Low internal energy

The other situation in which energetic factors predominate is when ions have low internal energies and hence long reaction times. The competition among metastable ion decompositions is, for example, very sensitive to their relative critical energies, E0. This fact underlies recent rationalisations of decomposition pathways of metastable ions [101,111]. 

In addition to the above methods utilizing conventional ionization modes, the field ionization technique has appeared [75]. The very intense electric field (about 1 V/A), produced by an electrode, results in the ionization of molecules in the gas phase. This soft ionization technique is often used competitively with Cl, since it does not pollute the source and may yield sufficiently reproducible results. The transit time of ions in the source is on the order of 10 to 10 second. The radical molecular ions (M ) produced are characterized by a low internal energy, and thus can be detected easily. As a result of dispersion within the source, however, sensitivity is about two orders of magnitude lower than that of El. As in the case of El, the fragments produced by FI can furnish interesting structural data on carbohydrates, amino acids, peptides and cardenolides [76],... 

Other soft ionization methods, such as field ionization, may also be chosen. Levsen and Beckey [197a] initially attempted to use decomposition-inducing collisions on M" molecular ions produced with low internal energy (thus highly abundant) produced by field ionization. 

Under these conditions, the unimolecular decomposition spectrum of molecular ions with low internal energy is rich in fragment ions, thus reflecting the facility of their formation, favoring the characterization of the polysaccharides. 

Consider that the final effect of these collisionally induced internal energy deposition processes are the production of molecular species with a wide internal energy distribution, as shown in Fig. 3.14. Area A corresponds to the molecular species that have experienced a low internal energy uptake, while areas B and C correspond to molecular species experiencing internal energy deposition, so as to promote decomposition processes. In the case of C, the decomposition will take place inside the source prior to acceleration (and the decomposition products will be consequently detected in the usual MALDI spectrum),... 

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