Electron ionization mass spectrometry fragmentation

Since dithio- and selenocarbamates and their derivatives are used so widely in the deposition of thin films and nanoparticles that a mechanistic study of their decomposition behavior was carried out by O Brien et al. [ 107]. Wold et al. [78] studied the decomposition products of Zn(S2CNEt2)2 using gas-chromatography mass spectrometry (GC-MS) and their reported deposition path shows clean ehmination of ZnS from the precursor (Eqs. 1 and 2 below). However, the proposed decomposition route is somewhat different to the step-by-step fragmentation observed in the electron-ionization mass spectrometry (EI-MS) of the compoimd, (Eq. 3). This difference can be attributed to inherent differences between the two techniques. 

M], fragment ions are also formed. Electron ionization mass spectrometry is therefore classed as a hard technique. 

Diphenylthiirene 1-oxide and several thiirene 1,1-dioxides show very weak molecular ions by electron impact mass spectrometry, but the molecular ions are much more abundant in chemical ionization mass spectrometry (75JHC21). The major fragmentation pathway is loss of sulfur monoxide or sulfur dioxide to give the alkynic ion. High resolution mass measurements identified minor fragment ions from 2,3-diphenylthiirene 1-oxide at mje 105 and 121 as PhCO" and PhCS, which are probably derived via rearrangement of the thiirene sulfoxide to monothiobenzil (Scheme 2). 

Electron ionization (earlier called electron impact) (see Chapter 2, Section 2.1.6) occupies a special position among ionization techniques. Historically it was the first method of ionization in mass spectrometry. Moreover it remains the most popular in mass spectrometry of organic compounds (not bioorganic). The main advantages of electron ionization are reliability and versatility. Besides that the existing computer libraries of mass spectra (Wiley/NIST, 2008) consist of electron ionization spectra. The fragmentation mles were also developed for the initial formation of a radical-cation as a result of electron ionization. 

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

Insofar as electron impact mass spectrometry is concerned, the cycloproparenes almost always display a molecular ion. The primary source of fragmentation is by loss of a C(1) substituent radical to provide a cycloproparenyl cation. However, labelling studies have shown that loss of H from 1 and 11 (at least) occurs only after complete scrambling of the carbon atoms216217. The use of appearance energy measurements218 for the loss of Br from ionized 2-bromocyclopropabenzene (57a), when coupled with thermochemical data, have led to the prediction that cation 110 is more stable than the phenyl cation by at least 27.6 kcal mol"1 AHf of 110 is estimated at 311 kcalmol"1. 

Analytical pyrolysis is defined as the characterization of a material or a chemical process by the instrumental analysis of its pyrolysis products (Ericsson and Lattimer, 1989). The most important analytical pyrolysis methods widely applied to environmental samples are Curie-point (flash) pyrolysis combined with electron impact (El) ionization gas chromatography/mass spectrometry (Cp Py-GC/MS) and pyrolysis-field ionization mass spectrometry (Py-FIMS). In contrast to the fragmenting El ionization, soft ionization methods, such as field ionization (FI) and field desorption (FD) each in combination with MS, result in the formation of molecule ions either without, or with only very low, fragmentation (Lehmann and Schulten, 1976 Schulten, 1987 Schulten and Leinweber, 1996 Schulten et al., 1998). The molecule ions are potentially similar to the original sample, which makes these methods particularly suitable to the investigation of complex environmental samples of unknown composition. 

The use of an on-line Fourier transform infrared (FTIR) detector with GC has allowed for the identification of unknowns and the distinction between structurally similar compounds. Many compounds with structural similarities cannot be identified by electron impact mass spectrometry because the fragmentation patterns are (or are nearly) identical. An example is the identification of positional isomers of substituted chlorobenzenes, whose mass spectra are identical. In these cases, chemical ionization can be used to highlight structural differences. The infrared detector (IRD) gives quite different spectra for positional isomers, and when compared to library spectra of authentic compounds, it gives unequivocal identification. 

Electron impact mass spectrometry studies of 1,3-di- and 1,2,3-tri-substituted imidazolines show ionization preferences depending on the substitution pattern (Scheme 23). Ions corresponding to R N were detected for all substrates (path 2). In 2-substituted 1,3-diarylimidazolidines, loss of substituents at C-2 ([M-R], path 3) is favored over the loss of hydrogen (path 4). In 1,2,3-trisubstituted compounds, azirinium ions (c, path 4A) were abundant, especially when R = aryl. 2-Phenylimidazolidines fragment to the characteristic tropylium ion (C7H7+, mjz 91). l-Aryl-3-benzylimidazolidines readily lose a benzyl group as radical or cation ([M-benzyl], path 1) <2000JHC57>. 

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