Resonance-enhanced Raman spectroscopy for on-line quantitative analyses.

摘要

This report describes a series of studies carried out as part of an effort to develop simple and rugged methods for on-line monitoring based on Raman spectroscopy. A comparison of sensitivity for Raman and infrared measurements shows the Raman approach has a substantial advantage over infrared measurements for direct multicomponent analyses. The infrared results are strongly influenced by the presence of concomitants, even for the simple two component system employed in this study. Various fiber optic probe systems are considered and compared. A bifurcated optical fiber bundle system is developed for exciting and collecting Raman scattering. The criteria for the optical design and the properties of the system are discussed. Raman spectra obtained with this system are much less subject to interference of background emission from silica than single fiber systems that are investigated. Resonance enhanced Raman spectroscopy offers a considerable sensitivity advantage over normal Raman spectroscopy, and the utility of this approach for on-line monitoring is examined. It is a selective and sensitive technique which provides structural information on a molecule, but its utility for quantitative analysis is obscured by sample absorption. A correction method for absorption in resonance Raman measurements for the fiber system is presented. Measurements on solutions of a strongly absorbing azo dye have been used to test the approach. A quantitative continuous flow analysis is demonstrated by coupling the system with a liquid chromatograph. A chromatographically unresolved azo dye mixture is resolved in the spectral dimension. Individual chromatograms are then generated, and the limits of detection for the azo dyes are about 160 ng.