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This article discusses two types of analysers to measure nutrients: a wet chemical analyser and an optical nitrate analyser. A nutrient analyser is an example of an oceanographic instrument to measure the concentration of certain nutrients (e.g. nitrate, nitrite, ammonia, phosphate and silicate) in situ.
Nutrient analyzers are oceanographic instruments, which 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 have 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 analyzers are based on proven wet-chemical laboratory analysis methods. In recent years, nitrate analyzers, based on the absorbance of ultraviolet light by nitrate in water, have been introduced.
Wet chemical analyzers
A variety of wet chemical nutrient analyzers exist on the market. These analyzers draw in sample water, which is then mixed with a reagent (or reagents). The resulting solution develops an attributive property (e.g. color complex or fluorescence) depending on the concentration of the target analyte, which is then measured either in an absorption cell (color complex) or by a light source and photodetector (fluorescence). In some cases, heating of the solution is required to speed up the development.
Depending on the chemical protocols followed (i.e. if heating and/or pre-concentration 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 analyzers are reagent consumption, reagent degradation time, available electrical energy (batteries) and biofouling.
A distinct advantage of wet-chemical analyzers is their capability to conduct in situ calibrations by piping a blank or standard solution of known concentration into the analyzer instead of the sample. Any instrument drift can be detected and the measurements can be corrected for the drift.
Optical nitrate analyzers
Optical nitrate analyzers use the property of dissolved nitrate to absorb 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 a photo detector. The resulting absorption spectra have to be analyzed (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 analyzers do not require any chemical reagents and have a very fast response time (on the order of 1 s) making them very suitable for measurements conducted during profiling work, or those done on towed vehicles and AUV's. The detection limitations 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 the availability of electrical energy (batteries) and biofouling (though for some instruments anti-biofouling measures can be added).
- Instruments and sensors to measure environmental parameters
- Light fields and optics in coastal waters
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