APPLICATION OF A QUANTITATIVE METHOD TO CHARACTERIZE DOC USING ASYMMETRICAL FLOW FIELD-FLOW FRACTIONATION WITH ON-LINE UV AND DOC DETECTION

Naturally occurring dissolved organic carbon (DOC) is ubiquitous in all terrestrial and aquatic environments and plays an important role in its geochemical and biochemical evolution. A method of characterizing dissolved organic carbon (DOC) by Asymmetrical Flow Field-Flow Fractionation with on-line UV and DOC detection is described and applied to standards and natural waters. Poly(styrene sulphonate) polymer standards, Suwannee River humic standards, and naturally occurring surface water and groundwater DOC were analyzed using this coupled detection technique. Molecular weight (MW) determinations using UV analysis agreed with results of earlier studies of humic and fulvic acids. The MW determinations were, however, lower using DOC analysis than UV analysis. The difference was attributed to the insensitivity of the latter technique to non-aromatic carbon. Comparison of the two detector responses provided information about amounts of aromatic and aliphatic contents of different samples. A normalized intensity comparison (NIC) method, similar to SUVA254, was developed to distinguish differences in the relative amounts of aromatic and aliphatic carbon in DOC through a comparison of the two detector responses. The NIC method was applied to total DOC fractions in a given sample, showing an average aromatic content of the bulk DOC, as well as being used to identify aromatic content in a range of MWs from a single DOC fraction. Aromaticity of Suwannee River standards increased in the order: SRFA-SRNOM-SRHA. The aromaticity of surface waters (riverine and aquitard recharge) varied, with mean values approximating SRNOM. Groundwaters had the lowest aromaticity of all samples measured. All natural DOC samples showed increasing aromaticity with increasing MW. The lighter MW fractions of DOC were shown to be much more aliphatic, and could not be resolved with UV detection. A surface water sample of high MW and polydispersity, similar to SRHA, was shown to have very low aromaticity, similar to groundwater fulvic acids. DOC detection techniques are shown to produce more accurate estimates of MW than UV analysis. Through the coupling of the two detectors some characteristics of the structure of DOC can be obtained.