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

Salt cluster ions formed from 0.05 M solutions of CaCl2, CuCl2 and NaAB (wh ere A = 1 or 2 and B = CO32-, HCO3-, H2PO4- and HPO42-) were studied by ele ctrospray ionization tandem mass spectrometry. The effects on salt cluster ions of droplet pH and of redox reactions induced by electrospray provide i nformation on the electrospray process. CaCl2 solution yielded salt cluster ions of the form (CaCl2)(n)(CaCl)(x)(x+) and (CaCl2)(n)(Cl)(y)(y-), where x, y = 1-3, in positive- and negative-ion modes, respectively. Upon collisi on induced dissociation (CID), singly charged CaCl2 cluster ions fragmented , doubly charged cluster ions generated either singly or both singly and do ubly charged fragment ions, depending on the cluster mass, and triply charg ed 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 observ ed as a neutral component of positively and negatively charged cluster ions . Free electrons were formed in a visible discharge that bridged the gap be tween the electrospray capillary and the sampling cone brought about the re duction of Cu2+ to Cu+. Upon CID, these cluster ions fragmented to lose CuC l2, CuCl, Cl, and Cl-2. 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 bot h Na2HPO4 and NaH2PO4 solutions, ions of the form (Na2HPO4)(h), (NaH2PO4)(i ), (Na3PO4)(j), (NaPO3)(k) plus Na+, PO3- or H2PO4- were observed. In addit ion, 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 i ndicate that changes in pH occur in both directions during the electrospray process principally by solvent evaporation; the pH value of the acidic sol ution became lower and that of the basic solution higher. Copyright (C) 200 1 John Wiley & Sons, Ltd.