The mass spectra and ionization potentials of the neutral fragments produced during the electron bombardment of aromatic compounds

A mass spectrometer equipped with a dual ionization chamber ion source has been used to characterize directly the neutral species produced in the dissociative ionization of gases by electron impact. Neutral fragment mass spectra have been obtained for the electron ionization and fragmentation of benzene, toluene, o-xylene, m-xylene, p-xylene, mesitylene and isotopically labeled toluene. The neutral fragment mass spectra correlate well with the structures of the molecules. The abundant species in the neutral fragment mass spectra also correlate reasonably well with the abundant complementary positive ions of the normal mass spectra. Ionization potentials have been determined for the abundant neutral species produced. Where comparisons with values reported elsewhere are possible, the agreement is usually within ±0.2 eV or less.     

Electron impact and chemical ionization mass spectra of aryl ureas

The electron impact and chemical ionization mass spectra of a series of N,N′, -diaryl ureas have been compared. The electron impact mass spectra indicate rearrangements leading to two pairs of aromatic amines and isocyanates, either as ions or molecules. The chemical ionization mass spectra showed the formation of protonated amines and isocyanates via rearrangement.     

The electron impact mass spectrometry of some para substituted trans-2-styryl-6-carboxychromones. An unusual cleavage of the chromone ring

The electron impact mass spectrometry of some para substituted trans-2-styryl-6-carboxychromones shows a new fragmentation of the chromone ring in addition to the well known retro Diels-Alder reactions with and without hydrogen transfer. The observed primary fragments are supported by metastable transitions and exact mass measurements.     

Mass spectrometry of pyrazolium salts

A series of pyrazolium bromides were investigated by field desorption, field ionization and electron impact mass spectrometry. Dealkylation was found to be the predominant thermal decomposition. The nature of the substituents controls the pyrazole formed. The application of field ionization was found to facilitate the interpretation of the thermolytic decomposition products. Field ionization and electron impact mass spectra of N-phenylpyrazole and N-benzylpyrazole were obtained.     

The reactive states of ions: Antiaromatic seven-membered N-substituted-1H-azepines and isomeric aromatic substrates

Comparison of the electron impact and chemical ionization mass spectra of N-substituted-1H-azepines and isomeric aromatic substrates reveals significant differences. The data indicate that the substituted azepincs examined are stable to electron impact at low temperatures and retain their antiaromatic 8π electron seven-membered cyclic system prior to their fragmentation. The electron impact data are supported by low electron-volt and metastable scan techniques. Electron impact or chemical ionization induced transformation of azepines to aromatic substrates similar to the thermal transformation of neutral analogues is not observed at low temperatures. Chemical ionization mass spectra of N-ethoxycarbonyl-1H-azepines and N-phenylcarbamates were complicated owing to thermal decomposition reactions at high temperatures.     

Mass spectral fragmentation patterns of 2,3-diphenylindole derivatives and dibenzo[ac]carbazoles

The electron impact mass spectra of a variety of substituted 2,3-diphenylindoles have been examined and the major fragmentation routes ascertained. The mass spectra of several dibenzo[ac]carbazoles are also discussed. All of the results allow us to assume that 2,3-diphenylindoles would not cyclize in the spectrometer upon electron impact.     

The elimination of carbon monoxide from molecular ions of phenol and other [C6H6O]+˙ isomers

Phenol and five acyclic isomeric compounds have been investigated using electron impact and field ionization techniques, mass analysed ion kinetic energy spectrometry and collision induced dissociation mass analysed ion kinetic energy spectrometry. The kinetic energy release data corresponding to the elimination of carbon monoxide from the molecular ions show that at least two structures of the reactant ion are involved. The electron impact and field ionization collision induced dissociation mass analysed ion kinetic energy spectra indicate that under electron impact conditions, the phenol ion partially isomerizes to another ion structure. An isomerization of about 40% to the structure of cis-2-hexen-4-yn-1-al is in good accordance with the spectral data.     

Electron impact and chemical ionization mass spectra of some unsaturated anomeric C-glycosides

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.     

Pyrolysis mass spectrometry of methionine

The mechanisms of some fragmentation of methionine upon Curie-point pyrolysis have been studied by the use of deuterium labelling. They lead to products having the same nominal mass but essentially different elemental compositions compared with ions generated by electron impact on methionine. For exaple, upon pyrolysis the product with mass 116 is due to a 1,2 elimination of hydroxylamine from the title compound, whereas upon electron impact the ion with m/e 116 is generated by successive loss of a molecule of water and of a methyl radical from the molecular ion; the generated of product m/e 101 proceeds via successive loss of a molecule of water, carbon monoxide and of hydrogen upon pyrolysis, but via a one-step elimination of methanethiol upon electron impact. No evidence has been found for a diketopiperazine formation by a bimolecular process upon pyrolysis, although this type of reaction is well known for α-amino acids.     

Elucidation of the dissociation pathways of electro‐ionized estrone

With the future aim of using gas chromatography coupled with mass spectrometry to characterize the transformation products of estrone submitted to UV‐photolysis or to waste water treatment plants, an interpretation of the electron impact mass spectrum of estrone is presented. Fragmentation mechanisms are proposed on the basis of high‐resolution measurements performed with a magnetic sector analyzer. Multiple‐stage mass spectrometry experiments were carried out using an ion trap mass spectrometer. The structures proposed for product ions were confirmed by the m/z shifts observed in the estrone‐d₄ and estrone methyl ether electron ionization mass spectra. If the formation of some of the most abundant ions may easily be explained by α‐cleavages and retro‐Diels‐Alder type rearrangements, complex mechanisms need to be considered to rationalize the formation of others. Isotope labelling allows discrimination of isobaric ions. Copyright © 2010 John Wiley & Sons, Ltd.     

Mass spectrometry of 4N-pyrimidinyl amino acids

The electron impact mass spectra of N-(1H-2-oxo-4-pyrimidinyl) amino acids are discussed. It was established that thermal elimination of water took place prior to mass fragmentation of all these compounds with the formation of bicyclic structures. The fragmentation of bicyclic ions led to 1H-2-oxo-pyrimidinyl species.     

Discrimination among geometrical isomers of α-linolenic acid methyl ester using low energy electron ionization mass spectrometry

There is a consensus that electron impact ionization mass spectrometry is not capable of discriminating among geometrical isomers of unsaturated fatty acid methyl esters (and in general olefinic compounds). In this paper, we report the identification of all eight geometrical isomers of α-linolenic acid, one of the few essential ω-3 fatty acids that has attracted great attention, using low-energy electron ionization mass spectrometry. Three electron energies 70, 50, and 30 eV were studied and the mass spectrum of each isomer was obtained from the analysis of different concentrations of a standard mixture of α-linolenic acid methyl ester geometrical isomers to ensure the robustness of the method. Principal component analysis was employed to model the complex variation of m/z intensities across the isomers. Only using the data of 30 eV energy was complete differentiation among geometrical isomers observed. The unique cleavage pattern of the α-linolenic acid methyl ester isomers leading to a benzenium ion structure is discussed and general fragmentation rules are derived using the mass spectra of over 300 compounds with different kinds and levels of unsaturation. Application of the proposed method is not limited to α-linolenic acid. It can potentially be used to identify the geometrical isomers of any compounds with an olefinic chain.     

Über quantitative Beziehungen zwischen Fragmenten und ihren relativen Intensitäten im Massenspektrometer. V—Untersuchungen an N-t-Alkylaminen und -acetamiden

The ion intensity ratios from α-fissions of N-t-alkylethylamines are independent of electron energy above 18 eV and they directly follow the ‘ion mass effect’ in this region. The course of the intensity ratios over the whole energy range can be explained by combination of appearance energy differences and the ion mass effect. In the upper energy region the increase of the ion intensity ratios I(M? R_large): I(M? R_small) normally found with increasing electron energy is mainly caused by successive decomposition of the ions. This is shown from investigations of N-t-alkylacetamides. A correlation is found between the masses of the primary ions and the tendency towards secondary ion formation, and the internal energy of the primary ions is found to have an influence too. The ‘ion mass effect’ is not a direct effect of the ion masses.     

The mass spectrometry of polychlorinated dibenzo-p-dioxins

The negative ion chemical ionization mass spectra of polychlorinated dibenzo-p-dioxins using oxygen, methane and methane/oxygen are reported together with their methane positive ion chemical ionization mass spectra and conventional electron impact spectra. The methane/oxygen negative ion chemical ionization mass spectra proved to be the most useful of the negative ion spectra for structure determination.     

Electron impact and field desorption mass spectrometry of some macrocyclic-acyclic polyether-ester compounds

The electron impact mass spectrometric behaviour of a series of macrocyclic-acyclic polyether-ester compounds containing an isoxazole, benzene or pyridine ring is discussed in detail with the aid of exact mass measurements and metastable ions (linked scans B/E and B²/E). The nature of unusual [M+H]⁺ species, present in the crown moieties only, is elucidated by means of low electron energy measurements and labelling experiments. The field desorption spectra show only intense peaks corresponding to [M+H]⁺ species and rule out the presence of dimeric moieties.     

Electron impact mass spectra of synthetically modified milbemycins

The potent endectocide 23-(O-methyloximino)-F28249α and related compounds were identified and characterized by mass spectrometry. The fragmentation pathway of 23-(O-methyloximino)-F28249α was identified by its high-resolution mass spectrum and the electron impact unit mass spectra of its homologs. This fragmentation pathway is presented and discussed.     

Electron impact mass spectrometry of some naphthoxypolymethylenethiopyrimidines

The electron impact induced fragmentation of a large number of 2- and 4-S-(2-naphthoxy)-polymethyienethiouracils is discussed in detail, with the aid of exact mass measurements and observation of metastable decompositions. The 2- and 4-thiopyrimidine structures can be easily distinguished by their mass spectra.     

Mass spectrometry of β-keto aldehydes: Novel hydroxyl transfer reactions

The mass spectral fragmentation of 2,3-diaryl-3-oxo-1-propanals has been investigated by low electron energy, metastable defocusing and shift techniques. The o-nitro group strongly interacts with α-cleavage reactions of the carbonyl group. The cyclization/elimination processes, accompanied by oxygen and hydroxyl transfer in the mass spectra of o-nitro-β-keto aldehydes, are preceded by hydrogen abstraction by the nitro group.     

Massenspektrometrie kondensierter N-Heterocyclen. 1–Elektronenstoßinduzierte Bildung von Cyclaziniumionen aus 5-Aryl(Hetaryl)-9-methyl-s-triazolo[4,3-c]tetrazolo-[1,5-a]pyrimidinen

The electron impact mass spectra of 5-aryl(hetaryl)-9-methyl-s-triazolo[4,3-c]tetrazolo[1,5- a]pyrimidines have been investigated. There is a general fragmentation mechanism, which forms stable cyclazinium ions by cyclization. In the case of the 5-hetaryl derivatives the proposed structures were confirmed by means of a comparison of mass analysed ion kinetic energy spectra with suitable model compounds.     

Mass spectral fragmentation pattern of 2,3-dimethyl- and 2-methyl-3-phenyl-4-arylazoisoxazol-5-ones

2,3-Dimethyl-4-arylazoisoxazol-5-ones and 2-methyl-3-phenyl-4-aryiazoisoxazol-5-ones undergo considerable fragmentation on electron impact. The base peaks in the mass spectra are at mass 56 and 118 respectively attributed to the N-methylacetonitrile cation and the N-methylbenzonitrile cation.