Field Fluorometers for the Hydrogeology: Improved Separation of Uranine from Other Dye Tracers Using LASER Light

The capability of a field fluorometer to extract the individual concentrations in a cocktail of two or three dye tracers is achieved through illumination of the water by as many light-emitting diodes (LEDs) of various wavelengths. Color filters in the excitation and detection beams are employed for limitation of the spectral band. These light sources are handy, long lasting and inexpensive. However, their spectral purity is less good than expected, with tailings on both sides of their maximum emission peak. We replace the longer wavelength LED (525 nm) by a green laser diode (532 nm) while keeping the higher one (470 nm) for measurement of the uranine. We observe following improvement of the separating factor of uranine (fluorescein) from a second dye tracer: f = 2.3 for amidorhodamine G, f = 2.0 for sulforhodamine B, f = 2.0 for rhodamine WT and f = 1.4 for eosin. Beside a 4 to 5-fold increase in the detection limit of the three rhodamine dyes, the use of a true monochromatic source such as the laser makes it possible in the field the separation of the so far best dye tracers, otherwise hard to achieve without refined laboratory techniques: Uranine and eosin.