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 C ₁₈-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.