UV absorbance ratio index with size exclusion chromatography (URI-SEC) as an NOM property indicator

An advanced approach is presented for identifying and characterizing biopolymers of natural organic matter (NOM) in water samples. It involves a simple method encompassing the ratio of peak heights of high performance size exclusion chromatography (HPSEC) chromatograms based on ultraviolet absorbance (UVA) detection at 210 nm and 254 nm. The HPSEC system eliminates inorganic interferences by separation and makes it possible to obtain ratio values associated with organic components as a function of molecular weight (MW). Certain biopolymers show a distinctive ultraviolet absorbance ratio index (URI, UVA₂₁₀/UVA₂₅₄) because they contain different compositional proportions of UV-absorbing functional groups and sp²-hybridized carbon. URI values were found to be the lowest for humic acids (1.59 for a humic acid, highest aromaticity), intermediate for fulvic acids (1.88 for a fulvic acid, intermediate aromaticity), and highest for proteins (13.5 for BSA, lowest aromaticity). URI increases with the degree of eutrophication of natural waters by the increase of microbially derived components that have a high functional group proportion with a low sp²-hybridized carbon, and increases with oxidation (e.g. ozonation) by the cleavage of unsaturated bonds (decrease in unsaturated bonds and increase in functional group proportions).

A particular functional group displays its characteristic chemical behaviour when it is present in a compound. The chemical behaviour of NOM is highly related to its functional groups. The concept of URI considers the main functional groups of NOM and can act as an important water quality index. It may be used as a surrogate for assessing the effects of oxidation of NOM. The method is simple and can be easily applied to characterization of bulk water samples.