Figure 13: Nano-ESI-mass spectrum of 25 μM ubiquitin in 1 mM NaCl solution. The spectrum was obtained at low nozzle-skimmer potential, so that there was little collisional activation of the protein. It is obvious that each charge state is not a single peak but consists of multiple peaks due to the adduct formation of the salt ions (e.g., Na+ & Cl) with ubiquitin. (Reprinted with permission from Journal of the American Society for Mass Spectrometry [91], Copyright 2005, Elsevier).