Investigation of the Fundamentals of Electrospray Ionization and the Separation of Small Molecules using Ion Mobility Mass Spectrometry
Davidson, Kimberly Lauren
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Ion mobility mass spectrometry (IM-MS) is a gas-phase technique used to separate ions based on their mass-to-charge ratio and collision cross section (i.e. shape). Accurate MS analysis of analytes, such as proteins and protein complexes, relies on the preservation of solution-phase oligomeric distributions during ionization. Gas-phase ions in IM-MS are typically formed using electrospray ionization (ESI), which is a gentle ionization source that preserves noncovalent interactions. One challenge in ESI is that multiple analytes in a single droplet can aggregate during solvent evaporation, biasing the oligomeric states of proteins or protein complexes observed during gas-phase measurements. Initial droplet diameters using electrokinetic nano-ESI were measured and reported to be ~60 nm at an ionization current of 30 nA, which is a typical current for native IM-MS experiments. Nonspecific aggregation of a monomeric protein, myoglobin, was measured as a function of concentration of the protein in solution as well as ionization current (i.e., initial droplet size). Results from those measurements show that nonspecific aggregation increases with ionization current, initial droplet size, and concentration of the analyte in solution. The ionization mechanism of macromolecules from solution to the gas phase was also analyzed by measuring the average charge states of proteins, ranging in mass from 8 – 468 kDa, in four different buffers (ammonium acetate, methylammonium acetate, dimethylammonium acetate, and trimethylammonium acetate). Those results were analyzed according to both the charged residue model and the more recent charged residue-field emission model. In IM, ions are separated using a drift cell that is filled with a neutral gas (typically He or N2) and has an applied electric field. The collision cross sections of 20 common amino acids were measured using a radio-frequency confining drift cell and five different drift gases (He, Ar, N2, CO2 and N2O), which were chosen based on their range of masses and polarizabilities. Drift gas selectivity was quantified using peak capacity and peak-to-peak resolution. Those results show that the identities of the pairs of amino acids separated depends on the drift gas and the number of pairs separated depends on peak capacity. The collision cross sections of ten petroleum-like compounds (containing aromatic isomers and double-bond equivalents) were also measured in He, Ar, N2 and CO2. Those results demonstrate the potential to improve separations of compounds with the same elemental composition by using gases with greater mass and polarizability.
- Chemistry