Fragmentation pattern, electron impact

There was one further problem, namely the 1-0-ethyl-ll-hydroxy-A9-THC was susceptible to pyrolysis at the elevated temperatures of the ion source. This resulted in irreproducible mass spectra. Silylation of the allylic alcohol functionality overcame this difficulty and the resulting electron impact fragmentation pattern was quite simple showing only one major peak, base peak at m/e = 327. The trimethylsilyl ion appeared at m/e =73 (7). 

Confirmation by Mass Spectrometry. Extensive work was undertaken to confirm the Identity of all fluoroderlvatlves using mass spectrometry. Electron Impact fragmentation patterns confirmed the Identity of the mono-, dl- and trlfluoro derivatives of the octyl and octadecyl ligands and all of the species labeled In Figure 5. 

General.—Since the electron-impact fragmentation patterns of many classes of natural products are now known, mass spectrometry can be used in conjunction with heavy isotopes in biosynthetic studies. A study of the fragmentation pattern of a metabolite with and without isotopic labelling yields direct information on the position of the labelled atom in the molecule without recourse to wet chemical degradation. Essential requirements for the biosynthetic application of mass spectrometry are (0 thermal stability of the metabolite at the probe inlet temperatures and (ii) a reasonably simple fragmentation pattern. 

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). 

The molecular ions of some of the alkaloids isolated more recently have been obtained and have been useful indicators of the substituents present on a common nucleus, particularly for the Buchenavia alkaloids (6). All spectra so far reported have been obtained using electron impact fragmentation. Table III lists the fragmentation patterns reported. Thus far no studies have been made on the breakdown mechanisms, but the cleavage of the flavonoid and nitrogenous rings is obviously an important process. 

Figure 3. Top. Ion/molecule reaction product formed by reaction of cis-dichloro-trans-dihydroxo-bis-2-propanamine platinum (IV) with its 50 eV electron impact fragments. Bottom. Dissociation pattern produced upon irradiation with cw CO2 laser, same experimental conditions and delay times as in the top spectrum. (Reproduced from ref. 18. Copyright 1987 American Chemical Society.)...

It is well known that the electron-impact ionization mass spectrum contains both the parent and fragment ions. The observed fragmentation pattern can be usefiil in identifying the parent molecule. This ion fragmentation also occurs with mass spectrometric detection of reaction products and can cause problems with identification of the products. This problem can be exacerbated in the mass spectrometric detection of reaction products because diese internally excited molecules can have very different fragmentation patterns than themial molecules. The parent molecules associated with the various fragment ions can usually be sorted out by comparison of the angular distributions of the detected ions [8]. 

The base peak in the mass spectrum of the LM free metal-ligand ion and the fragmentation patterns of this parent ion are of particuliar significance since they illustrate the effect of coordination upon the properties of the thiazole ligand. The free thiazole fragments upon electron impact by two major routes (Scheme 86 also cf. Section II. 6). 

Understanding how molecules fragment upon electron impact permits a mass spec trum to be analyzed m sufficient detail to deduce the structure of an unknown compound Thousands of compounds of known structure have been examined by mass spectrome try and the fragmentation patterns that characterize different classes are well docu mented As various groups are covered m subsequent chapters aspects of their fragmentation behavior under conditions of electron impact will be descnbed... 

Electron impact mass spectrometry has been employed to study the fragmentation patterns of isoxazolylmethyl- and bis(isoxazolylmethyl)-isoxazoles and the results are in agreement with proposed pathways (79AC(R)8l). Electron impact studies of nitrostyryl isoxazole (6) show fragmentation in a variety of ways. The standard loss of NO2 from the molecular ion... 

The fragmentation pattern of isoxazoles on electron impact has been well studied. It has been used as an important tool for the structural assignment of isoxazoles obtained from the reaction of chromones with hydroxylamine 79MI41600, 77JOC1356). For example, the structures of the isoxazoles (387) and (388) were assigned on the basis of their fragmentation patterns. Ions at mje 121 (100%) and mje 93 (19.8%) were expected, and indeed observed, for the isoxazole (388), and an ion at mje 132 (39.5%>) was similarly predicted and observed for the isoxazole (387). 

The fragmentation patterns of relatively volatile derivatives of penicillins (e.g. benzyl-penicillin methyl ester) under electron impact (B-72MI51101) and chemical ionization (75MI51100) conditions have been described. For both techniques the primary fragmentation is that shown in Scheme 1. 

The mass spectra of l-acyl-l//-l-benzazepines have been recorded.23 The mass spectrum of 3-mesyl-3/7-3-benzazepine shows an intense base peak at m/e = 142duetothebcnzazepinylium ion and a peak (51 %) at m/e — 115 (-HCN) which is attributed to the indenium cation.26 Fragmentation patterns for 1H- and 5/7-2-benzazepines40 and for 5//-dibenz[c,e]azepine5 are available. The electron-impact induced fragmentation pattern of 5//-dibenz[6,/]azepine displays an intense molecular ion as the base peak, and a moderately intense (M + 1) peak.5 ... 

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 was applied in conjunction with thermolysis studies leading mainly to sulfines and rearranged products with four-membered sulfoxides and to a loss of sulfur dioxide with sulfones The fragmentation pattern of thietes under electron impact can be explained by the sequential loss of the elements of sulfur monoxide and oxygen from an intervening cyclic sulfmate intermediate . ... 

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... 

Traditionally, conventional electron impact techniques have been used to effect sample ionization. The often complex fragmentation pattern which results can provide invaluable information for elucidating the structure of unknown compounds. Vast computer libraries of electron impact spectra are available to assist the analyst with identifications. However, electron impact is an ionization technique that is somewhat limited in scope. By contrast, sample ionization by chemical ionization techniques offers the potential for selectivity, increased sensitivity, complimentary fragmentation, and confirmation or determination of molecular weight. (7). In this paper we will describe several examples in which the versatility of the chemical ionization technique has simplified or improved an analysis. 

Some information has been obtained on the fragmentation patterns of 1,4-thiazine derivatives. A group of 2- and 3-substituted A-methylated benzothiazines with the general structure 2 (R = Me) lost a methyl radical in electron impact mass spectrometry at 70eV (Scheme 2) <1982J(P1)831>. 

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... 

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