Cavity Ring-Down Spectroscopy coupled with helium microwave-induced plasma for fluorine detection

Cavity Ring-Down Spectroscopy with a tunable diode laser has been used for the detection of fluorine at trace levels. A homemade experimental setup was constructed to accommodate the microwave-induced plasma glow discharge inside the optical cavity. The energy delivered by helium plasma discharge was successfully used not only for the dissociation of fluorine molecules but, more importantly, also for the excitation of fluorine atoms into their metastable level 3s⁴P_5/2. The absorption observed at the 3s⁴P_5/2–3p⁴D^o_7/2 transition (corresponding to a 685.603-nm atomic line) was used for fluorine atom detection. Good agreement between theoretical and measured absorption line shapes confirmed the selectivity of fluorine atomic absorption. We observed an absorption coefficient of 4 ? 10⁻⁶ cm⁻¹, corresponding to a detection of F(3s⁴P_5/2) atoms at about 1.5 ? 10⁶ atoms cm⁻³. These data demonstrate that we achieved a detection limit of 100 ng g⁻¹ under the conditions of our initial homemade experimental setup. However, the optimization of the system may lead to the improvement of the detection limit by approximately two orders of magnitude.