not spatially resolved. Similar to LIF, CRDS has been applied to gas-phase, liquid and solid samples. The direct absorption measurement removes the issue of fluorescence quenching but introduces the problem of large, unwanted losses in liquid and solid samples (scattering and reflection). CRDS measurements are not background free as they are based on the detection of a change in signal. The dynamic range is defined by the minimal and maximal detectable change in ring-down time. In a typical CRDS setup, the dynamic range spans three orders of magnitude.[@Lehmann2008] The sensitivity of LIF is generally superior for spatially confined samples whereas CRDS can match or exceed this sensitivity for larger samples that fill the entire length of the cavity.

In this study, we present a novel, direct combination of the CRD and LIF techniques using a single, pulsed laser beam that we name cavity-enhanced laser-induced fluorescence or CELIF. The CRD and LIF signals are recorded simultaneously on a shot-to-shot basis. For the first time the fluorescence signal, cavity ring-down intensity and cavity ring-down time are cross-correlated in a way that significantly enhances both techniques. The CRD measurement provides the absolute calibration of the LIF signal. The combined techniques lower the detection 