Electron impact mass spectrometry characteristics

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>. 

Figure 8-4. Methylation analysis of in vitro P-D-glucans. (a) Gas chromatography of the permethyl-ated alditol acetate obtained from methylation analysis of the cellulose synthesized in vitro by the enzyme from blackberry. Peak 1, derivative characteristic of (1 4) linked glucosyl units. Peak 2, internal standard (mj o-inositol). The derivative characteristic of (1 3) linked glucosyl units usually elutes 1 min before the major derivative visible in the chromatogram (not shown see Bulone et al. 1995). (b) Structural characterization by electron impact mass spectrometry of the 1,4,5-tri-0-acetyl-2,3,6-tri-0-methyl-D-glucitol derivative corresponding to peak 1 in A and characteristic of (1 4) linked glucosyl units.

Verification of the molecular weight of thiirene dioxides by mass spectrometry, employing the conventional electron-impact (El) ionization method, has been unsuccessful due to the absence or insignificant intensity of molecular ion peaks in their mass spectra. The base peak is rather characteristic, however, and corresponds to the formation of the disubstituted acetylene ion by loss of sulfur dioxide91 (equation 3). 

Principles and Characteristics Electron impact (El) ionisation is the original ionisation method (1918). Before 1980, mass spectrometry was merely restricted to electron impact (El), with chemical ionisation (Cl) being applied mainly for those samples which resist generation of satisfactory El data. Nowadays, El is still a widely used universal and nonselective ionisation method. In El, the sample is introduced as a vapour... 

Fig. 15.14 Analytical techniques for time-resolved headspace analysis. An electronic nose can be used as a low-cost process-monitoring device, where chemical information is not mandatory. Electron impact ionisation mass spectrometry (EI-MS) adds sensitivity, speed and some chemical information. Yet, owing to the hard ionisation mode, most chemical information is lost. Proton-transfer-reaction MS (PTR-MS) is a sensitive one-dimensional method, which provides characteristic headspace profiles (detailed fingerprints) and chemical information. Finally, resonance-enhanced multiphoton ionisation (REMPI) TOFMS combines selective ionisation and mass separation and hence represents a two-dimensional method. (Adapted from [190])...

Mass Spectrometry. A review on biochemical applications of mass spectrometry deals largely with carotenoids. Field-desorption m.s. of carotenoids gives the molecular ion as the base peak, with very few fragment ions. Eighteen carotenoids were examined by this technique. A fragment ion at M-68 on electron impact seems to be characteristic for acyclic carotenoids with a 1,2,7,8-tetrahydro end-group. ... 

Mass spectrometry is a useful tool in diagnosing tlio characteristics of a withanolide. The molecular ion in electron impact is either weakly present or totally absent in some cases. 

SP-2401" and 3% SP-2250. ° Detectors used by EPA standards procedures, include photoionization (PID)," electron capture (ECD)," Eourier transform infrared spectrometry (PTIR), " and mass spectrometry detectors (MSD)." ° Method 8061 employs an ECD, so identification of the phthalate esters should be supported by al least one additional qualitative technique. This method also describes the use of an additional column (14% cyanopropyl phenyl polysiloxane) and dual ECD analysis, which fulfills the above mentioned requirement. Among MSDs, most of the procedures employ electron impact (El) ionization, but chemical ionization (CI) ° is also employed. In all MSD methods, except 1625, quantitative analysis is performed using internal standard techniques with a single characteristic m/z- Method 1625 is an isotope dilution procedure. The use of a FTIR detector (method 8410) allows the identification of specific isomers that are not differentiated using GC-MSD. 

These distinguishing characteristics of chemical ionization mass spectrometry have been discussed previously, but as a result of recent studies (to be discussed in more detail later) we have come to recognize another characteristic aspect of the chemical ionization process when the high-pressure mass spectrometric technique is used. When chemical ionization is effected at a pressure in the mass spectrometer ionization chamber on the order of 1 Torr, the ions comprising the mass spectra are produced by collision processes, and after formation the ions undergo a number of collisions with molecules of reactant gas before they pass out of the ionization chamber. Thus, unlike the conditions which obtain in conventional (low-pressure) electron-impact and photon-impact ionization, the ions produced by chemical ionization are not isolated. That is to say, they are not formed... 

An important aspect of MS (especially electron impact) is fragmentation, which is characteristic for each compound. With methods like FAB and FD, this characteristic feature is often lost. Fragmentation patterns do give useful information about the structure of a compound. Loss of certain groups, or specific fragments for a certain class of compounds, are important information for the structure elucidation. Furthermore, with high resolution mass spectrometry, the elemental composition of the molecule can be obtained. The MS of alkaloids has been extensively reviewed by Hesse (1974) and Hesse and Bernhard (1975). 

---