Evanescent-wave cavity ring-down spectroscopy applied to condensed-phase systems.

摘要

Evanescent-wave cavity ring-down spectroscopy (EW-CRDS) is a sensitive, surface-selective spectroscopic technique that combines cavity ring-down spectroscopy (CRDS) with attenuated total reflectance (ATR) spectroscopy. In EW-CRDS, a total internal reflection (TIR) element such as a prism is placed within an optical cavity. When an absorbing or scattering chemical sample is placed at the TIR surface, its interaction with the evanescent wave causes attenuation of the reflected light. The interfacial optical loss can be determined from the change in the decay lifetime for the light that is detected as it leaks out of the second mirror. In this dissertation work, an EW-CRDS apparatus was developed and was applied to the investigation of three different condensed-phase chemical systems that span both physical and analytical chemistry.; An EW-CRDS apparatus was designed for ease of use and for applicability to the chemical systems of interest. This apparatus allowed the light to impinge upon the same spot within the sample for each pass and therefore provided a spatial advantage over typical high-sensitivity ATR instruments, in which the light passes through a waveguide.; In the first application, adsorption of the aqueous chromophore crystal violet (CV) to the silica surface was monitored as a function of the pH and ionic strength of the chromophore solution. Adsorption increased as the silanol groups at the silica surface were deprotonated, and competition with other cations was evident at high ionic strength.; Second, a proof-of-principle investigation was conducted to show that EW-CRDS could be extended for use as an environmental sensor with liquid samples. The anionic surfactant sodium dodecylbenzene sulfonate (NaDBS), a common household detergent, was detected by its complexation with methylene blue. A detection limit of ∼500 nM NaDBS was achieved.; Finally, optical losses at a polymer/fused-silica interface were monitored as a function of penetration by a solvent through the polymer, poly(dimethylsiloxane). Other techniques such as surface plasmon resonance were used to determine that the interfacial optical losses resulted from scattering rather than absorption. Methanol permeation caused the polymer to delaminate from the substrate in a spatially inhomogeneous fashion, increasing the interfacial scattering in comparison with that of the water system.