ESI mass spectra of cytochrome c (10−5 M) ions formed from aqueous solutions containing (a) 0, (b) 1.0 × 10−3, (c) 1.0 × 10−1, and (d) 8 M ammonium acetate. In the analyte ions that have sodium adducts, each sodium cation replaces a proton; i.e., the net increase in mass is 22 Da per sodium ion adducted.