Oversimplification of Dissolved Organic Matter Fluorescence Analysis: Potential Pitfalls of Current Methods

O ver the past 25 years, there has been a significant increase in the use of fluorescence spectroscopy to characterize the physicochemical properties of dissolved organic matter (DOM) in natural and engineered systems.Fluorescence intensity measured across a range of excitation and emission wavelengths produce thousands of data points per sample that are displayed using a three-dimensional contour plot, known as an excitation-emission matrix (EEM) (see Figure 1a).Fluorescence is an attractive analytical approach due to its ease of use, but the complexity associated with the interpretation of an EEM is often oversimplified.In many cases, the use of EEMs has replaced targeted analytical methods (e.g., fractionations) used to assess the chemical composition of DOM, at the expense of certainty regarding the chemical identity of specific constituents. 1he photophysics of DOM are complex.Only a subset of the molecules that comprise DOM contain functional groups capable of absorbing light in the ultraviolet-visible wavelengths greater than 200 nm (this subset is known as chromophoric DOM (CDOM)).In turn, only a subset of CDOM fluoresces (FDOM) upon relaxation from an excited singlet state.Different models could be invoked to describe the optical properties of DOM, including superposition of individual, noninteracting fluorophores or charge transfer (CT) interactions between electron donor-acceptor complexes. 2o simplify the analysis of DOM EEMs, several qualitative and quantitative approaches have been developed.One of the