bluuebunny/crossref_metadata_2025 huggingface dataset:
Fragmentation selectivity in electron impact ionization mass spectra of substituted dimethoxybenzenes
Authors: Nakata Hisao , Jitsukawa Fumie , Toyama Hitomi , Kato Yuko
Published: May 2005

Several 1‐X‐sabstitirted‐3‐methoxy‐4‐trideuteromethoxybenzens were synthesized and their electron impact ionization mass spectra were measured with an ionizing energy of 20 eV. From the peak intensity ratio of [M  CD3 ] and [M  CH3] the fragmentation‐directing ability of the substituent X was evaluated. The most powerful group was found to be NH2, which expelled a methoxy methyl group only from its para position. The CH3 group and four halogen atoms, F, Cl, Br and I, exerted a moderate effect Electron‐withdrawing groups such as NO2, CHO and CN had only a little influence on the fragmentation selectivity. These results were interpreted in terms of the effect of X on the distribution of both the unpaired electron and the positive charge in the molecular ion.
In situ N‐phosphorylation of oligopeptides for fast atom bombardment mass spectrometry
Authors: Yang Hou‐Jun , He Mei‐Yu , Ye Yun‐Hua , Zhao Yu‐Fen
Published: May 2005

Positive ion fast atom bombardment mass spectrometry (FABMS) of in situ N‐phosphorylated oligopeptides showed intense quasi‐molecular ions together with the successive alkene loss fragment ions, which afford multiple checks of the unequivocal reality of the relative molecular mass of the tested samples. More interesting, in a novel cleavage pattern only the N‐phosphoryl fragment ions gave intense peaks, the C‐terminal series ions being suppressed. For each of the N‐terminal ions, losses of alkenes also occur to provide multiple checks for the existence of these ions. The FABMS of the in situ N‐ phosphorylated oligopeptides might provide an easily accessible routine method for peptide sequencing.
Internal energy deposition in doubly charged tungsten hexacarbonyl on charge exchange at high energy with atomic and molecular targets
Authors: Susič R. , Lu L. , Riederer D. E. , Žigon D. , Cooks R. G. , Ast T.
Published: May 2005

Charge exchange of W(CO) in the course of 7 keV collisions with various targets results in singly charged ions which are highly internally excited. The distribution of internal energies estimated by the degree to which consecutive fragmentations by CO loss occur is broad and includes very high energies (up to 15 eV). This result is inconsistent with exclusive operation of long‐range electron transfer e.g. by either a curve crossing or Demkov mechanism; rather it suggests that direct excitation to a high‐energy repulsive state of the products also occurs perhaps by an electronic excitation mechanism. The nature of the internal energy distribution suggests mechanistic analogies with simple collisional activation. Different target gases give rise to different average internal energy depositions monatomics and diatomics yielding higher energy depositions than do polyatomic targets. There is an approximate correlation between energy deposition and target ionization energy which is consistent with the proposed excitation mechanism considering the shapes of the potential energy surfaces. When the detailed internal energy distributions are compared characteristic differences are seen for individual targets. The efficiencies of the various targets at producing charge exchange were also compared. Large differences were found with the polyatomic targets having the greatest efficiencies. In addition a rough correlation was observed between the extent of charge exchange and target ionization energy and this is interpreted in terms of greater contributions from the long‐range electron transfer process for targets of lower ionization energy. All the results are also consistent with contributons from more violent collisions which involve electron transfer at relatively small internuclear distances where the shapes of the potential surfaces are strongly dependent on distance.
Reactive collisions in quadrupole cells. Part I. Reaction of [CH3NH2]+˙ with the isomeric butenes and pentenes
Authors: Usypchuk Laurie L. , Harrison Alex. G. , Wang Jianyao
Published: May 2005

The reactions of mass‐selected [CH3NH2]+˙ ions with the isomeric butenes and pentenes were studied at low collision energies in the radiofrequency‐only quadrupole collision cell of a hybrid BEqQ tandem mass spectrometer. Characteristic iminium ions arising by addition of the methylamine to the olefin followed by fragmentation are observed for but‐1‐ene pent‐1‐ene and 3‐methylbut‐1‐ene. However, for but‐2‐ene pent‐2‐ene 2‐methylpropene 2‐methylbut‐1‐ene and 2‐methylbut‐2‐ene the major reaction channel of [CH3NH2]+˙ is charge exchange to form the olefinic molecular ion. The isomeric olefins are characterized to a considerable extent by the characteristic ion–molecule reactions that these molecular ions undergo with the neutral olefin.
Preservation of non‐covalent associations in electrospray ionization mass spectrometry: Multiply charged polypeptide and protein dimers
Authors: Smith Richard D. , Light‐Wahl Karen J. , Winger Brian E. , Loo Joseph A.
Published: May 2005

The ‘softness’ of the electrospray ionization (ESI) method provides a direct link between solution chemistry and the inherent gas‐phase environment of mass Spectrometry. Available results related to the preservation of non‐covalent associations into the gas phase after ESI are reviewed. These associations include the possible retention of elements of higher order protein structure, non‐covalent polypeptide–heme associations and enzyme complexes. Experimental results are presented showing that non‐covalently bound polypeptide and protein dimer ions are relatively common as low level contributions to ESI mass spectra. It is argued that these dimers are reflective of multimeric species in solution since Coulombic barriers preclude dimerization after ESI although uncertainty remains regarding whether they exist prior to the formation of highly charged droplets. The dissociation of dimers is facile and for proteins can yield monomers having a broad distribution of charge states. The detection of non‐covalently associated dimers requires gentle ESI mass spectrometer interface conditions, yielding relatively low levels of internal excitation. Under such conditions incomplete molecular ion desolvation can result in experimental artifacts for tandem mass spectrometric experiments. ESI mass Spectrometry may have broad potential for the study of noncovalent liquid phase associations.
Negative‐ion fast atom bombardment mass spectra of some bisanionic hypervalent germanium, silicon and titanium salts
Authors: Cerveau G. , Chuit C. , Reye C. , Aubagnac J.‐L.
Published: May 2005

Negative‐ion fast atom bombardment mass spectra of hypervalent germanates silicates and titanate with the use of two matrices, glycerol and m‐nitrobenzyl alcohol, were studied. In the case of germanates a complete reaction of the studied anion with glycerol is observed, so it is impossible to use such a matrix to identify these compounds. The use of m‐nitrobenzyl alcohol allows spectra to be obtained that are significant with respect to the structure for all the studied compounds. However, for each compound a partial reaction of this matrix is observed. Further, in the case of doubly charged anions, the adduct ion obtained with an alkali metal cation does not show glycerol reactivity whereas such reactivity occurs from the adduct ion obtained with proton.
Stereochemical effects in the mass spectra of some cyclopropylsilanes explained by the MNDO molecular orbital method. I. Silicon‐oxygen interactions
Authors: Vékey Károly , Somogyi Árpád , Tarnás József , Pócsfalvi Gabriella
Published: May 2005

Large differences have been found between the mass spectra of cis‐ and trans‐1‐carbomethoxy‐2‐trimethylsilylcyclopropane. To help in explaining these differences, quantum chemical calculations were performed by the MNDO method on the neutral molecules, on the vertical and adiabatic molecular ions and also the [M – methyl]+ ions. To evaluate the results, heats of formation, bond orders, molecular geometry, spin and charge densities and orbital coefficients were used. These indicate the formation of a very strong bonding interaction between silicon and oxygen in the molecular ion of the cis but not the trans isomer, leading to spontaneous methyl loss and a stabilization energy of ∼ 2.6 eV.
Electron impact mass spectrometry of [C7H8N]+ and [C6H7]+ and [C6H7]+ fragment ions produced from o‐toluidine and N‐methylaniline
Authors: Selim Ezzat T. M. , Rabbih M. A. , Fahmey M. A.
Published: May 2005

An energetic study of the production of [C7H8N]+ and [C6H7]+ fragment ions from o‐toluidine and N‐methylaniline is reported. The mechanisms for the formation of the ions are suggested. Metastable peaks associated with the formation and fragmentation of reactive [C7H8N]+ and [C6H7]+ ions were detected and kinetic energy released were determined. The results indicate that the [C7H8N]+ ion is formed at threshold from o‐toluidine with an aminotropylium structure whereas for N‐methylaniline the ion is formed with anN‐phenylmethaniminium structure. [C6H7]+ ions are believed to be formed at threshold from the two precursors with a protonated benzene structure.
Electrospray mass spectrometric studies of some phosphonium cations
Authors: Colton Ray , Traeger John C. , Harvey Justin
Published: May 2005

A series of methylphosphonium cations derived from mono‐ and polyphosphines were generated by interaction of the phosphine with methyl iodide. Electrospray mass spectra of the cations were obtained directly from dichloromethane–methanol solutions. In all cases the intact phosphonium cations were observed, often as the base peak, emphasizing the ‘soft’ nature of this type of ionization. Some of the polyphosphines showed additional peaks at m/z 16 and 32 units higher than the molecular ions due to oxidation of the non‐methylated phosphine groups. Collisionally activated dissociation mass spectra show a consistent mode of fragmentation with the formation of alkene ions being dominant. Where comparisons arc possible, there is a close correlation with the electron impact mass spectra of the neutral phosphines.
Decompositions of ammonium adducts of some isomeric sesquiterpene alcohols under collisional activation
Authors: Madhusudanan K. P. , Srivastava Rashmi , Kulshreshtha D. K. , Fraisse D.
Published: May 2005

The decompositions of the ammonium adducts of some sesquiterpene alcohols have been studied using collisional activation mass spectrometry. Losses of ammonia and water are the two primary fragmentation pathways of these adduct ions. Some of the isomeric pairs show differences in the abundances of the fragment ions, whereas others give similar ion abundances in the two isomers.
Electrospray mass spectrometry of lanthanide β‐diketone complexes. II‐Ligand‐exchange processes involving acetate ions and Ln(tfc)3 complexes
Authors: Curtis Jonathan M. , Derrick Peter J. , Schnell André , Constantin Emilie , Gallagher Richard T. , Chapman John R.
Published: May 2005

Electrospray ionization on Ln(tfc)3 complexes (Ln = Eu, Yb; tfc = D‐3‐trifluoroacetylcamphorate) was performed with samples dissolved in methanol–water containing acetic acid. The spectra were obtained with a four‐sector mass spectrometer. The mass spectra exhibit ions containing tfc ligand in addition to ions with acetato ligand. Electro‐spray and tandem mass spectra are presented.
Mass spectrometric studies of platinum(II) complexes of carbonyl‐stabilized arsonium ylides
Authors: Facchin G. , Zanotto L. , Seraglia R. , Traldi P.
Published: May 2005

A fast atom bombardment (FAB) mass spectrometric study was carried out on the two O‐coordinated carbonyl‐stabilized arsonium ylide Pt(II) complexes {(dppe)PtCI[OC(R)C(H)AsPh3]}BF4 ((dppe) = Ph2PCH2CH2Ph2; R = CH3 (1), OCH3 (3)) and on the two corresponding C‐coordinated isomers {(dppe)PtCl[Ph3AsC(H)COR]}BF4 (R = CH3 (2), OCH3 (4)). The mass spectral analysis of complexes 1 and 2 reveals that it is possible to distinguish the two isomeric forms mainly on the basis of the relative abundance of the protonated triphenylarsine oxide at m/z 323 while the different coordination modes of the ylide in complexes 3 and 4 are evidenced by the different fragmentation pathways of these two derivative;. Further, the reaction between [(dppe)PtCl]2(BF4)2 and the appropriate ylide performed under FAB conditions indicates that in both cases the first complex formed is the C‐coordinated isomer, in agreement with that observed in the condensed phase.
Calculation of the kinetic energy release of charge separation processes
Authors: Vékey Károly , Pócsfalvi Gabriella
Published: May 2005

Kinetic energy release (KER) was studied by experimental methods and semiempirical (MNDO and AM1) molecular orbital calculations in the case of various charge separation processes: loss of a methyl ion from [CH3C4CH3]2+, [CH3C3CH3]2+ and [N,N‐dimethyl‐p‐phenylenediamine]2+. It was found that the KER corresponding to the width of a dish‐topped peak at half‐height is very close to the mean KER of the process. The calculated potential energy curves of these reactions show significant reverse critical energies, a large part of which was found to be due, in agreement with conventional assumptions, to electric repulsion between the two separating singly charged products. The bond order between the two separating ions is nearly zero in the transition state, so exchange of internal energy between them is unlikely. These explain the good agreement between the (calculated) reverse critical energy and the measured kinetic energy release.
Comparison of ion trap and sector instruments in the study of fragmentation patterns of coumarins
Authors: Podda G. , Bonsignore L. , Loy G. , Catinella S. , Traldi P.
Published: May 2005

The mass spectra of a series of differently substituted coumarins were obtained by high‐ and low‐energy collision experiments. The results obtained by the two techniques show peculiar differences, mainly in the presence, under ion trap conditions, of a high relative abundance of M+·. The results support the validity of the ion trap technique basic studies of mass Spectrometry.
Mass spectrometry of some fluorinated alcohols
Authors: Carbini M. , Conte L. , Gambaretto G. , Catinella S. , Traldi P.
Published: May 2005

The 70 eV electron impact mass spectrometric behaviour of some fluorinated alcohols, i.e. 2‐fluoroethanol, 2,2,2,‐trifluoroethanol, 2,2,3,3‐tetrafluoropropan‐1‐ol and 1.3‐difluoropropan‐2‐ol, was studied in detail by means of mass‐analysed ion kinetic energy Spectrometry, deuterium‐labelling experiments and comparison with unfluorinated analogues. Composite metastable peaks related to HF losses were rationalized by invoking the presence of two concurrent decomposition pathways, the first consisting in a 1,2‐elimination and the second requiring the implication of hydroxyl hydrogen. This last process and other decomposition routes suggest the presence of H. bridging between the hydroxyl oxygen atom and fluorine.
Liquid chromatography/thermospray mass spectrometry of monosaccharides and their derivatives
Authors: Lipták Miklós , Dinya Zoltán , Sztaricskai Ferenc J. , Litkei György , Jekö Jáozsef
Published: May 2005

Thermospray (TSP) mass spectrometry has proved to be a useful technique for analysing various highly polar compounds. The use of a quadrupole mass spectrometer equipped with a thermospray/plasmaspray interface in the analysis of more than 30 monosaccharides and sugar derivatives is described. A 1:1 mixture of methanol or acetonitrile and 0.1 M aqueous ammonium acetate as eluent at 1 cm3 min−1 affords the best results. The correct setting of the capillary tip temperature and repeller voltage was fundamental for the ionization of carbohydrates. The/optimum values of these parameters were 240–250°C and 220–240 V, respectively. The thermospray mass spectra of most carbohydrates exhibit strong [M + NH4]+ ions which provide molecular mass information. The underivatized and derivatized monosaccharides could be grouped according to the presence or absence and the relative abundances of [M + NH4 − H2O]+, [M + H]+ and [M + NH4]+ ions.
Electron impact ionization mass spectra of some substituted dipyrido[1,2‐a:4,3‐d]pyrimidinones
Authors: Mäki Esa , Pihlaja Kalevi , Huber Imre , Fülöp Ferenc , Bernáth Gábor , Vainiotalo Pirjo
Published: May 2005

The electron impact ionization mass spectra of dipyrido [1,2‐a:4,3‐d]pyrimidinones are strongly influenced by the degree of aromaticity of the fused rings. The molecular ions of the compounds are fairly stable. The main routes of fragmentation involve formation of the [M – H]+ ion and loss of substituents, H2CN˙, CO and alkyl radicals.
Fast atom bombardment mass spectrometric study of alkali metal cationized unsaturated C‐glycosides
Authors: Brakta Mohamed , Chaguir Brahim , Sinou Denis , Becchi Michel , Bobenrieth Marie Joëlle , Banoub Joseph
Published: May 2005

Fast atom bombardment mass spectrometry of alkali metal cationized unsaturated C‐glycosides is a useful method for the determination of the molecular masses of these compounds. The mass‐analyzed ion kinetic energy and collision‐activated dissociation mass‐analyzed ion kinetic energy spectra of the pairs of anomers are distinguishable and are characteristic of each anomer.
Ion–molecule reactions by chemical ionization: Determination of rate constants
Authors: Torroni Sandro , Innorta Giuseppe , Scagnolari Francesco , Foffani Antonio , Curcuruto Ornella
Published: May 2005

Tungsten hexacarbonyl was studied by chemical ionization mass spectrometry using carbon monoxide as reagent gas. The variation of the relative abundances of ions as a function of reactant gas pressure was used for evaluating the relative rate constants for consecutive reactions. The results are in agreement with the rate constant ratios obtained from ion trap mass spectrometric data.
Protonation susceptibility and fragmentation capability of functional groups in chemical ionization mass spectrometry of simple bifunctional compounds. Semi‐quantitative interpretation of spectra
Authors: Nakata Hisao , Kadoguchi Kenji , Konishi Hideyuki , Takeda Naohito , Tatematsu Akira
Published: May 2005

Positive‐ion, methane‐mediated chemical ionization mass spectra were measured for simple bifunctional aromatic compounds of the type m‐XCH2C6H4CH2Y, where X = NH2 and N(CH3)2, and Y = OH and OCH3. Essentially only three peaks of ions, [MH]+, [MH – XH]+ and [MH – YH]+, have appeared for each compound. Since the two functional groups XCH2– and YCH2– do not interact with each other after protonation or after fragmentation, they are assumed to be protonated and to undergo fragmentations independently. The relative protonation susceptibility and fraction of fragmenting [MH]+ can be estimated for each functional group in these compounds. A semi‐quantitative interpretation of the observed spectra is presented.
Sputtered ammonium dinitramide: Tandem mass spectrometry of a new ionic nitramine
Authors: Doyle Robert J.
Published: May 2005

The recently synthesized ammonium dinitramide (ADN) is an ionic compound containing the ammonium ion and a new oxide of nitrogen, the dinitramide anion (O2NNNO2−). ADN has been investigated using high‐energy xenon atoms to sputter ions directly from the surface of the neat crystalline solid. Tandem mass spectrometric techniques were used to study dissociation pathways and products of the sputtered ions. Among the sputtered ionic products were NH4+, NO+, NO2−, N2O2−, N2O, N3O4− and an unexpected high abundance of NO3−. Tandem mass spectra of the dinitramide anion reveal the uncommon situation where a product ion (NO3−) is formed in high relative abundance from metastable parent ions but is formed in very low relative abundance from collisionally activated parent ions. It is proposed that the nitrate anion is formed in the gas phase by a rate‐determining isomerization of the dinitramide anion that proceeds through a four‐centered transition state. The formation of the strong gas‐phase acid, dinitraminic acid (HN3O4), the conjugate acid of the dinitramide anion, was observed to occur by dissociation of protonated ADN and by dissociation of ADN aggregate ions with the general formula [NH4(N(NO2)2)n] NH4+, where n = 1–30.
Influence of instrumental parameters and sample preparation on mass‐analysed ion kinetic energy spectra with fast atom bombardment exemplified with the oxonium ion of peracetylated ribopyranose
Authors: Blok‐Tip Leonore , Heerma Wigger , Haverkamp Johan
Published: May 2005

When mass‐analysed ion kinetic energy (MIKE) spectra are required to discriminate between isomeric ions formed under conditons of fast atom bombardment (FAB) in the ion source, severe interference may be observed. The interfering peaks in MIKE spectra obtained with a reversed‐geometry instrument can arise from different sources. Moreover, the intensity distribution of the true ions from the selected precursor ion may depend strongly on the instrument being used. This means that the FAB–MIKE or collisionally induced dissociation (CID) spectrum is not an absolute characteristic of a particular ion. The [M + H − HOAc]+ ion in the spectrum of peracetylated ribopyranose is used as an example to illustrate this and to trace and discuss the origin of the phenomena observed.
Reactions of propene and cyclopropane radical cations with neutral ethylene
Authors: Vollmer D. L. , Gross M. L.
Published: May 2005

The gas‐phase reactions of propene and cyclopropane radical cations with neutral ethylene were investigated by using Fourier transform, chemical ionization and tandem mass spectrometries. Both reactions form covalent C5H10 adduct ions. The adduct ions are hypothesized to form initially as distonic radical cations that isomerize via a substituted cyclopropane intermediate and are detected as the most stable C5H10 isomer, the 2‐methylbut‐2‐ene radical cation. The rate constant for each reaction is approximately 20% of the theoretical collision rate, indicating that product ions are formed in one out of every five collisions of the C3H6 radical cations with neutral C2H4.
A new ion source for liquid chromatography/thermospray mass spectrometry with a magnetic sector field mass spectrometer
Authors: Hau J. , Nigge W. , Linscheid M.
Published: May 2005

A new and simple ion source for thermospray on a sector field mass spectrometer has been designed, which is based on two sets of electrostatic quadrupole lenses to focus the divergent ion beam that emanates from the extraction cone into the ion source region. To illustrate the performance of the source, the on‐line separation and detection of a few samples are shown using scanning and single‐ion monitoring techniques. Examples are a mixture of phena‐zines and samples of caffeine and adenosine. With single ion motoring, 5 ng of a phenazine, 300 pg of caffeine and ∼1 ng of adenosine were detected. Further, it is shown that declustering and collision‐induced fragmentation occur inside the sampling cone, resulting in peak broadening on the low‐mass side of signals of protonated species. Minimizing the collision‐induced dissociation of erythromycin was used to optimize the flow‐rate of the liquid chromatograph and the pressure in the source housing.
Dependence on scattering angle of the internal energy distribution of products of charge‐changing collisions
Authors: Riederer D. E. , Lu L. , Cooks R. G.
Published: May 2005

The internal energy distributions arising from charge‐changing collisions were measured as a function of scattering angle, θ, for the ‘thermometer’ molecule W(CO)6. The experiments were performed by modifying a reverse‐geometry mass‐analyzed ion kinetic energy (MIKE) spectrometer by adding angle‐resolving slits which allow measurement of the scattering angle in the non‐focusing plane of the instrument. Charge exchange of W(CO) with benzene to give W(CO), and charge stripping of W(CO) on collision with O2 to give W(CO), were studied at 6 keV with the product ions being collected over laboratory scattering angles selected in the range 0–0.60°. The results show that charge‐changing collisions accompanied by scattering have the potential for depositing extremely large internal energies. The observation of the W(CO)2+ ion formed in dissociative charge stripping of W(CO) shows that it is possible to deposit at least 27 eV into the colliding W(CO) ion; of this energy, 15 eV is used for the charge‐stripping process, leaving 12 eV of internal energy in the nascent W(CO). Even greater internal energies (more than 15 eV) can be deposited into scattered W(CO) produced by charge exchange of the doubly charged ion. The availability of such high internal energies has potential use in causing dissociation of refractory ions such as those of biomolecules. The average internal energy, εAVE, deposited increases with the scattering angle at a rate of 10 eV degree−1 for charge exchange, and at approximately 5 eV degree−1 for charge stripping and for simple collision‐induced dissociation (CID). This observation suggests that non‐zero angle charge stripping and CID may occur via similar mechanisms in which direct vibrational activation occurs in small impact parameter collisions which also lead to angular scattering. The higher internal energies and larger εAVE vs. θ dependence observed for charge exchange are consistent with the formation of the products upon a highly repulsive surface associated with coulombic repulsion between the separating products.
Chemical ionization mass spectrometry of C2H4/C2D4 mixtures using iodine as reagent gas
Authors: Gable Kevin P. , Phan Thuy N. , Griffin Don
Published: May 2005

Gas‐phase ionization of ethylene with positive ions generated from molecular I2 leads to the formation of the iodiranium ion, m/z 155. No fragmentation or crossover is seen from a mixture of C2H4 and C2D4, although a net isotope effect (I155/I159 = 1.41 ± 0.02) is observed.
Mass spectra of aromatic 3‐methylisoxazolo‐ and 2‐methyloxazolopyridines
Authors: Ponticelli Fabio , Giomi Donatella , Papaleo Sandro , Tedeschi Piero
Published: May 2005

The fragmentation patterns of aromatic 3‐methylisoxazolo‐ and 2‐methyloxazolopyridines were investigated by high‐resolution mass spectrometry and collisionally activated B/E and B2/E linked scans. The isoxazolopyridines give mass spectra near identical with those of the corresponding oxazolopyridines, both showing mainly a competitive loss of CO and CH3CN from the molecular ion. However, the ratio of these concomitant pathways depends substantially on the type of fusion between the five‐ and six‐membered rings.
Large translational energy loss and scattering in collision‐induced dissociation processes
Authors: Vékey Károly , Czira Gábor
Published: May 2005

Large translational energy losses (TEL) of between 10 and 35 eV have been observed and studied in the collision‐induced decomposition (CID) of some CsI clusters, Cs3I2+, Cs5I4+ and Cs14I13+. The TEL was found to depend strongly on the collision energy. In some cases the TEL was greater than the centre of mass collision energy, showing that recoil energy of the target is undoubtedly important. The results suggest further that, at least for projectiles consisting of heavy atoms, most of the TEL is due to a scattering process. From the TEL and CID peak shapes approximate collision angle distributions were also determined.
Tandem mass spectrometric approaches for the analysis of alkylguanines in human urine
Authors: Cushnir J. R. , Naylor S. , Lamb J. H. , Farmer P. B.
Published: May 2005

Human exposure to carcinogenic alkylating agents can lead to the formation of covalently bound adducts in DNA, some of which are excreted in urine as alkylated purines following DNA degradation and repair. Tandem mass spectrometric methods have been developed for the qualitative and quantitative determination of such alkylpurines in human urine. Short‐chain alkyl‐ and hydroxyalkylguanines have been synthesized with the substituents at theN‐7‐,O6‐ andN2‐positions of guanine. Examination of the product ion scans of their molecular ions (electron impact (EI) ionization) revealed that the ion atm/z151, [guanine]+, was common to all of the alkylguanines studied, with the exception of the methylated analogues. Precursor ion scans of this ion on partially purified human urine extracts showed the presence of several ions (e.g.m/z179, 195) which were consistent with molecular ions for alkylguanines. The presence of these and other constituents was confirmed by product ion spectra of molecular ions (EI and fast atom bombardment), and by high‐performance liquid chromatographic separation prior to tandem mass spectrometry (MS/MS). Evidence was obtained for the presence ofN‐7‐methyl‐,N2‐dimethyl‐,N2‐dimethyl‐,N2‐ethyl‐ andN‐7‐(2‐hydroxyethyl)guanine. Quantitative methods were established for these five alkyl guanines using gas chromatography mass spectrometry (GC/MS) and GC/MS/MS. Deuterated internal standards were synthesized and added to the urine prior to extraction of alkylpurines by Sep‐Pak cartridge chromatography. The products were converted into theirtert‐butyldimethylsilyl derivatives and analysed by selected ion monitoring (SIM) of [M – 57]+or by multiple reaction monitoring (MRM) of the fragmentation M+˙→ [M – 57]+. The MRM method yielded values forN‐7‐methylguanine of 2.57 ± S.D. 1.32 mg day−1(n= 6),N2‐methylguanine of 0.31 ± 0.10 mg day−1(n= 10) andN2‐dimethylguanine of 0.21 ± 0.23 mg day−1(n= 10).N2‐Ethyl‐ andN‐7‐(2‐hydroxyethyl)guanine could only be detected by SIM at levels of ∼0.5 and 2 μg day−1, respectively. The MRM analyses, although inherently less sensitive than the SIM analyses, exhibit greater selectivity and consequently fewer contaminant ions.
Boundary‐effect activated dissociation in ion trap tandem mass spectrometry
Authors: Creaser C. S. , O'Neill K. E.
Published: May 2005

The potential of boundary‐effect activated dissociation (BAD) in ion trap tandem mass spectrometry (MS/MS) is discussed. Several classes of compounds were investigated and the BAD product ion tandem mass spectra were compared with those from collisionally activated dissociation (CAD) where an auxilliary r.f. ‘tickle’ voltage is used. The energy deposition in BAD MS/MS is generally lower than that for optimized CAD, but the experiments are easier to perform. An example of the BAD MS/MS of C10 alkylbenzenes and benzothiophene in diesel fuel using gas chromatographic introduction and predicted r.f. and d.c. voltages is presented.
The potential of composite pyrolysis gas chromatographic mass spectra in the study of lignocellulosics
Authors: Reeves James B. , Galletti Guido C.
Published: May 2005

The objective of this work was to evaluate the use of composite mass spectral (CMS) data from pyrolysis gas chromatography/mass spectrometry (PY–GC/MS) for lignocellulosic materials. Various forages, by‐products and fiber fractions derived from them were examined as CMS by PY–GC/MS. The PY–GC/MS system consisted of a heated platinum filament, a capillary gas chromatograph and an ion trap detector (ITD) mass spectrometer operated under electron impact conditions. Mass spectra were then composited in several ways by summing all the mass spectra acquired within retention times corresponding to major product classes. CMS data were entered in a dedicated library and compared using the ITD library editor software. The usefulness of such a simple procedure for studies related to lignocellulose analysis, such as forage recognition, development of analytical methods and digestibility/maturity correlation, is discussed.
Effects of functional group interactions on the dissociation reactions of protonated amino alcohols
Authors: Eichmann Erika S. , Brodbelt Jennifer S.
Published: May 2005

The dissociation reactions of protonated amino alcohols were examined in a quadrupole ion trap mass spectrometer. Multi‐stage collision‐induced dissociation techniques were used to characterize the ions and their fragments and to assist in the determination of the dissociation mechanisms. In addition, semi‐empirical calculations were used to rationalize the results on the basis of the thermodynamics of the reactions in question. The reaction of special interest was the double elimination of water and ammonia. For this high‐energy process, it is shown that the initial deamination step is the thermodynamically favored one in most cases. The enthalpies of formation for the various precursor and product ions and also those for some of the reaction intermediates were estimated using molecular modelling and semi‐empirical calculation methods. The values obtained indicated that the minimum endothermicity of the sequential deamination–dehydration reaction ranges from 209 to 460 kJ mol−1 for the compounds studied here. Moreover, protonation at the amine site was found to be energetically favored by 38–192 kJ mol−1 over protonation at the hydroxyl site.
Acetone chemical ionization studies VI: Hexenyl acetates
Authors: Vairamani M. , Kumar K. V. Siva
Published: May 2005

Acetone chemical ionization mass mass spectra of four isomeric hexenyl acetates were investigated. The characteristic decomposition of [M + CH3CO]+ adducts in the field‐free region reveals site‐specific adduct formation of acetyl ion with the double bond and the acetoxy group.
Fast atom bombardment and tandem mass spectrometry of quaternary pyridinium salt‐type tryptophan derivatives
Authors: Prokai Laszlo , Prokai‐Tatrai Katalin , Pop Emil , Bodor Nicholas , Lango Jozsef , Roboz John
Published: May 2005

Fast atom bombardment and collision‐induced dissociation tandem mass spectrometry were used to study the fragmentation of quaternary pyridinium salt‐type amides of tryptophan (α‐amino‐3‐indolepropionic acid) esters and their analogs which incorporate the α‐nitrogen into the quaternary pyridinium structure. By cleavage directly at the pyridine nitrogen, the 1‐alkyl‐substituted nicotinamides decompose exclusively to a carbocation, which then becomes the intermediate to further fragments. Rearrangement of the 3‐indolepropionate‐2‐yl carbocations may involve a five‐ to seven‐membered ring expansion, which generates alternative fragmentation pathways; the formation of an even‐electron and a radical cation, respectively. In trigonellyl amide‐type tryptophan derivatives, fragmentation of the pyridinium ion proceeds on multiple pathways induced by the positive charge which may not be localized on the quaternary nitrogen, and isomerization to a dihydropyridinyl structure is probably involved. Besides the formation of protonated nicotinamide and alkene from tryptophan amides that contain methylene or ethylene units between the amino and the quaternary pyridinium nitrogens, a fragmentation route leading to the carbocation identical with that of the 1‐alkyl‐substituted nicotinamides has also been revealed.
Mass spectra of oxazolidines derived from sugars
Authors: Vega‐Pérez José M. , Espartero José L. , Ruiz Francisco J. , Alcudia F.
Published: May 2005

The electron impact mass spectra of 29 oxazolidines derived from sugars were examined. Four general pathways of fragmentation were encountered, the importance of which depends on the nature of the different substitutents.
Rearrangement of aromatic sulfonate anions in the gas phase
Authors: Binkley Roger W. , Flechtner Thomas W. , Tevesz Michael J. S. , Winnik Witold , Zhong Boyu
Published: May 2005

Collisionally activated dissociation of deprotonated aromatic sulfonic acids in the gas phase causes rearrangement and fragmentation to produce the corresponding phenoxide ions. The mechanism for this reaction has been investigated and the results of this study favor initial intramolecular nucleophilic addition of a sulfonate oxygen atom to the aromatic ring, a process which is followed by heterolytic cleavage of the carbon–sulfur bond to rearomatize the ring. The product from this addition–elimination sequence is the anion of a sulfurous acid half‐ester, which loses SO2 to generate the corresponding phenoxide ion.
Mixed silicon carbide clusters studied by laser ablation Fourier transform ICR mass spectrometry
Authors: Greenwood Paul F. , Willett Gary D. , Wilson Michael A.
Published: May 2005

Ions produced by the 1064 nm Nd:YAG laser ablation of various Si–C samples include mixed silicon carbon (SixCy) cluster ions of low molecular mass. Unique SixCy cluster ion distributions are observed. A charge state effect for SixCy clusters is evident from differences between the respectively charged distributions. This may be accounted for by either differing energetics and/or differing formation mechanisms for positive and negative SixCy cluster ions. Several specific SixCy clusters of probable high stability consistently dominate the mass spectra. The chemistry of the more abundant SixCy ions was probed by collision‐induced dissociation and ion–molecule reactivity experiments. Similar SixCy+˙ dissociation results in the common loss of an Si ion. Parent SixCy ion and small neutral reagent reactants give rise to oxidation, addition and exchange reactions.
Effect of alkali metal cationization on the collision‐induced decomposition of alkyl per‐O‐acetyl‐2‐deoxy‐2‐halo‐α‐O‐mannopyranosides
Authors: Madhusudanan K. P. , Rani A. , Kumar B. , Suryawanshi S. N.
Published: May 2005

The effect of alkali metal cationization on the collision‐induced decomposition of alkyl per‐O‐acetyl‐2‐deoxy‐2‐bromo‐and‐iodo‐α‐O‐mannopyranosides was studied. The bromo sugars gave fairly abundant MH+, whereas for the iodo sugars the MH+ ions were insignificant. However, both the bromo and the iodo derivatives gave abundant M + alkali metal ion complexes. In contrast to the behaviour of the MH+ ion, the [M + Li]+, [M + Na]+ and [M + K]+ ions of these compounds do not decompose by loss of the C(1) substituent. Elimination of AcOH is the preferred fragmentation pathway of [M + Cat]+. Elimination of HX occurs only after loss of AcOH and CH2CO from MH+, whereas [M + Cat]+ directly loses HX. The elimination of HX is more pronounced from [M + Na]+ and [M + K]+ than from [M + Li]+. Loss of AcOLi is an additional fragmentation route observed in the case of the decomposition of [M + Li]+ ion.
Collision‐induced ionization of neutral peptides
Authors: Wu Zhuchun , Fenselau Catherine
Published: May 2005

A four‐sector mass spectrometer was employed to study fragments formed by the high‐energy collision of neutral peptides with molecular oxygen. Neutral peptides were generated in the gas phase from proton‐bound dimers by the method of Wesdemiotis and co‐workers [Anal Chem. 65, 1594 (1993)]. B/E scanning was used in MS‐2 to collected the fragment ions from neutral precursors. Molecular ions (radical ions) were not obtained in most of the spectra, but a variety of fragment ions were recorded. High resolution and mass accuracy allowed most fragment ions to be assigned. Most fragment ions are of the same types as those produced from protonated peptides by collisional activation. Formation of y‐type ions is significantly reduced in the processes induced by collisional ionization, compared with high‐ and low‐energy collisional activation of protonated peptide precursors.
Rearrangement of ethane‐1,2‐diol radical cations: Processes involving dipole‐catalyzed proton shifts and charge transfer
Authors: Ruttink Paul J. A. , Burgers Peter C.
Published: May 2005

Rearrangement and dissociation processes of solitary ethane‐1,2‐diol radical cations were investigated by ab initio MO calculations, executed at the SDCI/RHF/DZP level of theory, including Pople‐type size‐consistency corrections. In order to obtain an accurate description of the chemistry involved, part of the potential energy surface was investigated by using the multi‐reference CI method and also by using the valence bond (VB) method followed by SDCI calculations using the natural orbitals of the VB wavefunction. The ethane‐1,2‐diol radical cation is metastable with respect to CH3OH2+ + HCO˙; it has been shown recently that the isotopologue DOCH2CH2OD loses (exclusively) HCO˙ to produce CH2DOHD+, not the isotopomer CH3OD2+ expected from earlier mechanistic proposals. We have traced a low‐energy pathway which explains the observed label distribution and which takes place at the experimentally derived energy level. First, ionized ethane‐1,2‐diol collapses to the one‐electron bond species [HOCH2‥ + ˙‥CH2OH]+˙ which subsequently rearranges to the hydrogen‐bonded species CH2O…︁HO(H)CH. Next, transformation to the transient CH2O…︁HCH2OH+˙ takes effect and this rearrangement can be viewed as the 1,2‐hydrogen shift, CH2OH → CH3OH+˙, catalyzed by formaldehyde. Following this, charge transfer takes place from the methanol cation to the formaldehyde molecule which thus becomes charged; because it is now charged, the formaldehyde unit can rotate and donate a proton to the methanol molecule, after which dissociation follows. Our calculations and experimental results can be interpreted in terms of proton shifts rather than hydrogen shifts taking place in ion–molecule (proton‐bound) complexes.
Methyl group effect on the proton affinity of methylated acetophenones studied by two mass spectrometric techniques
Authors: Kukol A. , Strehle F. , Thielking G. , Grützmacher H.‐Fr.
Published: May 2005

The proton affinities (PA) of all isomeric dimethylacetophenones were determined using the ‘kinetic method’ with a tandem mass spectrometer and by measurements of the proton transfer equilibrium in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry. For both methods acetophenone and p‐methylacetophenone, with well known PA values, were used as reference bases. Both methods yielded identical PA values for all dimethylacetophenones. The PA of the dimethylacetophenones were in a narrow range between 872 and 880 kJ mol−1 except for 2,6‐dimethylacetophenone, for which a PA of 856 kJ mol−1 was found. The results are discussed in terms of possible mesomeric structures stabilizing the positive charge with regard to the substitution pattern in the phenyl ring. Obviously, the significant smaller PA of 2,6‐dimethylacetophenone compared with the other isomers is due to the distortion of the conjugation of the CO double bond with the aromatic π system.
Sequencing of cyclosporins by fast atom bombardment and linked‐scan mass spectrometry
Authors: Havlíček Vladimír , Jegorov Alexandr , Sedmera Petr , Ryska Miroslav
Published: May 2005

The mass spectrometric sequence determination of amino acid residues in cyclosporins using fast atom bombardment, collisionally activated dissociations in the first field‐free region and linked B/E scan is described. The general fragmentation scheme was derived from the spectra of cyclosporins A, B, C, D, F, G, L and [DH‐MeBmt1]CS. The main fragmentation pathways start by primary splitting between amino acids 2–3, 1–11 and 5–6. The corresponding N‐terminal b‐type ions are common fragment types in the mass spectra. The 1–11 splitting can be enhanced by the introduction of a lactone group into the peptide ring by conversion of cyclosporins into isocyclosporins. The fragmentation scheme was used for amino acid sequence determination in four new natural cyclosporins, [MeLeu1]CS, [Leu4]CS, [Ile4]CS and [Leu5]CS.
Laser‐induced electron impact ionization in a reflectron time‐of‐flight mass spectrometer
Authors: Moritz F. , Dey M. , Zipperer K. , Prinke S. , Grotemeyer J.
Published: May 2005

Some details of the generation of electrons by impinging a laser beam on a metal surface are described. It is shown that highly efficient electron generation is observed only during the laser pulse. Therefore, this technique delivers intense pulses of electrons. The process is investigated and different ion source set‐ups are discussed. In conjunction with a time‐of‐flight mass spectrometer this technique can be used to produce mass spectra of different samples ranging from simple organic molecules to peptides.
Ion–molecule reactions between organometallic ions and crown ethers in the gas phase
Authors: Zagorevskii Dmitri V. , Nekrasov Yurii S.
Published: May 2005

Ion–molecule reactions of the metal‐containing ions LM+ (L = (acac)2, acac, C6H6, C5H5; M = In, Ga, Co, Fe, Ni, Cr, Mn, Pd, Rh, Tl, La, Pr, Yb, Nd) with crown ethers in the gas phase were studied. Two major reactions were observed: adduct formation and substitution of a metal atom ligand by a crown ether. The relative abundances of the two reactions depends on the ease with which the metal atom may be reduced. Ligand substitution can involve hydrogen rearrangements with loss of acetylacetone or cyclopentadiene for crown ethers having mobile H atom(s). The use of ion–molecule reactions in the structural characterization of crown ethers and transition metalcontaining ions is discussed.
Mass spectrometric investigation of the equilibrium gas‐phase composition over InI3 at elevated temperatures
Authors: Bencze L. , Marsel J. , Popovič A. , Lesar A.
Published: May 2005

Mass spectra of the saturated vapour and partial pressures of different species over solid InI3 were measured between 350 and 450 K with a mass spectrometric Knudsen effusion technique. By measuring appearance energies (AE) for different ions in the mass spectra, it was proved that InI3 evaporates congruently from the solid phase as InI3(g) and In2I6(g). Values of AE for In2In+ ions (n = 0–5) increase as n decreases, showing an interesting periodicity with InI bond strengths. For these measurements an improved deconvolution of the ionization efficiency curves by fast Fourier transformation, using a combined Maxwellian and Gaussian electron energy distribution, was performed. By measuring ion abundances as a function of temperature, the Clausius–Clapeyron plots for all ions appearing in the mass spectra were drawn.
Application of site‐selective ion–molecule reactions to the analysis of the cinchona alkaloids
Authors: Eichmann Erika S. , Brodbelt Jennifer S.
Published: May 2005

Functional group interactions within biologically relevant molecules are among the most influential yet least understood factors in determining their reactive behaviors. Reactions of dimethyl ether ions, which have previously been shown to be site‐selective, with four cinchona alkaloids, cinchonine, cinchonidine, quinine and quinidine, have been examined. These reactions are each shown to produce qualitatively similar spectra for the stereoisomeric pairs cinchonidine–cinchonine and quinidine–quinine, but small variations in the relative abundances of the products indicate that some stereoselectivity can be observed. The site selectivity of each of the reagents was investigated by observing the reactions occurring with model subunits of the alkaloids.
Site specificity in the H–D exchange reactions of gas‐phase protonated amino acids with CH3OD
Authors: Gard Eric , Willard Dale , Bregar Jennifer , Green M. Kirk , Lebrilla Carlito B.
Published: May 2005

H–D exchange reactions of methanol‐d1 with protonated amino acids were performed in an external‐source Fourier transform mass spectrometer. Absolute rate constants were determined for the group which included glycine, alanine, valine, leucine, isoleucine and proline. By comparing reactivities with selected methyl esters, it was found that exchange on the carboxylic acid occurs 3–10 times faster than exchange on the amino group. No simple correlation is observed between the rates of H–D exchange on the acid group and the size of the alkyl group. However, the rates of exchange on the amine decrease with increasing gas‐phase basicity. Glycine, the least basic amino acid, exchanges its amine hydrogens the fastest. These results are useful for determining the interaction of methanol with protonated amino acids and can provide insight into the H–D exchange reactions observed with polyprotonated proteins produced by electrospray ionization.