4 Organic Chemistry of DOC
A fundamental difficulty in understanding the chemical properties of naturally occurring DOC is that it is chemically complex. There are literally thousands of combinations of organic carbon moieties (organic molecules that can combine to form larger organic molecules) present in DOC, including quinones, phenols, catechols, amino acids, and sugars. Longnecker and Kujawinski (2011), for example, used an analytical technique called electrospray ionization coupled with Fourier-transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to examine DOC found in groundwater. Figure 13 shows the thousands of peaks (individual moieties) found in a single groundwater sample of DOC, vividly illustrating its compositional complexity. This complexity, in turn, limits the utility of direct chemical analysis for understanding the properties of DOC. For that reason, studies of both groundwater and surface water have historically used indirect analytical methods for characterizing the properties of DOC.