Abstract
Compared with sporadic conventional water sampling, continuous water-quality monitoring with optical sensors has improved our understanding of freshwater dynamics. The basic principle in photometric measurements is the incident light at a given wavelength that is either reflected, scattered, or transmitted in the body of water. Here, we discuss the transmittance measurements. The amount of transmittance is inversely proportional to the concentration of the substance measured. However, the transmittance is subject to interference, because it can be affected by factors other than the substance targeted in the water. In this study, interference with the UV/Vis sensor nitrate plus nitrite measurements caused by organic carbon was evaluated. Total or dissolved organic carbon as well as nitrate plus nitrite concentrations were measured in various boreal waters with two UV/Vis sensors (5-mm and 35-mm pathlengths), using conventional laboratory analysis results as references. Organic carbon increased the sensor nitrate plus nitrite results, not only in waters with high organic carbon concentrations, but also at the lower concentrations (< 10 mg C L−1) typical of boreal stream, river, and lake waters. Our results demonstrated that local calibration with multiple linear regression, including both nitrate plus nitrite and dissolved organic carbon, can correct the error caused by organic carbon. However, high-frequency optical sensors continue to be excellent tools for environmental monitoring when they are properly calibrated for the local water matrix.
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Acknowledgements
This study was funded by the M. and T. Nessling Foundation. We thank the laboratory staff of Lammi Biological Station for conducting the laboratory work. This study was conducted under projects: ERDF-funded New Solutions for Environmental Monitoring (YMLI A30175), Profitability and Added-Value for Agriculture from Automatic, Real-Time Monitoring of Environment and the Extension Services (EnviSense), Measurement, Monitoring and Environmental Assessment (MMEA), Processes Controlling DOC-Fluxes in Boreal Catchments (PRO-DOC), and N-Sink (LIFE12 ENV/FI/597).
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Uusheimo, S., Tulonen, T., Arvola, L. et al. Organic carbon causes interference with nitrate and nitrite measurements by UV/Vis spectrometers: the importance of local calibration. Environ Monit Assess 189, 357 (2017). https://doi.org/10.1007/s10661-017-6056-6
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DOI: https://doi.org/10.1007/s10661-017-6056-6