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USGS Organic Matter Research

 

Methods


Excitation-Emission Matrices (EEMs)

An Excitation-Emission Matrix (EEM) is a three-dimensional (3-D) rendering of the fluorescence properties of carbon found within a water sample. It is a widely used technique for characterizing the composition of carbon, especially as present in dissolved organic matter. The production of EEMs, through fluorescence spectroscopy, is useful because it aids in the detection of pollutants and other water-quality constituents that often bind with organic matter. Since organic matter can be easily sampled in an aquatic system, EEMs are a quick and powerful tool for investigating the aquatic carbon dynamics in a watershed. In addition, the time it takes to run multiple samples in a laboratory is relatively fast, therefore, there is very little turnaround time before getting useful results. All these factors allow for a useful characterization of the type and distribution of carbon throughout a hydrologic system.

To create an EEM, a water sample is processed in a desktop fluorometer using fluorescence spectroscopy. To obtain fluorescence EEMs, excitation wavelengths are increased from 240 to 600 nanometers, at 3 nanometer intervals, and the fluorescence is measured at the re-emitted wavelengths of 240 to 620 nm, at 3 nm intervals. Concurrently, ultraviolet absorbance is also measured. The EEM that is output provides a rich source of information about the chemical components of a given sample and helps determine to what dissolved organic matter (DOM) pool it belongs [see table below]. By analyzing the differences in the chemical characteristics of the DOM pool, it is possible to link to changes in organic matter reactivity and, thus, determine where the organic matter originated. Additional information about the sample can also be inferred from the intensity peaks in the EEM. The intensity peaks relate to specific sources and components of organic matter in the water, such as humic or fulvic acids (protein-like material or phytoplankton-derived materials, respectively).

Example excitation-emission matrix showing the general locations of selected fluorescence peaks. Letters represent key peaks in excitation-emission responses and are associated with different sources and components of organic matter in water.
An example excitation-emission matrix (EEM) showing the general locations of selected fluorescence peaks. Letters represent key peaks in excitation-emission responses and are associated with different sources and components of organic matter in water.

Fluorescence Peak/Parameter

Excitation/Emission (nm)

Description

Reference

B

270/306

Tyrosine like-protein like, associated with autochthonous organic matter

Coble (1996,2007)

T

270/340

Tryptophan-like, protein like, associated with phytoplankton productivity and has been associated with wastewater

Coble (2007), Stedmon et al. (2003), Goldman et al. (2012)

A

260/450

Humic-like

Coble (2007), Stedmon et al. (2003)

M/N

300/390

Humic like, possibly marine, possible microbial reprocessing, more labile humic acids

Coble (2007), Stedmon et al. (2003)

C

340/440

Humic-like, terrestrial, wide spread

Coble (2007), Stedmon et al. (2003)

D

390/510

Soil fulvic acid

Stedmon et al. (2003)

Fluorescence Index (FI)

Ex370→Em470/Em520

Higher values indicate algal(microbial) vs. terrestrial derived DOC, values range from 1.3-1.9

McKnight et al. (2001)

SUVA254

Absorbance at 254nm normalized to DOC

Correlated to aromatic content, higher values associated with greater aromatic content

Weishaar et al. (2003)

Spectral Slope

Slope of absorbance over the range of 275-295nm

Lower S275-295 correlated with higher aromatic content and higher molecular weight

Helms et al. (2008)

S275-295

FDOM

370/460

In-Situ fluorescence that is highly correlated to DOC (dissolved organic matter) concentrations

Spencer et al. (2007), Downing et al. (2009)

 

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Page Last Modified: Tuesday - May 7, 2013 at 14:23:10 EDT