Application of Ultrahigh Resolution Mass Spectrometry and Deuterium Labeling to Determine the Contribution of Ketone/ Aldehyde-Containing Species to the Composition and Optical Properties of Dissolved Organic Matter

Abstract

Dissolved organic matter (DOM) is a complex, heterogeneous mixture comprised of thousands of chemical species, found in almost all aquatic environments and is one of Earth’s largest carbon reservoirs. DOM is known to affect many biogeochemical processes and may be a crucial component of the global carbon cycle. However, due to the inherent complexity of DOM, understanding and relating its photophysical and photochemical properties to its composition is difficult.

This dissertation describes and applies a mass labeling method using ultrahigh resolution mass spectrometry and deuterium labeling to determine the contribution of ketone/ aldehyde-containing species to the composition and optical properties of DOM. Sodium borodeuteride (NaBD4) selectively and irreversibly reduces ketone/aldehyde groups and the changes due to reduction were observed through mass spectrometry and ultraviolet/visible (UV-Vis) and fluorescence spectroscopy. The reduction of ketone/aldehyde-containing species by NaBD4 results in loss of absorption and creates unique mass markers (deuterated species at mass M+3.021927n) in the mass spectrum. The internal consistency of this algorithm for identifying reduced species was tested using two additional methods, both of which resulted in consistently identified reduced species in the mass spectra.

This method was applied to Suwannee River fulvic acid (SRFA) with increasing concentrations of NaBD4 to evaluate the extent of reduction which differed depending on the concentration used. Additionally, the extent of reduction resulting from the increasing concentrations of NaBD4, was shown to correlate well with changes in the absorption and emission spectra of the corresponding untreated and reduced samples; thus, providing evidence that ketone/aldehyde functional groups contribute substantially to the bulk optical properties of SRFA.

Furthermore, SRFA that was irradiated as well as reduced revealed insights into the structural components in SRFA that were lost or decreased due to irradiation and contributed to the observed optical properties. Also, it was demonstrated that irradiation made terrestrial material appear more like marine DOM.

DOM samples from several different aquatic environments were compared using this method to reveal differences and similarities within the composition of DOM. This method has proven to be a useful tool in relating the changes in the optical properties upon the reduction by NaBD4 to the changes observed by mass spectrometry to reveal information on the composition and well as source and structure of DOM.