Relationships between molecular weight and fluorescence properties for size-fractionated dissolved organic matter from fresh and aged sources

Relationships between the molecular weight (MW) and fluorescence properties of dissolved organic matter (DOM) are important considerations for studies seeking to connect these properties to water treatment processes. Relationships between the size and fluorescence properties of nine allochthonous DOM sources (i.e. leaf leachates, grass, and headwaters) were measured using asymmetrical flow field-flow fractionation (AF4) with on-line absorbance and fluorescence detectors. Correlations between optical properties and MW were readily apparent using parallel factor analysis (PARAFAC) coupled to self-organizing maps (SOM): protein/polyphenol-like fluorescence (peaks B and T) was highest at lower molecular weights (<0.5 kDa), fulvic/humic-like fluorescence (peaks A, C, and M) was highest at mid-weights (0.5-1 kDa), and humic-like fluorescence (Peaks A + C) was highest at larger molecular weights (>1 kDa). Proportions of peaks B, T, and A + C were significantly correlated with MW (p <0.001). The first principal component (PC1, 42% of variation in fluorescence properties) was a significant predictor of sample MW (R2 = 0.63, p < 0.05), while scores on PC2 (27% of total variance) traced a source-based gradient from deciduous leachates/headwaters through to coniferous leachates/headwaters. PC3 (13% of var.) was also correlated with MW (p <0.005). A secondary peak in peak T fluorescence was associated with larger size fractions in aged sources, and scores on PC1 also traced a path from the leachates of fresher leaves, through more humified leaves, to headwaters. Findings are consistent with the hypothesis that the structure of aged DOM arises through supramolecular assembly.