Miniaturization and application of direct-liquid cavity ring-down spectroscopy

Cavity ring-down spectroscopy (CRDS) is a relatively new analytical technique for making sensitive absorption measurements. CRDS involves the use of highly reflective mirrors to form an optical cavity into which the sample of interest is placed. Light injected into the optical cavity undergoes an exponential decay of intensity. Absorption measurements can be made by measuring a change in the lifetime of the exponential decay when the sample is present. This thesis describes the extension of CRDS to the liquid phase using a direct-liquid approach. Direct-liquid CRDS (DL-CRDS) entails filling the optical cavity completely with the solution of interest, allowing the mirrors to serve as the ends of the liquid container. This design eliminates losses caused by container interfaces and provides a long path-length through the sample. Design and characterization of the instrumentation are described and an in-depth analysis of the treatment of the experimental data is presented. Miniaturization towards eventual field-instrument production has resulted in a tabletop instrument that is quite compact and is estimated to cost less than {dollar}3500 when in production.; The direct-liquid CRDS instrument is shown to sensitively detect water in organic solvents in the 1- to 100-parts-per-thousand (ppt) range with excellent linearity. The absorptivity of water at 660 nm in acetone and acetonitrile is found to be 2.0 x 10-5 ppt -1 cm-1. As expected, the absorptivity of water in tetrahydrofuran is similar. Additional experiments use the DL-CRDS setup to study the kinetics of the reduction of methylene blue by ascorbic acid in acetonitrile. The sensitivity of DL-CRDS allows kinetic monitoring of reactions at extremely low concentrations and in solvents that would otherwise be unavailable due to solubility concerns. In this case, nanomolar concentrations of methylene blue and micromolar concentrations of ascorbic acid produced pseudo first-order kinetics. The overall reduction was found to have a first-order dependence on both methylene blue and ascorbic acid concentrations, with an experimentally determined rate constant of 2.7 +/- 0.5 x 10 3 M-1 s-1. Several additional applications are suggested to take advantage of the high sensitivity, robustness, and good time resolution of the tabletop, direct-liquid cavity ring-down spectroscopy instrument.