Negative- and positive-ion mass spectra of aryl glycoside acetates

The low specificity of the electron-impact positive-ion mass spectrum in relation to the structure of an aromatic aglycone has been shown. The glycosidic moiety is determined unambiguously from the pattern of the mass spectrum. The dissociative electron-capture mass spectra are sensitive to the nature of the substituents of the aromatic nucleus of the aglycone. The influence of the aglycone is most pronounced in cases with substituents having high electron affinities.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Scientific Center, Academy of Sciences of the USSR, Vladivostok, Institute of Biochemistry, Academy of Sciences of the Armenian SSR, Erevan, Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 623–628, September–October, 1984.     

Comparative chemical ionization mass spectra of tricyclic antidepressant amines and related compounds

Positive-ion methane chemical ionization (CI) mass spectra were obtained for seven underivatized tricyclic amines: amitriptyline, nortriptyline, protriptyline, imipramine, desipramine, cyclobenzaprine and cyproheptadine. Some discrepancies in previously reported spectra were noted. Spectra of protriptyline, cyclobenzaprine and cyproheptadine are reported for the first time. Comparisons are made among the CI spectra and with electron ionization spectra, and relative abundances of major CI ions among the seven compounds are rationalized in terms of substituent and geometric effects. In most cases low-mass iminium ions from anion abstraction were more abundant than [MH]⁺. Hydride abstraction and adduct formation with reagent-gas ions were important. The three heterocyclic amines gave abundant [M]⁺˙ by electron transfer. Protonation at the nitrogen atom on the side chain followed by amine elimination gave tricyclic aromatic fragment ions.     

Quantitative aspects of and ionization mechanisms in positive-ion atmospheric pressure chemical ionization mass spectrometry

The behavior in atmospheric pressure chemical ionization of selected model polycyclic aromatic compounds, pyrene, dibenzothiophene, carbazole, and fluorenone, was studied in the solvents acetonitrile, methanol, and toluene. Relative ionization efficiency and sensitivity were highest in toluene and lowest in methanol, a mixture of molecular ions and protonated molecules was observed in most instances, and interferences between analytes were detected at higher concentrations. Such interferences were assumed to be caused by a competition among analyte molecules for a limited number of reagent ions in the plasma. The presence of both molecular ions and protonated analyte molecules can be attributed to charge-transfer from solvent radical cations and proton transfer from protonated solvent molecules, respectively. The order of ionization efficiency could be explained by incorporating the effect of solvation in the ionization reactions. Thermodynamic data, both experimental and calculated theoretically, are presented to support the proposed ionization mechanisms. The analytical implications of the results are that using acetonitrile (compared with methanol) as solvent will provide better sensitivity with fewer interferences (at low concentrations), except for analytes having high gas-phase basicities.     

Rearrangements in chemical ionization mass spectra

Rearrangements reported in the literature for positive ions formed by chemical ionization are briefly reviewed, with particular emphasis on illustrative examples of hydrogen and skeletal rearrangements.     

Anomalous ions in the chemical ionization mass spectra of aromatic nitro and nitroso compounds

The appearance of [MH-30]⁺ ions in the chemical ionization mass spectra of aromatic nitro compounds may be due to their initial reduction to the corresponding amines within the ion source. Aromatic nitroso compounds may be similarly reduced to yield [MH-14]⁺ ions. The hydroxy derivatives of the nitroso compounds yield further anomalous ions at [MH-16]⁺ probably due to the reduction of the hydroxy groups.     

Evaluation of methane negative ion chemical ionization mass spectra of polycyclic aromatic hydrocarbons and related compounds

Negative ion chemical ionization (NICI) mass spectra with methane as reagent gas and the ion abundance ratios of the negative to the positive base peak for 51 polycyclic aromatic hydrocarbons and related compounds were measured and evaluated for highly sensitive detection and isomer differentiation. Either [M ? H]^?, M^?˙ or MH^? was the base peak, except for one compound with [M ? H₂]^?˙ as its base peak. The numbers of compounds with [M ? H]^?, M^?˙ or MH^? as their base peaks were 17, 26 and 7, respectively. Many of the compounds with [M ? H]^? as the base peak had an aliphatic part in their structure. The average value of N/P (negative/positive ion abundance ratio at the base peaks) was < 1. Many of the compounds with M^?˙ as the base peak had a relatively high electron affinity. A correlation between electron affinities and ion abundances was found. In most cases, the N/P ratios were > 1, and even reached 400 in benzo [a] pyrene. Many of the compounds with MH^? as their base peaks had a phenyl group, in which cases the N/P ratios were < 1. In the case of compounds with 18 or fewer carbon atoms, in particular, it was easy to distinguish isomers by comparing their NICI mass spectra. The N/P values served as a guideline in sensitive detection. Nine compounds achieved an N/P of ≥50.     

Electron ionization and chemical ionization mass spectra of pyridinium and isoquinolinium ylides

Electron ionization (EI) spectra and both positive and negative chemical ionization (CI) spectra have been obtained for four isoquinolinium ylides and two pyridinium ylides. Electron transfer reactions dominate the CI mass specra. The base peak in negative chemical ionization is the [M]^?· ion, formed by electron capture. In the positive methane CI spectra the molecular ion, [M]^+·, is relatively more intense than [MH]⁺ showing electron transfer to be the main positive ionization process. In the positive ammonia CI spectra, proton transfer to give [MH]⁺ is the main ionization process, but electron transfer is also observed. The EI spectra show fragmentations in which the aromatic nitrogen moiety retains the charge and fragmentation is by loss of radicals or small neutral molecules from the side-chains. Radical driven reactions are proposed to explain these spectra.     

Direct exposure chemical ionization mass spectra of explosives

Mass spectra of explosives, including TNT, tetryl, nitroglycerin, PETN and RDX have been recorded by direct exposure chemical ionization with isobutane as reagent at source temperatures of 50–100°C. The mass spectra contain major [MH]⁺ ions, adduct ions and some fragment ions. The configuration of the relative abundances of these ions has been found to be a function of temperature and source pressure. Maximum [MH]⁺ ion abundance has been obtained at source pressures much lower than normal chemical ionization pressures.     

Isobutane chemical ionization mass spectra of unsaturated alcohols

Analysis of the isobutane chemical ionization mass spectra of hexenols, cyclohexenols and various syn/anti pairs of bicyclic and tricyclic homoallylic alcohols shows that: (i) the spectra of the allylic alcohols are dominated by [M + H – H₂O]⁺ and [M + C₄H₉–H₂O]⁺ ions and contain traces of [M + H]⁺ ions; (ii) [M + H]⁺ ions are prominent in the spectra of acyclic and certain cyclic homoallylic alcohols; and (iii) [M + H]⁺ ions dominate the spectra of other acyclic unsaturated alcohols. The [M + H]⁺ ions may result from either: (a) protonation of the hydroxyl group, followed by a very rapid intramolecular proton transfer from the protonated hydroxyl group to the carbon–carbon double bond or internal solvation of the protonated hydroxyl group by the carbon–carbon double bond; and/or (b) direct protonation of the carbon–carbon double bond with significant internal solvation of the resulting carbocation by the hydroxyl group, which may lead to carbon–oxygen bond formation to give a protonated cyclic ether. The consequences of placing various geometric constraints on the possible intramolecular interactions between the hydroxyl group and the carbon–carbon double bond in unsaturated alcohols are explored.     

Electrospray ionization mass spectrometric analysis of chemical reactions of dissolution of selenium in strongly basic amines

When elemental selenium was added to a strongly basic amine, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), the selenium started to dissolve and the solution turned dark brown. We investigated the chemical reaction of this selenium dissolution process by electrospray ionization mass spectrometry (ESI-MS). The study reveals for the first time that cleavage of Se-Se bonds by the amine initiates the reaction to form molecular Se2, which then abstracts hydrogen from the amine molecule. ESI-MS with the use of a nanospray technique was shown to be a useful tool for studying the dissolution of elements in strongly basic or acidic solvents.     

Studies on the positive-ion mass spectra from atmospheric pressure chemical ionization of gases and solvents used in liquid chromatography and direct liquid injection

Detailed studies have been made using different source gases and solvents in a Micromass Quattro mass spectrometer under positive ion atmospheric pressure chemical ionization conditions. The major background ions from nitrogen, air, or carbon dioxide were investigated by tandem mass spectrometry, followed by similar studies on solvents commonly employed in normal- and reversed-phase high-performance liquid chromatography, namely, water-acetonitrile, acetonitrile, and dichloromethane, with nitrogen, air, or carbon dioxide; hydrocarbon solvents were studied using nitrogen. Spectra were interpreted in terms of the gases, solvents, and their impurities. The acetonitrile spectra provided clear evidence for both charge exchange and proton transfer, the former being facilitated by the introduction of some air into a flow of nitrogen. Radical cations of acetonitrile dimers, trimers, and tetramers were observed, as were protonated dimer and trimer species. Examination of the analytical response of four polycyclic aromatic hydrocarbons in various hydrocarbon solvents, with nitrogen gas, showed that the sensitivity of detection for an analyte and its ionization mechanism are dependent on both the analyte structure and the solvent, with pyrene showing the highest sensitivity, phenanthrene and fluorene being intermediate, and naphthalene having the lowest sensitivity. The degree of protonation followed the same trend. Signal intensity and degree of protonation were dependent on the alkane solvent used, with isooctane providing the best overall sensitivity for the sum of protonated molecules and molecular ions. The ions observed in these studies appeared to be the most stable ions formed under equilibrium conditions in the source.     

Determination of heterocyclic aromatic amines in meat extracts by liquid chromatography-ion-trap atmospheric pressure chemical ionization mass spectrometry

When protein-rich foods are processed under normal cooking conditions, heterocyclic aromatic amines (HAAs) can be generated at a few parts per billion level. In this work, we have analyzed the HAAs present in a lyophilized meat extract by means of a simplified solid-phase extraction procedure. All the analytes were collected in a single extract with recoveries in the range of 45.6-75.2%, so the analysis time has been greatly reduced. Problems derived from the less exhaustive purification of the extract have been solved by using MS(ion trap) detection. The RSD for quantification ranged from 2.1% to 5.1% for run-to-run precision and from 5.2% to 11% for day-to-day precision. The limits of detection for standard solutions ranged from 20 to 150 pg injected. For the meat extract analyzed limits of detection from 0.9 to 11.2 ng g(-1) were obtained. Results of the quantification are in agreement with those obtained using different clean-up procedures.     

Negative chemical ionization mass spectra of polycyclic chlorinated insecticides

Negative ion mass spectra obtained under chemical ionization conditions (NCI) employing methane, isobutane or methylene chloride as the enhancement gas are presented for a series of chlorinated polycyclic insecticides. All of the compounds examined except 1-hydroxychlordene yielded molecular anions of substantial relative abundance (6 to 39%). The most significant features of the spectra are the prominent peaks at masses greater than that of the molecule ion formed via ionmolecule association reactions. Peaks representing association of the parent molecule with ionic species such as H?, O?, OH?, Cl?, H₂OCl?, HCl₂?, ClO? and Cl₃? were observed in some cases. The base peak in all spectra was associated with the isotopic group of the [M + Cl]? on if contributions from other negative, even electron ions of low mass values present in high concentrations (Cl?, H₂OCl? Cl₂? and HCl₂?) are neglected. Fragmentation processes were limited to elimination reactions involving loss of combinations of the even electron neutral species H, Cl and HCl. In addition, fragmentation resulting from a nucleophilic radical displacement of Cl by O? from the parent molecule was observed in all cases except 1-hydroxychlordene when the source was modestly wet (methane as reactant gas). NCI mass spectra of polycyclic chlorinated pesticides are reproducible, intense, interpretable in terms of classical carbanion chemistry and thus may have important analytical utility, particularly when used in conjunction with positive electron-impact and chemical ionization mass spectral methods and selective use of different enhancement gases.     

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.

     

Positive chemical ionization mass spectra of polycyclic chlorinated pesticides

Methane chemical ionization (CI) mass spectra for a series of ten polycyclic chlorinated insecticides and metabolites have been examined. In all cases except heptachlor epoxide the base peak corresponded to elimination of Cl, or OH from the molecule ion. In the spectrum of heptachlor epoxide the [M + H]⁺ and [M ? Cl]⁺ clusters were of approximately equal intensity. The CI spectra were remarkably simple, invariably less complex than the corresponding electron-impact (EI) mass spectra and the intensity of the ions with high information content, e.g. [M ? CI]⁺ was uniformly high. All of these features are important to the analytical potential of these studies. Retro Diels-Alder (RDA) fragments were observed for the chlordanes, aldrin, isodrin, nonachlor and heptachlor epoxide. The reported preliminary data suggest that the relative intensity of RDA ions in CI mass spectra may be useful in establishing molecular configurations.     

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.     

Surface-assisted laser desorption and ionization mass spectrometry using low-cost matrix-free substrates

Surface-assisted laser desorption/ionization (SALDI) substrates have been fabricated using nanospiked polyurethane (PU) substrates that are replicated by a low-cost soft nanolithography method from silicon nanospike structures formed with femtosecond laser irradiations. The strongest mass spectrometry (MS) signal of Angiotensin II was obtained on 45-nm Au-coated nanospiked PU substrates. The effective ionization appears to be due to surface plasmon excitation. Such low-cost and identical SALDI substrates can be used for MS analysis of various molecules with high reproducibility.     

Multiple ionization,charge separation and charge stripping reactions involving polycyclic aromatic compounds

The 70 eV electron ionization mass spectra of polycyclic aromatic compounds are characterized by the presence of relatively stable multiply charged molecular ions [M]ⁿ⁺ (n=2–4). When generated from the compounds benzene, napthalene, anthracene, phenanthrene, 2,3-benzanthracene, 1,2-benzanthracene, chrysene, 9,10-benzophenanthrene and pyrene, the relative abundances of the multiply charged ions increase dramatically with the number of rings. These compounds form multiply charged molecular ions (n=2, 3) which undergo unimolecular decompositions indicative of considerable ionic rearrangement. The main charge separation processes observed here [M]²⁺→m₁⁺+m₂⁺, [M]³⁺˙→m₃⁺+m→+m₄²⁺) involve, in almost every case, one or more of the products [CH₃]⁺, [C₂H₃]⁺˙ and [C₃H₃]⁺. This suggests the existence of preferred structures amongst the metastable parent ions. Information on the relative importance of the various fragmentation pathways is presented here along with translational energy release data. Some tentative structural information about the metastable ions has been inferred from the translational energy release on the assumption that the released energy is due primarily to coulombic repulsion within the transition state structure. For the triply charged ions these interpretations have necessitated the use of a coulombic repulsion model which takes account of an extra charge. Vertical ionization energies for the process [M]ⁿ⁺+G→[M](ⁿ⁺¹)+G+e^? (charge stripping) have also been determined where possible for n=1 and 2 and the results from these experiments allow the derivation of simple empirical equations which relate successive ionization energies for the formation of [M]²⁺ and [M]³⁺˙ to the appearance energy of [M]⁺˙.     

Sites of protonation in phenothiazines: Methane and ammonia chemical ionization mass spectra

Positive ion methane and ammonia chemical ionization mass spectra for ten phenothiazine derivatives are reported. The fragmentations observed in the chemical ionization mass spectra are rationalized in terms of the location of the added proton. High-resolution measurements are used to confirm empirical formulae of the ions in the mass spectra. Changes in the mass spectra with a change in the chemical ionization reagent gas from methane to ammonia are described. A comparison with positive ion secondary ion mass spectra of the same compounds show that the amount of fragmentation is higher in the secondary ion mass spectra, but the same types of ions are observed in spectra produced by both ionization methods.