A critical evaluation of the cross-flow ultrafiltration technique for sampling colloidal organic carbon in seawater

The retention characteristics, integrity and performance of cross-flow ultrafilters [Amicon S10N1 with 1-kilodalton (kDa) cutoff] were examined in the laboratory. In addition, the effects of concentration factors and sample storage on dissolved organic carbon (DOC) mass balance and size fractionation were investigated using natural seawater and macromolecular solutions containing compounds of different molecular weights (MWs). The concentration of DOC in the permeate and the percentage of colloidal organic carbon (COC) retained by the ultrafilter change with the concentration factor during the ultrafiltration process. Thus, the DOC concentration in the entire permeate is necessary to reliably evaluate the DOC mass balance during ultrafiltration and for the calculation of the COC fraction. Replicate ultrafiltration experiments show that reproducible results can be achieved (±2%), and that losses or contamination problems during ultrafiltration were negligible. However, reproducible and accurate results for size fractionation of DOC require rigorous cleaning and strict sampling protocols. Short-term storage of coastal seawater samples (salinity ˜ 28; DOC ˜ 170 μM) did not significantly affect the overall results of DOC size fractionation but could have severe effects on minor constituents. A certain percentage of lower-MW (< 1 kDa) DOC can be retained by a 1-kDa ultrafilter, and a varying fraction of higher-MW macromolecules (> 1 kDa) can pass through the ultrafilter, depending on the concentration factor and bulk DOC concentrations. Inter-molecular interactions and/or steric effects of low-MW compounds and the slow breakthrough of high-MW macromolecules are likely the major processes responsible for these observations. Ultrafiltration of DOC in seawater can be characterized by a permeation model with a constant permeation coefficient of < 1. A high concentration factor is suggested for size fractionation of DOC in order to minimize the retention of low-MW DOC. In addition, results from isotopic (13C and 14C) and elemental (C and N) characterization of colloidal samples are presented.