Isolation and characterization of estuarine dissolved organic matter: Comparison of ultrafiltration and C18 solid-phase extraction techniques

Characterization of dissolved organic matter (DOM) from aquatic environments has always been constrained by the ability to obtain a representative fraction of the DOM pool for analysis. Ultrafiltration or extraction, commonly using XAD or C18 sorbents, is therefore generally used to concentrate and desalt DOM samples for further analyses. In this study, we compared ultrafiltration and C18 solid-phase extraction disks (SPE) as DOM isolation methods for estuarine samples. We also evaluated the use of the C18 SPE disks to isolate low-molecular-weight DOM (LMW-DOM) in the filtrate from ultrafiltration. The isolates from both methods and the LMW-DOM C18-extracts were characterized using Fourier transform infrared spectroscopy (FTIR) and direct temperature-resolved mass spectrometry (DT-MS). Based on mass balance and blank measurements, we found that the C18 SPE disks can be used to isolate bulk DOM and LMW-DOM from estuarine samples. FTIR and DT-MS analysis show that C18-extracted DOM and ultrafiltered high-molecular-weight DOM (HMW-DOM) differ markedly in chemical composition. The HMW-DOM is enriched in (degraded) polysaccharides along with aminosugars when compared with the C18-extracted DOM. The C18-extracted DOM appears enriched in aromatic compounds, probably from lignin and/or aromatic amino acids in proteins. C18 SPE of LMW-DOM samples from ultrafiltration increases the recovery of DOM from the total sample up to about 70%, compared to around 50% using ultrafiltration alone. Thus, a majority of the DOM can be isolated from estuarine samples by a combination of these techniques.