Limnol. Oceanogr., 44(5), 1999, 1316–1322

A 1,000-Dalton tangential-flow ultrafiltration (TFUF) membrane was used to isolate dissolved organic matter (DOM) from several freshwater environments. The TFUF unit used in this study was able to completely retain a polystyrene sulfonate 1,800-Dalton standard. Unaltered and TFUF-fractionated DOM molecular weights were assayed by high-pressure size exclusion chromatography (HPSEC). The weight-averaged molecular weights of the retentates were larger than those of the raw water samples, whereas the filtrates were all significantly smaller and approximately the same size or smaller than the manufacturer-specified pore size of the membrane. Moreover, at 280 nm the molar absorptivity of the DOM retained by the ultrafilter is significantly larger than the material in the filtrate. This observation suggests that most of the chromophoric components are associated with the higher molecular weight fraction of the DOM pool. Multivalent metals in the aqueous matrix also affected the molecular weights of the DOM molecules. Typically, proton-exchanged DOM retentates were smaller than untreated samples. This TFUF system appears to be an effective means of isolating aquatic DOM by size, but the ultimate size of the retentates may be affected by the presence of metals and by configurational properties unique to the DOM phase. The isolation and characterization of aquatic dissolved organic matter (DOM) in both limnological and oceanic environments has been the focus of a number of studies (Aiken et al. 1985, 1992; Orem et al. 1986; Orem and Hatcher 1987; Chen et al. 1993; Amon and Benner 1994, 1996; Chin et al. 1994, 1998; plus many others). The most common methods are comprised of either chromatographic fractionation using a hydrophobic resin (i.e., XAD 8 and 4) (Aiken et al. 1992) or physical separation by some form of ultrafiltration (Thurman et al. 1982; Aiken 1984). In particular tangential-flow ultrafiltration (TFUF, also known as cross-flow ultrafiltration) has been used extensively to isolate marine DOM from both coastal and open oceanic sties (Whitehouse et al. 1990; Benner et al. 1992; Buesseler et al. 1996; Mopper et al. 1996; Guoard and Santchi 1996; plus many others) and DOM from groundwater (Backhus and Gschwend 1990). The TFUF method is a physical separation process by which solvent and solute molecules smaller than the membrane pores pass through when a hydrostatic pressure is created. Molecules larger than the membrane pores are retained and concentrated. It is typically assumed that most of the retained DOM molecules have molecular weights greater than the manufacturer-specified membrane cutoff size. These isolates subsequently are used to characterize the various properties of DOM or are utilized in a wide range of geochemical and 1 To whom correspondence should be addressed.