Secondary Ion Mass Spectra of Neutral Sphingoglycolipids
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Abstract:
Secondary ion mass spectra of underivatized neutral sphingoglycolipids are presented. In the spectra of mono- and di-glycosylceramide, ions (M + H)+ and (M + H-H2O)+ were observed as relatively intense quasimolecular ions, whereas in the spectra of higher glycolipids, the quasimolecular ion species were predominantly (M + Na)+. Ions due to the ceramide moiety were observed as intense peaks comparable to quasimolecular ions. Ions derived from the fragments cleaved at the glycosidic linkages were hardly detected due to their low intensities. In general, secondary ion mass spectrometry provides good stable spectra for a long time during analysis.Keywords:
Glycosidic bond
Time-of-Flight
Reflectron
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Doubly charged ion mass spectra of alkylbenzenes were obtained by setting the electro static sectortwice as much voltage as a normal value. These spectra are different from those of singly charged ion spectra. Main fragment ions are shown Table1. Some structural limitations for the appearance of these ions are discussed .
Alkylbenzenes
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Abstract Positive and negative ion laser desorption (LD) mass spectra of organic acids are characterized by the emission of the quasimolecular ions (M+H)+ and (M-H)−. Generation of (M+H)+ ions is interpreted as evidence for the occurence of high pressure proton transfer reactions in LD. Fragment ions can be rationalized by loss of stable neutral molecules from quasimolecular ions although decomposition may also be occuring prior to ionization. Features unique to LD, including the detection of pyrolysis products along with ions characteristic of the sample, are discussed in terms of the internal energy distribution in the irradiated microvolume. Negative ion mass spectra of acids are dominated by (M-H)−, while positive ion spectra contain abundant fragment ions, underlining the utility of detecting negative ions for acidic compounds.
Proton affinity
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Polyatomic ion
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Abstract The mass spectra of isomeric 3‐ and 7‐cyclohexyl‐2‐norbornanols have been studied. The 3‐substituted norbornanols give very similar mass spectra, whereas large differences were observed between the mass spectra of 7‐ syn and 7‐ anti isomers. While the mass spectra of these bicyclic alcohols show extensive rearrangements to give rise to many odd‐electron ions, the Wagner‐Meerwein rearrangement of the molecular ion prior to fragmentation or during the process of dehydration is not important. The origins and the mechanisms for the formation of some major ions are discussed in terms of low voltage spectra, defocused metastables and their relative abundances, and ionization or appearance potentials of the ions of interest.
Fragmentation
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The oxidation catalysts (VO2)P207 and β-VOPO4 are studied by laser desorption time-of flight mass spectrometry. Mass spectra of both positive and negative ions are reported for these compounds. Individual scans are tested for ion association, that is, the consistent appearance of a given pair of ions in many scans. The structurally simpler [Symbol: see text]-VOPO4 shows fewer associations than either (VO)2P207 or an (18)O-substituted analogue of β-VOPO4 even though the overall spectra from the three compounds do not differ greatly. These results indicate that the associations observed between ions may be sensitive to small differences in structure in the original specimen. Thus, the identification of ion association may provide information to supplement the mass spectrum of the sample.
Time-of-Flight
Association (psychology)
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The ion kinetic energy (IKE) and mass spectra of 1- and 2-chloronapthalenes and seven dichoronaphthalenes have been recorded and the spectra of each set of isomers are strikingly similar; comparison of the daughter ion peaks obtained from the IKE spectra indicate chlorine randomisation over at least one of the rings in the monosubstituted compounds and over both rings in the isomeric dichloronaphthalenes.
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The relative abundances of M + 1 and M + 2 ions help to identify the elemental composition of the molecular ion (M). But scan speed, sensitivity, and resolution limitations of mass spectrometers have impeded determination of these abundances. Mass peak profiling from selected ion recording data (MPPSIRD) provided faster sampling and enhanced sensitivity, which permitted use of higher resolution. M + 2 profiles having only a few percent of the ion abundance of M were monitored at 20 000 resolution. The relative abundances, exact masses, and shapes of M, M + 1, and M + 2 mass peak profiles were determined. By applying five criteria based on these quantities, elemental compositions were determined even for ions too large (up to 766 Da) to be uniquely assigned from their exact mass and accuracy limits alone. A profile generation model (PGM) was written to predict these resolution-dependent quantities by considering all M + 1 and M + 2 ions for each candidate composition. The model also provided assurance that no other compositions were possible. Characterization of the M + 1 and M + 2 profiles by MPPSIRD and the PGM greatly expanded the practical ability of high-resolution mass spectrometry to determine elemental compositions.
Mass
Natural abundance
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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.
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The charge state of ions produced in electrospray ionization (ESI) was reduced in a controlled manner to yield predominantly singly charged species by exposure of the aerosol to a bipolar ionizing gas. Analysis of the resulting ions on an orthogonal time-of-flight mass spectrometer yielded mass spectra greatly simplified compared with conventional ESI spectra. The decreased spectral complexity afforded by the charge reduction facilitates the analysis of mixtures by ESI mass spectrometry.
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