A free-flowing soap film combined with cavity ring-down spectroscopy as a detection system for liquid chromatography

We have shown that a free-flowing soap film has sufficiently high-quality optical properties to allow it to be used in the cavity of a ring-down spectrometer (CRDS). The flow rates required to maintain a stable soap film were similar to those used in liquid chromatography and thus allowed interfacing with an HPLC system for use as an optical detector. We have investigated the properties of the system in a relevant analytical application. The soap film/CRDS combination was used at 355 nm as a detector for the separation of a mixture of nitroarenes. These compounds play a role in the residue analysis of areas contaminated with explosives and their decomposition products. In spite of the short absorption path length (9 μm) obtained by the soap film, the high-sensitivity of CRDS allowed a limit of detection of 4 × 10⁻⁶ in absorption units (AU) or less than 17 fmol in the detection volume to be achieved.