Practical Consequences of Internal Energy

Fragmentation trees similar to that shown here can be constructed from any El mass spectrum, providing ten thousands of examples for the sixth assumption of QET that fragment ions may again be subject to dissociation, provided their internal energy suffices. 

Note The assumptions of QET have turned into basic statements governing the behavior of isolated ions in the gas phase and thus, in mass spectrometry in general. 

A mass analyzer normally brings only those ions to detection that have been properly formed and accelerated by the ion source beforehand (Chap. 4). Therefore, a reaction needs to proceed within a certain period of time - the dwelltime of ions within the ion source - to make the products detectable in the mass spectrum, and for this purpose there is a need for some excess energy in the transition state. 

The dwelltime of ions within the ion source is defined by the extraction voltages applied to accelerate and focus them into an ion beam and by the dimensions of that ion source. In standard El ion sources the freshly formed ions dwell about 1 ps before they are forced to leave the ionization volume by action of the accelerating potential. [41] As the ions then travel at speeds of some 10 m s they pass the mass analyzer in the order of 10-50 ps (Fig. 2.9). [9] Even though this illustration has been adapted for a double focusing magnetic sector mass spectrometer, an ion of m/z 100, and an acceleration voltage of 8 kV, the effective time scales for other types of instruments (quadrupole, time-of-flight) are very similar under their typical conditions of operation (Table 2.4). 

Mass Analyzer Flight Path [ml Acceleration Voltage [V] Typical m/z Flight Time [ps] 

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