(1) Physics Department, Moscow State University
119899 Moscow, Russia
(2) ENEA, INN-FIS, C.P. 65, 00044 Frascati, Italy
(3) Physics Department, University of Oldenburg,
26111 Oldenburg, Germany
Significant problems of environmental measurements require sensitive and accurate techniques for dissolved organic matter (DOM) remote monitoring in natural water. Dissolved organic matter is a significant part of natural water systems and determines their ecological state. Currently, the fluorescence technique using water Raman scattering for fluorescence standardization is widely applied for estimating the DOM content in natural water. The fluorescence technique is based on the hypothesis of a linear regression between normalized fluorescence intensity and DOM concentration. However, a more careful investigation of DOM spectral characteristics has led us to conclude that different factors can break this correlation.
One of these factors is the fluorescence saturation, caused by limited life-time of molecules in their excited state, depending on the intensity of laser pulses exciting the sample. It is noticeable at conditions of 10^24 photons per second per square centimetre, which we usually have with laser excitation in laboratory conditions. During remote sensing the excitation photon flux is several orders of magnitude lower, and this leads to overestimation of DOM concentration if laboratory data are used as calibration measurements.
The recent work presents the theory of the effect and summarises the experimental results on fluorescence saturation for samples of natural water at different wavelengths of excitation. It was experimentally observed in some cases that the normalised fluorescence signal is increasing almost three times for an attenuated laser beam when compared to high-intensity laser excitation.