Desorption electron impact studies

Mass Spectrometry. Mass spectrometry holds great promise for low-level toxin detection. Previous studies employed electron impact (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), and cesium ion liquid secondary ion mass spectrometry (LSIMS) to generate positive or negative ion mass spectra (15-17, 21-23). Firm detection limits have yet to be reported for the brevetoxins. Preliminary results from our laboratory demonstrated that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data). We expect significant improvement by manipulation of the DCI conditions and selected monitoring of the molecular ion or the ammonia adduction. 

Due to the high mass, low volatility, and thermal instability of chlorophylls and derivatives, molecular weight determination by electron impact (El) MS is not recommended. Desorption-ionization MS techniques such as chemical ionization, secondary ion MS, fast-atom bombardment (FAB), field, plasma- and matrix-assisted laser desorption have been very effective for molecular ion detection in the characterization of tetrapyrroles. These techniques do not require sample vaporization prior to ionization and they are effective tools for allomerization studies. 

Mass spectrometry (MS) in its various forms, and with various procedures for vaporization and ionization, contributes to the identification and characterization of complex species by their isotopomer pattern of the intact ions (usually cation) and by their fragmentation pattern. Upon ionization by the rough electron impact (El) the molecular peak often does not appear, in contrast to the more gentle field desorption (FD) or fast-atom bombardment (FAB) techniques. An even more gentle way is provided by the electrospray (ES) method, which allows all ionic species (optionally cationic or anionic) present in solution to be detected. Descriptions of ESMS and its application to selected problems are published 45-47 also a representative application of this method in a study of phosphine-mercury complexes in solution is reported.48... 

Schulten, H.-R. Nibbeiing, N.M.M. An Emission-Controlled Field Desorption and Electron Impact Spectrometry Study of Some A-Substituted Propane and Butane... 

The production of stable anions by low-energy electron impact has been studied by ESD [22,41,43,44,122,147,154,156,157,166-219] and charge-trapping [154,156,217,220-236] experiments. Below 20 eV, the desorption of stable anions from condensed systems is attributed to DEA, which produces oscillatory structures in the anion yield functions, and... 

Structural Studies of Neutral Glycosphingolipids of Human Neutrophils by Electron Impact/Desorption Mass Spectrometry... 

Chemical ionization (CI)-MS can be used to study alkaloids that are not amenable to examination by electron impact (EI)-MS. For example, the quaternary alkaloid thalirabine (Section II,C, 123), undergoes fragmentation under the conditions of EI-MS and does not show a parent ion however, the CI-MS shows a double Hofmann elimination product which retains the skeletal atoms (32). Field desorption (FD)-MS has similar utility, as in the case of cycleanine IV-oxide (Section II,C,17) for which FD-MS shows the parent ion not detectable by EI-MS (65). Desorption/CIMS (D/CIMS) was used on dihydrosecocephar-anthine (Sec. H,C,30) and related bases (80,292a). 

The analytically important features of Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry (1) have recently been reviewed (2-9) ultrahigh mass resolution (>1,000,000 at m/z. < 200) with accurate mass measurement even 1n gas chromatography/mass spectrometry experiments sensitive detection of low-volatility samples due to 1,000-fold lower source pressure than in other mass spectrometers versatile Ion sources (electron impact (El), self-chemical ionization (self-Cl), laser desorption (LD), secondary ionization (e.g., Cs+-bombardment), fast atom bombardment (FAB), and plasma desorption (e.g., 252cf fission) trapped-ion capability for study of ion-molecule reaction connectivities, kinetics, equilibria, and energetics and mass spectrometry/mass spectrometry (MS/MS) with a single mass analyzer and dual collision chamber. 

Mass spectrometry has been an extremely useful tool for the characterization of neutral organometallic clusters, except for those few cases which have extremely high molecular weights or possess such ligands as PPh3 that reduce the volatility of the compound. Mass spectrometry has not been a useful characterization technique for ionic clusters because these compounds are insufficiently volatile to permit study by conventional electron-impact techniques. However, there is some hope that with the development of field-desorption techniques ionic clusters as well as neutrals will be capable of being analyzed by mass spectrometry (146). 

Regrettably, alkyne-substituted cluster complexes seem particularly prone to fragmentation and very few accurate mass spectroscopic studies have been reported. A recent exception has been the field-desorption and electron-impact mass spectral investigation of mono-and oligo-nuclear ferracyclic ring systems of the from Fei(CO)J,(C2R2)2 (x = 1, 2, 3 y = 6, 8) (384). These species show intense molecular ion peaks, which enable ready recognition of the molecular composition. 

At the outset of the structure investigation it was hoped that single-crystal X-ray methods might be successful, particularly with the large alkaloids however, as a number of trials were not promising, the approaches to structures have relied on spectroscopic and chemical methods. Spectroscopic examination involved electron impact, chemical ionization, and field desorption mass spectrometry, H- and C-nuclear magnetic resonance study (sometimes NOE and correlation spectroscopy), and this was followed by isolation of the polyhydroxylated core by alcoholysis or hydrolysis. Core 30 was novel and its structure established spectroscopically 48), whereas euonyminol was characterized as its octaacetate and compared with an authentic sample. 

Mass spectrometry (MS) has not been applied extensively to the study of naturally occurring xanthones, but the mass spectral data provide valuable information about the structure elucidation of xanthones. As well as electron impact MS, which is a routine technique for the structure elucidation of xanthones, recently developed soft ionization techniques, such as desorption-chemical ionization MS (D/CI-MS) and fast atom bombardment MS (FAB-MS), are of great interest for the analysis of glycosides. Molecular ion peaks can be observed without derivatization. Tandem MS/MS can be extensively employed in directly characterizing constituents of complex mixtures. Recently, xanthone profiles of H. perforatum cell cultures were identified by HPLC-MS/MS analysis [106]. 

Breath samples have been analyzed for tetrachloroethylene in several studies. Preconcentration on a solid sorbent followed by thermal desorption onto a cryogenic trap connected to the gas chromatograph was used to analyze exhaled air in several TEAM studies (Wallace 1986 Wallace et al. 1986a, 1986b, 1986c, 1986d). Vapors were thermally released directly onto the chromatographic column for separation and detection by electron impact MS (EIMS). 

As a result of numerous studies a basic understanding of the mechanism of electron impact induced desorption was achieved in the past decade In contrast, few papers have been published on the electron impact induced chemical evaporation. Besides the above-mentioned etching of Si and some other materials by XeFj only the C/H2-, TiC/Hz-, TiB2/H2- and the Si/Hj-systems have been investigated. In the case of silicon etching by hydrogen the effect of the electron impact cannot be considered as a purely catalytic one since the reaction product, silane, is thermodynamically unstable with respect to solid silicon and Hj. Nevertheless, it provides some insight into the mechanism of such processes and, therefore, we shall discuss it briefly. 

Electron impact (El), the method outlined in Section 9.6.1, is the conventional method of ionizing samples for MS. However, El is not free from disadvantages, such as inability to provide molecular weight information from thermally labile samples, difficulty for study of low-volatility materials, complex spectra arising from interference between molecular and fragment ions, etc. In order to overcome these problems, a large number of alternative ionization methods, collectively known as soft ionization methods have been developed. Reviews of the available techniques have been compiled by Milne and Lacey (1974) and Games (1978). Chemical ionization (Cl), field ionization (FI) and field desorption (FD)... 

Puzo G, Tissie G, Lacave C, Aurelle H, Prome JC. Structural determination of cord factor from a Corynebacterium diphtheriae strain by a combination of mass spectral ionization methods field desorption cesium cationization and electron impact mass spectrometry studies. Biomed Mass Spectrom. 1978 5 699-703. 

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