Detecting Structural Environments of Carboxyl Groups in Dissolved Natural Organic Molecules

The carboxyl group is one of the most abundant and important functional groups in natural organic molecules (NOM). Its structural environment determines its reactivity and stability and has been examined previously using Fourier-transform infrared spectroscopy (FTIR), specifically the asymmetric stretch of the deprotonated carboxylate group. In this study, we provide a comparative analysis of the carbonyl stretch of the protonated carboxyl group (ν(C═O)_carboxyl) and show that it is useful for the detection of carboxyl structural environments where ambiguity exists. When compared to model carboxylic acids, the ν(C═O)_carboxyl frequency of dissolved NOM from various locations and climatic conditions shows a narrow distribution in 1720-1732 cm^-1. The majority of carboxyls in NOM are α-substituted (hydroxyl, ether, ester, ketone, carboxyl, etc.) aliphatic carboxylic acids, agreeing with previous studies. Additionally, the ν(C═O)_carboxyl frequency, as suggested here, provides a greater distinction between aliphatic and aromatic carboxylic acids and is better suited for peptide-rich biological samples. Environmental water samples often contain high salt concentrations that interfere with the detection of carboxyls. Solid-phase extraction using PPL cartridges is an efficient desalting method and can be applied to organic-poor salt-rich samples for proper FTIR analysis.