Cavity-enhanced absorption measurements across broad absorbance and reflectivity ranges

Cavity-enhanced spectrometry constitutes an important and highly sensitive technique for absorbance measurements. The current practice generally involves very high reflectivity mirrors and hence intense light sources (typically lasers) to have enough light transmitted. Available theory describes the situation only for high-finesse cavities (high-reflectance mirrors) and generally for systems with very low absorbances. We develop the general expression for absorbance regardless of mirror reflectivity or the absorbance and show that in the limit of high reflectivities and low absorbances it predicts the same numerical values as that derived by O'Keefe (Chem. Phys. Lett. 1998, 293, 331-336; Chem. Phys. Lett. 1999, 307, 343-349). Signal to noise in any photometric system is also dependent on the amount of light reaching the detector because of shot noise limitations. We show that a small aperture in the entrance mirror greatly improves light throughput without significant departure from the theoretically predicted amplification of absorbance; such simple modifications result in real improvement of detection limits, even with mirrors of modest reflectivity and inexpensive detectors. This allows the merits of cavity enhancement measurements to be demonstrated for pedagogic purposes.