Electrospray ionization fragment generation

Fig. 8 Product ion spectra of diverse tropane alkaloids after collision-induced dissociation. Spectra were obtained after positive electrospray ionization and collision-induced dissociation (CID) with nitrogen in a QTrap 4000 instrument. Common structural elements cause generation of corresponding diagnostic fragments, e.g. m/z 124 and 93. For structural assignment of biotransformation products similar fragmentation pattern were produced and interpreted as shown by Chen et al. [6, 7, 51] and He et al. [59]...

Soft ionization methods produce few fragments under relatively mild conditions. The ionization method that has received the most attention in terms of its applicability to protein and DNA analysis is the electrospray ionization (ES) technique. This is a soft method that is capable of generating molecular ions from biological macromolecules present in solution. Table 12.3 gives examples of the charge and m/z ranges that have been observed with some biopolymer species in electrospray ionization mass spectrometers. 

The unimolecular fragmentation of calixarene-derived ions will not be treated here, especially as studies on this topic are much restricted due to the fact that classical El and Cl techniques cannot be applied to these mvolatile and often quite polar polyphenols. Rather, mass spectrometric analysis is limited to the detection of positively or negatively charged quasi-molecular ions, such as [M + H]+ and [M — H], or molecular adduct ions, such as [M + NR4]+ and [M - - metal]+. In general, these ions can be readily generated by using matrix-assisted laser desorption (MALDI) and/or electrospray ionization (ESI) mass spectrometry. 

The reaction of sodium azide with aryldiazonium salts yields arylpentazoles (equation 14.49), from which it has been possible to generate the cychc anion psig] through molecular fragmentation in an electrospray ionization mass spectrometer. ... 

The ionization source is the mechanical device that allows ionization to occur. Matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) are now the most common ionization sources for biomolecular MS, because both of them are soft ionization techniques that is, the sample ionization process generates few or no fragments even for large biomolecules, such as proteins and oligosaccharides, so that the intact molecular ions can be easily observed. 

In soft ionization, e.g., chonical and electrospray ionization, the energy associated with the added charge is usually only sufficient to generate adducted ions, such as [M + HJ, i.e., M + proton (Section 2.2.2), but not to cause fragmentation. Note that the ions formed are protonated molecules and not protonated molecular ions, as the latter would have two charges, i.e., the already ionized molecular ion and the proton that is itself an ion. 

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