ESI mass spectra of ubiquitin (10−5 M) ions formed from aqueous solutions containing 2.0 × 10−2 M sodium chloride and (a) 0, (b) 1.0 × 10−1, or (c) 7 M ammonium acetate. Cluster ions Na(NaCl)n+ and Na2(NaCl)m2+ are indicated by closed circles and open circles, respectively. In (a), the open and closed arrows denote ubiquitin 5+ ions with an adduction of +1200 and 4030 Da, respectively. In the insets of (c), the asterisks denote peaks that are +36 Da relative to sodium adduct peaks and presumably represent sodium adducts that contain a single adducted NCl. Noise peaks are denoted by the letter “N”.