Electron impact/desorption mass

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

Field desorption and electron impact ionization mass spectra were taken by using a Matsuda type double focussing spectrometer... 

Direct probe mass spectrometry. Glycosphingolipids (30-100 pg) were permethylated as described (12). The samples (less than 5 p g) were subjected to electron impact/desorption analysis with a Varian MAT CH-5 DF mass spectrometer under the following conditions emission current, 300pA electron energy, 70 eV acceleration voltage, 3KV ion source temperature, 160° C emitter wire current, programed from 0 to 35mA. 

This information was obtained from samples of less than 5 pg by electron impact/desorption direct probe mass spectrometry. On the basis of complete structural analyles, to be presented elsewhere, we have been able to determine that the four fractions isolated thus far actually contain six different glycosphingolipids with the following structures ... 

Soltmann B, Sweeley C C, Holland J F 1977 Electron impact ionization mass spectrometry using field desorption activated emitters as solid sample probes. Anal Chem 49 1164-1166... 

Mass spectroscopy is a useful technique for the characterization of dendrimers because it can be used to determine relative molar mass. Also, from the fragmentation pattern, the details of the monomer assembly in the branches can be confirmed. A variety of mass spectroscopic techniques have been used for this, including electron impact, fast atom bombardment and matrix-assisted laser desorption ionization (MALDI) mass spectroscopy. 

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that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data).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. 

Alternative approaches consist in heat extraction by means of thermal analysis, thermal volatilisation and (laser) desorption techniques, or pyrolysis. In most cases mass spectrometric detection modes are used. Early MS work has focused on thermal desorption of the additives from the bulk polymer, followed by electron impact ionisation (El) [98,100], Cl [100,107] and field ionisation (FI) [100]. These methods are limited in that the polymer additives must be both stable and volatile at the higher temperatures, which is not always the case since many additives are thermally labile. More recently, soft ionisation methods have been applied to the analysis of additives from bulk polymeric material. These ionisation methods include FAB [100] and LD [97,108], which may provide qualitative information with minimal sample pretreatment. A comparison with FAB [97] has shown that LD Fourier transform ion cyclotron resonance (LD-FTTCR) is superior for polymer additive identification by giving less molecular ion fragmentation. While PyGC-MS is a much-used tool for the analysis of rubber compounds (both for the characterisation of the polymer and additives), as shown in Section 2.2, its usefulness for the in situ in-polymer additive analysis is equally acknowledged. 

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... 

Mass spectrometry is used to identify unknown compounds by means of their fragmentation pattern after electron impact. This pattern provides structural information. Mixtures of compounds must be separated by chromatography beforehand, e.g. gas chromatography/mass spectrometry (GC-MS) because fragments of different compounds may be superposed, thus making spectral interpretation complicated or impossible. To obtain complementary information about complex mixtures as a whole, it may be advantageous to have only one peak for each compound that corresponds to its molecular mass ([M]+). Even for thermally labile, nonvolatile compounds, this can be achieved by so-called soft desorption/ionisation techniques that evaporate and ionise the analytes without fragmentation, e.g. matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). 

An electron impact mass spectrum of calcium leuco-vorin has not been obtained because the compound is not sufficiently volatile. It would be difficult to isolate the free acid without first dehydrating the compound. Due to its ionic nature, calcium leucovorin will not dissolve in common silylating reagents. Field desorption, another mass spectral technique, generally lends itself more to compounds like leucovorin. Indeed, this technique has been applied successfully to methotrexate and other folic acid analogs.1 ... 

The carbinolamine metabolite 64 was unstable in organic solvents and on routine chromatography. It was impossible to obtain molecular ions in the mass spectra by electron impact, chemical ionization, Held desorption, or fast-atom bombardment techniques. The apparent molecular ion occurred at m/e 456, consistent with a carbinolamine such as 64, which eliminates water to form an [M — 18] peak. The H-NMR spectrum of this metabolite was similar to that of... 

Hogg, A.M. Payzant, J.D. Design of a Field lonization/Field Desorption/Electron Impact Ion Source and its Performance on a Modified AEIMS9 Mass Spectrometer. Int. J. Mass Spectrom. Ion Phys. 1978, 27, 291-303. 

The purpose of the MS techniques is to detect charged molecular ions and fragments separated according to their molecular masses. Most flavonoid glycosides are polar, nonvolatile, and often thermally labile. Conventional MS ionization methods like electron impact (El) and chemical ionization (Cl) have not been suitable for MS analyses of these compounds because they require the flavonoid to be in the gas phase for ionization. To increase volatility, derivatization of the flavonoids may be performed. However, derivatization often leads to difficulties with respect to interpretation of the fragmentation patterns. Analysis of flavonoid glycosides without derivatization became possible with the introduction of desorption ionization techniques. Field desorption, which was the first technique employed for the direct analysis of polar flavonoid glycosides, has provided molecular mass data and little structural information. The technique has, however, been described as notorious for the transient... 

Figure 14.1 Schematic view of a mass spectrometer. Its basic parts are ion source, mass analyzer, and detector. Selected principles realized in modern mass spectrometers are assigned El—electron impact. Cl—chemical ionization, FAB—fast atom bombardment, ESI—electrospray ionization, MALDI—matrix-assisted laser desorption/ionization. Different combinations of ion formation with mass separation can be realized.

FIGURE 2.17. Mass spectra of leucine, (a) Electron impact (El). (b) Chemical ionization (Cl), (c) Field desorption (FD). 

A direct mass spectrometric method for simultaneous detection of five benzimidazoles including levamisole, thiabendazole, mebendazole, fenbendazole, and febantel in sheep milk was reported (377). The method, which involves injection of crude milk extracts and selection and collision of the most abundant ionic species obtained under electron impact ionization, was highly sensitive and rapid. Another direct mass spectrometric approach for rapid and quantitative determination of phenothiazine in milk was also described (323). This method involves an extraction step using a Cig microcolumn disc, followed by thermal desorption of the analyte from the disc directly into an ion trap mass spectrometer. 

TD-IT, Thermal desorption-ion trap CAD MIKE, collisionally activated decomposition mass-analyzed ion kinetic energy MID, multiple ion detection FID, flame ionization detector NPD, nitrogen/phosphorus detector SIM, selected ion monitoring El, electron impact TMS, trimethylsilane MTBSTFA, N-methyl-N-(tetr.-butyldimethylsilyl)trifluoroacetamide. 

Electron impact mass spectral data for conjugated, reduced and higher flavonoids have been summarized (B-75MI22201) and a review of electron impact, chemical ionization and field desorption mass spectra of flavones, flavonols, isoflavones and flavanones has been published (B-80MI22201). 

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