometry. Part II. Substituent effects in the ioinization of [CH3COR] compounds and their enol tautomers
Authors: Hrušák Jan , Tkaczyk Martin
Published: May 2005

A semiempirical MNDO method was used in calculations on monosubstituted neutral and cationic compounds of the [CH3COR] type with a wide range of substituents (R = H, F, Cl, CH3, NH2, OH, OCH3, NHCH3, CHCH2). The results obtained are interpreted with respect to the effect of the individual substituents on the geometry and electron distribution in the systems studied and allow the conclusion that the ratios of partial charges in the individual substructures in a radical cation correspond to those of the respective peak intensities in the mass spectrum.
Effect of collision gas pressure and collision energy on reactions between oxygen and the negative molecular ion of tetrachlorodibenzo‐p‐dioxins in the collision cell of a triple ‐quadrupole mass spectrometer
Authors: Kostiainen R. , Auriola S.
Published: May 2005

Low‐energy reactive collisions between the negative molecular ion of a tetrachlorodibenzo‐p‐dioxin (TCDD) and oxygen inside the collision cell of a triple‐stage quadrupole mass spectrometer produce a substitution ion [M  Cl + O]−, a phenoxide ion [C6H4‐nO2Cln]−·, [M  HCl]−·, and Cl− by which 1,2,3,4‐, 1,2,3,6/1,2,3,7‐ and 2,3,7,8‐TCDD isomers can be distinguished either directly or on the basis of intensity ratios. The collision conditions have an important effect on the relative abundances. Energy‐ and pressure‐resolved curves show that the ions formed by a collisionally activated reaction (CAR) process, i.e. [M  Cl + O]− and [C6H4‐n,O2Cln]−·, are favoured by a high pressure of oxygen (3‐6 mTorr) (1 Torr = 133.3 Pa) and a low collision energy (0.1‐7 eV), whereas the ions formed by a collisionally activated dissociation (CAD) process, i.e. [M  HCl]−· and Cl−, are favoured by high pressure and high energy. By choosing a relatively low collision energy (5 eV) and high pressure (4 mTorr), the CAR and CAD ions can be clearly detected.
Mechanism of ionization and initial fragmentation in electron impact mass spectra of 3‐halogenobenzanthrones
Authors: Abliz Zeper , Ueda Toyotoshi , Kubogata Kazuhiko , Iwashima Satoshi , Takekawa Minoru , Aoki Junji , Kan Teruo
Published: May 2005

Electron‐impact mass spectra of 3‐halogenobenzanthrones (halogen X = Cl, Br, I) were measured and ionization efficiency curves and three kinds of linked‐scan spectra were obtained for several fragment ions. The fundamental mechanisms of ionization and initial fragmentation were interpreted by the penetration length of an impacting electron or the density distribution on the molecular surface of a rejected electron and its orbital energy. The apparent ionization energy (IE) of a singly charged molecular ion seems to be the lower one of non‐bonding electrons on O or X, and that of a doubly charged molecular ion the sum of three terms, the IE of non‐bonding electron on O, that on X and the electrostatic repulsion between two positive charges. Two competing pathways of decomposition from the molecular ion M+· to an ion [M ‐ CO,‐ X]+ were observed: one is the initial detachment of CO in chloro and bromo compounds and the other is the initial elimination of the iodine atom in the iodo compound. The sequence of these reactions was confirmed by metastable ion analysis with linked‐scan spectra and the relative magnitudes of the appearance energies. They can be explained by the driving force of a localized positive charge or unpaired electron on a heteroatom.
Acetone chemical ionization studies: Olefins
Authors: Vairamani M. , Kumar K. V. Siva , Rao G. K. Viswanadha
Published: May 2005

Acyclic, cyclic and bicyclic olefins have been found to undergo acetylation in the gas phase under acetone chemical ionization, giving rise to diagnostic ions. Terminal olefins show enhanced loss of water from the [M + 43]+ adduct. The [M + 43]+ ion of Δ3− Δ4− and Δ5‐olefins give characteristic collision‐induced dissociation spectra.
Surface effects in fast atom bombardment mass spectrometry of ethylene glycol‐glycerol mixtures
Authors: Dube Gunther , Vollhardt Dieter
Published: May 2005

Binary mixtures of ethylene glycols HO(CH2CH2O)xH (x = 3–5) with glycerol were investigated by fast atom bombardment mass Spectrometry. The relative intensities in the ethylene glycol partial mass spectra as a function of concentration exhibit a steep slope in the low concentration range. With increasing concentration the curves asymptotically approach the 100% line. This effect increases from triethylene to pentaethylene glycol. Comparison of these results with those of surface tension measurements leads to the conclusion that the high sensitivity of fast atom bombardment mass Spectrometry for ethylene glycols is caused by the surface‐active properties of these compounds, which lead to an enrichment of the surface investigated with the substance molecules.
Mass spectra of 1‐aryl‐2, 2‐dibromocyclopropanes
Authors: Lin Shaw‐Tao , Lin Mei‐Ling , Lin Wen‐Chung
Published: May 2005

The electron impact (EI) mass spectra of sixteen 1‐aryl‐2,2‐dibromocyclopropanes give an ion at m/z 115, the structure of which is proposed to be the indenium ion based on a collision‐activated decomposition experiment.
Collision‐activated decomposition of peptides by Fourier transform ion cyclotron resonance spectrometry
Authors: Sheil Margaret M. , Guilhaus Michael , Derrick Peter J.
Published: May 2005

The dynamics of the collision‐activated decomposition (CAD) of large peptide ions by Fourier transform ion cyclotron resonance (FT‐ICR) spectrometry have been investigated in detail. The results presented concern [ M + H]+ ions of the cyclic depsipeptides beauvericin (M = 783.4 u) and valinomycin (M = 1110.6 u) and demonstrate that these peptide ions with less than 100 eV translational energy can undergo collisions with netrual gas atoms and decompose to yield abundant fragment ions. The effects of varying two parameters, the duration of the radiofrequency pulse used to accelerate the precursor ions and the time allowed for collisions, are illustrated. Increasing the former is shown to increase the extent and nature of CAD fragmentation; however, the same effect was not observed on increasing the collision time, and hence the number of collisions, at the pressures used in these experiments.
Fast atom bombardment mass spectrometry of some alkoxv‐ and chloro‐silanes
Authors: Kallury Krishna M. R. , Krull Ulrich J. , Thompson Micliael
Published: May 2005

The positive and negative FAB mass spectra of a series of alkoxy‐ and chloro‐silanes Xm(CH3)3‐mSi(CH2)nR [m = 1 or 3, n = 3, 10 or 17, X = Cl or OMe or OEt, R = Me, NH2, glycidoxy, COOMe, NHCO(CH2)7COOMe or NHCO(CH2)10CH2OAc] were recorded in NBA and NPOE matrices. The chlorosilanes underwent rapid hydrolysis into silanols which condense to form siloxanes, the process being complete in NBA and partial in NPOE, yielding siloxane‐based fragment ions in the positive spectra and silyloxyanions in the negative spectra. The alkoxysilanes were more resistant to hydrolysis, affording abundant [MH – HX]+ ions (X = OMe or OEt) in their positive FAB spectra and moderate to high intensity [M – H]− ions in the negative mode, the latter undergoing characteristic sequential loss of C2H4, EtOH and C2H4. Significant variations were observed in the positive spectra of all the silanes with change of matrix.
Electron impact and chemical ionization mass spectra of some unsaturated anomeric C‐glycosides
Authors: Brakta Mohamed , Lhoste Paul , Sinou Denis , Banoub Joseph
Published: May 2005

Electron impact mass spectra at 70 eV electron energy and chemical ionization mass spectra with ammonia as the reagent gas are reported for certain unsaturated C‐glycosides. Comparisons are made between the mass spectra of anomeric pairs of these glycosides.
Electron impact‐induced fragmentation of some aminonitropyridines
Authors: Gawinecki Ryszard , Rasała Danuta
Published: May 2005

The fragmentation behaviour of 18 vicinally substituted aminonitropyridines was studied under electron impact conditions. Tbe decomposition patterns were found to be strongly dependent on the position of substituents. The formation of the aza analogue of carbazole seems to affect considerably the fragmentation of 3‐nitro‐2‐phenylaminopyridine.
Mass spectral fragmentation pathways of N‐acetylnitramines: 1‐acetylhexahydro‐3,5‐dinitro‐1,3,5‐triazine and 1‐acetyloctahydro‐3,5,7‐trinitro‐1,3,5,7‐tetrazocine
Authors: Burrows Elizabeth P.
Published: May 2005

Mass spectra of the N‐acetylnitramines l‐acetylhexahydro‐3,5‐dinitro‐l,3,5‐triazine (TAX) and 1‐acetyloctahydro‐3,5,7‐trinitro‐l,3,5,7‐tetrazocine (SEX) were recorded in electron impact (EI) and positive and negative chemical ionization (PCI and NCI) modes, and the fragmentation pathways were compared with those of other nitramines which have been well documented and characterized. Unexpectedly, for both acetylnitramines in the EI mode (and in the PCI mode) proton adducts were the only molecular ion species observed; in neither mode was there evidence for higher adducts. In contrast, for TAX in the NCI mode the [M + NO2]− adduct was the second most abundant ion (70%); relatively small amounts of the [M + NO]− adduct (2%) and the hydrogen adduct [MH]− (3%) were observed. Under identical NCI conditions no molecular ion species or adduct ions were detected for SEX; the ion of highest m/z corresponded to loss of NO2 or HNO2 from a molecular ion species. The findings of collision‐induced dissociation experiments are also discussed.
Electron impact mass spectrometry of some 3‐substituted 1,2,3,4‐tetrahydroisoquinolines
Authors: Ibañez Adriana F. , Iglesias Graciela Y. Moltrasio
Published: May 2005

The electron impact mass spectral fragmentation of a series of 3‐substituted‐l,2,3,4‐tetrahydroisoquinolines are reported. The principal fragmentation modes of these compounds comply with the main generalizations reached earlier with simpler systems. Complete aromatizations to form the isoquinolinium ion are observed only for compounds 5 and 7 and no retro‐Diels‐Alder cleavage is observed when R1 is a benzyl group.
Determination of the proton affinity of carbon dioxide by photoionization mass spectrometry
Authors: Traeger John C. , Kompe Barbara M.
Published: May 2005

The adiabatic ionization energies and COOH+ appearance energies for a series of carboxylic acids have been measured by dissociative photoionization mass Spectrometry. Using the stationary electron convention, a heat of formation of 143.5 ± 0.7 kcal mol−1 (1 kcal = 4.184 kJ) at 298 K is derived for the carboxyl cation in the gas phase, which corresponds to an absolute proton affinity for carbon dioxide of 128.1 ± 0.7 kcal mol−1.
Low‐Energy, low‐temperature mass spectra. 14—A simple model for the rate coefficient
Authors: Maccoll A.
Published: May 2005

The Kassel quantum model for the unimolecular rate coefficient is applied to mass spectral fragmentation. The inadequacy of the classical model is emphasized. The kinetic effect, the competitive effect and the thermal effect are discussed. Finally, the mass spectrum is presented as a function of energy. The quantum model, involving summation over states rather than integration over an energy range of the classical model, is ideally suited to simple computation.