ETD product ion spectrum of the [M + 26H]26+ charge state of apomyoglobin followed by sequential ion/ion proton transfer (IIPT) reaction for (a) 0 ms, (b) 20 ms, (c) 40 ms, (d) 80 ms, and (e) 160 ms. Reprinted from Int. J. Mass Spectrom., Vol. 377, Anderson, L. C.; English, A. M.; Wang, W.; Bai, D. L.; Shabanowitz, J.; Hunt, D. F. Protein derivatization and sequential ion/ion reactions to enhance sequence coverage produced by electron transfer dissociation mass spectrometry, pp. 617–624 (ref 49). Copyright 2015, with permission from Elsevier.