A Deuterium Labeling Method for the Characterization of (Chromophoric) Dissolved Organic Matter Using Ultrahigh Resolution Electrospray Ionization Mass Spectrometry

Abstract

Dissolved organic matter (DOM) is a complex ensemble of naturally occurring organic compounds found in virtually all aquatic environments. The overwhelming diversity of DOM makes it extremely difficult to understand the relationship between its bulk physicochemical properties and its molecular structure and composition.

This dissertation describes the development of a novel method to identify ketone/aldehyde-containing species within DOM, which are known to contribute substantially to the ultraviolet/visible (UV-vis) absorption and emission of chromophoric DOM. In this method, an aqueous sample is treated with sodium borodeuteride (NaBD4) and is analyzed via ultrahigh resolution electrospray ionization (ESI) mass spectrometry. Ketone/aldehyde-containing species (at mass m) in the untreated sample are identified by searching the mass spectrum of the reduced sample for peaks corresponding to deuterated derivatives (at mass m+3.021927n). Initial experiments demonstrated that this method reliably discriminates among mass spectral peaks in an untreated DOM sample that comprise species with zero, one, and/or two reducible moieties.

The reactivity and optical properties of reducible species within Suwannee River fulvic acid (SRFA) were studied by treating an aqueous sample with several amounts of NaBD4. This study demonstrated that most species with at least one ketone/aldehyde moiety were reduced a single time under low [NaBD4], while higher [NaBD4] resulted primarily in additional reductions on multi-ketone/aldehyde species. Furthermore, the changes in UV-vis absorption and emission of the reduced aliquots relative to that of the untreated were correlated with the number of ketone/aldehyde-containing species reduced and identified by this method.

The fully developed protocol was used to compare DOM extracted from several aquatic environments. Two pools of ketone/aldehyde-containing species were tentatively identified: A terrestrially-produced group of lignin/tannin-derivatives and a microbially-produced group of carboxyl-rich alicyclic molecules. While the first pool has previously been shown to contribute substantially to the absorption/emission of chromophoric DOM, the second pool most likely would not. The mass labeling method developed here revealed compositional features that are not observable by common ESI mass spectrometric analyses and may serve as a useful way to link the physicochemical properties of DOM to its structure and composition.