NMR spectroscopy of dissolved organic matter: a review

The presence of liquid water makes our planet habitable. Water in soils, sediments, lakes, rivers and the ocean forms the largest habitat for life on Earth. During life and upon death, all organisms release dissolved organic matter (DOM) to their environment. These molecular traces of life travel with water through land- and seascapes. DOM in the ocean and freshwaters contains more carbon than all living biomass on Earth combined. An in-depth knowledge of the molecular composition of the DOM pool is crucial to understand its role in the global carbon cycle. DOM is one of the most diverse mixtures known. So far, only the structure of a few components has been elucidated, thus, its molecular composition remains largely unknown. NMR spectroscopy is a promising tool for the molecular-level characterization of complex mixtures such as DOM. Major drawbacks of this spectroscopic technique in the past were the lack of sensitivity and insufficient spectral resolution. Large amounts of DOM were required and overlapping signals of the manifold DOM constituents resulted in broad unresolved spectral features. Recent technical and methodical improvements, the application of multivariate statistical analyses and the development of new chemical derivatization strategies may overcome these limitations. Here, we review the application of NMR spectroscopy within the quickly emerging field of the structural characterization of marine DOM. In the first section, this review covers fundamental aspects of NMR spectroscopy and its application to the analysis of DOM. The major points in the following are (1) a comprehensive overview of the current state of NMR spectroscopy for the analysis of marine DOM, (2) a discussion of the most important technical and methodical improvements and (3) suggestions for future implementations of NMR for the characterization of DOM. This review provides an overview for experts but also serves as a starting point for beginners.