Apparatuses and methods are provided for quantifying an analyte in a gas or liquid phase. A light source with an emission spectrum overlapping an absorption of the analyte, and a pair of reflective mirrors located on an optical axis to form an optical cavity can be included. An off-axis photon detector can be configured as a fluorescence detector and located off the cavity axis and arranged to provide a first signal in response to fluorescence within the cavity. An axial photon detector can be located axially, and external, to the cavity and arranged to provide a second signal. The portion of the apparatus including the light source, the cavity, and the axial photon detector can be configured with a cavity-enhanced absorption spectrometer, and the portion of the apparatus including the off-axis photon detector can be configured with a cavity-enhanced laser-induced fluorescence (CELIF) spectrophotometer.
