A New Differential Method for Determining the Light Spectral Absorption by Suspended and Dissolved Organic Matter Directly in Seawater

M. E. Lee✉, E. B. Shybanov

Marine Hydrophysical Institute of RAS, Sevastopol, Russian Federation

✉ e-mail: michael.lee.mhi@gmail.com

Abstract

Purpose. The purpose of the study is to introduce a new method for measuring the spectral absorption of light using suspended and dissolved organic matter directly in seawater, without its prior concentration or filtration onto a substrate.

Methods and Results. A new method is proposed for determining light absorption by impurities in seawater. It is based on the application of two reflective double-walled conical cuvettes of large length (connected at the input to an integrating sphere) and assembled according to the optical scheme of a two-beam differential spectrophotometer. The cuvettes are identical thin-walled quartz cones coaxially inserted inside the same outer cones, the latter having a mirror coating. When seawater is placed in the measuring channel and the deionized water is placed in the reference channel for comparison, only the total absorption by suspended matter and colored dissolved organic matter will be defined. It is necessary to pour the filtrate of the same seawater that was passed through the 0.2 µm-pore filter into the reference cuvette to determine only the light absorption by suspended matter in seawater. The seawater filtrate placed in the measuring channel and compared to the optically pure water in the reference channel permits determination of the spectrum of dissolved organic matter absorption in the original seawater.

Conclusions. For the first time, the spectral characteristics of light absorption by suspended matter in an aqueous medium are being defined directly in its natural state. This is being achieved by significantly increasing sensitivity, provided that all the scattered rays are collected completely by the receiving device. This is made possible by a method that uses double-walled conical quartz glass cuvettes, which increase the reflection angle by the value of the cone apex with each subsequent reflection. Therefore, the combination of conical cuvettes in the optical scheme of a two-beam differential photometer with an integrating sphere allows to obtain accurate data that require no more corrections for the influence of scattered rays. The principle of a two-beam differential spectrophotometer proposed and applied in the study makes it possible to perform spectral analyses of the absorption characteristics of various suspended matter components separately from all other substances and the properties of the seawater in which they are present.

Keywords

suspended particles, dissolved organic matter, light absorption, scattering medium, total internal reflection, double-walled cone cuvette, scattering angle

Acknowledgements

The study was conducted as part of the state assignment FNNN-2024-0012, funded by FSBSI FSC MHI, titled “Analysis, Diagnosis and Operational Forecast of the State of Hydrophysical and Hydrochemical Fields of Marine Waters Based on Mathematical Modeling Using the Data of Remote and Contact Measurement Methods”.

Original russian text

Original Russian Text © M. E. Lee, E. B. Shybanov, 2025, published in MORSKOY GIDROFIZICHESKIY ZHURNAL, Vol. 41, Iss. 4, pp. 399–416 (2025)

For citation

Lee, M.E. and Shybanov, E.B., 2025. A New Differential Method for Determining the Light Spectral Absorption by Suspended and Dissolved Organic Matter Directly in Seawater. Physical Oceanography, 32(4), pp. 429-445.

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