We reassessed the molecular weight of dissolved organic matter (DOM) determined by high pressure size exclusion chromatography (HPSEC) using measurements made with different columns and various generations of polystyrenesulfonate (PSS) molecular weight standards. Molecular weight measurements made with a newer generation HPSEC column and PSS standards from more recent lots are roughly 200 to 400 Da lower than initial measurements made in the early 1990s. These updated numbers match DOM molecular weights measured by colligative methods and fall within a range of values calculated from hydroxyl radical kinetics. These changes suggest improved accuracy of HPSEC molecular weight measurements that we attribute to improved accuracy of PSS standards and changes in the column packing. We also isolated DOM from wetlands in the Prairie Pothole Region (PPR) using XAD-8, a cation exchange resin, and PPL, a styrene-divinylbenzene media, and observed little difference in molecular weight and specific UV absorbance at 280 nm (SUVA280) between the two solid phase extraction resins, suggesting they capture similar DOM moieties. PPR DOM also showed lower SUVA280 at similar weights compared to DOM isolates from a global range of environments, which we attribute to oxidized sulfur in PPR DOM that would increase molecular weight without affecting SUVA280.
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We thank two anonymous reviewers for their helpful comments and the following individuals. Rob Lindberg, Dan Dunlap, and Brent Curtiss from Ohio State’s IT department helped recover data generated with MS-DOS compatible software (Figure 1) for this paper. Jill Kerrigan ran many of the absorbance measurements reported in this paper. Dave Mushet and Matthew Solensky of the USGS NPWRC along with Paula Dalcin-Martins and Garret Smith provided vital help with sample collection from PPR wetlands. Sara Breitmeyer and Brett Poulin (USGS) performed DOM isolation by XAD-8. This work was supported by NSF grants EAR 1246594 and EAR 1245135.