Improving the sensitivity of HPLC absorption detection by cavity ring-down spectroscopy in a liquid-only cavity

A previously described liquid-only cavity flow cell is used to assess the feasibility of improving absorbance detection limits in liquid chromatography (LC) using cavity ring-down spectroscopy (CRDS). In this miniaturized cavity there is an optical path length of only 2 mm between the mirrors, which at the same time form the walls of the flow cell. Typical ring-down times are 65–75 ns for the eluent blank. The performance of the presented flow cell compares favorably to conventional absorbance detection: the baseline noise is determined to be 2.7 × 10−6 A.U. using averaging over 1 s. The concentration detection limits are between 15 and 20 nM (injected concentrations) for compounds with a molar extinction coefficient of 1.0–1.4 × 104 M−1 cm−1 at the laser wavelength of 532 nm. The baseline noise as well as the absolute concentration detection limit is lowered by a factor of 30 as compared to measurements with a typical conventional absorbance detector. With an extra band broadening of only 15%, the flow cell is suitable for LC analysis.