Nutrient analysers are oceanographic instruments to measure the concentration of certain nutrients in situ. While most measurements of nutrients are still made by taking water samples for later analysis in the lab a variety of in situ instruments has become available that automatically measure nutrient concentrations at pre-programmed intervals. These instruments allow a much higher temporal resolution of measurements than what can be achieved by taking samples.
Most of the nutrient analysers are based on proven wet-chemical laboratory analysis methods. In recent years nitrate analysers based on the absorbance of ultraviolet light by nitrate in water have been introduced.
Wet chemical analysers
A variety of wet chemical nutrient analysers exist on the market. These analysers draw in sample water and mix it with a reagent (or reagents). The resulting solution develops a characteristic property (e.g. colour complex or fluorescence) depending on the concentration of the target analyte, that is then measured in an absorption cell (colour complex) or by a light source and photodetector (fluorescence). In some cases heating of the solution is required to speed up development.
Depending on the chemical protocols followed (i.e. if heating and/or preconcentration steps are needed), the time response (time between independent measurements) is on the order of a few seconds to minutes.
Parameters limiting the deployment time of wet-chemical analysers are reagent consumption, reagent degradation time, available electrical energy (batteries) and biofouling.
A distinct advantage of wet-chemical analysers is the capability of conducting in situ calibrations by piping a blank or standard solution of known concentration into the analyser instead of the sample. Any instrument drift can be detected and the measurements corrected for the drift.
Nutrients that can be measured in situ include dissolved nitrate, nitrite, ammonia, phosphate and silicate (see links to companies below for details).
Optical nitrate analysers
Optical nitrate analysers use the property of dissolved nitrate that it absorbs ultraviolet light. The instrument consists of a light source (deuterium lamp of flash lamp), collimating optics, a light path through the sample water and a spectrometer with photodetector. The resulting absorption spectra have to be analysed (either by an on-board computer or after data recovery) as other constituents in the seawater also absorb ultraviolet light. (For details see Johnson & Colleti (2002))
Optical nitrate analysers do not require any chemical reagents and have a very fast response (on the order of 1 s) and therefore are also suitable for measurements during profiling work, on towed vehicles and AUV's. The detection limits depend on the length of the optical absorption path, generally these instruments are not well suited for low nitrate concentrations (< 1 umol).
The deployment time of the optical instruments is limited by available electrical energy (batteries) and biofouling (though for some instruments anti-biofouling measures can be added).
- Grasshoff, K., Kremling, K., Erhardt, M. (eds.) (1999), Methods of Seawater Analysis, Wiley-VCH, 600 pp., ISBN: 978-3527295890
- Hanson, A.K., Donaghay, P.L., 1998. Micro- to fine-scale chemical gradients and layers in stratified coastal waters. Oceanography, 11(1), 10-17.
- Johnson, K.S., Coletti, L.J., 2002. In situ ultraviolet spectrophotometry for high resolution and long-term monitoring of nitrate, bromide and bisulfide in the ocean. Deep-Sea Research I 49, 1291-1305.
- Johnson, K.S., J.A. Needoba, S.C. Riser, W.J. Showers, 2007. Chemical Sensor Networks for the Aquatic Environment, Chem. Rev., 107, 623-640.
- Satlantic Optical nitrate analysers, water quality monitor. Accessed 14.5.2007
- TriOS Optical Sensors Optical nitrate analysers. Accessed 14.5.2007
- Systea S.p.a., wet chemical nutrient analysers. Accessed 14.5.2007
- SubChem Systems Inc., submersible chemical analysers for nutrients, trace metals. Accessed 14.5.2007
- YSI Inc., nutrient analysers. Accessed 14.5.2007
- EnviroTech LLC, nutrient analysers. Accessed 14.5.2007
- Alliance for Coastal Technologies, database of instruments for studying and monitoring of the coastal environment, technology evaluations, needs & use assessments. Accessed 14.5.2007