  We consider theoretically the energy loss of electrons scattered to high
angles when assuming that the primary beam can be limited to a single atom. We
discuss the possibility of identifying the isotopes of light elements and of
extracting information about phonons in this signal. The energy loss is related
to the mass of the much heavier nucleus, and is spread out due to atomic
vibrations. Importantly, while the width of the broadening is much larger than
the energy separation of isotopes, only the shift in the peak positions must be
detected if the beam is limited to a single atom. We conclude that the
experimental case will be challenging but is not excluded by the physical
principles as far as considered here. Moreover, the initial experiments
demonstrate the separation of gold and carbon based on a signal that is related
to their mass, rather than their atomic number.
