Applications of Ionic Clusters in High Resolution Mass Spectrometry
This dissertation reports a series of experiments undertaken to determine new uses for gas-phase cluster ions by investigation of their formation and thermochemistry. Clusters of ions and neutrals can be formed from electrospray ionization of peptides, amino acids, metals, and metal complexes in solution, resulting in varied chemical distributions including both homogeneous analyte clusters and heterogeneous clusters, such as hydrated metal ions. Large hydrated ions are ideal chemical thermometers, which can be used to measure the adiabatic internal energy deposition from a gas-phase reaction simply by determining the change in mass resulting from the evaporation of water from the cluster ion. Ideally, each of these waters removes a discrete amount of energy - approximately the energy of two hydrogen bonds, or 10 kcal/mol - resulting in a "ladder" or "scale" of measured energies for a given reaction, as well as a width for the energy distribution imparted to the ion during the reaction. These robust and adjustable "nanocalorimeters" are introduced here and used to determine the thermochemistry of one-electron recombination reactions with metal ions and ion complexes. These results have key implications for the magnitude and character of the energy deposition in electron capture dissociation, a fundamental technique in top-down proteomics. Of further interest is the fact that these nanocalorimetry measurements should become more comparable to bulk measurements made in solution as cluster ion size increases. Initial experiments using these gas-phase measurements to obtain bulk values, such as the absolute value of the standard hydrogen electrode, that are not measureable in solution-phase electrochemistry are demonstrated.
Additionally, clusters of ions formed from a mixture of analyte components should be, in principle, indicative of the stoichiometry of the mixture if they are formed statistically. Here, statistical analyses of cluster ion distributions are used to obtain reasonably accurate and rapid measurements of peptide and amino acid molar fractions in solution mixtures over a three order of magnitude range in lieu of traditional standards. These measurements are not possible using individual ions, due to differences in ionization and detection efficiency of the discrete analytes which cause preferential enhancement or suppression of mixture components. However, within a cluster composed primarily of like components, i.e., a clustering agent, these differences become small, and mixtures of peptides and amino acids containing up to ten components are quantified. Taken together, these experiments reveal a robust series of applications for cluster ions previously regarded as a detriment to the efficient formation of ions.