Quantitative surface studies of peptides and self assembled monolayers by time of flight secondary ion mass spectrometry: Effects of oxidation reactions

Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a surface analysis technique providing molecular information from outmost surface. The theme of this thesis is to develop new methodologies of using quantitative SIMS to study organic molecules on surfaces, including peptides and alkanethiols in self-assembled monolayer. The focus of these SIMS studies were on the kinetics of solid state reactions that occurred at the surface of analyte or at the interface between analyte and a substrate. The general approach utilized for the quantitative SIMS analysis was internal standard methodology.; A kinetic study on solid state oxidation of methionine-enkephalin (YGGFM), a neuropeptide was carried out using both ToF-SIMS and X-ray photoelectron spectroscopy (XPS). Oxidation of methionine residue of protein is one of big concern for chemical instability of methionine containing proteins used as protein drugs. The aim for developing the quantitative SIMS methodology to study solid state oxidation of methionine peptides is to build a model system to evaluate factors that influence the reaction. Apparent oxidation kinetic equation was built based on peak ratio of the intensity of the oxidized molecular ion divided by the intensity of the unoxidized molecular ion. The deprotonated molecular ions in negative SIMS were chosen in this study. The real oxidation kinetic equation was constructed by combining a calibration curve and the apparent oxidation kinetic equation. Apparent oxidation rates showed that thioether group was oxidized faster to sulfoxide in UV environment than in laboratory air.; Ozone assisted ToF-SIMS methodology was developed to quantify the mixed alkylthiol SAMs on gold. In this methodology, the mixed alkylthiol SAMs on gold is first oxidized by ozonolysis then analyzed by SIMS. Ozonolysis reaction converts analyte (thiolate) to sulfanate, which intrinsically has higher secondary ion yield. Using oxidized molecular ions for quantifying the composition of alkylthiol mixture not only increases sensitivity but also provides a possibility to directly determine the composition of mixed SAMs on gold. A case study of competitive adsorption between three butanethiol isomers to gold was carried out. 1-Methyl-1-propanethiol and 2-Methyl-2-propanethiol has much less adsorption abilities than 1-butanethiol due to the steric effects.