Chapter 9: Laser Fluorimetry Check Access

The uranyl ion gives green fluorescence on excitation with UV light of a suitable wavelength. Fluorimetry is one of the most sensitive and widely used methods for the determination of uranium in geochemical samples containing uranium of the order of 10 ppm in a solid sample by pellet fluorimetry and in ppb ranges in hydrogeochemical (water) samples by laser or LED fluorimetry. Laser or LED fluorimetry methods could equally be utilized for solid samples. Fluorescence is most useful at much lower concentrations than absorptiometry, especially because in fluorescence measurements PMT observes faint light against a dark background. The fluorescence intensity depends on the intensity of exciting radiation. It is, therefore, imperative to use exciting radiation of adequate high intensity, such as a short-lived (3–4 ns) pulsed, self-terminating N₂ laser with a repetition rate of 15 pulses per second, emitting UV radiation at 337.1 nm radiation to fluoresce uranium down to ppb level with adequate accuracy. Interference due to organic matter and humic acid is avoided utilizing their shorter half-life fluorescence by electronic gating.