Seasonal and temporal characterization of dissolved organic matter in rainwater by proton nuclear magnetic resonance spectroscopy

Dissolved organic carbon in rainwater was concentrated by two separate techniques and characterized by 1H-NMR. The total 1H-NMR integral of lyophilized rainwater was higher in every rain sample than that of C18 solid phase extracted samples and always contained a higher percentage integral in the region of protons bound to carbon atoms singly bound to oxygen (H–C–O), such as alcohols, polyols and carbohydrate-like compounds. C18 extracted samples had higher % integral in the alkyl region, consistent with reverse phase extraction of more hydrophobic components in rainwater. Differences in both the 1H-NMR integration and spectral pattern of lyophilized rainwater samples were especially apparent between spring and winter rains, with spring samples having higher percent carbohydrate (H–C–O) signal and winter rains having higher percent alkyl protons and a spectral pattern consistent with the presence of saturated fatty acids. Spring rains are characterized by lower % Alkyl signal coupled with higher % H–C–O than all other events while marine winter events appear in regions characterized by relatively high % Alkyl and average % H–C–O, consistent with increased abundance of fatty acids or fatty acid oxidation products. The 1H-NMR data presented in this manuscript are important because they provide spectral data relating to the source and chemical characteristics of dissolved organic carbon in rainwater as a function of season and air mass back trajectory.