Science

By Organic Matter Research Laboratory

Optical properties such as absorbance and fluorescence are used to gain insight into dissolved organic matter (DOM) composition, and can also serve as proxies for more expensive and difficult to obtain measurements. These techniques are relatively rapid and inexpensive and allow for the comprehensive tracking of DOM dynamics in a wide range of aquatic ecosystems.

What are optical properties?

Optical characterization is, by definition, the use of light to understand the chemical make-up of materials. Using different wavelengths of light, we can learn about different molecular properties. This technique is commonly referred to as spectroscopy.

Optical characterization of DOM provides reliable information about its composition which can be related to its source, environmental processing, redox state, and reactivity. The analysis of DOM optical properties is rapid, reliable and has emerged as an essential tool for the characterization of DOM quality.

The fraction of DOM that interacts with light in the visible spectrum is referred to as chromophoric dissolved organic carbon (CDOM). The extent to which a specific sample absorbs light depends upon the wavelength of light used. For this reason, spectrophotometry is performed using monochromatic light where all photons have the same wavelength. The intensity of absorbed light at a single wavelength (e.g. A254) can then be related to a sample's DOM concentration and composition. By analyzing a wide array of wavelengths (e.g. 240-600 nm), we obtain a better picture of what the organic matter pool is made up of.

Optical measurements can be broken up into two different classes. Absorbance is a measurement of the amount of light of a specific wavelength that a material absorbs, or takes up. Fluorescence is a measurement of the light that a material gives off following absorbance. Fluorescence occurs when the light energy a molecule absorbs causes an electron to be excited to a higher energy level, and subsequently when the electron returns to ground state, energy is lost as light emitted at a higher wavelength. For each wavelength used to "light up" or "excite" a sample, a range of wavelengths can be "emitted." By measuring the intensity of the light that is emitted, fluorescence spectrospocy provides paired excitation/emission data (e.g. ex260/em450), commonly referred to as an Excitation-Emission Matrix (EEMs). The response from these excitation and emission wavelengths can be related to specific molecular structures.

Absorbance

The extent to which a sample absorbs light depends upon the wavelength of light. For this reason, spectrophotometry is performed using monochromatic light where all photons have the same wavelength. The intensity of absorbed light at a single wavelength (e.g. A-254) can then be related to a sample's DOM concentration.

Figure 1: Chromophoric DOM components absorb light thereby decreasing the amount of energy exiting the sample (Credit: Tamara Kraus, USGS).

Response at a single wavelength related to DOM concentration — Figure 2: The response at a single wavelength is related to DOM concentration (e.g. absorbance at 254 nm) while the slope between two wavelengths or the ratios between them provides information about the composition of the DOM (credit: Tamara Kraus, CAWSC).

Fluorescence

Fluorescence occurs when a molecule absorbs energy causing an electron to be excited to a higher energy level, and as the electron returns to ground state, energy is lost as an emission of a photon. Thus, the excitation and emission wavelengths (e.g. Ex260Em450) at which fluorescence occurs are characteristic to specific molecular structures.