Electrospray ionization tandem mass spectrometric study of salt cluster ions: Part 2 — Salts of polyatomic acid groups and of multivalent metals

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Abstract
Salt cluster ions formed from 0.05 M solutions of CaCl2, CuCl2 and NaAB (where A = 1 or 2 and B = CO32−, HCO3−, H2PO4− and HPO42−) were studied by electrospray ionization tandem mass spectrometry. The effects on salt cluster ions of droplet pH and of redox reactions induced by electrospray provide information on the electrospray process. CaCl2 solution yielded salt cluster ions of the form (CaCl2)n(CaCl)xx+ and (CaCl2)n(Cl)yy−, where x, y = 1–3, in positive‐ and negative‐ion modes, respectively. Upon collision induced dissociation (CID), singly charged CaCl2 cluster ions fragmented, doubly charged cluster ions generated either singly or both singly and doubly charged fragment ions, depending on the cluster mass, and triply charged clusters fragmented predominantly by the loss of charged species. CuCl2 solution yielded nine series of cluster ions of the form (CuCl2)n(CuCl)m plus Cu+, CuCl+, or Cl−. CuCl, the reductive product of CuCl2, was observed as a neutral component of positively and negatively charged cluster ions. Free electrons were formed in a visible discharge that bridged the gap between the electrospray capillary and the sampling cone brought about the reduction of Cu2+ to Cu+. Upon CID, these cluster ions fragmented to lose CuCl2, CuCl, Cl, and Cl2. Na2CO3 and NaHCO3 solutions yielded cluster ions of the form (Na2CO3)n plus Na+ or NaCO3−. Small numbers of NaHCO3 molecules were found in some cluster ions obtained with the NaHCO3 solution. For both Na2HPO4 and NaH2PO4 solutions, ions of the form (Na2HPO4)h, (NaH2PO4)i, (Na3PO4)j, (NaPO3)k plus Na+, PO3− or H2PO4− were observed. In addition, ions having one or two phosphoric acid (H3PO4) molecules were observed from the NaH2PO4 solution while ions containing one sodium hydroxide (NaOH) molecule were observed from the Na2HPO4 solution.The cluster ions observed from these four salts of polyatomic acid groups indicate that changes in pH occur in both directions during the electrospray process principally by solvent evaporation; the pH value of the acidic solution became lower and that of the basic solution higher. Copyright © 2001 John Wiley & Sons, Ltd.