These thermometer ions were developed to characterize the internal energy distribution in negative ion mode . Figure values previously determined by time-and energy-resolved surface-induced dissociation , were also used in internal energy and characteristic temperature determination in electrospray sources . Finally, the in-source fragmentation of LPISASHpSpSKTR was studied by the Xia group in nano-electrospray , although not for quantitative internal energy determination. Note that the experimental E0 values of cationized amino acids and small peptides have been determined by guided ion beam mass spectrometry (reviewed in ). However, these were not used for ionization source characterizations. BzPy and leucin enkephalin than for the larger peptides. Retrospectively, we conclude that the difference could be due either to differences in the ionization mechanism (charged residue mechanism for benzylpyridiniums versus ion evaporation or chain ejection for larger peptides; see section 3.5), or to biases in the E0 values used at the time. The E0 values of the benzylpyridinium ions used in MassKinetics were indeed underestimated compared to the values we can now recommend (Tables and herein).