Abstract The chemical ionization mass spectra of a group of commonly occuring steroids are discussed. Intense signals (base peak) are observed in the molecular ion region for hydrocarbons and ketones. Functional group elimination is observed with sterols and especially their methyl and TMSi ethers, with the result that base peaks for these compounds are found at m/e values well below the expected molecular ion region. These observations are compared with those from the corresponding electron impact spectra. 相似文献
Desorption/chemical ionization mass spectrometry (D/CI.-MS.) is a recently developed technique especially indicated for highly polar and non volatile compounds. Various naturally occurring glycosides such as saponins, iridoid and secoiridoid glycosides, cardenolides and flavone-O-glycosides have been investigated by this method. All the measurements were carried out on underivatized compounds. In addition to the structural informations generally furnished by field-desorption mass spectrometry (molecular ion and sugar sequence), the molecular ion and pertinent fragments of the aglycone could also be obtained. 相似文献
As a reagent gas for positive- and negative-mode chemical ionization mass spectrometry (CI-MS), isobutane (i-C4H10) produces superior analyte signal abundance to methane. Isobutane has never been widely adopted for CI-MS because it fouls the ion source more rapidly and produces positive CI spectra that are more strongly dependent on reagent gas pressure compared with methane. Isobutane was diluted to various concentrations in argon for use as a reagent gas with an unmodified commercial gas chromatograph-mass spectrometer. Analyte spectra were directly compared using methane, isobutane, and isobutane/argon mixtures. A mixture of 10% i-C4H10 in argon produced twice the positive-mode analyte signal of methane, equal to pure isobutane, and reduced spectral dependence on reagent gas pressure. Electron capture negative chemical ionization using 1% i-C4H10 in argon tripled analyte signal compared with methane and was reproducible, unlike pure isobutane. The operative lifetime of the ion source using isobutane/argon mixtures was extended exponentially compared with pure isobutane, producing stable and reproducible CI signal throughout. By diluting the reagent gas in an inert buffer gas, isobutane CI-MS experiments were made as practical to use as methane CI-MS experiments but with superior analytical performance.
Laser-induced acoustic desorption (LIAD) was successfully coupled to a conventional atmospheric pressure chemical ionization
(APCI) source in a commercial linear quadrupole ion trap mass spectrometer (LQIT). Model compounds representing a wide variety
of different types, including basic nitrogen and oxygen compounds, aromatic and aliphatic compounds, as well as unsaturated
and saturated hydrocarbons, were tested separately and as a mixture. These model compounds were successfully evaporated into
the gas phase by using LIAD and then ionized by using APCI with different reagents. From the four APCI reagent systems tested,
neat carbon disulfide provided the best results. The mixture of methanol and water produced primarily protonated molecules,
as expected. However, only the most basic compounds yielded ions under these conditions. In sharp contrast, using APCI with
either neat benzene or neat carbon disulfide as the reagent resulted in the ionization of all the analytes studied to predominantly
yield stable molecular ions. Benzene yielded a larger fraction of protonated molecules than carbon disulfide, which is a disadvantage.
A similar but minor amount of fragmentation was observed for these two reagents. When the experiment was performed without
a liquid reagent (nitrogen gas was the reagent), more fragmentation was observed. Analysis of a known mixture as well as a
petroleum cut was also carried out. In summary, the new experiment presented here allows the evaporation of thermally labile
compounds, both polar and nonpolar, without dissociation or aggregation, and their ionization to predominantly form stable
molecular ions. 相似文献
The characterization of polymers by pyrolysis directly in the ion source of a double focusing magnetic sector mass spectrometer, operating in the chemical ionization mode, is described. Pyrolysis is achieved by two different probe techniques. A low temperature, slow heating rate direct insertion probe (DIP) is used at 400°C, and a specifically constructed high temperature, fast heating rate, high temperature pyrolysis (HTP) probe is used at 1000°C. This probe is capable of achieving pyrolysis temperatures of 1200°C at controlled heating rates up to 20,000°C/s. The mass spectrometric analysis of the pyrolysis products was achieved under chemical ionization (CI) conditions utilizing methane, isobutane, and ammonia as reagent gases. Under CI conditions the molecular ions formed in the mass spectrometer show little tendency to fragment. The CI mass pyrograms are very simple, with each peak in the spectra ascribable to a particular component in the pyrolysis product mixture. The results of the two probe pyrolysis techniques are compared and the utility of each technique for the characterization of polymers is demonstrated using the vinyl polymers polymethyl methacrylate, polyvinyl chloride, and polystyrene. 相似文献
Mass spectrometry both complements other analytical techniques and allows for types of analyses and experiments not possible with common analytical methods, such as NMR, IR, and UV/Vis spectroscopy. Electrospray constitutes one of the mildest forms of ionization, making it the preferred method for the analysis of large fragile or reactive ions. There is particular promise for mass spectrometry in aiding the characterization of polyoxometalates and their solutions, but caution must be taken in designing the experiments in order to yield reliable data and to avoid the temptation of over‐interpreting the relevance of gas‐phase data to solution chemistry. 相似文献
Abstract A quantitative analysis of fatty acids in micro-samples of dried blood spots by chemical ionization mass spectrometry has been developed. Isotope determination was used as the quantitating technique using the corresponding deuterium labeled internal standards. The procedure yields excellent precision and accuracy as demonstrated by the analysis of known fatty acid mixtures and of both C12:0 to C18:0 acids and phytanic acid in the blood from patients. 相似文献
Abstract A new synthetic route for the photocleavable molecular tag for laser desorption ionization mass spectrometry (LDI-MS) was achieved using the Fries reaction of 2,6-dimethylphenyl ester as its key reaction. Zirconium chloride was used as uniquely efficient adjuvant to promote the reaction. The molecular tag was obtained in five steps without chromatographic purification. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file. 相似文献
Limonene and its ozone-initiated reaction products were investigated in situ by low temperature plasma (LTP) ionization quadrupole time-of-flight (QTOF) mass spectrometry. Helium was used as discharge gas and the protruding plasma generated ~850 ppb ozone in front of the glass tube by reaction with the ambient oxygen. Limonene applied to filter paper was placed in front of the LTP afterglow and the MS inlet. Instantly, a wide range of reaction products appeared, ranging from m/z 139 to ca. 1000 in the positive mode and m/z 115 to ca. 600 in the negative mode. Key monomeric oxidation products including levulinic acid, 4-acetyl-1-methylcyclohexene, limonene oxide, 3-isopropenyl-6-oxo-heptanal, and the secondary ozonide of limonene could be identified by collision-induced dissociation. Oligomeric products ranged from the nonoxidized dimer of limonene (C20H30) and up to the hexamer with 10 oxygen atoms (C60H90O10). The use of LTP for in situ ozonolysis and ionization represents a new and versatile approach for the assessment of ozone-initiated terpene chemistry.
