Fluorescein Sensors: Water Quality Monitoring

Why would I want to measure Fluorescein?

Fluorescent dyes like Fluorescein, are often added to water systems to provide water discharge and velocity data. Dye tracing studies can provide useful information for modeling surface and groundwater systems in addition to tracing contaminants. Fluorometers allow researchers to measure ultra-low concentrations of dye. With extreme sensitivity and instrument flexibility, researchers can glean powerful data that could potentially be missed when using less sensitive methods.

​Fluorescein was the first fluorescent dye used for water tracing work, and is still used for detection by fluorometers, and for qualitative (visual) studies of underground contamination of wells.  Fluorescein is used for flow measurements and circulation, dispersion, and plume studies, and can also be used for masking, hydraulic model studies, and underground water studies.

How is Fluorescein Measured?

Solinst Eureka’s Fluorescein sensor is a fluorometric sensor.  Fluorescence occurs when a molecule absorbs light energy at one wavelength and then emits that energy at a different wavelength.  Fluorometric sensors emit light at a certain wavelength, and look for a very specific, different wavelength in return.

There are many variations of fluorescein dye and each compound will produce slightly shifted excitation and emission maxima. However, these small shifts shouldn’t greatly affect the signal detected by the fluorometer, especially if the compound used in the study is the same compound used to calibrate your instrument.

Solinst Eureka Fluorescein sensor is configured to report directly proportional to the concentration of the dye present in the sample, up to 500 parts per billion (500 µg/L). Relative fluorescence readings, dye concentrations, dilution factors, dye travel time, and other parameters provide valuable data used to draw conclusions regarding the water system being studied.

What should I know about fluorescein measurement in the field?

Dye tracers are compounds used for measuring, mapping, and monitoring water systems. Tracers are used for measuring water flows, studying, and modeling surface and ground water systems, tracing contaminants in emergency response situations, detecting leaks, and measuring tank retention times. Fluorescent tracers like fluorescein are chosen because they are cost-effective, and easily and accurately measured on-site with a portable field-ready fluorometer.

Fluorescein emits a brilliant green fluorescence, which gives an excellent visual or photographic contrast against the backgrounds normally encountered in water transport studies. Therefore, it is easy to visualize the progress of an experiment. It is more aesthetic than the red dyes. This is psychologically important, especially in ocean areas subject to the blooms of certain dinoflagellates, called “red tides.” Less public resistance may be encountered using a dye that does not resemble red tide.

For many studies, Rhodamine has become the dye of choice over Fluorescein, due to fluorescein’s rapid degradation in sunlight.  Also, many naturally occurring fluorescent materials have similar characteristics, and thus interfere with fluorescein measurement.  Fluorescein is also more pH-sensitive than rhodamine dyes. Fluorescence drops very sharply at pH values below 5.5. For optimum results, pH should be between 6 and 10.

Fouling is the biggest field problem for fluorescence sensors.  Any foreign material that accumulates on the active surface of the sensor will either reduce the amount of emitted light or received light, or both. Fouling typically does not pose a problem when profiling or spot checking for daily surveys; only when deployed for extended periods.  For continuous deployment, anti-fouling accessories may be added, such as Eureka’s universal wiper system, and copper mesh sensor guard.