5.2.3 The Energetics of Fragmentation

Most commercial analytical mass spectrometers use a nominal electron beam energy of 70 eV. As stated, most organic molecules require about 10 eV to ionize. What does this imply about the energy available to drive decomposition reactions?

• In organic compounds, many bond strengths are in the range 200-500 kJ/mol, which corresponds to roughly 2-5 eV per molecule.
• Many core electron processes occur at energies greater than about 25 eV. We are more interested in valence electrons (those typically involved in bonding) than in the core electrons.
• The rate of a decomposition reaction depends on the potential energy available. To produce a measurable product, a reaction must occur on the time scale of a the mass spectrometry experiment, which is typically about 1-100 microseconds.
• To achieve reasonably fast reactions, the molecule requires about 15-20 eV of excess potential energy (above the ion ground state). Since only a fraction of the electron beam energy is transferred, the beam energy must be considerably larger than the internal energy desired.
• The rate of ion formation is dependent on nominal incident electron beam energy; in general, there is a trade-off between transferred potential energy and ionization rate.

Putting these pieces together, to observe considerable decomposition products (but not core electron processes), an electron beam energy much greater than 20 eV is required. A 70 eV electron beam yields considerable decomposition reactions as well as an acceptable overall ionization rate.