Varations of molecular weight estimation by HP-size exclusion chromatography with UVA versus online DOC detection

High performance size exclusion chromatography (HPSEC) with ultraviolet absorbance (UVA) detection has been widely utilized to estimate the molecular weight (MW) and MW distribution of natural organic matter (NOM). However, the estimation of MW with UVA detection is inherently inaccurate because UVA at 254 nm only detects limited components (mostly pi bonded molecules) of NOM, and the molar absorptivity of these different NOM constituents is not equal. In comparison, a SEC chromatogram obtained with a DOC detector showed significant differences compared to a corresponding UVA chromatogram, resulting in different MW values as well as different estimates of polydispersivity. The MWs of Suwannee River humic acid (SRHA), Suwannee River fulvic acid (SRFA), and various mixtures thereof were estimated with HPSEC coupled with UVA and DOC detectors. The results show that UVA is not an adequate detector for quantitative analysis of MW estimation but rather can be used only for limited qualitative analysis. The NOM in several natural waters (Irvine Ranch, California groundwater, and Barr Lake, Colorado surface water) were also characterized to demonstrate the different MWs obtained with the two detectors. The results of the SEC-DOC chromatograms revealed NOM constituent peaks that went undetected by UVA. Utilizing online DOC detection, a better representation of NOM MWs was suggested, with NOM displaying higher weight-averaged MW (Mw) and lower number-averaged MW (Mn) as well as higher polydispersivity. A method for estimation of the MWs of NOM fractional components and polydispersivities is presented.