Cavity Ring-Down Spectroscopy as a Detector for Liquid Chromatography

We have demonstrated the use of cavity ring-down spectroscopy (CRDS) as a detector for high performance liquid chromatography (HPLC). For this use, we have designed and implemented a Brewsterʹs angle flow cell such that cavity ring-down spectroscopy can be performed on microliter volumes of liquids. The system exhibits a linear dynamic range of 3 orders of magnitude (30 nM to 30 (mu)M quinalizarin at 470 nm) for static measurements and 2 orders of magnitude (0.5 (mu)M to 50 (mu)M) for HPLC measurements. For the static measurements, the baseline noise is 2.8 × 10^-6 AU rms and 1.0 × 10^-5 AU peak-to-peak, and for the HPLC separations, it is 3.2 × 10^-6 AU rms and 1.3 × 10^-5 AU peak-to-peak. The baseline noise is determined after the data are smoothed by an 11-point boxcar average. The peak areas detected from HPLC separations are reproducible to within 2-3%. The HPLC mass detection limit for a molecule with (epsilon)= 9 × 103 M-1 cm-1 in a 300-(mu)m path length cell (illuminated volume, 0.5 (mu)L) is reported as 2.5 × 10^-8 g/mL. These results were obtained using a simple pulsed CRDS system and are comparable to, if not better than, a high-quality commercial UV-vis absorption detector for the same path length.