摘要:

AbstractA series of small peptides has been studied by negative‐ion fast‐atom bombardment mass spectrometry with collision‐induced dissociation. It has been found that by derivatizing peptides with 4‐aminonaphthalenesulphonic acid in a peptide linkage at the C‐terminus, negative‐ion formation can be enhanced and fragmentation in collision‐induced dissociation reactions controlled. The peptide‐naphthalenesulphonates show charge‐remote fragmentations and the resultant spectra give seq

ISSN:0951-4198    

DOI:10.1002/rcm.1290081002

出版商:Heyden&Son Limited    

年代:1994

数据来源: WILEY

摘要:

AbstractDihydroxycarbene is produced in the gas phase by neutralization of the HOCOH+·radical cation which is formed by dissociative electron ionization of oxalic acid. Reionization ∼0.3 μs later shows that HOCOH can survive intact and, thus, exists as a stable species with appreciable barriers for dissociation or rearrangement to formic acid (HCOOH). Within the time scale of the experiment, a small fraction of the carbene decomposes to H2O + CO. comprison of the experimental results withab initiotheory shows that the dissociating HOCOH molecules are generated in the electronically excited triplet state, while the klarge amount of surviving carbene molecules is formed in the singl

ISSN:0951-4198    

DOI:10.1002/rcm.1290081003

出版商:Heyden&Son Limited    

年代:1994

数据来源: WILEY

摘要:

AbstractThe Argonne set of coals cover the rank range from lignite to semi‐anthracite; these samples have been studied by matrix‐assisted laser desorption mass spectrometry (MALDI‐MS) in a time‐of‐flight mass spectrometer equipped with a nitrogen laser at 337 nm, using sinapinic acid as matrix. The coal particle size was less than 5 microns. The characteristics of the MALDI‐MS spectra of the set of coals were found to be rank‐related; desorption from high‐rank coals was found to take place with greater relative ease than from low‐rank coals. Two major features were found in all spectra: a homologous series of peaks in the 200–500 u mass range and an intense peak between 1000 and 5000 u, the particular shape of the peak depending on coal rank. A continuum of lower intensity peaks extending to very large molecular masses was found in all spectra, the upper limit of molecular masses increasing with coal rank at the same laser fluence. The effect of changes in laser power on spectra was investigated: upper mass limits were found to increase with power up to the detection limit of the instrument but low‐mass parts of spectra were found to distort, possibly due to detector overloading. A maximum laser fluence value acceptable over the coal‐rank range represented by these samples could therefore not be easily defined. None of the mass spectra showed evidence of the presence of either carbon clusters or fullerene formation, indicating that laser fluences did not reach intensities high enough to induce substantial secondary reactions. Comparing molecular mass distributions detected by MALDI of coal pyrolysis tars and directly from coals suggests the MALDI and pyrolytic mechanisms of volatile release to be structurally different; in particular, the preferential evaporation of lighter species which occurs during pyrolytic tar evolution (and during field‐ionization mass spectroscopy) appears to evole material with a more restricted range of molecular masses compared to la

ISSN:0951-4198    

DOI:10.1002/rcm.1290081004

出版商:Heyden&Son Limited    

年代:1994

数据来源: WILEY

摘要:

AbstractCoal pyrolysis tars and liquefaction extracts prepared from the set of eight Argonne coal samples have been characterized by MALDI‐MS. A Kratos Kompact MALDI III linear time‐of‐flight mass spectrometer was used, with a nitrogen laser operating at 337 nm and sinapinic acid as matrix. Spectra were collected by summing 50 laser pulses at low laser fluence to avoid fragmentation of desorbed ions. At low mass, 200–500 u, spectra from the pyrolysis tars and liquefaction extracts showed common features: intense peaks originating from the sinapinic acid matrix,m/z205, 224 and 246, and, a range of peaks in the mass range from about 250 u to 400 u, which probably corresponds to overlapping homologous series of apparently polar material. A peak of ion intensity between 1000 and 5000 u was systematically observed, which was sample dependent and not always similar for the tar and extract prepared from the same coal. At high masses, separate trends were observed for coal pyrolysis tars and liquefaction extracts: molar mass ranges of pyrolysis tars were smaller, and showed no particular trend with carbon content of the original coal; these findings are consistent with size‐exclusion chromatography derived findings. Spectra of liquefaction extracts extended over wider ranges of molar masses and increased with increasing coal rank; the highest masses extended to 50 000u. Overall, pyrolysis tars appeared structurally different from liquefaction extracts and more difficult to desorb. The effect of changes in laser fluence and ion extraction voltage on mass spectra have been investigated: high‐mass regions of the spectra were found to be very sensitive to the magnitude of the ion extraction voltage. No carbon clusters or fullerene structures we

ISSN:0951-4198    

DOI:10.1002/rcm.1290081005

出版商:Heyden&Son Limited    

年代:1994

数据来源: WILEY

摘要:

AbstractTwo kerogen samples from shales of different geological age, selected from each of Types I, II and III, have been characterized by matrix‐assisted laser desorption ionization mass spectroscopy (MALDI‐MS). A Kratos Kompact MALDI III time‐of‐flight mass spectrometer equipped with a nitrogen laser operated at 337 nm was used on samples mounted in a matrix of sinapinic acid. The spectrometer was used in linear mode with a mass range up to 270 000 u and an ion extraction voltage of 20 kV. Individual spectra from 50 laser shots at low laser fluence were summed for each spectrum. The present study is a first attempt at characterizing kerogens by MALDI‐MS. At low mass (200–500 u) families of peaks consisting of overlapping homologous series of apparently polar compounds have been observed: these peaks appear to be distinct from those due to the matrix material. In higher mass regions of the spectra, continua of masses extending from 1000 u up to around 10 000 u were observed; ions at higher masses were separated to baseline up to masses of 50 000 u. Evidence was found for a relationship between the minimum laser power sufficient for activating the MALDI‐ionization process and the geological age of the kerogens; the laser power decreased as the geological age increased. In the case of one kerogen, a high laser fluence was used to generate molecular ions up to the detection limit of the mass spectrometer, in excess of 260 000 u. This paper presents results from a preliminary study of kerogens which requires an in‐depth examination of the desorption process in c

ISSN:0951-4198    

DOI:10.1002/rcm.1290081006

出版商:Heyden&Son Limited    

年代:1994

数据来源: WILEY

摘要:

AbstractDespite the widespread use of matrix‐assisted laser desorption ionization, the processes involved in ion formation have yet to be satisfactorily explained. Several protonation processes have been suggested as possible methods for the creation of the analyte ions. In this work the existence of neutral hydrogen atoms in the ablation plume is reported. These are selectively ionized from the ground state through a (2+1) multiphoton resonant excitation scheme. The mass spectra at different delay times between the ablation and the post‐ablation‐ionization lasers have demonstrated the existence of hydrogen atoms with high velocities in the ablated

ISSN:0951-4198    

DOI:10.1002/rcm.1290081007

出版商:Heyden&Son Limited    

年代:1994

数据来源: WILEY

摘要:

AbstractProteinase K has been employed to proteolyze peptides before their mass spectrometric analysis. This combination of the two methods yields abundant sequence ions in the mass spectrum, thus leading to an easy prediction of the primary structure of the peptide. It was found that both the variety of available sequence ions and also their abundances are increased strongly after proteinase K digestion, when compared with the normal mass spectrum. The shortcoming of little fragmentation encountered using conventional mass spectrometric ionization methods can mostly be overcome. The H′nand Y"mions produced from proteolysis provided a double check of the peptide sequenc

ISSN:0951-4198    

DOI:10.1002/rcm.1290081008

出版商:Heyden&Son Limited    

年代:1994

数据来源: WILEY

摘要:

AbstractWe have studied isotopic fractionation effects which occur during the analysis of oxygen isotope ratios by secondary ionization mass spectrometry. Variable instrumental isotopic fractionation has been well documented in the past as making reliable ion microprobe analyses of oxygen isotopes elusive. We report here techniques for minimizing these effects by careful optimization of an Isolab 54 ion microprobe and a method of integrating the secondary‐ion beam to eliminate a major source of fractionation caused by unequal focusing of isotopes on the source slit of the mass spectrometer. A further improvement has been the installation of new ion detectors for measuring18O−and17O−secondary ion beams in which the ions strike a conversion dynode (Conversion dynode system—CDS) to release electrons into a channeltron. The CDS detectors have a uniform, high and stable gain which, coupled with the beam integration techniques referred to above, yield reproducible oxygen isotope ratios on both conducting and insulating mineral standards (⩽1.2‰ on18O/16O during a day and<2‰ over periods of da

ISSN:0951-4198    

DOI:10.1002/rcm.1290081009

出版商:Heyden&Son Limited    

年代:1994

数据来源: WILEY

摘要:

AbstractThe conjugation products of E‐destruxin (cyclodepsipeptide) with glutathione have been identified by fast‐atom bombardment mass spectrometry. Appropriate conditions were developed for cyclodepsipeptide substrate, which enabled the direct, dynamic mass spectra analysis of spontaneous as well as glutathione‐s‐transferase catalyzed conjugation reactions. Application to the most active cyclopeptide in the series of destruxins yielded glutathionyl

ISSN:0951-4198    

DOI:10.1002/rcm.1290081010

出版商:Heyden&Son Limited    

年代:1994

数据来源: WILEY

摘要:

AbstractOn analyzing 11 pure individual toxaphene congeners, it was noted that a nonachloro congener (2,2,5,5,8,8,9,10,10‐nonachlorobornane, Parlar's No. 62) failed to produce a reasonable [M–Cl]−response under our electron‐capture negative‐ion (ECNI) chemical ionization conditions. On the other hand, this congener gave a stronger response than other nonachlorobornane congeners in electron impact (EI) mode. Based on EI mass spectra data as well as electron capture gas chromatographic results, this congener is one of the major components in commercial toxaphene and is also present in fish and cod liver oil samples. This congener‐specific discriminatory effect raises concern about possible false negative results from the ECNI/selective‐ion monito

ISSN:0951-4198    

DOI:10.1002/rcm.1290081011

出版商:Heyden&Son Limited    

年代:1994

数据来源: WILEY