Alkali metal ions laser ionization

Keki, S., Deak, G., and Zsuga, M., Fragmentation study of rutin, a naturally occurring flavone glycoside cationized with different alkali metal ions, using post-source decay matrix-assisted laser desorption/ionization mass spectrometry, J. Mass Spectrom., 36, 1312, 2001. 

Wang, Y., Rashidzadeh, H., and Guo, B. (2000) Structural effects on polyether cationization by alkali metal ions in matrix-assisted laser desorption/ ionization. J. Am. Soc. Mass Spectrom., 11, 639-643. 

In desorption ionization methods, like field desorption [11], fast-atom bombardment [12], and matrix-assisted laser desorption (MALDI) [13], adding small amoimts of alkali metal salts to the sample on the emission wire (in field desorption) or target (in fast-atom bombardment and MALDI) may result in the observation of adduct ions [M + Alkali] (see also Sect. 7.2). This is generally termed doping with alkali metal salts. In principle, when a platimrm or tungsten wine, to be used in thermoionic emission, is doped with alkali metal salts, gas-phase alkali metal ions may be produced as well. It may be questioned whether this is due to a thermoionic or desorption/ionization effect. 

The laser intensity determines the rate of the excitation process. Certain threshold intensities are required in order to promote chemical surface processes at observable rates and yields. For evaporation and desorption of neutrals with CO2 laser pulses threshold intensities of about 10 Wcm were measured. At 10 to 10 Wcm very fast desorption and evaporation takes place at rates of about 10 monolayers per second and more, followed by ionization probably just above the surface due to chemical reactions such as proton transfer and attachment of alkali metal ions. Saturation is expected at a certain intensity, above which the process considered cannot be promoted further. 

Soft ionization MS techniques, such as matrix-assisted laser desorption ionization (MALDI), ESI, and fast atom bombardment (EAB), allow cryptand and cryptate molecular ions to be seen. The choice of matrix is important because many of the cryptands easily bind alkali metals, even displacing a cryptate s guest species. 

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