An alternative approach used for higher sensitivity and higher time-response measurement of co is resonance fluorescence (RF) the technique in which a sample of the species of interest, held within the instrument, and emits (usually vacuum UV) radiation characteristic of its energy level structure, this radiation is used to excite any ambient molecules of the same species (whose energy levels will exactly match the excitation radiation, hence the term resonance) which will subsequently fluoresce, providing a measurable signal. The RF approach improves typical detection limits time response to approximately 2 ppb in 10 s. Alternative approaches to CO measurement include gas chromatography (GC) and tunable diode laser spectroscopy