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Cavity Ring-Down Spectroscopy in Analytical Techniques

Cavity ring-down spectroscopy (CRDS) is a sensitive absorption spectroscopy technique, used in the detection of very weak transitions and trace species. A laser pulse is trapped in an optical cavity, in which the intensity decays exponentially. The decay is faster in the presence of an absorbing or scattering material. In this work the method is applied in two analytical techniques. In the first, a liquid chromatography detector, the evaporative light scattering detector (ELSD), was modified to include CRDS. Solutes are detected in the ELSD by measuring light scattering. The modification was successful and detection was achieved with CRDS. However, the detection limit of CRDS (1 g L−1 ) was not better than before modification (< 1 mg L−1 ). Reasons for the poor sensitivity were explored and the result is confirmed by calculations. The second technique to which CRDS was applied, was headspace analysis. To circumvent common problems with this spectroscopic method in liquid samples, the vapour phase of a dye in solution was probed. Not enough dye molecules were present to make detection possible, supported by calculations. In specific applications with more volatile compounds this method might be successful.