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  • 1
    ISSN: 1040-7685
    Keywords: microcolumn liquid chromatography ; liquid chromatography/mass spectrometry ; temperature programming ; optimization methods ; fatty acids ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A temperature-programmed microcolumn liquid chromatography/mass spectrometry (LC/MS) system has been developed as a practical alternative to conventional solvent-programmed LC/MS. Good reproducibility of solute retention was achieved in a specially modified oven that permitted linear temperature programming between 40 and 100°C at rates of 0.1 to 0.5°C min-1. Through theoretical modelling studies performed under isocratic and isothermal conditions, the optimum separation under temperature-programmed conditions was rapidly and accurately predicted. This method enabled the complete resolution of saturated and unsaturated fatty acids in a fish oil dietary supplement with identification from their characteristic fragmentation patterns under electron-impact MS conditions. The mass spectra from the LC/MS system correlated very well with those from a standard reference library to facilitate spectral searching methods.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1052-9306
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A quantitative assay for dexamethasone in human plasma is described as based on methodology employing electron capture negative ionization mass spectrometry (ECNI/MS). The unique feature of this assay is the sample preparation involving chemical oxidation which transforms dexamethasone to a highly electrophilic species while not significantly affecting the electrophilic character of the biochemical matrix, thereby permitting selective and sensitive detection of the analyte. Optimized chemical procedures and instrumental parameters are described for the detection of oxidized dexamethasone. Finally, the newly developed methodology, based on gas chromatography/mass spectrometry (GC/MS) with ECNI, is evaluated by comparison with a conventional GC/MS assay using electron impact of an 11,17,21-tris-trimethylsilyl ether 20-enol-trimethylsilyl ether derivative of dexamethasone and with a radioimmunoassay in the analysis of pooled samples of human plasma containing widely varying concentrations of the drug as obtained from patients at different stages of the dexamethasone suppression test.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1520-6882
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1052-9306
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Fast atom bombardment collisionally activated dissociation tandem mass spectrometry (FAB CAD MS/MS) of a disulfide-containing peptide, [2-D-penicillamine, 5-D-penicillamine]-enkephalin ([Pen]-enkephalin), is described. Unlike those of most other disulfide-containing peptides investigated, CAD of the native, unreduced protonated molecule of [Pen]-enkephalin yields a relatively large number of fragment ions. Most of the peaks in the CAD spectrum represent fragmentations of the peptide backbone with ‘unsymmetric’ cleavage of the disulfide bond with charge retention on the N-terminus; the fragment ions generally do not contain a sulfur atom or a disulfide group. The CAD spectrum of the N-terminal ethyl-triphenylphosphonium derivative, on the other hand, shows few fragment ions and is dominated by the single peak at m/z 523. This dominant ion also results from ‘unsymmetric’ cleavage and corresponds to the analogous protonated species (m/z 235) in the spectrum of the underivatized peptide. The chemical method of charge localization by triphenylphosphonium derivative formation at one end of the peptide is shown to be useful for investigating fragmentation mechanisms in FAB CAD MS/MS. Comparison of the CAD spectra of derivatized and underivatized [Pen]-enkephalin suggests that charge-remote fragmentation plays a significant role in the high-energy dissociation of this disulfide-bonded peptide.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ion-molecule reactions of selected hydrocarbon cations with methanol which lead to the production of hydroxylated odd-electron molecular ions in the high-pressure ion source of a mass spectrometer or in the central quadrupole of a tandem quadrupole mass spectrometer are described. A wide variety of hydrocarbon cations were investigated, including aliphatic and aromatic cations; of these, only those having a vacant site on an aromatic system appear to undergo the hydroxylation reaction in high yield. A mechanism is proposed for the formation of the molecular ion of phenol from the ion-molecule reaction involving the phenyl cation with methanol. In addition, thermochemical data are provided which support the formation of the postulated species.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ion-molecule reactions with oxygen promote fragmentation in the electron capture negative ionization mass Spectrometry of methylated abscisic acid. It is shown that low-energy collisions of the molecular anion (m/z 278) with O2 in the center quadrupole of a triple quadrupole mass spectrometer result in the formation of an adduct ion which subsequently fragments to yield two prominent ions, one at m/z 141 which contains the side-chain and a second at m/z 152 which includes the ring. Two minor products are represented by peaks at m/z 179 and 136. These ion-molecule reactions can also occur in the ion source when oxygen is present, which may cause fluctuations in relative intensities of the fragment ion peaks. The principal fragments of mass 141 and 152, and also those of mass 179 and 136, do not form in the absence of O2. Product ion mass spectra following collisions of M-· with 18O2 in the center quadrupole show that the side-chain fragment ion shifts from m/z 141 to m/z 143, indicating retention of an oxygen atom from the gas-phase reaction with molecular oxygen. It is proposed that on reaction with oxygen, a peroxy radical anion intermediate at m/z 310 is formed which then fragments to yield an enolate anion (m/z 141) or a quinone-like radical anion (m/z152). The induced fragmentations of the 2-cis and 2-trans isomers of methylated abscisic acid appear to be identical. Analysis of isotopically labeled compounds of both abscisic acid and structurally similar metabolites reveal that oxygen-activated fragmentation in these selected cases provides diagnostic ions which allow one to determine the position(s) of isotopic enrichment in the molecule.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The key to enhanced electron-capture negative ion mass spectrometry (ECNI-MS) response of ketosteroids is extensive α,β-unsaturation. Combinations of double bonds, carbonyl groups, epoxides and halogens within a steroid nucleus were correlated with their ECNI responses. The greatest ECNI responses result from extensive conjugation, such as the linearly conjugated 4-ene-3,6-dione system and the cross-conjugated 1,4-diene-3,11-dione system. In general, an epoxy ketosteroid has a relative response 40 times greater than that of its corresponding enone. In an α-halo ketone, an axial halogen substitution will increase the relative response by an order of magnitude, but an equatorial halogen substituent will not affect the response. The fragmentation of ketosteroids as related to ECNI response is discussed.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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