Small-Volume Raman Spectroscopy with a Liquid Core Waveguide

Abstract

A liquid-core waveguide based on a new fluoropolymer tube is proposed as the basis of a novel, small probe-volume Raman-scattering-based detector. Illumination is transverse to the axis of the tube, with end-on detection through a transparent window at the tube terminus. Using this approach and a ∼6-nL probe volume, we have measured benzene, carbon tetrachloride, and aqueous solutions of 2-propanol (in 5−100% concentration) as test compounds. We show successful collection of Raman spectra to shifts of ∼200 cm-1 collected without a holographic notch filter. Additionally, the apparatus naturally discriminates against Raman signals from the capillary material. This permits simplified detection of the liquid sample without the need to subtract the otherwise omnipresent contribution of the capillary material. A simple, multiple-pass, laser illumination scheme is found to enhance the Raman signal intensities; the gain in S/N increases slightly less than linearly with the number of passes. In the case of 2-propanol, experiments were conducted in both static and in a flow injection mode. With a correction applied for RI change due to the solute, the Raman intensity was linearly dependent on the analyte concentration.