Spectral Properties And Structural Basis Of Chromophoric Dissolved Organic Matter

Chromophoric dissolved organic matter（CDOM）is a kind of complex organic molecules with a large number of chromophores.In order to clarify the spectral properties and structural basis of CDOM,and understand the environmental behavior of CDOM better,four different sources of CDOM were conducted in this thesis.By measuring and analyzing their absorption spectra,fluorescence spectra and fluorescence lifetime,the spectral characteristics of CDOM and the relationship between transient fluorescence spectra and steady-state fluorescence spectra were explored,and the applicability of different data fitting methods to describe the spectral properties of CDOM was discussed.The chromophore/fluorophore structure and interaction of CDOM were analyzed by the fluorescence quenching effect of lignin monomers on CDOM.The main conclusions are as follows:（1）Integrating the fluorescence attenuation curves of CDOM at different emission wavelengths can obtain the steady-state fluorescence spectrum.The coincidence between the steady-state fluorescence spectrum and the directly measured steady- state fluorescence spectrum is mainly related to the quantum efficiency of photodetectors,and is also affected by the spectral performance of the monochromator and the time resolution of the detector.（2）For the system containing multiple fluorescent components,there are two ways to calculate the average fluorescence lifetime.The amplitude weighted average lifetime（τα）represents the fluorescence decay rate of CDOM,a large value ofτα indicates that the fluorescence decay rate of CDOM is slow;The intensity weighted average lifetime（τ_f）represents the average retention time of ¹CDOM*in the excited state,and the large value ofτ_f indicates that the long-lived components have more contribution to the steady-state fluorescence of CDOM（3）CDOM contains a variety of different fluorophores.The fluorescence components with different lifetimes obtained by multi-exponential fitting have no clear physical meaning and could not represent the specific fluorophores in CDOM. The PARAFAC method can only analyze the components that have the main contribution to the steady-state fluorescence,and may ignore the information of the components with smaller fluorescence intensity.（4）By analyzing the transient fluorescence spectrum of CDOM,it is found that the short-lived fluorescence component（τ<0.2 ns）plays a major role in the fluorescence attenuation.The fluorescence decay rate of CDOM is faster at long emission wavelength（Em>650 nm）.The steady-state fluorescence intensity of CDOM increases significantly after reduction with Na BH4,but the change of amplitude weighted mean lifetime is less than 30%,which indicates that the intramolecular fluorescence quenching of CDOM is mainly caused by charge transfer Intermolecular..（5）The results show that lignin monomers have static fluorescence quenching effect on CDOM and redcuced CDOM by Na BH₄.The quenching efficiency is related to the types of CDOM and aldehydes or alcohols.The environmental conditions such as solvent polarity and p H also affect the fluorescence quenching efficiency.The changes of the spectral properties and fluorescence lifetime of CDOM during quenching reveal that there are electron transfer interactions among the fluorescent molecules of CDOM,and the molecular conformation and binding ability betweenthe fluorescent molecules are closely related to the environmental conditions.