GroEL mass spectra acquired under the following conditions: (A) signal obtained by trapping an intact 14-mer GroEL complex in the C-trap; (B) signal of the GroEL monomer subunit obtained upon collisional activation between the funnel exit electrode and inject flatapole. The GroEL monomer ions were accumulated in the C-trap; (C) subunit backbone-level spectrum upon 200 V collisional activation in the HCD cell; The inset shows the signal of one of the GroEL fragment ions (b63) identified at a mass measurement accuracy of 2.3 ppm. Reproduced from: Belov, M.E., Damoc, E., Denisov, E., Compton, P.D., Horning, S., Makarov, A.A., Kelleher, N.L.. Anal. Chem., 2013, 85, 11163–11173 (ref 246). Copyright 2013 American Chemical Society