A common feature of the positive- and negativeion mass spectra of metal carboxylates measured with the use of different ionization methods is the prevalence of cluster ions composed of the metal (M) and the anion (A). Positive cluster ions consist of(<i>n</i> + 1) singly charged positive metal ions bound to <i>n</i> anions. Negative cluster ions consist of <i>n</i> metal ions and (<i>n</i> + 1) anions. For mass spectra recorded from metal acetates, we have previously shown that the types and intensities of the cluster ions change with different pressures and temperatures of a chemical ionization source, and that a wider variety of cluster ions can be created under controlled chemical ionization conditions than has been observed in previous electron ionization and field desorption mass spectrometric studies.
Yu Cao and Kenneth L. Busch, "Enhanced Stability of Gas-Phase Dimer Molecules of Group I Metal Acetates Deduced from Direct-Exposure Probe Chemical Ionization Mass Spectra," Appl. Spectrosc. 48, 1559-1561 (1994)