Because CDOM is an optical description of the photoreactive component of bulk DOM, several methods exist to characterize CDOM physically and chemically. These properties exhibit wide variations among CDOM source (e.g., freshwater vs. marine CDOM). At present, no universally accepted method for isolating CDOM from the bulk fraction exists, and here we briefly review several methods for its isolation. Each method has its benefits and limitations and we urge readers to carefully consider the procedure used to isolate CDOM for measurement or use in experiments.
Physically, CDOM is operationally defined as that material which passes through a 0.2 /¿m filter. However, many studies have shown that the range of CDOM size in natural waters is variable. The distinction of high molecular weight (HMW) DOM from low molecular weight (LMW) is determined using ultrafiltration with a 1000 Dalton (1 kDa) cutoff filter, and some studies have partitioned DOM among several size classes [24,59-63]. Though ultrafiltration may be used to represent CDOM, Osburn and Boyd have unpublished results that show recoveries of UV absorbance at 320 nm vary with both the source of DOM and the method of CDOM extraction (tangential flow filtration and solid phase extraction, Figure 3). This is not surprising since fulvic acids, which have conjugated double bonds and absorb readily in the UV region of the solar spectrum, are less than 1 kDa. Reverse osmosis is another physical method for isolating organic solutes from natural waters, but this method also concentrates salts and may actually polymerize DOM compounds, thus altering the original DOM material. Further, use of reverse osmosis techniques make additional chemical analysis of DOM with high salt content analytically difficult, especially if lyophilization is involved as the high salt content may preclude complete drying of the sample [Osburn, unpublished results].
Chemical characterization of DOM has been studied extensively, providing a wealth of information regarding its chemical properties [17,20,25,60,64-89], CDOM is usually characterized as aquatic humic substances, such as humic and fulvic acids, owing to the presence of multiple double bonds in aromatic, aldehyde, and ketone groups. Isolation of humic substances involves their separation by adsorption on macroporous resins (e.g., XAD-8 or XAD-4) and elution at various pH . Humic acids are soluble above a pH of 2, while fulvic acids are soluble at any pH. Solid phase extraction (SPE) onto C18 resin is also employed to isolate CDOM [44,73]. Amador and coworkers have shown that
Mar = Marine
Est = Estuarine
Fresh = Freshwater
ER = Elizabeth River
TFF = Tangential Flow Filtration
C|8 Nexus = Solid phase extractants
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