Houben-Weyl Methoden der organischen Chemie vol.5-4 Bromine by Roedig A., et al. (eds.)

By Roedig A., et al. (eds.)

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The ions are transferred into the vacuum of the mass analyser using an atmospheric pressure interface (API). To assist the transport of ions produced from the atmospheric pressure ionization region towards the high vacuum, a high voltage (typically, 2–3 kV) is applied on the surface of the target plate (MALDI probe) and a stream of dry nitrogen is applied to the area surrounding the target plate. As the transfer of ions into the mass spectrometer is relatively inefficient, the total sample consumption is higher for AP-MALDI than for vacuum MALDI.

However, the lowering of the ionization voltage may favour some fragmentation processes. A modification implies desorbing the sample from a heated rhenium filament near the electronic beam. This method is called desorption electron ionization (DEI). Under conventional electron ionization conditions, the formation of negative ions is inefficient compared with the formation of positive ions. 2 Chemical Ionization Electron ionization leads to fragmentation of the molecular ion, which sometimes prevents its detection.

Finally, molecules will be ‘transparent’ to the electrons with higher energies: here we enter the field of electron microscopy. 2 CHEMICAL IONIZATION 27 As already discussed, negative ions can also be formed through ion–molecule reactions with one of the plasma ions. These reactions can be an acid–base reaction or an addition reaction through adduct formation. The mixture, CH4 –N2 O 75:25, is very useful because of the following reactions involving low-energy electrons: N2 O + e− −→ N2 O•− N2 O•− −→ N2 + O•− O•− + CH4 −→ CH3 • + OH− This plasma also contains other ions.

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