The electron impact and chemical ionization mass spectra of dimethyltrisulfide

Summary The mass spectra obtained by electron impact ionization (EI) of dimethyltrisulfide, both at constant sample pressure and during elution from a GC column, are essentially identical, with the molecular ion Me₂S ₃ ⁺ providing the basis peak. Masses heavier than the molecular ion are not observed. Chemical ionization, using nitrogen, methane or isobutane, gives rise to numerous ions of larger mass than that of the molecular ion. Particularly characteristic are sulfonium type structures Me₃S _n ⁺ , with n=3–6. In addition, radical cations of the type Me₃S_nCH ₂ ⁺ and protonated trisulfide, Me₂S₃H⁺, are observed, even with N₂ as ionizing gas, together with a variety of ions of lower hydrogen content. Further, a large number of ion types of lower mass than the parent molecule are formed. The mass distribution of ions in the spectrum is found to be highly dependent on the partial pressure of dimethyltrisulfide in the ion source. These phenomena were investigated and accounted for semiquantitatively.

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Die EIund CI-Massenspektren des Dimethyltrisulfids

Zusammenfassung Dimethyltrisulfid liefert nach Elektronenstoßionisation bei konstantem Probendruck und bei GC-Probeneinlaß im wesentlichen identische Massenspektren. Das Molekülion Me₂S ₃ ⁺ stellt dabei den Basispeak dar. Größere Massen als die des Molekülions wurden nicht beobachtet. Bei chemischer Ionisation unter Verwendung von Stickstoff, Methan oder Isobutan als Reaktantgas entstehen zahlreiche Ionen mit höherer Masse als der des Molekülions. Charakteristisch sind hierbei sulfoniumartige Strukturen Me₃S _n ⁺ mit n=3–6. Daneben beobachtet man Radikalkationen des Typs Me₂S_nCH ₂ ⁺ und protoniertes Trisulfid Me₂S₃H⁺ auch bei Ionisation mittels N₂ sowie Ionenarten mit einem geringeren Wasserstoffgehalt. Weiterhin wird eine größere Anzahl von Ionenarten gebildet, deren Massen kleiner sind als die des Stammoleküls. Die Massenverteilung der Ionen im Spektrum hängt außerordentlich stark vom Partialdruck des Dimethyltrisulfids in der CI-Quelle ab. Diese Abhängigkeiten wurden semiquantitativ untersucht und dargestellt.

     

Fragmentation selectivity in electron impact ionization mass spectra of substituted dimethoxybenzenes

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.     

The electron impact and chemical ionization mass spectra of some glycidyl ethers

The electron impact and chemical ionization mass spectra of a number of aliphatic and aromatic glycidyl ethers are reported. For electron impact spectra, partial fragmentation pathways have been determined by metastable linked scan techniques.     

Positive electron impact and chemical ionization mass spectra of some nitramine nitrates

The positive electron impact (EI) and isobutane chemical ionization (CI) mass spectra of six nitramine nitrates were studied with the aid of some accurate mass measurements. In the EI spectra, β fission relative to both the nitramine and nitrate ester is important. In the CI spectra a major ion occurs at [MH – 45]⁺ and was found to be mainly due to [M + 2H ? NO₂]⁺. All of the compounds except N-(2 hydroxyethyl)-N-(2′,4′,6′-trinitrophenyl)nitramine nitrate gave an [MH]⁺ ion. The [MH – 45]⁺ ion in the isobutane CI mass spectra of tetryl is also due to [M + 2H ? NO₂]⁺.     

Comparison of electron impact and chemical ionization mass spectra of some arenetricarbonylchromium complexes

The mass spectrometric behavior of a) the tricarbonylchromium complexes of a series of aromatic hydrocarbons, b) the dimethyldiphenyl compounds of the Group IV elements (i.e., diphenylpropane, dimethyldiphenylsilane, etc.) and c) the mono- and bis-tricarbonylchromium complexes of these ligands under electron impact and chemical ionization conditions are reported. The MH⁺ ion is base peak for all of the simple arenetricarbonylchromium complexes using chemical ionization, whereas [M — 3 CO]⁺ or ⁵²Cr⁺ dominate the spectra with electron impact ionization. The chemical ionization spectra of the series of Group IV element ligands do not exhibit signals in the molecular ion region, the base peak being [M — Ph]⁺. [M — CH₃]⁺ is the electron impact base peak for each of the ligands except the lead-containing compound, for which the base peak is ²⁰⁸Pb⁺. The mono-tricarbonylchromium complexes yield chemical ionization molecular ion clusters, but their base peaks arise via fragmentation of the Group IV element—aromatic ring bonds. Electron impact ionization spectra of the mono complexes are characterized by losses of CO and the production of Cr⁺ ions, neither of which occurs with chemical ionization. For the series of bis-tricarbonylchromium complexes, an MH⁺ ion is prominent only in the chemical ionization spectrum of the diphenylpropane complex. The electron impact induced spectra of the bis-tricarbonylchromium complexes are similar to those of the mono-complexes in that loss of CO is a prominent feature.     

Chemical ionization and electron impact mass spectra of thiosulfinates,thiosulfonates and sulfinyl sulfones

The chemical ionization and electron impact mass spectra of some thiosulfinates, thiosulfonates and sulfinyl sulfones have been studied. The electron impact mass spectra of four of the six thiosulfonates show molecular ions of less than 1%. Inconclusive evidence was obtained for sulfenyl sulfinate type intermediates in the electron impact spectra of thiosulfonates. The electron impact spectra of thiosulfonates were similar to those of thiosulfonates. The chemical ionization (isobutane) mass spectra of thiosulfinates and thiosulfonates generally show protonated molecular ions [MH]⁺ as base peaks and [MH+1]⁺ and [MH+2]⁺ peaks.     

Mechanism of ionization and initial fragmentation in electron impact mass spectra of 3-halogenobenzanthrones

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.     

Chemical ionization and electron impact mass spectra of symmetrical S-alkyl alkanesulfonothioates (thiosulfonates)

The chemical ionization (CI) and electron impact (EI) mass spectra of eight symmetrical S-alkyl alkanesulfonothioates (thiosulfonates) are reported.     

Chemical ionization and electron impact mass spectra of alicyclic substituted 2-aryl-1,3-dithianes

The methane and isobutane chemical ionization mass spectra of alicyclic substituted 2-aryl-1,3-dithianes were examined by gas chromatography mass spectrometry. The protonated molecular ion was found to be of low abundance in the methane spectra, while a protonated cyclic sulfide cation (m/z 107) appeared as the base peak. A protonated molecular ion was the base peak when isobutane was used as the reagent gas. Electron impact mass spectra displayed weak molecular ions and were characterized by the m/z 106 fragment.     

Field ionization mass spectra of photopolymers of thymine

The field ionization mass spectra of cis-syn thymine dimer, tymine-thymine adduct and a new thymine phototrimer have been measured. Results are presented to illustrate that this technique is more efficacious than the electron impact method in structural elucidation of photopolymers of thymine and related compounds.     

Chemical ionization and electron impact mass spectra of S-1-chloroalkyl alkanesulfonothioates (thiosulfonates)

The chemical ionization (CI) and electron impact (El) mass spectra of six S-1-chloroalkyl alkanesulfonothioates (thiosulfonates) are reported.     

Electron impact,electron capture negative ionization and positive chemical ionization mass spectra of organophosphorus flame retardants and plasticizers

Phosphate esters are important commercial products that have been used both as flame retardants and as plasticizers. To analyze these compounds by gas chromatographic mass spectrometry, it is important to understand the mass spectra of these compounds using various ionization modes. This paper is a systematic overview of the electron impact (EI), electron capture negative ionization (ECNI) and positive chemical ionization (PCI) mass spectra of 13 organophosphate esters. These data are useful for developing and optimizing analytical measurements. The EI spectra of these 13 compounds are dominated by ions such as H₄PO₄⁺, (M ? Cl)⁺, (M ? CH₂Cl)⁺ or (M)⁺ depending on specific chemical structures. The ECNI spectra are generally dominated by (M ? R)^?. The PCI spectra are mainly dominated by the protonated molecular ion (M + H)⁺. The branching of the alkyl substituents, the halogenation of the substituents and, for aromatic phosphate esters, ortho alkylation of the ring are all significant factors controlling the details of the fragmentation processes. EI provides the best sensitivity for the quantitative measurement of these compounds, but PCI and ECNI both have considerable qualitative selectivity. Copyright © 2013 John Wiley & Sons, Ltd.     

Simulation of electron ionization mass spectra of cycloalkyl fluorophosphonates

The mass spectra of highly toxic cycloalkyl alkylfluorophosphonates can be divided into two components: organophosphorus and hydrocarbon. The organophosphorus component is a generalized spectral image of a taxonomic group among those forming a homologous series of cycloalkyl alkylfluorophosphonates. The hydrocarbon component is interpreted as a spectrum of the cycloolefin which is the main product of cycloalkyl alkylfluorophosphonate fragmentation. A method for simulation of the electron ionization mass spectra of these compounds is proposed.     

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.     

Electron impact and chemical ionization mass spectra of alkyldiphenylphosphine oxides

In the 70 e V electron impact mass spectra of a series of alkyldiphenylphosphine oxides (R?₂PO, R = Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, t-Bu, neopentyl, n-decyl), molecular ions of low abundance are observed and [M + H]⁺ ions are formed to a small extent at high sample pressures. The major ions include [?₂PO]⁺, [?₂POH]⁺; [?₂CH₂PO]⁺ and [?₂CH₂POH]⁺ which are formed by rearrangement and cleavage processes. The chemical ionization mass spectra obtained with methane and isobutane reagents consist of [M + H]⁺ ions. The proton affinity of R?₂PO was found to be 219 ± 2.5 kcal mol^?1.     

Intensity of molecular ion peak in electron ionization mass spectra

Compounds from NIST??08 and Wiley 8th databases were considered as a representative subset of the general population of organic compounds which can be analyzed using mass spectrometry with electron ionization. The percentage of organic compounds as a function of intensity of molecular ion (M^+?) peak normalized to the base peak was determined for the first time. It was shown that only 26% compounds have high-intensity M^+? peaks (greater than 50% of base peak). Intensity of M^+? peak is less than or equal to 1 or 5% of base peak for 24 or 37% compounds respectively. It means that in case of trace-level analysis M^+? peak may not be registered for quarter (or even more) of organic compounds. It is well-known that the absence of M^+? peaks in electron ionization mass spectra reduces reliability of unknown compound identification based on library search. Therefore determination of molecular mass by independent technique (e.g., mass spectrometry with chemical ionization) is necessary for increasing the identification reliability.     

Electron impact and chemical ionization mass spectra of nitro-substituted polyfunctional isomers

The electron impact and methane and ammonia chemical ionization mass spectra of some selected nitro-substituted isomeric benzalacetophenones, benzyl ketones and aromatic epoxides have been examined. The isomeric pairs show significant differences in the electron impact and chemical ionization spectra. The EI spectra show cleavage α to the carbonyl as the major fragmentation mode. Under CI conditions subtle differences in the fragmentation modes of isomeric pairs are more enhanced, and elimination reactions are more favoured in the o-nitro-substituted compounds than in the para isomers.