Mass spectral techniques electron impact

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 ... 

Various mass spectral techniques have been used in characterizing various metal-containing macromolecules. The focus will be on two techniques. The first one is the most commonly employed mass spectrophotometer setup, called high-resolution electron impact mass spectrometry (HREl-MS). The second system is the matrix-assisted laser desorption ionization (MALDl) system. Illustrative examples of these two procedures are given for the products of Group IVB metallocene dichlorides and the antibacterial drug norfloxacin, scheme 8. 

A comparison of the electron impact (El) and chemical ionization (Cl-methane) mass spectra of 1//-azepine-1-carboxylates and l-(arylsulfonyl)-l//-azepines reveals that in the El spectra at low temperature the azepines retain their 8 -electron ring structure prior to fragmentation, whereas the Cl spectra are complicated by high temperature thermal decompositions.90 It has been concluded that Cl mass spectrometry is not an efficient technique for studying azepines, and that there is no apparent correlation between the thermal and photo-induced rearrangements of 1//-azepines and their mass spectral behavior. 

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). 

Electron impact mass spectral analysis of sulfuric acid and sulfate salts (most notably ammonium sulfate) from filter samples has not been totally successful. Although the sensitivity of the technique was quite high (10 9 gms), both sulfuric acid and ammonium sulfate gave only SO3+. S02. SO+ fragments when analyzed by electron impact mass spectrometry. Since their spectra were very similar, accurate quantitation would require that only one species be introduced into the source at a... 

A number of systematic structural analyses have been described for families of saturated oxazolones. First, as mentioned previously, detailed smdies of NMR long-range coupling in 2,4-disubstimted-5(47/)-oxazolones and in 5(27/)-oxazo-lones have been reported." Similarly, detailed NMR studies of the kinetics of racemization of 2,4-disubstimted-5(47/)-oxazolones have been performed. A theoretical study of the spectral-luminescence properties of some 4-alkyl-2-phenyl-5(47/)-oxazolones has been reported and an investigation of the infrared (IR) and Raman spectra of 5(4//)-oxazolones, particularly of the carbonyl group vibration, has been reported. Electron impact mass spectra of saturated 5(47/)-oxazolones have been published. More recently this technique has been used to distinguish between the stereoisomers of some spirocyclopropane oxazolones 352 (Fig. 7.36). Finally, several studies of the HPLC behavior of 5(47/)-oxazolones complete a general view for this family of compounds. " " ... 

Various ionization techniques applied in association with Py-GC/MS are reported in literature (see e.g. [12]). However, the most common ionization method is electron impact with the detection of positive ions (EI+). The chemical ionization (Cl) is sometimes used, but Cl spectra interpretation is difficult because of the lack of fragmentation and because the reproducibility in Cl is affected by the experimental conditions in which the spectra are generated. However, Cl spectra provide valuable information regarding the molecular mass of the analyte, and this can be very useful in combination with EI+ spectral information. 

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]. 

A target may resist hydrolysis or chemical degradation, or the degradation products may not yield useful information. It is also common that insufficient material exists for proper analysis. In these cases, an alternative degradation technique is available that uses the ionizing electron beam of a mass spectrometer. The ionization pathways available from electron impact in the mass spectmm are bond fission processes that occur by known and predictable pathways. Indeed, each pathway usually follows analogous chemical reaction pathways in a retro-synthetic manner. It therefore follows that an examination of mass spectral ionization patterns can give clues for suitable disconnections and a synthetic tree. 

Historically, ions for mass analysis were produced by electron impact, in Ihis process, the sample is brought to a temperature high enough to produce a molecular vapor, which is then ionized by bombarding the resulting molecules with a beam of energetic electrons. Despite certain disadvantages, this technique is still of major importance and is the one on which many libraries of mass spectral data are ba,scd. 

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