Number-Average Molecular Weights of Natural Organic Matter, Hydrophobic Acids, and Transphilic Acids from the Suwannee River, Georgia, as Determined Using Vapor Pressure Osmometry

Abstract

Vapor pressure osmometry (VPO) was used to measure the number-average molecular weights (M n ) of natural organic matter (NOM) that was isolated using reverse osmosis, hydrophobic acids (HPOA) that were isolated by adsorption on XAD-8 resin, and transphilic acids (TPIA) that were isolated by adsorption on XAD-4 resin. All samples were isolated from the headwaters of the Suwannee River in southeastern Georgia, USA, in May 2012. NOM, HPOA, and TPIA samples had M n of 634 -11, 583 -8, and 498 -7 g/mol, respectively. Novel methodology was introduced wherein VPO measurements were made at room temperature, and M n values were rendered from a robust analysis of data that considered not only the well-known effect of hydronium ion but also the contributions of inorganic solutes present in the samples. The method was validated by results on a known sample (benzenehexacarboxylic acid), whose molecular weight was determined accurately to within 0.5% of the true value. The VPO-determined M n are compared to those from noncolligative methods to highlight uncertainty regarding a fundamental parameter of matter found in nearly all terrestrial waters.

Probing the organization of fulvic acid using a cationic surfactant

“…Still, the detailed organization of HS remains a matter of debate: molecular weight measurements performed at different stages of extensive fractionation schemes suggest that HS can be regarded as macromolecules or slightly branched polymers that can coil or adopt an extended conformation according to solution properties [1] [22] [23] [24], whereas some Size Exclusion Chromatography experiments and NMR results suggest that HS correspond to supramolecular associations of small heterogeneous molecules that can be disrupted in the presence of organic acids [6] [10] [25] [71] [72] [73]. To account for the surface-active properties of HS and the enhanced solubility of hydrophobic organic compounds in their presence [26], such molecular aggregates have also been described as micelles with a hydrophilic exterior and a hydrophobic core [9].…”

What is driving the NF-κB response in environmental water extracts?

“…Raeke et al (2016) recently found poor recovery of higher molecular weight organic matter by Oasis HLB, which was attributed to stronger sorption to the sorbent and consequently incomplete elution. Therefore, the poor recovery of HA compared to NOM may be due to differences in molecular weight, with HA typically having a larger molecular weight than NOM (Pavlik and Perdue, 2015).…”

Mobility of Dissolved Organic Matter from the Suwannee River (Georgia, USA) in Sand-Packed Columns

“…M n values determined by asymmetrical flow field-flow fractionation (AsFlFFF) using absorbance detection at 254 nm were 1,461 -7 Da for RFSW, 1,517 -8 Da for NOM, 1,471 -7 Da for HPOAs, and 1,095 -5 Da for TPIAs (Kuhn et al, 2014). AsFlFFF values tend to be much higher for DOM than are values determined by VPO (Pavlik and Perdue, 2014). The AsFlFFF values may be useful for comparison between samples but should not be thought of as true MWs.…”

“…The ''molar absorptivity'' (e 280 ), which has been shown to correlate with HS aromaticity, was calculated from absorbance at k at 280 nm and DOC concentrations to be in L/ (mol C$cm): 486 for NOM, 461 for HPOAs, and 262 for TPIA. The number average MW (M n ) values determined by vapor pressure osmometry (VPO) (Pavlik and Perdue, 2014) were 634 g/mol for NOM, 583 g/mol for HPOAs, and 498 g/mol for TPIA. M n values determined by asymmetrical flow field-flow fractionation (AsFlFFF) using absorbance detection at 254 nm were 1,461 -7 Da for RFSW, 1,517 -8 Da for NOM, 1,471 -7 Da for HPOAs, and 1,095 -5 Da for TPIAs (Kuhn et al, 2014).…”