Absorption Measurements in Liquid Core Waveguides Using Cavity Ring-Down Spectroscopy

Short liquid core waveguides (LCWs) were included into a fiber-loop cavity ring-down absorption spectrometer to reduce the detection limit over, both, single pass absorption in a LCW and cavity-enhanced absorption using a conventional fiber-loop cavity. LCWs of 5 and 10 cm length were interfaced with a pressure-flow system and a multimode fiber loop cavity using concave fiber lenses with matching numerical apertures and diameters. Two red dyes, Allura Red AC and Congo Red, were detected with a 532 nm pulsed laser at a 5 nM limit of detection in a detection volume of less than 1 mu L, corresponding to a minimal detectable absorbance of less than 4 x 10(-4) cm(-1) and a minimal detectable change in absorption cross section, sigma(min) = V-det x epsilon x C-LOD, of about 14 mu m(2) (Allura Red AC) and 37 mu m(2) (Congo Red).