Molecular fragments, mass spectrometr

A mass spectrometric study was carried out to establish tbe structure of compoimd 69. Its mass spectrum contains tbe molecular ion peak m/z 252 (16.98%) and a base peak (100%) at m/z 210, corresponding to 2-(2-hydroxypbenyl)benzimidazole (70). A tendency towards decreasing the heterocycle size is characteristic of the mass spectrometric behavior of 1,5-benzodiazepin-2-ones [61] and consequently the mass spectra of these compounds contains intense peaks of the corresponding benzimidazoles. It is also known that the mass spectrometric fragmentation of 1,5-benzodiazepines is similar to their thermal or acid decomposition. In fact, refluxing compound 69 in concentrated sulfuric acid yields benzimidazole 70 as the main product. 

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. 

The electron ionization (El) mass spectra of TMS ethers and esters are generally characterised by weak or absent molecular ions. The [M—15]+ ion formed by loss of a methyl radical is generally abundant and in the case of alcoholic functions, the loss of a trimethylsilanol molecule [M—90]+ is also diagnostic. The peak at mJz 73, corresponding to the TMS group, is important in nearly all the TMS-derivative mass spectra. Figure 8.2 shows the fragmentation of TMS esters and ethers in mass spectrometric analyses. 

Neomycin is insufficiently volatile for direct mass spectrometric analysis. To overcome this problem Inouye- - prepared the volatile N-salicylidene Schiff s base, the M.S. of which, however, did not exhibit a peak for the molecular ion. To observe the molecular ion it was necessary to use the o-trimethylsilyl ether of the N-salicylidene Schiff s base. The spectrum of N-salicylidene neomycin was found to be dependant on the ion-chamber temperature indicating that thermal decomposition plays a significant part in the fragmentation process. 

There are certain rules determining fragmentation of organic compounds in a mass spectrometer. That is why on the basis of the fragmentation pattern it is possible to define the molecular mass, elemental composition, presence of certain functional groups, and often the structure of an analyte. There are a lot of similarities in the mass spectrometric behavior of related compounds. This fact facilitates manual interpretation of a mass spectrum, although it requires some experience. It is also worth mentioning that mass... 

Mass spectrometric detection of AG (Fig. 2.7.9) was achievable in an ESI interface in positive as well as negative ionisation modes [8], Figure 2.7.10 displays the spectra of C12-AG, which are characterised by several molecular ions and fragments. The peak assignment with the corresponding masses of all ions is listed for the Ci2- and C14-homologues in... 

A further common problem the analyst faces when integrating SPC into analytical procedures for the determination of LAS is the scarcity of available reference compounds, thereby complicating their determination. Therefore, the identification of the analytes has to be performed by comparison of retention time and absolute peak area ratio between the deprotonated molecular ion and the fragment ion, relative to the ratio obtained from the authentic standard ( 20%). Retention times of SPC, for which no standards were available, can be determined once by mass spectrometric identification in full-scan mode. 

The mass spectrometric behavior (the types and abundance ratios of the fragments) shows variations from compound to compound depending on the changes of both the metal and halogen atoms. Substitution of aluminium by gallium or that of chlorine by bromine considerably decreases the stability of the molecular ion. 

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