Field desorption ionic analytes

Analytes of very high polarity are not anymore ionized by field ionization. Here, the prevailing pathways are protonation or cationization, i.e., the attachment of alkali ions to molecules. [78] The subsequent desorption of the ions from the surface is effected by the action of the electric field. As [M-t-Na]" and [M-i-K] quasi-molecular ions are already present in the condensed phase, the field strength required for their desorption is lower than that for field ionization or field-induced [Mh-H]" ion formation. [37,79] The desorption of ions is also effective in case of ionic analytes. 

The precursor model of FAB applies well to ionic analytes and samples that are easily converted to ionic species within the liquid matrix, e.g., by protonation or deprotonation or due to cationization. Those preformed ions would simply have to be desorbed into the gas phase (Fig. 9.6). The promoting effect of decreasing pH (added acid) on [M+H] ion yield of porphyrins and other analytes supports the precursor ion model. [55,56] The relative intensities of [Mh-H] ions in FAB spectra of aliphatic amine mixtures also do not depend on the partial pressure of the amines in the gas phase, but are sensitive on the acidity of the matrix. [57] Furthermore, incomplete desolvation of preformed ions nicely explains the observation of matrix (Ma) adducts such as [M+Ma+H] ions. The precursor model bears some similarities to ion evaporation in field desorption (Chap. 8.5.1). 

With FAB, TOP, MALDI, ESI-MS, field desorption (FD) MS and GC-MS technique the analytical capabilities for non-ionic gemini surfactants were compared [157]. Parees et al. reported on the analysis of a series of oligomeric ethoxylated surfactants of this type which showed an improved surface activity. Even an antibacterial lipopeptide biosurfactant, lichenysin A, cultured and isolated, was analysed by EAB-MS and EAB-MS/MS, ESI-MS and various other methods [158]. The compound was characterised and the lipid moiety contained a mixture of 14 linear and branched P-hydroxy fatty acids from C12 to C17. 

Because of the experimental difficulty of the technique and because more user-friendly and to some extent more powerful alternatives have become available, FDI is not frequently applied anymore, except for some specific applications. In this respect, an important development is liquid injection field desorption ionization (LIFDI), which enables sample application to the emitter without breaking the vacuum (see Fig. 7.1) [7, 8]. The specific applications where FDI and LIFDI are still applied comprise the analysis of some oiganometallic compounds [9,10], ionic liquids [11], and compound classes, such as (cyclo)paraffins, aromatic hydrocarbons, and nonpolar sulfur compounds (thiophenes) [7, 12-14], not readily amenable to ESI or MALDI. For such nonpolar analytes, mainly molecular ions M+ are observed, whereas for some more polar compounds, [M+H]+ and/or sodiated molecules ([M-l-Na] ) may be observed, e g., for glycosides (Sect. 7.5.2), lipids (Sect. 7.5.4), and peptides (Sect. 7.5.5). A detailed overview on technology and applications of FDI-MS was provided by Schulten et al. [15, 16]. 

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