Application of Fluorescence Spectroscopy in Chemical Oceanography: Tracing Colored Dissolved Organic Matter (CDOM) Erika Mae A. Espejo 3rd year, BS Chemistry, University of the Philippines, Diliman Abstract Dissolved organic matter (DOM), the fraction passing through a 0. 45 µm membrane filter, is considered poorly understood mixture of organic polymers because of its complexity. Although it largely influences a lot of biogeochemical processes in aquatic environments, its characterization is not that simple.
However, due to the fact that it comprises optically active fraction called colored dissolved organic matter (CDOM) together with the help of its colloidal components, tracing of DOM can be possible. Through different methods and instruments such as fluorescence excitation-emission spectroscopy, parallel factor analysis (PARAFAC), isolation-fractionation technique (pairing of fluorescence and absorbance spectroscopy), and satellite remote sensors, analysis of DOM can be done which can help elucidate its dynamics in aquatic environments.
Introduction When a molecule absorbs light (energy), an electron is excited and promoted to an unoccupied orbital. Figure 1 shows a Jablonski diagram which describes what happens when an electron is excited: Fig. 1 Jablonski diagram The energy difference between the ground (S 0) and excited singlet states (S1, S2 or higher) determines the wavelengths at which light is absorbed. Absorption (excitation) can result in a range of transitions to various vibrational sublevels of excited singlet states, which is then followed by nonradiative relaxation to the lowest sublevel of the S 1 state, via vibrational relaxation and internal conversion.
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Internal conversion, singlet–triplet intersystem crossing and fluorescence then compete for relaxation to the ground state (S 0). The wavelength of the fluorescence emission is determined by the difference in energy between S1 and S0 states. The greater the conjugation in the molecule, the lesser the difference in energy resulting in a longer wavelength of fluorescence. Discussion The fraction passing through a 0. 45 µm filter includes material in true solution, together with some colloidal components, and is termed dissolved organic matter (DOM).
It could be autochthonous/external (from degradation of terrestrial plant matter which is dissolved and transported through river systems and estuaries to the marine environment), or allochthonous/internal (from exudation by phytoplankton, excretion by zooplankton, and post-death organism decay process). DOM influences different aspects of aquatic environments like microbial and plankton (aquatic) ecology, trace metal speciation and transport, polycyclic aromatic hydrocarbons (PAHs) toxicity, trace water masses, mobilization of organic and inorganic pollutants, photo degradation, drinking water treatment, and carbon budgeting.
This implies that tracing and characterization of DOM is essential to understand its dynamics; however, since DOM is a complex and poorly understood heterogeneous mixture of aliphatic and aromatic polymers, and its composition varies in time and space depending on proximity to sources and exposure to degradation process, characterization is arduous (involves large sample volumes and many stages) . The optically active fraction of DOM (passing through a 0. 2 µm filter) is called the colored dissolved organic matter (CDOM). It absorbs ultraviolet and blue light radiation in 350-500 nm range and also fluoresces when excited by light .
Its presence gives water a yellow/brown color (and often described as yellow substance or gelbstoff) and its light absorption is highest in the ultraviolet (UV) region and declines to near-zero levels in the red region of the spectrum . It plays an important role in determining the underwater light fields, represents a significant component of ocean optical signals for satellite-based measurements of ocean color and can interfere in global and regional estimates of primary production; affects the ocean color, underwater light fields and aquatic chemistry through a suite of sunlight-initiated photochemical processes .
Thus, using spectroscopy, it can be used as a tracer for the characterization of the DOM pool. This review discusses four approaches in fluorescence spectroscopy for tracing CDOM. The first one is the Fluorescence Emission-Excitation Spectroscopy. Fluorescence excitationemission matrices (EEMs) are emission scans from excitations over a range of wavelengths (? ) which provide information on number, types and abundance of fluorophores present in CDOM  . It can also ifferentiate between CDOM of terrestrial and marine origin (marine CDOM has a fluorescence maximum at shorter wavelengths than terrestrial). For multivariate analysis of EEMs, Principal Component Analysis (PCA), a two-way data analysis method is used (for example 45 excitation ? times 150 emission ? equals 6750 variables). However, Stedmon et. al said that Parallel Factor Analysis (PARAFAC) is better suited to EEMs since it is a three-way version of the PCA where the data are composed into tri-linear components. Equation 1 describes the PARAFAC model (the second approach): xijk = ? ifbifckf + ? ijk (1) where xijk is the intensity of the fluorescence for the ith sample at emission wavelength j and excitation wavelength k, aif is directly proportional to the concentration (moles) of the fth analyte in sample I, b jf is linearly related to the fluorescence quantum efficiency (fraction of absorbed energy emitted as fluorescence), ckf is linearly proportional to the specific absorption coefficient (molar absorbtivity) at excitation wavelength k, F defines the number of components in the model, and a residual matrix ? jk represents the variability not accounted for by the model. Figure 2 and figure 3 show that the model reproduces the main features of the measured EEMs when they sampled in the east coast of Jutland, Denmark: This implies that PARAFAC modeling is an effective method of characterizing CDOM with EEMs. This approach was able to trace CDOM to help elucidate its dynamics: Stedmon et. al said that the model was successful in grouping the fluorophores present into groups with similar structure. They have found out that excitation at longer ? uggests that the fluorophores responsible for this fluorescence are more aromatic in nature or contain several functional groups, the ratio of fluorescence in this region (~500 nm) relative to the fluorescence at 450 nm, varies depending on the number of aromatic groups and, hence, the source of the material, and ratios twice as large in the estuary than in the terrestrial samples, suggests that the fluorescence is not only due to terrestrially derived matter but also CDOM produced/transformed in estuarine processes.
As with the behavior of CDOM, results show that this approach distinguishing is capable between of CDOM derived from different sources since there are considerable differences in the composition of CDOM from sources of DOM. Table 1 shows the behavior of CDOM from different sources: Table 1. Behavior of CDOM from different sources High fluorescence intensity Low fluorescence intensity Lakes: there is a net production of ? Transported out of the forest and again autochthonous DOM during estuarine mixing (where the freshwater input from the stream mixes with the saline waters of the inner estuary) ?
In freshwater: due to mixing (dilution), and degradation/transformation ? In forest stream: photochemical degradation due to exposure to sunlight (photochemical degradation bleaches the DOM fluorescence and causes the specific fluorescence to decrease) ? Results show that this approach enables us to establish relationships between general characteristics of the DOM pool and its fluorescent properties. The third approach is the isolation-fractionation based techniques ((ion-exchange resins, reverse osmosis, rotary evaporation, and tangential flow ultrafiltration).
However this approach uses isolates which may not completely reflect the actual structure, behaviour, interactions and reactivity of DOM in the natural environment due to alterations in the structure of the DOM during extraction and concentration and due to their removal from the original environment in which they were situated. Nevertheless, the paired fluorescence and absorbance measurements can still distinguish CDOM from different sources. Figure 4 shows that DOC against a340 for all sample sites and demonstrates a strong correlation (r=0. 9, n=30); a340 was found to be the best proxy for DOC from all the optical measurements taken, where a340 is absorption coefficient at 340 nm (provide a check for inner-filtering effects when highly absorbent DOM quenches fluorescence, resulting in a decrease in intensity): Fig. 4 Relationship of DOC and a340 measured in River Tyne, northern England The last approach is through satellite remote sensing, a method that could estimate the amount of CDOM in surface waters over large geographic areas would be highly desirable.
Satellite remote sensing has the potential to CDOM observation with high spatial and temporal resolution and enables scaling up to the level of large ecosystems and biomes which implies that match-ups have really high correlation (hence approach is  . Figure 5 below shows satellite measurements of CDOM successful and verified): Satellite-derived CDOM products will allow us to estimate processed such as ecosystem production of DOM and sunlight decomposition of CDOM  . The new odel will also allow us to validate the remote sensing estimates of phytoplankton (chlorophyll concentration) and productivity, and may open up new possibilities for using ocean color remote sensing with studies in areas such as photochemistry, the photobiology of ultraviolet radiation and even ocean circulation . Conclusion The importance of CDOM in tracing and characterizing DOM has been showed through the use of its optical properties; thus enabling us to explain the dynamics of its pool.
The use of fluorescence spectroscopy makes it possible to distinguish the properties of CDOM which can enlighten us on how it influences the biogeochemical processes in the aquatic environments (for example the absorbance measurements can tell us what components of CDOM are present, its molecular weight, it sources, etc), and how it behaves in different environments. References:  Andy Bakera, Robert G. M. Spencer. Characterization of dissolved organic matter from source to ea using fluorescence and absorbance spectroscopy  C. A. Stedmon*, S. Markager . Behaviour of the optical properties of coloured dissolved organic matter under conservative mixing  S. P. Tiwari, P. Shanmugam. An optical model for the remote sensing of coloured dissolved organic matter in coastal/ocean waters  Colin A. Stedmona, Stiig Markagera, Rasmus Bro. Tracing dissolved organic matter in aquatic environments using a new approach to fluorescence spectroscopy  Claude Belzile, Laodong Guo.
Optical properties of low molecular weight and colloidal organic matter: Application of the ultrafiltration permeation model to DOM absorption and fluorescence  C. Romera-Castillo, M. Nieto-Cid, C. G. Castro , C. Marrase, J. Largier, E. D. Barton, X. A. Alvarez-Salgado. Fluorescence: Absorption coefficient ratio — Tracing photochemical and microbial degradation processes affecting coloured dissolved organic matter in a coastal system  http://neptune. gsfc. nasa. gov/science/slides. php? sciid=73
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