Mass spectrometry of gas phase cluster ions between acid and water
Kubasiewicz, Paul J.
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When an analyte seeded in a high pressure inert gas is expanded into a vacuum chamber, an intense beam of molecular clusters is formed. The consequential skimming and ionization of the cluster beam allows for the analysis of molecular cluster ions using tandem mass spectrometry. Various process that occur within certain heterocluster ions, including reactivity and fragmentation pathways, may be observed by inducing collisions of the cluster ions with inert gas atoms. In certain situations, a reaction channel is only accessed when a minimum number of solvent molecules are present in a heterocluster ion. These reactions are "solvent catalyzed" in that a critical number of solvent molecules effectively lowers the activation barrier such that the reaction becomes the dominant process occurring within that cluster ion. A series of mass spectrometric investigations have been performed. The first was focused on formic acid/water cluster ions. It was found that the loss of a dimer was almost always preferred except when there were six formic acid molecules present in a cluster. The structure that was formed between these six formic acid molecules and the water was so preferred that it was extremely difficult for the dimer to be removed, so usually only one formic acid molecule would be removed. The second investigation focused on the reactivity of acetic acid/water cluster ions. It was discovered that the preferred cluster structure was that containing five acetic acid molecules with the water. The acetic acid/water clusters seemed to prefer the loss of a monomer unit. A third investigation performed was focused on the reactivity of acetic acid/ammonia cluster ions. It seems that once the cluster size is greater than six acetic acid molecules, the loss of a dimer is preferred to the loss of a monomer, this would make the clusters of six acetic acids and ammonia the magic number. A fourth investigation was between formic acid/ammonia cluster ions. No new surprises were discovered from this data.