Fiber-loop ring-down spectroscopy: A novel online detection method

The application of both pulsed-laser and phase-shift fiber-loop ring-down spectroscopy (FLRDS) as an on-line detector for capillary electrophoresis (CE) and microfluidic devices is demonstrated. Fiber-loop ring-down spectroscopy is suited for the detection of picoliter-sized samples of dilute analyte through direct absorption.; Four interfaces between a capillary flow system and an optical waveguide loop were investigated. A simple fiber-loop/capillary coupling scheme was used on pulsed FLRDS whereas three different coupling schemes were used on phase-shift FLRDS. In the simplest case a capillary was inserted between the two ends of the fiber loop and was optically coupled to the loop using transparent and index-matched PDMS polymer. A plug of sample dye in dimethyl sulfoxide solvent, carried through the detector region by electroosmotic flow, was detected by time-dependent determination of the ring-down times with good reproducibility and fair dynamic range of the dye concentration.; Phase-shift FLRDS is demonstrated to provide much faster response and permit higher data acquisition rates. A commercial microcross provided the simplest and most convenient coupling scheme, whereas the other two schemes required drilling through the walls of the capillary and a microfluidic device, respectively, to introduce the fiber ends. Compared to the Pulsed-FLRDS system, lower detection limit (30 muM) and wider linear range (up to 5 mM) were achieved on these phase-shift FLRDS systems.; The incorporation of a micro lens onto the emitting fiber end has been demonstrated to be an effective method to enhance transmission through the detection gap and lead to an improvement of the sensitivity by up to 80% compared to the straight-cut fiber ends. 5.3-femtomole absorbers were experimentally detected on the lensed-fiber drilled-capillary FLRDS system.; Phase-shift FLRDS coupled with capillary electrophoresis was applied to the analysis of bio-molecules, human serum albumin (HSA) and DNA from herring testes. The labeling coefficient between the dye and HSA molecules was determined to be 6-7. The detection limits of 300 nM and 440 mug/mL were achieved for labeled HSA and DNA, respectively. In addition, based on the scattering effect, the FLRDS system was also demonstrated to be a reliable method for particle characterization and microorganism detection.