Nutrient analysers

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This article discusses two types of analyzers to measure nutrients: a wet chemical analyzer and an optical nitrate analyzer. A nutrient analyzer is an example of an oceanographic instrument to measure the concentration of certain nutrients (e.g. nitrate, nitrite, ammonia, phosphate and silicate) in situ. A section on recent (2020) advances in nutrient sensing has been added at the end of the article.

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Introduction

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 feeding 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.

Nutrients that can be measured in situ include dissolved nitrate, nitrite, ammonia, phosphate, and silicate.

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[1]))

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).

See also


Further reading

  • Grasshoff, K., Kremling, K. and Erhardt, M. (eds.) 1999. Methods of Seawater Analysis, Wiley-VCH, 600 pp., ISBN: 978-3527295890
  • Hanson, A.K. and Donaghay, P.L. 1998. Micro- to fine-scale chemical gradients and layers in stratified coastal waters. Oceanography 11(1): 10-17.
  • Johnson, K.S., Needoba, J.A., Riser, S.C. and Showers, W.J. 2007. Chemical Sensor Networks for the Aquatic Environment, Chem. Rev. 107: 623-640.
  • Chen, X., Zhou, G., Mao, S. and Chen, J. 2018. Rapid detection of nutrients with electronic sensors: a review. Environmental Science: Nano 5: 837-862.
  • Mahmud, M.A.P., Ejeian, F., Azadi, S., Myers, M., Pejcic, B., Abbassi, R., Razmjou, A. and Asadnia, M. 2020. Recent progress in sensing nitrate, nitrite, phosphate, and ammonium in aquatic environment. Chemosphere 259, 127492

Update 2020 of developments in nutrient analyzers


The highly recommended review of Mahmud et al. (2020) highlights recent improvements in aquatic chemical sensors to monitor nitrate (NO3-), nitrite (NO2-), ammonium (NH4+), and phosphate (PO43-) ion concentrations in water. The review critically analyses and compares the performance of these chemical sensors with a particular emphasis on their capability for long-term in situ (sea)water monitoring. It discusses the following topics:

  • Sensors for detection of nitrate
    • Electrochemical sensors for detection of nitrate
      • Nitrate potentiometric sensors
      • Nitrate amperometric/voltammetric sensors
    • Optical sensors for nitrate detection (UV spectrophotometry)
  • Sensors for detection of nitrite
    • Electrochemical sensors for ditection of nitrite
      • Nitrite potentiometric sensors
      • Nitrite amperometric/voltammetric sensors
      • impedimetric/conductometric sensors
    • Optical sensors for nitrite detection
  • Sensors for detection of phosphate
    • Electrochemical sensors for detection of phosphate
      • Phosphate potentiometric sensors
      • Phosphate amperometric/voltammetric sensors
      • Phosphate impedimetric/conductimetric sensors
    • Optical sensors for phosphate detection
  • Sensors for detection of ammonium
    • Electrochemical sensors for ditection of ammonium
      • Ammonium potentiometric sensors
      • Ammonium amperometric/voltammetric sensors
      • Ammonium impedometric/conductimetric sensors
    • Optical sensors for ammonium detection

The authors point to the following promising developments in nutrient sensor technology: For nitrate sensing: devices based on UV spectrometry; for nitrite sensing: devices based on ion-selective field-effect transistor systems; for phosphate sensing: devices based on microelectromechanical systems; for ammonium sensing: devices based on chemical field-effect transistor systems. Another major development is the use of graphene-based materials that present exceptional specific surface area. Electrodes using these materials have a higher density of sensing elements and greater efficiency of sensing, which is particularly important for monitoring trace levels of nutrients in aquatic ecosystems[2]. Major issues still requiring further research and development are: (1) calibration-free sensing (2) improved selectivity to specific nutrients, and (3) biofouling prevention and resistance to biofouling.


References

  1. 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.
  2. Zhu, C., Du, D., Lin, Y. 2017. Graphene-like 2D nanomaterial-based biointerfaces for biosensing applications. Biosens. Bioelectron. 89 (1): 43-55


The main authors of this article are Schroeder, Friedhelm and Prien, Ralf
Please note that others may also have edited the contents of this article.

Citation: Schroeder, Friedhelm; Prien, Ralf; (2021): Nutrient analysers. Available from http://www.coastalwiki.org/wiki/Nutrient_analysers [accessed on 25-02-2021]