Fluorescence, originally developed within the laboratory for determining concentrations of fluorescent compounds of interest, is recognized as a technique that can combine selectivity with extremely high sensitivity. The technique moved from the laboratory into the field with in-situ fluorometers becoming available in the latter half of the 20th century, providing the benefits of instant readings of targeted fluorophors in the environment. These original in-situ fluorometers, working in the visible range and targeted to detect chlorophyll, initially were costly and were therefore adopted mainly for scientific research in the sectors of marine biology and environmental sciences for algae studies. These in-situ visible wavelength fluorometers also were made available to detect dye tracers for time and travel studies within the water environment. Later, ultraviolet (UV)-based in-situ fluorometers became available to monitor polyaromatic hydrocarbons (PAH), as well as colored dissolved organic compounds (CDOM), or gelbstoff. These original in-situ fluorometers used xenon flash lamps for excitation, and photomultipliers were required for detection rather than photodiodes, which have poor sensitivity in this region, impacting their price to the market.
Recently, the introduction of low cost LEDs enabled manufacturers to offer new in-situ fluorometers that benefit from lower manufacturing costs. This has resulted in the wider adoption of in-situ fluorescence measurements and employement in large smart networked systems monitoring the aquatic environment. The scientific and environmental monitoring sectors, as well as industries involved with water processing, have benefited from the use of in-situ fluorescence with the advantages of real-time data and reduced requirements for laboratory analysis.
Challenges
The transition from the laboratory to the field has not been straightforward, unlike laboratory conditions. When looking to monitor specific fluorophors within natural aquatic environments, the waters under investigation can contain other non-targeted fluorophors that can contribute to the detected signal. For instance, PAH and CDOM have similar fluorescence characteristics, with CDOM “bleeding” into the PAH space. Depending on levels of PAH and CDOM, this can, at best, reduce the accuracy and sensitivity of the data provided for the targeted fluorophore, and, at worst, render the data to limited use.
Another concern when monitoring PAHs and tryptophan-like fluorescence within the environment is the potential of signal interference from algae present within the sample, which can contribute UV fluorescence from UV excitation in these regions. The fluorescent technique, being an optical system, also can be limited in its application where turbidity levels within the water are relatively low. High turbidity levels will cause saturation and deflection of the fluorometer excitation light, and render the resulting signal non-linear and misaligned to the instruments’ calibration. Where concentrations of the targeted fluorophore are so high as to absorb the fluorometer excitation light, a similar effect occurs, which curtails the use of the in-situ fluorescence technique to a limited range.
Finally, if the benefits of high sensitivity are to be gained, then small changes in fluorescence yield from temperature changes within the environment also need to be considered, as the additional energy obtained from higher temperatures will impact an in-situ fluorometer’s signal.
A New Approach
These challenges were addressed in the development of a new multiparameter in-situ fluorometer, which was part funded within the EU FP7 project SenseOcean, the Chelsea Technologies Group ‘V-Lux.’ It was identified that the use of fluorescence in the field could be significantly improved in relation to data reliability and range by incorporating a number of fluorescence channels into one sensor, and augmenting these with turbidity, absorption and temperature channels. With careful design, it was recognized that a single multiparameter fluorometer could provide a flexible platform that could be easily configured to detect a number of required fluorometric parameters. Based on demand for the Lux range of fluorometers, it was apparent that there would be calls for four main variants on the new instrument: detection of algae; benzene, toluene, ethylbenzene and xylene; crude hydrocarbons; and tryptophan-like fluorescence.
The new sensor provides the advantages of the latest developments in optical sensing technologies using sensitive and strong solid state detectors. The sensor is packaged within a small 50-mm-diameter housing of 158-mm length, is rated to 6,000 meters and has integrated anti-biofouling protection. It comes with an internal logger and provides real-time data in a choice of data output protocols, including MODBUS, SDI-12 and other digital formats, and includes quality control channels.
A key driver was to provide the user with a robust measurement for the targeted fluorometric compound. This only can be done by comparing the fluorescence yield directly to both the targeted and non-targeted fluorophors. This has been achieved by a combination of techniques, including use of common excitation wavelengths, reporting of data in the same units, and use of developed algorithms for reporting concentrations of the resultant targeted compound with interferences removed.
This combined use of fluorescence channels provides an advance in in-situ fluorescence monitoring, and in addition, the sensor addresses the previous limitations imposed when monitoring waters of high turbidity levels. The inclusion and use of the absorption and turbidity channels provides a sensor that can provide robust fluorescence data to levels up to 1,000 fnu, previously unachievable using single channel fluorescence. These corrections also increase the useable linear range of targeted fluorophors to levels that are more than 10 times of that which can currently be achieved with single-channel fluorometers.
These improvements in in-situ fluorometry place this technology into new areas, including oil and water mixes with the potential of challenging incumbent infrared and gas chromatography-mass spectrometry techniques in the oil and gas and industrial sectors. The new tryptophan-like fluorescence variant also is suitable for monitoring further up the wastewater treatment process into secondary and primary tanks, as well as wastewater treatment input.
