Positive and negative ion mass spectra of oxygen,sulphur and selenium 2,1,3-benzodiazoles

The 70 eV positive and negative ion mass spectra of the oxygen, sulphur and selenium 2,1,3-benzodiazoles have been studied. The influence of hetero atom on the fragmentation modes of the benzodiazole molecular ions has been discussed.     

Why are electron capture negative ion mass spectra not reproducible? An ion source problem

Differences in the electron capture negative ion mass spectra of environmentally related organic compounds acquired on a VG 30-250 triple quadruple mass spectrometer and on an HP 5985B gas chromatography/mass spectrometry system were investigated with respect to the ion formation process. Neither ion source temperature nor pressure was responsible for the differences. The populations of thermal electrons in both ion sources were experimentally determined and found to be similar, suggesting that electron capturing reactions should proceed with comparable efficiencies in both ion sources. The ion extraction efficiencies of the two instruments were examined by monitoring the transmission profiles of low- and high-mass ions as a function of lens potentials. Results indicated that the HP 5985B extraction lens significantly suppressed low-mass ions. Further, theoretical evaluation of ion trajectories using SIMION suggested that on the HP 5985B, low-mass ions entered the mass analyzer as a defocused beam, but high-mass ions entered the analyzer as a well-collimated beam. On the VG 30–250, low- and high-mass ions were transmitted to the analyzer with equal efficiency by the ion extraction system.     

The photochemical characteristics of aromatic enediyne compounds substituted with electron donating and electron withdrawing groups

trans- and cis-1-(4-Dimethylaminophenyl)-6-(4-nitrophenyl)hex-3-ene-1,5-diynes (trans- and cis-DANE) were synthesized and their photochemical properties were studied. The absorption spectra of trans-DANE red-shifted compared with the parent compound bisphenylethynylethene (BEE) due to intramolecular charge transfer. The fluorescence spectra, Stokes shift, fluorescence lifetime, fluorescence quantum yield, and quantum yield of trans-to-cis photoisomerization of trans-DANE showed strong dependence upon the solvent polarity in the less-polar region. No fluorescence emission from trans-DANE was observed in medium-polar and polar solvents. The quantum yield of cis-to-trans isomerization was almost solvent independent. The donor-acceptor substituents shifted the equilibrium between the trans perpendicular triplet state and the trans planar triplet state to the trans triplet state, and resulted in an increase in the triplet lifetime. Comparison of the photochemical properties of trans-DANE with trans-4-dimethylamino-4'-nitrostilbene (DANS) suggests that trans-DANE is a possible fluorescent probe in the non-polar region.     

Comparison of the positive and negative ion electrospray mass spectra of some small peptides containing pyroglutamate

The positive ion electrospray mass spectra of [M+H](+) and the negative ion electrospray mass spectra of [M-H](-) ions of selected pyroglutamate containing peptides both provide sequencing data. The negative ion spectra show the normal alpha and beta backbone cleavages in addition to delta and gamma backbone cleavages initiated by the side chains of Glu and Phe residues. For example, the [M-H](-) ion of pGlu Pro Gln Val Phe Val-NH(2) shows delta and gamma peaks at m/z 224 (delta, Gln3), 244 (gamma, Phe4), 451 (delta, Phe4), 471 (gamma, Gln3). Some of the negative ion spectra show unusual grandaughter peaks that originate by alpha and beta, or delta and gamma backbone cleavages of a beta(1) cleavage ion.     

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.     

Doubly charged ion mass spectra of organophosphorus compounds

Doubly charged ion mass spectra have been obtained for 11 organophosphorus compounds. Methane has been used as a target gas to increase the probability of single electron transfer collisions in the first field-free region of an Hitachi RMU-7L mass spectrometer. In general, the spectra of organophosphorus compounds do not exhibit molecular ions but are dominated by fragment ions, many of which must be formed by rearrangement processes. A geometry-optimized self-consistent field molecular orbital method has been employed to compute energies and structural parameters for prominent ions. In addition, a diabatic curve crossing model has been used to examine the single electron transfer reactions responsible for intense ions in the doubly charged ion mass spectra. Appearance energies measured for ions prominent in the 2E spectra of organophosphorus compounds have ranged from 23 to 38 eV.     

Negative ion mass spectra of highly fluorinated compounds

The negative ion mass spectra of highly fluorinated molecules with varying functionality are discussed. Attempts to obtain negative molecular ions where no positive molecular ion was observed met with little success. An interesting structural correlation for fluorinated ethers was developed based upon the observation of the abundant perfluoro alkoxide anions, CF₃O^? and CF₃(CF₂)₂O^?, in the negative ion mass spectrum.     

The gas enhanced negative ion mass spectra of polychloroanisoles

Methane or a methane–oxygen mixture was used as an enhancement gas to obtain negative ion mass spectra of polychloroanisoles. Dichloroanisoles did not react with oxygen but the more highly chlorinated anisoles did. Compounds with hydrogen ortho to the methoxy group had [M? 1]^? ions, while others gave . The fragment arose through loss of an ortho chlorine and amethyl hydrogen. The loss of HCl followed by oxygen displacement of a remaining ortho or para chlorine produced [M? 55]^? ions; the para position was the preferred site of displacement. Another ion-molecule reaction with oxygen leads to [M? CH₂Cl]^?. The fragmentations resemble those of chlorinated aromatics such as the polychlorodibenzodioxins.     

The negative ion mass spectra of methanol and deuterated methanol

The negative ion mass spectra of methanol, methanol-d and trideuteromethanol have been measured at 70 eV. Use of deuterated methanols has enabled the ion at m/e 31 to be identified as CH₃O^? and not CH₂OH^?. Isotope effects have been determined for H^? and D^? formation, and for OH^? and OD^? formation. It has been shown that OH^? formation occurs as a rearrangement reaction as well as by simple C? O bond fission.     

Negative ion mass spectra of various fluorine containing molecules

Negative ion mass spectra are reported for the compounds (CF₃CO)₂O, (C₂F₅CO)₂O, (C₃F₇CO)₂O, C₃F₇C(F)(CF₃)₂, n-C₆F₁₄, CF₃COCL, (CF₃)₂CO, BF₃, N₂F₄ and SiF₄.     

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.     

Common substructures in groups of compounds exhibiting similar mass spectra

Summary Principal component projections of sets of mass spectra show clusters that contain compounds with common structure properties. The similarity of structures is investigated by an automatic search for large common substructures within the compounds of a cluster. Resulting spectra-structure-relationships are helpful in interpretation of spectra.     

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]⁺.     

Negative ion field desorption mass spectra of some inorganic and organic compounds

Field desorption of negative ions can be achieved below the threshold of field electron emission. To this end a mixture of the sample with polyethylene oxide and water was applied to smooth wire cathodes. The mass spectra of some inorganic and organic compounds are reported. Anionization by [CI]^? ion attachment is demonstrated with the examples of 20-hydroxycholesterol and sucrose.     

Fragmentation in doubly charged ion mass spectra of anhydropisatin and related compounds

The doubly charged ion mass spectra of anhydropisatin, 4-methoxyanhydropisatin, 3, 8, 9-trimethoxypterocarpen and 3, 4, 8, 9-tetramethoxypterocarpen were determined, and the fragmentation was explained by assuming that the paired electrons were partially localized in the fragmentations and by comparing the spectra with that of 3-(CD₃)-anhydropisatin. Conventional mass spectra of these compounds were very simple, but the doubly charged ion spectra were sufficiently characteristic for the reliable identification.     

Positive and negative ion desorption from PVC as studied by electron stimulated desorption

With the aim of performing time-of-flight studies of electron stimulated ion desorption (ESID) from polymers bombarded with a variable energy electron beam source, an experimental set-up including a homemade time-of-flight mass spectrometer has been developed for positive and negative ion analysis. This system uses as a trigger for the experiments either the pulsed electron beam or the pulsed (positive/negative) extraction high voltage applied to the sample. Results for both positive and negative ion desorption from poly(vinyl chloride) (PVC) obtained in ESID measurements with these two different modes of operation will be presented and discussed.     

Rearrangement reactions in the positive and negative ion mass spectra of clausmarin A

The positive and negative ion mass spectra of clausmarin A, a naturally occurring tetracyclic coumarin, have been studied. The base peaks in the positive and negative ion spectra correspond to the complementary ions at m/z 143 and m/z 255, respectively, formed by a rearrangement reaction. The fragmentation pathways have been elucidated with the help of exact mass measurements and in situ deuterium labelling.     

Gas phase electron attachment reactions and negative ion mass spectra of tris- and bis(N,N-diethyldithiocarbamato) metal complexes

Results are presented for the gas phase thermalized electron attachment reactions of transition metal dithiocarbamates Metal·[S₂CN·Et₂]_n, with n = 3 for the trivalent metals Cr, Mn, Fe, Co and n = 2 for the divalent metals Fe, Co, Ni, Pd and Cu. Details of negative ion mass spectra are given, and these were obtained by use of the complementary methods of in-beam direct insertion and desorption chemical ionization of samples into temperature and pressure controlled ion source methane gas plasmas. For the tris-complexes, only small abundances of [M]^? were observed, being consistent with the electron entering a metal-based orbital to give a reduced [Metal^II·L₃]*^? species—the precursor of the observed [Metal^II·L₂]^? or [L]^? product ions. For the bis-complexes, a much higher proportion of the total ion current was carried by [M]^?, with [L]^? being the next most abundant species in all cases and the principal product in the reaction sequence \documentclass{article}\pagestyle{empty}\begin{document}${[{\rm Metal}^{{\rm II}} \cdot {\rm L}_{\rm 2}] + {\rm e} \to [{\rm Metal}^{\rm I} \cdot {\rm L}_{\rm 2}]^{*-} \mathop {\rightarrow}\limits^* [{\rm L]}^{\rm - } + [{\rm Metal}^{\rm I} \cdot {\rm L}].}$\end{document}. Rearrangement ions [L·Metal^I·X]^? with X=SH, SCN, NEt₂ were also identified for this series which can be accounted for by rearrangement reactions occurring in ligands not fulfilling a bidentate function towards the relevant metal.