Spectral Features in Cationization Mass Spectrometry

Cationization processes in MS clearly demonstrate, that the appearance of the mass spectmm of a particular analyte strongly depends on the experimental conditions, e.g., on the presence or absence of certain additives in the sample or, when an LC mobile phase is concerned, in the solvents. 

For an ion with a known elemental composition, its OE+ and EE+ character can be readily derived using the nitrogen rule, which states that a molecule, M+ or atty other OE+ with an even mass or mass-to-charge ratio m/z) should contain zero or 

The most important ionization process with the soft ionization techniques in positive-ion mode is protonation, i.e., formation of [M+H]+. Next to protonation, ion-attachment or cationization proeesses with sample-related Na - or K -ions may contribute to the ionization of polar analytes in these ionization techniques. [M+Na] and [M+K] have been reported for virtually all soft ionization techniques discussed in Sect. 7.2. The ESl-MS mass spectrum of the iridoid glycoside globttlarinmay serve as an example of this behavior, showing [M+H]+, [M+NH ], [M+Na], and M+K]+ (see Fig. 7.7, see also Sect. 7.5.2). It should be emphasized that the terms protonated molecule, sodiated molecule, and potassiated molecule should be applied rather than the frequently, but erroneous term molecular iorr. The term moleeular ion is reserved for radieal ions Also the terms qua-simolecular ion or pseudomolecular ion should not be used [92]. 

In mass spectra, where both [M+H]+ and [M+Na]+ and/or [M+K]+ are present, the eationized speeies are readily recognized by a 22-Da (21.982 Da) or 38 Da (37.956 Da) mass shift, respectively, relative to the [M+H]+. Sodimn is monoiso-topic, whereas potassirrm, being an A+2 -element, shows an M+2 isotopic contribution of 7.2%. Often, the occurrence of [M+Na] and [MJ-K] can actually be attributed to the presence of Na - and/or K -ions in the (often) biological matrices analyzed, but also residttal alkali-metal ions (typically 1 mM) in the solvents 

For MALDI-MS, van Kampen et al. [98], and their references therein] showed the importance of (1) the affinity of the analyte toward the Alkali+, whieh seems to be primarily governed by the charge density of the Alkali (2) the gas-phase availability of Alkali, whieh seems to be related to the lattice energies in the alkali-cation salt, and thus to the eounter anion used and (3) the matrix applied, which in turn may affect ion separation within the salt. For polyethylene glyeol (PEG) with an alkali-halide salt, the most abundant [M+Alkali]+-ion observed depends 

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