The fragmentation of ammonium-containing metabolites ) also inspired the development of a series of benzylammonium thermometer ions, with different benzyl substituents. These thermometer ions were more adequate than benzylpyridinium salts to characterize the internal energies of ions generated by Low Temperature Plasma (LTP) and Atmospheric Pressure Chemical Ionization (APCI) , which require sufficient analyte volatility. The activation of benzylammonium species leads to the formation of a benzyl cation via C-N cleavage (Figure ). Calculations at the CCSD(T)/cc-PVTZ//M06-2X-D3/6-311++G(d,p) level of theory give their dissociation energies (Table ). As detailed further in section 4, benzylammonium ions have been adopted widely to estimate the internal energies of ions in other sources than the ESI .