Fluorometers: Water Quality Sensors

Fluorometer Operating Principles

Excitation: The fluorometer emits light at a specific wavelength to excite fluorescent molecules in water.

Emission: These molecules then emit light at a different wavelength.

Detection: The fluorometer measures the intensity of the emitted light (filters ensure only the desired wavelength is detected), which indicates the concentration of the fluorescent substance.

Fluorometer Applications

• Surface Water Quality: Monitor lakes, rivers, and reservoirs for pollutants and organic matter to maintain healthy ecosystems.
• Groundwater Monitoring: Detect and measure contaminants in groundwater, ensuring safe drinking water and environmental protection.
• Effluent Monitoring: Measure the quality of wastewater discharged from facilities and industrial processes to comply with environmental regulations.
• Environmental Research: Support scientific studies on water quality and pollution, contributing to environmental conservation efforts.
• Tracer Tests: Detect and track fluorescent tracers in water systems, helping to understand flow patterns, identify leaks, and assess environmental impacts.

Solinst Eureka offers a wide range of fluorometers:

Chlorophyll Sensors

Chlorophyll, the green pigment in phytoplankton and algae, indicates the level of photosynthesis occurring. This primary production forms the base of the aquatic food web, supporting everything from zooplankton to fish. While moderate chlorophyll levels suggest a healthy ecosystem, excessive levels can signal eutrophication, leading to unpleasant aesthetics, drinking water taste and odour issues, and potential fish kills.

Solinst Eureka offers two chlorophyll sensors: blue and red excitation. The blue version is commonly chosen because algae absorb blue light well, providing high sensitivity for fluorescence detection. However, dissolved and particulate organic matter (DOM/POM) can reduce the sensitivity of the blue sensor. The red chlorophyll sensor, designed to minimize interference from DOM/POM, allows for accurate detection of algal abundance since red light doesn’t excite these materials. While it is less sensitive for detecting eukaryotic algae, its effectiveness against interference may outweigh this drawback. Additionally, red excitation fluorometry is more sensitive to prokaryotic algae (cyanobacteria), making it ideal for freshwater systems rich in DOM and blue-green algae.

Blue-Green Algae Sensors

Blue-green algae, actually bacteria known as cyanobacteria, are photosynthetic and responsible for over 20% of the earth’s photosynthesis. While cyanobacteria contribute to the oxygen cycle, they can proliferate and exhaust their nutrients, leading to decay that lowers oxygen levels and causes fish kills. Additionally, dying cyanobacteria may release toxins harmful to humans and animals.

Phycocyanin and phycoerythrin are key indicators of harmful cyanobacteria blooms; therefore, Solinst Eureka offers two blue-green algae sensors available for monitoring in marine and freshwater applications: Phycoerythrin, which is dominant in marine species such as Synechococcus spp., and Phycocyanin, which is abundant in freshwater taxa such as Anabaena, Microcystis, and Spirulina.

CDOM/FDOM Sensors

Dissolved Organic Material (DOM) encompasses various forms, including humic acids and organic secretions. It is a significant reservoir of reactive carbon and serves as a dynamic substrate for bacteria, plants, and animals. DOM can photodegrade into volatile compounds that harm the environment and contains chromophores that absorb light, leading to the term Chromophoric Dissolved Organic Materials (CDOM).

CDOM also fluoresces (FDOM), allowing researchers to measure its abundance in water systems through fluorometry. Monitoring DOM levels is crucial for understanding changes in primary productivity, phytoplankton dynamics, algal blooms, and overall environmental conditions. Because there are different CDOM sources that may emit a range of wavelengths, Solinst Eureka chose a broad band emission filter that will detect various types of CDOM found in the natural environment.

Tryptophan Sensors

Tryptophan is an amino acid dissolved in water with specific excitation and emission. It is classified as protein-like organic matter, and sources may include water systems with high biological activity and wastewater or industrial discharge. Tryptophan is another parameter researchers can measure to track wastewater effluent, which may significantly impact habitats and wildlife. The Tryptophan fluorometer outputs a 0-5 volt analog signal proportional to the fluorescence response of the Tryptophan dissolved in water. Solinst Eureka Probes convert the signal to a digital format to measure Tryptophan in concentrations 0 – 5000 ppb. To identify the presence/absence of protein-like organic matter that may indicate wastewater discharge, Solinst Eureka’s Tryptophan fluorometer enables users to detect Tryptophan within a low-level range (limit of detection of 3 ppb) needed for this environmental study.

Rhodamine Sensors

Fluorescent dyes like Rhodamine are often added to water systems to provide water discharge and velocity data. Rhodamine is a highly fluorescent material with the unique ability to absorb green light and emit red light. Very few compounds have this property, so interferences from other substances are very rare. This makes Rhodamine a highly specific tracer. The fluorometer is configured to shine green light on the sample and detect the red light emitted. The amount of red light emitted is directly proportional to the concentration of the dye, up to 1000 parts per billion (1000 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.

Fluorescein Sensors

Fluorescein was the first fluorescent dye used for water tracing work and is still used for qualitative studies of underground contamination of wells. Fluorescein is used for flow measurements, circulation, dispersion, plume studies, hydraulic models and groundwater studies. The 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). Fluorescein emits a brilliant green fluorescence, which gives an excellent visual or photographic contrast against the backgrounds commonly encountered in water transport studies. Therefore, it is easy to visualize the progress of an experiment. It is more aesthetic than red dye, which is important in ocean areas subject to the blooms of certain dinoflagellates, called “red tides.”

PTSA Fluorescence Sensors

PTSA (Pyrenetetrasulfonic acid) dye is non-toxic and chemically stable, making it an ideal trace additive for water treatment systems and agricultural applications. When adding PTSA to treatment formulations, the fluorescent response of the PTSA tracer dye is proportional and graphically linear between specific concentration ranges and the concentration of the chemical with which the system is dosed. Measurements from the PTSA fluorometer may be made available online, in the field, and in the laboratory to be expedited quickly and routinely, enabling real-time product dose analysis. This allows improved accuracy of feed rate monitoring, continuous system monitoring, and control of system characteristics, all ensuring that companies can improve their results and meet challenging system requirements. This UV optical sensor detects the fluorescence of PTSA in water up to 650 ppb. PTSA dye emits wavelengths between 400 and 500 nm when irradiated with UV light.

Crude Oil Sensors

“Crude oil” refers to unprocessed natural petroleum, with various classes differing in chemical composition. Not all crude oil in water comes from spills; natural seepage also occurs. High concentrations can harm aquatic life. Measuring crude oil is challenging due to varying fluorescence responses from different sources, making sensor calibration difficult. The best method is to prepare a standard with known gravimetric concentration similar to the type of oil expected in the field. Crude oil also “weathers,” as lighter fractions like benzene, xylene, and toluene volatilize. Analyzing multiple field samples can help determine oil content; if sensor responses are linear, a single calibration is sufficient. If not, a calibration table should be created to align sensor readings with expected concentrations.

Fluorometers and Biofouling

Fouling can hinder fluorescence sensors by accumulating on the active surface, decreasing emitted or received light. Foreign materials may also produce false signals by fluorescing at similar wavelengths. While it’s primarily a concern for long-term deployments, anti-fouling accessories like Solinst Eureka’s wiper system and copper mesh sensor guards can help during continuous use. Wiper systems are also helpful when detecting crude or refined oils to keep the sensor from getting coated.

Also an option, the Manta+ F35 Water Quality Multiprobe is fitted with a copper-filled, silicone “nose cone” covering all exposed sensors except the critical measurement surfaces. A central wiper keeps those measurement surfaces clean.

Calibrating Fluorometers

There are three ways to calibrate fluorometer sensors, depending on the type. First, you can calibrate to a known concentration or lab-qualified sample of the target analyte; the sample can be prepared gravimetrically or purchased. Second, you can calibrate with a “cal cube” or solid secondary standard, an optical device available from Solinst Eureka that provides a consistent output for a given type of fluorescence. Third, you can calibrate with a transfer standard, such as rhodamine; PTSA is also an ideal lab standard that can be used to standardize all of Solinst Eureka’s fluorometers. The latter two methods are indirect but fast, inexpensive, and practical.

CAUTION: Never look directly at a fluorometer sensor.
The UV rays emitted by the sensor can cause eye damage.

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