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Thesis (M.S.)--Lehigh University, 2009. / Adviser: James Roberts.

Thesis (Ph.D.)--Lehigh University, 2009. / Adviser: James E. Roberts.

Thesis (Ph. D.)--University OF MICHIGAN.

Understanding the interactions at the modified silica interface used as a stationary phase in various chromatographic techniques is of great importance in elucidating the mechanism of solute retention. Investigating the factors that control the selectivity and efficiency for retention of a solute is also important as it can lead to the manipulation of the interfacial properties to give improved separations. In this research, solid phase extraction was used to obtain information about silica based anion-exchange stationary phases and their interaction with acidic analytes. Solid Phase Extraction (SPE) experiments using strong anion-exchange sorbents containing a fixed positive charge illustrated the importance of the counter-ion present at the surface. From these studies, it was determined that different counter-anions have different affinities for the ion-exchange site. Lower selectivity counter-anions (i.e. acetate) are more easily displaced from the sorbent by acidic analytes than a higher selectivity counter-anion (i.e. citrate). The general trend amongst counter-ions beginning with the counter-ion with the greatest affinity for the ion-exchange site is shown here: citrate > maleate > sulfate > formate > phosphate > chloride > hydroxide > nitrate > propionate > acetate. Overall, selectivity was not only determined to be a function of ionic interactions, but was also found to be a function of the extent of hydration of both the counter-ion and the surface. Weak anion-exchange sorbents consisting of primary and secondary amines were also investigated. In order for weak sites to contain ion-exchange sites, the pH needs to be selected so that the surface is ionized. Due to the pH dependence of weak anion exchangers, studies were undertaken to determine the effect that pH has on the extraction of acidic analytes. It was determined that the pH at the surface is not necessarily that of the bulk solution. It was also concluded that ionic, hydrophobic, strong dipole, and charge-induced dipole interactions contribute to the extraction of acidic compounds. SPE was also applied to the isolation and purification of acidic compounds. With a better understanding of the surface/solvent environment, a generic approach was developed for the extraction of toxicologically relevant compounds from biological matrices. By understanding the influence of pH, counter-anion, and degree of hydrophobic character of both analyte and surface, an enhancement in extraction efficiency and selectivity was achieved.

Low energy (tens of eV) ion-surface collisions carried out in a tandem mass spectrometer are investigated as a tool to characterize self-assembled monolayer (SAM) films. The target films are prepared by spontaneous chemisorption of thiol-based (HS-R) compounds onto Au (111) substrates. Most of the films used as targets contain alkane or fluoro-alkane backbones, some with unique groups in the terminal position (e.g., -CD₃, -OH, -OC(O)CF₃). Pyrazine is the most frequently used probe ion, however in certain cases other small organic molecules are also used. Common interactions between the impinging ion and the target film that vary as a function of film characteristics include, but are not limited to, reactive scattering, neutralization and T → V conversion. Pyrazine ion readily reacts when colliding with hydrocarbon films at 20-eV, forming product ions that incorporate a hydrogen atom or a methyl group. Several examples of the utility of these processes to characterize film properties are presented. For hydrocarbon films, ion-surface reactions of pyrazine ion resulting in addition of a hydrogen atom or a methyl group are shown to vary with the quality, chemical composition and orientation of the target film. Experiments with isotopically labeled films show that the ion beam interacts predominantly with the end groups of the film, however interactions with underlying groups increase as the film or substrate quality decreases. The orientation difference of odd and even chain length n-alkanethiols produces a measurably different degree of hydrogen addition with the higher free energy odd chain length orientation being more reactive. The composition of mixed component films (H, D or H, F) is tracked by measuring the abundance of unique reaction products, energy transfer (translational to vibrational conversion) and charge exchange properties. When mixed films containing deuterium labeled and unlabeled n-alkanethiols are subjected to collisions of 20-eV pyrazine ion, the D-addition ion abundance increases linearly with the surface concentration of D-containing alkane chains. When mixed films containing different ratios of H and F components are the target, several processes track with the changing population of surface species. As the target films become more fluorocarbon in nature H-addition decreases, total ion current reaching the detector increases, and dissociation increases. Several properties of electron transfer from the film to the ion are examined. When the probe ion and collision energy remain consant, charge exchange is shown to be primarily governed by the work function of the film and the thickness of the adsorbed layer. Fluorocarbon films, which have a higher work function than hydrocarbon films, consistently show less charge exchange. When comparing hydrocarbon films of varying chain lengths (ranging from 15 to 18 carbons), a increase of ∼1% in total ion current measured at the detector is observed for each additional methylene in the chain.

Organic thin films (alkanethiolates chemisorbed on gold) were employed in low-energy (eV) ion-surface collisions to validate the technique as a surface analysis tool and to further investigate processes associated with ion-surface interactions. Low-energy ion surface collisions of small polyatomic and atomic ions with self-assembled monolayers (SAMs) ascertain the chemical composition, structure, and quality of SAMs utilizing four processes: energy transfer (fragmentation of projectile ions: surface-induced dissociation (SID)), electron transfer (neutralization of the projectile ions), atom/group transfer (reaction between the projectile ion and atom/groups from SAMs), and chemical sputtering. Low-energy ion-surface collisions were used to investigate newly synthesized fluorinated compounds where the degree of fluorination of the thiolate tail group increases. Data indicate that substitution of CH₃ with CF₃ as the terminal group has a substantial influence on energy transfer, electron transfer, and atom/group transfer. Slight penetration into a depth of SAM films is illustrated by the formation of certain ion-surface reaction products (a result not observed for previously characterized Langmuir-Blodgett (L-B) films). A novel neutralization mechanism for reaction between methyl cation and hydrocarbon and fluorocarbon SAMs was established. Ion neutralization (besides direct electron transfer) results from a hydride ion transfer, methyl anion transfer, or fluoride transfer between hydrocarbon and fluorocarbon SAMs and incoming methyl cations. Experimental ion-surface and ion-molecule data support the ion neutralization mechanism originally proposed by ab initio and thermochemical calculations. Ion-surface processes were also used to characterize three mixed SAM systems (system 1: hydroxyl/hydrocarbon mixed SAMs and systems 2 and 3: fluorocarbon/hydrocarbon mixed SAMs). The mixed SAMs were prepared from binary thiol solutions and uniform solutions of asymmetrical disulfides. These ion-surface data can be useful for qualitative (identification of the sample's chemical composition) and quantitative analysis (calculation of the surface concentration of a chemical species for a mixed SAM). An in-line Sector-Time-of-Flight (TOF) tandem mass spectrometer with low-energy ion-surface collisions was characterized. Research involved testing the versatility of the instrument in terms of effective ion activation (peptide fragmentation) and surface analysis of organic thin films. This prototype will aid further implementation of SID into commercial TOF instruments for efficient ion activation and surface analysis capabilities.